LOFAR early-time search for coherent radio emission from GRB 180706A

Abstract

The nature of the central engines of gamma-ray bursts (GRBs) and the composition of their relativistic jets are still under debate. If the jets are Poynting flux dominated rather than baryon dominated, a coherent radio flare from magnetic re-connection events might be expected with the prompt gamma-ray emission. There are two competing models for the central engines of GRBs; a black hole or a newly formed milli-second magnetar. If the central engine is a magnetar it is predicted to produce coherent radio emission as persistent or flaring activity. In this paper, we present the deepest limits to date for this emission following LOFAR rapid response observations of GRB 180706A. No emission is detected to a 3σ limit of 1.7 mJy beam −1 at 144 MHz in a two-hour LOFAR observation starting 4.5 minutes after the gamma-ray trigger. A forced source extraction at the position of GRB 180706A provides a marginally positive (1 sigma) peak flux density of 1.1 ± 0.9 mJy. The data were time-sliced into different sets of snapshot durations to search for FRB like emission. No short duration emission was detected at the location of the GRB. We compare these results to theoretical models and discuss the implications of a non-detection.

Pulsar revival in neutron star mergers: multi-messenger prospects for the discovery of pre-merger coherent radio emission

“…LOFAR has performed successful triggered observations of GRB 180706A (Rowlinson et al 2019) & GRB 181123B (Rowlinson et al 2021). The former was a long GRB but the latter was a short GRB, and its afterglow has been associated with a galaxy at z=1.8 (Paterson et al 2020) with a chance alignment of 0.44%.…”

“…Making use of direction-dependent calibration and source subtraction techniques, Rowlinson et al (2019) were able to place deep fluence limits, corresponding to 3 × 10 3 Jy ms for a typical sGRB of redshift 𝑧 = 1 7 . Given this, and the results of Tian et al (2022), we find that triggered observations by LOFAR and MWA will detect pre-merger coherent emission to a distance of approximately 3 − 5 𝜂 1/2 −2 𝐵 𝑠,12 Mpc, using detection limits in Table 2.…”

“…ter Veen et al 2019), or making use of early time alerts from gravitational wave detectors (James et al 2019). Rapid radio observations of sGRBs can test theories of coherent radiation after the merger (Rowlinson et al 2019) predicted in shocks (Usov & Katz 2000;Sagiv & Waxman 2002), or from a short-lived magnetar remnant (Rowlinson et al 2013). The constraints set by past observations of kind are discussed with respect to the model presented here in Section.…”

Multiwavelength Observations of Fast Radio Bursts

“…The prominent role of magnetars as promising candidates for extragalactic FRB sources has fostered a number of complementary attempts to identify counterparts or associations with other classes of known sources: since magnetars are believed to represent the endpoint of some core-collapsed progenitors of long GRBs (e.g., [56][57][58]), as well as the result of a compact binary merger signalled by a short GRB (e.g., [59][60][61]), some of these GRB sources were targeted by radio follow-up observations, either within hours of the GRB or years later, to search for FRB emission [62][63][64][65][66][67]. Systematic and sensitive searches for emission compatible with MGFs from well localised FRB sources have also been carried out in parallel, both independently of and simultaneously with radio observations, whose results and implications are presented in Section 6.…”

A targeted search for repeating fast radio bursts with the MWA

“…To date, there have been many searches for FRB emission below 300 MHz, including simultaneous, multiband, targeted (Law et al 2017;Sokolowski et al 2018;Houben et al 2019) and widefield blind (Coenen et al 2014;Karastergiou et al 2015;Tingay et al 2015;Rowlinson et al 2016;Sanidas et al 2019) searches using LOFAR, the Murchison Wide-field Array (MWA) and the Long-wavelength Array. In addition, searches for prompt and dispersed (FRB-like) signals predicted to be associated with gammaray bursts have also been conducted with MWA and LOFAR via rapid-response observations (Kaplan et al 2015;Rowlinson et al 2019Rowlinson et al , 2021Anderson et al 2021;Tian et al 2022a,b), none of which have yielded a detection. However, recently Pleunis et al (2021a) reported the detection of 18 bursts from FRB 20180916B between 110-188 MHz using LOFAR, the only detections of any FRB below 300 MHz to date.…”