The failed state transition of the ATOLL source GRS 1724-308

TL;DR

This paper reports a rare, long-lasting high-flux hard state event in the neutron star LMXB GRS 1724-308, exploring its possible origins and its relation to super-orbital modulation, expanding understanding of ATOLL source behaviors.

Contribution

It presents the first observation of a peculiar, long-lasting flaring event in GRS 1724-308 and discusses potential causes, including a failed state transition and super-orbital phase coincidence.

Findings

Detected a 300-day flaring event in 2008 with a high-flux hard state.

Confirmed the 90-day orbital period of GRS 1724-308.

Suggested the event may be linked to super-orbital modulation phases.

Abstract

The 2004-2012 X-ray time history of the NS LMXB GRS 1724-308 shows, along with the episodic brightenings associated to the low-high state transitions typical of the ATOLL sources, a peculiar, long lasting (about 300 d) flaring event, observed in 2008. This rare episode, characterised by a high-flux hard state, has never been observed before for GRS 1724-308 , and in any case is not common among ATOLL sources. We discuss here different hypotheses on the origin of this peculiar event that displayed the spectral signatures of a failed transition, similar in shape and duration to those rarely observed in Black Hole binaries. We also suggest the possibility that the atypical flare occurred in coincidence with a new rising phase of the 12-years super-orbital modulation that has been previously reported by other authors. The analysed data also confirm for GRS 1724-308 the already reported orbital period of about 90 d.