The fast spiral-in of the companion star to the black hole XTE J1118+480

TL;DR

This paper reports the detection of an unexpectedly rapid orbital decay in the black hole X-ray binary XTE J1118+480, challenging existing models and suggesting new physics or extreme magnetic effects in the system.

Contribution

It presents the first measurement of a rapid orbital period decay in XTE J1118+480, indicating a need to revise models of binary evolution and magnetic braking in such systems.

Findings

Orbital period decay rate of -1.83 ms/yr detected

Decay rate is 150 times larger than gravitational wave emission predictions

Conventional magnetic braking models cannot explain the observed decay

Abstract

We report the detection of an orbital period decay of $\dot P=-1.83\pm0.66$ ms yr$^{-1}$ in the black hole X-ray binary XTE J1118+480. This corresponds to a period change of $-0.85\pm0.30$ $\mu $s per orbital cycle, which is $\sim 150$ times larger than expected from the emission of gravitational waves. These observations cannot be reproduced by conventional models of magnetic braking even when including significant mass loss from the system. The spiral-in of the star is either driven by magnetic braking under extremely high magnetic fields in the secondary star or by a currently unknown process, which will have an impact on the evolution and lifetime of black hole X-ray binaries.