PSR J1306-40: An X-Ray Luminous Redback with an Evolved Companion

Abstract

PSR J1306-40 is a millisecond pulsar binary with a non-degenerate companion in an unusually long ∼1.097 day orbit. We present new optical photometry and spectroscopy of this system, and model these data to constrain fundamental properties of the binary such as the component masses and distance. The optical data imply a minimum neutron star mass of 1.75 ± 0.09 M (1-sigma) and a high, nearly edge-on inclination. The light curves suggest a large hot spot on the companion, suggestive of a portion of the pulsar wind being channeled to the stellar surface by the magnetic field of the secondary, mediated via an intrabinary shock. The Hα line profiles switch rapidly from emission to absorption near companion inferior conjunction, consistent with an eclipse of the compact emission region at these phases. At our optically-inferred distance of 4.7 ± 0.5 kpc, the X-ray luminosity is ∼10 33 erg s -1 , brighter than nearly all known redbacks in the pulsar state. The long period, subgiant-like secondary, and luminous X-ray emission suggest this system may be part of the expanding class of millisecond pulsar binaries that are progenitors to typical field pulsar-white dwarf binaries.

Identifying the formation mechanism of redback pulsars

“…As we can see from the Table, most of the RBs do exhibit high filling factor values. In the cases where F F has an estimation with F F < 1, the minimum value is of 0.70(6) for PSR J1431-4715 whereas the maximum value is of 0.92(6) for PSR J1306-40 (Swihart, et al 2019). 4 The minimum mass is computed considering a pulsar mass of 1.4 M ⊙ and an inclination of 90 o .…”

Discovery of a New Redback Millisecond Pulsar Candidate: 4FGL J0940.3-7610

“…Overall, the spectra of the optical source are consistent with that of a moderately cool, late-G to early-K type dwarf star (we reach similar conclusions from our light curve fitting in § 3.2). This spectral type is typical of redback secondaries (e.g., Bellm et al 2016;Linares et al 2018;Swihart et al 2019Swihart et al , 2020. There is no evidence for a second set of absorption lines in any of the optical spectra, as would be expected if this star was in a binary with a main sequence or evolved star, suggesting the companion is a compact object.…”

“…When setting the companion mass equal to the approximate median redback companion mass (0.4 M ; Strader et al 2019), the best fit inclination is 81.9 +7.9 −12.1 , corresponding to a primary mass of 1.26 +0.15 −0.03 M . If the companion is on the more massive end of the redback distribution, approaching 0.6 M , such as the confirmed redback PSR J1306-40 (Keane et al 2018;Linares 2018;Swihart et al 2019) or the candidate redback 3FGL J0212.1+5320 (Li et al 2016;Shahbaz et al 2017), then the best fit inclination is i = 87.7 +2.3 −13.9 corresponding to M 1 = 1.43 +0.11 −0.01 M . These models are very good fits to the data and are nearly identical statistically, with χ 2 /d.o.f.…”

“…Given the evidence for irradiative heating and the likely strong magnetic fields induced on the tidally-locked (and thereby rapidly-rotating) companion's surface, it is possible that accelerated intrabinary shock particles are magnetically-ducted to the surface, mimicking one or more hot-spots (e.g., Sanchez & Romani 2017). These spots are one way to cause irregular heating patterns like these in the light curves of redback secondaries if the spots are shifted in azimuth to be off the line connecting the primary and secondary (e.g., Strader et al 2019;Swihart et al 2019). We do not attempt to model these spots in this discovery paper, but we suggest these more complex heating models be explored when future data become available.…”

4FGL J1120.0-2204: A Unique Gamma-ray Bright Neutron Star Binary with an Extremely Low Mass Proto-White Dwarf

“…One approach that has been used successfully to identify MSP counterparts in the unassociated Fermi sources is to search for X-ray/optical matches inside the LAT error ellipses. X-rays in these binary systems are often produced by an intrabinary shock at the interface between the pulsar and companion winds (e.g., Gentile et al 2014;Romani & Sanchez 2016;Wadiasingh et al 2017), while the non-degenerate stellar companions typically display optical periodic variability due to tidal distortion of the secondary and/or irradiative heating of the tidally-locked companion surface (e.g., McConnell et al 2015;Sanchez & Romani 2017;Cho et al 2018;Swihart et al 2018;Draghis et al 2019;Swihart et al 2019;Yap et al 2019;Swihart et al 2021).…”