One question has polarised the black hole X-ray binary community for many years: is the inner accretion disc truncated in the low-hard state?
The variability timescales and brightness temperatures of radio flares from stars to supermassive black holes
4PiSky Authors: Gosia Pietka / Rob Fender
See the full publication on Astro-ph.
Synchrotron-emitting radio sources can undergo flaring events that cause dramatic variability in their lightcurves. One of the earliest models for these radio flares describes a cloud of relativistic particles and magnetic fields expanding into a surrounding medium. In this model, at a given frequency, the evolution of the light curve of an expanding cloud is determined by increasing transparency, causing an increase in the flux density, with expansion losses reducing the internal energy.
A prompt radio transient associated with a gamma-ray superflare from the young M dwarf binary DG CVn
4PiSky Authors: Rob Fender / Gemma Anderson / Tim Staley
On April 23, 2014, the Swift satellite detected a gamma-ray superflare from the nearby star system DG CVn. This system comprises a M-dwarf binary with extreme properties: it is very young and at least one of the components is a very rapid rotator. The gamma-ray superflare is one of only a handful detected by Swift in a decade. As part of our AMI-LA Rapid Response Mode, ALARRM, we automatically slewed to this target, were taking data at 15 GHz within six minutes of the burst, and detected a bright (~100 mJy) radio flare. This is the earliest detection of bright, prompt, radio emission from a high energy transient ever made with a radio telescope, and is possibly the most luminous incoherent radio flare ever observed from a red dwarf star.
Utilising over 500 (~daily) archival observations of the black hole X-ray binary GX 339-4 with the Rossi X-ray Timing Explorer, we have been able to map out quantitatively how accretion on to a black hole evolves.
4PiSky Authors: Gemma Anderson / Tim Staley / Rob Fender
AMI-LA Rapid Response Mode (ALARRM) observations of the nearby bright gamma-ray burst GRB 130427A allowed the 4 Pi Sky team to obtain one of the earliest radio detections of a GRB to date. As soon as this GRB had risen above the horizon the AMI-LA quickly slewed to its position detecting radio emission within 8 hours post-burst. Further follow-up AMI observations showed the radio flux to increase in brightness before rapidly declining one day later. Such a sudden decline in radio emission is very rare and has only been observed from a few GRBs.