The Origin of the Ultraluminous X-Ray Sources

Abstract

Recently, several ultraluminous X-ray (ULX) sources were shown to host a neutron star (NS) accretor. We perform a suite of evolutionary calculations which show that, in fact, NSs are the dominant type of ULX accretor. Although black holes (BH) dominate early epochs after the star-formation burst, NSs outweigh them after a few 100 Myr and may appear as late as a few Gyr after the end of the star formation episode. If star formation is a prolonged and continuous event (i.e., not a relatively short burst), NS accretors dominate ULX population at any time in solar metallicity environment, whereas BH accretors dominate when the metallicity is sub-solar. Our results show a very clear (and testable) relation between the companion/donor evolutionary stage and the age of the system. A typical NS ULX consists of a ∼ 1.3 M NS and ∼ 1.0 M Red Giant. A typical BH ULX consist of a ∼ 8 M BH and ∼ 6 M main-sequence star. Additionally, we find that the very luminous ULXs (L X 10 41 erg/s) are predominantly BH systems (∼ 9 M ) with Hertzsprung gap donors (∼ 2 M ). Nevertheless, some NS ULX systems may also reach extremely high X-ray luminosities ( 10 41 erg/s).

On the formation of galactic black hole low-mass X-ray binaries

“…Previous studies have already demonstrated that it is hard to produce compact BHXRBs consisting of an initially low-mass companion with the standard formation scenario, since the companion star is unable to expel the CE (e.g., Portegies Zwart et al 1997;Kalogera 1999;Podsiadlowski et al 2003;Kiel & Hurley 2006, see however, Yungelson & Lasota 2008Wiktorowicz et al 2013). However, the orbital periods, the donors' spectral types, and the measured spins of quite a few BHs strongly suggest that in the incipient binaries the donor stars are likely to be of low-mass.…”

“…The solid and dashed red lines represent the results with the λs obtained with the Wind1 and Wind2 prescriptions, respectively. Also displayed is the distribution of the observed BH LMXBs with the black line (data taken from Wiktorowicz et al 2013;Fragos & McClintock 2015). The number of the BH XRBs is calculated by multiplying the formation rate with the duration in a specific stage.…”

“…However, the envelope of such stars are generally too tightly bound for a low-mass secondary to expel , thus a merger is more likely to take place, unless the primary has lost a significant fraction of its envelope before the CE phase through stellar winds (Wiktorowicz et al 2013), or abnormally high values for the CE ejection efficiency are adopted (Kiel & Hurley 2006;Yungelson & Lasota 2008). …”

Compact Binary Merger Rates: Comparison With Ligo/Virgo Upper Limits

“…We then assume that each of these binaries will eventually undergo RLOF and become an X-ray binary. We do not model RLOF phase (there seem to be a general issue with Galactic BH binary modeling; i.e., Wiktorowicz et al 2014). Instead we assume that some fraction (randomly chosen value between 0 and 0.2) of the donor star can accrete onto a BH.…”

The nature of donors in ultraluminous X-ray binaries powered by neutron stars

“…The donor type of NS-ULXs are studied with population synthesis computations by Wiktorowicz et al (2017). They suggested that the largest population of the donors of NS-ULXs is red giants with ∼ 1M ⊙ , which belong typically old populations.…”