Whispering in the dark
Faint X-ray emission from black holes with OB star companions
Sen, K.; El Mellah, I.; Langer, N.; Xu, X. -T.; Quast, M.; Pauli, D.
Context. Recently, astrometric and spectroscopic surveys of OB stars
revealed a few stellar-mass black holes (BHs) with orbital periods of as
low as 10 days. Contrary to wind-fed BH high-mass X-ray binaries
(HMXBs), no X-ray counterpart was detected, probably because of the
absence of a radiatively efficient accretion disc around the BH.
Nevertheless, dissipative processes in the hot, dilute, and strongly
magnetised plasma around the BH (so-called BH corona) can still lead to
non-thermal X-ray emission (e.g. synchrotron).
Aims: We determine the X-ray luminosity distribution from BH+OB star
binaries up to orbital periods of a few thousand days.
Methods. We used detailed binary evolution models computed with MESA for
initial primary masses of 10–90 M⊙ and orbital periods of 1–3000 d. We
computed the X-ray luminosity for a broad range of radiative
efficiencies that depend on the mass accretion rate and flow geometry.
Results:
For typical conditions around stellar-mass BHs, we show that particle
acceleration through magnetic reconnection can heat the BH corona. A
substantial fraction of the gravitational potential energy from the
accreted plasma is converted into non-thermal X-ray emission. Our
population synthesis analysis predicts that at least 28 (up to 72) BH+OB
star binaries in the Large Magellanic Cloud (LMC) produce X-ray
luminosities of above 1031 erg s‑1, which are observable with focused
Chandra observations. We identify a population of SB1 systems in the LMC
and HD96670 in the Milky Way comprising O stars with unseen companions
of masses of above 2.3 Msun, which aligns well with our predictions and
may be interesting sources for follow-up observations. The predicted
luminosities of the OB companions to these X-ray-emitting BHs are
104.5 ‑ 5.5 Lsun.
Conclusions: These findings advocate for prolonged X-ray observations of
the stellar-mass black hole candidates identified in the vicinity of OB
stars. Such long exposures could reveal the underlying population of
X-ray-faint BHs and provide constraints for the evolution from single to
double degenerate binaries and identify the progenitors of gravitational
wave mergers.
This paper in ADS