GRB Orphan Afterglows in Present and Future Radio Transient Surveys

Orphan 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 gamma rays) has already ceased. Given the typically estimated jet opening angle of GRBs theta<SUB>jet</SUB> ~ 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 muJy 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<SUP>- 3</SUP> OA deg<SUP>- 2</SUP> yr<SUP>- 1</SUP>, MeerKAT and EVLA at 8.4 GHz should see ~ 3 10<SUP>- 1</SUP> OA deg<SUP>- 2</SUP> yr<SUP>- 1</SUP>. The SKA, reaching the muJy flux limit, could see up to ~ 0.2 - 1.5 OA deg<SUP>- 2</SUP> yr<SUP>- 1</SUP>. 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.