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

Abstract

The question "what is the observable electromagnetic (EM) signature of a compact binary merger?" is an intriguing one with crucial consequences to the quest for gravitational waves (GW). Compact binary mergers are prime sources of GW, targeted by current and next generation detectors. Numerical simulations have demonstrated that these mergers eject energetic sub-relativistic (or even relativistic) outflows. This is certainly the case if the mergers produce short GRBs, but even if not, significant outflows are expected. The interaction of such outflows with the surround matter inevitably leads to a long lasting radio signal. We calculate the expected signal from these outflows (our calculations are also applicable to short GRB orphan afterglows) and we discuss their detectability. We show that the optimal search for such signal should, conveniently, take place around 1.4 GHz. Realistic estimates of the outflow parameters yield signals of a few hundred µJy, lasting a few weeks, from sources at the detection horizon of advanced GW detectors. Followup radio observations, triggered by GW detection, could reveal the radio remnant even under unfavorable conditions. Upcoming all sky surveys can detect a few dozen, and possibly even thousands, merger remnants at any give time, thereby providing robust merger rate estimates even before the advanced GW detectors become operational. In fact, the radio transient RT 19870422 fits well the overall properties predicted by our model and we suggest that its most probable origin is a compact binary merger radio remnant.

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

“…Radio emission from post-merger events are produced via synchrotron radiation of accelerated electrons in shocks formed between expanding outflows and circum-merger material (Nakar & Piran 2011;Piran et al 2013;. We briefly review synchrotron radiation from sub-relativistic and ultra-relativistic outflows.…”

“…As indicated by SPH and numericalrelativity simulations, we choose these particular values in masses such that the mergers of the two objects will form some dynamic ejecta and the NS will not be swallowed entirely by the gravitational potential of a too massive BH (see e.g., Foucart 2012; Kawaguchi et al 2015;Kyutoku et al 2015). The ejecta create matter outflows and therefore are potentially responsible for radio afterglows and long-lasting radio remnants (Nakar & Piran 2011;Piran et al 2013;. For simplicity, we assume in our simulations that the NSs and BHs are non-spinning; in the physical universe we expect BHs in BH-NS systems to have a considerable spin, so there may be more dynamical ejecta produced for a NS orbiting in a prograde orbit around a BH with a maximal spin.…”

“…These include sGRBs and their afterglows (Eichler et al 1989;Paczynski 1991;Narayan et al 1992), optical-near-IR counterparts called macronovae or kilonovae (e.g., Li & Paczyński 1998;Kulkarni 2005;Metzger et al 2010;Roberts et al 2011;Metzger & Berger 2012;Barnes & Kasen 2013;Berger et al 2013;Tanaka & Hotokezaka 2013;Tanvir et al 2013;Yang et al 2015;Jin et al 2016) and longlasting radio merger remnants (Nakar & Piran 2011;Piran et al 2013;. Additional radio emission and, in particular, a coherent prompt radio pulse from a magnetically driven, relativistic plasma outflow prior to the DNS merger have also been proposed (Hansen & Lyutikov 2001;Pshirkov & Postnov 2010;Palenzuela et al 2013;Totani 2013).…”

“…Recent observations suggest that the radio transient sky is far quieter compared to the optical one, so we expect far fewer false positives by a factor of a hundred or more depending on the sky location than in the optical-IR (e.g., Frail et al 2012;Mooley et al 2013Mooley et al , 2016. This paper focuses on strategies for detecting and identifying, following the detection of a GW event, two different radio post-merger counterparts: (i) the longer duration radio remnants that may last from months to years and (ii) the faster ultrarelativistic radio afterglow components that last weeks to months (Nakar & Piran 2011;Piran et al 2013;Margalit & Piran 2015). Importantly, we do not consider here the ideal case in which a sGRB or an optical counterpart to a GW merger has been detected.…”

“…The interaction of the merger ejecta with the circum-merger medium results in a radio remnant with timescales of a few months to years (Nakar & Piran 2011). The radio luminosity can be brighter by a couple of orders of magnitude than that of a typical radio supernova.…”