Multiwavelength Observations of a New Redback Millisecond Pulsar 4FGL J1910.7−5320

Abstract

We present the study of multiwavelength observations of an unidentified Fermi Large Area Telescope (LAT) source, 4FGL J1910.7−5320, a new candidate redback millisecond pulsar binary. In the 4FGL 95% error region of 4FGL J1910.7−5320, we find a possible binary with a 8.36 hr orbital period from the Catalina Real-Time Transient Survey, confirmed by optical spectroscopy using the SOAR telescope. This optical source was recently independently discovered as a redback pulsar by the TRAPUM project, confirming our prediction. We fit the optical spectral energy distributions of 4FGL J1910.7−5320 with a blackbody model, inferring a maximum distance of 4.1 kpc by assuming that the companion fills its Roche lobe with a radius of R = 0.7 R ☉ . Using a 12.6 ks Chandra X-ray observation, we identified an X-ray counterpart for 4FGL J1910.7−5320, with a spectrum that can be described by an absorbed power law with a photon index of 1.0 ± 0.4. The spectrally hard X-ray emission shows tentative evidence for orbital variability. Using more than 12 yr of Fermi-LAT data, we refined the position of the γ -ray source, and the optical candidate still lies within the 68% positional error circle. In addition to 4FGL J1910.7−5320, we find a variable optical source with a periodic signal of 4.28 hr inside the 4FGL catalog 95% error region of another unidentified Fermi source, 4FGL J2029.5−4237. However, the γ -ray source does not have a significant X-ray counterpart in an 11.7 ks Chandra observation, with a 3 σ flux upper limit of 2.4 × 10 ⁻¹⁴ erg cm ⁻² s ⁻¹ (0.3–7 keV). Moreover, the optical source is outside our updated Fermi-LAT 95% error circle. These observational facts all suggest that this new redback millisecond pulsar powers the gamma-ray source 4FGL J1910.7−5320 while 4FGL J2029.5−4237 is unlikely the γ -ray counterpart to the 4.28 hr variable.

Full text

Multiwavelength Observations of a New Redback Millisecond Pulsar 4FGL
J1910.7−5320
Ka-Yui Au
1
, Jay Strader
2
, Samuel J. Swihart
3
, Lupin C. C. Lin
1
, Albert K. H. Kong
4
, Jumpei Takata
5
,
Chung-Yue Hui
6
, Teresa Panurach
2
, Isabella Molina
2
, Elias Aydi
2
, Kirill Sokolovsky
2
, and Kwan-Lok Li
1
1
Department of Physics, National Cheng Kung University No. 1 University Road, Tainan City 70101, Taiwan; kyau@phys.ncku.edu.tw,lilirayhk@phys.ncku.edu.tw
2
Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA
3
National Research Council Research Associate, National Academy of Sciences, Washington, DC 20001, USA resident at Naval Research Laboratory, Washington,
DC 20375, USA
4
Institute of Astronomy, National Tsing Hua University Hsinchu 30013, Taiwan
5
School of Physics, Huazhong University of Science and Technology Wuhan 430074, Peopleʼs Republic of China
6
Department of Astronomy and Space Science, Chungnam National University Daejeon 34134, Republic of Korea
Received 2022 November 7; revised 2022 December 16; accepted 2022 December 21; published 2023 January 31
Abstract
We present the study of multiwavelength observations of an unidentified Fermi Large Area Telescope (LAT)
source, 4FGL J1910.7−5320, a new candidate redback millisecond pulsar binary. In the 4FGL 95% error region of
4FGL J1910.7−5320, we find a possible binary with a 8.36 hr orbital period from the Catalina Real-Time
Transient Survey, confirmed by optical spectroscopy using the SOAR telescope. This optical source was recently
independently discovered as a redback pulsar by the TRAPUM project, confirming our prediction. We fit the
optical spectral energy distributions of 4FGL J1910.7−5320 with a blackbody model, inferring a maximum
distance of 4.1 kpc by assuming that the companion fills its Roche lobe with a radius of R=0.7 R
☉
. Using a 12.6
ks Chandra X-ray observation, we identified an X-ray counterpart for 4FGL J1910.7−5320, with a spectrum that
can be described by an absorbed power law with a photon index of 1.0 ±0.4. The spectrally hard X-ray emission
shows tentative evidence for orbital variability. Using more than 12 yr of Fermi-LAT data, we refined the position
of the γ-ray source, and the optical candidate still lies within the 68% positional error circle. In addition to 4FGL
J1910.7−5320, we find a variable optical source with a periodic signal of 4.28 hr inside the 4FGL catalog 95%
error region of another unidentified Fermi source, 4FGL J2029.5−4237. However, the γ-ray source does not have a
significant X-ray counterpart in an 11.7 ks Chandra observation, with a 3σflux upper limit of 2.4 ×10
−14
erg cm
−2
s
−1
(0.3–7 keV). Moreover, the optical source is outside our updated Fermi-LAT 95% error circle. These
observational facts all suggest that this new redback millisecond pulsar powers the gamma-ray source 4FGL
J1910.7−5320 while 4FGL J2029.5−4237 is unlikely the γ-ray counterpart to the 4.28 hr variable.
Unified Astronomy Thesaurus concepts: Millisecond pulsars (1062);Gamma-ray sources (633);Compact binary
stars (283)
1. Introduction
Millisecond pulsars (MSPs)are neutron stars with a very
short spin period on the order of one thousandth of a second.
One widely accepted explanation, known as the recycling
scenario, is that MSPs were in binaries with donor stars and the
accretion from a companion in a binary system continuously
transfers the angular momentum to the pulsar (Alpar et al.
1982; some MSPs still remain in binaries after the recycling
process). If the binaries started the recycling phase at long
orbital periods, then the MSPs will be formed with a complex
evolution dynamics. In these cases, the donors are far away
from the pulsars, and did not fill the Roche lobes during the
process until it became a (sub)giant, which typically leads to an
MSP-He white dwarf (WD)system (Tauris 2011; Hui et al.
2018). For the low-mass X-ray binaries (LMXBs)that started
the recycling process in tighter orbits, special subclasses of
pulsar binaries can be formed: redback and black widow MSPs.
The two classes have compact orbits, and hence, short
periods (1 day)with very low-mass companions
(M
C
0.1 M
☉
for redbacks and <0.1 M
☉
for black widows;
Roberts 2011; Chen et al. 2013). They are called redbacks and
black widows in analogy to the conduct of real spiders: the
female redback and black widow spider cannibalizes their
companion after copulation, just like the central neutron stars
create energetic pulsar winds and high-power radiation fields,
which ablate the nearby companions. This scenario could explain
how isolated MSPs are formed (Van den Heuvel & Van
Paradijs 1988). In recent years, some redback MSPs, PSR
J1227-4853 (Roy et al. 2015), PSR J1023 +0038 (Archibald
et al. 2009; Patruno et al. 2013;Stappersetal.2014), and M28I
(Papitto et al. 2013), showed a transition between the LMXB state
and the radio pulsar state, providing strong evidence supporting the
recycling scenario of the MSPs formation (Alpar et al. 1982).
Before the launch of the Fermi Large Area Telescope (LAT),
discovering redback and black widow MSPs was challenging
because the material blown off from the companion by the
high-power pulsar wind/radiation can hide the radio emission
of the pulsar. Therefore, it is difficult to find the MSP radio
pulsations in blind all-sky radio surveys. However, the GeV γ-
ray emission is not affected by the obscuring material, making
LAT a great tool to discover redback and black widow MSP
candidates with follow-up observations in other wavelengths,
given that many redback/black widow MSPs have γ-ray
The Astrophysical Journal, 943:103 (8pp), 2023 February 1 https://doi.org/10.3847/1538-4357/acae8a
© 2023. The Author(s). Published by the American Astronomical Society.
Original content from this work may be used under the terms
of the Creative Commons Attribution 4.0 licence. Any further
distribution of this work must maintain attribution to the author(s)and the title
of the work, journal citation and DOI.
1

emission (Abdo et al. 2013; Swihart et al. 2022). Nowadays, it
is also possible to find MSP binaries by using optical data from
all-sky surveys (ZTF and Gaia; Gaia Collaboration et al. 2016;
Bellm et al. 2018; Burdge et al. 2022).
LAT, the main instrument on the Fermi Gamma-ray Space
Telescope, is an imaging telescope for high-energy γ-rays in
the range of 100 MeV–300 GeV (Atwood et al. 2009). Using
12 yr of LAT data, the Fermi-LAT science team made the
Fermi-LAT Fourth Source Catalog Data Release 3 (4FGL-
DR3; Abdollahi et al. 2022)to list the known properties of all
the detected point-like and extended γ-ray sources of Fermi-
LAT; 4FGL (Abdollahi et al. 2020; the first data release)and
4FGL-DR3 share the same data analysis method and model for
the Galactic interstellar emission. One of the major differences
between them is that 4FGL-DR3 used four more years than
4FGL, and so the inferred parameters of the 4FGL-DR3 γ-ray
sources are generally more accurate.
In this paper, two pulsar-like unassociated Fermi sources,
4FGL J1910.7−5320 and 4FGL J2029.5−4237, were investi-
gated. For 4FGL J1910.7−5320, we present the multiwave-
length observations, including the data from Fermi-LAT,
Chandra X-ray observatory, the Southern Astrophysical
Research (SOAR)telescope, and the Catalina survey, strongly
suggesting that it is a redback MSP candidate. In addition, the
Transients and Pulsars with the MeerKAT (TRAPUM)team
has independently found the radio pulsations associated with
4FGL J1910.7−5320, which is in line with our result. We also
present the γ-ray, X-ray, and optical analyses of 4FGL J2029.5
−4237. However, we find no evidence that this second
unassociated GeV source is a pulsar binary system.
2. Searching for Spider MSP Candidates with 4FGL-
DR3/CRTS
Since the first catalog release of Fermi-LAT, there have been
numerous attempts to expand the redback and black widow
MSP population by searching the LAT catalog for new pulsar
systems (e.g., Kong et al. 2014; Hui et al. 2015; Li et al. 2016;
Strader et al. 2016). These searches are based on the well-
known fact that most MSPs have (i)stable γ-ray light curves on
a monthly timescale, and (ii)curved γ-ray spectra rather than a
simple power law. In this project, we did not use the spectral
and timing properties of the γ-ray sources for picking up
candidates. We cross checked the 4FGL-DR3 and the Catalina
Surveys Southern periodic variable star catalogs of the Catalina
Real-Time Transient Surveys (CRTS; Drake et al. 2009,2017;
Abdollahi et al. 2022)and selected possible γ-ray emitting
compact binaries with the following criteria.
1. The 4FGL-DR3 γ-ray sources must be “unassociated
sources”with detection significances higher than 5σand
high Galactic latitudes (i.e., |b|>5°).
2. For all the selected 4FGL-DR3 sources, the semimajor
axes of the 95% error circles must be smaller than0°.1 to
minimize the contamination (i.e., unrelated CRTS
sources).
3. The periods of the selected CRTS variables must be
shorter than 24 hr.
4. The selected CRTS sources must be fainter than 15 mag.
Redback and black widow systems are generally fainter
than the threshold, although there are counterexamples,
e.g., 3FGL J0212.1+5320 (Li et al. 2016). If a redback/
black widow has an apparent magnitude of <15 mag, it is
probably a very nearby pulsar system (i.e., 1 kpc), and
might have been discovered by the pulsar surveys.
Ten candidates were selected using the above method. We
then further cross checked the candidates with the SIMBAD
7
database, and found that six of them are RR Lyrae variable and
one of them is a known redback MSP candidate, 3FGL J0954.8
−3948 (Li et al. 2018). We examined visually the CRTS light
curves of the rest of the candidates. One of them is an Algol
type variable, which has two obvious eclipse dips seen in the
light curve. The other two, 4FGL J1910.7−5320 and 4FGL
J2029.5−4237, show sinusoidal-like modulations, possibly
caused by pulsar heating and/or ellipsoidal variation, if they
are pulsar systems (Romani & Sanchez 2016; Draghis et al.
2019; Hui & Li 2019; Yap et al. 2019). We therefore started a
multiwavelength follow-up campaign for the two systems.
3. 4FGL J1910.7−5320
In the following subsections, we will focus on the results of
the multiwavelength observations of 4FGL J1910.7−5320.
3.1. CRTS Surveys Data and VizieR Photometry Viewer
We downloaded and reanalyzed the optical data of 4FGL
J1910.7−5320 from the CRTS (Drake et al. 2009)to confirm,
and perhaps improve, the period from the CRTS catalog. The
phased light curves were also investigated to see if they are
consistent with that of a pulsar binary. In addition, the spectral
energy distribution (SED)in the optical band obtained from the
VizieR Photometry viewer
8
was used to estimate the color
temperature of the source as well as the distance to the system.
The CRTS observations were taken from 2005 August 1 to
2013 June 21, and the variable source is located at R.A.
(J2000)=19
h
10
m
49 12, decl.(J2000)=
-
¢53 20 57.
1
.Wefit
the CRTS data with a sinusoidal function and found that the
period is 0.3484776(10)days (roughly 8.36 hr)with a mean
magnitude of 19.08 ±0.01 mag and an amplitude of
0.54 ±0.02 mag. We note that the period in the CRTS catalog
is 0.697 days (twice our best-fit period; Drake et al. 2017), and
at this period, the folded light curve shows a double-peaks
feature. Given that the 0.697 days period is inconsistent with
the SOAR observations (see Section 3.2), we conclude that the
8.36 hr period is the real one. More detailed investigations will
be discussed in the next subsection.
For the SED, we included data from Gaia (Gaia
Collaboration et al. 2016; Brown et al. 2018; Gaia
Collaboration 2020), the Galaxy Evolution Explorer (Bianchi
et al. 2011), POSS (Lasker et al. 2008), VISTA (McMahon
et al. 2013), and the Wide-field Infrared Survey Explorer
(Marocco et al. 2021), and corrected for the absorption using
the extinction function from Cardelli et al. (1989)with the total
absorption in magnitudes of A
v
=0.1857 mag and the ratio of
total to selective absorption of R
v
=3.1 (Schlegel et al. 1998).
Then we used a blackbody radiation model to fit the SED data
(Figure 1). The best-fit color temperature is T=5154 ±164 K,
and the inferred distance is =-
+
D
5.8 0.4
0.5 (R/R
☉
)kpc where Ris
the companion radius.
Assuming that the companion nearly fills up the Roche lobe,
we used the approximate formula from Eggleton (1983), which
7
http://simbad.cds.unistra.fr/simbad/
8
http://vizier.unistra.fr/vizier/sed/
2
The Astrophysical Journal, 943:103 (8pp), 2023 February 1 Au et al.

is
=´ ++
Ra q
qq
0.49
0.6 ln 1 ,1
23
23 13
() ()
where q=M
C
/M
MSP
(M
C
: mass of companion; M
MSP
: mass of
MSP)and ais the distance between the binary members, to
calculate the Roche lobe radius. By assuming the masses of the
MSP and the companion are 1.4 and 0.4 M
☉
, respectively, and
using Kepler’s Third Law to calculate a,wefind R0.7 R
☉
,
and hence, D4.1 kpc. If we assumed the companion size is
similar to that of a black widow (i.e., M
C
=0.03 M
☉
), then
R0.3 R
☉
and D1.8 kpc.
Nevertheless, the SED data employed were not obtained
simultaneously while the optical source is strongly variable. To
check whether the effect is huge, we tried to find the blackbody
model parameters using only the VISTA J- and K-band data
(McMahon et al. 2013), which were taken nearly simulta-
neously. Similar to our original results, the parameters are
T=4550 K, D=5.2 (R/R
☉
)kpc. The distance is D3.6 kpc
by assuming the system is redback-like, i.e., R0.7 R
☉
.
We also searched the Gaia Catalog DR3 (Gaia Collabora-
tion 2020; Moss et al. 2022)for further distance information,
but the parallax is not well constrained (Bailer-Jones et al.
2021).
3.2. SOAR Spectroscopy
We obtained optical spectroscopy of the candidate
counterpart to 4FGL J1910.7−5320 using the Goodman
Spectrograph (Clemens et al. 2004)on the SOAR telescope
over six nights from 2022 April 10 to 2022 June 10, typically
taking multiple spectra per night. For all spectra we used a 1 2
slit and a 400 l mm
−1
grating covering a wavelength range of
∼3950–7850 Å, giving a resolution of about 7.3 Å for the
FWHM. The spectra were reduced and optimally extracted
using standard routines in IRAF (Tody 1986). We obtained 20
total usable spectra in this setup (Table 1).
The spectra generally appear consistent with a late G-/early
K-type star (Figure 2). The most prominent absorption lines are
Mgband Na D, along with several Fe lines. Hαand Hβare
present in absorption in some of the SOAR spectra, while in
others Hαis weak or absent. There are no clear emission lines
in any spectra.
We derived barycentric radial velocities (RVs)through cross
correlation with a high signal-to-noise template spectrum in the
region of Mgb.Wefit a circular Keplerian model to the
velocities. As the fitting spectroscopic period is consistent with
the photometric period, we fix it to the latter as the time span of
the photometry is much longer than that of the spectroscopy.
The model parameters of the fitting model are
K
2,obs
=219 ±14 km s
−1
,γ=−17 ±12 km s
−1
, and T
0
=
BJD 2459700.8091(41), where K
2,obs
is semiamplitude, γis
systemic velocity, and T
0
is the ascending node of the pulsar in
Barycentric Julian Date (BJD). This fit has a χ
2
/degree of
freedom (dof)of 38/17 and an rms of 30.9 km s
−1
, suggesting
an imperfect fit. Two of the most negative velocity measure-
ments seem to be unexpected outliers and could have
underestimated uncertainties; if these points were excluded,
the quality of the fit would be substantially improved. But we
have no specific justification for such a change, so we retain the
full data set, and acknowledge this is a preliminary character-
ization that could be improved with more data in the future.
Figure 1. Optical spectral energy distribution (SED)of 4FGL J1910.7−5320
with extinction correction. The red line is the best-fit blackbody model.
Table 1
Radial Velocities of 4FGL J1910.7−5320 from SOAR
BJD Radial Vel. Unc.
(days)(km s
−1
)(km s
−1
)
2459679.82228114 −46.4 23.6
2459679.84012259 −138.8 27.6
2459679.85806185 −171.1 24.2
2459680.82744858 94.5 27.0
2459680.84495725 66.9 31.4
2459680.86457697 −19.9 26.5
2459700.80192541 −193.2 19.9
2459700.81948426 −214.2 16.7
2459700.83923644 −191.6 18.9
2459722.66462979 15.0 23.9
2459722.68212232 −37.5 25.2
2459724.84289214 −215.2 27.0
2459724.86042189 −284.8 23.3
2459724.88435799 −237.7 33.6
2459740.69388734 220.7 22.6
2459740.71174963 175.3 24.2
2459740.80735656 −29.4 23.8
2459740.82483399 −174.2 20.1
2459740.88603764 −192.1 22.4
2459740.90351553 −287.8 21.1
Figure 2. A sample spectrum of the optical counterpart to 4FGL J1910.7
−5320 from 2022 April 11. A relative flux calibration has been applied, and
the spectrum smoothed with a 3 pixel boxcar for display. Prominent metal and
Balmer absorption lines are apparent, as described in Section 3.2.
3
The Astrophysical Journal, 943:103 (8pp), 2023 February 1 Au et al.

We refit both the CRTS and SOAR data assuming a pulsar
heating scenario (i.e., the CRTS light curve leads the SOAR
RV curve by p
2). The best-fit parameters are an orbital period
of 0.34847592(21)days, a mean CRTS magnitude of
19.015 ±0.018 mag, a CRTS amplitude of 0.506 ±0.029 mag,
K
2,obs
=218 ±8kms
−1
,γ=−17 ±6kms
−1
, and the phase
zero at BJD 2453584.0121(31). We use the best-fit parameters
of P=0.34847592 days and phase zero at BJD 2453584.0121
to fold both the CRTS light and the RV curves, which are
plotted in Figures 3and 4, respectively.
It is clear that we have incomplete phase coverage of the
source in Figure 3; missing data around f=0.25 when the
secondary is the faintest. This is not solely chance, but reflects
the observational biases induced by the observational window
available. Nonetheless, the main orbital parameters are
relatively well constrained.
Because of irradiation, K
2,obs
is not necessarily the same as
the center of mass K
2
, though no extreme changes are observed
in the optical spectra at different phases. The observed mass
function implied by the optical spectroscopy is f=0.37 ±0.02
M
e
, which is a typical value for a spider binary. Even
accounting for the uncertainty in the true K
2
value, this mass
function (which approximately represents the minimum mass
of the primary)implies that an edge-on binary inclination for a
neutron star is ruled out, and instead an intermediate inclination
is more likely.
3.3. Chandra X-Ray Analysis
The γ-ray source 4FGL J1910.7−5320 was observed with
Chandra (Weisskopf et al. 2002)for 12.6 kson 2019
November 20, and we used the Chandra data to check whether
the CRTS source has an X-ray counterpart. If so, the X-ray data
can constrain the X-ray spectral shape and X-ray variability for
the putative X-ray counterpart to 4FGL J1910.7−5320.
We used CIAO (version 4.13; Fruscione et al. 2006)to
extract the source and background spectra from the Chandra
data. We made an auxiliary response file (for both source and
background), which is an effective area calibration file where
we also applied an energy-dependent point-source aperture
correction. We generated a response matrix file to map between
the properties of the incoming photons and the electronic
signals obtained from the detector. After performing a spectral
binning with at least 20 counts per bin, we used XSPEC
9
(version 12.12.0; Arnaud 1996)from HEASARC to measure
the hydrogen column density (which will be a fixed parameter
in our spectral model)and the photon index of the X-ray source
assuming an absorbed power-law model. We also used the
dmextract task to generate the light curve with a 2″radius
circular region and 2000 s bin time. Barycentric corrections
were performed using axbary. In Figure 3, we folded
the X-ray light curve using the optical period of
P=0.34847592 days.
A significant X-ray counterpart was detected at the optical
position of the variable CRTS source. Its location is at R.A.
(J2000)=19
h
10
m
49 10 and decl.(J2000)=
-
¢53 20 57.
2
with a 90% uncertainty of 0 8. This is only 0 17 from the
CRTS variable described in Section 3.1, strongly suggesting
that they are the same source. There are 106 source counts in a
2″radius aperture and 164 counts in a nearby source-free
circular background region with a radius of 10″.Wefit the
X-ray spectrum with an absorbed power-law model with the
Galactic hydrogen column density of N
H
=5.22 ×10
20
cm
−2
(fixed; Blackburn et al. 1999; HI4PI Collaboration et al.
2016).
10
The best-fit parameters to the X-ray spectrum
(Figure 5)are a photon index of Γ=1.0 ±0.4 and an
energy flux of F
0.3–7keV
=(1.7 ±0.2)×10
−13
erg cm
−2
s
−1
Figure 3. (a)The CRTS folded light curve and (b)the X-ray folded light curve
observed by Chandra. The red line in (a)is the best fit with a sinusoidal
function. Both light curves are folded on the orbital period of P=0.34847592
days with phase zero at BJD 2453584.0121, which is the ascending node of the
pulsar.
Figure 4. Radial velocity (RV)curve of 4FGL J1910.7−5320. We folded it on
the orbital period of P=0.34847592 days with the phase zero at BJD
2453584.0121.
9
https://heasarc.gsfc.nasa.gov/xanadu/xspec
10
http://heasarc.gsfc.nasa.gov/ftools
4
The Astrophysical Journal, 943:103 (8pp), 2023 February 1 Au et al.

(χ
2
/d. o. f. =3.7/3). The 0.3–7 keV X-ray luminosity is
L
x
(3.4 ±0.4)×10
32
erg s
−1
by assuming D4.1 kpc.
We also fit the spectrum with a blackbody model, and the
result is F
0.3–7keV
=(1.26 ±0.15)×10
−13
erg cm
−2
s
−1
and a
temperature of kT =0.9 ±0.1 keV (χ
2
/dof =13.2/3). The
absorbed power-law model is statistically preferred by
comparing the χ
2
values.
3.4. Fermi-LAT Gamma-Ray Analysis
Here, we used Fermitools (version v11r5p3; Fermi Science
Support Development Team 2019)from the Fermi Science
Support Center (FSSC)
11
with the 4FGL-DR3 (Abdollahi et al.
2022)and Pass8 data (P8R3)to refine the γ-ray position and
the γ-ray spectral properties of 4FGL J1910.7−5320.
We downloaded the LAT event files and spacecraft data
from FSSC. The P8R3 data downloaded starts from 2008
August 4 to 2021 November 9 with energies in 0.1–300 GeV.
We chose the SOURCE class events (FRONT and BACK)with
a zenith angle smaller than 90°. The center of the 14°×14°
region of interest is at (α,δ)=(287°.705, −53°.349): the
4FGL-DR3 position of 4FGL J1910.7−5320. We used the
4FGL-DR3 cataloged sources located within 10°from the
target to establish the spatial and spectral model of the γ-ray
emission. The model includes the latest Galactic interstellar
(gll_iem_v07.fits)and isotropic (iso_P8R3_SOURCE_V3_v1.
txt)diffuse components. We employed a LogParabola model
for 4FGL J1910.7−5320 as suggested in the 4FGL-DR3,
which is
=
ab-+
dN
dE NE
E,2
b
0
log E
Eb
⎜⎟
⎛
⎝⎞
⎠()
(())
where αcharacterizes the photon index and βdefines the
degree of curvature for the LogParabola model. There is a total
of 33 free parameters from the source in the emission model by
allowing the background diffuse components and the sources
inside a 5°radius circle from 4FGL J1910.7−5320 to vary. We
performed a binned likelihood analysis with 37 logarithmically
uniform energy bins, which gives a test statistic (TS)value, the
significance of a certain source, of 146 (∼11.7σ
detection significance with 3 extra parameters), a 0.1–100 GeV
energy flux of F
0.1–100 GeV
=(2.6 ±0.4)×10
−12
erg cm
−2
s
−1
,
α=2.2 ±0.2, and β=0.3 ±0.2 (Figure 6). The 0.1–100 GeV
γ-ray luminosity is L
γ
(5.3 ±0.8)×10
33
erg s
−1
by assum-
ing D4.1 kpc. Using gtfindsrc,werefined the 68% error
circle of 4FGL J1910.7−5320 to a circular region with 2 1
radius centered at (α,δ)=(287°.691, −53°.330), which
includes the CRTS optical source (Figure 7).
4. 4FGL J2029.5−4237
The possible optical counterpart to 4FGL J2029.5−4237 is
located at R.A.(J2000)=20
h
29
m
34 21 and decl.(J2000)=
-
¢42 33 17. 6, inside the 95% error region of the γ-ray source.
Following a similar procedure to the analysis of 4FGL
J1910.75320, the best-fit period of the optical variable is
0.178235614(93)days with a mean magnitude of
14.264 ±0.002 mag and an amplitude of 0.183 ±0.003 mag.
We find no X-ray counterpart in a 11.7 ks Chandra observation
on 2019 September 1, leading to a 3σflux upper limit of
F
0.3–7 keV
=2.4 ×10
−14
erg cm
−2
s
−1
, where we assumed the
photon index to be 2 and the Galactic column density of N
H
=
3.7 ×10
20
cm
−2
(fixed; Blackburn et al. 1999; HI4PI
Collaboration et al. 2016).
12
The corresponding 3σluminosity
upper limit is L
0.3–7 keV
=(3.8 ±0.2)×10
30
erg s
−1
by assum-
ing the Gaia DR3 (Gaia Collaboration 2020)distance of
D=1.14 ±0.03 kpc.
In the Fermi-LAT analysis, the P8R3 data used spans from
2008 August 4 to 2020 March 25 for 4FGL J2029.5−4237. Using
a LogParabola model suggested in 4FGL-DR3, the best-fitTS
value is 119 (equivalent to a >10σdetection significance), with a
0.1–100 GeV energy flux of F
0.1–100 GeV
=(2.0 ±0.2)×10
−12
erg cm
−2
s
−1
,α=2.4 ±0.2, and β=9.998 (fixed in 4FGL-DR3,
likely because it reaches the maximum limit of the parameter
space). The updated 68% error circle of 4FGL J2029.5−4237
is located at (α,δ)=(307°407, −42°651)with 2 4radius.
However, the CRTS optical source is outside this improved 95%
error circle (shown in Figure 8), strongly suggesting that the
CRTS source is unrelatedto4FGLJ2029.5−4237.
5. Discussion
We find a candidate optical/X-ray counterpart to 4FGL
J1910.7−5320, and the observational results strongly suggest
Figure 5. The X-ray spectrum in the 0.3–7 keV energy band of 4FGL J1910.7
−5320. It was obtained by the best-fit power-law model.
Figure 6. The γ-ray spectrum (Fermi-LAT; 0.1–100 GeV)of 4FGL J1910.7
−5320 with the best-fit LogParabola spectral model indicated by the red line.
11
https://fermi.gsfc.nasa.gov/ssc
12
http://heasarc.gsfc.nasa.gov/ftools
5
The Astrophysical Journal, 943:103 (8pp), 2023 February 1 Au et al.

that it is a redback MSP binary. We summarize the properties
of this redback candidate below:
1. An updated γ-ray 68% error circle with 2 1 radius is
presented in Figure 7and the optical source is still inside
the error circle.
2. A compact orbit of P=8.36 hr and K
2,obs
=218 ±8
km s
−1
, which is consistent with the orbital properties of
many redbacks (Hui & Li 2019; Strader et al. 2019).
3. A single peak orbital light curve of 4FGL J1910.7−5320
(Figure 3)implies that the optical emission probably is
pulsar heating dominated.
4. An X-ray counterpart to the optical source is also found
in Chandra. A hard photon index of Γ=1.0 ±0.4 was
shown in X-rays and the emission exhibits tentative
evidence for periodic modulation.
5.1. X-Ray Orbital Modulation of 4FGL J1910.7−5320
As we mentioned in Section 3.3 and Figure 3, the X-ray and
optical sources probably are the counterparts to 4FGL J1910.7
−5320. This association can be further confirmed, if the X-ray
and optical phased light curves have any relation.
In order to explore the X-ray modulation, we folded the
Chandra X-ray light curve on the orbital period P=
0.34847592 days (Figure 3). Although the observation does
not cover a complete orbit, the folded light curve shows a
possible dip at phase 0.75 (Figure 3). If the X-ray orbital
modulation is real, it could be due to the Doppler boosting
effect in the intrabinary shock (Li et al. 2014; Takata et al.
2014; Kong et al. 2017). At phase 0.75 (Figure 3), the stellar
companion is behind the presumed pulsar as seen from Earth. If
the momentum flux of the pulsar wind is stronger than that of
the stellar wind from the companion, the intrabinary shock can
wrap the companion. This means the shocked wind does not
point toward Earth, and the X-ray emission therefore decreases,
consistent with the Chandra light curve of 4FGL J1910.7
−5320. Furthermore, the intrabinary shock could produce
synchrotron X-ray emission that can be described by a power-
law spectral model. This naturally explains the observed
Chandra X-ray emission, which appears nonthermal with
Γ=1.0 ±0.4. Unfortunately, the current X-ray and optical
data sets do not allow a detailed investigation. Deeper and
longer observations in X-rays and the optical band (with color
information)are required in the future.
5.2. 4FGL J1910.7−5320 as a Redback or Black Widow?
From Figure 3, the optical phased light curve of 4FGL
J1910.7−5320 shows an obvious one-peak signature. Since the
pulsar heating effect is usually more prominent for black
widows than for redbacks, this light curve feature is more
consistent with that of a black widow. The optical peak and
X-ray peak are shifted by half an orbit, which is similar to the
original black widow PSR B1957 +20 (Fruchter et al. 1988).
However, there are also several black widow examples that
show zero phase shift between the optical and X-ray peaks,
such as PSR J1124−3653, PSR J1653−0158, and PSR J2256
−1024 (Gentile et al. 2014; Long et al. 2022). Therefore, the
evidence is not particularly strong to say that 4FGL J1910.7
−5320 is a black widow candidate.
In Section 3.1, we assumed the size of the companion in the
case of the redback (i.e., R0.7 R
☉
)and black widow (i.e.,
R0.3 R
☉
)based on a Roche lobe description, and estimated
the distance to be D4.1 kpc and 1.8 kpc, respectively. If the
system is a black widow MSP, it will be relatively close to us.
In fact, the system will be much closer if it is a black widow,
because the companion size of a black widow is generally
smaller than the Roche lobe radius (Draghis et al. 2019).We
also performed another independent distance estimation by
comparing the magnitude and the distance of 4FGL J1910.7
−5320 to the original black widow, PSR B1957 +20, of which
the distance is D=1.5–2.5 kpc (Taylor & Cordes 1993; Cordes
& Lazio 2002; van Kerkwijk et al. 2011)and the minimum
optical magnitude in the Rband is 24.6 mag (Reynolds et al.
2007). In 4FGL J1910.7−5320, the minimum best-fit magni-
tude in the CRTS catalog is 19.62 mag (∼20.5 for the faintest
data), which is about ∼5 mag brighter than the minimum
magnitude of PSR B1957 +20. This implies that if 4FGL
J1910.7−5320 is a black widow MSP similar to PSR
B1957 +20, the distance to 4FGL J1910.7−5320 is
D∼0.15–0.25 kpc, which is extremely close to us. At this
Figure 7. The Digitized Sky Survey (DSS)image (left)and the Chandra X-ray image (right)of 4FGL J1910.7−5320. The blue ellipses show the 95% error ellipse of
4FGL catalog. The magenta circle is the updated LAT error circle at a 68% confidence level. The small red cross is the X-ray/optical position determined by Chandra
(Section 3.3). The two upper middle inset boxes are the zoomed-in view of the optical and X-ray counterparts.
6
The Astrophysical Journal, 943:103 (8pp), 2023 February 1 Au et al.

short distance, Gaia should be able to measure the parallax, and
hence, the distance of 4FGL J1910.7–5320 easily. However,
the parallax is not well constrained in the Gaia Catalog DR3
(Gaia Collaboration 2020). Therefore, interpreting 4FGL
J1910.7−5320 as a redback MSP system is favored.
5.3. Strong Irradiation Signature of 4FGL J1910.7−5320?
We notice that the irradiation signature is relatively strong
(the amplitude is over 1 mag)compared to other redbacks. To
check this phenomenon systematically, we used the CRTS
catalog (Drake et al. 2009)to find the modulation amplitude of
other known redback systems. We find that there are only two
redbacks, PSR J2215 +5135 (Breton et al. 2013)and PSR
J2339−0533 (Romani & Shaw 2011)that have a high
irradiation signature (amplitude >1 mag)among the 14
known redbacks (Hui & Li 2019; Strader et al. 2019).A
dedicated statistical study of irradiation signatures of redbacks
will be published elsewhere.
5.4. 4FGL J2029.5−4237
We found a variable optical source inside the 95% error ellipse
of 4FGL J2029.5−4237 and speculated that it is another MSP
binary candidate. In the 11.7 ks Chandra observation, we found
no significant X-ray source spatially coincident with the optical
variable, with a 3σflux upper limit of 2.4 ×10
−14
erg cm
−2
s
−1
(0.3–7keV). The corresponding luminosity is L=3.8 ×10
30
erg s
−1
by using the Gaia distance of 1.14 kpc (Gaia
Collaboration 2020), which is considerably lower than in many
other MSPs. The optical source is also outside the updated 95%
LAT error circle (Figure 8). These results strongly suggest that the
variable optical source is probably not the counterpart to 4FGL
J2029.5−4237 and unlikely a redback MSP.
While we were in the late stages of preparing this paper,
TRAPUM discovered the radio pulsations associated with
4FGL J1910.7−5320 (i.e., PSR J1910−5320; C. Clark 2022,
private communication),
13
and confirms that 4FGL J1910.7
−5320 is a redback MSP. They also found that 4FGL J2029.5
−4237 is an isolated MSP, which is unrelated to the optical
source reported in this paper.
The Fermi-LAT Collaboration acknowledges generous
ongoing support from a number of agencies and institutes that
have supported both the development and the operation of the
LAT as well as scientific data analysis. These include the
National Aeronautics and Space Administration and the
Department of Energy in the United States, the Commissariatà
l’Energie Atomique and the Centre National de la Recherche
Scientifique/Institut National de Physique Nucléaire et de
Physique des Particules in France, the Agenzia Spaziale
Italiana and the Istituto Nazionale di Fisica Nucleare in Italy,
the Ministry of Education, Culture, Sports, Science and
Technology (MEXT), High Energy Accelerator Research
Organization (KEK)and Japan Aerospace Exploration Agency
(JAXA)in Japan, and the K. A. Wallenberg Foundation, the
Swedish Research Council and the Swedish National Space
Board in Sweden. Additional support for science analysis
during the operations phase from the following agencies is also
gratefully acknowledged: the Istituto Nazionale di Astrofisica
in Italy and and the Centre National d’Etudes Spatiales in
France. This work performed in part under DOE contract DE-
AC02-76SF00515.
This research has made use of data obtained from the
Chandra Data Archive and the Chandra Source Catalog, and
software provided by the Chandra X-ray Center (CXC)in the
application packages CIAO and Sherpa.
Based on observations obtained at the Southern Astrophy-
sical Research (SOAR)telescope, which is a joint project of the
Ministério da Ciência, Tecnologia e Inovações (MCTI/LNA)
do Brasil, the US National Science Foundations NOIRLab, the
University of North Carolina at Chapel Hill (UNC), and
Michigan State University (MSU).
The CRTS survey is supported by the U.S. National Science
Foundation under grants AST- 0909182 and AST-1313422.
K.Y.A. and K.L.L. are supported by the National Science
and Technology Council of the Republic of China (Taiwan)
through grant 111-2636-M-006-024, and K.L.L. is also a
Figure 8. The DSS image (left)and the Chandra X-ray image (right)of 4FGL J2029.5−4237. The blue ellipses show the 95% error ellipse of the 4FGL catalog. The
magenta concentric circles present the updated LAT error circles at a 68% (inside)and a 95% (outside)confidence level. The small red cross is the position determined
by the CRTS survey.
13
http://www.trapum.org/discoveries/
7
The Astrophysical Journal, 943:103 (8pp), 2023 February 1 Au et al.

Yushan Young Fellow supported by the Ministry of Education
of the Republic of China (Taiwan).
J.S. acknowledges support by NSF grant AST-2205550 and
the Packard Foundation. This research was performed while
SJS held a NRC Research Associateship award at the Naval
Research Laboratory. Work at the Naval Research Laboratory
is supported by NASA DPR S-15633-Y.
C.Y.H. is supported by the National Research
Foundation of Korea through grants 2016R1A5A1013277
and 2022R1F1A1073952.
Facilities: Fermi, CXO, SOAR.
Software: CIAO (version 4.13; Fruscione et al. 2006),
HEASOFT (Nasa High Energy Astrophysics Science Archive
Research Center, Heasarc, 2014), FERMITOOLS (version
v11r5p3; Fermi Science Support Development Team 2019).
ORCID iDs
Jay Strader https://orcid.org/0000-0002-1468-9668
Samuel J. Swihart https://orcid.org/0000-0003-1699-8867
Lupin C. C. Lin https://orcid.org/0000-0003-4083-9567
Albert K. H. Kong https://orcid.org/0000-0002-5105-344X
Jumpei Takata https://orcid.org/0000-0002-8731-0129
Chung-Yue Hui https://orcid.org/0000-0003-1753-1660
Teresa Panurach https://orcid.org/0000-0001-8424-2848
Elias Aydi https://orcid.org/0000-0001-8525-3442
Kirill Sokolovsky https://orcid.org/0000-0001-5991-6863
Kwan-Lok Li https://orcid.org/0000-0001-8229-2024
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