# A NICER view on the 2020 Magnetar-Like Outburst of PSR J1846$-$0258

Chin-Ping Hu  Department of Physics, National Changhua University of
Education, Changhua 50007, Taiwan  Lucien Kuiper  SRON-Netherlands Institute
for Space Research, Niels Bohrweg 4, 2333 CA, Leiden, The Netherlands  Alice
K. Harding  Theoretical Division, Los Alamos National Laboratory, Los Alamos,
NM 87545 USA  George Younes  Department of Physics, The George Washington
University, Washington, DC 20052, USA  Astronomy, Physics and Statistics
Institute of Sciences (APSIS), The George Washington University, Washington,
DC 20052, USA  Harsha Blumer  Department of Physics and Astronomy, West
Virginia University, Morgantown, WV 26506, USA  Center for Gravitational Waves
and Cosmology, West Virginia University, Chestnut Ridge Research Building,
Morgantown, WV 26505, USA  Wynn C. G. Ho  Department of Physics and Astronomy,
Haverford College, 370 Lancaster Avenue, Haverford, PA 19041, USA  Teruaki
Enoto  Extreme Natural Phenomena RIKEN Hakubi Research Team, Cluster for
Pioneering Research, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
Cristóbal M. Espinoza  Departamento de Física, Universidad de Santiago de
Chile (USACH), Av. Victor Jara 3493, Estación Central, Chile  Center for
Interdisciplinary Research in Astrophysics and Space Sciences (CIRAS),
Universidad de Santiago de Chile, Santiago, Chile  Keith Gendreau  X-Ray
Astrophysics LAboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771
A. K. Harding<EMAIL_ADDRESS>
###### Abstract

We report on our monitoring of the strong-field magnetar-like pulsar PSR
J1846$-$0258 with the Neutron Star Interior Composition Explorer (NICER) and
the timing and spectral evolution during its outburst in August 2020. Phase-
coherent timing solutions were maintained from March 2017 through November
2021, including a coherent solution throughout the outburst. We detected a
large spin-up glitch of magnitude $\Delta\nu/\nu=3\times 10^{-6}$ at the start
of the outburst and observed an increase in pulsed flux that reached a factor
of more than 10 times the quiescent level, a behavior similar to that of the
2006 outburst. Our monitoring observations in June and July 2020 indicate that
the flux was rising prior to the Swift announcement of the outburst on August
1, 2020. We also observed several sharp rises in the pulsed flux following the
outburst and the flux reached quiescent level by November 2020. The pulse
profile was observed to change shape during the outburst, returning to the
pre-outburst shape by 2021. Spectral analysis of the pulsed emission of NICER
data shows that the flux increases result entirely from a new black body
component that gradually fades away while the power-law remains nearly
constant at its quiescent level throughout the outburst. Joint spectral
analysis of NICER and simultaneous NuSTAR data confirms this picture. We
discuss the interpretation of the magnetar-like outburst and origin of the
transient thermal component in the context of both a pulsar-like and a
magnetar-like model.

††facilities: _NICER_ , _NuSTAR_ , _Swift_ (XRT)††software: HEASoft, XSPEC

##  1 Introduction

The dividing line between the two neutron star populations, rotation-powered
pulsars (RPPs) and magnetars, has been blurring over the past ten years with
the discovery of magnetar-like outbursts from the RPPs PSR J1846$-$0258 and
J1119-6127. Both of these pulsars possess very strong surface dipole magnetic
fields that are at the upper end of what is observed for RPPs, with $5\times
10^{13}$ G for J1846$-$0258 and $4\times 10^{13}$ G for J1119-6127. The first
observed outburst from J1846$-$0258 occurred in 2006 (Kumar & Safi-Harb, 2008;
Gavriil et al., 2008), during which the pulsed flux increased by a factor of
five, decaying over several months, and a transient thermal component appeared
in addition to the persistent non-thermal emission, whose flux rose by 35%
(Kuiper & Hermsen, 2009). PSR J1119$-$6127 experienced a magnetar-like
outburst in 2016 (Archibald et al., 2016), but in this case a transient non-
thermal power law component appeared and the quiescent purely thermal pulsed
flux increased by a factor of $>160$. In both outbursts, large spin-up
glitches occurred, with $\Delta\nu/\nu=5.74\times 10^{-6}$ for J1119$-$6127
and $\Delta\nu/\nu=2-4.4\times 10^{-6}$ for J1846$-$0258 (Livingstone et al.,
2010).

PSR J1846$-$0258 was discovered through 0.324 s X-ray pulsations by Gotthelf
et al. (2000) and with its high spin-down rate of $\dot{P}=7\times
10^{-12}\,\rm s\,s^{-1}$ has a characteristic age of $\tau=723$ yr, making it
the youngest known RPP. It is located in the Kes 75 supernova remnant and
surrounded by a compact pulsar wind nebula (Helfand et al., 2003; Ng et al.,
2008). Its pulsed emission is still only seen in the X-ray band from $3-200$
keV with RXTE INTEGRAL, (Kuiper & Hermsen, 2015), and recently also in soft
$\gamma$-rays below 100 MeV with Fermi (Kuiper et al., 2018). Its spectral
energy distribution (SED) is very similar to that of PSR B1509$-$58, another
young, energetic RPP, with an SED peak around 1 - 10 MeV and a decline above
30 MeV (Kuiper et al., 2018). However, B1509$-$58 is a radio pulsar while
J1846$-$0258 is radio quiet. These two pulsars are members of a class of so-
called “MeV pulsars” whose SEDs peak at MeV energies, contrary to most other
energetic RPPs peaking at GeV energies. The MeV pulsars are a group of 11
rotation-powered pulsars that exhibit strong, non-thermal hard X-ray emission,
have broad single pulse profiles, and no detected gamma-ray pulsations above 1
GeV (Kuiper et al., 2018; Harding & Kalapotharakos, 2017). All except one are
radio quiet.

In this paper, we will describe our long-term monitoring and timing of
J1846$-$0258 using NICER (and Swift for the pre-2018 era) in Section 2. As a
result of this program we have obtained phase-coherent timing solutions from
April 2017 through November 2021. We will also report on the magnetar-like
outburst of August 2020 and its aftermath, during which we obtained Target-of-
Opportunity (ToO) observations following the Swift announcement (Krimm et al.,
2020) of a magnetar-like $\sim 0.1$ s flare/burst. Near the start of the
outburst we detected a large spin-up glitch (Kuiper et al., 2020) and
maintained coherent timing over the entire outburst. Timing through the
outburst and the evolution of the pulsed flux and pulse profile are described
in Section 3.1. The spectral evolution before and during the outburst are
discussed in Section 3.2. Finally in Section 4 we will discuss the
implications of these results and how J1846$-$0258 fits in to the RPP-magnetar
connection.

##  2 Observations and Data Reduction

###  2.1 NICER Monitoring

We have been timing J1846$-$0258 with NICER starting on May 23, 2018
(Observation id. 1033290101; MJD 58261) through a series of Guest Observer
proposals, AO1 - AO4 GO. The last NICER observation used in this work was
performed on November 13, 2021 (Observation id. 4607021501; MJD 59531), and so
our NICER monitoring covered a $\sim 3.5$ year period subjected to $\sim 3$
months (approximately lasting from end November till end February) regular
episodes in which no data can be taken due to visibility constraints resulting
in (large) data gaps.

The NICER data were processed using HEASOFT version 6.27.2 and NICERDAS
version 7a. We used the latest calibration files obtained from the standard
CALDB release for NICER downloaded from NASA’s High Energy Astrophysics
Science Archive Research Center (HEASARC). We start our data reduction from
level 1 event files. We create good time intervals using standard filtering
criteria as described in the NICER Data Analysis Guide. For spectral studies,
we excluded module numbers 14 and 34 to avoid potential contamination by
instrumental noise in the soft energy band.

For the timing analysis, requiring less restrictive selections as for the
spectral analysis, we used predominantly the screened event files as stored at
HEASARC. Next, we applied a time filter based on the observed count rate in
the 12-15 keV band, and ignored events from periods with high rates in that
band. Also events from high-rate modules are ignored. Finally, to search for
the pulsed emission from J1846$-$0258 we used only events with energies in the
2.5–10 keV band, the optimum band. Sometimes the default screening criteria
resulted in very small exposure times in which case we used the unfiltered
level 1 event files, and followed the same screening steps as outlined before.

###  2.2 NuSTAR Observations

During the 2020-outburst NuSTAR observed J1846$-$0258 on 2020 August 5 (ObsID:
80602315002), 2020 August 20 (ObsID: 80602315004), 2020 September 17 (ObsID:
80602315006), and 2020 October 09 (ObsID: 80602315008). We also used NuSTAR
data of J1846$-$0258 from a $\sim 95$ ks observation taken on 2017 September
17 (ObsID: 40301004002) with the source in quiescent state.

The NuSTAR data were reduced using the standard data reduction process with
NuSTAR DAS v1.9.2 and CALDB 20191219. We extracted on-source spectra from
circular regions of 1$\arcmin$-radius centered at the source position in the
3–70 keV energy band from the NuSTAR observations. For the spectral fitting,
we grouped source spectra using the grppha tool of HEASoft, such that each
spectral bin would have a minimum of 25 counts.

###  2.3 Swift Monitoring

In this work we also analysed data from the XRT-telescope (Burrows et al.,
2005) aboard the Neil Gehrels Swift observatory (Gehrels et al., 2004),
operating in Windowed-Timing (WT) mode, providing a time resolution of 1.7675
ms. The Swift monitoring campaign of J1846$-$0258 started on July 25, 2011 and
ended on November 19, 2018. Data from the early phase of the monitoring
program up to and including September 4, 2016 has already been used and
analysed in Kuiper et al. (2018). In order to characterize the spin-behavior
of J1846$-$0258 during its quiescent state before the 2020-outburst we
analysed the monitoring data collected between September 17, 2016 and November
19, 2018. More detailed information is given in Sect. 3.1.1.

##  3 The August 2020 Outburst

On August 1, 2020, the Swift telescope reported a short-duration ($\sim 0.1$
s) flare in the BAT detector from J1846$-$0258 (Krimm et al., 2020). We
started ToO observations on August 2 with NICER , obtaining densely sampled
observations of the source until November 22, 2020. From our AO2 GO monitoring
observations, that started on March 26, 2020, we saw that during the
observation performed on June 26, 2020 (MJD 59026) the pulsed count rate in
the 2.5-10 keV band had already increased by a factor of $\sim 4$ with respect
to the (pre-outburst) quiescent level (Kuiper et al., 2020). The July 25, 2020
(MJD 59055) observation, performed about a week before the magnetar-like
burst, showed an even larger, 10 times higher, 2.5-10 keV pulsed count rate
than the pre-outburst value. Apparently, the source had already gone into
outburst somewhere between May 27, 2020 (MJD 58996; the last AO2 GO
observation of the quiescent period) and June 26, 2020.

Figure 1 shows the 2.5 - 10 keV pulsed count rate starting from our AO2
observation on May 27, 2020 through the outburst until the count rate reached
near quiescent level on November 22, 2020. The Swift-BAT burst announced on
August 1, 2020 (Krimm et al., 2020) is marked by the vertical dotted line,
which already occurred after the first recorded rate peaked on July 25, 2020.
During the ToO observations, we recorded at least three more count rate peaks
with the highest level reached on August 21, 2020. The very dense time
sampling reveals that the outburst decay was very non-steady with possibly
many unresolved variations in flux.

Figure 1: Pulsed count rate ($2.5–10$ keV) of J1846$-$0258 from NICER data
taken between June 26, 2020 and November 22, 2020\. The vertical dotted line
indicates the date of the magnetar - like $\sim 0.1$s burst detected by Swift.
The horizontal long-dashed line is the quiescent count rate level. The grey
areas - indicated by the numbers I, II and III - covering the 2020-outburst
era specify the three data periods for which we have phase-coherent timing
solutions (see Table 1 entries 5–7 ). The four black bars at the bottom of the
plot refer to the four NuSTAR observations performed during the full outburst
period.

Blumer et al. (2021) presented radio observations with the Green Bank
Telescope (GBT) and X-ray observations with Chandra of J1846$-$0258 and its
pulsar wind nebula (PWN) following the 2020 outburst. Their GBT observations
took place August 5 - 6, 2020, only a few days after the Swift announcement
but actually about ten days after the initial outburst peak flux. They
reported no detectable pulsed radio emission and place an upper limit of 55
mJy. They also presented results of Chandra ACIS-S observations taken on
September 12, 2020, more than one month after the outburst. Fitting the total
(=pulsed plus unpulsed/DC) X-ray spectrum with a power law plus blackbody,
they reported a softer power law index of $\Gamma=1.7\pm 0.3$ compared to the
quiescent value of $\Gamma=1.2\pm 0.1$ and the appearance of a thermal
component with temperature $kT=0.7\pm 0.1$ keV. Their measured 0.5 - 10 keV
flux was 2.4 times the quiescent flux level, consistent with our NICER count
rate on that date (see Fig. 1). They detected no significant increase in X-ray
flux or spectral index of the PWN. Livingstone et al. (2011) also found no
change in X-ray flux or spectrum of the PWN following the 2006 outburst.

Table 1: Phase-coherent ephemerides for PSR J1846$-$0258 as derived from NICER-XTI and Swift-XRT (monitoring) data covering the time period MJD 57871-59532 (April 28, 2017 – November 14, 2021). Entry∗ | Start | End | t0, Epoch | $\nu$ | $\dot{\nu}$ | $\ddot{\nu}$ | $\dddot\nu$ | RMS† | $\Phi_{0}^{\ddagger}$
---|---|---|---|---|---|---|---|---|---
# | [MJD] | [MJD] | [MJD,TDB] | [Hz] | [$10^{-11}$ Hz/s] | [$10^{-21}$ Hz/s2] | [$10^{-24}$ Hz/s3] |  |
Pre-Outburst 2020
1 | 57871 | 58061 | 57956.0 | 3.044 250 906(4) | -6.636 32(7) | 4.63(79) | 0.0 | 0.044 | 0.4167
2 | 58167 | 58422 | 58402.0 | 3.041 697 558(20) | -6.619 07(50) | 3.67(54) | 0.0 | 0.025 | 0.4258
3 | 58261 | 58671 | 58312.0 | 3.042 212 358(3) | -6.621 40(4) | 2.93(3) | 0.0 | 0.029 | 0.1074
4 | 58698 | 58996 | 58893.0 | 3.038 892 369(2) | -6.604 60(3) | 5.72(9) | 0.0 | 0.042 | 0.9539
Outburst 2020
5 | 59055 | 59086 | 59064.0 | 3.037 919 719(23) | -6.725 6(25) | -2659(45) | 0.0 | 0.038 | 0.8216
6 | 59087 | 59129 | 59099.0 | 3.037 707 087(50) | -7.018 0(52) | 1776(98) | -1.37(14) | 0.026 | 0.1456
7 | 59129 | 59175 | 59145.0 | 3.037 432 343(15) | -6.849 8(12) | 298(22) | 0.0 | 0.038 | 0.4163
Post-Outburst 2020
8 | 59281 | 59418 | 59318.0 | 3.036 433 103(8) | -6.709 6(2) | 5.6(1.0) | 0.0 | 0.022 | 0.0535
9 | 59434 | 59532 | 59523.0 | 3.035 250 253(16) | -6.667 2(4) | 0.0 | 0.0 | 0.018 | 0.7372
∗ Entries 1-2 are solely Swift-XRT ToA based covering April 28 – November 4,
2017 (entry-1) and February 18 – November 19, 2018 (entry-2),
entry 3 is based on a combination of Swift-XRT and NICER ToA’s starting at the
first NICER observation performed on
May 23, 2018 (MJD 58261) and entries 4–9 are solely NICER TOA based.
∗ Solar System planetary ephemeris DE200 has been used in the barycentering
process.
† The RMS is the root-mean-square of the deviation from the best fit in phase
units.
‡ $\Phi_{0}$ is the phase offset to apply in the phase calculation to obtain
consistent alignment (see e.g. Eq. 1 in Kuiper et al., 2018).

###  3.1 Timing analysis

In this work, constituting an extension of the timing analyses presented in
earlier work by Kuiper & Hermsen (2009) and Kuiper et al. (2018), we performed
timing analyses of the pre-outburst, outburst and post-outburst episodes of
the 2020-outburst using data from NICER\- and (for a part of the pre-outburst
period, 2017-2018) Swift-monitoring observations to make comparisons between
quiescent- and outburst states possible. Swift and NICER monitoring overlapped
between May–November 2018 enabling cross-checking and verification of the
timing characteristics.

These analyses yielded phase-coherent timing models (ephemerides) in terms of
three, sometimes four, timing parameters specifying the frequency $\nu$,
frequency derivative $\dot{\nu}$, frequency second $\ddot{\nu}$ and third
order $\dddot\nu$ derivatives evaluated at a certain epoch t0. These models
accurately describe the rotation behavior of the pulsar as a function of time,
and so tracking the spin-evolution of the source.

Pulse-phase folding timing data from any instrument using these ephemerides,
adopting different energy selections, yielded energy-resolved pulse-phase
distributions, often called pulse-profiles. From these the pulse shape as a
function of energy can be determined for any emission state of the pulsar.

The models also enable us to separate at any epoch the pulsed emission
component from the total emission which has contributions from the surrounding
PWN and SNR, and the pulsar itself either pulsed or DC. So, we can study the
pulsed flux (or count rate) as a function of time, probing the outburst.

Figure 2: Left: Spin frequency evolution $\Delta\nu(t)$ of J1846$-$0258
(relative to the pre-2006 outburst ephemeris, see e.g. Kuiper & Hermsen, 2015;
Gavriil et al., 2008; Livingstone et al., 2010, 2011) since its discovery in
RXTE-PCA data from April 1999 up to November 2021. The 2006 and 2020 glitches
are clearly visible near MJD 54000 and MJD 59000, as well as an (accelerated)
spin-down rate feature/glitch that pops up around MJD 57700 near/during the
Swift 2016–2017 observation gap. Right: Zoom in of the rotation frequency
evolution during the period of Swift-XRT and NICER timing from early 2014
until November 2021. Swift and NICER data gaps are shown as grey hatched
vertical areas.

####  3.1.1 Timing models (ephemerides)

The first step in the timing analysis is the conversion of spacecraft arrival
times (usually evaluated in TT or TDT time system) to solar-system
barycentered times (TDB time frame) using the instantaneous spacecraft
ephemeris (specifying the position and velocity of the spacecraft), solar-
system ephemeris (in this work JPL-DE200), a Chandra X-ray based accurate
location of the pulsar (Helfand et al., 2003) and for Swift up-to-date clock-
correction information. We performed this barycentering process for Swift \-
XRT (WT-mode) observations 00032031149 – 00032031216 executed between
September 15, 2016 and November 19, 2018 (MJD 57646 – 58441). An equivalent
(IDL-based) barycentering procedure was used for the NICER observations.

From the barycentered Swift and NICER event lists of each observation (or
combination of observations if the exposure of a single observation was too
low) we derived so-called pulse arrival times (ToA’s). For Swift-XRT (2.5–10
keV) on average we typically needed 15-20 ks net exposure time to detect the
timing signal of J1846$-$0258 above $3\sigma$ in a $25^{\prime\prime}$
extraction area centered on the source (see e.g. Sect. 3.1.2 of Kuiper et al.,
2018). For NICER (2.5–10 keV) the required net exposure is typically 5–8 ks to
reach a significant pulsed signal detection depending on the instantaneous
background rate.

From these ToA’s we constructed phase-coherent timing models covering a $\sim
4.5$ year period from April 28, 2017 till November 14, 2021 according to the
method outlined in Sect. 4 of Kuiper & Hermsen (2009). The results are listed
in Table 1 and consist of eight separate models: four covering the pre-
outburst episode (entries 1–4 of which 1–2 are solely based on Swift
monitoring), three covering the (densely sampled) outburst period (entries
5–7), and finally two covering the post-outburst era (entries 8–9). The spin-
evolution of J1846$-$0258 since its discovery in April 1999 with respect to
the pre-2006 outburst ephemeris is shown in the left panel of Fig. 2, while
the right panel zooms in on the time period analysed in this work. In the
latter period from early September 2016 till mid November 2021 we discovered
several timing discontinuities in both $\nu$ and $\dot{\nu}$ which are
summarized below:

  * (i)

MJD 57635–57871 (September 4, 2016 - April 28, 2017): A clear glitch in
$\dot{\nu}$ showing an acceleration in the spin-down rate occuring just before
or in the 2016–2017 Swift monitoring data gap.

  * (ii)

MJD 58061–58167 (November 4, 2017 - February 18, 2018): Phase-coherence loss
during the 2017–2018 Swift monitoring data gap.

  * (iii)

MJD 58671–58698 (July 7, 2019 - August 3, 2019): A small glitch of fractional
size $\Delta\nu_{g}/\nu_{g}=(7.3\pm 0.4)\times 10^{-8}$ occurred during the
NICER monitoring. In spite of its small amplitude, the glitch could be well
characterized because of the densely sampled NICER ToO-observations in the
post-glitch period. Changes in both $\dot{\nu}$ and $\ddot{\nu}$ were also
detected, with values $\Delta\dot{\nu}{{}_{g}}/\dot{\nu}{{}_{g}}=(4.5\pm
0.3)\times 10^{-4}$ and $\Delta\ddot{\nu}_{g}=(2.66\pm 0.13)\times 10^{-21}$
Hz/s2, all evaluated at an assumed glitch epoch $t_{g}$ of MJD 58685. This
timing glitch could easily be distinguished from red noise.

  * (iv)

MJD 58996–59055 (May 27, 2020 - July 25, 2020): A strong glitch of fractional
size $\Delta\nu_{g}/\nu_{g}\sim 3\times 10^{-6}$, comparable in size to the
2006 outburst (see Sect.4.2 of Kuiper & Hermsen, 2009, for more details),
heralded very likely the start of the 2020 outburst of J1846$-$0258 .

  * (v)

MJD 59175–59281 (November 11, 2020 - March 8, 2021): Phase-coherence loss
during the NICER 2020–2021 data gap of $\sim 3.4$ month duration due to one or
more glitches.

  * (vi)

MJD 59418–59434 (July 23, 2021 - August 8, 2021): A small glitch of fractional
size $\Delta\nu_{g}/\nu_{g}\sim(0.5-0.66)\times 10^{-6}$ occured during
regular monthly NICER monitoring.

A linear fit to the spin-down rates in Table 1 gives an indication of the
effective evolution of this quantity on time, with $\ddot{\nu}=3.9(3)\times
10^{-21}$ Hz/s2. This indicates a braking index before the outburst (Swift \+
NICER data) of $n=2.7\pm 0.2$. Interestingly, the braking index after the 2006
outburst was seen to decrease from $2.65\pm 0.01$ to $2.16\pm 0.13$
(Livingstone et al., 2011). Our preliminary measurement indicates that between
2006 and 2020 the rotation somehow went back to its original pre-2006-outburst
evolution. There is still not enough data to measure the braking index after
the 2020 outburst.

Figure 3: Pulse profiles of J1846$-$0258 for different stages prior to the
2020-outburst, indicated by Pre-II (Swift 2017–2018; MJD 57871–58442; April
28, 2017 – November 20, 2018 totaling an exposure time of 319.4 ks, NuSTAR
September 17–20, 2017; exposure time $\sim 95$ ks) and Pre-I (NICER 2018 –
2020 pre-outburst; MJD 58261–58996; May 23, 2018 – May 27, 2020; exposure time
$\sim 166.7$ ks), for different energy bands as observed by Swift, NICER and
NuSTAR. Along the y-axis the count rate (in cts/s) is shown. Error bars are at
$1\sigma$ confidence.

####  3.1.2 Pulse profile evolution

We applied the timing models listed in Table 1 in a pulse-phase folding
procedure to obtain pulse-phase distributions for several different energy
bands for the pre-outburst period (split into two parts related to Swift and
NICER monitoring), for the outburst episode subdivided into three parts driven
by ephemeris validity constraints, and finally for the post-outburst era. The
resulting phase-distributions are shown in Figs. 3, 4 and 5 for the pre-
outburst, outburst and post-outburst periods, respectively.

Figure 4: Pulse profiles of J1846$-$0258 during different phases of the
2020-outburst, indicated by I (MJD 59055–59086; July 25 – August 24, 2020), II
(MJD 59087–59127; August 26 – October 5, 2020) and III (MJD 59129–59175;
October 7 – November 22, 2020, for different energy bands as observed by NICER
and NuSTAR. NuSTAR observed J1846$-$0258 on August 5 and August 20, 2020,
during segment-I, on September 17, 2020 during II, and October 9, 2020 during
III. Along the y-axis the count rate (in cts/s) is shown. Error bars are at
$1\sigma$ confidence. The columns show the pulse profiles for a certain energy
band for either NICER or NuSTAR with a fixed range in ordinate making
comparisons between profiles from different outburst phases more comfortable.
Significant changes in shape of the pulse phase distributions and intensity of
the pulsed emission are only detectable below $\lesssim 10$ keV.

The pre-outburst period running from April 28, 2017 to May 27, 2020 was
subdivided into two parts with Swift (Pre-II; MJD 57871–58442) and NICER
(Pre-I; MJD 58261–58996) monitoring. Pulse phase folding 51 Swift
observations, totaling 319.402 ks exposure time, resulted in the phase
distributions shown in the upper-left two frames of Fig. 3 with $Z_{2}^{2}$
significances of $2.8\sigma$ and $13.0\sigma$ for the 1–2.5 and 2.5–10 keV
bands, respectively. During the Swift monitor period also NuSTAR observed
J1846$-$0258 during September 17–20, 2017 for $\sim 95$ ks while the source
was in quiescent state. The well-known pulse-shape (see e.g. Fig. 3 of Kuiper
& Hermsen, 2009) is clearly visible up to $\sim 60$ keV in these NuSTAR data
(see upper right three frames of Fig. 3). The second part of the pre-outburst
period (Pre-I) referred to solely NICER monitoring observations comprising
166.67 ks of (screened) exposure time. The corresponding pulse-phase
distributions are shown in the lower-left frames of Fig. 3 for the 1–2.5
($Z_{2}^{2}$ significance of $4.8\sigma$) and 2.5–10 keV bands.

The post-outburst period from March 8 – November 13, 2021 (MJD 59281–59531)
using merely NICER monitoring observations, totalling 103.55 ks of screened
exposure time, shows pulse-phase distributions for the 1–2.5 ($Z_{2}^{2}$
significance of $4.1\sigma$) and 2.5–10 keV bands (see Fig. 5) equivalent in
shape and strength to the corresponding NICER pre-outburst distributions.

It is clear from the pre- and post-outburst observations, representing the
quiescent state, that the pulsed emission in the 1–2.5 keV band is weak,
mainly coming from events with measured energies in the range 2–2.5 keV.

The situation changed dramatically during the outburst episode (July 25 –
November 22, 2020; MJD 59055-59175) that was split into three smaller time
segments, indicated by I, II and III (NICER screened exposures 86.014, 40.694
and 20.064 ks, respectively), driven by phase-coherency111During a regular
NICER monitoring observation on June 26, 2020 (MJD 59026) the source was
already in outburst, however, we could not phase-connect its timing data (see
Table 1).

Figure 4 shows the evolution of the pulse morphology and strength for
different energy bands during different stages of the outburst as observed by
NICER and NuSTAR. The most left column showing the NICER 1–2.5 keV lightcurves
demonstrates the emergence of a very strong (soft) pulsed emission component
which gradually fades away towards its quiescent level. The pulsed
significance $Z_{2}^{2}$ in the 1–2.5 keV band evolves from $31.5\sigma$ to
$15.2\sigma$ to $6.0\sigma$ for the three segments I, II and III,
respectively.

Both the NICER 2.5–10 keV and NuSTAR 3–10 keV bands show a similar fading
evolution, however, contrary to the quiescent pulse morphology a prominent
‘shoulder’ or bump appears during segment I observations near phase $\sim 0.2$
before primary maximum at phase 0.37. This feature also fades away in course
of the outburst. It is also clear from the 1–2.5, 2.5–10 and 3–10 keV pulse-
phase distributions that an underlying enhanced DC-component appeared that
fades away across the outburst.

On the contrary, the pulsed emission above 10 keV as shown by NuSTAR in the
two right most columns of Fig. 4 seems to not vary dramatically and is
comparable to its quiescent state properties (c.f. Fig.3; Pre-II - right two
panels).

Figure 5: Pulse profiles of J1846$-$0258 for the Post-outburst 2020 period
(MJD 59281–59531; March 8 – November 13, 2021) as observed by NICER in two
different energy bands. Along the y-axis the count rate (in cts/s) is shown.
Error bars are at $1\sigma$.

####  3.1.3 Pulsed flux evolution during the 2020-outburst

From the pulse-phase distributions per observation (or combination of
observations) pulsed excess counts can be determined for any energy band by
estimating the excess counts above the DC or unpulsed level evaluated from the
0.8–1.05 phase window (off-pulse phase; see e.g. Fig. 3 of Kuiper & Hermsen,
2009). These pulsed excess counts are subsequently converted to rates using
the (screened) exposure times of the involved observation(s). The 2.5–10 keV
pulsed count-rate across the 2020-outburst episode as observed by NICER is
shown in Fig. 1. The different segments - I, II and III - are hatched while
solid vertical bars at the bottom of the plot indicate the performed NuSTAR
observations. The horizontal dashed line represents the quiescent 2.5–10 keV
level and the vertical dotted line indicates the Swift detection of the
magnetar-like $\sim 0.1$s burst (Krimm et al., 2020), triggering our NICER ToO
program. It is clear from this plot that J1846$-$0258 was already in outburst
before the occurrence of the magnetar-like burst. Several re-flarings are also
visible in Fig. 1 in particular the one on August 21, 2020 (MJD 59082).
Initially, we suspected that this was a statistical fluke, however, a deeper
study reveals that this enhancement of about 1 hour duration was also clearly
detectable in a contemporaneous NuSTAR observation (ObsID: 80602315004), and
so demonstrates the validity of the event. The feature mimics a mini-outburst,
for the first time introduced by Kuiper et al. (2012) (see their Sect. 3.1.1)
in the study of the high-energy characteristics of the 2008 and 2009-outbursts
of magnetar 1E1547.0$-$5408.

The quiescent state level is reached near MJD 59175 (November 22, 2020), the
last NICER observation before the 2020–2021 data gap.

###  3.2 Spectral Evolution

We carried out spectral fits on the pulsed component only, meaning that the
photons collected during off-pulse phases were treated as background and
subtracted from the on-pulse spectra. To examine the broadband spectral
behavior of the pulsed emission, we first jointly fit NICER and NuSTAR spectra
at four different epochs. We used the tbabs model with corresponding
interstellar medium abundance as described in Wilms et al. (2000) to determine
the hydrogen column density ($N_{\scriptsize{\textrm{H}}}$) and linked this
parameter across all data sets. The spectra were fit with a two-component
model consisting of a hard power law and a soft blackbody. The parameters of
these two components are linked between data sets obtained at the same epoch.
An additional constant component was introduced to account for the cross-
calibration between instruments and the flux difference between data sets.

Figure 6: Spectra extracted from NICER data (open points) and NuSTAR data
(closed points). The solid lines indicate the best-fitting model to data with
the corresponding color. The bottom panel shows the corresponding residuals in
terms of $\sigma$.

The first NuSTAR observation (ObsID 80602315002) was made together with NICER
ObsID 3033290103 on MJD 59066, and we fit these two data sets by linking all
the spectral parameters. The other three NuSTAR observations did not have
corresponding NICER observations on the same dates, so we performed
simultaneous fits with nearby NICER observations that bracketed the NuSTAR
ones. For example, we jointly fit the NuSTAR observation on MJD 59081 (ObsID
80602315004) with NICER observations on MJD 59080 and 59082. Similar work was
done for observations on MJD 59109 (ObsID 80602315006) and MJD 59131 (ObsID
80602315008). The results are shown in Table 2 and the unfolded spectra of
these four data sets are shown in Figure 6. The $N_{\textrm{H}}$ was
constrained to be $(6.1\pm 0.8)\times 10^{22}$ cm-2. The photon indices of
these four data sets varied slightly but remained consistent at around
$\Gamma\approx 1.2$. On the other hand, the blackbody component varied
significantly, indicating that the flux variability during the outburst is
dominated by changes in the blackbody component. This can also be seen from
the pulse profiles as a function of energy.

Figure 7: Spectral evolution of the pulsed emission of J1846$-$0258 observed
with NICER following the outburst on 2020 August 01. From upper to lower,
panels show the evolution of the unabsorbed flux in the 1–10 keV band (in
units of erg cm-2 s-1), the blackbody temperature (in units of keV), the
emitting radius (assuming a distance of 6.5 kpc), the BB flux, PL flux, and
the ratio of BB flux to the total flux. The horizontal dashed line denotes the
mean value of the PL flux across all data sets.

We then traced the spectral evolution in the soft X-ray band using all NICER
data sets. We tried to fit individual data sets by freezing the
$N_{\textrm{H}}$ at $6.1\times 10^{22}$ cm-2 and allowing all the blackbody
and power law parameters to be free. However, the NICER energy range is not
sensitive to the hard power law component, and the spectral parameters could
not be well constrained. Therefore, we froze the $\Gamma$ at 1.2 and allowed
the $kT_{\textrm{BB}}$, $R_{\textrm{BB}}$, and power-law normalization to be
free parameters in the following analysis. Additionally, the number of X-ray
photons decreased as the flux decreased and the exposure time of each
observation was different. Therefore, we jointly fit one to three data sets by
linking the aforementioned three spectral parameters. The resulting unabsorbed
fluxes in the 1-10 keV band, fluxes of individual components in the same
energy band, and three spectral parameters are shown in Figure 7, and the
parameters are shown in Table 3. The overall flux, $kT_{\textrm{BB}}$, and
blackbody flux decreased over time. A mini flare on MJD 59082 (August 21) is
observed. During the flare, the $kT_{\textrm{BB}}$ and flux increased. On the
other hand, the power law flux remained constant throughout the entire
observation run, suggesting that the outburst is dominated by the increase of
the surface emission. A few days after the flare, the blackbody component
could not be well constrained and the power law dominated the spectrum in the
soft X-ray band.

Table 2: Best-Fit parameters of joint NuSTAR – NICER observations  Time (MJD) | NuSTAR ObsID | NICER ObsID | $\Gamma$ |  $kT_{\textrm{BB}}$ (keV) |  $R_{\textrm{BB}}$ (km)
---|---|---|---|---|---
59066 | 80602315002 | 3033290103 | $1.4_{-0.3}^{+0.5}$ | $0.93\pm 0.08$ | $0.6\pm 0.3$
59081 | 80602315004 | 3033290115,116 | $1.3_{-0.5}^{+0.3}$ | $1.01_{-0.05}^{+0.06}$ | $0.9_{-0.2}^{+0.3}$
59109 | 80602315006 | 3033290126,127 | $1.4\pm 0.2$ | $0.5_{-0.1}^{+0.2}$ | $4_{-3}^{+14}$
59131 | 80602315008 | 3033290132,135,136 | $1.1\pm 0.1$ | $0.8\pm 0.1$ | $0.6_{-0.3}^{+1.2}$
Table 3: Best-Fit parameters of NICER observations

Time | NICER ObsID | $kT_{\textrm{BB}}$ | $R_{\textrm{BB}}$ | Flux | BB Flux
---|---|---|---|---|---
(MJD) |  | (keV) | (km) | ($10^{-11}$ erg s-1 cm-2) | ($10^{-11}$ erg s-1 cm-2)
59056 | 3598010801 | $1.0\pm 0.1$ | $1.0_{-0.3}^{+0.4}$ | $3.5_{-0.4}^{+0.5}$ | $2.1_{-0.7}^{+0.8}$
59056 | 3598010802 | $1.03_{-0.09}^{+0.07}$ | $1.1_{-0.2}^{+0.3}$ | $3.0_{-0.3}^{+0.4}$ | $2.5_{-0.5}^{+0.4}$
59064 | 3033290101,102 | $0.9\pm 0.1$ | $1.0_{-0.3}^{+0.5}$ | $2.6\pm 0.3$ | $1.3_{-0.4}^{+0.5}$
59067 | 3033290103 | $1.0\pm 0.1$ | $0.9_{-0.3}^{+0.4}$ | $2.2\pm 0.3$ | $1.7_{-0.4}^{+0.3}$
59068 | 3033290105 | $0.9\pm 0.1$ | $0.9_{-0.3}^{+0.6}$ | $2.2\pm 0.4$ | $1.1\pm 0.5$
59070 | 3033290106,107 | $0.9\pm 0.1$ | $0.8_{-0.3}^{+0.4}$ | $2.3_{-0.3}^{+0.2}$ | $1.0_{-0.3}^{+0.4}$
59072 | 3033290108 | $0.8\pm 0.2$ | $0.9_{-0.4}^{+0.9}$ | $2.0_{-0.5}^{+0.3}$ | $0.9\pm 0.4$
59073 | 3033290109,110 | $0.7\pm 0.1$ | $1.3_{-0.5}^{+0.9}$ | $2.2_{-0.4}^{+0.2}$ | $0.9\pm 0.3$
59075 | 3033290111,112 | $0.9\pm 0.2$ | $0.6_{-0.3}^{+0.7}$ | $2.0_{-0.4}^{+0.3}$ | $0.8_{-0.4}^{+0.5}$
59079 | 3033290113,114 | $0.9\pm 0.1$ | $1.1_{-0.3}^{+0.6}$ | $2.4_{-0.4}^{+0.3}$ | $1.4_{-0.4}^{+0.5}$
59081 | 3033290115,116 | $1.06_{-0.11}^{+0.09}$ | $0.7\pm 0.2$ | $3.0_{-0.3}^{+0.4}$ | $2.1\pm 0.6$
59086 | 3598010901 | $0.96_{-0.10}^{+0.08}$ | $0.8_{-0.2}^{+0.3}$ | $2.0_{-0.2}^{+0.3}$ | $1.6_{-0.4}^{+0.3}$
59088 | 3033290117,118 | $0.8\pm 0.1$ | $1.3_{-0.5}^{+0.9}$ | $1.9_{-0.4}^{+0.3}$ | $1.1\pm 0.2$
59092 | 3033290119,120 | $0.7\pm 0.2$ | $2_{-1}^{+4}$ | $1.7_{-0.6}^{+0.3}$ | $0.7\pm 0.4$
59098 | 3033290122,123 | $0.5\pm 0.2$ | $3_{-2}^{+14}$ | $2.2_{-0.5}^{+0.2}$ | $0.5_{-0.4}^{+0.3}$
59102 | 3033290124,125,126 | $0.5_{-0.1}^{+0.2}$ | $4_{-2}^{+9}$ | $1.7_{-0.5}^{+0.2}$ | $0.6_{-0.4}^{+0.3}$
59122 | 3033290128,131,132 | $0.8_{-0.2}^{+0.3}$ | $0.7_{-0.4}^{+2.0}$ | $1.3_{-0.5}^{+0.3}$ | $0.5_{-0.3}^{+0.4}$
59137 | 3033290135,136,137 | $0.8_{-0.1}^{+0.2}$ | $0.8_{-0.4}^{+0.9}$ | $1.4_{-0.4}^{+0.3}$ | $0.6\pm 0.3$
59145 | 3598011101 | $0.5_{-0.2}^{+0.5}$ | $<28$ | $1.2\pm 0.2$ | $<0.4$
59154 | 3033290139,141,143 | $0.3_{-0.1}^{+0.2}$ | $<6000$ | $1.5\pm 0.4$ | $<0.8$
59165 | 3033290114,146,147 | $0.14_{-0.04}^{+0.12}$ | $<1\times 10^{8}$ | $1.8_{-0.8}^{+1.4}$ | $<2.2$
59174 | 3033290150,3598011201,202 | $0.3_{-0.2}^{+0.4}$ | $<3\times 10^{7}$ | $0.8\pm 0.2$ | $<1.1$

Note. — The fluxes are calculated in the energy range of 1–10 keV.

##  4 Discussion

The August 2020 X-ray outburst was the second magnetar-like outburst from PSR
J1846$-$0258 since its discovery in 1999, after a 14 year hiatus since its
first recorded outburst in 2006. Our NICER GO and ToO observations provided
continuous monitoring and timing of the source starting about three and a half
years before the 2020 outburst until more than a year following the outburst
in November 2021. Our monitoring observations prior to the August 1, 2020
Swift announcement of the outburst showed that the flux was already increasing
above quiescent level by late June. Due to our high cadence observations
during and following the outburst, we were able to resolve rapid changes in
the pulsed flux, which our measurements showed increased by more than a factor
of ten above the quiescent level. While this flux increase is much higher than
the five-times increase reported for the 2006 outburst (Kuiper & Hermsen,
2009), the flux during that outburst was less densely sampled and timing
coherence was lost (Gavriil et al., 2008), so higher flux peaks may have been
missed. Both outbursts were accompanied by spin-up glitches of very similar
size and in both cases, transient thermal components appeared that were
responsible for most of the flux increases. The thermal components and the
flux also decayed over similar timescales of several months. However, we
observed a change in the X-ray pulse profile during the 2020 outburst, while
no change in profile was reported for the 2006 outburst. Overall, the
characteristics of the two magnetar-like outbursts of J1846$-$0258 are very
similar.

PSR J1846$-$0258 thus appears to be a unique and fascinating source with a
dual nature, behaving most of the time like a RPP and occasionally like a
magnetar. It is also an unusual young RPP, having no detected radio or GeV
emission. So is it a RPP that is becoming a magnetar, or a magnetar
masquerading as a RPP? As we discuss below, the quiescent behavior as well as
the outbursts can be interpreted in either scenario but involve very different
physical mechanisms for the emission.

First, we consider a pulsar-like model for PSR J1846$-$0258 . Harding &
Kalapotharakos (2017) presented a model for MeV pulsars like J1846$-$0258 in
which their lack of radio or GeV emission is due to geometry. In this model,
MeV pulsars are the same as the rest of the young RPP population: they have
global dipole magnetospheres that are nearly force-free, emit radio emission
from above the magnetic poles and $\gamma$-ray and non-thermal X-ray emission
from near the current sheet outside the light cylinder (Kalapotharakos et al.,
2018; Harding et al., 2021). However, they have small magnetic inclination
angles and we view them along lines-of-sight, away from the magnetic poles,
that miss both the radio beams and the center of the current sheet where the
highest energy acceleration occurs and where the GeV emission is produced. In
this model then, the quiescent X-ray emission is synchrotron radiation (SR)
from electron-positron pairs near the current sheet, where the shape of the
SED derives from the smoothly falling spectrum of the pairs (Harding &
Kalapotharakos, 2015; Harding et al., 2021). The magnetar-like outbursts of
J1846$-$0258 are due to the appearance of or a change in a toroidal surface
magnetic field component that provides a field twist, and more particle
acceleration and pair production near the surface. The increased pair
multiplicity increases the polar cap heating and opens up more field lines,
increasing the size of the polar caps. If our line-of-sight then crosses
nearer to the larger heated polar cap, we will temporarily see a thermal
component, but we can still miss the radio emission since it is more tightly
beamed than the thermal emission. However, in this case the thermal component
will not be in phase with the non-thermal SR pulse which is a major drawback
for this model. The measurements of a braking index in J1846$-$0258
(Livingstone et al., 2011) with values typical of RPPs may add support to the
RPP model. However, it is unclear what we should expect for magnetar braking
indices and why they have not been detected.

Next, we consider a magnetar-like model in which the quiescent X-ray emission
from J1846$-$0258 is resonant inverse-Compton scattering emission (RICS) from
particles accelerated by field twists and scattering thermal photons from the
hot neutron star surface (Baring & Harding, 2007; Beloborodov, 2013). We do
not see any GeV emission because photon splitting and pair production
attenuate the RICS spectrum above about 1 MeV (Wadiasingh et al., 2019). The
outbursts are then typical magnetar bursts where crustal motions cause
increases in the magnetic field twists, particle acceleration and RICS
emission. The increased particle acceleration and pair/splitting cascades
produce visible heated spots due to particle surface bombardment (Beloborodov,
2013). Since this emission occurs on closed field lines near the equator, we
could miss any transient radio emission along open field lines. There are
several potential drawbacks in this model. If the quiescent emission is RICS,
why don’t we see a quiescent thermal component as appears in magnetars?
Supporting the non-observed thermal component, many/most young magnetars have
X-ray luminosities $10^{34}-10^{35}\mbox{ erg s${}^{-1}$}$ (see eg. magnetar
review by Kaspi & Beloborodov (2017)), which would translate to fluxes $\sim
2\times 10^{-12}-10^{-11}\,\rm erg\,s^{-1}\,cm^{-2}$ at 6.5 kpc, lower than
the observed fluxes in Table 3. Perhaps this component is much weaker or non-
existent for J1846$-$0258 because its magnetic activity is not persistent
enough to produce heated spots from particle bombardment, and with the
geometry for J1846$-$0258 proposed above, we would not see the heated polar
caps in quiescence. In addition, there is no significant change in flux or
spectrum of the power-law component during the outburst. Also, magnetar pulse
profiles are typically more complex than that of J1846$-$0258 (Hu et al.,
2019) but this could result from less persistent magnetic field activity as
well. In addition, the X-ray luminosity of J1846$-$0258 is less than its spin
down power, whereas for magnetars the X-ray luminosity exceeds the spin-down
power by several orders of magnitude. Finally, one could argue that the SED of
J1846$-$0258 is closer to the smooth SR-like spectrum of RPPs like the Crab
and B1509$-$58 (Kuiper et al., 2018) that do not show the sharp cutoff
predicted for magnetar SEDs (Wadiasingh et al., 2019). However, at the moment
there are no detected spectral points in the SED of J1846$-$0258 between 200
keV and 30 MeV so a sharp attenuation cutoff could exist. In addition, both
the quiescent and outburst X-ray spectra of J1846$-$0258 is very close to the
spectrum of magnetar RXS J1708$-$40 (Kuiper & Hermsen, 2009). The true test
then of a pulsar-like vs. a magnetar-like model for PSR J1846$-$0258 is a
measurement of its SED between 200 keV and 30 MeV.

Regardless of whether the pulsar-like or magnetar-like model for J1846$-$0258
is more plausible, the existence and behavior of J1846$-$0258 and J1119$-$6127
suggest a continuum of neutron stars that lie between RPPs and magnetars. The
enhanced quiescent thermal emission observed from them and several other
high-B RPPs supports such a continuum (Kaspi & McLaughlin, 2005; Ng et al.,
2012; Olausen et al., 2013; Hu et al., 2017). In fact, the characteristics of
J1846$-$0258 are similar to those of transient Anomalous X-Ray Pulsars
(Livingstone et al., 2010) and may indicate a link between RPPs and magnetars.
An additional similarity between RPPs and magnetars is the discovery of a wind
nebula around the canonical magnetar Swift J1834.9$-$0846 (Younes et al.,
2016). What is less clear is their direction of evolution. Espinoza et al.
(2011, 2022) showed that the high-B pulsar J1734$-$333 is evolving towards the
magnetars (based on its braking index). There is no evidence for magnetar
X-ray behavior for this pulsar, but this result may indicate the direction of
evolution. Since both J1846$-$0258 and J1119$-$6127 are very young, it is
possible that a stronger interior magnetic field is in the process of emerging
over a timescale $<10^{3}-10^{4}$ yr (Bhattacharya & Soni, 2008). After the
2006 outburst of J1846$-$0258 , its braking index was observed to decrease by
18% (Livingstone et al., 2011), which could be caused by an increase in
magnetic field strength (Blandford et al., 1983; Ho, 2015). Yet, such sources
may have been born with stronger magnetic fields that are in the final stage
of decaying (Pons et al., 2009; Viganò et al., 2013; Gourgouliatos et al.,
2016). Continued X-ray monitoring of J1846$-$0258 as well as expanded spectral
coverage will be crucial in understanding of its true nature and place in
neutron star evolution.

This work was supported by the National Aeronautics and Space Administration
(NASA) through the NICER mission, the Astrophysics Explorers Program and
through the NICER Guest Investigator Program. This work partly made use of
data supplied by the UK Swift Science Data Centre at the University of
Leicester. This research has also made use of data obtained with _NuSTAR_ , a
project led by Caltech, funded by NASA and managed by NASA/JPL, and has
utilized the NUSTARDAS software package, jointly developed by the ASDC (Italy)
and Caltech (USA). C.-P.H. acknowledges support from the National Science and
Technology Council in Taiwan through grant MOST 109-2112-M-018-009-MY3.
W.C.G.H. acknowledges support through grants 80NSSC21K2035 and 80NSSC22K1308
from NASA. C.M.E. acknowledges support from the grant ANID FONDECYT 1211964.

## References

  * Archibald et al. (2016) Archibald, R. F., Kaspi, V. M., Tendulkar, S. P., & Scholz, P. 2016, ApJ, 829, L21, doi: 10.3847/2041-8205/829/1/L21
  * Baring & Harding (2007) Baring, M. G., & Harding, A. K. 2007, Ap&SS, 308, 109, doi: 10.1007/s10509-007-9326-x
  * Beloborodov (2013) Beloborodov, A. M. 2013, ApJ, 762, 13, doi: 10.1088/0004-637X/762/1/13
  * Bhattacharya & Soni (2008) Bhattacharya, D., & Soni, V. 2008, in Bulletin of the Astronomical Society of India Proceedings, Vol. 25, Bulletin of the Astronomical Society of India Proceedings, 47
  * Blandford et al. (1983) Blandford, R. D., Applegate, J. H., & Hernquist, L. 1983, MNRAS, 204, 1025, doi: 10.1093/mnras/204.4.1025
  * Blumer et al. (2021) Blumer, H., Safi-Harb, S., McLaughlin, M. A., & Fiore, W. 2021, ApJ, 911, L6, doi: 10.3847/2041-8213/abf11d
  * Burrows et al. (2005) Burrows, D. N., Hill, J. E., Nousek, J. A., et al. 2005, Space Sci. Rev., 120, 165, doi: 10.1007/s11214-005-5097-2
  * Espinoza et al. (2011) Espinoza, C. M., Lyne, A. G., Kramer, M., Manchester, R. N., & Kaspi, V. M. 2011, ApJ, 741, L13, doi: 10.1088/2041-8205/741/1/L13
  * Espinoza et al. (2022) Espinoza, C. M., Vidal-Navarro, M., Ho, W. C. G., Deller, A., & Chatterjee, S. 2022, A&A, 659, A41, doi: 10.1051/0004-6361/202142598
  * Gavriil et al. (2008) Gavriil, F. P., Gonzalez, M. E., Gotthelf, E. V., et al. 2008, Science, 319, 1802, doi: 10.1126/science.1153465
  * Gehrels et al. (2004) Gehrels, N., Chincarini, G., Giommi, P., et al. 2004, ApJ, 611, 1005, doi: 10.1086/422091
  * Gotthelf et al. (2000) Gotthelf, E. V., Vasisht, G., Boylan-Kolchin, M., & Torii, K. 2000, ApJ, 542, L37, doi: 10.1086/312923
  * Gourgouliatos et al. (2016) Gourgouliatos, K. N., Wood, T. S., & Hollerbach, R. 2016, Proceedings of the National Academy of Science, 113, 3944, doi: 10.1073/pnas.1522363113
  * Harding & Kalapotharakos (2015) Harding, A. K., & Kalapotharakos, C. 2015, ApJ, 811, 63, doi: 10.1088/0004-637X/811/1/63
  * Harding & Kalapotharakos (2017) Harding, A. K., & Kalapotharakos, C. 2017, in Proceedings of the 7th International Fermi Symposium, 6.  https://arxiv.org/abs/1712.02406
  * Harding et al. (2021) Harding, A. K., Venter, C., & Kalapotharakos, C. 2021, ApJ, 923, 194, doi: 10.3847/1538-4357/ac3084
  * Helfand et al. (2003) Helfand, D. J., Agüeros, M. A., & Gotthelf, E. V. 2003, ApJ, 592, 941, doi: 10.1086/375838
  * Ho (2015) Ho, W. C. G. 2015, MNRAS, 452, 845, doi: 10.1093/mnras/stv1339
  * Hu et al. (2019) Hu, C.-P., Ng, C.-Y., & Ho, W. C. G. 2019, MNRAS, 485, 4274, doi: 10.1093/mnras/stz513
  * Hu et al. (2017) Hu, C.-P., Ng, C. Y., Takata, J., Shannon, R. M., & Johnston, S. 2017, ApJ, 838, 156, doi: 10.3847/1538-4357/aa67e3
  * Kalapotharakos et al. (2018) Kalapotharakos, C., Brambilla, G., Timokhin, A., Harding, A. K., & Kazanas, D. 2018, ApJ, 857, 44, doi: 10.3847/1538-4357/aab550
  * Kaspi & Beloborodov (2017) Kaspi, V. M., & Beloborodov, A. M. 2017, ARA&A, 55, 261, doi: 10.1146/annurev-astro-081915-023329
  * Kaspi & McLaughlin (2005) Kaspi, V. M., & McLaughlin, M. A. 2005, ApJ, 618, L41, doi: 10.1086/427628
  * Krimm et al. (2020) Krimm, H. A., Lien, A. Y., Page, K. L., et al. 2020, GRB Coordinates Network, 28187, 1
  * Kuiper et al. (2020) Kuiper, L., Harding, A. K., Enoto, T., et al. 2020, The Astronomer’s Telegram, 13985, 1
  * Kuiper & Hermsen (2009) Kuiper, L., & Hermsen, W. 2009, A&A, 501, 1031, doi: 10.1051/0004-6361/200811580
  * Kuiper & Hermsen (2015) —. 2015, MNRAS, 449, 3827, doi: 10.1093/mnras/stv426
  * Kuiper et al. (2018) Kuiper, L., Hermsen, W., & Dekker, A. 2018, MNRAS, 475, 1238, doi: 10.1093/mnras/stx3128
  * Kuiper et al. (2012) Kuiper, L., Hermsen, W., den Hartog, P. R., & Urama, J. O. 2012, ApJ, 748, 133, doi: 10.1088/0004-637X/748/2/133
  * Kumar & Safi-Harb (2008) Kumar, H. S., & Safi-Harb, S. 2008, ApJ, 678, L43, doi: 10.1086/588284
  * Livingstone et al. (2010) Livingstone, M. A., Kaspi, V. M., & Gavriil, F. P. 2010, ApJ, 710, 1710, doi: 10.1088/0004-637X/710/2/1710
  * Livingstone et al. (2011) Livingstone, M. A., Ng, C. Y., Kaspi, V. M., Gavriil, F. P., & Gotthelf, E. V. 2011, ApJ, 730, 66, doi: 10.1088/0004-637X/730/2/66
  * Ng et al. (2012) Ng, C. Y., Kaspi, V. M., Ho, W. C. G., et al. 2012, ApJ, 761, 65, doi: 10.1088/0004-637X/761/1/65
  * Ng et al. (2008) Ng, C. Y., Slane, P. O., Gaensler, B. M., & Hughes, J. P. 2008, ApJ, 686, 508, doi: 10.1086/591146
  * Olausen et al. (2013) Olausen, S. A., Zhu, W. W., Vogel, J. K., et al. 2013, ApJ, 764, 1, doi: 10.1088/0004-637X/764/1/1
  * Pons et al. (2009) Pons, J. A., Miralles, J. A., & Geppert, U. 2009, A&A, 496, 207, doi: 10.1051/0004-6361:200811229
  * Viganò et al. (2013) Viganò, D., Rea, N., Pons, J. A., et al. 2013, MNRAS, 434, 123, doi: 10.1093/mnras/stt1008
  * Wadiasingh et al. (2019) Wadiasingh, Z., Younes, G., Baring, M. G., et al. 2019, BAAS, 51, 292.  https://arxiv.org/abs/1903.05648
  * Wilms et al. (2000) Wilms, J., Allen, A., & McCray, R. 2000, ApJ, 542, 914, doi: 10.1086/317016
  * Younes et al. (2016) Younes, G., Kouveliotou, C., Kargaltsev, O., et al. 2016, ApJ, 824, 138, doi: 10.3847/0004-637X/824/2/138