Abstract
ABSTRACT The mergers of two neutron stars are typically accompanied by broad-band electromagnetic emission from either a relativistic jet or a kilonova. It has also been long predicted that coherent radio emission will occur during the merger phase or from a newly formed neutron star remnant; however, this emission has not been seen to date. This paper presents the deepest limits for this emission from a neutron star merger, following triggered LOFAR observations of the short gamma-ray burst 181123B, starting 4.4 min after the GRB occurred. During the X-ray plateau phase, a signature of ongoing energy injection, we detect no radio emission to a 3σ limit of 153 mJy at 144 MHz (image integration time of 136 s), which is significantly fainter than the predicted emission from a standard neutron star. At a redshift of 1.8, this corresponds to a luminosity of 2.5 × 1044 erg s−1. Snapshot images were made of the radio observation on a range of time-scales, targeting short-duration radio flashes similar to fast radio bursts. No emission was detected in the snapshot images at the location of GRB 181123B enabling constraints to be placed on the prompt coherent radio emission model and emission predicted to occur when a neutron star collapses to form a black hole. At the putative host redshift of 1.8 for GRB 181123B, the non-detection of the prompt radio emission is two orders of magnitude lower than expected for magnetic reconnection models for prompt GRB emission and no magnetar emission is expected.
Highlights
The detection and association of the gravitational wave event, GW170817, and the short Gamma-Ray Burst (SGRB) 170817A confirmed the theory that the progenitor of many SGRBs is the merger of two neutron stars (Abbott et al 2017)
The model proposed by Usov & Katz (2000), in which the radio and gamma-ray emission originate from magnetic reconnection in a strongly magnetized jet, can be constrained using this ratio and it is equivalent to δ 0.1εB where εB is the proportion of energy contained in the magnetic fields
We find that the rest-frame light curve, at a redshift of 0.7, can be fitted with an unstable 1.4M magnetar that collapses at ∼400 seconds with a magnetic field of 2.4+−11..33 × 1014 G and spin period of 0.095+−00..001210 ms (note Sarin et al (2020) modelled this GRB using a Bayes inference fitting technique and found an earlier collapse time of 250 seconds)
Summary
The detection and association of the gravitational wave event, GW170817, and the short Gamma-Ray Burst (SGRB) 170817A confirmed the theory that the progenitor of many SGRBs is the merger of two neutron stars (Abbott et al 2017). Rowlinson et al (2013) showed that the plateau phases in X-ray light curves following many SGRBs are consistent with the central engine being a magnetar While support for this model has increased, there is currently no ‘smoking gun’ observation to prove that a magnetar was formed via the merger of two neutron stars. At 1.4 GHz, the Australian Square Kilometer Array Pathfinder (ASKAP; Hotan et al 2014) has followed up 20 GRBs (including four SGRBs) with their rapid response system (Bouwhuis et al 2020) After searching their data for FRBs, they concluded there was no pulsed radio emission above 26 Jy ms. 1 8 th of the image and is plotted with the dashed line
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