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Register a new userX - Ray Flares and Their Connection With Prompt Emission in GRBs
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We use a wavelet technique to investigate the time variations in the light curves from a sample of GRBs detected by Fermi and Swift. We focus primarily on the behavior of the flaring region of Swift-XRT light curves in order to explore connections between variability time scales and pulse parameters (such as rise and decay times, widths, strengths, and separation distributions) and spectral lags. Tight correlations between some of these temporal features suggest a common origin for the production of X-ray flares and the prompt emission.
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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 $E_{iso}gtrsim10^{52}$~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 $sim 10^{15}$--$10^{17}$~cm with Lorentz gamma factor $Gammasim10^{2}$--$10^{3}$. 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 $sim10^{12}$~cm with $Gammalesssim 4$. 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 $simeq10^{10}$~cm reaching transparency at $sim10^{12}$~cm with $Gammalesssim4$. 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.
We study the observed correlations between the duration and luminosity of the early afterglow plateau and the isotropic gamma-ray energy release during the prompt phase. We discuss these correlations in the context of two scenarios for the origin of the plateaus. In the first one the afterglow is made by the forward shock and the plateau results from variations of the microphysics parameters while in the second one the early afterglow is made by a long-lived reverse shock propagating in a low Lorentz factor tail of the ejecta.
From a sample of GRBs detected by the $Fermi$ and $Swift$ missions, we have extracted the minimum variability time scales for temporal structures in the light curves associated with the prompt emission and X-ray flares. A comparison of this variability time scale with pulse parameters such as rise times,determined via pulse-fitting procedures, and spectral lags, extracted via the cross-correlation function (CCF), indicate a tight correlation between these temporal features for both the X-ray flares and the prompt emission. These correlations suggests a common origin for the production of X-ray flares and the prompt emission in GRBs.
The prompt emission in long gamma-ray bursts arises from within relativistic outflows created during the collapse of massive stars, and the mechanism by which radiation is produced may be either magnetically- or matter-dominated. In this work we suggest an observational test of a magnetically-dominated Poynting flux model that predicts both gamma-ray and low-frequency radio pulses. A common feature among early light curves of long gamma-ray bursts are X-ray flares, which have been shown to arise from sites internal to the jet. Ascribing these events to the prompt emission, we take an established Swift XRT flare sample and apply a magnetically-dominated wind model to make predictions for the timing and flux density of corresponding radio pulses in the ~100-200 MHz band observable with radio facilities such as LOFAR. We find that 44 per cent of the X-ray flares studied would have had detectable radio emission under this model, for typical sensitivities reached using LOFARs rapid response mode and assuming negligible absorption and scattering effects in the interstellar and intergalactic medium. We estimate the rate of Swift gamma-ray bursts displaying X-ray flares with detectable radio pulses, accessible to LOFAR, of order seven per year. We determine that LOFAR triggered observations can play a key role in establishing the long debated mechanism responsible for gamma-ray burst prompt emission.
We analyze GRB 151027A within the binary-driven hypernova (BdHN) approach, with progenitor a carbon-oxygen core on the verge of a supernova (SN) explosion and a binary companion neutron star (NS). The hypercritical accretion of the SN ejecta onto the NS leads to its gravitational collapse into a black hole (BH), to the emission of the GRB and to a copious $e^+e^-$ plasma. The impact of this $e^+e^-$ plasma on the SN ejecta explains {the} early soft X-ray flare observed in long GRBs. We here apply this approach to the UPE and to the hard X-ray flares. We use GRB 151027A as a prototype. From the time-integrated and the time-resolved analysis we identify a double component in the UPE and confirm its ultra-relativistic nature. We confirm the mildly-relativistic nature of the soft X-ray flare, of the hard X-ray flare and of the ETE. We show that the ETE identifies the transition from a SN to the HN. We then address the theoretical justification of these observations by integrating the hydrodynamical propagation equations of the $e^+ e^-$ into the SN ejecta, the latter independently obtained from 3D smoothed-particle-hydrodynamics simulations. We conclude that the UPE, the hard X-ray flare and the soft X-ray flare do not form a causally connected sequence: Within our model they are the manifestation of textbf{the same} physical process of the BH formation as seen through different viewing angles, implied by the morphology and the $sim 300$~s rotation period of the HN ejecta.
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