GRB Orphan Afterglows in Present and Future Radio Transient Surveys

Abstract

AbstractOrphan Afterglows (OA) are slow transients produced by Gamma Ray Bursts seen off–axis that become visible on timescales of days/years at optical/NIR and radio frequencies, when the prompt emission at high energies (X and γ rays) has already ceased. Given the typically estimated jet opening angle of GRBs θjet~ 3°, for each burst pointing to the Earth there should be a factor ~ 700 more GRBs pointing in other directions. Despite this, no secure OAs have been detected so far. Through a population synthesis code we study the emission properties of the population of OA at radio frequencies. OAs reach their emission peak on year-timescales and they last for a comparable amount of time. The typical peak fluxes (which depend on the observing frequency) are of few μJy in the radio band with only a few OA reaching the mJy level. These values are consistent with the upper limits on the radio flux of SN Ib/c observed at late times. We find that the OA radio number count distribution has a typical slope − 1.7 at high fluxes and a flatter ( − 0.4) slope at low fluxes with a break at a frequency–dependent flux. Our predictions of the OA rates are consistent with the (upper) limits of recent radio surveys and archive searches for radio transients. Future radio surveys like VAST/ASKAP at 1.4 GHz should detect ~ 3 × 10− 3OA deg− 2yr− 1, MeerKAT and EVLA at 8.4 GHz should see ~ 3 × 10− 1OA deg− 2yr− 1. The SKA, reaching the μJy flux limit, could see up to ~ 0.2 − 1.5 OA deg− 2yr− 1. These rates also depend on the duration of the OA above a certain flux limit and we discuss this effect with respect to the survey cadence.