Fast Radio Bursts from Magnetars Born in Binary Neutron Star Mergers and Accretion Induced Collapse

Abstract

Recently born magnetars are promising candidates for the engines powering fast radio bursts (FRBs). The focus thus far has been placed on millisecond magnetars born in rare core-collapse explosions, motivated by the star forming dwarf host galaxy of the repeating FRB 121102, which is remarkably similar to the hosts of superluminous supernovae (SLSNe) and long gamma-ray bursts (LGRB). However, long-lived magnetars may also be created in binary neutron star (BNS) mergers, in the small subset of cases with a sufficiently low total mass for the remnant to avoid collapse to a black hole, or in the accretion-induced collapse (AIC) of a white dwarf. A BNS FRB channel will be characterized by distinct host galaxy and spatial offset distributions than the SLSNe/LGRB channel; we anticipate a similar host population, although possibly different offset distribution for AIC events. We show that both the BNS and AIC channels are consistent with the recently reported FRB 180924, localized by ASKAP to a massive quiescent host galaxy with an offset of about 1.4 effective radii. FRBs from magnetars born in BNS mergers and AIC will be accompanied by persistent synchrotron radio emission on timescales of months to years, powered by the nebula of relativistic electrons and magnetic fields inflated by the magnetar flares, or on longer timescales through interaction of the merger ejecta with the interstellar medium; this timescale is shorter than for the SLSN/LGRB channel. Using models calibrated to FRB 121102, we make predictions for the dispersion measure, rotation measure, and persistent radio emission from magnetar FRB sources born in BNS mergers or AIC, and show these are consistent with upper limits from FRB 180924 for reasonable parameters. Depending on the rate of AIC, and the fraction of BNS mergers leaving long-lived stable magnetars, the birth rate of repeating FRB sources associated with older stellar populations could be comparable to that of the core-collapse channel. We also discuss potential differences in the repetition properties of these channels, as a result of differences in the characteristic masses and magnetic fields of the magnetars.

Magnetars from Neutron Star–White Dwarf Mergers: Application to Fast Radio Bursts

“…Moreover, its RM decreases with time. These results are generally consistent with Margalit et al (2019) and the RM observation of FRB 190824 (Bannister et al 2019).…”

“…If this is the case, FRBs could be divided into two populations: FRB 121102-like bursts stem from young magnetars born in SLSNe/LGRBs channels, while FRB 180924-like cases come from those young magnetars born in BNS/BWD/AIC channels. Additionally, most FRB 180924-like bursts should also be repeating in the flaring magnetar framework due to the event rate comparison between magnetars and total FRB events (Nicholl et al 2017;Margalit et al 2019;Ravi 2019), which is supported by FRB 171019 followed by faint bursts (Kumar et al 2019).…”

“…On the other hand, young millisecond magnetars could also be born in binary neutron star (BNS) mergers (Rosswog et al 2003;Price & Rosswog 2006;Giacomazzo & Perna 2013), binary white dwarf (BWD) mergers (King et al 2001;Yoon et al 2007;Schwab et al 2016), or accretion-induced collapse (AIC) of white dwarfs (WDs) (Nomoto & Kondo 1991;Tauris et al 2013;Schwab et al 2015). These magnetars could produce FRBs analogous to FRB 180924, as suggested by Margalit et al (2019). Compared with FRBs created from magnetars born in SLSNe/LGRBs channels, the FRBs produced from magnetars born in BNS/BWD/AIC channels could have a distinct observational properties.…”

“…

It is widely believed that magnetars could be born in core-collapse supernovae (SNe), binary neutron star (BNS) or binary white dwarf (BWD) mergers, or accretion-induced collapse (AIC) of white dwarfs. In this paper, we investigate whether magnetars could also be produced from neutron star-white dwarf (NSWD) mergers, motivated by FRB 190824-like fast radio bursts (FRBs) possibly from magnetars born in BNS/BWD/AIC channels suggested by Margalit et al (2019). By a preliminary calculation, we find that NSWD mergers with unstable mass transfer could result in the NS acquiring an ultra-strong magnetic field via the dynamo mechanism due to differential rotation and convection or possibly via the magnetic flux conservation scenario of a fossil field.

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On the FRB luminosity function – – II. Event rate density

“…One possible scenario would be to invoke an NS-NS post-merger stable magnetar that can survive the merger (e.g. Zhang & Mészáros 2001;Metzger et al 2008;Bucciantini et al 2012;Rezzolla & Kumar 2015;Margalit et al 2019;Wang et al 2020). However, this requires a stiff NS equation of state and small masses in the NS-NS merger systems.…”