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.

Exponential rise timescale as a function of peak radio luminosity for the entire set of radio flares studied. Overplotted are lines corresponding to a fixed minimum brightness temperature (TB,min ), demonstrating that TB,min is an increasing function of luminosity, peaking just above the theoretical limit of 10^12 K for the most luminous AGN.

We have compiled a sample of ~200 synchrotron flares observed at 5-8 GHz, originating from a wide range of astrophysical objects, including flare stars, X-ray binaries, supernovae, gamma ray burst afterglows and supermassive black holes. For each of the flares studied, we measured the rise/decline timescales by fitting exponential functions, and compared them against peak radio luminosities. The figure below shows the broad correlation between rise timescales and the peak radio luminosities that we find for these events. Overplotted are lines corresponding to constant minimum brightness temperatures, showing that if based on the variability timescales, minimum brightness temperature is a strongly rising function of source luminosity.
A clear spread in characteristic timescale between different types of transients can be seen, suggesting that the variability timescales could be used as an early diagnostic of source class in future radio-transient surveys.