# Fast orbital shrinkage of black hole X-ray binaries driven by circumbinary disks

## TL;DR

This study explores how interactions with circumbinary disks can cause rapid orbital decay in black hole X-ray binaries, explaining observed anomalies and providing new insights into their evolution.

## Contribution

It demonstrates that circumbinary disk interactions can account for the rapid orbital shrinkage observed in certain black hole X-ray binaries, a phenomenon not explained by standard stellar evolution.

## Key findings

- Orbital period derivatives can be explained by tidal torque from circumbinary disks.
- Disks with masses around 10^{-9} to 10^{-7} solar masses are consistent with observations.
- Simulations match observed donor star properties but predict higher temperatures than observed.

## Abstract

Recently, the black hole X-ray binary (BHXB) Nova Muscae 1991 has been reported to be experiencing an extremely rapid orbital decay. So far, three BHXBs have anomalously high orbital period derivatives, which can not be interpreted by the standard stellar evolution theory. In this work, we investigate whether the resonant interaction between the binary and a surrounding circumbinary (CB) disk could produce the observed orbital period derivatives. Analytical calculations indicate that the observed orbital period derivatives of XTE J1118+480 and A0620-00 can originate from the tidal torque between the binary and a CB disk with a mass of $10^{-9}~\rm M_{\odot}$, which is approximately in agreement with the dust disk mass detected in these two sources. However, Nova Muscae 1991 was probably surrounded by a heavy CB disk with a mass of $10^{-7}~\rm M_{\odot}$. Based on the CB disk model and the anomalous magnetic braking theory, we simulate the evolution of the three BHXBs with intermediate-mass donor stars by using the MESA code. Our simulated results are approximately consistent with the observed donor star masses, orbital periods, and orbital-period derivatives. However, the calculated effective temperatures of the donor stars are higher than indicated by the observed spectral types of two sources.