Precursors in Short Gamma-Ray Bursts as a Possible Probe of Progenitors

Abstract

We extract 18 candidate short gamma-ray bursts (SGRBs) with precursors from 660 SGRBs observed by the Fermi and Swift satellites, and carry out a comprehensive analysis of their temporal and spectral features. We obtain the following results: (1) for a large fraction of candidates, the main burst durations are longer than their precursor durations, comparable to their quiescent times from the end of precursors to the beginning of their main bursts. (2) The average flux of precursors tends to increase as their main bursts brighten. (3) As seen from the distributions of hardness ratio and spectral fitting, the precursors are slightly spectrally softer with respect to the main bursts. Moreover, a significant portion of precursors and all main bursts favor a non-thermal spectrum. (4) The precursors might be a probe of the progenitor properties of SGRBs such as the magnetic field strength and the crustal equation of state if they arise from some processes before mergers of binary compact objects rather than post-merger processes.

Precursor flares of short gamma-ray bursts from crust yielding due to tidal resonances in coalescing binaries of rotating, magnetized neutron stars

“…Strong magnetic fields may be important in this scenario, as there is some evidence to suggest that those binaries which emit precursor flares contain magnetars. For example, the majority of precursor flares exhibit a nonthermal spectrum [9], which would be expected if Alfvén waves propagating along open field lines are the primary means of the associated energy transport, though this requires a high surface field strength B 10 13 G [33,35]. Recent estimates (between ∼ 0.4% [8] and 2.7% [9]) for the proportion of SGRBs that show precursor activity are also consistent with magnetar birth rates expected from population synthesis models [36].…”

“…This proposal is strongly supported by the combined detection of the gravitational wave (GW) event GW170817 from coalescing NSs and the subsequent SGRB that was observed by Fermi and INTEGRAL ∼ 1.7 s after [4,5]. Although not observed for this particular event, some SGRBs are preceded by 'precursor' flares: energetically weaker but spectrally similar flashes are seen several seconds prior to the main burst in some cases [6][7][8][9]. If the main episode arises within 2 seconds after the merger, this would imply that some precursors occur before coalescence.…”

“…A more recent analysis involving a study of 519 SGRBs, which enforced the above condition, concluded that only 0.4% of bursts show precursor activity [8], while the analysis of Ref. [9], which did not enforce this condition, found that 18 of the 660 bursts ( 2.7%) within their sample exhibited precursors, though these latter authors also included events of lower significance (σ 2).…”

“…In this paper, we investigate tidally-forced oscillations and the induced crustal strains of rotating, magnetised NSs, to then compare quake energetics and resonance times with the luminosities and orbital frequencies observed for precursor flares from SGRBs [6][7][8][9]. Strong magnetic fields may be important in this scenario, as there is some evidence to suggest that those binaries which emit precursor flares contain magnetars.…”

“…For example, the majority of precursor flares exhibit a nonthermal spectrum [9], which would be expected if Alfvén waves propagating along open field lines are the primary means of the associated energy transport, though this requires a high surface field strength B 10 13 G [33,35]. Recent estimates (between ∼ 0.4% [8] and 2.7% [9]) for the proportion of SGRBs that show precursor activity are also consistent with magnetar birth rates expected from population synthesis models [36]. Oscillating NSs also emit gravitational radiation [37], so precursors attributable to large mode amplitudes should be accompa-nied by appreciable GW signals [38][39][40], especially if the stars have intrinsic quadrupole moments from magnetic deformations [41][42][43], which may be detectable with existing and upcoming GW observatories [44].…”