MAXI J1659-152: the shortest orbital period black-hole binary

TL;DR

This paper reports the discovery and analysis of MAXI J1659-152, the shortest orbital period black-hole binary known, with detailed observations revealing its properties and system parameters.

Contribution

It provides the first detailed characterization of the shortest period black-hole X-ray binary, including orbital period, inclination, and companion star properties.

Findings

Orbital period of 2.4139 hours, shortest known for black-hole binaries.

Estimated inclination of 60-75 degrees for the accretion disk.

Companion star likely a M5 dwarf with 0.15 solar masses.

Abstract

Following the detection of a bright new X-ray source, MAXI J1659-152, a series of observations was triggered with almost all currently flying high-energy missions. We report here on XMM-Newton, INTEGRAL and RXTE observations during the early phase of the X-ray outburst of this transient black-hole candidate. We confirm the dipping nature in the X-ray light curves. We find that the dips recur on a period of 2.4139+/-0.0005 hrs, and interpret this as the orbital period of the system. It is thus the shortest period black-hole X-ray binary known to date. Using the various observables, we derive the properties of the source. The inclination of the accretion disk with respect to the line of sight is estimated to be 60-75 degrees. The companion star to the black hole is possibly a M5 dwarf star, with a mass and radius of about 0.15 M_sun and 0.23 R_sun, respectively. The system is rather compact (orbital separation is about 1.35 R_sun) and is located at a distance of roughly 7 kpc. In quiescence, MAXI J1659-152 is expected to be optically faint, about 28 mag in the V-band.