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

Abstract

Aims.We model the present-day number and properties of ultracompact X-ray binaries (UCXBs) in the Galactic bulge. The main objective is to compare the results to the known UCXB population as well as to data from the Galactic Bulge Survey, in order to learn about the formation of UCXBs and their evolution, such as the onset of mass transfer and late-time behavior. Methods. The binary population synthesis code SeBa and detailed stellar evolutionary tracks have been used to model the UCXB population in the Bulge. The luminosity behavior of UCXBs has been predicted using long-term X-ray observations of the known UCXBs as well as the thermal-viscous disk instability model. Results. In our model, the majority of UCXBs initially have a helium burning star donor. Of the white dwarf donors, most have helium composition. In the absence of a mechanism that destroys old UCXBs, we predict (0.2−1.9) × 10 5 UCXBs in the Galactic bulge, depending on assumptions, mostly at orbital periods longer than 60 min (a large number of long-period systems also follows from the observed short-period UCXB population). About 5−50 UCXBs should be brighter than 10 35 erg s −1 , mostly persistent sources with orbital periods shorter than about 30 min and with degenerate helium and carbon-oxygen donors. This is about one order of magnitude more than the observed number of (probably) three. Conclusions. This overprediction of short-period UCXBs by roughly one order of magnitude implies that fewer systems are formed, or that a super-Eddington mass transfer rate is more difficult to survive than we assumed. The very small number of observed longperiod UCXBs with respect to short-period UCXBs, the surprisingly high luminosity of the observed UCXBs with orbital periods around 50 min, and the properties of the PSR J1719−1438 system all point to much faster UCXB evolution than expected from angular momentum loss via gravitational wave radiation alone. Old UCXBs, if they still exist, probably have orbital periods longer than 2 h and have become very faint due to either reduced accretion or quiescence, or have become detached. UCXBs are promising candidate progenitors of isolated millisecond radio pulsars.

MAXI J1957+032: An Accreting Neutron Star Possibly in a Triple System

“…An attractive feature of both models is the expected lack of Balmer-series hydrogen or HeI emission lines in the spectrum of J1957 during outburst. These species are generally observed in LMXB outbursts (e.g., van Paradijs & McClintock 1995), and may be expected to be present if the donor star in J1957 were a late-K/early-M dwarf as assumed above (although not necessarily for short-period binaries; Degenaar et al 2016). The outburst characteristics of J1957 are in fact quite similar to those of the transient ultracompact LMXB candidate 1RXS J180408.9−34205 (Baglio et al 2016;Degenaar et al 2016).…”

“…One of these systems is the LMXB 4U 2129+47, which consists of an NS accreting from a main-sequence dwarf star in a 4.27 hr orbit, with an additional F7V star in a 175 > day orbit (Garcia et al 1989;Bothwell et al 2008). As the orbital period of the interacting binary is smaller than a day, the binary will likely shrink through the loss of energy and angular momentum to magnetic-dipole and gravitational radiation to form an ultracompact LMXB with a degenerate donor (Pylyser & Savonije 1988;van Haaften et al 2013). Such a system, but with a lower-mass tertiary, would be analogous to the scenario that I favor for J1957.…”

Revealing Black Holes with Gaia

“…Mass ratios are sampled uniformly between 0.001 and 1 (Mazeh et al 1992;Goldberg & Mazeh 1994). We assume a primary-mass-dependent binary fraction propor-tional to 1/2+1/4(log m) (van Haaften et al 2013). We distribute the orbital separations uniformly in log-space up to 5.75 × 10 6 R ⊙ and as (a/10 ⊙ ) 1.2 below 10 R ⊙ (Han 1998).…”

Evolution of Intermediate-Mass X-Ray Binaries Driven by The magnetic Braking of Ap/Bp Stars. I. Ultracompact X-Ray Binaries

“…In a compact binary, the stellar wind from the donor star induced by the X-ray radiation cannot be ignored during the accretion of the NS (Ruderman 1989;Tavani & London 1993). van Haaften et al (2013) found that irradiation of the donor star plays a vital role in forming UCXBs with an orbital period longer than 40 min. A fraction of the X-ray luminosity (L X = ηṀ NS c 2 ) that the donor star receives is assumed to drive a wind from the surface of the donor star and converted into the kinetic energy of the wind (with a velocity given by the escape speed from the donor's surface).…”