Millisecond oscillations in the bursting flux of SAX J1810.8-2609

TL;DR

This paper reports the discovery of millisecond oscillations in a thermonuclear X-ray burst from SAX J1810.8-2609, revealing the neutron star's spin frequency and aiding in understanding its physical properties.

Contribution

It is the first detection of burst oscillations from SAX J1810.8-2609, providing key data on its neutron star spin and aiding spectral and evolutionary modeling.

Findings

Detected oscillations at 531.8 Hz during a burst

Oscillation frequency increased from 531.4 to 531.9 Hz

Oscillations lasted about 6 seconds

Abstract

SAX J1810.8-2609 is a faint X-ray transient, mostly known for its abnormally low quiescent thermal luminosity, which disagrees with standard cooling models. It is also one of a small sample of stars whose mass and radius have been estimated using spectral modeling of one of its thermonuclear bursts. Here we report the discovery of millisecond oscillations in a type I thermonuclear X-ray burst from SAX J1810.8-2609 observed by RXTE during the 2007 outburst. A strong signal (Leahy-normalized power of 71.5, 4.5e-9 chance of coincidence with a conservative estimate for the number of trials) was present at 531.8 Hz during the decay of one out of six bursts observed. Oscillations were detected for about 6 seconds, during which their frequency increased from 531.4 to 531.9 Hz in a manner similar to other burst oscillation sources. The millisecond oscillations discovered pinpoint the spin frequency of the neutron star, which is important for the spectral modeling, associated mass-radius inference, and understanding the evolutionary status and cooling behavior of the star. As of April 2018 the source is in outburst again, providing a fleeting opportunity to acquire new material for the burst oscillation searches.