Pre-merger Electromagnetic Counterparts of Binary Compact Stars

Abstract

We investigate emission signatures of binary compact star gravitational wave sources consisting of strongly magnetized neutron stars (NSs) and/or white dwarfs (WDs) in their late-time inspiral phase. Because of electromagnetic interactions between the magnetospheres of the two compact stars, a substantial amount of energy will be extracted, and the resultant power is expected to be ∼ 10 38 − 10 44 erg/s in the last few seconds before the two stars merge, when the binary system contains a NS with a surface magnetic field 10 12 G. The induced electric field in the process can accelerate charged particles up to the EeV energy range. Synchrotron radiation is emitted from energetic electrons, with radiative energies reaching the GeV energy for binary NSs and the MeV energy for NS-WD or double WD binaries. In addition, a blackbody component is also presented and it peaks at several to hundreds keV for binary NSs and at several keV for NS-WD or double WD binaries. The strong angular dependence of the synchrotron radiation and the isotropic nature of the blackbody radiation lead to distinguishable modulation patterns between the two emission components. If coherent curvature radiation is presented, fast radio bursts could be produced. These components provide unique simultaneous electromagnetic signatures as precursors of gravitational wave events associated with magnetized compact star mergers and short gamma ray bursts (e.g., GRB 100717).

How Bright Are Fast Optical Bursts Associated With Fast Radio Bursts?

“…The existence of such a bright supernova is excluded by the data for at least some FRBs, e.g., FRB 140514 (Petroff et al 2015). If some FRBs are associated with some violent catastrophic events such as double neutron star mergers (e.g., Totani 2013;Wang et al 2016Wang et al , 2018 or GRBs (Zhang 2014), a bright optical component, either from the kilonova or the GRB afterglow, would be expected. The cosmic comb model (Zhang 2017) attributes an FRB to a sudden reconfiguration of the magnetosphere of a neutron star by a nearby "astrophysical stream."…”

Neutron star mergers and how to study them

“…Signals at these luminosities would only be detectable by all-sky monitors if the events are particularly nearby, precluding these models as the origin of some claimed precursors (e.g., the precursor for GRB 090510, which occurred at a redshift of 0.9; Ackermann et al 2010). Isotropic precursor emission may be expected in these wavelengths from magnetospheric interactions (Hansen and Lyutikov 2001;Metzger and Zivancev 2016;Wang et al 2018), disruption of the NS crust could produce a short gamma-ray flash (Tsang et al 2012), or emission from the crust can power an EM chirp (Schnittman et al 2018). These could give unique constraints on the magnetic fields of the progenitors or on the NS EOS (Sect.…”

The capability of the Australian Square Kilometre Array Pathfinder to detect prompt radio bursts from neutron star mergers

“…For that, we need to understand the flux density predictions of different models. Some models (e.g., Pshirkov & Postnov 2010;Hansen & Lyutikov 2001) predict that BNS mergers can produce FRB-like emission at low radio frequencies, 100 MHz, but this is too low for ASKAP, and some of them have a hard cut-off at the high frequency end (e.g., Wang et al 2018). Moreover, no FRBs have been detected at those frequencies (e.g., Rowlinson et al 2016;Chawla et al 2017;Tingay et al 2015;Sokolowski et al 2018, although new detections are coming closer, Pilia et al 2020;Chawla et al 2020), which may influence the BNS/FRB rate comparison (e.g., Callister et al 2016).…”

“…Many models (including some that predate the discovery of fast radio bursts (FRBs; Lorimer et al 2007; Thornton et al 2013; Cordes & Chatterjee 2019)) predict prompt radio emission associated with compact object mergers. This emission could be generated by the magnetic field interactions during the inspiral (Hansen & Lyutikov 2001; Lai 2012; Totani 2013; Metzger & Zivancev 2016; Wang et al 2016, 2018), interaction between a relativistic jet and interstellar medium (Pshirkov & Postnov 2010), or the collapse of a supramassive neutron star remnant into a black hole (Ravi & Lasky 2014; Falcke & Rezzolla 2014). In particular, prompt emission may be in the form of short coherent radio pulses like FRBs.…”