Swift Observations of the 2014 Outburst of the X-ray Nova/Black Hole Candidate GRS 1739-278
Abstract
The bright X-ray transient and black hole candidate GRS 1739-278 is currently undergoing its second outburst since its discovery in 1996 (Vargas 1997). The 1996 outburst lasted nearly 300 days and showed rapid X-ray variability early in the outburst when the source was in the high and very high states (Borozdin 2000), and a strong 5-Hz quasi-periodic oscillation (QPO) was discovered in a single Rossi X-ray Timing Explorer observation during the very high state (Borozdin 2000; Wijnands 2001). Based on these variations and spectral state changes, Borozdin (2000) proposed that GRS 1739-278 contains a black hole candidate. The current outburst began in early March 2014 and continues through mid-May, reaching a peak of ~300 mCrab in the 15-50 keV band, a factor of ~3 lower than the peak in 1996. The source has exhibited a complicated light curve in Swift observations, both in the Burst Alert Telescope (BAT) where it shows flux variations of a factor of two on time scales as short as five days, and in the X-Ray Telescope (XRT), which is more sparsely sampled, but shows strong variability in intensity and hardness ratio both on long (multi-day) and short minute) time scales. This is suggestive of an outburst that has remained in the hard state. The long series of Swift observations enables us to track the evolution of spectral parameters with fine temporal resolution through a series of spectral states. We also search the XRT data for evidence of QPOs or other timing features. These spectral and timing studies and the 18-year gap between outbursts give new insight into the nature of this system and its evolution, and will help to confirm the black hole nature of the compact source. We will also compare this system to other X-ray novae black-hole binary systems in outburst like V404 Cyg.
- Publication:
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AAS/High Energy Astrophysics Division #14
- Pub Date:
- August 2014
- Bibcode:
- 2014HEAD...1410903K