The return of the bursts: Thermonuclear flashes from Circinus X-1

TL;DR

This paper reports the first detection of thermonuclear X-ray bursts from Circinus X-1 in 25 years, confirming its nature as a low magnetic field neutron star and exploring reasons for the recent burst activity.

Contribution

It provides the first identification of thermonuclear bursts from Circinus X-1 and confirms its classification as a low magnetic field neutron star.

Findings

Detection of 15 X-ray bursts from Cir X-1 in 2010

Identification of 9 bursts as type I thermonuclear

Confirmation of Cir X-1 as a low magnetic field neutron star

Abstract

We report the detection of 15 X-ray bursts with RXTE and Swift observations of the peculiar X-ray binary Circinus X-1 during its May 2010 X-ray re-brightening. These are the first X-ray bursts observed from the source after the initial discovery by Tennant and collaborators, twenty-five years ago. By studying their spectral evolution, we firmly identify nine of the bursts as type I (thermonuclear) X-ray bursts. We obtain an arcsecond location of the bursts that confirms once and for all the identification of Cir X-1 as a type I X-ray burst source, and therefore as a low magnetic field accreting neutron star. The first five bursts observed by RXTE are weak and show approximately symmetric light curves, without detectable signs of cooling along the burst decay. We discuss their possible nature. Finally, we explore a scenario to explain why Cir X-1 shows thermonuclear bursts now but not in the past, when it was extensively observed and accreting at a similar rate.