Large amplitude variability from the persistent ultracompact X-ray binary in NGC 1851

TL;DR

This paper reports unprecedented large amplitude X-ray flux variations in an ultracompact binary in NGC 1851, challenging existing models of mass transfer driven by gravitational radiation.

Contribution

It presents the first observation of significant flux variability in an ultracompact X-ray binary, questioning current understanding of disk stability and mass transfer mechanisms.

Findings

Flux variations of over a factor of 10 on days to weeks.

Long-term luminosity changes by factors of two.

Variability patterns are inconsistent with standard models.

Abstract

Using archival RXTE data, we show that the ultracompact X-ray binary in NGC 1851 exhibits large amplitude X-ray flux varations of more than a factor of 10 on timescales of days to weeks and undergoes sustained periods of months where the time-averaged luminosty varies by factors of two. Variations of this magnitude and timescale have not been reported previously in other ultracompact X-ray binaries. Mass transfer in ultracompact binaries is thought to be driven by gravitational radiation and the predicted transfer rates are so high that the disks of ultracompact binaries with orbits as short as that of this object should not be susceptible to ionization instabilities. Therefore the variability characteristics we observe were unexpected, and need to be understood. We briefly discuss a few alternatives for producing the observed variations in light of the fact that the viscous timescale of the disk is of order a week, comparable to the shorter time scale variation that is observed but much less than the longer term variation. We also discuss the implications for interpretation of observations of extragalactic binaries if the type of variability seen in the source in NGC 1851 is typical.