Abstract
Swift J1745-26 is an X-ray binary towards the Galactic Centre that was detected when it went into outburst in September 2012. This source is thought to be one of a growing number of sources that display "failed outbursts", in which the self-absorbed radio jets of the transient source are never fully quenched and the thermal emission from the geometrically-thin inner accretion disk never fully dominates the X-ray flux. We present multifrequency data from the Very Large Array, Australia Telescope Compact Array and Karoo Array Telescope (KAT- 7) radio arrays, spanning the entire period of the outburst. Our rich data set exposes radio emission that displays a high level of large scale variability compared to the X-ray emission and deviations from the standard radio--X-ray correlation that are indicative of an unstable jet and confirm the outburst's transition from the canonical hard state to an intermediate state. We also observe steepening of the spectral index and an increase of the linear polarization to a large fraction (~50%) of the total flux, as well as a rotation of the electric vector position angle. These are consistent with a transformation from a self-absorbed compact jet to optically-thin ejecta -- the first time such a discrete ejection has been observed in a failed outburst -- and may imply a complex magnetic field geometry.
Highlights
Once thought to be an anomaly, relativistic jets are accepted to be a standard feature of stellar mass black holes in activelyC 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical SocietyRemillard 2006; Belloni 2010)
In the hard state, when the X-ray spectrum is dominated by power-law emission from the optically thin, geometrically thick inner regions, the radio jets are described by self-absorbed synchrotron emission with a flat (α ∼ 0) or inverted spectrum (α > 0), where Fν ∝ να; this is interpreted as an optically thick, compact jet
We present the full sets of radio data from Australia Telescope Compact Array (ATCA), Very Large Array (VLA) and Karoo Array Telescope (KAT-7) monitoring observations of Swift J1745−26 obtained over the period of the outburst
Summary
Once thought to be an anomaly, relativistic jets are accepted to be a standard feature of stellar mass black holes in actively. In an increasing number of ‘failed outbursts’, the system never transitions to the soft state, the selfabsorbed radio jets are never fully quenched and the geometrically thin accretion disc likely remains truncated at a radius greater than the ISCO and never fully dominates the observed X-ray flux (e.g. nine sources in Brocksopp, Bandyopadhyay & Fender 2004, and references therein; Wijnands & Miller 2002; Capitanio et al 2009; Ferrigno et al 2012; Soleri et al 2013). It was suggested that the source was an LMXB black hole system on the basis of X-ray (Swift and INTEGRAL) spectral and timing observations These were used to classify its state at various epochs throughout the outburst as being either hard or hard intermediate (e.g. Belloni et al 2012; Grebenev & Sunyaev 2012; Tomsick, DelSanto & Belloni 2012; Vovk et al 2012; Sbarufatti et al 2013) suggesting that this was a ‘failed outburst’.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
