SWIFT OBSERVATIONS OF GAMMA-RAY BURST PULSE SHAPES: GRB PULSE SPECTRAL EVOLUTION CLARIFIED

Abstract

We present a detailed analysis of GRB 151006A, the first GRB detected by Astrosat CZT Im-ager (CZTI). We study the long term spectral evolution by exploiting the capabilities of Fermi and Swift satellites at different phases, which is complemented by the polarization measurement with the CZTI. While the light curve of the GRB in different energy bands show a simple pulse profile, the spectrum shows an unusual evolution. The first phase exhibits a hard-to-soft (HTS) evolution until ∼ 16 − 20 s, followed by a sudden increase in the spectral peak reaching a few MeV. Such a dramatic change in the spectral evolution in case of a single pulse burst is reported for the first time. This is captured by all models we used namely, Band function, Blackbody+Band and two blackbodies+power law. Interestingly, the Fermi Large Area Telescope (LAT) also detects its first photon (> 100 MeV) during this time. This new injection of energy may be associated with either the beginning of afterglow phase, or a second hard pulse of the prompt emission itself which, however, is not seen in the otherwise smooth pulse profile. By constructing Bayesian blocks and studying the hardness evolution we find a good evidence for a second hard pulse. The Swift data at late epochs (> T 90 of the GRB) also shows a significant spectral evolution consistent with the early second phase. The CZTI data (100-350 keV), though having low significance (1σ), show high values of polarization in the two epochs (77% to 94%), in agreement with our interpretation.

A Comprehensive Analysis of Fermi Gamma-Ray Burst Data. IV. Spectral Lag and its Relation to Ep Evolution

“…Some authors also pointed out that the spectral evolution is related to the pulse profile (Hakkila & Preece 2011;Hakkila et al 2015Hakkila et al , 2018. For example, the hard-to-soft spectral evolution may primarily occur in hard and/or asymmetric pulses, and the intensity-tracking spectral evolution may be more prevalent in soft and/or symmetric pulses (Hakkila et al 2015(Hakkila et al , 2018. In addition, Hakkila & Preece (2014) found that the residuals of single pulse fits to GRB lightcurves leave behind a mysterious triple-peaked structure.…”

Time-reversed Gamma-Ray Burst Light-curve Characteristics as Transitions between Subluminal and Superluminal Motion

“…• Pulses evolve from hard-to-soft, with asymmetric pulses showing more pronounced spectral evolution than symmetric pulses (e.g. Crider et al (1999); Ryde & Svensson (1999); Hakkila & Preece (2011); Hakkila et al (2015)). When early pulse emission can be observed, pulses are shown to start near-simultaneously at all energies (Hakkila & Nemiroff 2009).…”

“…These analyses have identified "hard-to-soft" and "intensity tracking" behaviors (e.g., Wheaton et al (1973); Golenetskii et al (1983); Norris et al (1986); Paciesas et al (1992)), but this oversimplified bimodal classification scheme has been shown to represent a continuous range of behaviors (e.g., Kargatis et al (1994); Bhat et al (1994); Ford et al (1995); Borgonovo, & Ryde (2001)). Treating each GRB pulse as a structured episode rather than one in which pulses are statistically-significant intensity peaks, several studies (Liang & Kargatis 1996;Hakkila et al 2015Hakkila et al , 2018a have demonstrated that most pulses undergo and stretched at the time of reflection to show how well they match one another (the solid line represents the folded and stretched residuals preceding the time of reflection, while the dashed line represents the residuals following that time). The process used to identify time-reversality, as well as the fitted characteristics of this pulse, is described in Hakkila et al (2018b).…”

“…• Pulse asymmetry correlates with hardness evolution: asymmetric pulses are harder overall and have more pronounced hard-to-soft evolution than symmetric pulses Hakkila et al (2015), which more commonly exhibit "intensity tracking" behaviors. Structured features, or "spikes" representing strong variability in GRB pulses are generally harder than the underlying smooth pulse emission.…”

“…The stretching factor s mirror measured from GRB pulse residuals is the best linear fit obtained by folding the timeforward part of the residual wave and stretching it until it matches the time-reversed part of the wave. Historically (Hakkila & Preece 2014;Hakkila et al 2015Hakkila et al , 2018a the fitted stretching factor s (and subsequently its modelindependent counterpart s mirror ) was inconveniently defined to be a compression of the residuals duration following the time of reflection relative to the residuals duration preceding it. In other words, small s mirror and s values refer to residual waves that need to be stretched significantly preceding the time of reflection in order to match the component following the time of reflection.…”