Early X-Ray Flares in GRBs

Abstract

We analyze the early X-ray flares in the GRB "flare-plateau-afterglow" (FPA) phase observed by Swift-XRT. The FPA occurs only in one of the seven GRB subclasses: the binary-driven hypernovae (BdHNe). This subclass consists of long GRBs with a carbon-oxygen core and a neutron star (NS) binary companion as progenitors. The hypercritical accretion of the supernova (SN) ejecta onto the NS can lead to the gravitational collapse of the NS into a black hole. Consequently, one can observe a GRB emission with isotropic energy E iso 10 52 erg, as well as the associated GeV emission and the FPA phase. Previous work had shown that gamma-ray spikes in the prompt emission occur at ∼ 10 15 -10 17 cm with Lorentz gamma factor Γ ∼ 10 2 -10 3 . Using a novel data analysis we show that the time of occurrence, duration, luminosity and total energy of the X-ray flares correlate with E iso . A crucial feature is the observation of thermal emission in the X-ray flares that we show occurs at radii ∼ 10 12 cm with Γ 4. These model independent observations cannot be explained by the "fireball" model, which postulates synchrotron and inverse Compton radiation from a single ultra relativistic jetted emission extending from the prompt to the late afterglow and GeV emission phases. We show that in BdHNe a collision between the GRB and the SN ejecta occurs at 10 10 cm reaching transparency at ∼ 10 12 cm with Γ 4. The agreement between the thermal emission observations and these theoretically derived values validates our model and opens the possibility of testing each BdHN episode with the corresponding Lorentz gamma factor.

Multivariate Analysis of BATSE Gamma-Ray Burst Properties Using Skewed Distributions

“…Acuner & Ryde (2018) employed the Gaussian mixture model to analyze Fermi GRBs in a different fivedimensional space of the Band et al (1993) spectral parameters (α, β, E peak ), the duration T 90 and the fluence, and also claimed evidence for five groups. For completeness, it should be noted that Ruffini et al (2018) specified seven GRB classes.…”

Induced Gravitational Collapse, Binary-Driven Hypernovae, Long Gramma-ray Bursts and Their Connection with Short Gamma-ray Bursts

“…The SN shock breakout and the hypercritical accretion can be observed as X-ray precursors [52]. The e + e − feedback onto the SN ejecta also produces gamma-and X-ray flares observed in the early afterglow [40]. The synchrotron emission by relativistic electrons from the νNS in the expanding magnetized HN ejecta and the νNS pulsar emission explain the early and late X-ray afterglow [91].…”

“…It was recently addressed in [40] the role of X-ray flares as a powerful tool to differentiate the BdHN model from the "collapsar-fireball" model [114].…”

“…Second, the thermal emission observed in the X-ray flares of the early (rest-frame time t ∼ 10 2 s) afterglow of BdHNe, implies occurrence radii ∼10 12 cm expanding at mildly-relativistic velocity, e.g., Γ 4 [40] (see below). The latter observational fact evidences that the X-ray afterglow is powered by a mildly-relativistic emitter.…”

“…It was recently shown in [40] that the observed thermal emission in the X-ray flares present in the early (rest-frame time t ∼ 10 2 s) afterglow implies an emitter of size ∼10 12 cm expanding at mildly-relativistic velocity, e.g., Γ 4. This is clearly in contrast with the "collapsar-fireball" scenario in which there is an ultrarelativistic emitter (the jet) with Γ ∼ 10 2 -10 3 extending from the prompt emission all the way to the afterglow.…”