# A 350-MHz Green Bank Telescope Survey of Unassociated
Fermi LAT Sources: Discovery and Timing of Ten Millisecond Pulsars

P. Bangale Department of Physics and Astronomy, West Virginia University,
Morgantown, WV 26506, USA  Department of Physics and Astronomy, Bartol
Research Institute, University of Delaware, Newark, DE 19716, USA  B.
Bhattacharyya National Centre for Radio Astrophysics, Tata Institute of
Fundamental Research, Pune 411 007, India  F. Camilo South African Radio
Astronomy Observatory (SARAO), 2 Fir Street, Observatory, Cape Town, 7925,
South Africa  C. J. Clark Max Planck Institute for Gravitational Physics
(Albert Einstein Institute), D-30167 Hannover, Germany  Leibniz Universität
Hannover, D-30167 Hannover, Germany  I. Cognard Station de radioastronomie de
Nançay, Observatoire de Paris, CNRS/INSU, F-18330  M. E. DeCesar College of
Science, George Mason University, Fairfax, VA 22030, resident at Naval
Research Laboratory, Washington, DC 20375, USA  E. C. Ferrara  Department of
Astronomy, University of Maryland, College Park, MD 20742  Center for Research
and Exploration in Space Science and Technology, NASA/GSFC, Greenbelt, MD
20771  NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA  P. Gentile
Department of Physics and Astronomy, West Virginia University, Morgantown, WV
26506, USA  L. Guillemot Laboratoire de Physique et Chimie de l’Environnement
et de l’Espace – Université d’Orléans / CNRS, F-45071 Orléans Cedex 02, France
Station de radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU,
F-18330 Nançay, France  J. W. T. Hessels ASTRON, the Netherlands Institute for
Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands  Anton
Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904,
1098 XH Amsterdam, The Netherlands  T. J. Johnson  College of Science, George
Mason University, Fairfax, VA 22030, resident at Naval Research Laboratory,
Washington, DC 20375, USA  M. Kerr Space Science Division, U.S. Naval Research
Laboratory, Washington, DC 20375, USA  M. A. McLaughlin Department of Physics
and Astronomy, West Virginia University, Morgantown, WV 26506, USA  Center for
Gravitational Waves and Cosmology, Chestnut Ridge Research Building,
Morgantown, WV 26505, USA  L. Nieder Max Planck Institute for Gravitational
Physics (Albert Einstein Institute), D-30167 Hannover, Germany  Leibniz
Universität Hannover, D-30167 Hannover, Germany  S. M. Ransom National Radio
Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
P. S. Ray Space Science Division, U.S. Naval Research Laboratory, Washington,
DC 20375, USA  M. S. E. Roberts  Eureka Scientific Inc, Oakland, CA. USA  J.
Roy  National Centre for Radio Astrophysics, Tata Institute of Fundamental
Research, Pune 411 007, India  S. Sanpa-Arsa  National Astronomical Research
Institute of Thailand (Public Organization), 260 Moo 4, T. Donkaew, A. Maerim,
Chiang Mai, 50180, Thailand  G. Theureau Laboratoire de Physique et Chimie de
l’Environnement et de l’Espace – Université d’Orléans / CNRS, F-45071 Orléans
Cedex 02, France  Station de radioastronomie de Nançay, Observatoire de Paris,
CNRS/INSU, F-18330 Nançay, France  LUTH, Observatoire de Paris, Université
PSL, Université Paris Cité, CNRS, 92195 Meudon, France  M. T. Wolff Space
Science Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA

###### Abstract

We have searched for radio pulsations towards 49 Fermi Large Area Telescope
(LAT) 1FGL Catalog $\gamma$-ray sources using the Green Bank Telescope at 350
MHz. We detected 18 millisecond pulsars (MSPs) in blind searches of the data;
10 of these were discoveries unique to our survey. Sixteen are binaries, with
eight having short orbital periods $P_{B}<1$ d. No radio pulsations from young
pulsars were detected, although three targets are coincident with apparently
radio-quiet $\gamma$-ray pulsars discovered in LAT data. Here, we give an
overview of the survey and present radio and $\gamma$-ray timing results for
the 10 MSPs discovered. These include the only isolated MSP discovered in our
survey and six short-$P_{B}$ binary MSPs. Of these, three have very low-mass
companions ($M_{c}\ll 0.1M_{\odot}$) and hence belong to the class of black
widow pulsars. Two have more massive, non-degenerate companions with extensive
radio eclipses and orbitally modulated X-ray emission consistent with the
redback class. Significant $\gamma$-ray pulsations have been detected from
nine of the discoveries. This survey and similar efforts suggest that the
majority of Galactic $\gamma$-ray sources at high Galactic latitudes are
either MSPs or relatively nearby non-recycled pulsars, with the latter having
on average a much smaller radio/$\gamma$-ray beaming ratio as compared to
MSPs. It also confirms that past surveys suffered from an observational bias
against finding short-$P_{B}$ MSP systems.

††facilities:  Green Bank Telescope (GBT), Fermi Gamma-ray Space Telescope
(Fermi), Giant Metrewave Radio Telescope (GMRT), Westerbork Synthesis Radio
Telescope (WSRT), Nançay Radio Telescope (NRT), Neil Gehrels Swift Observatory
(Swift), Chandra X-ray Observatory (CXO), X-ray Multi-Mirror Mission (XMM-
Newton), ††thanks: Corresponding authors:
P. Bangale (priyadarshini.bangale@gmail.com),
M. Kerr<EMAIL_ADDRESS>
##  1 Introduction

One of the major results to come out of over a decade of observations with the
Fermi Gamma-ray Space Telescope (hereafter Fermi) is that the majority of
Galactic high-energy ($E\sim 1$ GeV) $\gamma$-ray sources so far detected are
pulsars. At low Galactic latitudes ($b\lesssim 10^{\circ}$), it had been
suspected since the EGRET era that most discrete sources might be young
($\tau\lesssim 10^{5.5}$ yr), spin-powered pulsars (Roberts, 2005). Before the
launch of Fermi, the most likely population of Galactic $\gamma$-ray sources
at higher Galactic latitudes was thought to be nearby ($d\lesssim 2$ kpc)
middle-aged pulsars like PSR B1055$-$52 and Geminga, perhaps associated with
the Gould Belt of nearby star-forming regions (Gonthier et al., 2005).
Millisecond pulsars (MSPs, which, for the purpose of this paper, are defined
as having spin periods $P<10$ ms) were considered a possibly important class
as well, despite only one marginal detection with EGRET (Kuiper et al., 2000).
Radio pulsar surveys targeting EGRET source error boxes (Nice & Sayer, 1997;
Champion et al., 2005; Crawford et al., 2006) were notoriously unsuccessful,
with only three pulsars discovered in these surveys—the MSPs J0751$+$1807
(Lundgren et al., 1995) and J1614$-$2230 (Hessels et al., 2005; Demorest et
al., 2010), and the young PSR J1028$-$5819 (Keith et al., 2008)—that were
plausibly the counterparts of the $\gamma$-ray source targeted (Abdo et al.,
2010a). Very deep radio searches of low-latitude extended X-ray sources
discovered within EGRET error boxes were somewhat more efficient at
discovering potential $\gamma$-ray pulsars. Two young and energetic pulsars,
PSR J2021+3651 (Roberts et al., 2002a) and PSR J2229+6114 (Halpern et al.,
2001) were discovered this way, both of which proved to be powering their
respective coincident $\gamma$-ray sources (Halpern et al., 2008; Pellizzoni
et al., 2009).

With the launch of the Fermi satellite, it was quickly determined that MSPs
were efficient $\gamma$-ray emitters, with eight MSP detections reported
within the first few months of operations (Abdo et al., 2009b). In addition,
blind searches of the Fermi Large Area Telescope (LAT, Atwood et al., 2009)
data soon discovered 16 young pulsars, three of which were at mid-Galactic
latitudes ($10^{\circ}<|b|<30^{\circ}$ Abdo et al., 2009a). The 46
$\gamma$-ray pulsars reported in the First Fermi-LAT Pulsar Catalog (Abdo et
al., 2010a) include 15 pulsars at $|b|>5^{\circ}$. Of these, eight are MSPs,
four are young radio pulsars, and three are detected only in $\gamma$ rays. It
was therefore expected that deep radio searches of unassociated LAT sources at
mid latitudes should result in a mix of MSPs and young pulsars.

A sensitive 820-MHz radio search with the 100-m Green Bank Telescope (GBT) of
25 of the brightest LAT sources not associated with previously known sources
of potential $\gamma$ rays (i.e., energetic pulsars or blazars) discovered no
young pulsars. However, out of the eight sources searched at $|b|>5^{\circ}$,
three were found to be binary radio MSPs (Ransom et al., 2011). One of these
four, PSR J2214+3000, is in a tight ($P_{B}\sim 10$ hr) orbit around a very
low-mass (minimum companion mass $M_{c}\sim 0.02M_{\odot}$) companion and
exhibits regular eclipses, becoming at that time only the fourth known “black
widow” system discovered in the Galactic field (outside of a globular
cluster).

Between the discovery of the first MSP in 1982, the isolated 1.6-ms PSR
B1937+21 (Backer et al., 1982), and the launch of Fermi in 2008,
psrcat111http://www.atnf.csiro.au/research/pulsar/psrcat/ (Manchester et al.,
2005) lists the discovery of approximately 70 MSPs in the Galactic field. In
comparison, since data from Fermi became publicly available in 2009, over 100
MSPs (including the ones reported here) were discovered in the Galactic field
by targeting Fermi LAT sources (Cognard et al., 2011; Keith et al., 2011;
Guillemot et al., 2012; Kerr et al., 2012; Ray et al., 2012; Barr et al.,
2013; Camilo et al., 2015; Cromartie et al., 2016; Pleunis et al., 2017;
Deneva et al., 2021; Bhattacharyya et al., 2013, 2021; Wang et al., 2021;
Clark et al., 2023a). Surprisingly, only one young $\gamma$-ray pulsar has
been discovered through targeted radio searches of Fermi sources (Camilo et
al., 2012). Surveys such as these are rapidly expanding the known population
of MSPs, which may help to better understand the nature of binary and isolated
MSPs. Also, continued long-term timing has shown that some of these new MSPs
are useful in pulsar timing arrays for gravitational wave detection (Hobbs et
al., 2010; Arzoumanian et al., 2015; FERMI-LAT Collaboration et al., 2022).
Here we present an overview of the most prolific of the early Fermi targeted
searches, reporting the discovery of 10 MSPs and the detection of eight more
MSPs that were found in subsequent surveys.

This paper presents our survey results in detail (a brief overview of the new
discoveries was published previously in Hessels et al., 2011). Here, we give
detailed radio timing solutions for the ten MSPs discovered in this survey. We
also present the detection of pulsed $\gamma$-ray emission from nine of these.
Six of these were previously reported in the second LAT pulsar catalog (Abdo
et al., 2013, hereafter 2PC), and all were included in the third LAT pulsar
catalog (Smith et al., 2023, hereafter 3PC). The new discoveries include one
isolated MSP and six in close binaries with periods under one day. Three of
these have very low mass companions ($M_{c}\ll 0.1M_{\odot}$), and an
additional two exhibit extensive eclipses. One of the newly discovered MSPs is
a binary which appears to be a chance discovery, well outside the target LAT
positional error ellipse. In §2 we describe the survey. In §3 we describe the
subsequent timing program and summarize the survey results as well as describe
follow-up radio, X-ray, and $\gamma$-ray studies of these sources. In §4 we
discuss the implications of the new discoveries for different pulsar
populations with detailed discussions of the new black widow and redback
systems.

##  2 Search observations and results

Candidate sources for our radio searches were drawn from a preliminary version
of the first Fermi-LAT source catalog (1FGL, Abdo et al., 2010b). We
considered the fraction of the sky which is visible from the GBT for more than
$\sim$1 hr, or equivalently Dec. $>-$40∘. We chose 350 MHz as the observation
frequency due to the steep spectra of MSPs (Kramer et al., 1998) and the
larger beam size of the GBT at low frequencies. The positional uncertainty of
the LAT sources was typically less than the 35′ full-width-half-max (FWHM)
beam of the 350-MHz system. We chose sources well away from the Galactic
plane, i.e., $|b|$ $>$ 5∘, where sky temperature and the effects of
interstellar scattering are reduced. We excluded sources that had viable
Active Galactic Nuclei (AGN) counterparts (Abdo et al., 2010c) and sources
with statistically significant variability as defined in 1FGL, as pulsars had
long been recognized as a non-variable $\gamma$-ray population (e.g.
McLaughlin et al., 1996, however, see Roberts et al. (2002b); Mayer et al.
(2013); Stappers et al. (2014) for discussions of $\gamma$-ray variability
from pulsar wind and accretion flows; and Allafort et al. (2013) for the
discovery of a rare example of $\gamma$-ray pulsar variability). We also
excluded sources in the Fermi-LAT Bright Source List (Abdo et al., 2009c)
since those had largely been surveyed with the GBT at 820 MHz by Ransom et al.
(2011). There is no clear relation between $\gamma$-ray and radio pulsed flux
(3PC), which likely depends to a large degree on beaming geometry, and so
targeting a fainter $\gamma$-ray population would not be expected to yield a
smaller (or larger!) fraction of radio pulsar counterparts.

Among the sources that passed these criteria, we generally preferred sources
that had an obviously ‘pulsar-like’ spectrum, i.e., a power-law behavior at
lower energies but with significant curvature/cutoff at higher energies.
However, we did not exclude sources based solely on spectra but balanced
spectral desirability with considerations of visibility and observing
efficiency within each scheduled observation session. By following this
procedure, 49 sources were finally observed throughout 2009. The 1FGL and 3FGL
source names, pointing positions, 3FGL $\gamma$-ray fluxes, variability
indices, observation dates, exposure lengths, and minimum detectable fluxes
are listed in Table 1, along with source classifications.

Table 1: 350-MHz Observations of Fermi-LAT Sources

1FGL | $l$ | $b$ | 3FGL | 3FGL off | 3FGL Flux | Var. Idx. | Date | $t_{\mathrm{obs}}$ | Smin | Notes
---|---|---|---|---|---|---|---|---|---|---
| deg. | deg. |  | deg. |  $10^{-9}$ ph cm-2 s-1 |  |  | min. | mJy |
J0008.3$+$1452 | 107.65 |  $-$46.70 | J0008.3+1456 | 0.046 | 0.41 | 34.44 | 2009-10-24 | 22 | 0.15 | NVSS J000825$+$145635
J0023.5$+$0930 | 111.53 |  $-$52.79 | J0023.4+0923 | 0.082 | 1.12 | 51.37 | 2009-10-25 | 32 | 0.13 | PSR J0023$+$0923
J0046.8$+$5658 | 122.27 |  $-$5.94 | J0047.0+5658 | 0.077 | 1.58 | 74.44▲ | 2009-10-25 | 32 | 0.21 | GB6 J0047$+$5657
J0103.1$+$4840 | 124.97 |  $-$14.15 | J0102.8+4840 | 0.100 | 2.19 | 55.01 | 2009-11-04 | 32 | 0.16 | PSR J0102$+$4839
J0106.7$+$4853 | 125.50 |  $-$13.89 | J0106.5+4855 | 0.025 | 3.36 | 41.67 | 2009-10-25 | 32 | 0.16 | PSR J0106$+$4855∙
J0226.3$+$0937 | 158.20 |  $-$46.63 | J0226.3+0941 | 0.041 | 0.68 | 95.92▲ | 2009-11-04 | 32 | 0.14 | NVSS J022634$+$093843
J0305.0$-$0601 | 185.36 |  $-$51.88 | None | – | – | – | 2009-11-04 | 32 | 0.10 | PMN J0304–0608
J0308.6$+$7442 | 131.73 | 14.23 | J0308.0+7442 | 0.016 | 3.04 | 50.41 | 2009-10-25 | 32 | 0.17 | PSR J0307$+$7443
J0311.3$-$0922 | 191.40 |  $-$52.54 | None | – | – | – | 2009-11-04 | 32 | 0.10 |
J0340.4$+$4130 | 153.81 |  $-$11.02 | J0340.3+4130 | 0.029 | 3.66 | 60.97 | 2009-10-25 | 32 | 0.16 | PSR J0340$+$4130
J0523.5$-$2529 | 228.24 |  $-$29.80 | J0523.3–2528 | 0.043 | 1.77 | 50.15 | 2009-10-25 | 18 | 0.13 | CRTS J052316.9$-$252737
J0533.9$+$6758 | 144.78 | 18.16 | J0534.0+6759 | 0.037 | 1.40 | 60.02 | 2009-10-24 | 28 | 0.17 | PSR J0533$+$6759★
J0545.6$+$6022 | 152.45 | 15.74 | J0545.6+6019 | 0.045 | 1.27 | 35.95 | 2009-10-25 | 32 | 0.17 |
J0547.0$+$0020c | 205.14 |  $-$14.15 | J0546.4+0031c | 0.244 | 0.57 | 53.31 | 2009-10-27 | 32 | 0.16 |
J0622.2$+$3751 | 175.84 | 10.96 | J0622.2+3747 | 0.061 | 2.18 | 53.97 | 2009-10-27 | 32 | 0.15 | PSR J0622$+$3749∙
J0803.1$-$0339 | 224.67 | 14.09 | J0803.3–0339 | 0.088 | 0.99 | 69.53 | 2009-10-27 | 18 | 0.16 | TXS 0800$-$034
J0843.4$+$6718 | 147.70 | 35.58 | J0843.4+6713 | 0.085 | 0.64 | 55.97 | 2009-10-27 | 32 | 0.13 | PSR J0843$+$67†
J0902.4$+$2050 | 206.66 | 37.74 | J0902.4+2050 | 0.025 | 1.23 | 101.83▲ | 2009-10-27 | 32 | 0.10 | NVSS J090226$+$205045
J0929.0$-$3531 | 263.05 | 11.21 | J0928.9–3530 | 0.030 | 0.64 | 37.60 | 2009-10-24 | 32 | 0.17 | NVSS J092849$-$352947
J0953.6$-$1505 | 251.85 | 29.66 | J0953.7–1510 | 0.094 | 1.25 | 40.21 | 2009-10-24 | 32 | 0.11 |
J0955.2$-$3949 | 269.96 | 11.49 | J0954.8–3948 | 0.118 | 1.30 | 51.34 | 2009-10-24 | 32 | 0.14 | PSR J0955$-$3947†
J1119.9$-$2205 | 276.51 | 36.04 | J1119.9–2204 | 0.038 | 2.70 | 62.62 | 2009-10-24 | 32 | 0.11 | CRTS J111958.3$-$220456
J1124.4$-$3654 | 284.17 | 22.79 | J1123.9–3653 | 0.085 | 2.18 | 34.58 | 2009-10-24 | 32 | 0.13 | PSR J1124$-$3653
J1142.7$+$0127 | 267.51 | 59.44 | J1142.9+0120 | 0.033 | 1.00 | 70.84 | 2009-10-24 | 32 | 0.12 | PSR J1142$+$0119★
J1302.3$-$3255 | 305.60 | 29.90 | J1302.3–3259 | 0.075 | 1.99 | 39.75 | 2009-10-24 | 32 | 0.18 | PSR J1302$-$3258
J1312.6$+$0048 | 314.73 | 63.20 | J1312.7+0051 | 0.066 | 2.41 | 46.35 | 2009-10-24 | 32 | 0.13 | PSR J1312$+$0051★
J1544.5$-$1127 | 356.22 | 32.96 | J1544.6–1125 | 0.026 | 1.01 | 47.47 | 2009-10-24 | 32 | 0.17 | 1RXS J154439.4$-$112820
J1549.7$-$0659 | 1.23 | 35.01 | J1549.7–0658 | 0.006 | 0.96 | 48.92 | 2009-10-24 | 32 | 0.15 | PSR J1551$-$0658
J1600.7$-$3055 | 344.06 | 16.46 | J1600.8–3053 | 0.031 | 1.08 | 42.59 | 2009-11-04 | 32 | 0.21 | PSR J1600$-$3053△
J1627.6$+$3218 | 52.99 | 43.28 | J1627.8+3217 | 0.074 | 0.57 | 33.03 | 2009-11-06 | 32 | 0.13 | PSR J1627$+$3219†
J1722.4$-$0421 | 18.30 | 17.60 | J1722.7-0415 | 0.203 | 1.02 | 34.03 | 2009-11-04 | 32 | 0.22 |
J1730.7$-$0352 | 19.92 | 15.98 | J1730.6–0357 | 0.113 | 1.33 | 29.97 | 2009-11-04 | 32 | 0.23 |
J1806.2$+$0609 | 33.32 | 12.83 | J1805.9+0614 | 0.127 | 1.12 | 40.32 | 2009-11-04 | 32 | 0.25 | PSR J1805$+$0615†
J1810.3$+$1741 | 44.61 | 16.86 | J1810.5+1743 | 0.054 | 2.54 | 51.86 | 2009-10-25 | 32 | 0.20 | PSR J1810$+$1744
J1858.1$-$2218 | 13.57 |  $-$11.40 | J1858.2–2215 | 0.047 | 1.65 | 55.58 | 2009-10-24 | 32 | 0.25 | PSR J1858$-$2216★
J1903.8$-$3718c | 359.76 |  $-$18.39 | None | – | – | – | 2009-10-24 | 32 | 0.22 |
J1921.2$+$0132 | 37.70 |  $-$5.93 | J1921.2+0136 | 0.090 | 1.27 | 35.34 | 2009-11-03 | 32 | 0.36 | PSR J1921$+$01
J2023.7$-$1141 | 32.62 |  $-$25.69 | J2023.6–1139 | 0.007 | 1.05 | 62.82 | 2009-10-24 | 32 | 0.14 | PMN J2023$-$1140
J2043.2$+$1709 | 61.89 |  $-$15.32 | J2043.2+1711 | 0.021 | 4.40 | 59.09 | 2009-10-24 | 32 | 0.15 | PSR J2043$+$1711△
J2055.2$+$3144 | 75.35 |  $-$8.62 | None | – | – | – | 2009-10-24 | 32 | 0.18 |
J2057.4$+$3057 | 75.00 |  $-$9.43 | None | – | – | – | 2009-10-25 | 32 | 0.18 |
J2107.5$+$5202c | 92.23 | 3.07 | J2108.1+5202 | 0.117 | 1.66 | 57.21 | 2009-10-25 | 32 | 0.33 |
J2112.5$-$3044 | 14.90 |  $-$42.44 | J2112.5–3044 | 0.009 | 3.26 | 51.84 | 2009-10-24 | 32 | 0.14 |
J2129.8$-$0427 | 48.97 |  $-$36.96 | J2129.6–0427 | 0.071 | 0.91 | 60.25 | 2009-10-24 | 32 | 0.14 | PSR J2129$-$0429
J2139.9$+$4715 | 92.62 |  $-$4.03 | J2140.0+4715 | 0.024 | 3.86 | 39.37 | 2009-10-25 | 32 | 0.22 | PSR J2139$+$4716∙
J2204.6$+$0442 | 64.80 |  $-$38.66 | J2204.4+0439 | 0.059 | 0.49 | 59.90 | 2009-11-04 | 32 | 0.12 | 4C $+$04.77
J2216.1$+$5139 | 99.92 |  $-$4.18 | J2215.6+5134 | 0.048 | 2.17 | 56.86 | 2009-10-25 | 32 | 0.19 | PSR J2215$+$5135
J2256.9$-$1024 | 59.21 |  $-$58.29 | J2256.7–1022 | 0.054 | 1.32 | 33.45 | 2009-10-24 | 32 | 0.12 | PSR J2256$-$1024†
J2257.9$-$3643 | 4.03 |  $-$64.21 | J2258.2–3645 | 0.070 | 0.27 | 33.94 | 2009-11-04 | 32 | 0.12 | MRSS 406$-$025483

Note. — Columns include the 1FGL name of the source, Galactic longitude $l$
and latitude $b$, the 3FGL name of the source, the offset of our pointing
center from the center of the 3FGL position ellipse in degrees (note that all
beams with 3FGL counterparts completely cover the 3FGL error ellipse), the
1$-$100 GeV $\gamma$-ray flux in photons cm-2 s-1, the $\gamma$-ray
variability index (with higher values indicating greater probabilities of
being a variable source, with a typical range for non-variable sources of
35–70), the date of the observation, the minimum detectable flux (in mJy),
calculated from Eq. 1, and a note indicating a source classification, if
identified. The pulsars discovered in this survey are indicated with bold
face.

$\bigstar$: $\gamma$-ray MSP discovered in 820 MHz survey and subsequently
detected in 350 MHz data.

$\vartriangle$: MSP already discovered in other pulsar survey.

$\bullet$: Young blind search $\gamma$-ray pulsar.

$\blacktriangle$: $\gamma$-ray variability index above 72.44, which indicates
the source is variable on the scale of months with $>99\%$ confidence. This
information was not available at the time of source selection.

$\dagger$: Recently-discovered MSPs:
http://astro.phys.wvu.edu/GalacticMSPs/GalacticMSPs.txt

The observations were performed using the 4096-channel 100-MHz bandwidth mode
of the GUPPI (Green Bank Ultimate Pulsar Processing Instrument) backend on the
GBT (DuPlain et al., 2008) with a central frequency of 350 MHz. The
integration times were generally 32 minutes, making this about an order of
magnitude more sensitive than any prior large-scale surveys of the Northern
sky and five to six times more sensitive than the Green Bank North Celestial
Cap pulsar survey (Stovall et al., 2014), at least for sources near the center
of our beam and assuming typical pulsar spectral indices. The sensitivity of
the search is obtained by using the modified radiometer equation:

$S_{\mathrm{min}}=\frac{(S/N)_{\mathrm{min}}~{}(T_{\mathrm{sys}}+T_{\mathrm{sky}})}{G\sqrt{n_{p}t_{\mathrm{obs}}\triangle
f}}\sqrt{\frac{W}{P-W}}$ (1)

where (S/N)min = 8 is the threshold signal to noise ratio used, G = 2 K Jy-1
is the effective gain of the GBT, np = 2 is the number of polarizations
summed, $\triangle f$ = 100 MHz is the total observing bandwidth,
$T_{\mathrm{sys}}$ = 25 K is the system temperature, $T_{\mathrm{sky}}$ is the
sky background temperature, $t_{\mathrm{obs}}$ is the integration time, P is
the period of the pulsar, and W is the pulse width. For example, given W =
0.1P, typical for MSPs, a sky background temperature of 50 K, typical for
sources off of the Galactic plane at this frequency (Haslam et al., 1982), and
an integration time of 32 minutes, the minimum detectable flux is 0.2 mJy. We
list the nominal sensitivity towards each pointing position in Table 1,
assuming W = 0.1P.

Our actual sensitivity at the beam center was likely somewhat, and perhaps
substantially, worse than what would be inferred from the radiometer equation.
This equation does not take into account the effect of radio frequency
interference (RFI), which results in parts of the bandpass and some intervals
in time being unusable. It also assumes full sensitivity across the entire
observing bandwidth, whereas, in reality, the sensitivity decreases at the
edges of the band, resulting in a smaller effective bandwidth. The assumption
of W = 0.1P will not be true for all pulsars. Some may have intrinsically
broadened pulse profiles, and pulsars with higher dispersion measures (DMs)
may experience appreciable pulse broadening due to interstellar scattering and
dispersion. In addition, it is important to note that pulsars with average
fluxes greater than the quoted minimum detectable flux could be missed due to
interstellar scintillation, which can cause the fluxes of pulsars to vary
dramatically from epoch to epoch. Sensitivity is also worse for very short
orbit binaries ($P_{b}\lesssim 5$ hr) because we only use an acceleration
search over the first-period derivative (see below), which cannot fully
correct for the orbital acceleration over a 32-minute observation. And
finally, many binary pulsars are in eclipse for a significant fraction of
their orbit, and during this survey, we only observed each source one time and
so could have missed systems that, on average, are quite bright.

We de-dispersed the data over the range from 0 pc cm-3 to twice the maximum DM
predicted by the NE2001 Galactic electron density model in the direction of
each source (Cordes & Lazio, 2002); our search was prior to the release of the
newer YMW16 electron density model Yao et al. (2017). We performed
acceleration searches up to $z_{\mathrm{max}}$ (the maximum Fourier frequency
derivative) of 200 to improve sensitivity to short-orbit binaries. Up to eight
harmonics were summed in the Fourier power spectra using the standard tools
found in PRESTO222http://www.cv.nrao.edu/$\sim$sransom/presto/ (Ransom et al.,
2002).

In our survey data, we discovered 10 MSPs and detected four more that had
recently been discovered by other surveys. A subsequent survey using the GBT
at a central frequency of 820 MHz targeted more than 100 LAT sources (Sanpa-
Arsa, 2016), several of which we had also targeted with our 350-MHz survey.
These discoveries are summarized in Table 2. This higher frequency survey
discovered five pulsars within our sample. Subsequent folding of our data
using the 820-MHz determined periods and DMs resulted in clear detections of
four of these pulsars, although mostly at significance levels below our
criteria for a discovery. Of these four, PSRs J0533+6759, J1312+0051, and
J1858$-$2216 have broad pulses, detected with low signal-to-noise in our data.
The other, PSR J1142+0119, was discovered first in the 820-MHz data. The final
820-MHz discovery, PSR J1921+0137, is not detectable in our survey, likely due
to either a flat spectrum, an eclipse, or scintillation.

All of the 10 discoveries are MSPs, with the longest spin period being 7.6 ms.
Our survey also included three LAT sources which were later determined to be
young, slow pulsars through blind searches of the LAT $\gamma$-ray data
(Pletsch et al., 2012). Folding our data with the $\gamma$-ray period over a
plausible range of DMs failed to detect any of these three. These results are
summarized in Table 2, and the 350-MHz pulse profiles of the 10 discoveries
are shown in Figure 1. The mean flux densities in the table have been
calculated using the radiometer equation (Eq. 1) and the appropriate sky
temperatures for each source (Haslam et al., 1982). No estimates for
statistical uncertainty for variability due to scintillation are available.

Figure 1: Pulse profiles from the 350-MHz discovery observations for newly
discovered MSPs. These are shifted so that the main peak is at phase 0.5. Two
cycles are shown for clarity. Image credit: Hessels et al. (2011)

As described above, our pointing positions were based on an early version of
1FGL. Improved positions and $\gamma$-ray spectra were published in the second
and third Fermi-LAT catalogs (2FGL and 3FGL, Nolan et al., 2012; Acero et al.,
2015), and so we list the associated 3FGL source and the offset from our
pointing position (in degrees) in Table 1. In most cases, our pointing
positions were consistent with the 3FGL positions, and the entire 3FGL error
ellipse was contained within the half-power beamwidth. However, five of our
pointings have no 3FGL counterpart within the FWHM beam, presumably due to
either improved background modeling or previously confused sources being
resolved. Our large beam also allowed for detecting pulsars well outside the
LAT error ellipses, as was the case for our chance discovery of PSR
J1551$-$0658\. For the other 18 MSPs, the pulsars are apparently associated
with the 3FGL sources. Seven 3FGL sources that we searched have now been
plausibly associated with AGN; 3FGL J0523.5$-$2529 (Strader et al., 2014) and
J1119.9$-$2205 (Swihart et al., 2022) are low-mass binary candidates; and 3FGL
J1544.6$-$1125 is associated with a transitional MSP candidate (Bogdanov et
al., 2015). We list these new associations in Table 1.

In the long interval between the survey observations and the finalization of
this manuscript, the 4FGL Fermi point source catalogs (Abdollahi et al., 2020)
have been released. Because the $\gamma$-ray sources reported in this work are
relatively bright, the properties are already well-determined in the 3FGL
catalog, and we determined that revising all quantities to their 4FGL values
is not warranted.

##  3 Radio timing observations and results

Following each discovery, we began regular timing observations with the GBT as
soon as was feasible, which in a few cases was up to six months after the
initial discovery observation. All GBT observations used the GUPPI
spectrometer333https://wikio.nrao.edu/bin/view/CICADA/GUPPiUsersGuide,
typically at a central frequency of 820 MHz due to receiver availability, and
when possible at 350 MHz. There were additional timing observations done at
central frequencies of 1500 and 1900 MHz for several of the pulsars. For PSRs
J0340+3140 and J1551$-$0568, the GBT campaigns were supplemented by
observations with the Nançay telescope in France at frequencies of 1376, 1408,
and 1598 MHz. For PSR J1551$-$0568, the position outside the Fermi error box
was first determined with an imaging/pulsed observation by the Giant Metrewave
Radio Telescope (GMRT) at a frequency of 322 MHz (Roy & Bhattacharyya, 2013).
Two of our sources, PSRs J0023+0923 and J0340+4130, have been included as
targets in the NANOGrav Pulsar Timing Array and so are being extensively timed
(Alam et al., 2021a, b). For more details of exact observations obtained for
each pulsar, see the accompanying files, which contain the pulse time-of-
arrival (TOA) data.

We observed each pulsar at least once a month for a span of no less than one
year, with at least one densely sampled set of observations over a period of a
few weeks to obtain phase-connected solutions. Individual integration times
were adjusted according to pulsar strength, and ranged from 5 minutes to 0.5
hr. In some instances, we performed gridding observations at 820 or 1420 MHz
to obtain immediately improved positions that increased observing efficiency
and simplified the determination of timing solutions. In a few cases, X-ray
observations from a Neil Gehrels Swift Observatory (Swift) campaign to
systematically survey unassociated Fermi-LAT sources provided higher precision
candidate positions (Falcone et al., 2011). Until obtaining enough timing
points to yield an adequate orbital solution for each pulsar, we recorded data
in search mode, which were analyzed using PRESTO (Ransom et al., 2002). In
some cases, once a reasonable initial solution was obtained, we recorded data
in full-Stokes (polarimetry) fold mode, with 256 bins per pulse period. A
1-minute observation with a 25-Hz pulsed noise diode was performed before each
observation for polarization calibration. These data were analyzed with
Psrchive (Hotan et al., 2004). In either case we used 2048 frequency channels
typically spanning 100 MHz of band at a central frequency of 350 MHz, 200 MHz
of band at 820 MHz, or 800 MHz of band at the higher frequencies (1500 and
1900 MHz).

To determine the TOAs, a high S/N profile at each frequency was used as a
reference template. TOAs were then calculated by cross-correlating each pulse
profile with the reference template profile at each frequency (Taylor, 1992).
A model ephemeris was then fitted to the topocentric TOAs. In the case of a
binary, an initial binary solution ephemeris was obtained by fitting a
sinusoid to the measured barycentric frequencies. Timing solutions
accompanying this paper were obtained with Tempo2 (Hobbs et al., 2006) and
generally using the JPL DE421 Solar System model (Standish 2004). Exceptions
are noted below.

Six of our discoveries and two of the re-detections are in tight binaries with
orbital periods $P_{B}<1$ day. Three of the short-period binaries have very
low-mass companions ($M_{c}\ll 0.1M_{\odot}$), and so we identify them with
the ‘black widow’ class of MSPs. Two of these black widows, PSRs J1124$-$3653
and J1810+1744, also exhibit eclipses. Two of the other tight binaries, PSRs
J2129$-$0429 and J2215+5135, have minimum companion masses in the $M_{c}\sim
0.2-0.4~{}M_{\odot}$ range, exhibit extensive eclipses, and have very strong
orbital period variation. We classify them as ‘redbacks’ (Roberts, 2011). The
final short-period binary, PSR J1302$-$3258, occasionally seems to eclipse at
low frequencies for $\sim$10% of the orbit but has a companion mass consistent
with a white dwarf and shows no other characteristics of being a redback.
Three of the other discoveries–PSRs J0102+4839, J0307+7443, and
J1551$-$0658–are in long-period ($P_{B}>1$ day) binaries around companions
whose minimum masses are consistent with being white dwarfs. Only two of the
detected MSPs are isolated; one of them, PSR J0533+6759, is a weak re-
detection of a pulsar first discovered at 820 MHz. The other, PSR J0340+4130,
is an isolated MSP which we study in this paper.

Because the LAT data span over 10 years, we found that the timing solutions
established solely via these radio pulsar timing campaigns were sometimes
inadequate to accurately predict the pulse phase over the full duration. This
is particularly true for the compact binaries exhibiting nondeterministic
orbital period variations. Consequently, we extended the timing solutions
using the Fermi-LAT data. In brief, we used maximum likelihood methods similar
to those in Ray et al. (2011) and Kerr et al. (2015). However, these
approaches are based on first estimating TOAs from LAT data. Here, we directly
optimize the unbinned likelihood using PINT444https://github.com/nanograv/PINT
(Luo et al., 2021) and estimate parameters and their uncertainties using
standard methods. Parameters like spin frequency and its derivative and proper
motions, whose effect on pulsar timing residuals is cumulative in time, are
often best estimated using LAT data, while others benefit from the generally
higher precision of the radio TOAs. For each pulsar, we determined the optimal
set of parameters to fit using each data set, and we iteratively fit the
timing solution to the two data sets independently, yielding a timing solution
that adequately describes both data sets and provides optimal parameter
estimates. For the two redback pulsars, J2129$-$0429 and J2215+5135, the
orbital period variations were so substantial that we were unable to obtain a
timing solution in this manner. Instead, we used only LAT data and followed
the method of Clark et al. (2020) to estimate the orbital period evolution.

The timing solutions555We exclude the complex orbital period variations for
PSR J2129$-$0429 and PSR J2215+5135; see
https://fermi.gsfc.nasa.gov/ssc/data/access/lat/ephems/ for these values. for
the 10 newly discovered MSPs are presented in Tables 3 and 4. For some
pulsars, an “EFAC” term is included to describe additional scatter in the
radio data beyond that indicated simply by radiometer noise. Errors reported
by Tempo2 and given in these tables are scaled such that the $\chi^{2}$ of the
residuals is unity. For all of the MSPs, in addition to the measured $\dot{P}$
we give an “intrinsic” $\dot{P}$ which has been corrected for acceleration due
to proper motion when available (i.e., the Shklovskii (1970) effect),
acceleration toward the Galactic plane, and differential acceleration parallel
to the Galactic plane (Nice & Taylor, 1995). We also calculate a transverse
velocity, given the DM-derived distance. Because of uncertainties in the
distance and the Galactic rotation model, there is additional uncertainty in
these estimates, which is hard to quantify. We use our best estimate of the
intrinsic $\dot{P}$ for all derived quantities ($\dot{E}$, $B$, and $\tau$) in
Tables 3 and 4. The flux densities given are mean values of detections (not
including eclipse times) calculated using the modified radiometer equation at
350, 820, and 1500 MHz.

Table 2: Properties of pulsars observed in this survey  Name | $P$ | $S_{350}$ |  $F_{\gamma}$11Gamma-ray fluxes in the first group are from this work. Others are taken from the 3FGL catalog. |  $\dot{E}$22The $\dot{E}$ values have been corrected for the Shklovskii (Shklovskii, 1970) effect (except for those marked by * ) when proper motion information is available, and all have been corrected for the perpendicular and parallel components of acceleration with respect to the Galactic plane (Nice & Taylor, 1995). | DM | Dist. 33The distance is estimated using the YMW16 electron distribution model (Yao et al., 2017). The distances reported in Hessels et al. (2011) were based on the NE2001 model, as YMW16 model was not complete then. | $P_{B}$ |  $M_{c}$ 44The minimum companion masses ($M_{c}$) were calculated assuming a pulsar mass of 1.4 M⊙ and an orbital inclination of 90∘.
---|---|---|---|---|---|---|---|---
| ms | mJy | $10^{-11}{\rm erg}\,{\rm cm}^{-2}{\rm s}^{-1}$ |  $10^{34}\,$ergs ${\rm s}^{-1}$ | pc cm-3 | kpc | days | $M_{\odot}$
Pulsars discovered in searches of 350 MHz data
J0023+0923 | 3.05 | 5.4 | $0.7\pm 0.1$ | 1.3 | 14.3 | 1.2 | 0.14 | 0.01
J0102+4839 | 2.96 | 2.8 | $1.6\pm 0.1$ | 1.6 | 53.5 | 2.3 | 1.6 | 0.18
J0307+7443 | 3.16 | 1.8 | $1.6\pm 0.1$ | 2.2 | 6.3 | 0.4 | 37.1 | 0.20
J0340+4130 | 3.30 | 6.5 | $2.1\pm 0.1$ | 0.7 | 49.6 | 1.6 | Isolated | N/A
J1124$-$3653 | 2.41 | 2.2 | $1.3\pm 0.1$ | 1.5 | 44.9 | 1.0 | 0.23 | 0.02
J1302$-$3258 | 3.77 | 1.3 | $1.2\pm 0.1$ | 0.4 | 26.2 | 1.4 | 0.78 | 0.15
J1551$-$065855Not associated with the LAT source. | 7.09 | 0.7 | – | 0.2 | 21.6 | 1.3 | 5.2 | 0.20
J1810+1744 | 1.66 | 78 | $2.3\pm 0.1$ | 3.1 | 39.6 | 2.4 | 0.15 | 0.04
J2129$-$0429 | 7.61 | 2.1 | $0.8\pm 0.1$ | 2.9 | 16.9 | 1.4 | 0.64 | 0.37
J2215+5135 | 2.61 | 16 | $1.3\pm 0.1$ | 6.1 | 69.2 | 2.8 | 0.17 | 0.21
Pulsars detected in searches of 350 MHz data, discovered previously or
concurrently
J1600$-$3053 | 3.60 | 2.7 | $0.6\pm 0.1$ | 0.7 | 52.3 | 2.5 | 14.3 | 0.20
J1805+0615 | 2.13 | 1.1 | $0.6\pm 0.1$ | 7.3 | 64.9 | 3.8 | 0.34 | 0.02
J2043+1711 | 2.38 | 0.8 | $3.0\pm 0.1$ | 1.2 | 20.7 | 1.5 | 1.4 | 0.17
J2256$-$1024 | 2.29 | 8.3 | $0.8\pm 0.1$ | 3.4 | 13.8 | 1.3 | 0.21 | 0.03
Pulsars discovered in 820-MHz survey detected by folding 350 MHz data
J0533+6759 | 4.39 | 2.1 | $1.0\pm 0.1$ | 0.5* | 57.4 | 2.4 | Isolated | N/A
J1142+0119 | 5.07 | 0.4 | $0.6\pm 0.1$ | 8.5 | 19.2 | 2.2 | 1.6 | 0.15
J1312+0051 | 4.23 | 0.4 | $1.7\pm 0.1$ | 0.4 | 15.3 | 1.4 | 38.5 | 0.18
J1858$-$2216 | 2.38 | 2.0 | $0.8\pm 0.1$ | 1.1* | 26.6 | 0.9 | 46.1 | 0.21
Pulsars discovered in 820-MHz survey not detected in 350MHz data
J0843+67 | 2.84 | – |  | – | 20.7 | 1.5 | 7.4 | 0.27
J1921+0137 | 2.50 | – | $1.6\pm 0.2$ | 4.9* | 104.9 | 5.0 | 9.9 | 0.23
Fermi-LAT pulsars observed but not detected in 350 MHz data
J0106+4855 | 83.16 | $<0.15$ | $1.9\pm 0.1$ | 2.9* | 70.9 | 3.0 | – | –
J0622+3749 | 333.21 | $<0.15$ | $2.0\pm 0.1$ | 2.7* | – | – | – | –
J1627+3219 | 2.18 | – |  | – | 28.1 |  | 0.18 | 0.02
J2139+4716 | 282.85 | $<0.22$ | $2.3\pm 0.2$ | 0.3* | – | – | – | –

###  3.1 Radio polarization observations and results

For some of the discoveries, full-Stokes polarization profiles were obtained
using the coherent fold mode of the GBT GUPPI backend, and the data were
analyzed using Psrchive (Hotan et al., 2004). We performed RFI excision in
frequency and in time, and the data were calibrated by ensuring the noise
power was the same in both polarizations and combined. We fit for rotation
measures (RMs) where possible, and include them as RM-corrected polarization
profiles in our multi-frequency profiles in Figures 3–5. Two of the pulsars
show high degrees of linear polarization ($\sim$70% in the main peak for PSR
J0340+4130 and nearly 100% in the secondary peak for PSR J0102+4839). PSR
J0307+7443’s profile shows significant, $\sim$20% linear polarization. The
main peak of PSR 1551$-$0658’s 820-MHz profile is $\sim$40% linearly
polarized. None of the pulsars show high degrees of circular polarization.
Position angles (P.A.s) are displayed for bins surpassing a S/N threshold of
5. For PSR J0102+4839, the position angle shows an orthogonal jump at the peak
of the main pulse, suggesting another emission mode at the emission peak
(Johnson et al., 2014). For PSRs J0340+4130 and J1551$-$0658, the P.A. swing
is smooth across the profile, suggesting the emission originates from a
continuous region. The P.A. could not be measured for PSR J0307+7443.

It is sometimes possible to estimate the magnetic inclination angle $\alpha$
and the observer’s viewing angle with respect to the rotation axis $\zeta$
from the rotating-vector model (Radhakrishnan & Cooke, 1969). However, MSP
polarization profiles often contain features—like orthogonal jumps—which
cannot be adequately described by the model. In none of our data sets is a
direct constraint on $\alpha$ or $\zeta$ available. Rough constraints on
$|\alpha-\zeta|<\approx 30^{\circ}$ are consistent with visible radio beams.

Figure 2: Phase-aligned weighted $\gamma$-ray (black) and 430 and 1500-MHz
radio polarimetric (blue and orange) pulse profiles for PSR J0023+0923 (from
Arecibo NANOGrav observations). The $\gamma$-ray pulse profile indicates the
pedestal of background emission as estimated by spectral analysis. Note the
clear precursor pulse and double peaked main pulse at 1500 MHz as compared to
the 430 MHz pulse here and the 350 MHz pulse shown in Figure 1. By 2 GHz, the
second peak of the main pulse dominates (see Figure 22.2 in 2PC), shifting the
apparent phase by $\sim 0.05$ as compared to the 350 MHz main peak. The radio
and $\gamma$-ray peaks are misaligned with the main radio peak at pulse phase
$\phi\sim 0.25$ preceding the broad $\gamma$-ray peak centered $\sim 0.65$.
Figure 3: Phase-aligned $\gamma$-ray (black) and 820-MHz radio polarimetric
(blue) pulse profiles for PSR J0102+4839. In the radio profile, linear
(circular) polarization is shown in red (green), while the polarization
position angle appears in orange. The radio and $\gamma$-ray peaks are
misaligned. The main radio peak at $\phi\sim 0.35$ is followed by at least one
additional component, a “shoulder” at $\phi\sim 0.45$. We do not believe this
is due to scatter-broadening as this feature is less prominent at lower
frequencies. There is a second peak at $\phi\sim 0.90$; it is not clear
whether we are seeing emission from one or both magnetic poles. Note the
orthogonal mode jump at the peak of the main pulse.  Figure 4:  Phase-aligned
$\gamma$-ray (black) and radio (blue) pulse profiles for PSR J0307+7443. The
main $\gamma$-ray and radio peaks are roughly aligned. In the radio profile,
linear (circular) polarization is shown in red (green). Due to the low signal-
to-noise ratio, a position angle measurement cannot be made. Figure 5:  Phase-
aligned $\gamma$-ray (black) and 820-MHz radio polarimetric (blue) pulse
profiles for PSR J0340$+$4130\. In the radio profile, linear (circular)
polarization is shown in red (green), while the polarization position angle
appears in orange. The radio peak at $\phi\simeq 0.55$ is aligned with one of
the $\gamma$-ray peaks.

###  3.2 X-Ray observations and analysis

Gentile et al. (2014) presented Chandra observations covering a full orbit of
the three new black widows from this survey (PSRs J0023+0923, J1124$-$3553,
and J1810+1744) as well as the redback PSR J2215+5135. They found clear
indications of orbitally varying hard X-ray emission from PSR J2215+5125 (see
below for an XMM-Newton analysis) and tentative evidence for such variations
in PSR J1124$-$3653\. All four of these systems also had a soft X-ray
component consistent with the thermal emission seen from other MSPs. The
remaining redback from our survey, PSR J2129$-$0429, was observed by XMM-
Newton over an entire orbit. Preliminary results were presented in Roberts et
al. (2015) (see also Hui et al., 2015), and a more detailed analysis,
including NuSTAR hard X-ray observations, in Al Noori et al. (2018). It has a
predominantly non-thermal spectrum with a flat power-law component extending
out to at least 40 keV. The flux of this hard component varies by more than a
factor of 10 over the orbit.

XMM-Newton observed PSR J2215+5125 on 2016-06-17 for 54 ks (around 3.5
orbits). For all three imaging detectors, we extracted and barycentered 0.5-7
keV events from a $20^{\prime\prime}$ radius region centered on the timing
position and from a nearby $40^{\prime\prime}$ radius background region. We
constructed binned background subtracted source intensity light curves and 68%
confidence regions using the Bayesian method outlined by Loredo (1992) for the
summed light curves of the MOS 1 and 2 detectors and for the PN detector shown
in Figure 6. We also produced a light curve folded on the orbital period using
the same method and show that light curve in Figure 7.

The XMM-Newton data was spectrally fit to an absorbed blackbody plus power law
model over the $0.2-10$ keV band. The hydrogen column density ($n_{H}$) was
fixed at $2.41\times 10^{21}{\tt cm^{-2}}$ derived from an $A_{V}$ calculated
from the Milky Way dust model found in Green et al. (2015). We fit a flux
$F_{bb}\sim 10^{-14}~{}{\rm erg~{}cm^{-2}~{}s^{-1}}$, $kT=0.26\pm 0.05$ keV to
the thermal component, typical for the surface of a neutron star (Marelli et
al., 2011). The power-law component is very hard, with a photon index of
$\Gamma=1.0\pm 0.2$ and a flux $F_{pl}\sim 10^{-13}\,{\rm
erg~{}cm^{-2}~{}s^{-1}}$, similar to other known redbacks. A more complete
analysis of these data will be presented elsewhere.

Figure 6: XMM-Newton background-subtracted 0.5-7 keV light curve of PSR
J2215+5135, produced from the full observation, which extends for about 3.5
orbits. Orbital phase is defined so that pulsar superior junction is at 0.25.
Green indicates the PN data, and red indicates the summed MOS1 and MOS 2 data.
The error bars represent the 68% confidence region of the Bayesian posterior
probability distribution of the count rate using an exposure corrected
background region.  Figure 7: Background-subtracted XMM-Newton 0.5-7.5 keV
light curve folded on the orbital period, repeated twice for clarity.

Here, we also report on X-ray observations of the other five discoveries from
this survey. Because in none of these cases a detailed spectral analysis
possible, we first describe our general approach and assumptions. To estimate
expected X-ray absorption, we used the model of optical extinction of the
Galaxy by Drimmel et al. (2003) to estimate the visual extinction $A_{V}$
towards the source at the nominal DM-derived distance, and then the empirical
relationship of $N_{H}\sim 2.21\times 10^{21}A_{V}~{}{\rm cm^{-2}}$ of Güver &
Özel (2009). To estimate the expected emission, we assumed a blackbody of
$kT=0.2$ keV, which is typical for the majority of MSPs which do not have
intrabinary shock emission or strong pulsed X-ray emission. Flux limits quoted
are for the 0.3–8 keV range. Note that assuming a power law of 1.5 (reasonable
for magnetospheric emission or intrabinary shock emission) would increase the
flux limit by a factor of $\sim$2–3.

PSR J0102+4839 is one of many sources which has benefited from a Swift
campaign to monitor Fermi sources Falcone et al. (2011) with the XRT (Burrows
et al., 2000). Observations in October and November 2010 resulted in a total
exposure of 4412 s. No excess emission was seen at the pulsar position,
placing a conservative upper limit of $10^{-14}\,{\rm erg~{}cm^{-2}~{}s^{-1}}$
on the 0.3–8 keV flux.

PSR J0307+7443 was observed by XRT three times in 2009–2010 for a total
exposure of 10458 s. No clear detection was made, although there is a local
$\sim 1.5\sigma$ excess at the pulsar position, which would correspond to a
0.3–8 keV flux of $\sim$$10^{-14}\,{\rm erg~{}cm^{-2}~{}s^{-1}}$.

XMM-Newton observed PSR J0340+4130 twice in August 2009 for a total of 41 ks.
However, the observations were affected by background flaring, and after
excising flares, there was a total exposure kept for analysis of 21.6, 25.9,
and 11.6 ks for the MOS1, MOS2, and PN detectors, respectively. No significant
emission was found at the position of the pulsar. Our best estimate of the
count rate from the PN is $0.0007\pm 0.0006$ cts/s. From this we derive an
upper limit of $\sim$2$\times 10^{-15}{\rm erg~{}cm^{-2}~{}s^{-1}}$.

Chandra observed PSR J1302$-$3258 for 9.9 ks on 30 March 2011 with the ACIS-S
instrument. A total of 16 counts were detected in the 0.3–2.0 keV range within
$2^{\prime\prime}$ of the pulsar position, with an estimated background of
$\sim$0.1 counts. No photons above 2 keV were detected, suggesting the
emission to be predominantly thermal emission from the pulsar surface. Using
an estimated neutron hydrogen column density $n_{H}=5\times 10^{20}{\rm
cm^{-2}}$, this corresponds to a 0.2 keV blackbody flux of $1.2\times
10^{-14}\,{\rm erg~{}cm^{-2}~{}s^{-1}}$ or an unabsorbed flux of $1.6\times
10^{-14}{\rm erg~{}cm^{-2}~{}s^{-1}}$, yielding an estimated isotropic X-ray
luminosity of $1.9\times 10^{30}\,d_{\mathrm{1kpc}}^{2}$ erg s-1.

PSR J1551$-$0658, being a chance detection, is $21.7^{\prime}$ from the
nearest Swift pointing of the nearby Fermi source, and was hence out of the
field of view. We therefore have no X-ray data on the source.

Given the DM-derived distance estimates, these luminosities and limits are all
consistent with what would be expected from the rough relationship between
spin-down power and total X-ray emission derived for MSPs with reliable
parallax derived distances, ${\rm log}L_{x}=(0.27\pm 0.20){\rm
log}\dot{E}+20.90\pm 6.53$ (Bognar et al., 2015). They are also in line with
the relation for just the blackbody emission component, ${\rm
log}L_{x}=(0.25\pm 0.16){\rm log}\dot{E}+21.28\pm 5.36$, which tends to
dominate, although that of PSR J1302$-$3258 is towards the high end of the
scatter in the data.

To summarize, while one of the black widows and both redbacks show clear
evidence for non-thermal emission, there is no evidence for either bright
magnetospheric or intrabinary shock emission from any of the other pulsars,
with all the data being consistent with the thermally dominated pulsar surface
emission typically seen from MSPs.

###  3.3 $\gamma$-ray observations and analysis

To characterize the $\gamma$-ray emission and pulsations, we analyzed $\sim$8
years (2008 August 4 to 2016 July 20) of P8R2 Fermi-LAT data. All LAT data
analyses were carried out with v10r00p05 of the Fermi Science Tools
(STs)666http://fermi.gsfc.nasa.gov/ssc/data/analysis/software/. We selected
SOURCE class events, as defined under the P8R2_V6 instrument response
functions (IRFs), which had energies between 100 MeV and 100 GeV,
reconstructed directions within 15∘ of the radio position of each MSP, and
zenith angles $\leq$ 90∘. We used the ST gtmktime to exclude times when the
LAT was not in nominal science operations mode, when the LAT data were flagged
as bad, and during bright LAT-detected solar flares and gamma-ray bursts.

We performed a binned maximum-likelihood analysis on a $21\fdg 2\times 21\fdg
2$ region for each MSP using the P8R2_SOURCE_V6 IRFs, combining events which
convert in the front and back section of the LAT. We calculated binned
exposure maps with 30 logarithmic energy bins and spatial pixels that were
$0\fdg 1$ on a side. The source model for each region included all 3FGL point
and extended sources within $25^{\circ}$ of each MSP and the diffuse
background models. The Galactic diffuse emission was modeled using the
gll_iem_v06.fits template, and the isotropic diffuse spectrum, which includes
the extragalactic diffuse and residual instrument backgrounds, was modeled
using iso_P8R2_SOURCE_V6_v06.txt. The spectral parameters of point sources
within 6∘ of each MSP and with average significance $\geq 10\sigma$, in four
years, were left free as well as the normalizations of the Galactic and
isotropic diffuse components and point sources within 8∘ that were flagged as
a variable in 3FGL. All other spectral parameters were kept fixed to the 3FGL
values.

Since we are using twice as much (8 years vs. 4 years) and improved (P8 vs.
P7REP) data compared to the 3FGL Catalog, it is possible that our models of
the regions may be incomplete. After our initial fits, we looked at the
spatial and spectral residuals to see if additional sources needed free
parameters or if additional sources needed to be included. For the region
around PSR J2215+5135, we did find a few sources outside the 6∘ radius that
were over or undersubtracted. We refit this region with those additional
source normalizations free and found satisfactory fits. In the region around
PSR J0340+4130, we saw evidence for a source not included in 3FGL. Using the
ST gtfindsrc, we localized this source to (R.A., Dec.) = ($59\fdg 88$, $50\fdg
96$) with a 95% confidence-level error radius of $0\fdg 03$, positionally
associating this source with the flat-spectrum radio quasar, 4C $+$50.11,
which flared in $\gamma$ rays in 2014 January (The Fermi-LAT Collaboration,
2014). After including it in the model, we found satisfactory fits.

All of the new MSPs, with the exception of PSR J1551$-$0658, were found as
significant point sources in the 3FGL catalog, with positions consistent with
the timing positions within the LAT error ellipses. We fixed them to their
timing positions in the region models and modeled their spectra as a power law
(Equation 2) and an exponentially-cutoff power law (Equation 3 with $b\
\equiv\ 1$).

$\frac{dN}{dE}=K\left(\frac{E}{E_{0}}\right)^{-\Gamma}$ (2)
$\frac{dN}{dE}=K\left(\frac{E}{E_{0}}\right)^{-\Gamma}\exp{\left(-\frac{E}{E_{\mathrm{C}}}\right)^{b}}$
(3)

For both spectral models $K$ is a normalization factor, with units of cm-2 s-1
MeV-1; $E_{0}$ is a scale energy, fixed to the 3FGL pivot energy value for
each MSP; and $\Gamma$ is the photon index. For the cutoff model,
$E_{\mathrm{C}}$ is the cutoff energy and $b$ is an exponential index
controlling the sharpness of the cutoff. We define a test statistic (TS) as
twice the difference in the log likelihood to assess the overall source
significance and to test nested models. The power-law model is rejected (large
TScutoff) in favor of the cutoff model with $>$ 5.5 $\sigma$ significance for
all detected MSPs. We also performed analysis with the $b$ parameter free but
found no significant improvement (small TS${}_{\mathrm{b\,free}}$,
$\lesssim$2.5$\sigma$) of the fit for any MSP when doing so. The best-fit
spectral parameters and TS values for all nine MSPs are given in Tables 3 and
4.

We used the best-fit models and gtsrcprob to calculate spectral weights
(probability that each event came from the respective MSP based on the
likelihood model) for each event. We then folded the events for each MSP using
the ephemerides described in §3 using the fermi plugin to tempo2 (Ray et al.,
2011) and calculated a weighted H-test (Kerr, 2011) for each MSP. We formed
$\gamma$-ray profiles by generating weighted histograms from events lying
within 3∘ of the source and display them in Figures 2–5, and FIgures 8–12.
Estimates of the background level (dashed line) and error flags for the
intensity in a given phase bin are computed via a bootstrap process described
in 2PC. In each panel, we show one or more radio pulse profiles to place the
$\gamma$-ray emission in context.

Table 3: Timing and $\gamma$-ray Spectral Parameters for the Discovered MSPs

Timing Parameters | PSR J0023$+$0923 | PSR J0102$+$4839 | PSR J0307$+$7443 | PSR J0340$+$4130 | PSR J1124$-$3653
---|---|---|---|---|---
Fermi-LAT 3-Year Source | 3FGL J0023.4$+$0923 | 3FGL J0102.8$+$4840 | 3FGL J0308.0$+$7442 | 3FGL J0340.3$+$4130 | 3FGL J1123.9$-$3653
Right Ascension (J2000) | $00^{\rm h}\;23^{\rm m}\;16\fs 88027(1)$ | $01^{\rm h}\;02^{\rm m}\;50\fs 66874(6)$ | $03^{\rm h}\;07^{\rm m}\;55\fs 886(1)$ | $03^{\rm h}\;40^{\rm m}\;23\fs 288229(6)$ | $11^{\rm h}\;24^{\rm m}\;01\fs 1160(2)$
Declination (J2000) | $+09^{\circ}\;23^{\prime}\;23\farcs 8902(5)$ | +48${}^{\circ}\;39^{\prime}\;42\farcs 7635(5)$ | $+74^{\circ}\;43^{\prime}\;13\farcs 426(5)$ | $+41^{\circ}\;30^{\prime}\;45\farcs 2892(1)$ | $-36^{\circ}\;53^{\prime}\;19\farcs 087(4)$
Pulsar Period (ms) | 3.05020310344768(3) | 2.9641124215887(4) | 3.1560886884993(7) | 3.29933936625009(2) | 2.4095726733227(3)
Period Derivative, Ṗ (10-20 s s-1) | 1.14238(3) | 1.1406(3) | 1.7279(7) | 0.70485(8) | 0.60151(1)
Reference Epoch (MJD) | 55521 | 55527 | 55780 | 56279 | 55128
Dispersion Measure (pc cm-3) | 14.326553(4) | 53.5041(6) | 6.3430(8) | 49.57583(3) | 44.8560(2)
Proper motion in RA $\mu_{\alpha}\cos\delta$ (mas yr-1) |  $-$11.00(7) |  $-$3.4(4) |  $-$1(1) |  $-$0.77(5) | 3.1(4)
Proper motion in Dec $\mu_{\delta}$ (mas yr-1) |  $-$8.8(1) |  $-$1.9(5) | 3(1) |  $-$3.1(1) |  $-$4.1(6)
Orbital Period (days) | 0.13879914308(1) | 1.672149564(1) | 37.10775488(9) | … | 0.226987946(8)
Projected Semi-Major Axis (lt-s) | 0.03484228(5) | 1.8558825(7) | 16.108338(8) | … | 0.079631(6)
Orbital Eccentricity |  $<$3.4 $\times$ 10-5 |  $<$2.3 $\times$ 10-6 |  $<$3.3 $\times$ 10-3 | … | 0.0
Epoch of Ascending Node (MJD) | 55186.1134208(1) | 55514.5773299(1) | 55599.363937(4) | … | 55128.586598(3)
Span of Timing Data (MJD) | 55130.08$-$57375.95 | 54682.78$-$59050.55 | 54682.77$-$59033.74 | 55186.08$-$57378.23 | 55128.59$-$56854.65
Timing-Derived Parameters
Mass Function ($10^{-3}$ $M_{\odot}$) | 2.35$\times$10-3 | 2.45 | 3.25 | … | 0.0105
Minimum Companion Mass ($M_{\odot}$) |  $\geq$ 0.01 |  $\geq$ 0.18 |  $\geq$ 0.20 | … |  $\geq$ 0.028
Galactic Longitude (deg) | 111.3 | 124.7 | 131.7 | 153.7 | 284.1
Galactic Latitude (deg) |  $-$52.8 |  $-$14.2 | 14.2 |  $-$11.1 | 22.8
DM-derived Distance (YMW16, kpc) | 1.2 | 2.3 | 0.4 | 1.6 | 1.0
Surface Mag. Field, $B$ ($10^{8}$ G) | 1.8 | 1.8 | 2.3 | 1.5 | 1.2
Characteristic Age (Gyr) | 4.2 | 4.1 | 2.9 | 7.4 | 6.3
Spin-down Luminosity, $\dot{E}$ ($10^{34}$ erg s-1) | 1.58922(4) | 1.7291(5) | 2.1699(9) | 0.77477(8) | 1.6974(5)
Transverse velocity (km s-1) | 80(16) | 43(12) | 6(2) | 25(5) | 25(6)
Acceleration $\perp$ to plane (10-14 s s-1) |  $-$5.8(4) |  $-$4.9(3) | 2.1(3) |  $-$4.0(2) | 4.4(2)
Acceleration $\parallel$ to plane (10-14 s s-1) |  $-$0.7(1) |  $-$0.73(2) |  $-$0.071(8) | 2.1(4) |  $-$2.0(4)
Corrected Ṗ (10-20 s s-1) | 0.91(4) | 1.096(9) | 1.718(2) | 0.65(8) | 0.55(8)
Corrected $\dot{E}$ ($10^{34}$ erg s-1) | 1.27(6) | 1.66(1) | 2.158(3) | 0.72(1) | 1.57(2)
Corrected $B$ ($10^{8}$ G) | 1.7 | 1.8 | 2.3 | 1.5 | 1.2
Observed Parameters
Rotation Measure (RM) (rad m-2) | … |  $-$86.3(8) | 13(3) | 56(1) | …
Flux Density at 350 MHz (mJy) | 5.4 | 2.8 | 1.8 | 6.5 | 2.2
Flux Density at 820 MHz (mJy) | 2.4 | 0.8 | 0.19 | 1.6 | …
Flux Density at 1400 MHz (mJy) | 0.5 | … | … | 0.5 | …
Flux Density at 2000 MHz (mJy) | 0.5 | … | … | … | …
Spectral Fit Parameters
K ($10^{-12}$ cm-2 s-1 MeV-1) | 3.0(5) | 2.4(2) | 3.6(6) | 2.9(2) | 1.8(2)
Photon Index $\Gamma$ | 1.3(2) | 1.7(1) | 1.0(1) | 1.2(1) | 1.4(1)
EC (GeV) | 1.9(5) | 5(1) | 1.6(2) | 3.6(4) | 3.7(7)
F100 ($10^{-8}$ cm-2 s-1) | 0.9(2) | 2.0(2) | 1.5(1) | 1.5(1) | 1.2(1)
G100 ($10^{-11}$ erg cm-2 s-1) | 0.7(1) | 1.6(1) | 1.6(1) | 2.1(1) | 1.3(1)
TS | 470 | 1349 | 1920 | 2448 | 1149
TScutoff | 48 | 66.5 | 254 | 200 | 85
TS${}_{\mathrm{b\ free}}$ | 0.9 | 0.0 | 0.2 | 0.8 | 0.5
Efficiency | 0.10 | 0.62 | 0.013 | 0.90 | 0.10

Note. — Numbers in parentheses represent 2-$\sigma$ uncertainties in the last
digit as determined by TEMPO2. Minimum companion masses were calculated
assuming a pulsar mass of 1.4 $M_{\odot}$. The gamma-ray spectral parameters
are from fits of a power-law with exponential cutoff shape, given in Equation
3 with $b$=1. The first three parameters are as defined in Equation 3: F100
and G100 give the integrated photon or energy flux above 0.1 GeV,
respectively, while the last two parameters are gamma-ray detection
significance of the source and significance of an exponential cutoff (as
compared to a simple power law). Gamma-ray efficiency is estimated as $4\pi
G_{100}d^{2}/\dot{E}$ using YMW16 (Yao et al., 2017) distances assuming
spatially uniform emission. The uncertainty on DM produces a negligible
$<$0.0P uncertainty in radio/$\gamma$ alignment at 820 MHz, with a maximum of
$<$0.02P at 350 MHz for an uncertainty of 0.001 DM units.

Table 4: Timing and $\gamma$-ray Spectral Parameters for the Discovered MSPs

Timing Parameters | PSR J1302$-$3258 | PSR J1551$-$0658 | PSR J1810$+$1744 | PSR J2129$-$0429 | PSR J2215$+$5135
---|---|---|---|---|---
Fermi-LAT 3-Year Source | 3FGL J1302.3$-$3259 | (3FGL J1549.7$-$0658) | 3FGL J1810.5$+$1743 | 3FGL J2129.6$-$0427 | 3FGL J2215.6$+$5134
Right Ascension (J2000) | $13^{\rm h}\;02^{\rm m}\;25\fs 5262(2)$ | $15^{\rm h}\;51^{\rm m}\;09\fs 5279(2)$ | $18^{\rm h}\;10^{\rm m}\;37\fs 28478(5)$ | $21^{\rm h}\;29^{\rm m}\;45\fs 05(8)$ | $22^{\rm h}\;15^{\rm m}\;32\fs 6(2)$
Declination (J2000) | $-32^{\circ}\;58^{\prime}\;36\farcs 843(7)$ | $-06^{\circ}\;58^{\prime}\;07\farcs 83(1)$ | $+17^{\circ}\;44^{\prime}\;37\farcs 380(1)$ | $-04^{\circ}\;29^{\prime}\;06\farcs 81(8)$ | $+51^{\circ}\;35^{\prime}\;36\farcs 3(3)$
Pulsar Period (ms) | 3.770853091420(1) | 7.09375570886(1) | 1.6627549909837(1) | 7.613937470415(5) | 2.6096197252726(2)
Period Derivative, Ṗ (10-20 s s-1) | 0.656(1) | 2.05(6) | 0.4476(1) | 32.775(5) | 2.8204(3)
Reference Epoch (MJD) | 55925 | 55997 | 55530 | 55750 | 56362
Dispersion Measure (pc cm-3) | 26.179(1) | 21.578(1) | 39.65952(6) | 16.8771(1) | 69.1947(1)
Proper motion in RA $\mu_{\alpha}\cos\delta$ (mas yr-1) | … | … | 7.5(2) | 12.3(1) | 0.3(5)
Proper motion in Dec $\mu_{\delta}$ (mas yr-1) | … | … |  $-$3.6(4) | 10.1(1) | 1.8(6)
Orbital Period (days) | 0.78444160(2) | 5.20657070(3) | 0.148170285(1) | 0.63522773(2) | 0.172501860(1)
Projected Semi-Major Axis (lt-s) | 0.92792(4) | 4.331018(6) | 0.095378(1) | 1.85219(2) | 0.468131(4)
Orbital Eccentricity | 3.2 $\times$ 10-5 | 1.1 $\times$ 10-5 | 4.7 $\times$ 10-5 | 0.0 | 0.0
Epoch of Ascending Node (MJD) | 55924.803600(5) | 55999.906196(1) | 55529.9597206(2) | 55702.111161(7) | 55702.111161(7)
Span of Timing Data (MJD) | 54683.52$-$59050.18 | 55623.28$-$56371.38 | 54682.96$-$59050.59 | 54682.84$-$58900.40 | 54682.84$-$58900.40
Timing-Derived Parameters
Mass Function ($10^{-3}$ $M_{\odot}$) | 1.39 | 3.21 | 0.042 | 16.9 | 3.70
Minimum Companion Mass ($M_{\odot}$) |  $\geq$ 0.15 |  $\geq$ 0.20 |  $\geq$ 0.04 |  $\geq$ 0.37 |  $\geq$ 0.21
Galactic Longitude (deg) | 305.5 | 1.5 | 44.6 | 48.8 | 99.8
Galactic Latitude (deg) | 29.9 | 34.8 | 16.9 |  $-$36.8 |  $-$4.1
DM-derived Distance (YMW16, kpc) | 1.4 | 1.3 | 2.4 | 1.4 | 2.8
Surface Mag. Field, $B$ ($10^{8}$ G) | 1.6 | 3.9 | 0.8 | 15.9 | 2.7
Characteristic Age (Gyr) | 9.0 | 5.5 | 5.8 | 0.36 | 1.4
Spin-down Luminosity, $\dot{E}$ ($10^{34}$ erg s-1) | 0.483(1) | 0.227(7) | 3.8443(9) | 2.9314(5) | 6.2654(8)
Transverse velocity (km s-1) | … | … | 96(19) | 106(21) | 25(9)
Acceleration $\perp$ to plane (10-14 s s-1) | 5.2(3) | 5.3(3) | 5.2(3) |  $-$5.5(3) |  $-$3.3(3)
Acceleration $\parallel$ to plane (10-14 s s-1) |  $-$1.2(3) | 2.5(6) |  $-$1.5(7) |  $-$0.5(1) |  $-$4.9(6)
Corrected Ṗ (10-20 s s-1) | 0.608(6) | 1.92(4) | 0.36(1) | 32.2(1) | 2.770(8)
Corrected $\dot{E}$ ($10^{34}$ erg s-1) | 0.448(5) | 0.213(4) | 3.1(1) | 2.86(1) | 6.15(1)
Corrected $B$ ($10^{8}$ G) | 1.5 | 3.7 | 0.8 | 15.8 | 2.7
Observed Parameters
Rotation Measure (RM) (rad m-2) | … | … | … | … | …
Flux Density at 350 MHz (mJy) | 1.3 | 0.7 | 78 | 2.1 | 16
Flux Density at 820 MHz (mJy) | 2.4 | 0.3 | 7 | … | 0.8
Flux Density at 1400 MHz (mJy) | … | … | 1.3 | … | 0.14
Flux Density at 2000 MHz (mJy) | … | … | 0.4 | … | 0.11
Spectral Fit Parameters
K ($10^{-12}$ cm-2 s-1 MeV-1) | 1.8(2) | … | 1.1(6) | 1.6(2) | 1.7(2)
Photon Index $\Gamma$ | 1.2(2) | … | 2.0(1) | 1.7(1) | 1.3(2)
EC (GeV) | 3.1(6) | … | 3.8(7) | 4(1) | 4.2(8)
F100 ($10^{-8}$ cm-2 s-1) | 0.9(1) |  $<$ 0.5 | 4.1(2) | 1.0(1) | 1.0(1)
G100 ($10^{-11}$ erg cm-2 s-1) | 1.2(1) | … | 2.3(1) | 0.8(1) | 1.3(1)
TS | 1132 | 4 | 1958 | 541 | 887
TScutoff | 101 | … | 72 | 31 | 83
TS${}_{\mathrm{b\ free}}$ | 0.2 | … | 0.8 | 0.1 | 6.2
Efficiency | 0.64 | … | 0.49 | 0.064 | 0.19

Note. — Numbers in parentheses represent 2-$\sigma$ uncertainties in the last
digit as determined by TEMPO2. Minimum companion masses were calculated
assuming a pulsar mass of 1.4 $M_{\odot}$. The gamma-ray spectral parameters
are from fits of a power-law with exponential cutoff shape, given in Equation
3 with $b$=1. The first three parameters are as defined in Equation 3: F100
and G100 give the integrated photon or energy flux above 0.1 GeV,
respectively, while the last two parameters are gamma-ray detection
significance of the source and significance of an exponential cutoff (as
compared to a simple power law). Gamma-ray efficiency is estimated as $4\pi
G_{100}d^{2}/\dot{E}$ using YMW16 distances assuming spatially uniform
emission. The uncertainty on DM produces a negligible $<$0.01P uncertainty in
radio/$\gamma$ alignment at 820 MHz, with a maximum of $<$0.02P at 350 MHz for
an uncertainty of 0.001 DM units.

Figure 8:  Phase-aligned $\gamma$-ray (black) and radio (blue) pulse profiles
for PSR J1124$-$3653\. The radio peak leads the first $\gamma$-ray peak by
about 0.35 cycles.  Figure 9:  Phase-aligned $\gamma$-ray (black) and radio
(blue and orange) pulse profiles for PSR J1302$-$3258\. The $\gamma$-ray and
radio peaks are misaligned, with a radio interpulse becoming clearly
detectable at 820 MHz.  Figure 10:  Phase-aligned $\gamma$-ray (black) and
radio (blue, orange, green, and red) pulse profiles for PSR J1810+1744. The
$\gamma$-ray and radio peaks are roughly aligned, with a radio precursor
becoming clearly detectable at 1500 and 2000 MHz. Figure 11:  Phase-aligned
$\gamma$-ray (black) and radio (blue) pulse profiles for PSR J2129$-$0429\.
The radio and $\gamma$-ray peaks are misaligned. Figure 12:  Phase-aligned
$\gamma$-ray (black) and radio (blue and orange) pulse profiles for PSR
J2215+5135. The radio peak is aligned with one of the $\gamma$-ray peaks.
Figure 13:  Radio pulse profile (blue) for PSR J1551$-$0658 at 820 MHz. The
linear (circular) polarization is shown in red (green), while the polarization
position angle appears in orange.

##  4 Discussion

This paper presents a 350-MHz GBT survey for pulsars, carried out in 2009,
which targeted 49 unidentified Fermi-LAT sources. We detected 18 MSPs in our
survey; 10 of these were discoveries unique to our survey. PSR J0340$+$4130 is
an isolated MSP. PSRs J0023$+$0923, J1124$-$3653, and J1810$+$1744 are short
orbital period binaries with very low mass companions ($M_{c}\ll
0.1M_{\odot}$), with the latter two showing eclipses at frequencies
$\lesssim$2 GHz, and hence can be classified as black widow systems. PSRs
J2129$-$0429 and J2215$+$5135 have heavier, non-degenerate (Breton et al.,
2013; Bellm et al., 2013) companions of masses $M_{c}>0.2M_{\odot}$, are
eclipsed for a large fraction of their orbits, and have orbitally modulated
X-ray emission (Gentile et al., 2014; Roberts et al., 2015). Moreover, this
X-ray modulation is centered on pulsar inferior conjunction (Wadiasingh et
al., 2017). These are hallmarks of “redback” systems (Roberts, 2011).

PSR J1302$-$3258 also has a short binary period and shows evidence for brief
($\sim 10$% of the orbit) eclipses at low frequencies, but does not have
excess X-ray emission or a bright optical companion. PSRs J0102$+$4839,
J0307+7443, and J1551$-$0658 are longer period ($P_{B}>1$ day) binaries with
minimum companion masses typical of helium white dwarf stars orbiting MSPs.
This final discovery, PSR J1551$-$0658, seems to be a chance detection since
its position is well outside of the Fermi-LAT error box, and there is no
evidence of pulsed emission in the $\gamma$-ray data. We show its radio pulse
profile and polarimetry in Figure 13.

###  4.1 Implications of the Number of Spiders

One major surprise of these Fermi searches has been the number of compact
($P_{b}<1$ day) binary systems which show evidence of the pulsar wind
interacting with the companion. Such compact, often eclipsing systems come in
two varieties, now commonly referred to as ‘black widows’ (see Swihart et al.,
2022, for a census) and ‘redbacks’ (see Strader et al., 2019).

In black widow systems, the pulsar’s intense, relativistic wind is ablates the
companion star, stripping it of mass, and potentially ‘devouring’ it entirely
over the course of billions of years777Hence the association with black widow
spiders, who, according to some reports, sometimes eat their partner as part
of the mating process.. PSR B1957$+$20, with a degenerate $\sim$0.02
$M_{\odot}$ companion, was the first such system to be found and is considered
the prototype of the class. At radio wavelengths, this pulsar is eclipsed for
$\sim$10% of its orbit, likely due to screening by material blown off of the
companion star (i.e., not occultation from the companion star itself). It also
shows timing irregularities, especially around eclipse ingress/egress
(Fruchter et al., 1988), commonly referred to as “eclipse delays”, as well as
an orbital period derivative due to tidal effects. Whether such systems can
fully ablate their companion and turn into the observed isolated MSPs is a
matter of ongoing debate. Plausibly, some systems can ablate their companion
star entirely, while others may not have sufficiently strong winds to do this
within a Hubble time.

Additionally, some eclipsing systems have companions which are significantly
more massive ($\sim 0.2-0.4$ $M_{\odot}$) and probably non-degenerate. These
systems were first identified in globular clusters (PSR J1740$-$5340 in
NGC6397; D’Amico et al., 2001) and have now been termed redbacks888Redbacks
are Australian spiders whose similar markings and habit of devouring mates
make them cousins of the North American black widow. from black widows
(Roberts, 2011). Such systems possibly represent an earlier stage in the
recycling process, as argued in the case of PSR J1023$+$0038, which has
displayed the characteristics of both an accreting low-mass X-ray binary and a
normal radio millisecond pulsar in the last decade (Archibald et al., 2009;
Stappers et al., 2014). It is also possible that such systems have different
progenitors and a different evolution than the black widows (e.g., Chen et
al., 2013; Benvenuto et al., 2014; Smedley et al., 2015). Prior to Fermi, only
four of the known MSPs in the Galactic field were in compact/eclipsing
systems.

####  4.1.1 The New Black Widows

PSR J0023+0923 has a spin period of 3.05 ms and DM of 14.33 pc cm-3, with an
implied DM-distance of 1.2 kpc using the YMW16 model of Yao et al. (2017).
With $S_{350}$ $\sim$2 mJy, the source was easily detected in a search of the
first 200 s of the observation. The orbital period is 3.3 hr, with a minimum
companion mass of 0.016 $M_{\odot}$. These orbital properties are very similar
to those of the black widow PSR J2051$-$0827\. However, it does not show any
evidence of an eclipse at either 350 MHz or 2 GHz. The radio profile is quite
narrow ($W_{50}\sim 0.05$ ms, see Figure 1), and despite its black-widow-like
very low mass companion, extensive timing shows no evidence for orbital period
variations.

Early optical observations by Breton et al. (2013) showed that the companion
is strongly heated but that it appears to have a small radius that is
significantly underfilling its Roche lobe. More extensive observations by Mata
Sánchez et al. (2023) show the Roche lobe fill factor is $\sim 50\%$.
Therefore the surface gas may be too strongly gravitationally bound for the
pulsar wind to ablate the surface, explaining the lack of eclipses. This may
also explain why it does not exhibit orbital variations. The optical emission
suggests a moderate inclination of $\sim 42^{\circ}$, and so even if the
pulsar is as heavy as $2~{}M_{\odot}$, the companion would still be a very
light $0.025~{}M_{\odot}$, and is therefore consistent with other black-widow
masses.

The narrow pulse profile and lack of orbital variations allow for high-
precision timing and has led to this source being included in the NANOGrav and
EPTA pulsar timing arrays (e.g., van Haasteren et al., 2011).

PSR J1124$-$3653 has a spin period of 2.41 ms and DM of 44.86 pc cm-3, with an
implied DM-distance of 1.0 kpc. With $S_{350}\sim 0.3$ mJy, it is one of the
weaker sources presented here and required a search of the full 32-min
discovery observation in order to be found. The orbital period is 5.45 hr,
with a minimum companion mass of 0.028 $M_{\odot}$. It is eclipsed around 40%
of the time at 350MHz, 20% of the time at 820MHz, but apparently not eclipsed
at 1400MHz. However, the 1400MHz TOAs seem to have excess delays around
superior conjunction. As noted above, Gentile et al. (2014) found significant
hard X-ray emission suggestive of an intrabinary shock. Optical observations
of the system show that the companion is very strongly irradiated by high
energy emission from the pulsar and/or shock (Draghis et al., 2019).

PSR J1810+1744 has a very short999It is the 4th fastest-spinning pulsar known
in the Galactic field. spin period of 1.66 ms and DM of 39.66 pc cm-3, which
implies a DM-distance of 2.4 kpc. Relative to other known MSPs, the source is
quite strong at low frequencies, $S_{350}=20$ mJy, and it was easily detected
in a search of the first 200 s of the observation. At even lower frequencies,
it is one of the brightest pulsars in the sky (Kondratiev et al., 2016). The
orbital period is 3.6 hr and the minimum companion mass is 0.045 $M_{\odot}$.
Like the original black widow B1957$+$20, J1810+1744 has a very fast spin
rate, though its orbital period is significantly shorter. From 350 to 1500
MHz, the radio profile evolves to have a sharp preceding peak (Figure 1),
which is reminiscent of the trailing notch seen in the pulse profile of PSR
B1937+21 (McKee et al., 2019). The $\gamma$-ray and radio peaks are roughly
aligned (Figure 10), with a radio precursor becoming clearly detectable at
1500 MHz.

J1810+1744 is eclipsed for roughly 10% of its orbital period, which is a
similar eclipse fraction to the original black-widow B1957$+$20 (Fruchter et
al., 1988). The eclipse boundaries are very sharp; the observed pulse
intensity drops from its full non-eclipse value to undetectable within $\sim$2
min. Eclipse delays, i.e. pulsed emission which has been shifted to later
pulse longitude, are also observed at eclipse egress. Eclipse studies at lower
frequencies with LOFAR and WEPSR show the eclipse edges to be highly variable,
and that the mass loss rate is high enough to evaporate the companion within
roughly a Hubble time (Polzin et al., 2018) J1810+1744 has one of the highest
minimum companion masses ($M_{c}>0.045$ $M_{\odot}$) of all the known black
widows. Extensive obervations with Keck indicate a very high neutron star mass
of $M_{ns}\sim 2.1M_{\odot}$ and a nearly Roche lobe filling companion with a
mass $M_{c}\sim 0.065M_{\odot}$ (Romani et al., 2021). This suggests
J1810+1744 may be an evolutionary bridge between redbacks and black widows.

####  4.1.2 The New Redbacks

The longer eclipse fractions (typically 50%) and higher companion masses
($0.2-0.4$ $M_{\odot}$) of these systems identify them as redbacks, a class of
system that before the discovery of the X-ray binary / MSP “missing link”
J1023$+$0038 (Archibald et al., 2009) was only observed in globular clusters.
The companions of these systems also show strong orbital modulation in optical
(Breton et al., 2013) and X-ray (Gentile et al., 2014; Archibald et al., 2009;
Bogdanov et al., 2015). It is unclear whether the redbacks are an evolutionary
precursor of the black widows.

PSR J2129$-$0429 has a spin period of 7.61 ms and DM of 16.88 pc cm-3, with an
implied DM-distance of 1.4 kpc. With $S_{350}\sim 0.5$ mJy, it also required
the full 32-min observation to be detected. It’s high spin-down rate, which
implies a larger dipole surface magnetic field $B\sim 1.6\times 10^{9}$ and a
lower characteristic age than other redbacks. PSR J2129$-$0429 also has a
relatively long orbital period ($P_{B}=15.2$ hr) and large minimum companion
mass ($P_{C}\gtrsim 0.37M_{\odot}$). The high companion mass and relatively
slow rotation period argue that this is a redback that is likely in an early
stage of recycling.

The pulsar has large orbital period variations and is eclipsed $\sim 50\%$ of
the time at low frequencies. The double peaked orbital X-ray light curve shows
there is a strong shock which is apparently wrapped around the pulsar, which
may indicate the companion magnetic field is dynamically important (Al Noori
et al., 2018). Optical observations show the companion is nearly Roche lobe
filled, and also show long term trends in the brightness and temperature of
the companion, suggesting the Roche lobe filling factor may be significantly
changing on monthly to yearly time-scales (Bellm et al., 2016; Al Noori et
al., 2018). The system has a high inclination ($i\sim 77^{\circ}$), which is
shown through both optical modeling and eclipses of the pulsar seen in
$\gamma$-ray and X-ray data (Clark et al., 2023b).

PSR J2215+5135 has a spin period of 2.61 ms and DM of 69.19 pc cm-3, with an
implied DM-distance of 2.8 kpc. It has the highest spin-down luminosity of any
of the sources presented here and is the closest source to the Galactic plane.
The orbital period is 4.1 hr, with a minimum companion mass of 0.21
$M_{\odot}$. The pulsar has highly frequency dependant eclipses, with it being
eclipsed for $\sim 1/2$ the orbit at 350 MHz but only for $\sim 1/4$ the orbit
at 2 GHz. LOFAR studies have shown the eclipse fraction to increase to $\sim
3/4$ at 54MHz (Broderick et al., 2016). The orbital and eclipse properties are
very similar to those of PSR J1023$+$0038, and consequently PSR J2215+5135
fits well within the redback category. The radio pulse profile morphology
shows strong evolution with frequency (Figure 1), which makes interpretation
of the radio/$\gamma$-ray alignment challenging.

The XMM-Newton light curves (Figures 6 and 7) show the double peaked orbital
variations centered at pulsar inferior conjunction that seem to be a common
feature of redbacks. The non-thermal emission also has the very hard (photon
index $\Gamma\sim 1$) typical of these sources.

This source has garnered significant interest since optical studies have
suggested a neutron star mass $M_{ns}>2.1M_{\odot}$ (Linares et al., 2018;
Kandel & Romani, 2020) orbiting the Roche lobe filling $M_{c}\sim
0.33M_{\odot}$ companion.

PSR J1302–3205, with an orbit of 18.8 hr, minimum companion mass of
$0.15M_{\odot}$, and evidence for a brief radio eclipse around superior
conjunction, is potentially of the redback class of systems where the
companion has not yet fully evolved and there is the potential for further
accretion episodes. We obtained multiple images of PSR J1302$-$3205 with the
Las Cumbres Observatory (LCOGT) network of 1 m and 2 m telescopes using
various filters. Using the XB-News pipeline analysis (Russell et al., 2019),
there were no firm detections of the source. An observation with the Spectral
Camera on the LCOGT 2 m at Siding Springs taken near pulsar inferior
conjunction, when heating should produce a maximum, puts a conservative upper
limit magnitude of $m_{i^{\prime}}>21.5$. An observation with the MuSCAT3
instrument on the LCOGT 2 m at Haleakala taken near pulsar superior
conjunction put upper limits on the night side of the companion of
$m_{i^{\prime}}>22.3$, $m_{g^{\prime}}>22.3$, $m_{r^{\prime}}>22.6$, and
$m_{z_{s}}>21.9$. With an extinction estimate of $A_{v}\sim 0.22$ from Drimmel
et al. (2003) for the nominal distance of 1 kpc, the companion must be no
brighter than a M8 sub-dwarf. The minimum companion mass is consistent with
that of a fully evolved helium white dwarf expected for the orbital
period–companion mass relationship derived by several authors (e.g. Tauris &
Savonije, 1999). We further note that PSR J1816$+$4510 has redback-like
eclipses, but optical spectroscopy indicates the companion is more consistent
with a proto-white dwarf (Kaplan et al., 2013). Combined with modest,
apparently thermal X-ray emission seen from the system, we find no convincing
evidence that PSR J1302$-$3205 should be classified as a redback at this time.

###  4.2 Implications for $\gamma$-ray Pulsar Population Statistics

Our survey contains 22 MSPs (all known as of publication) which are apparently
associated with Fermi-LAT sources. Six are black widows, with the four
discoveries reported here joined by PSR J2256$-$1024, discovered in the GBT
drift scan survey (Crowter et al., 2015), and PSR J1805+0615, discovered in a
survey of Fermi-LAT sources with Arecibo (Cromartie et al., 2016). Four are
redbacks; and three are redback candidates (Strader et al., 2019). Four of
these show evidence of intrabinary shocks in X-rays (Gentile et al., 2014;
Roberts et al., 2015), and five of them have been shown to significantly heat
their companion (Breton et al., 2013; Bellm et al., 2013). This might suggest
that the remarkably high fraction of these systems in this $\gamma$-ray
selected sample could be due to excess $\gamma$-ray emission from the pulsar
wind interacting with the companion. However, it is clear from the estimated
background levels in the $\gamma$-ray pulse profiles of our sample that
essentially all of the observed $\gamma$-ray flux can be attributed to pulsed
emission, and the $\gamma$-ray efficiencies (see Tables 3 and 4) are
compatible with those of other MSPs detected by Fermi-LAT (see 2PC and 3PC).

We note that all four of our detected pulsars with $\dot{E}>3\times 10^{34}$
erg s-1 are eclipsing sources, suggesting that such sources are more energetic
than the general MSP population. In general, $\gamma$-ray luminosity is
correlated with $\dot{\rm E}$, albeit mildly for MSPs in the 1033–1034 erg s-1
range (see Figure 14 and 3PC). This means that high-$\dot{\rm E}$ MSPs will be
detectable in $\gamma$ rays over a larger volume compared to low-$\dot{\rm E}$
pulsars. Conversely, radio luminosity is only weakly coupled with $\dot{\rm
E}$. Moreover, the eclipsing systems are also harder to detect in radio pulsar
surveys since their short orbital periods can require acceleration searches
for even short observations, and they can be eclipsed for a significant
fraction of the time. Given these considerations, we expect $\gamma$-ray
searches to be more efficient at detecting ‘spider’ systems than radio
surveys.

To examine the $\dot{E}$ dependence more concretely, we compared the
measurements in 3PC, which provide the best-available estimates of
$\gamma$-ray luminosity and Shklovskii-corrected $\dot{E}$, for isolated,
short-period, and long-period binaries, with results shown in Figure 14.
Visually, there seems to be a lack of low-$\dot{E}$ short-period binaries.
However, a two-sample Kolmogorov-Smirnov test indicates only modest evidence
for a difference between the binary and isolated populations (p-value 5%) and
no evidence for a difference between short- and long-period binaries (p-value
15%).

Figure 14: $\gamma$-ray luminosity ($L_{\gamma}$) as a function of corrected
spin-down luminosity ($\dot{\rm E}$) for the sample of $\gamma$-ray MSPs
reported in 3PC.

Of the 19 sources that we observed that are listed in the 3FGL catalog and
remain unidentified or unassociated with possible AGN counterparts, most
remain viable pulsar candidates. If they are MSPs, their non-detection is
likely due to large accelerations due to orbital motion, large eclipse
fractions, or simply radio faintness, as most MSPs appear to have radio and
$\gamma$-ray beams sweeping similar regions of the sky, with $>75$% of
$\gamma$-ray MSPs being radio-loud (3PC), although in the case of black widows
and redbacks they may be eclipsed more than half of the time at low
frequencies. Redbacks may also spend some fraction of their time in a low
accretion state with no observable radio pulsations but can have observable
unpulsed $\gamma$-ray emission in this state (Stappers et al., 2014; Johnson
et al., 2015). We therefore estimate that about 5 of the unidentified sources
are MSPs. Some of these may be detectable with re-observations or at higher
frequencies (note that PSR J1921+0137, discovered in an 820-MHz survey, is
undetectable in our data). In fact, three of our targets, 1FGL J0523.5$-$2529
(Strader et al., 2014), 1FGL J1119.9$-$2205 (Swihart et al., 2022), and 1FGL
J1544.5$-$1127 (Strader et al., 2019), have been suggested to be redbacks
through the identification of a plausible optical counterpart. PSR
J0955$-$3947, another redback candidate (Braglia et al., 2020), was recently
confirmed as a short-orbit pulsar by
TRAPUM101010http://trapum.org/discoveries/ (Clark et al., 2023a).

Three of our targets are now known to be non-recycled, middle-aged pulsars
discovered through blind $\gamma$-ray searches. These appear to be radio-quiet
pulsars, and none are detectable in our radio data. Compared to the 15 sources
that have not yet been identified in our sample, these three sources are on
average 2.6 times brighter in $\gamma$ rays. Assuming the $\gamma$-ray blind
search detectability is largely flux limited and that there is a roughly
spherical distribution of nearby, high Galactic latitude pulsars, this would
suggest that roughly a dozen of these fainter sources could be middle-aged
($\tau$$\sim$$10^{5}$ yr), radio-quiet pulsars. We note that nearby
$\gamma$-ray pulsars are likely to have fairly low spin-down powers and hence
are much more likely to be radio faint, compared to younger pulsars deep in
the plane (3PC). We therefore find it plausible that essentially all of our
remaining unassociated sources are either MSPs or nearby middle-aged pulsars.

Johnson et al. (2014) divide $\gamma$-ray pulsars into three classes based on
their pulse profiles: $\gamma$-ray peaks trail radio (Class I); are aligned
(Class II); $\gamma$-ray peaks leading radio (Class III). (See also Espinoza
et al. (2013) for a similar classification scheme.) Outer gap and slot gap
(two-pole caustic) models best fit roughly equal numbers of Class I and III,
while Class II are exclusively fit with pair-starved polar cap models.
According to this scheme, PSR J0102+4839 is a Class III pulsar, with the
$\gamma$-ray peak leading the radio peak. PSR J0307+7443 is a Class II, with
the $\gamma$-ray and radio peaks aligned. PSR J0340+4130 is a Class III, with
the primary $\gamma$-ray peak leading the primary radio peak, though it should
be noted that the primary radio peak is aligned with the secondary
$\gamma$-ray peak. Johnson et al. (2014) found that Class II MSPs had the
shortest spin periods, with five of the six members of this class having spin
periods less than 2 ms, while Class I and Class III MSPs had longer spin
periods. PSR J0307+7443, with a spin period of 3.2 ms, does not fit that
trend.

The close-orbit pulsars are more difficult to classify since nearly all show
strong frequency dependence in their radio pulse profiles. At 350 MHz, PSR
J1302$-$3258 is a Class I, with the main radio peak leading the $\gamma$-ray
peak. However, we note that an interpulse becomes apparent at 820 MHz which
trails the secondary $\gamma$-ray peak and becomes even more prominent at
higher radio frequencies. The Fermi-LAT 2PC catalog used the brightest profile
peak (at 2 GHz) of PSRs J0023+0923, J1810+1744, and J2215+5135 when
calculating phase lags. However, both PSRs J0023+0923 and J1810+1744 show a
narrow “pre-cursor” peak that is not visible at 350 MHz but whose peak
brightness begins to approach the main peak’s brightness by 2 GHz, and would
presumably become the “main peak” at higher frequencies. At low frequencies,
PSR J0023+0923 is a Class I and PSR J1810+1744 a Class II, but if measured
from the flatter spectrum pre-cursor peak, the classifications would be
reversed. The pulse profile evolution of PSR J2215+5135 is even more
pronounced. At 350 MHz, it has two peaks which are fairly well aligned with
the $\gamma$-ray peaks, implying this would be a Class II source, but at 2 GHz
there is a single peak that is neither of the 350-MHz peaks and which trails
the $\gamma$-ray peaks, and so would be a Class III. Pulse profiles which
strongly evolve with frequency seem to be a feature of many black widows and
redbacks. We note that this has also been noted in the original black widow
pulsar B1957+20 (Johnson et al., 2014) as well as in the redbacks PSR
J1048+2339 (Deneva et al., 2016) and PSR J1023+0038 (Archibald et al., 2009).
For these systems, at least, classification schemes based on relative phases
of $\gamma$-ray and radio peaks could break down. The pulse-profile evolution
common to these also brings up the possibility that the companion is somehow
influencing the radio emission of the pulsar if this frequency dependence is
truly more common among these systems than in the general millisecond pulsar
population.

###  4.3 Implications for General Millisecond Pulsar Population Statistics

In the standard theory of MSP formation, a neutron star in a binary system is
‘recycled’ by accreting matter from its companion star (Alpar et al., 1982;
Stairs, 2004), and consequently, most MSPs are expected to be in binary
systems with a white dwarf, the natural end state of the evolution of the
light companion following accretion and spinup. However, psrcat
currently111111MSP class counts obtained with psrcat use version 1.70 and
require period $P<10$ ms and a well-measured $\dot{P}$. lists 52 MSPs in the
Galactic field without binary companions, about 23% of the total. The
formation of these isolated MSPs is still not well understood, though one
possibility is that they ablated their companions with their strong
relativistic particle winds (Ruderman et al., 1989). Another $\sim$32
MSPs121212https://apatruno.wordpress.com/about/millisecond-pulsar-catalogue/
in the Galactic field do not have ordinary white dwarf companions but instead
have either under-massive companions or non-degenerate companions.
Understanding the various sub-populations of MSPs is critical for improving
our view of the pulsar recycling process and whether it proceeds via only one
channel or a variety of routes. The rapid rate of MSP discovery is can greatly
help in this endeavor by rounding out the population with respect to previous
observational biases.

Deep searches targeting $\gamma$-ray sources have a very different set of
observational biases than shallow, wide-field radio surveys. Due to there
being a much stronger correlation between spin-down luminosity and apparent
$\gamma$-ray luminosity than there is with apparent radio luminosity, a
$\gamma$-ray targeted survey will be more biased towards nearby, energetic
MSPs and less biased towards high radio luminosities. Older low-frequency
surveys had much less spectral resolution and were, therefore, more
susceptible to DM smearing of the pulse profile, degrading their sensitivity
to MSPs. Higher DM and scattering in the Galactic plane would also inhibit the
discovery of MSPs. In the past, there were also strong biases against
detecting pulsars with short orbital periods since acceleration searches only
became common practice after 2000.

Only two out of 18 MSPs (11%) detected in this survey are isolated. 3PC
reports 91 MSPs discovered by targeting unidentified Fermi sources, and of
these, only 16 are isolated131313We exclude two MSPs which were discovered in
a targeted search of LAT data (Clark et al., 2018)., about 18%. Both are lower
than the roughly 25% isolated fraction of the Galactic plane population found
before 2009. Furthermore, eight of the pulsars detected in this survey have
orbital periods $P_{b}<1$ d. In the same 3PC sample, the fraction of short-
period binaries is 54%. In stark contrast, only $\sim$14% of MSPs found before
2009 have such short binary periods. As a result of the Fermi searches, there
are now more known MSPs with short orbits than isolated ones. If we compare
the Galactic field population of $P_{b}<1$ day MSPs to the ones in globular
clusters, in both cases, the fraction is now $\sim$25%. This calls into
question the importance of binary exchange in the formation of these systems
in globular clusters.

Compared to binary MSPs, isolated MSPs could in principle be older or sustain
a higher spin-down energy loss ($\dot{E}$) in order to fully disintegrate
their companions. To investigate this, we have compared the $\dot{E}$ and
characteristic ages of binary and isolated MSPs both in full and restricted to
Galactic latitudes $|b|>5^{\circ}$. In order to obtain a larger sample, we use
values of $\dot{P}$ which have not been corrected for the Shklovskii effect to
estimate $\dot{E}$ and the characteristic age. (In addition to this caveat, it
is important to note that these are derived quantities of $P$ and $\dot{P}$
and are only rough proxies of the true ages and spindown luminosities.) The
distribution function for these two quantities is shown in Figure 15,
inspection of which indicates that the distributions for isolated and binary
MSPs is broadly compatible. We quantitatively compared the distributions using
a two-sample Kolmogorov-Smirnov test, finding modest evidence (p-value 5%)
that the $\dot{E}$ distributions differ. Further examination indicates that
the $|b|>5$ pulsars have only a 2% probability of being drawn from the same
distribution, with isolated pulsars generally having lower values of
$\dot{E}$, while the $|b|<5$ populations are compatible. There is no
substantial difference in any of the age distributions. Further work will be
necessary to determine if the observed difference in spindown luminosities is
intrinsic or an observational selection effect against low-$\dot{E}$, high-
latitude binaries.

Figure 15: The cumulative distribution functions (CDFs) of the spin-down
energy loss rates (top) and ages (bottom) for isolated and binary MSPs. The
different line styles show the full, low-latitutde (dashed) and high-latitude
(dotted) samples. These CDFs are broadly compatible, but a Kolmogorov-Smirnov
test provides modest evidence that the high-latitude $\dot{E}$ distributions
differ. See main text for details of the sample construction and analysis.

##  5 Conclusion

This work reports on a groundbreaking survey conducted more than ten years
ago. Although we wish it had been published sooner, much of the delay was due
to the success of the project. The MSP discoveries necessitated timing
observations, and the discovery of such an unusually—at the time—large number
of ‘spider’ pulsars demanded multiwavelength follow-up. It was always tempting
to acquire more observational data and better understand this growing source
class and its relation to $\gamma$ rays. The haul of new MSPs also created its
own distractions, because it inspired numerous follow-up surveys, many cited
above, of different parts of the sky, at different frequency ranges, and with
different Fermi source lists. The success of this method continues through the
present day, with powerful new instruments like FAST and MeerKAT and finding
ever fainter radio pulsars in Fermi sources (Wang et al., 2021; Clark et al.,
2023a).

Despite the long gap between observations and report, this work is the
definitive record of the radio observations carried out; the follow-up timing
observations; and the phase-aligned $\gamma$-ray and radio pulse profiles.
Together with a synthesis of multi-wavelength observations, particularly X-ray
but also benefiting from optical analyses of redback candidates that lack
pulsations, we can argue that $\gamma$-ray targeted searches correct a long-
standing bias in radio surveys, which seem to have overproduced isolated MSPs
relative to the general population. And it is only with the benefit of the
substantial followup that we can appreciate how efficient the original target
selection was. It is thus fitting that this report finally appear in the
literature, and we conclude: better late than never!

This work was partially supported by NASA Fermi Guest Investigator Program
grants NNG10PB13P, NNX10AP81G, NNX12AE32G, NNX12AP21G, and NNX13AO96G. The
Green Bank Telescope is operated by the National Radio Astronomy Observatory,
a facility of the National Science Foundation operated under cooperative
agreement by Associated Universities, Inc. The WSRT is operated by ASTRON/NWO.
This work makes use of observations from the Las Cumbres Observatory global
telescope network, with support from New York University Abu Dhabi. This paper
includes observations made with the MuSCAT3 instrument, developed by
Astrobiology Center and under financial supports by JSPS KAKENHI (JP18H05439)
and JST PRESTO (JPMJPR1775), at Faulkes Telescope North on Maui, HI, operated
by the Las Cumbres Observatory. We thank Gyula Jozsa for help scheduling the
WSRT observations. JWTH acknowledges funding from an NWO Vidi fellowship and
ERC Starting Grant “DRAGNET” (337062). MSER acknowledges funding from the NYU
Abu Dhabi faculty research fund. Work at NRL is supported by NASA and by ONR
6.1 basic research funding. ECF is supported by NASA under award number
80GSFC21M0002. SMR is a CIFAR Fellow. MAM is supported by NSF award 2009425.
MED, ECF, MK, MAM, SMR, and PSR and members of the NANOGrav Physics Frontiers
Center supported by NSF award 2020265. The Fermi LAT Collaboration
acknowledges generous ongoing support from a number of agencies and institutes
that have supported both the development and the operation of the LAT as well
as scientific data analysis. These include the National Aeronautics and Space
Administration and the Department of Energy in the United States, the
Commissariat à l’Energie Atomique and the Centre National de la Recherche
Scientifique / Institut National de Physique Nucléaire et de Physique des
Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale
di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), High Energy Accelerator Research Organization
(KEK) and Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A.
Wallenberg Foundation, the Swedish Research Council and the Swedish National
Space Board in Sweden. Partially based on observations obtained with XMM-
Newton, an ESA science mission with instruments and contributions directly
funded by ESA Member States and NASA Additional support for science analysis
during the operations phase is gratefully acknowledged from the Istituto
Nazionale di Astrofisica in Italy and the Centre National d’Études Spatiales
in France. This work performed in part under DOE Contract DE-AC02-76SF00515.
We thank the NANOGrav collaboration for contributing data on PSR J0340+4130
that were used for this paper. We also thank Duncan Lorimer and Elizabeth
Ferrara for maintaining the list of published and unpublished Galactic field
MSPs.

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