The evolution of ultracompact X-ray binaries

Abstract

Context. Ultracompact X-ray binaries (UCXBs) typically consist of a white dwarf donor and a neutron star or black hole accretor. The evolution of UCXBs and very low mass ratio binaries in general is poorly understood. In particular, the dynamical behavior of an accretion disk extending to a large radius (relative to the orbit) is unclear. Aims. We investigate the evolution of UCXBs in order to learn for which mass ratios and accretor types these systems can exist, and if they do, what are their orbital and neutron star spin periods, mass transfer rates and evolutionary timescales. Methods. We compute tracks of a binary containing a Roche-lobe overflowing helium white dwarf in which mass transfer is driven by gravitational wave emission. For different assumptions concerning accretion disk behavior we calculate for which system parameters dynamical instability, thermal-viscous disk instability or the propeller effect emerge. The significance of these processes during the evolution of an UCXB is considered. Results. At the onset of mass transfer, the survival of the UCXB is determined by how efficiently the accretor can eject matter in the case of a super-Eddington mass transfer rate. At later times, the evolution of systems strongly depends on the binary's capacity to return angular momentum from the disk to the orbit. We find that this feedback mechanism most likely remains effective even at very low mass ratio. In the case of steady mass transfer, the propeller effect can stop accretion onto recycled neutron stars completely at a sufficiently low mass transfer rate, based on energy considerations. However, mass transfer will likely be non-steady because disk instability allows for accretion of some of the transferred matter. Together, the propeller effect and disk instability cause the low mass ratio UCXBs to be visible a small fraction of the time at most, thereby explaining the lack of observations of such systems. Conclusions. Most likely UCXBs avoid late-time dynamically unstable mass loss from the donor and continue to evolve as the age of the Universe allows. This implies the existence of a large population of low mass ratio binaries with orbital periods ∼70-80 min, unless some other mechanism has destroyed these binaries. Even though none have been discovered yet, black hole UCXBs could also exist, at orbital periods of typically 100-110 min.

Disc reflection and a possible disc wind during a soft X-ray state in the neutron star low-mass X-ray binary 1RXS J180408.9–342058

“…UCXBs are X-ray binaries in which the donor is an evolved star in a tight P orb 90 min orbit. Baglio et al (2016) estimated the time-averaged mass-accretion rate of RXS J1804 based on RXTE/ASM and MAXI monitoring, and concluded that this is broadly consistent with the source having a He-rich donor in a ≃40 min orbit, based on a comparison with the disk instability model (Lasota (2008); see also the evolutionary tracks of van Haaften et al (2012)). …”

Population synthesis of ultracompact X-ray binaries in the Galactic bulge

“…For each donor composition, the evolution after the stage described in Sect. 2.3.1 follows the tracks described in van Haaften et al (2012b). Initially, the white dwarf donor has not yet cooled and therefore is larger than a zerotemperature white dwarf of the same mass.…”

“…For a 1.4 M neutron star companion, white dwarfs with a mass higher than ∼0.83 M experience dynamically unstable mass transfer, assuming a zero-temperature (completely degenerate) mass-radius relation for the donor (e.g. Yungelson et al 2002;van Haaften et al 2012b). This assumption implies that these white dwarfs have cooled considerably by the time they eventually fill their Roche lobe, although tidal heating and irradiation before the onset of mass transfer may counteract this for a short time 2 .…”

“…van den Heuvel & Bonsema 1984;Fryer et al 1999;Paschalidis et al 2011;Metzger 2012). Furthermore, in systems with a donor mass larger than ∼0.38 M (Yungelson et al 2002;van Haaften et al 2012b) (this value is only weakly sensitive to accretor mass) the accretor will be unable to eject enough transferred matter from the system by isotropic re-emission, where most arriving matter leaves the vicinity of the accretor in a fast, isotropic wind powered by accretion (Soberman et al 1997;Tauris & Savonije 1999). This also leads to a merger.…”

Long-term luminosity behavior of 14 ultracompact X-ray binaries

“…For two sources the average count rate is negative so the circle is not shown. The vertical dotted line at 28 min represents the orbital period above which a helium accretion disk becomes subject to the thermalviscous instability, based on the zero-temperature white dwarf donor model by van Haaften et al (2012b).…”