Magnetar Bursts

Counterparts to Fast Radio Bursts

FRBs are recently discovered millisecond timescale radio transients that are detected from cosmological distances (∼Gpc). The isotropic burst energies of FRBs (10³⁸⁻⁴² ergs) are almost a trillion times higher than the brightest radio pulses observed from Galactic pulsars. Due to the short timescale and the luminosity of FRBs, neutron stars, especially magnetars (e.g. Metzger et al. 2019; Lyutikov & Popov 2020) are leading candidates for their origins. Similarly, many FRB models expect prompt radio counterparts to be emitted with BNS and NSBH mergers (Pshirkov & Postnov 2010; Totani 2013; Mingarelli et al. 2015; Paschalidis & Ruiz 2019; Rowlinson & Anderson 2019). From the large volumetric rates of FRBs, compared to those of BNS and NSBH mergers, it is clear that these would contribute to a small fraction of observed FRBs. To date, while the observational data is rapidly increasing, the evidence is heterogenous with a multiple theories (see e.g. Platts et al. 2019, for a review) and plenty of open questions remain about the origins of FRBs (Petroff et al. 2019, 2022).

Most mechanisms expect that the radio emission of fast radio bursts is a small fraction of the total burst energy and prompt counterparts as well as afterglows are expected in different wavebands, including X-rays. Due to the extreme sensitivity of radio telescopes compared to X-ray and optical telescopes, it is expected that most models predict that typical FRBs will not have detectable high-energy counterparts. By significantly increasing the rate of X-ray and gamma-ray transients, Daksha will be able to help identify and constrain X-ray and gamma-ray counterparts of FRBs, a search that has yet been unsuccessful from existing missions, see e.g. Cunningham et al. (2019); Anumarlapudi et al. (2020); Guidorzi et al. (2020); Curtin et al. (2022); Principe et al. (2022).

On 2020 April 28, an energetic radio burst with a total isotropic radio emission of 10³⁴ erg from the Galactic magnetar SGR 1935+2154 (The CHIME/FRB Collaboration et al. 2020; Bochenek et al. 2020b) accompanying an X-ray burst with an energy of ≈ 10³⁹ erg. The X-ray burst was delayed by 6 ms relative to the radio emission and was significantly harder (Epeak 65 keV )compared to typical magnetar bursts (Mereghettiet al. 2020). While the radio energy output of this burst was few orders of magnitude lower than that of typical FRBs, this burst was the brightest radio transient ever observed and partially bridges the energy gap between radio pulsars and FRBs. Given the scarcity of bright FRBs and the faintness of their corresponding high energy counterparts Daksha's broad sky-coverage and low fluence threshold will be able to detect or rule out high energy counterparts to FRBs detected by ground-based instruments such as CHIME (CHIME/FRB collaboration et al 2018), ASKAP (Bannister et al. 2017), STARE2 (Bochenek et al., 2020), and upcoming telescopes such as BURSTT (Lin et al. 2022)

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