Black hole X-ray binaries (BHXBs) are systems composed of a black hole and a companion star. In these systems, the material from the companion star falls onto the black hole, forming an accretion disk. The accretion disk is very hot and emits X-rays.

Sometimes, BHXBs show a phenomenon called "outliers." Outliers are sudden increases in the X-ray emission from the system. These increases can last for a few seconds to a few minutes. The cause of outliers is not well understood.

A new study by a team of astronomers from the University of Southampton and the University of Warwick in the UK has found that outliers are caused by changes in the accretion disk. The team studied a BHXB called GRS 1915+105 and found that the outliers occurred when the inner edge of the accretion disk moved closer to the black hole.

This movement of the inner edge of the accretion disk is thought to be caused by changes in the magnetic field around the black hole. The magnetic field can change the shape of the accretion disk and cause it to move closer to the black hole.

The study's findings are published in the journal Monthly Notices of the Royal Astronomical Society.