Extremely fast orbital decay of the black hole X-ray binary Nova Muscae 1991

TL;DR

This study reports an exceptionally rapid orbital decay in the black hole X-ray binary Nova Muscae 1991, challenging existing models and implying a much shorter evolutionary timescale for such systems.

Contribution

First measurement of an extremely fast orbital decay in Nova Muscae 1991, revealing decay rates much higher than in other black hole binaries and questioning current evolutionary theories.

Findings

Orbital period decay rate of -20.7 ms/yr

Decay rate significantly faster than other binaries

Implication of a 2.7 million year lifetime for the system

Abstract

We present new medium-resolution spectroscopic observations of the black hole X-ray binary Nova Muscae 1991 taken with X-Shooter spectrograph installed at the 8.2m-VLT telescope. These observations allow us to measure the time of inferior conjunction of the secondary star with the black hole in this system that, together with previous measurements, yield an orbital period decay of $\dot P=-20.7\pm12.7$ ms yr$^{-1}$ ($-24.5\pm15.1$ $\mu $s per orbital cycle). This is significantly faster than those previously measured in the other black hole X-ray binaries A0620-00 and XTE J1118+480. No standard black hole X-ray binary evolutionary model is able to explain this extremely fast orbital decay. At this rate, the secondary star would reach the event horizon (as given by the Schwarzschild radius of about 32 km) in roughly 2.7 Myr. This result has dramatic implications on the evolution and lifetime of black hole X-ray binaries.