[1]Fengqiu Adam Dong

[1]Department of Physics & Astronomy, University of British Columbia, 325 -
6224 Agricultural Road, Vancouver, V6T 1Z1, British Columbia, Canada

2]U.S. Naval Research Laboratory, 4555 Overlook Ave, SW Washington, 20375, DC,
USA

3]Department of Physics, McGill University, 3600 rue University, Montréal, H3A
2T8, QC, Canada

4]Trottier Space Institute, McGill University, 3600 rue University, Montréal,
H3A 2A7, QC, Canada

5]Tata Institute of Fundamental Research, National Centre for Radio Astronomy,
Savitribai Phule Pune University Campus Spicer College Road, Ganeshkhind,
Pune, HR6G+3G5, 411007, Maharashtra, India

6]Cornell Center for Astrophysics and Planetary Science, Cornell University, ,
Ithaca, 14853, NY, USA

7]David A. Dunlap Department of Astronomy & Astrophysics, University of
Toronto, 50 St. George Street, Toronto, M5S 3H4, Ontario, Canada

8]Dunlap Institute for Astronomy & Astrophysics, University of Toronto, 50 St.
George Street, Toronto, M5S 3H4, Ontario, Canada

9]Department of Physics and Astronomy, West Virginia University, , Morgantown,
26506-6315, WV, USA

10]Center for Gravitational Waves and Cosmology, West Virginia University,
Chestnut Ridge Research Building, Morgantown, 26505, WV, USA

11]University of California Santa Cruz, 1156 High St, Santa Cruz, 95064, CA,
USA

12]ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4,
Dwingeloo, PD, 7991, The Netherlands

13]Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science
Park 904, Amsterdam, 1098, XH, The Netherlands

14]Department of Physics, Massachusetts Institute of Technology, 77
Massachusetts Ave, Cambridge, 02139, MA, USA

15]MIT Kavli Institute for Astrophysics and Space Research, Massachusetts
Institute of Technology, 77 Massachusetts Ave, McNair Building (MIT Building
37), Cambridge, 02139, MA, USA

16]E.A. Milne Centre for Astrophysics, University of Hull, Cottingham Road,
Kingston-upon-Hull, HU6 7RX, UK

17]Centre of Excellence for Data Science Artificial Intelligence and Modelling
(DAIM), University of Hull, Cottingham Road, Kingston-upon-Hull, HU6 7RX, UK

18]International Centre for Radio Astronomy Research, Curtin University,
Bently, 6102, WA, Australia

19]National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville,
22903, VA, USA

20] York University, 4700 Keele Street, Toronto, M3J 1P3, ON, Canada

21]Perimeter Institute for Theoretical Physics, 31 Caroline St N, Waterloo,
N2L 2Y5, ON, Canada

# The discovery of a nearby 421 s transient with CHIME/FRB/Pulsar

<EMAIL_ADDRESS>Tracy Clarke  Alice P. Curtin  Ajay Kumar  Ingrid
Stairs  Shami Chatterjee  Amanda M. Cook  Emmanuel Fonseca  B. M. Gaensler
Jason W.T. Hessels  Victoria M. Kaspi  Mattias Lazda  Kiyoshi W. Masui  James
W. McKee  Bradley W. Meyers  Aaron B. Pearlman  Scott M. Ransom  Paul Scholz
Kaitlyn Shin  Kendrick M. Smith  Chia Min Tan  *  [  [  [  [  [  [  [  [  [  [
[  [  [  [  [  [  [  [  [  [

###### Abstract

Neutron stars and white dwarfs are both dense remnants of post-main-sequence
stars. Pulsars, magnetars and strongly magnetised white dwarfs have all been
seen to been observed to exhibit coherent, pulsed radio emission in relation
to their rotational period. Recently, a new type of radio long period
transient (LPT) has been discovered. The bright radio emission of LPTs
resembles that of radio pulsars and magnetars. However, they pulse on
timescales (minutes) much longer than previously seen. While minute timescales
are common rotation periods for white dwarfs, LPTs are much brighter than the
known pulsating white dwarfs, and dipolar radiation from isolated (as opposed
to binary) magnetic white dwarfs has yet to be observed. Here, we report the
discovery of a new $\sim$421 s LPT, CHIME J0630+25, using the CHIME/FRB and
CHIME/Pulsar instruments. We used standard pulsar timing techniques and
obtained a phase-coherent timing solution which yielded limits on the inferred
magnetic field and characteristic age. CHIME J0630+25 is remarkably nearby
($170\pm 80$ pc), making it the closest LPT discovered to date.

###### keywords:

keyword1, Keyword2, Keyword3, Keyword4

Recently, a new class of objects known as radio long-period transients
(LPTs)111Note that these sources have also been given some other names in the
literature such as ultra long period transients (ULPT), long period radio
transients (LPRT). In this study, we use the term long period transient
throughout. has been discovered. Four such objects have been confirmed:
GLEAM-X J162759.5-–523504.3 [1] and GPM 1839–10 [2], both discovered by the
Murchison Widefield Array (MWA), ASKAP J1935+2148 [3], discovered by the
Australian Square Kilometre Array Pathfinder (ASKAP) telescope, and PSR
J0901–4046 [4], discovered by the MeerKAT telescope (1091 s for GLEAM-X
J162759.5-–523504.3, 1318 s for GPM 1839–10, 3225 s for ASKAP J1935+2148, and
76 s for PSR J0901–4046). However, their nature remains mysterious.

These LPTs are characterised by their exceptionally long periods, wide burst
widths (up to 60 seconds), and complex temporal and spectral microstructure.
Two types of sources, white dwarfs and neutron stars, have emerged as the
favoured models for LPTs due to their emission characteristics and rotation
periods.

Radio pulsars are the most common form of detectable neutron star. They are
remarkably accurate celestial clocks. By carefully measuring pulse times of
arrival (TOAs), we can fully account for every rotation of a pulsar through a
process called pulsar timing. The rotational period and period derivative
obtained via pulsar timing provide critical constraints [5, 6] on the
mechanisms of coherent radio emission [7, 8, 9]. The constraints derived from
pulsar timing assume that the emissions are powered by rotational energy loss.
However, not all emission from neutron stars is powered by spin-down. A subset
of magnetically-powered young neutron stars, namely magnetars, exhibits noisy
rotational behaviour and drastically changing pulse-to-pulse emissions
punctuated by high-energy outbursts [see 10, 11, for reviews]. Accomplishing
phase-coherent timing for magnetars can be challenging due to their apparent
rotational instability that often but not exclusively occurs contemporaneously
with X-ray outbursts [e.g. 12, 13]. The complex temporal and spectral
structure of LPT radio emission is somewhat similar to that of magnetars, and
some have argued [14] that this favours a neutron-star model for LPTs [15,
16]. However, radio emission from neutron stars with periods as long as those
of LPTs has not yet been observed. The longest period pulsar, PSR J0250+5854,
has a rotational period of 23.5 s [17]. There is one magnetar candidate, 1E
161348-5055, with a period of 6.67 h at the centre of supernova remnant RCW103
[18]. However, only high-energy emission has ever been detected from 1E
161348-5055 [19, 20].

White dwarfs are an alternative explanation for LPTs due to their longer
rotational periods. Most white dwarf emission is not periodic or detectable in
radio [21]. However, recently two white dwarfs have been discovered to emit
pulsed emission in the radio bank like pulsars, AR Scorpii and J1912–4410 [22,
23]. In the case of J1912–4410, the emission was detectable down to 1.4 GHz
with duty cycles similar to radio pulsars. Additionally, the periods of the
white dwarfs producing coherent pulsed radio emissions are similar to those of
the discovered LPTs. Yet, a couple of issues remain with the white dwarf model
for LPTs. Despite optical searches, no optical counterparts have been
identified in the localisation regions of any LPT [4]. In addition, only white
dwarfs in binary systems have been seen to emit coherent radio emission. At
least one LPT, PSR J0901–4046, has well-constrained timing parameters, which
show it to be an isolated source. While it is theoretically possible for an
isolated white dwarf to produce coherent radio pulses, this has yet to be
observed.

We have discovered a new $P=421.35542$ s LPT using the Canadian Hydrogen
Intensity Mapping Experiment (CHIME) telescope, CHIME J0630+25, utilising
search strategies developed for Galactic pulsar searches [24]. We have also
obtained a phase-coherent timing solution for CHIME J0630+25. The discovery
marks the closest LPT to date at 170(80) pc. Additionally, we have identified
two potential X-ray counterparts to CHIME J0630+25 using the Neil Gehrels
Swift telescope, although the radio localisation region is large.

##  1 Results

CHIME, located near Penticton, British Columbia, Canada, is a transit
telescope comprised of four cylindrical dishes and three distinct backend
instruments: CHIME/Cosmology [25], CHIME/FRB [26], and CHIME/Pulsar [27]. The
unique cylindrical design of CHIME offers a wide field of view (FOV) of
approximately 200 square degrees at any given time. Additionally, as an
interferometric telescope, CHIME/FRB can form 1024 static synthesised beams
within its large primary beam, allowing for the localisation of astrophysical
transients with a typical precision of $\sim$0.5 degrees. The extensive FOV
enables CHIME/FRB to survey the entire northern sky daily. Of particular
interest is CHIME/FRB’s capability to perform continuous single pulse surveys,
enabling the detection of even highly sporadic sources, including repeating
and one-off FRBs [28], rotating radio transients [24], and LPTs. We identified
an astrophysical candidate with the first detection occurring on MJD 58772 at
12:54:51 UTC (see Methods) at Right Ascension (RA) 06:30:43$\pm$6’,
Declination (Dec) 25:23:24$\pm$11’ where the quoted uncertainty is at the
1$\sigma$ level.222All coordinates in this study are given in the J2000
coordinate system. This candidate, CHIME J0630+25, was then followed up by the
more sensitive CHIME/Pulsar instrument with 688 observations, totalling
$\sim$176 hours between MJDs 59300 and 60116. These are in the form of high-
time resolution Stokes-I spectra. We detected 11 bursts with the CHIME/Pulsar
observations. We also detected 6 bursts with CHIME/FRB. The CHIME/FRB
detections only had metadata recorded. However, we confirmed that these bursts
are indeed from CHIME J0630+25, as they are consistent with its rotation
period. Additionally, one of the bursts was a co-detection with both backends.
All the detections are provided in Table 1, which includes the fluence,
effective width, spectral index, and burst category for each burst (see
Methods). The burst category is a classification system that we used for the
quasiperiodicity analysis described in the Methods section.

Table 1: Properties for the pulses for CHIME J0630+25 detected by the
CHIME/FRB and CHIME/Pulsar systems. For the CHIME/FRB detections, we report
the TOA in UTC time at CHIME referenced to 400 MHz, signal-to-noise ratio
(S/N), and the automated pipeline DM. For the CHIME/Pulsar detections, we
report the pulse TOAs referenced to 800 MHz and provide calibrated
characteristics from the total intensity data stream. This includes the
effective width ($W_{\text{eff}}$), fluence ($F$), spectral index ($\alpha$),
DM, and a burst category that facilitates the quasiperiodicity analysis
described further in the Methods. The CHIME/FRB detections lack
$W_{\text{eff}}$, $F$, $\alpha$ and Burst Category values as Stokes-I
intensity data do not exist for these bursts. This is discussed in more detail
in the main text. Note that we omit the detection S/N of the pulses detected
by CHIME/Pulsar as the detection pipeline is different from CHIME/FRB, and the
detection S/N is not directly comparable. Therefore, to avoid confusion, we
only provide the fluence.

Burst | Instrument | TOA | $W_{\text{eff}}$ | $F$ | $\alpha$ | DM | Burst | Detection
---|---|---|---|---|---|---|---|---
|  | MJD | ms | Jy ms |  | pc cm-3 | Category | S/N
58772A | CHIME/FRB | 58772.53808(1) | – | – | – | 23(3) | – | 12.9
58855A | CHIME/FRB | 58855.30794(1) | – | – | – | 23(3) | – | 9.7
58860A | CHIME/FRB | 58860.29698(1) | – | – | – | 24(3) | – | 15.0
58871A | CHIME/FRB | 58871.26526(1) | – | – | – | 24(3) | – | 9.4
59167A | CHIME/FRB | 59167.45383(1) | – | – | – | 24(3) | – | 10.7
59553A∗ | CHIME/FRB | 59553.39770(1) | – | – | – | 23(3) | – | 8.7
59341A | CHIME/Pulsar | 59341.97314(2) | 770(230) | 270(80) | -1.8(3) | 20(3) | C1 | –
59341B | CHIME/Pulsar | 59341.97803(1) | 280(90) | 90(30) | -1.5(3) | 23(3) | C1 | –
59456A | CHIME/Pulsar | 59456.65990(1) | 320(100) | 90(30) | -2.7(3) | 23(3) | C1 | –
59456B | CHIME/Pulsar | 59456.66474(1) | 400(120) | 90(30) | -2.0(3) | 23(3) | C1 | –
59460A | CHIME/Pulsar | 59460.64875(1) | 280(90) | 310(90) | -2.1(3) | 20(3) | C3 | –
59463A | CHIME/Pulsar | 59463.64282(2) | 910(270) | 670(200) | -2.1(3) | 22(6) | C3 | –
59548A | CHIME/Pulsar | 59548.41384(1) | 470(140) | 200(60) | -2.2(3) | 20(3) | C1 | –
59553A∗ | CHIME/Pulsar | 59553.39770(1) | 470(140) | 550(160) | -2.3(3) | 20(7) | C2 | –
59563A | CHIME/Pulsar | 59563.37042(2) | 650(200) | 500(150) | -2.5(3) | 23(3) | C2 | –
59565A | CHIME/Pulsar | 59565.36500(1) | 180(60) | 90(30) | -1.2(3) | 21(3) | C1 | –
59574A | CHIME/Pulsar | 59574.34304(2) | 200(60) | 60(20) | -1.9(3) | 25(4) | C2 | –

∗ This pulse was simultaneously detected with both the CHIME/Pulsar and
CHIME/FRB systems. We report the measurement from each instrument seperately.

The dynamic spectra for the CHIME/Pulsar detections are shown in Figure 1 and
show a more complex temporal and spectral structure than in conventional radio
pulsars. The bursts can span widths up to $\sim$4 s (1% of the period, in the
case of 59463A) and can show erratic sub-structures reminiscent of the radio
bursts seen from radio-loud magnetar XTE 1910–197 [15, 16] (in the case of
59460A).

An initial periodicity analysis was performed with the rrat_period package
from PRESTO 333https://github.com/scottransom/presto. rrat_period works by
iterating over many different trial periods to find the period most likely to
be an integer factor of the spacing between successive bursts. Using
barycentre corrected arrival times with rrat_period yielded $P\approx 421$ s.
This was then refined using TEMPO2 [29] and PINT [30] to obtain a phase-
coherent timing solution with period of $P=421.35542(1)$ s and
$\dot{P}=-2.5(1.6)\times 10^{-12}$ ss-1 at 1-$\sigma$ uncertainty (see
Methods). The full set of phase-coherent timing parameters is given in Table
2. As a result of the low numbers of TOAs and the large uncertainties on each
TOA, we emphasise that the negative $\dot{P}$ could contain significant
covariances with position. To date, most pulsars which are spinning up have
been found in binaries [e.g. 31]. Therefore, if the negative $\dot{P}$ were to
be believed, then CHIME J0630+25 likely exists in a tight binary like the AR
Scorpii and J1912–4410 systems. This is given further analysis in the
Discussion section. More detections are required to determine $\dot{P}$
robustly. Consequently, we adopted a 1-$\sigma$ shifted upper limit on
$\dot{P}$ as discussed below.

Dispersion measure (DM) is a frequency dependent delay from the ionized
component of the interstellar medium. This affects all radio pulses from
pulsars and white dwarfs. The DM allows us to estimate the distance to the
source using interstellar medium models. There exist two models for the
Galactic electron density, NE2001 [32] and YMW16 [33]. We estimate the
distance to CHIME J0630+25 using the YMW16 Galactic electron density model
[33]. Analyses of Galactic interstellar medium models have shown that the
YMW16 model is better than the NE2001 electron density model [32] for nearby
pulsars and is likely accurate to a factor of $\sim$1.5 of the YMW16 estimate
[34, 35]. Furthermore, we systematically compared pulsars with annual parallax
measurements (or other more reliable distance measurements such as globular
cluster association) at similar Galactic latitudes as CHIME J0630+25 to obtain
the uncertainties for the YMW16 derived distances (see Methods). This results
in the low dispersion measure distance of 170(80) pc. The next closest LPT is
PSR J0901–4046 with a YMW16 dispersion measure distance of 330(90) pc. The
uncertainties are derived with a similar method as CHIME J0630+25. CHIME
J0630+25, along with PSR J0901–4046, are remarkably close and are prime
candidates for multiwavelength follow-up. The existence of these two sources
suggests a large Galactic population yet to be discovered.

Figure 1: The collection of pulses from CHIME J0630+25 detected by
CHIME/Pulsar. The top panel for each burst contains frequency averaged and
dedispersed time series. The second panel shows the dynamic spectrum of each
burst, and the bottom panel show the dedispersion heat map for each burst. The
dedispersion heat map shows the power for many different DM trials. All
astrophysical pulses of CHIME J0630+25 pulses centre on DM$\approx 22.5~{}$pc
cm-3. Figure 2: continued

###  1.1 Periodicity and timing solution

Table 2: The phase-coherent timing parameters for CHIME J0630+25. The uncertainties quoted are the 1-$\sigma$ confidence intervals. PEPOCH is the epoch for period determination. TIMEEPH is the time ephemeris that is used. NTOA is the number of times of arrivals used in the fit. CLOCK is the timescale that is used. JUMP1 gives the jump between the CHIME/FRB and CHIME/Pulsar instruments and is mainly determined by the single concurrent detection with both instruments. EFAC is a multiplicative factor in the TOA uncertainties. We also provide the derived parameters of $\tau$, the characteristic age, $B_{\text{surface}}$, the surface magnetic field, and $\dot{E}$, the spin-down luminosity. However, these are strictly limits based on the shifted upper limit on $\dot{P}$ as described in the text. The uncertainties given are those obtained from the timing fit. We did not fit the declination (see text).  Property | Value
---|---
Name | CHIME J0630+25
R.A. (hh:mm:ss) | 06h30m43s$\pm$6’
Dec (dd:mm:ss) | 25∘23’14”
$P$(s) | 421.35542(1)
$\dot{P}$($\times 10^{-12}$ ss-1) | –2.5(1.6)
PEPOCH(MJD) | 59173
TIMEEPH | FB90
NTOA | 17
CLOCK | TT(TAI)
JUMP1(s) | 0.247
EFAC | 2.476
reduced $\chi^{2}$ | 1.0
Derived Values |
Galactic Longitude (deg) | 188.0
Galactic Latitude (deg) | 7.1
$\tau$ ($\times 10^{6}$ yr) |  $>$4.2
B${}_{\text{surface}}$ ($\times 10^{15}$ G) |  $<$0.8
$\dot{E}$ ($\times 10^{26}$ erg/s) |  $<$8.5

Figure 3: The $P-\dot{P}$ diagram. 1E161348–5055, a central compact object
that shows evidence of magnetar-like emission, is shown with a grey star [20].
The grey circles show the LPTs discovered by the MWA. The longest period
pulsar PSR J2050+5854 and LPT PSR J0901–4046 are shown by the coloured dots
and labelled as such. The grey-shaded region shows the death valley for
coherent pulsed radio emission. The black dashed line corresponds to a pure
dipole, dotted lines for a twisted dipole and solid lines for a twisted
multipole configuration [5, 6]. The blue dashed lines correspond to lines of
constant age and the grey solid lines correspond to lines of constant magnetic
field. The code to create this plot was adapted from [2] 1 with more source
categories (i.e. millisecond pulsars and RRATs). All downward-facing arrows
correspond to upper limits and are at the 1-$\sigma$ level. The CHIME J0630+25
arrow is longer to represent the shifted upper limit used (see text)

1https://github.com/nhurleywalker/GPMTransient

We present the phase-connected timing solution in Table 2. Despite an 802-day
timing baseline, due to the large uncertainties on the arrival times of each
burst and the sparse sampling, we could not robustly constrain the declination
of CHIME J0630+25 through timing alone. We note that fitting a timing jump
(JUMP1) to account for the clock offset for the CHIME/FRB and CHIME/Pulsar
systems resulted in a value consistent with 0. This is expected as the two
systems have an offset of a few hundred milliseconds, less than the arrival
time uncertainty of the bursts. Therefore, we used a fixed static jump of
0.247 s, derived from timing other pulsars. The clock differences between the
two systems are given a full discussion in the Methods. We also note that an
additional jump at MJD 59158 can be added to reduce the residuals by a factor
of $\sim$2\. However, we find no physical motivation to do so as the global
fit is already a small fraction of the pulse period ($<1.5\%$).

Almost all theories for radio pulses from pulsars require the production of
electron-positron pairs near the neutron star’s polar caps [36, 5, 6]. Within
the context of these theories, the minimum magnetic field strengths and
configurations govern the ability to generate the electron-positron pairs
given a rotation period [5]. The different field configurations give rise to
sets of permitted P-$\dot{P}$ values. Possible configurations can include
dipolar, multipolar, or even twisted magnetic fields. They are complicated
further as the field configuration is likely different for every pulsar due to
their formation or evolution. Conventional pulsar radio emission should cease
beyond some “death valley” of P-$\dot{P}$ combination where pair production is
no longer efficient [5, 6]. In Figure 3, we show the Death Valley shaded in
grey for the extremes of these magnetic field configurations.

To place a conservative $\dot{P}$ upper limit on CHIME J0630+25, we take the
shifted upper limit approach as described in [37] given by
$\dot{P}_{\text{upper}}=\textrm{MAX}(\dot{P}_{\mu},0)+\sigma$ where $\sigma$
is the uncertainty given in Table 2. This results in
$\dot{P}_{\text{upper}}=1.6\times 10^{-12}$ss-1. Such a shifted upper limit
regime is resilient to statistical fluctuations if one believes that the true
value is near 0. As all $\dot{P}$ measurements of other LPTs have either been
positive or consistent with zero, we consider this a prudent upper limit on
the $\dot{P}$ of CHIME J0630+25. The shifted upper limit constrains CHIME
J0630+25’s magnetic field to be $<0.8\times 10^{15}$ G. Assuming that the
emission is powered by spin-down, the position of CHIME J0630+25 in Figure 3
shows that it is only allowable by the twisted multiple configuration.

Magnetic white dwarfs may emit coherent radio pulses with similar mechanisms
as radio pulsars [38]. Due to their larger moment of inertia, for the same
spin-down, white dwarf pulsars have much more spin-down energy to draw upon
for their emission. For a white dwarf, assuming that the emission mechanisms
are similar to that of radio pulsars, the shifted upper limit on $\dot{P}$ of
CHIME J0630+25 would yield an upper limit on the magnetic dipole field of
$\sim 3.4\times 10^{9}$ G by following equation 1 in [39]. The magnetic dipole
death line for a white dwarf of $\sim$421 s is $\sim 3.3\times 10^{9}$ G [38].
Other field configurations, such as multipole and twisted magnetic fields
lower this death line. Therefore, assuming the detected emission is similar to
that in regular radio pulsars, white dwarf models are much more favourable for
death valley limits of CHIME J0630+25.

###  1.2 Quasiperiodicity

Quasiperiodicity is a feature where substructures within a burst are separated
by almost an integer number of a quasiperiod. It is a common feature in radio
transient phenomena on the timescales of milliseconds to seconds [40, 41, 16].
It has been suggested that quasiperiodicity may be a universal feature in
neutron stars that span across multiple subclasses such as pulsars, magnetars,
Rotating Radio Transients, or LPTs [41]. Some bursts of J0630+25 contain large
amounts of substructure, such as 59460A. For this reason, we performed an
autocorrelation analysis to search for quasiperiodicity in all bursts that
show more than one peak in the burst time series (see Methods). We found no
significant indications of quasiperiodicity in the 7 bursts with multiple
peaks.

###  1.3 Swift X-ray observations

Figure 4: The Swift-XRT observations with detected sources and the CHIME J0630+25 localisation region overlaid. The localisation uncertainty region for right ascension is from pulsar timing, and the declination uncertainty region is from multi-beam detections of J0630+25 with CHIME/FRB (see Methods). The sources are numbered to match those in Table 3. Table 3: The four X-ray sources within the CHIME J0630+25 localisation region were detected by the Swift XRT. see Methods for the estimated $N_{\mathrm{H}}$. The maximum Galactic line of sight $N_{\mathrm{H}}$ is $\sim 3\times 10^{21}$cm-2. Source | RA | Dec | S/N | 0.1-1.5 keV | 1.5-10 keV | $N_{\mathrm{H}}$
---|---|---|---|---|---|---
| hms(J2000) | dms(J2000) |  | counts/s $(10^{-4})$ | counts/s $(10^{-4})$ | cm-2
1 | 06:30:49$\pm$6.8” | 25:23:01$\pm$6.8” | 2.8 | 2.4(1.9) | 5.7(2.7) | $\sim 8\times 10^{21}$
2 | 06:30:23$\pm$6.3” | 25:25:24$\pm$6.3” | 2.8 | 6.4(3.2) | 0(2) | $<2\times 10^{20}$
3 | 06:30:28$\pm$5” | 25:17:01$\pm$5” | 3.2 | 12(4) | 0.3(8) | $<2\times 10^{20}$
4 | 06:30:17$\pm$5.3” | 25:19:50$\pm$5.3” | 2.7 | 6.6(2.4) | 0.0(2) | $<2\times 10^{20}$

Neutron stars often emit thermal and non-thermal X-rays. Thermal emission is
produced by a neutron star’s cooling surface or by hotspots such as the polar
caps on the neutron star. These spots are reheated by magnetospheric return
currents of accelerated charges [10]. The particles accelerated by the
magnetic fields can also generate non-thermal X-ray emission independently in
the form of synchrotron and curvature radiation [42, 43, 44]. Another form of
non-thermal X-ray emission comes from young, energetic pulsars via the
interactions between the pulsar wind and the surrounding interstellar medium.
The interactions can produce shocks that accelerate pulsar wind particles,
which in turn produce X-rays [45]. All these mechanisms contribute to a vast
array of neutron star high-energy emission phenomenology.

A particularly interesting subset of X-ray bright neutron stars are X-ray Dim
Isolated Neutron Stars (XDINS) [46, 47]. These neutron stars are particularly
close, between 100 and 500 pc away. Due to their proximity, XDINs provided
precious insights into the formation pathways and thermal properties of
neutron stars [48].

Driven by a similar proximity motivation as XDINS, on the 5th of November
2023, we began a 32 ks observation campaign of CHIME J0630+25 with the Neils
Gehrels Swift Observatory X-ray Telescope (Swift XRT). This led to full
coverage of the 1-$\sigma$ localisation area of CHIME J0630+25 and partial
coverage of the 2-$\sigma$ localisation area. In total, we detected four X-ray
sources within the 1-$\sigma$ localisation area. The sources are shown in
Figure 4 and Table 3. We note that source 3 is coincident with a known
variable star, ATO J097.6166+25.2833. The fastest rotating stars like $\alpha$
Regulus and Vega possess spin periods of
$\mathcal{O}($10\text{\,}\textrm{h}$)$ [49, 50]. Significantly faster rotation
will cause the star to lose its outer layers as it lacks sufficient
centripetal acceleration [51]. Therefore, CHIME J0630+25 is unlikely to be
associated with ATO J097.6166+25.2833.

As the source counts are too low to build a spectrum, we measured the counts
in two different energy bands, 0.1-1.5 keV and 1.5-10 keV. We find that the
majority of photons emitted by sources 2, 3, and 4 are in the lower energy
bands. To compare the $N_{\mathrm{H}}$ of the Swift candidates with the DM of
CHIME J0630+25, we assumed a neutron star blackbody spectrum with a
temperature of 0.5 keV. This choice of blackbody spectrum was based on the
data reported in the McGill Online Magnetar Catalog
444https://www.physics.mcgill.ca/p̃ulsar/magnetar/main.html. We then estimated
the neutral hydrogen along the line of sight using WebPIMMS
555https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3pimms/w3pimms.pl (see
Methods) and found that sources 2, 3 and 4 possess low $N_{\mathrm{H}}$ values
while source 1 is likely extragalactic. As it is likely that source 3 is
associated with ATO J097.6166+25.2833, we argue that sources 2 and 4 are
likely nearby and could be the X-ray counterparts of CHIME J0630+25. Deeper
X-ray campaigns are required to characterise their spectra robustly.

##  2 Discussion

###  2.1 Timing model

From the upper limit on $\dot{P}$, we derived the maximum spin-down luminosity
available to CHIME J0630+25 and subsequently compared this with the observed
radio luminosity for both the neutron star and white dwarf models.

In the neutron star model, we derived the upper limit on the spin-down
luminosity to be $8.5\times 10^{26}$erg s-1. Using an approximate mean flux
density of 0.6 mJy 666This is the flux density averaged across the period.,
the radio luminosity is estimated to be $2.2^{+2.5}_{-1.6}\times 10^{26}$erg
s-1, within the spin-down luminosity range. We note, that the radio efficiency
is higher than any known radio pulsar.

The emission mechanisms could also be powered by the magnetic fields of a
neutron star such as the case in a magnetar. This is because the emission of
CHIME J0630+25 is highly sporadic. One such explanation is the reconnection of
magnetic field lines in a neutron star’s magnetosphere, invoked to explain the
emission of fast radio bursts [52]. Emission produced by drawing energy from
the neutron star’s magnetic field occurs when some mechanism (e.g.
magnetohydrodynamics waves) causes the reconnection of magnetic field lines in
magnetospheric plasma. Once reconnection occurs, the magnetic field energy is
converted into plasma kinetic and thermal energy, accelerating charged
particles and releasing radiation. This could be through mechanisms such as
curvature radiation [53]. Magnetars are known to emit sporadic radio
emissions, for example PSR J1622–4950 [54], XTE J1810–197 [16], and SGR
1935+2154 [55]. Therefore, if the origin of the radio emission for CHIME
J0630+25 is similar, it could explain the highly intermittent pulses of CHIME
J0630+25. Longer timing baselines and more detections will significantly
improve the $\dot{P}$ constraint to determine a robust dipole magnetic field.

White dwarfs possess a moment of inertia roughly $1\times 10^{5}$–$5\times
10^{5}$ times larger than a neutron star (assuming a range of 3000–7000 km
radius for a white dwarf and 10 km radius for a neutron star). The spin-down
luminosity is $0.9$–$4.3\times 10^{32}$erg s-1, significantly higher than the
emitted radio luminosity of CHIME J0630+25. Therefore, there is more than
sufficient energy in the white dwarf spin-down model.

###  2.2 The nature of CHIME J0630+25

Table 4: All sources that are similar in nature to CHIME J0630+25.

Source | Width | $P$ | $\dot{P}$ | $b$ | Duty | Micro- | Binary | ref
---|---|---|---|---|---|---|---|---
| FWHM (s) | s | s s-1 | ∘ | Cycle | structure |  |
ASKAP J1935+2148 | 10-50 | 3225 | $<1.2\times 10^{-10}$ | 0.74 | 0.3–1.5% | ✓ | N/A | [3]
GPM 1839–10 | 30–300 | 1318 | $<3.6\times 10^{-13}$ | $-2.06^{\circ}$ | 2.2–22.7% | ✓ | N/A | [2]
GLEAM-X1 | 30-60 | 1091 | $<10^{-9}$ | $-2.6^{\circ}$ | 2–5% | ✓ | N/A | [1]
CHIME J0630+25 | 0.2-3.2 | 421 | $<9\times 10^{-13}$ | $+7^{\circ}$ | 0.4–0.8% | ✓ | N/A |
J1912–4410 |  $<$4 | 318 | – | -22.06 |  $<$1.2% | – | ✓ | [23]
AR Scorpii | $\sim 60$ | 117 | $3.9\times 10^{-13}$ | 18.7 |  $\sim$50% | – | ✓ | [22]
PSR J0901–4046 |  $\sim$0.3 | 76 | $\sim 2.21\times 10^{-13}$ | $+3.7^{\circ}$ |  $\sim$0.70% | ✓ | $\times$ | [4]
PSR J0250+5854 |  $\sim$0.070 | 23.5 | $2.72\times 10^{-14}$ | $-0.5^{\circ}$ | 0.3–0.4% | – | $\times$ | [17]

1 Abbreviation for GLEAM-X J162759.5-–523504.3.

Clearly, the new population of LPTs is unexpected among the decades of pulsar,
magnetar and white dwarf discoveries. Comparing CHIME J0630+25 to sources that
occupy the same parameter space can offer insight into the emission source
[56, 38, 57]. We provide a collection of sources which produce similar
emission mechanisms in Table 4 for easy comparison. In the following section,
we discuss the possibility of CHIME J0630+25 being the same class of object as
actively fusing stars, Galactic Center Radio Transients, white dwarfs and
neutron stars.

Many objects exhibit periodic radio emission at
$\mathcal{O}($1\text{\,}\mathrm{h}$)$ timescales. These include flaring ultra-
cool dwarfs, like TLV 513-46546 (1.96 h), main sequence stars like CU Virginis
(12.5 h) [58], and sources of unknown origin like the Galactic Centre Radio
Transients, GCRT J1745–3009 (1.3h) [59] and GCRT J1742–3001 (no period) [60].
For ultra-cool dwarfs, it has been suggested that the limit to their spin
periods cannot be much less than $\sim$1 h due to rotational break-up [61]. A
similar argument can be given for main sequence stars at about a $\sim$10 h
rotational period. GCRTs are pulsing radio sources towards the Milky Way
Centre. Like LPTs, their nature remains mysterious. The pulse widths of GCRT
J1745–3009 are $\sim$11 minutes wide and much longer than CHIME J0630+25 or
any LPT [59]. Similarly, GCRT J1742–3001’s flares have widths on timescales of
months, again much longer than any LPT [60]. Therefore, we conclude that none
of the known source types with hour long periods can explain the properties of
CHIME J0630+25.

A white dwarf origin is attractive as their rotation periods are more aligned
to that of LPTs, and in particular, CHIME J0630+25 [22, 23]. AR Scorpii and
J1912–4410 are the only two known radio-pulsating white dwarfs, and their
rotation periods (117 s and 318 s, respectively) are within a few factors of
CHIME J0630+25 [22, 23]. While AR Scorpii shows a high-duty cycle at all
observed frequencies, J1912–4410 shows similar duty cycles to radio pulsars at
1.4 GHz and a high-duty cycle at higher frequencies. Therefore, at least some
white dwarfs can produce radiation akin to that of radio pulsars at similar
observational frequencies. We show this in Table 4. However, pulsating white
dwarfs emit pulses of $\mathcal{O}($220\text{\,}\textrm{Jy
pc}^{2}\mathrm{/}\textrm{beam}$)$, more than an order of magnitude less
luminous than CHIME J0630+25 [22, 23]. Note that this is only a sample of 2,
and the deviations in the population of pulsating white dwarfs remain to be
seen.

Both AR Scorpii and J1912–4410 are in tight binary systems, and their radio
emission is thought to be an interaction between the spin frequency and the
orbital frequency [39, 23]. In the case of CHIME J0630+25, we can not rule out
a binary system. Indeed, if we assume circular orbits and that the timing
residuals are caused by orbital motion, then an estimate of the projected
semimajor axis is possible. Assuming a maximum timing residual of $\sim 5$ s
and that Roemer delay is the main contributor to the timing residuals, we
estimate that if CHIME J0630+25 were to exist in a binary, the maximum semi-
major axis is $\sim$0.01 AU (using equation 8.28 in [62]). This is a factor of
2 larger than the semimajor axis of AR Scorpii and J1912–4410 both at $\sim
0.0055$ AU. Therefore, more timing data and deeper optical surveys are
required to investigate the binary white dwarf scenario for CHIME J0630+25.

We performed a cross-check with known white dwarfs compiled from Gaia EDR3
data [63] and found one viable candidate, WDJ063117.11+252250.59, within the
CHIME J0630+25 localisation region. WDJ063117.11+252250.59 is located at
RA=06:31:17.09$\pm 0.0004"$, Dec=25:22:50.12$\pm 0.0007"$ with a parallax
distance of 272(56) pc. The position of WDJ063117.11+252250.59 is within 2
$\sigma$ of CHIME J0630+25. However, despite being within $2\sigma$ of the
localisation region of CHIME J0630+25, if a source is $\sim 0.5^{\circ}$ or
more deviated from the pointing of CHIME/Pulsar beam, then we should see
significant beam attenuation effects at the higher observing frequencies of
CHIME/Pulsar. There are no evidence of beam effects in any of the bursts
detected by CHIME/Pulsar (Figure 1), and therefore, we conclude that CHIME
J0630+25 is unlikely to be associated with WDJ063117.11+252250.59. Assuming
the bursts are evenly distributed in time, CHIME J0630+25 has a burst rate of
$\sim$0.06 bursts/h. Thus, with $\sim$17 hours one can expect to make a
detection at CHIME’s sensitivity. With better localisation by utilising the
CHIME/FRB baseband system [64] or other interferometric telescopes, we will be
able to examine the white dwarf model more conclusively.

Neutron star models for LPTs often invoke an old magnetar. Magnetars are
theorised to be the early stage of neutron star evolution, and their strong
magnetic fields are predicted to decay on a timescale of $\sim 10^{4}$ years.
From spin-down alone, the maximum period of a magnetar can only reach $\sim
13$ s in this scenario [65]. Therefore, it has been theorised that magnetars
can only reach longer periods via another mechanism, such as angular momentum
kicks via giant flares [66]. Indeed, CHIME J0630+25 possess a characteristic
age $>10^{6}$ yr, which disfavours the magnetar model. To support the
theoretical ultra long period magnetar, there is one candidate magnetar with a
period of 6.67 h, 1E161348-5055 [20]. However, no radio pulsations have been
seen to date. This is unsurprising as only a subset of known magnetars are
radio emitters [10]. The upper limit for the surface magnetic field of CHIME
J0630+25 is 0.8$\times 10^{15}$ G, which allows for a magnetar-like field.

CHIME J0630+25 exhibits many similar characteristics to known Galactic
magnetars and radio pulsars. Firstly, the duty cycle of CHIME J0630+25 is
$\sim$0.4–0.8%, which is similar to known long-period pulsars such as PSR
J0250+5854 and magnetar candidate PSR J0901–4046 at 0.3–0.4% and $\sim$0.7%
respectively [17, 4]. The burst morphology of J0630+25, characterised by
complex temporal and spectral structure (such as that seen in 59460A), has
been documented in radio-loud magnetars such as XTE J1810–97 [16], and also
the Galactic centre magnetar PSR J1745–2900 [67]. To the best of our
knowledge, they have not been seen in single pulses of radio pulsars or white
dwarfs.

There are some characteristics of CHIME J0630+25 which favour a regular radio
pulsar rather than a magnetar. The spectral index of most bursts from CHIME
J0630+25 is steep, with an average of -2.0. Radio magnetars emit spectra which
are flat even over a wide range of frequencies, from 1.4–45 GHz [68, 69, 70,
71]. However, most radio pulsars have steep spectra, with a mean spectral
index of $\sim-1.8$ [72]. If CHIME J0630+25 is a neutron star, then
multiwavelength observations of the source region – including spectroscopy of
the Swift-identified X-ray sources discussed above – are essential for
discerning the nature of CHIME J0630+25.

##  3 Methods

###  3.1 Source Identification

CHIME/FRB is a trigger-based FRB-detection instrument on the CHIME telescope
[25] that constantly scans the overhead sky with 1024 FFT-formed beams between
400–800 MHz and a field of view of approximately $2^{\circ}$ in RA and
$100^{\circ}$ in Dec [26]. The instrument triggers on any impulsive signal
that passes the initial radio frequency interference (RFI) check [26]. Then,
bespoke software will determine if the incoming signal is terrestrial or
astrophysical in nature. Data are then saved to disk if a certain S/N
threshold (currently 8.5) is met. Due to the substantial data volume, data for
sources within the Milky Way Galaxy were not saved by CHIME/FRB until October
2022. However, metadata were saved for each “event” regardless of origin. The
metadata contains real-time pipeline-derived information such as the RA, Dec,
DM, TOA, and S/N. We used the CHIME Metadata Clustering Analysis (CHIMEMCA),
to identify CHIME J0630+25 [see 24, for a full description]. After filtering
out RFI, a cluster was identified by CHIMEMCA, with the first event of the
cluster detected on MJD 58772 at 12:54:51 UTC, at RA=06:30:19$\pm 30^{\prime}$
and Dec=25:23:14$\pm 30^{\prime}$ with a DM of (22.6$\pm$3.2) pc cm-3.

The CHIME/Pulsar instrument forms ten steerable phased array tracking beams to
track sources as they pass through the CHIME field of view. It produces high-
time resolution spectra, packaged as conventional SigProc-style filterbank
data777https://sigproc.sourceforge.net/. CHIME/Pulsar can also correct for
intrachannel dispersion smearing through coherent dedispersion and can record
data at significantly higher time resolutions than CHIME/FRB. These
advantages, along with the phased array tracking beams, contribute to an
increase in sensitivity for CHIME/Pulsar compared to CHIME/FRB.

Follow-up observations were conducted using the CHIME/Pulsar system from MJD
59300 to MJD 60116. These observations were carried out nearly daily,
consisting of about 10 minutes per scan, resulting in 688 observations and
175.946 hours of Stokes I data for this study. To process all the CHIME/Pulsar
data, we have employed the CHIME/Pulsar Single-pulse PIPEline
(CHIPSPIPE)888https://github.com/CHIME-Pulsar-Timing/CHIME-
Pulsar_automated_filterbank, an automated single-pulse search pipeline
designed to handle the large data volume of CHIME/Pulsar. The pipeline is
based on PRESTO [73]. It further utilises SPEGID [74] and FETCH [75] to filter
out spurious candidates. Finally, the pulses that are graded as astrophysical
by FETCH are examined by a human. For a detailed discussion of CHIPSPIPE,
refer to [24]. For all the bursts that passed the human check, we manually
removed the RFI-contaminated channels and generated the dynamic spectrum,
pulse profile, and dedispersion-time plot for each burst using the Your [76]
package. These are shown in Figure 1.

CHIPSPIPE has limited sensitivity to wider pulses. Therefore, we visually
inspected the dynamic spectrum and pulse profile around each detected pulse
within a 10-second window. This manual inspection aims to identify additional
sub-pulses missed by CHIPSPIPE that may provide evidence of quasiperiodicity.
59460A, 59463A, 59553A, 59563A, and 59574A exhibited distinct second peaks not
flagged by the initial CHIPSPIPE detection.

In total, with CHIME/FRB, we detected 6 bursts above S/N 8.5, and with
CHIME/Pulsar, we detected 11 bursts with Stokes-I intensity data showing
extended widths and complex structures, typical of other LPTs. Despite the
lack of stokes-I intensity data, we confirmed the CHIME/FRB bursts to be real
astrophysical events as the arrival time is consistent with the rotation
period of CHIME J0630+25. All bursts are detailed in Table 1. Subsequent
localisation with the CHIME/FRB metadata [28] yielded RA=06:31:00$\pm$6’,
Dec=25:23:24$\pm$11’ in J2000 coordinates at the 1$\sigma$ level. Considering
that the disparity between the CHIME/Pulsar pointing and the CHIME/FRB
localisation is roughly equivalent to one full width at half maximum of the
CHIME/Pulsar beam at 400 MHz [27], it is likely that the true location of the
source lies between the localisation made by CHIME/FRB and the CHIME/Pulsar
pointing. Indeed, the long-term timing of CHIME J0630+25 suggests that the
best fit RA is 06:30:43$\pm$6’ (see Section 1.1), which is taken to be the
preferred RA localisation throughout this study. Interferometric telescopes
with low-noise environments, such as the Karl Jansky Very Large Array or
MeerKAT, should be able to localise CHIME J0630+25. Furthermore, we are
currently saving CHIME/FRB baseband data, which will allow for more precise
localisation of future detections.

###  3.2 Quasiperiodicity

Table 5: This table shows the results of fitting for the quasiperiodic peaks in the autocorrelation function. The $X_{i}$ parameters are the location parameters for the Gaussian peak in the ACF. The $\sigma$ parameters are the width parameters for the peaks in the ACF. 59460A is the only burst with two peaks in the intensity time series and, therefore, is the only burst with two $X_{i}$ parameters. Burst | $X_{0}$ | $\sigma_{0}$ | $X_{1}$ | $\sigma_{1}$
---|---|---|---|---
| ms | ms | ms | ms
59460A | 206${}_{-8}^{+7}$ | 88${}_{-7}^{+8}$ | 440${}_{-10}^{+10}$ | 123${}_{-7}^{+7}$
59463A | 2490${}_{-10}^{+10}$ | 504${}_{-10}^{+20}$ | – | –
59553A | 376${}_{-2}^{+2}$ | 209${}_{-2}^{+3}$ | – | –
59563A | 760${}_{-10}^{+10}$ | 270${}_{-20}^{+20}$ | – | –
59574A | 1460${}_{-60}^{+60}$ | 410${}_{-80}^{+100}$ | – | –

(a) 59460A

(b) 59463A

(c) 59553A

(d) 59563A

Figure 5: Autocorrelation on the band-averaged time series of C2 and C3 type
bursts. The top panel for each burst is the band averaged intensity, and the
second panel is the autocorrelation, where the dashed red line is the Gaussian
modulated exponential decay that has been fitted. The third panel subtracts
the exponential decay from panel two, leaving only the Gaussian component. The
bottom panel is the residual after all components of the fit have been
subtracted. The red vertical lines represent the location of the fitted
Gaussian. The residuals still exhibit a small amount of structure. This is
generally an order of magnitude smaller than the subtracted signal and is the
result of two factors: the variable CHIME/Pulsar baseline and not being able
to exactly describe the ACF peak as a Gaussian (such as is the case for 59463A
and 59553A). (a) 59574A Figure 6: continued

This section aims to identify any quasiperiodicity in the bursts emitted by
CHIME J0630+25. We began our analysis by visually examining the bursts. This
initial step allowed us to identify distinct burst morphologies, which we then
categorised into three groups: C1, C2, and C3. C1 represents bursts with a
single clear peaked envelope. C2 bursts are double-peaked, where the second
peak builds on top of the first without returning to the baseline. Finally, C3
bursts occur when the pulse rises, reaches a peak, falls back to near
baseline, and exhibits a second peak (and, in the case of 59460A, a third).
Among the bursts analysed, we found three (59553A, 59563A, and 59574A)
exhibiting C2 behaviour and two (59460A and 59463A) exhibiting C3 behaviour
and — the rest exhibit C1 type behaviour. The categories are tabulated in
Table 1.

In the subsequent analysis, only the bursts belonging to the C2 and C3
categories are used. We examined the separation between the peaks in C2 and C3
bursts by performing autocorrelation on each burst. The autocorrelation
results are presented in Figure 5. The second panel shows the autocorrelation
function (ACF) result for each burst. To extract the separation between peaks,
we fitted an exponential decay modulated by a Gaussian function to the ACF
using Bayesian Markov Chain Monte Carlo (MCMC) with uniform priors. The fit
model is described by

$f(T)=A\exp{(-BT)}+C\exp{\bigg{(}-\frac{(T-X_{0})^{2}}{2\sigma_{0}^{2}}\bigg{)}}$
(1)

for all bursts apart from 59460A or

$f(T)=A\exp{(-BT)}+C\exp{\bigg{(}-\frac{(T-X_{0})^{2}}{2\sigma_{0}^{2}}\bigg{)}}+D\exp{\bigg{(}-\frac{(T-X_{1})^{2}}{2\sigma_{1}^{2}}\bigg{)}}$
(2)

in the case of 59460A. $A,B,C,D,X_{0},\sigma_{0},X_{1},\sigma_{1}$ are fit
parameters and $T$ is the time-lag parameter. The $X$ parameters give the
separation between sub-bursts. The $\sigma$ parameters give the width of the
fitted Gaussian. 59460A requires a two Gaussian fit due to the three peaks in
the intensity time series. The exponential baseline is removed for each burst
and is shown in the respective third panels of Figure 5. This leaves only the
Gaussian (peak) component behind. $A,B,C$, and $D$ are nuisance parameters and
are marginalised over to provide the fit results in Table 5.

Interestingly, we found that the peak time lag in 59563A is a factor of
2.02(4) greater than that of 59553A, and the peak time lag in 59574A is a
factor of 1.9(1) greater than that of 59563A. We then tested the significance
of this apparent factor of two. We simulated five different sub-burst
separations between 180 ms, the narrowest burst detected, and 10,000 ms, the
search range for additional sub-bursts. We then assigned a sub-burst arrival
time for each sub-burst, i.e., if one of the simulated burst separations was
760 ms, the TOAs assigned would be 0 ms and 760 ms. Therefore, for five sub-
burst separations, we get five pairs of TOAs. We then found every combination
of three sub-burst separations pairs. There are ${5\choose 3}=10$
combinations. For each combination, the TOA pairs were inputted into
rrat_period_multiday, a sub-program of PRESTO. rrat_period_multiday was
designed to find any underlying periodicity for many sets of single-pulse
arrival times and will provide the most likely period and the separation
between bursts [77, 78]. When the period calculated by rrat_period_multiday
results in all separations within 0.1 of an integer number, we counted that as
a success. If there is one success within the ${5\choose 3}$ combinations,
then that trial was successful. We performed 1,000,000 trials with 982,002
successes. This indicates that the situation where there is a common integer
factor between the three different burst separations is highly likely and,
thus, is not significant.

###  3.3 Distance determination and dispersion measure

For the distance determination, the dispersion measure used is the error-
weighted average of all the CHIME/Pulsar detections. This is given by

$DM_{av}=\frac{\sum_{i}DM_{i}/\sigma_{i}^{2}}{\sum_{i}1/\sigma_{i}^{2}}.$ (3)

Where $DM_{i}$ is the dispersion measure of each individual pulse and
$\sigma_{i}$ is the corresponding uncertainty. The average uncertainty is
given by

$\sigma_{DM}=\frac{1}{\sqrt{\sum_{i}\sigma_{i}^{2}}}.$ (4)

The resultant DM value is 22(1) pc cm-3.

The distance was determined by the YMW16 electron density model [33]. To find
the uncertainties associated with CHIME J0630+25, we queried the ATNF pulsar
catalogue and applied a series of cuts to the full catalogue. First, we
isolated those pulsars with similar Galactic latitudes as the $b=7^{\circ}$ of
CHIME J0630+25. We selected pulsars with $b>4^{\circ}$ and $b<10^{\circ}$. We
then limited the DM of these sources to be less than 60 pc cm-3. Finally, we
only considered pulsars with an independent distance measure, such as parallax
or globular cluster association. In total, 10 pulsars met this criteria. We
then defined the uncertainty of CHIME J0630+25 as the standard deviation of
CHIME J0630+25 if it were to possess the same fractional uncertainty as the 10
pulsars. Explicitly this is

$\sigma_{\text{J0630+25}}=\sqrt{\sum_{i}^{N}\frac{\bigg{(}170~{}\text{pc}\times\frac{d_{i}}{y_{i}}-\mu\bigg{)}^{2}}{N}}$
(5)

where $d_{i}$ is the independently measured distance to the pulsar, $y_{i}$ is
the YMW16 derived distance, $\mu$ is the mean of all 170
pc$\times\frac{d_{i}}{y_{i}}$, and N is the total number of pulsars which meet
the defined criteria. For CHIME J0630+25, we found that the distance is
170(80) pc. Note that the 1 pc cm-3 uncertainty on the DM translates to a
deviation of 2 pc according to the YMW16 electron density model and,
therefore, is negligible compared to the uncertainty due to the electron
density model itself. Therefore, is not included in the final value. Applying
this error determination method to the 10 pulsars in our sample, we find that
the true position of all but one source is within 2-$\sigma$ of the YMW16
distance estimate.

###  3.4 Timing

The time of arrival (TOA) extraction is vital for period determination and
timing solution extraction. This is necessary for both the CHIME/FRB and
CHIME/Pulsar datasets. Unfortunately, for the CHIME/FRB detections, the only
data products saved were the metadata. These contain a timestamp but no total
intensity data, and therefore it is difficult to know if the system triggered
on one of the microstructure peaks. The complex and long burst widths of CHIME
J0630+25 mean that CHIME/FRB could have triggered on any one of the
microstructure peak components within the burst envelope. Therefore, we took a
conservative TOA error of 1 s for all metadata-only detections. The CHIME/FRB
TOAs are referenced at the bottom of the CHIME band at 400 MHz. For the
CHIME/Pulsar detections where the total intensity data are recorded, we
followed the prescription outlined in [2]. Specifically, we smoothed the
bursts’ pulse profile using a 1-s Gaussian kernel. The maximum of the smoothed
profile is taken as the TOA, and the full-width-half-max (FWHM) is taken as
the uncertainty of the TOA. The CHIME/Pulsar TOAs are referenced at the top of
the CHIME band at 800 MHz. In total, we collected 11 TOAs spread over 802 days
between MJD 58772 and 59574. All TOAs are then corrected to the solar system
barycentre arrival time using Astropy for use with rrat_period.

The next step is to generate the full timing ephemeris. Using the rrat_period
module of PRESTO with the barycentre corrected TOAs of all CHIME/Pulsar
bursts, we found that the most likely period for CHIME J0630+25 is $\sim 421$
s. The derived period is only approximate as rrat_period provides only an
estimated period without accounting for period derivatives. On two occasions,
CHIME/Pulsar observed two bursts during the same transit. These are MJD
59341A, B and MJD 59456A, B, separated by 422.5(2.2) s and 418.2(1.4) s,
respectively, driving the convergence to 421 s. Taking this as a starting
point, we created a timing ephemeris, with the period derivative set to 0 and
one phase jump (JUMP1) between the CHIME/Pulsar and CHIME/FRB instruments. We
tested the validity of fitting the phase jump between the two instruments by
performing the same TOA extraction procedure described above on multiple
pulsars, namely J0012+54, J0209+5759, and J1838+5051. During this process, we
ensured that the timespan covered was similar to that of CHIME J0630+25. Then,
using PINT, a jump is fit between the CHIME/FRB and CHIME/Pulsar TOAs of the
three pulsars. The fitted jumps for all three pulsars are 0.215(2) s, 0.250(6)
s, and 0.272(2) s, respectively. For the timing of CHIME J0630+25 we use a
fixed 0.247 s clock offset between the CHIME/FRB and CHIME/Pulsar TOAs. This
is the mean of the jumps fit for the three pulsars.

We then used two standard pulsar timing software, PINT
999https://github.com/nanograv/PINT[30] and TEMPO2
101010https://ascl.net/1210.015 [29], to perform a least squares fit to all
the TOAs of J0630+25. The results for the fit are provided in Table 2, and the
residuals are provided in Figure 7. We noticed a slight downward drifting
trend before MJD 59168. Therefore, we tried to fit the second period
derivative, however it did not provide significant improvements.

To test the significance of the 421 s period, we calculated the probability
that an alternative period will always be detected as an integer multiple of
421 s. We defined the alternative period as $P2=P_{J0630+25}/i$ where
$P_{J0630+25}$ is the period given in Table 2 and i is an integer factor. We
also accounted for the timing noise shown in Figure 7 by allowing the
detection to be made within 6 s (roughly the worst residual of all the TOAs)
of an integer multiple of 421.35542 s. If CHIME J0630+25 had a true period of
421.35542 s/i then the probability that a pulse lands precisely on a multiple
of 421.35542 is 1/i. Because we allowed it to land anywhere within 6 s of an
integer multiple of 421.35542 s the probability is therefore
$(2\times\rm{int}(6/P2)+1)/i$. Where int is the integer flooring function. The
first burst does not contribute to the probability as it sets the starting
point. Therefore, the probability that a source with an alternative period
will always be detected within 6 s of an integer multiple of 421.35542 s is

$P_{i}=\bigg{(}\frac{2\times\textrm{int}(6/P2)+1}{i}\bigg{)}^{N-1}$ (6)

where N is the number of detections made. The alternative period can vary
between i=2 to i=210 such that the smallest alternative period is the largest
FWHM of any burst. We then account for the look-elsewhere effect by
calculating the probability of any alternative period defined by

$P_{\textrm{any alternative}}=1-\Pi_{i=2}^{210}(1-P_{i}).$ (7)

Only using the 11 bursts from CHIME/Pulsar, the probability of any alternative
period is 0.00099. This is dominated by $P_{2}=0.00097$ and rapidly decays for
all other alternative periods. Therefore, we conclude that 421.35542 s is the
correct period for CHIME J0630+25.

Figure 7: The timing residuals for CHIME J0630+25. The plot shows the
residuals of the phase-connected timing parameters in Table 2.

###  3.5 DM Measurement and Flux Calibration

Using the total intensity data obtained by CHIME/Pulsar, we measured some
essential characteristics such as effective pulse width, fluence, and DM. For
the bursts detected from CHIME J0630+25, the fluence is defined by the
integrated flux density over the duration of the burst, and the effective
width is calculated via $W_{\rm eff}=\text{F}/\text{S}_{peak}$ where F is the
fluence and Speak is the peak flux density.

CHIME/Pulsar has declination-dependent sensitivity. Therefore, we used 3C133
as a calibrator source to determine the system equivalent flux density (SEFD).
The SEFD is calibrated by fitting the telescope temperature and is defined in
the following way

$\text{SEFD}(\nu)=\frac{T_{\rm telescope}(\nu)+T_{\rm sky}(\nu)}{G(\nu)}$ (8)

where $T_{\rm sky}$ is obtained from the Haslam 408-MHz all-sky map [79, 80],
$G\approx 1.16$ K/Jy is the telescope gain and $T_{\rm telescope}$ is the
system temperature of all telescope components (i.e. receiver, structure,
ground etc). The flux density of a steady source is given by

$S(\nu)=\frac{T_{\rm on}(\nu)-T_{\rm off}(\nu)}{T_{\rm
off}(\nu)}\times\text{SEFD}(\nu),$ (9)

where $S(\nu)$ is the flux density. $T_{\rm on}$ and $T_{\rm off}$ are the
temperatures of the calibrator source and a blank patch of nearby sky,
respectively. $T_{\rm telescope}$ is an unknown, thus, we performed a maximum
likelihood reduced $\chi^{2}$ fit of the CHIME/Pulsar measured 3C133 spectrum
against the catalogued flux density measurements of 3C133. For the catalogue
values, we used the VLA calibrator list
111111https://science.nrao.edu/facilities/vla/observing/callist. We found that
the best functional form of $T_{\rm telescope}$ is a 5th-order polynomial. We
then used the single pulse radiometer equation to convert from S/N units to Jy
units for each burst,

$S(\nu)=\frac{\text{S/N}}{\sqrt{n_{p}\Delta\nu\Delta t}}\cdot\text{SEFD}(\nu)$
(10)

where S/N is the signal to noise ratio, $n_{p}$ is the number of
polarisations, $\Delta\nu$ is the bandwidth, and $\Delta t$ is the time
resolution.

We measured the DM of each pulse using DM_phase
121212https://github.com/danielemichilli/DM_phase which is a brute force
algorithm that maximises the coherent power across the bandwidth by trying
many different DMs. Due to the wide burst widths, the DM uncertainty for each
pulse is large; this is exemplified in Figure 1 by the large hot spot that the
dedispersion panel covers.

###  3.6 Spectral index

Using the total intensity data for the CHIME/Pulsar bursts, we measured the
spectral index of CHIME J0630+25. We first flux calibrated the bursts using
the method described above. This process will also serve to calibrate the
spectrum of CHIME J0630+25. Then, each burst is integrated over its duration
to obtain a spectrum. Maximum likelihood is used to fit the spectra with a
power law model of the form

$S(\nu)=A\nu^{\alpha}$ (11)

where $\alpha$ is the spectral index, and A is the amplitude parameter. Both
$\alpha$ and A are fit parameters. The spectral indices are provided in Table
1. All spectral index fits are provided in the Appendix.

To place an uncertainty on the fit, we measured the spectral index of 23 other
calibrator sources with known spectral indices using the same technique. We
found that the mean uncertainty on the spectral index calibrated in this way
is 0.3.

###  3.7 Energy Budget

The upper limit of the spin-down energy is given by [62]

$\dot{E}=-4\pi^{2}I\dot{P}/P^{3}$ (12)

where $I$ is the moment of inertia of the neutron star, P is the period, and
$\dot{P}$ is the period derivative. The moment of inertia is assumed to be
$I=10^{45}$ g cm2, the period is 421.35542 s, and the period derivative is
given by the conservative shifted upper limit of $1.6\times 10^{-12}$ s s-1.
This yields an upper limit on the spin-down energy of 8.5$\times 10^{26}$ erg
s-1.

We followed the prescription laid out in Equation 3.40 of [62] for the
estimated radio luminosity. This is given by

$L=\frac{2\pi
d^{2}}{\delta}(1-cos{\rho})S_{\textrm{mean}}(f_{0})\frac{f_{0}^{-\zeta}}{\zeta+1}(f_{2}^{\zeta+1}-f_{1}^{\zeta+1}),$
(13)

where L is the total radio energy output, d is the distance, $\rho$ is the
opening angle, $\delta$ is the duty cycle, $f_{0}$ is 600 MHz for CHIME,
$S_{\textrm{mean}}$ is the mean flux density at $f_{0}$, $\zeta=-2$ is the
mean spectral index, and $f_{1}\approx 10^{7}$ Hz and $f_{2}\approx 10^{11}$
Hz are reference frequencies. For the duty cycle, we took the largest FWHM of
the collection of CHIME J0630+25 pulses, i.e., $\delta=W/P=\sim
4~{}s/421~{}s=9.5\times 10^{-3}$. Assuming typical opening angles of
$\rho\approx 6^{\circ}$ [62], an observing frequency of 600MHz, mean flux
density of 0.0006 Jy (calculated by the average fluence/period), and a
distance of 0.17 kpc, we found

$L\approx 1.2\times 10^{31}\text{erg~{}s}^{-1}(0.17\pm 0.08\text{
kpc}/\text{kpc})^{2}(0.0006\text{ Jy}/\text{Jy})=2.2^{+2.5}_{-1.6}\times
10^{26}\text{erg~{}s}^{-1}$ (14)

Therefore, the total radio luminosity output of CHIME J0630+25 is within the
spin-down luminosity range.

###  3.8 Follow up of CHIME J0630+25 with other telescopes

We use data from both archival observations near CHIME J0630+25 and targeted
follow-up campaigns to explore CHIME J0630+25 across many wavelengths. In the
radio band, we used the Green Bank Telescope (GBT) because of its increased
sensitivity and longer observation tracks compared to CHIME. We also used the
upgraded Giant Metrewave Radio Telescope (uGMRT) and archival VLA Low-band
Ionosphere and Transient Experiment (VLITE) to localise CHIME J0630+25. Both
uGMRT and VLITE can reach arcsecond angular resolution.

Pulsars and magnetars are known to emit X-rays. Therefore, we also performed
targetted observations with the Neils Gehrels Swift Observatory’s X-Ray
Telescope (XRT). Finally, some magnetars are soft $\gamma$-ray repeaters.
Thus, we searched the known $\gamma$-ray archives for as-of-yet-unknown
magnetar candidates.

####  3.8.1 Radio

Table 6: Properties of all telescopes used for this study | CHIME/FRB | CHIME/Pulsar | GBT L-band | GBT 800MHz | VLITE 338 MHz
---|---|---|---|---|---
Receiver noise temperature |  $\sim$50K |  $\sim$50K |  $\sim$18K |  $\sim$20K |  $\sim$180K
Frequency Range | 400-800MHz | 400-800MHz | 1150-1730MHz | 680-920MHz | 321.9-360MHz
Number of beams | 1024 (Static) | 10 (Tracking) | 1 | 1 | 1
Beam width (FWHM) | 40’-20’ | 30’-15’ | 9’ | 15’ | 5”
Time resolution | 1ms | 327.68$\mu$s | 20.48$\mu$s | 20.48$\mu$s | 2s
Search Frequency Resolution | 24.4 kHz | 390.625 kHz | 781 MHz | 195 MHz | 47.1 MHz
Coherent Dedispersion | No | Yes | Yes | Yes | No

We were awarded 16 hours of observations using the GBT at 800 MHz and 16 hours
at 1440 MHz to observe CHIME J0630+25. We used the VEGAS back end at 20.48
$\mu$s time resolution and coherently dedispersed to 22.5 pc cm-3. The
observations were taken between MJD 59861 and 59914. The Stokes I data were
processed using CHIPSPIPE to search for single pulses. No pulses were
detected.

We were also awarded 12 hours of data with the upgraded Giant Metrewave Radio
Telescope to localise the source. We performed the observations between MJD
59700 and 59837 from 950 MHz to 1460 MHz. These had an integration time of
0.67 s in incoherent array mode. Unfortunately, due to the variable baseline
and the high levels of radio frequency interference (RFI), we were not able to
make use of the uGMRT data.

Finally, we searched through archival data from VLITE [81,
82]131313https://vlite.nrao.edu for high-resolution observations ($\sim$ 5”)
at 340 MHz covering the region of interest. We identified two observations
where CHIME J0630+25 was located within 2∘ of the phase centre and made short
time interval images at the VLITE sample time of 2 s. The short VLITE images
were catalogued using PyBDSF [83], and components associated with all
persistent radio sources were eliminated. The remaining catalogued sources
were low signal-to-noise (S/N $\sim$ 4), and visual inspection revealed these
remaining candidates were likely associated with poorly cleaned sidelobes in
the images. As the CHIME/FRB localisation is large compared to the VLITE
resolution, the chance coincidence for a low S/N candidate is large.
Therefore, it is difficult to associate any low S/N VLITE candidates with
CHIME J0630+25. Unfortunately, VLITE’s highest time resolution is 2 s and so
bursts from CHIME J0630+25 are predominantly less than one bin long, resulting
in low S/N. This makes it challenging to differentiate potential CHIME
J0630+25 bursts in VLITE data from remaining uncleaned sidelobe structures.

####  3.8.2 X-ray

Our X-ray observations of CHIME J0630+25 consisted of 32ks of Swift XRT time
under target ID 97140 and 97203. The two targeted observations allowed
comprehensive RA coverage over the large uncertainty area of CHIME J0630+25.
To process the data, we used the tools provided by the UK Swift Science Data
Centre 141414https://www.swift.ac.uk/user_objects/ to create the images. Then,
we used Ximage 151515https://heasarc.gsfc.nasa.gov/docs/software.html to
detect the sources and provide S/N estimates. The final image (Figure 4) was
produced using Saoimage DS9
161616https://sites.google.com/cfa.harvard.edu/saoimageds9.

To measure the count rates at 0.1-1.5keV and 1.5-10keV, we created the images
using the UK Swift Science Data Centre image creation package. Then, using
Ximage and the sosta program, we estimated the localised count rate and
background in the local region of the sources. We then used the WebPIMMS
171717https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3pimms/w3pimms.pl software
to estimate the neutral hydrogen along the line of sight for each of the
sources. This was done by assuming a black body spectrum of 0.5 keV. Then, we
entered an input energy range of 0.1-1.5keV and an output energy range of
1.5-10keV. We then adjusted the Galactic $N_{\mathrm{H}}$ value until we found
a reasonable match with the observed hardness ratio.

####  3.8.3 $\gamma$-ray

We also searched for possible as-of-yet unknown soft $\gamma$-ray repeater
counterparts to CHIME J0630+25. These would reside in the same databases as
$\gamma$-ray bursts (GRB), albeit with an unknown classification. We first
cross-match the coordinates and times of arrivals of CHIME J0630+25 with all
$\gamma$-ray sources reported in GRBWeb [84]. We limit the GRBWeb triggers to
those that are well localised (e.g., 1$\sigma$ spatial error $<$1 degree), as
it is challenging to claim significant spatial coincidences for triggers with
either unknown or large uncertainty regions. In our cross-match, we
conservatively assume a 1$\sigma$ positional error in RA of 1 degree and a
1$\sigma$ positional error in DEC of 0.5 degrees for CHIME J0630+25. We then
cross-match the localisation region of CHIME J0630+25 with that of all known
sources in GRBWeb, requiring the localisations to be consistent within the
3$\sigma$ uncertainties. Within one week of each burst, we did not find any
sources to be coincident with CHIME J0630+25. However, given GRBWeb’s focus on
cosmological GRBs and not Galactic $\gamma$-ray sources such as soft $\gamma$
repeaters, we also cross-match the position of CHIME J0630+25 and its bursts
with all triggers reported in the $\gamma$-ray Coordination Network
(GCN)181818www.gcn.gsfc.nasa.gov circulars. We again limit our search to well-
localised triggers, e.g., ($\sigma<1$ degree), and do not find any trigger-
burst pairs with the given criteria. When considering solely spatial
coincidence, however, we find one trigger spatially coincident with CHIME
J0630+25. The trigger is GRB110414A, which was detected long before CHIME was
built.

However, as noted in [85], there is a high chance probability of finding
spatial coincidences given CHIME’s current localisation capabilities.
Accordingly, we conclude that no significant coincidences exist between CHIME
J0630+25 and any known $\gamma$-ray triggers.

## Acknowledgements

F.A.D is support by the UBC Four Year Fellowship.

Basic research in radio astronomy at the U.S. Naval Research Laboratory is
supported by 6.1 Base funding. Construction and installation of VLITE was
supported by the NRL Sustainment Restoration and Maintenance fund.

Pulsar and FRB research at UBC are supported by an NSERC Discovery Grant and
by the Canadian Institute for Advanced Research.

K.S. is supported by the NSF Graduate Research Fellowship Program.

A.B.P. is a Banting Fellow, a McGill Space Institute (MSI) Fellow, and a Fonds
de Recherche du Quebec – Nature et Technologies (FRQNT) postdoctoral fellow.

V.M.K. holds the Lorne Trottier Chair in Astrophysics & Cosmology, a
Distinguished James McGill Professorship, and receives support from an NSERC
Discovery grant (RGPIN 228738-13), from an R. Howard Webster Foundation
Fellowship from CIFAR, and from the FRQNT CRAQ.

The Dunlap Institute is funded through an endowment established by the David
Dunlap family and the University of Toronto. B.M.G. acknowledges the support
of the Natural Sciences and Engineering Research Council of Canada (NSERC)
through grant RGPIN-2022-03163, and of the Canada Research Chairs program.

A.M.C. is funded by an NSERC Doctoral Postgraduate Scholarship.

S.M.R. is a CIFAR Fellow. and is supported by the NSF Physics Frontiers Center
award 2020265.

K.W.M. holds the Adam J. Burgasser Chair in Astrophysics.

E.F., I.S., S.C., S.M.R. are members of the NANOGrav Physics Frontiers Center,
supported by the NSF award 2020265.

A.P.C is a Vanier Canada Graduate Scholar.

##  Appendix A Appendix

(a) 59341A

(b) 59341B

(c) 59456A

(d) 59456B

Figure 8: The calibrated spectral fit of all bursts with intensity data.

(a) 59460A

(b) 59463A

(c) 59548A

(d) 59553A

Figure 9: Continued

(a) 59563A

(b) 59565A

(c) 59574A

Figure 10: Continued

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