Soft state of GX 339-4 in its 2014-2015 outburst
ATel #7009; Devraj Pawar (R. J. College, Mumbai), Diego Altamirano (Southampton Univ.), Sara Motta (ESA/ESAC), Tomaso Belloni (INAF-OAB), James Miller-Jones (ICRAR Curtin), Peter Curran (ICRAR Curtin)
on 30 Jan 2015; 14:01 UT
Credential Certification: Devraj Pawar (devrajdp@gmail.com)
Subjects: Radio, X-ray, Black Hole, Transient
On October 31, 2014 (MJD 56961.05) Yan et al. reported a new outburst of GX 339-4 (ATel #6649). A Swift/XRT observation on the same day indicated that the source was in the hard state (photon index = 1.32+/-0.02). The source was sun constrained for Swift/XRT from November 11, 2014 to January 20, 2015. Motta et al. recently reported a probable transition of GX 339-4 to the soft state (ATel #6960), based on Swift/BAT and MAXI observations.
To understand the X-ray evolution of the source we triggered Swift/XRT observations from January 21, 2015 (MJD 57043.03) when it came out of the Swift/XRT sun constraint. The observations indicate that the source has been in the soft state since at least January 21, 2015 (MJD 57043.03); its spectrum is well fit by a multicoloured blackbody (using model wabs*diskbb and abundance "wilm" in XSPEC). The temperature has remained at ~1 keV since the monitoring began on MJD 57043.03 through to MJD 57051.02. On MJD 57051.02, the 0.3-10.0 keV unabsorbed flux is (2.6+/-0.1)e-8 erg/cm2/s for a temperature of 1.11+/-0.04 keV, nH = (5.23+/-0.03)e21/cm2. Using the minimum distance estimate of 6 kpc (Hynes et al. 2004 ApJ 609, 317) we estimate a 0.3-10 keV luminosity of at least ~8.6e37 erg/s.
GX 339-4 will be monitored every other day by Swift/XRT through February 20.
We thank the Swift team for scheduling our observations.
