Black Hole high mass x-ray binary

A stellar mass black hole (BH) binary system consists of a BH with a mass function in range 5 – 100Msun with a companion star (all stellar species from dwarves to giants). Most of these binaries have a low mass companion star (low mass x-ray binaries or LMXBs) but a few of them have massive companions like O, B or WR stars, these are the high mass x-ray binaries (HMXBs). Unlike their population their impact on the BH physics and understanding of BH binary evolution is HUGE. Also, BH-HMXBs are progenitors of major gravitational wave source BH+BH binaries, which enhances the need to study them.

The small family of known BH-HMXBs consists of IC 10 X-1(Crowther et al. 2004, Prestwich et al. 2007, Laycock et al. 2015), Cyg X1 (Bolton 1972; Hutchings et al. 1973), Cyg X-3 (Hanson, Still & Fender 2000), LMC X-1 (Orosz et al. 2009), LMC X-3 (Nowak et al. 2001), M33 X-7 (Orosz et al. 2007), NGC 300 X-1 (Carpano et al. 2007; Crowther et al. 2010; Binder et al. 2011), and CXOU J123030.3+413853 in NGC 4490 (Esposito et al. 2013), CG X-1(Qiu et al. 2019), MWC 656(Casares et al. 2012,2014).

The binary parameters of some these systems are tabulated in Table A1, Laycock et al. 2015.

Animation of the Period Determination of IC 10 X-1 (Laycock et al. 2015).

IC 10 X-1 as prototype WR+BH HMXB

The extragalactic High Mass X-ray binary IC 10 X1 consists of a black hole (BH) and a massive Wolf–Rayet (WR) star. The exciting nature of this system has been revealed by a series of observations with Chandra and XMM over the last decade (Laycock et al. 2015). The following are some of the key properties of this system:

  • The optical counterpart is a WN3 type Wolf-Rayet star ([MAC92] 17A, Clark et al. 2004, Bauer et al. 2004) of mass ~ 17-35Msun.
  • The companion mass was found to be 23- 32Msun indicating a black hole. (Prestwich et al 2007, Silverman & Filippenko 2008)
  • Deep eclipses are seen in the X-ray light curve with duration of 5 hours. The eclipses are asymmetric with egress duration(~0.9h), suggesting an accretion disk hotspot or corona.
  • The X-ray eclipse leads inferior conjunction of the RV curve by ∼90◦, so either the BH is being eclipsed by a trailing shock/plume, or the He II line does not directly trace the motion of the WR star and instead originates in a shadowed partially ionized region of the stellar wind. This makes the premise of using RV curve from HeII[4686] emission line for mass determination unreliable.

What is the Mass of IC 10 X-1?

The mass of the black hole in IC 10 X-1 was determined using RV curve made from HeII[4686] emission line of the optical spectra. But later our study reveals that The X-ray eclipse leads inferior conjunction of the RV curve by ∼90◦. This phase lag between the X-ray lightcurve and the optical RV curve indicates presence of a trailing shock/plume. The BH is most probably being eclipsed by the wind not the star itself. So, the HeII line isn’t appropriate for tracing the star’s motion and hence for mass determination.

In order to determine the mass of this system, the accretion-wind interaction needs to be studied in more detail. This will not only solve the mass-conundrum for this system but also for systems like NGC 300 X-1. Also, RV curves using more and more emission lines (preferably lines originating from the core like NIV etc) needs to be studied from the phase resolved spectra in order to “cheat” the wind.

Showing phase lag between X-ray eclipse and optical RV curve.

This figure shows the mentioned phase lag between x-ray light curve and the optical RV curve also the predicted UV-RV curve and IR light curve from the model shown above.


Model of IC-10 X-1 showing accretion-wind interaction in this system.

The above model shows the effect of wind in the IC 10 X-1 system.