Constraining the Type of Central Engine of GRBs with Swift Data

Abstract

The central engine of gamma-ray bursts (GRBs) is poorly constrained. There exist two main candidates: a fastrotating black hole and a rapidly spinning magnetar. Furthermore, X-ray plateaus are widely accepted to be the energy injection into the external shock. In this paper, we systematically analyze the Swift/XRT light curves of 101 GRBs having plateau phases and known redshifts (before 2017 May). Since a maximum energy budget (∼2×10 52 erg) exists for magnetars but not for black holes, this provides a good clue to identifying the type of GRB central engine. We calculate the isotropic kinetic energy E K,iso and the isotropic X-ray energy release E X,iso for individual GRBs. We identify three categories based on how likely a black hole harbors a central engine: "Gold" (9 out of 101; both E X,iso and E K,iso exceed the energy budget), "Silver" (69 out of 101; E X,iso less than the limit but E K,iso greater than the limit), and "Bronze" (23 out of 101; the energies are not above the limit). We then derive and test the black hole parameters with the Blandford-Znajek mechanism, and find that the observations of the black hole candidate ("Gold"+"Silver") samples are consistent with the expectations of the black hole model. Furthermore, we also test the magnetar candidate ("Bronze") sample with the magnetar model, and find that the magnetar surface magnetic field (B p ) and initial spin period (P 0 ) fall into reasonable ranges. Our analysis indicates that if the magnetar wind is isotropic, a magnetar central engine is possible for 20% of the analyzed GRBs. For most GRBs, a black hole is most likely operating.

Closure relations during the plateau emission of Swift GRBs and the fundamental plane

“…In addition to these classes, we also consider the cases of internal and external plateaus. While external plateaus are created by the deceleration of the external shock (Tang et al 2019), internal plateaus are likely powered by the GRB magnetar central engine and are characterized by values of α > (3, 4) according to the literature (see, e.g., Li et al 2018).…”

Multipulse Fermi Gamma-Ray Bursts. I. Evidence of the Transition from Fireball to Poynting-flux-dominated Outflow

“…It should be also noted that estimating Y from afterglow modeling may critically depends on the estimation of kinetic energy E K . Here we consider only the X-ray band (Wygoda et al 2016;Li et al 2018b) to estimate E K , which could also result in a significant underestimate of the true energy of the blast-wave (e.g., Beniamini et al 2015.…”

Deciphering the properties of the central engine in GRB collapsars

“…At the same time, GRBs with very large gammaray isotropic energies and/or prompt emission durations, are not readily accountable by the magnetar model (e.g. Cenko et al 2010;Li et al 2018). In contrast to black hole engines, the available energy to power a GRB is limited by the magnetar's rotational energy (∼2 × 10 52 erg) (e.g.…”

“…those powered by magnetars and those powered by black holes (e.g. Li et al 2018). Even if this is the case, it is not clear if these populations may be discernible from prompt emission observations.…”

“…Assuming that these features are indicative of late-time activity of the central engine, inferences about the nature of the latter can be made (e.g. Lü & Zhang 2014;Zhang et al 2014;Li et al 2018). It is still, however, possible to naturally account for the same features without invoking late-time central engine activity (Beniamini & Kumar 2016;Beniamini & Mochkovitch 2017;Beniamini et al 2020b).…”