A 15.65 solar mass black hole in an eclipsing binary in the nearby spiral galaxy Messier 33

TL;DR

This paper reports the discovery of a 15.65 solar mass black hole in an eclipsing binary system in galaxy M33, challenging existing stellar evolution models and providing new insights into massive star evolution.

Contribution

It presents the first precise mass measurement of a black hole in an eclipsing binary outside the Milky Way, highlighting the need to revise models of massive star evolution and mass loss.

Findings

Black hole mass measured at 15.65 +/- 1.45 solar masses.

Black hole in an eclipsing binary system in M33.

Mass loss during progenitor evolution was less than standard models assume.

Abstract

Stellar-mass black holes are discovered in X-ray emitting binary systems, where their mass can be determined from the dynamics of their companion stars. Models of stellar evolution have difficulty producing black holes in close binaries with masses >10 solar masses, which is consistent with the fact that the most massive stellar black holes known so all have masses within 1 sigma of 10 solar masses. Here we report a mass of 15.65 +/- 1.45 solar masses for the black hole in the recently discovered system M33 X-7, which is located in the nearby galaxy Messier 33 (M33) and is the only known black hole that is in an eclipsing binary. In order to produce such a massive black hole, the progenitor star must have retained much of its outer envelope until after helium fusion in the core was completed. On the other hand, in order for the black hole to be in its present 3.45 day orbit about its 70.0 +/- 6.9 solar mass companion, there must have been a ``common envelope'' phase of evolution in which a significant amount of mass was lost from the system. We find the common envelope phase could not have occured in M33 X-7 unless the amount of mass lost from the progenitor during its evolution was an order of magnitude less than what is usually assumed in evolutionary models of massive stars.