Long-Duration Radio Transients Lacking Optical Counterparts Are Possibly Galactic Neutron Stars

Abstract

Recently, a new class of radio transients in the 5 GHz band and with durations of the order of hours to days, lacking any visible-light counterparts, was detected by Bower and collaborators. We present new deep near-infrared (IR) observations of the field containing these transients, and find no counterparts down to a limiting magnitude of K = 20.4 mag. We argue that the bright (>1 Jy) radio transients recently reported by Kida et al. are consistent with being additional examples of the Bower et al. transients. We refer to these groups of events as "long-duration radio transients." The main characteristics of this population are: timescales longer than 30 minutes but shorter than several days; very large rate, ∼10 3 deg −2 yr −1 ; progenitor's sky surface density of >60 deg −2 (at 95% confidence) at Galactic latitude ∼40 • ; 1.4-5 GHz spectral slopes, f ν ∝ ν α , with α 0; and most notably the lack of any X-ray, visible-light, near-IR, and radio counterparts in quiescence. We discuss putative known astrophysical objects that may be related to these transients and rule out an association with many types of objects including supernovae, gamma-ray bursts, quasars, pulsars, and M-dwarf flare stars. Galactic brown dwarfs or some sort of exotic explosions in the intergalactic medium remain plausible (though speculative) options. We argue that an attractive progenitor candidate for these radio transients is the class of Galactic isolated old neutron stars (NSs). We confront this hypothesis with Monte Carlo simulations of the space distribution of old NSs, and find satisfactory agreement for the large areal density. Furthermore, the lack of quiescent counterparts is explained quite naturally. In this framework, we find: the mean distance to events in the Bower et al. sample is of order kpc; the typical distance to the Kida et al. transients are constrained to be between 45 pc and 2 kpc (at the 95% confidence level); these events should repeat with a timescale of order several months; and sub-mJy level bursts should exhibit Galactic latitude dependence. We discuss two possible mechanisms giving rise to the observed radio emission: incoherent synchrotron emission and coherent emission. We speculate that if the latter is correct, the long-duration radio transients are sputtering ancient pulsars or magnetars and will exhibit pulsed emission.

Detectable radio flares following gravitational waves from mergers of binary neutron stars

“…Among these typical radio SNe are the most abundant. Transient search over 1/17 of the sky with F lim = 6 mJy at 1.4GHz (Levinson et al 2002;Gal-Yam et al 2006;Ofek et al 2010) finds one radio SN. This rate translates to 10 3 − 10 4 SNe in a whole sky F lim = 0.1 mJy survey.…”

Reliability of Radio Transients Detected in the Nasu Sky Survey

“…However, in the drift-scanning observations of the Nasu sky survey, the area and time were not independent, which may have introduced anomalies in the discovery rates. In Section 5, we revise the discovery rate in terms of the definition in Ofek et al (2010) and the reliability evaluation results presented in Section 4. …”

“…In terms of duration time, discovery rate, and lack of clear counterparts, these events were similar to Bower's transients. This similarity inspired Ofek et al (2010) to argue the origins of the transients. They concluded that the likely origins were old Galactic neutron stars.…”

“…One source, labeled VLA 121550.2+130654, was identified as an optically obscured Type II supernova, but the other source was inconclusive. Later, Ofek et al (2010) revealed that the unknown source was a spurious event incited by an error in the VLA recording system. Therefore, they estimated the sky surface density or snapshot rate (number of events per unit sky area, denoted by Σ) as Σ >6 mJy 1.4 GHz < 1.5 × 10 −3 deg −2 at the 95% confidence level for transient sources with a flux density above 6 mJy at 1.4 GHz.…”

Rapid Fringe Detection Technique for Discovering Radio Transients in Nasu Drift-scan Data

“…Furthermore, the RRATS have much shorter durations than WJN transients. In another example, 10 radio transient signals have been discovered in archival data from the VLA spanning 22 yr with a typical 7-day epoch separation (Bower et al 2007;Ofek et al 2009). Reanalysis has revealed that six signals were derived from artifacts or side lobes of bright sources (Frail et al 2012).…”