Galactic Ultracompact X-Ray Binaries: Disk Stability and Evolution

Abstract

We study the mass-transfer rates and disk stability conditions of ultracompact X-ray binaries (UCXBs) using empirical time-averaged X-ray luminosities from Paper I and compiled information from the literature. The majority of UCXBs are consistent with evolutionary tracks for white dwarf donors. Three UCXBs with orbital periods longer than 40 minutes have mass-transfer rates above 10 −10 M yr −1 , inconsistent with white dwarf donor tracks. We show that if helium star donors can retain their initial high entropy, they can explain the observed mass-transfer rates of these UCXBs. Several UCXBs show persistent luminosities apparently below the disk instability limit for irradiated He accretion disks. We point out that a predominantly C and/or O disk (as observed in the optical spectra of several) lowers the disk instability limit, explaining this disagreement. The orbital period and low time-averaged mass-transfer rate of 2S 0918-549 provide evidence that the donor star is a low-entropy C/O white dwarf, consistent with optical spectra. We combine existing information to constrain the masses of the donors in 4U 1916-053 (0.064 ± 0.010 M ) and 4U 1626-67 (<0.036 M for a 1.4 M neutron star). We show that 4U 1626-67 is indeed persistent, and not undergoing a transient outburst, leaving He star models as the best explanation for the donor.

Novel modelling of ultracompact X-ray binary evolution – stable mass transfer from white dwarfs to neutron stars

“…3 and 4. The observed UCXBs (Heinke et al 2013) are shown with red (persistent source) and blue (transient source) symbols. UCXBs marked with squares, triangles and circles correspond to systems with no information of spectra, with helium and without helium lines, respectively.…”

“…2, we notice that our UCXB tracks can explain the location of the observed systems quite well. The data were taken from Heinke et al (2013) and the error bars on | M 2 | reflect bolometric correction uncertainty from the observed X-ray luminosity. The seven persistent and short-orbital period UCXBs (P orb ≃ 10 − 25 min) are either located on the ascending or the descending branch.…”

“…red track for P orb,i = 3.25 days). Alternatively, for a WD donor, a CB disc model can also account for these sources as a result of a significant increase in | M 2 | (Heinke et al 2013). All the five transient systems likely populate the declining branch of the evolutionary tracks.…”

“…Due to their wider orbits, the radius of their accretion disc is larger and its temperature lower, which causes thermal viscous instabilities and thus a transient behavior (Lasota et al 2008). Heinke et al (2013) suggested that this might be the reason why so relatively few UCXBs are seen in wide orbits with P orb > 30 min (keeping in mind that UCXBs should accumulate in wide orbits over time). The transient behavior allows radio MSPs to turn on, whereby the "radio ejection mechanism" (Burderi et al 2002) can prevent further accretion.…”

“…Due to continuous emission of gravitational waves, this system will "merge" in about 400 Myr. However, rather than resulting in a catastrophic event, once the WD fills its Roche lobe, the outcome is expected to be a long-lived ultracompact X-ray binary (UCXB, Webbink 1979;Nelson et al 1986;Podsiadlowski et al 2002;Nelemans et al 2010;van Haaften et al 2012a;Heinke et al 2013). These sources are tight low-mass X-ray binaries (LMXBs) observed with an accreting neutron star (NS) and a typical orbital period of less than 60 min.…”