CfA OIR Division Lunch Talks
Wednesday, May 29, 2013, 11:00 am, Pratt Conference Room

Stellar tidal disruptions: from optical to radio

Sjoert van Velzen (Radboud University Nijmegen)

Very rarely -- about once in 100,000 years -- some star's trajectory will carry it into the narrow region around the super-massive black hole at the center of its galaxy, where a star is shredded by the extreme tidal forces. The accretion of this debris produces a flare of thermal radiation, which peaks in the (far) UV band. Since jets are ubiquitous in accreting systems, we also anticipate the launch of a relativistic jet, producing non-thermal emission at radio and X-ray frequencies. This birth of a relativistic outflow was observed recently for two tidal disruption candidates, discovered by Swift. Yet no unambiguous radio transients have been observed from stellar tidal disruption flares with thermal signatures. The number of flares that have been followed-up by radio telescopes is small, so this discrepancy could be explained by: (i) Doppler boosting, which reduces the jet flux for off-axis observers, (ii) a delay of the radio emission with respect to the time of disruption, (iii) a radio-loud/radio-quiet dichotomy related to different accretion states. We present results from radio follow-up observations of a large number of thermal tidal disruption flares, allowing us to test the hypothesis that all tidal disruptions produce relativistic jets. We also present the first measurement of the rate of optical tidal disruptions, obtained from a systematic search of SDSS multi-epoch imaging data (Stripe 82). We show how this rate can be combined with results from upcoming radio transient surveys to determine what fraction of stellar tidal disruptions are radio-loud. These results will help to complete our understanding of jet formation in compact objects.