Radio and Optical Follow‐up Observations of a Uniform Radio Transient Search: Implications for Gamma‐Ray Bursts and Supernovae

Abstract

We present the first full characterization of the transient radio sky via radio and optical follow-up observations of all the possible radio transients we have discovered in a survey covering %1/17 of the sky. The two confirmed radio transients turn out to be an optically obscured radio supernova (SN ) in the nearby galaxy NGC 4216, the first such event to be discovered by a wide-field radio survey, and a source not associated with a bright host galaxy. We speculate that this second source may be a flare from a peculiar radio-loud AGN, or a burst from an unusual Galactic compact object, but its nature merits further study. We place an upper limit of 65 radio transients above 6 mJy over the entire sky (95% confidence level). The implications are as follows. First, we derive a limit on the typical beaming of GRBs; we find f À1 ]. Second, our results impose an upper limit on the rate of events that eject k10 51 ergs in unconfined relativistic ejecta, whether or not accompanied by detectable emission in wavebands other than the radio. Our estimated rate,ṅ 1000 yr À1 Gpc À1, is about 2 orders of magnitude smaller than the rate of core-collapse SNe (and Type Ib/c events in particular), indicating that only a minority of such events eject significant amounts of relativistic material, which are required by fireball models of long-soft GRBs. Finally, we show that wider and /or deeper radio variability surveys are expected to detect numerous orphan radio GRB afterglows and illustrate the great potential of new radio instruments to revolutionize the study of nearby SNe.

Radio Counterparts of Compact Binary Mergers Detectable in Gravitational Waves: A Simulation for an Optimized Survey

“…According to the identification of a radio supernova in a radio survey without any other counterparts Gal-Yam et al 2006), the areal density of radio supernovae is roughly estimated as -0.1 deg 2 at 0.1 mJy. Thus there will be a few to tens of type II radio supernovae in a GW localization field.…”

MILLIMETER TRANSIENT POINT SOURCES IN THE SPTpol 100 SQUARE DEGREE SURVEY

“…As this is the first transient survey in the millimeter band, comparison of these results to previous surveys (e.g., Levinson et al 2002;Gal-Yam et al 2006;Bower et al 2010;Bell et al 2011;Bower & Saul 2011;Croft et al 2011) is a complex and model-dependent task. The highest frequency of these (Bower et al 2010) was conducted at 5 GHz, a factor of 30 below our primary observing band, and correspondingly was focused on somewhat different sources.…”

“…This occurs late in the history of the burst, with the peak time occurring six months or more after the burst, and emission lasting for a period of a year (Levinson et al 2002;Ghirlanda et al 2013); a timescale to which the one year SPTpol survey described here has extremely limited or no sensitivity (Section 3). Additional data from the in-progress four-year, 500 square degree SPTpol survey (Section 6) will allow a direct comparison of the results from Levinson et al (2002) to millimeter-band data with similar timescales and corresponding limits on the allowed spectral index of sources, like the possible detection in Gal-Yam et al (2006). The more indirect comparison-connecting constraints on late-time isotropic afterglows to the constraints we place on early partially beamed bursts-is highly theoretically uncertain, for the reasons described in Section 3, and an interesting topic for future modeling work.…”

“…This requirement for an extremely flat spectrum over two orders of magnitude in frequency makes it very unlikely that the candidate from Gal-Yam et al (2006) is related to the candidate in Section 4. The population of bright (1 mJy) GRB afterglows is dominated, both at 1 and 150 GHz, by nearly on-axis selfabsorbed bursts with rising spectra (Ghirlanda et al 2014), which would make the majority of NVSS/FIRST-detectable afterglows visible to SPTpol, given sufficiently long observation times.…”

GRB Orphan Afterglows in Present and Future Radio Transient Surveys

“…Searches of transients in radio archival observations or radio surveys (Levinson et al 2002;Gal-Yam et al 2006;Bannister et al 2011;Bell et al 2011;Bower and Saul 2011;Bower et al 2007Bower et al , 2010Croft et al 2010;Frail et al 2012;Carilli et al 2003;Matsumura et al 2009;Lazio et al 2010) set upper limits on the sky density of transients. Detection of radio transients is, however, only the first step towards the identification of OA, because many other astronomical objects can produce radio transients (see e.g.…”