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Long-duration soft state of the Clocked Burster GS 1826-238

ATel #10117; David Palmer (LANL) on behalf of the Swift/BAT team
on 23 Feb 2017; 15:36 UT
Credential Certification: David M. Palmer (palmer@lanl.gov)

Subjects: X-ray, Gamma Ray, Binary, Cataclysmic Variable, Neutron Star

The LMXB GS 1826-238 transitioned to a soft state on 2015 July 9-10, and has remained in that state ever since, except for a few brief returns to the hard state (2015 Aug 21-Sep 19; and early 2016 Jan). This is in addition to a ~3-month transition to the soft state in 2014 June-August, following at least 25 years of consistent hard-state behavior since its discovery (Nakahira et al. ATEL #6250).

Before the transition, the flux of this source varied from 1.2e-2 to 2.2e-2 instrumental units (averaged over quarter-years) with slow variation over months-years timescales. After the transition, the count rate has dropped by an order of magnitude to ~2e-3 units. (Units are roughly counts/s/cm^2 in the 15-50 keV band.)

This source is known as the Clocked Burster for its quasi-periodic bursting cycle (Ubertini et al., 1999, ApJ 514 L27).

BAT detected no bursts from this source during its observations from 2005-2015 when the source was in its hard state. This is consistent with the previously observed behavior, where bursts (in the 1-10 keV band) are associated with corresponding decreases in the 15-50 keV flux (Ji et al., 2014, ApJ 782 40).

Since the transition to the soft state, there have been four detected bursts from the source location in BAT's 15-50 keV band (with individually marginal but cumulatively convincing significance), most recently on 2017-02-20.

Long term BAT monitoring for this source in the 15-50 keV band is shown in the page linked below.

BAT scaled map analysis of Ginga 1826-238