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Register a new userA millisecond pulsar candidate in a 21-hr orbit: 3FGL J0212.1+5320
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Manuel Linares
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We present the discovery of a variable optical counterpart to the unidentified gamma-ray source 3FGL J0212.1+5320, and argue this is a new compact binary millisecond pulsar (MSP) candidate. We show 3FGL J0212.1+5320 hosts a semi-detached binary with a 0.86955$pm$0.00015 d orbital period and a F6-type companion star at an estimated distance of D=1.1$pm$0.2 kpc, with a radial velocity curve semi-amplitude K$_2$=214.1$pm$5.0 km s$^{-1}$ and a projected rotational velocity of Vsin(i)=73.2$pm$1.6 km s$^{-1}$. We find a hard X-ray source at the same location with a 0.5$-$10 keV luminosity L$_mathrm{X}$=2.6$times$10$^{32}$ (D/1.1 kpc)$^2$ erg s$^{-1}$, which strengthens the MSP identification. Our results imply a mass ratio q=M$_2$/M$_1$=0.26$^{+0.02}_{-0.03}$ if the companion star fills its Roche lobe, and q$gtrsim$0.26 in any case. This classifies 3FGL J0212.1+5320 as a redback binary MSP; if its MSP nature is confirmed, this will be the brightest compact binary MSP in the optical band (r$simeq$14.3 mag) and will have the longest orbital period among Galactic field systems (nearly 21 hr). Based on the light curve peak-to-peak amplitude ($Delta$r=0.19 mag), we further suggest that the orbital inclination is high and the putative pulsar mass is close to canonical (M$_1$$simeq$1.3$-$1.6 M$_odot$). Finally, we discuss the lack of heating signatures and asymmetric optical light curves in the context of other redback MSPs.
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We present a multi-wavelength study of the unidentified Fermi object, 3FGL J0212.1+5320. Within the 95% error ellipse, Chandra detects a bright X-ray source (i.e., F(0.5-7keV) = 1.4e-12 erg/cm^2/s), which has a low-mass optical counterpart (M < 0.4 Msun and T ~ 6000 K). A clear ellipsoidal modulation is shown in optical/infrared at 20.87 hours. The gamma-ray properties of 3FGL J0212.1+5320 are all consistent with that of a millisecond pulsar, suggesting that it is a gamma-ray redback millisecond pulsar binary with a low-mass companion filling > 64% of the Roche-lobe. If confirmed, it will be a redback binary with one of the longest orbital periods known. Spectroscopic data taken in 2015 from the Lijiang observatory show no evidence of strong emission lines, revealing that the accretion is currently inactive (the rotation-powered pulsar state). This is consistent with the low X-ray luminosities (Lx ~ 10^32 erg/s) and the possible X-ray modulation seen by Chandra and Swift. Considering that the X-ray luminosity and the high X-ray-to-gamma-ray flux ratio (8%) are both comparable to that of the two known gamma-ray transitional millisecond pulsars, we suspect that 3FGL J0212.1+5320 could be a potential target to search for future transition to the accretion active state.
Linares et al. (2016) obtained quasi-simultaneous g, r and i-band light curves and an absorption line radial velocity curve of the secondary star in the redback system 3FGL J0212.1+5320. The light curves showed two maxima and minima primarily due to the secondary stars ellipsoidal modulation, but with unequal maxima and minima. We fit these light curves and radial velocities with our X-ray binary model including either a dark solar-type star spot or a hot spot due to off-centre heating from an intrabinary shock, to account for the unequal maxima. Both models give a radial velocity semi-amplitude and rotational broadening that agree with the observations. The observed secondary stars effective temperature is best matched with the value obtained using the hot spot model, which gives a neutron star and secondary star mass of $M_{rm 1}$=1.85$^{+0.32}_{-0.26}$ $M_{odot}$and $M_{rm 2}$=0.50$^{+0.22}_{-0.19}$ $M_{odot}$, respectively.
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We present the orbital solution for the donor star of the candidate transitional millisecond pulsar 3FGL J1544.6-1125, currently observed as an accreting low-mass X-ray binary. The orbital period is $0.2415361(36)$ days, entirely consistent with the spectral classification of the donor star as a mid to late K dwarf. The semi-amplitude of the radial velocity curve is exceptionally low at $K_2=39.3pm1.5$ km s$^{-1}$, implying a remarkably face-on inclination in the range 5--8$^{circ}$, depending on the neutron star and donor masses. After determining the veiling of the secondary, we derive a distance to the binary of $3.8pm0.7$ kpc, yielding a 0.3--10 keV X-ray luminosity of $6.1pm1.9times10^{33},{rm erg,s^{-1}}$, similar to confirmed transitional millisecond pulsars. As face-on binaries rarely occur by chance, we discuss the possibility that Fermi-selected samples of transitional milli-second pulsars in the sub-luminous disk state are affected by beaming. By phasing emission line strength on the spectroscopic ephemeris, we find coherent variations, and argue that these variations are most consistent with emission from an asymmetric shock originating near the inner disk.
We report on the detection of a 400.99018734(1) Hz coherent signal in the Rossi X-ray Timing Explorer light curves of the recently discovered X-ray transient, IGR J17498-2921. By analysing the frequency modulation caused by the orbital motion observed between August 13 and September 8, 2011, we derive an orbital solution for the binary system with a period of 3.8432275(3) hr. The measured mass function, f(M_2, M_1, i)=0.00203807(8) Msun, allows to set a lower limit of 0.17 Msun on the mass of the companion star, while an upper limit of 0.48 Msun is set by imposing that the companion star does not overfill its Roche lobe. We observe a marginally significant evolution of the signal frequency at an average rate of -(6.3 +/- 1.9)E-14 Hz/s. The low statistical significance of this measurement and the possible presence of timing noise hampers a firm detection of any evolution of the neutron star spin. We also present an analysis of the spectral properties of IGR J17498-2921 based on the observations performed by the Swift-X-ray Telescope and the RXTE-Proportional Counter Array between August 12 and September 22, 2011. During most of the outburst, the spectra are modeled by a power-law with an index Gamma~1.7-2, while values of ~3 are observed as the source fades into quiescence.
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