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.
An additional bright radio flare, peaking at around 90 mJy, occurred around one day later, and there may have been further events between 0.1–1 days when we had no radio coverage. The source subsequently returned to a quiescent level of 2–3 mJy on a timescale of about 4 days. Although radio emission is known to be associated with active stars, this is the first detection of large radio flares associated with a gamma ray superflare, and demonstrates both feasibility and scientific importance of rapid response modes on radio telescopes. The figure below illustrates the combined Swift and AMI-ALARRM lightcurves for DG CVn. Learning from this programme, we have further refined the ALARRM response sequence and have already managed to get on a burst within 70 sec (see 4 PI SKY twitter feed).