A peculiar thermonuclear X-ray burst from the transiently accreting neutron star SAX J1810.8-2609

TL;DR

This paper reports on a unique, long, and energetic thermonuclear X-ray burst from SAX J1810.8-2609, likely occurring during the early, colder stage of outburst, and also documents a new, brighter outburst of the source.

Contribution

It presents the first detailed analysis of a peculiar long X-ray burst from SAX J1810.8-2609 and reports a significantly brighter outburst, expanding understanding of this transient's behavior.

Findings

The X-ray burst was longer (~20 min) and more energetic (~6.5E39 erg) than previous bursts.

A new outburst in 2012 was brighter, with a peak luminosity >10 times previous outbursts.

The burst's properties suggest it occurred during the early, colder phase of the outburst.

Abstract

We report on a thermonuclear (type-I) X-ray burst that was detected from the neutron star low-mass X-ray binary SAX J1810.8-2609 in 2007 with Swift. This event was longer (~20 min) and more energetic (a radiated energy of Eb~6.5E39 erg) than other X-ray bursts observed from this source. A possible explanation for the peculiar properties is that the X-ray burst occurred during the early stage of the outburst when the neutron star was relatively cold, which allows for the accumulation of a thicker layer of fuel. We also report on a new accretion outburst of SAX J1810.8-2609 that was observed with MAXI and Swift in 2012. The outburst had a duration of ~17 days and reached a 2-10 keV peak luminosity of Lx~3E37(D/5.7kpc)^2 erg/s. This is a factor >10 more luminous than the two previous outbursts observed from the source, and classifies it as a bright rather than a faint X-ray transient.