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 3σ 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.
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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 (MC ≳ 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; Stappers et al. 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 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 investigated. For 4FGL J1910.7−5320, we present the multiwavelength 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 5σ and 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) = 19h10m4912, decl.(J2000) =
. We fit 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 Av
= 0.1857 mag and the ratio of total to selective absorption of Rv
= 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 (R/R☉) kpc where R is the companion radius.
Figure 1. Optical spectral energy distribution (SED) of 4FGL J1910.7−5320 with extinction correction. The red line is the best-fit blackbody model.
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Standard image High-resolution imageAssuming that the companion nearly fills up the Roche lobe, we used the approximate formula from Eggleton (1983), which is
![Equation (1)](https://content.cld.iop.org/journals/0004-637X/943/2/103/revision1/apjacae8aeqn1.gif)
where q = MC /MMSP (MC : mass of companion; MMSP: mass of MSP) and a is 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, we find 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., MC = 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 simultaneously. 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 Collaboration 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 12 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).
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 |
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The spectra generally appear consistent with a late G-/early K-type star (Figure 2). The most prominent absorption lines are Mgb and Na D, along with several Fe lines. Hα and Hβ are present in absorption in some of the SOAR spectra, while in others Hα is weak or absent. There are no clear emission lines in any spectra.
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.
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Standard image High-resolution imageWe derived barycentric radial velocities (RVs) through cross correlation with a high signal-to-noise template spectrum in the region of Mgb. We fit 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 K2,obs = 219 ± 14 km s−1, γ = − 17 ± 12 km s−1, and T0 = BJD 2459700.8091(41), where K2,obs is semiamplitude, γ is systemic velocity, and T0 is the ascending node of the pulsar in Barycentric Julian Date (BJD). This fit has a χ2/ 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 measurements 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 characterization that could be improved with more data in the future.
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 ). 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, K2,obs = 218 ± 8 km s−1, γ = − 17 ± 6 km s−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 3 and 4, respectively.
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.
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Standard image High-resolution imageFigure 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.
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Standard image High-resolution imageIt is clear that we have incomplete phase coverage of the source in Figure 3; missing data around ϕ = 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, K2,obs is not necessarily the same as the center of mass K2, 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⊙, which is a typical value for a spider binary. Even accounting for the uncertainty in the true K2 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) = 19h10m4910 and decl.(J2000) =
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''. We fit the X-ray spectrum with an absorbed power-law model with the Galactic hydrogen column density of NH = 5.22 × 1020 cm−2 (fixed; Blackburn et al. 1999; HI4PI Collaboration et al. 2016).
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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 F0.3–7keV = (1.7 ± 0.2) × 10−13 erg cm−2 s−1 (χ2/d. o. f. = 3.7/3). The 0.3–7 keV X-ray luminosity is Lx
≲ (3.4 ± 0.4) × 1032 erg s−1 by assuming D ≲ 4.1 kpc.
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.
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Standard image High-resolution imageWe also fit the spectrum with a blackbody model, and the result is F0.3–7keV = (1.26 ± 0.15) × 10−13 erg cm−2 s−1 and a temperature of kT = 0.9 ± 0.1 keV (χ2/dof = 13.2/3). The absorbed power-law model is statistically preferred by comparing the χ2 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) = (287705, − 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
![Equation (2)](https://content.cld.iop.org/journals/0004-637X/943/2/103/revision1/apjacae8aeqn2.gif)
where α characterizes the photon index and β defines 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 F0.1–100 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 Lγ
≲ (5.3 ± 0.8) × 1033 erg s−1 by assuming D ≲ 4.1 kpc. Using gtfindsrc, we refined the 68% error circle of 4FGL J1910.7−5320 to a circular region with 21 radius centered at (α, δ) = (287°.691, −53
330), which includes the CRTS optical source (Figure 7).
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.
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Standard image High-resolution imageFigure 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.
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Standard image High-resolution image4. 4FGL J2029.5−4237
The possible optical counterpart to 4FGL J2029.5−4237 is located at R.A.(J2000) = 20h29m3421 and decl.(J2000) =
, 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 3σ flux upper limit of F0.3–7 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 NH
= 3.7 × 1020 cm−2 (fixed; Blackburn et al. 1999; HI4PI Collaboration et al. 2016).
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The corresponding 3σ luminosity upper limit is L0.3–7 keV = (3.8 ± 0.2) × 1030 erg s−1 by assuming 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-fit TS value is 119 (equivalent to a >10σ detection significance), with a 0.1–100 GeV energy flux of F0.1–100 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 24 radius. However, the CRTS optical source is outside this improved 95% error circle (shown in Figure 8), strongly suggesting that the CRTS source is unrelated to 4FGL J2029.5−4237.
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.
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Standard image High-resolution image5. Discussion
We find a candidate optical/X-ray counterpart to 4FGL J1910.7−5320, and the observational results strongly suggest 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 7 and the optical source is still inside the error circle.
- 2.
- 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 R band is 24.6 mag (Reynolds et al. 2007). In 4FGL J1910.7−5320, the minimum best-fit magnitude 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 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 3σ flux upper limit of 2.4 × 10−14 erg cm−2 s−1 (0.3–7 keV). The corresponding luminosity is L = 3.8 × 1030 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 Astrophysical 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 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 - Fermi Gamma-Ray Space Telescope (formerly GLAST), 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).
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