2018ApJ...862L...4B

SAX J1810.8-2609 is a faint X-ray transient, mostly known for its low quiescent thermal luminosity, which disagrees with slow cooling in the core. It is also one of a small sample of stars with a mass and radius that has been estimated using spectral modeling of one of its thermonuclear bursts. Here we report the discovery of millisecond oscillation in a type I thermonuclear X-ray burst from SAX J1810.8-2609 observed by Rossi X-Ray Timing Explorer (RXTE) during the 2007 outburst. A strong signal (probability of false detection corresponding to 5.75σ of the normal distribution) was present at 531.8 Hz during the decay of one out of six bursts observed. An oscillation was detected for about 6 s, during which its frequency increased from 531.4 to 531.9 Hz in a manner similar to other burst oscillation sources. The millisecond oscillation establishes the spin frequency of the neutron star (NS), which is important for the spectral modeling, associated mass-radius inference, and the evolutionary status and cooling behavior of the star. The source goes into outburst semi-regularly (most recently in 2018 April), providing an opportunity to acquire new material for the burst oscillation searches.