Surprise in simplicity: an unusual spectral evolution of a single pulse GRB 151006A

Abstract

We present a detailed analysis of GRB 151006A, the first GRB detected by Astrosat CZT Imager (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.

Violation of Synchrotron Line of Death by the Highly Polarized GRB 160802A

“…The evolution of the peak energy (E p ) of GRBs has been found to show hard to soft evolution with time in some GRBs, E p tracks the intensity variations in others, while some GRBs show a mixture of both phenomena (Lu et al 2012). An exception to this behavior is found in the case of GRB 151006A (Basak et al 2017) where a hardening (increase) in E p is found in an apparently single pulse towards the end of the emission. The variation of the spectral parameters along with flux ( Figure 4) show the E p values for the first episode initially follow a hard to soft (HTS) trend, and then track the 8-900 keV flux.…”

Role of THESEUS in Understanding the Radiation Mechanism of GRB Prompt Emission

“…Our third example is GRB 151006A (Rao et al 2016;Basak et al 2017). This is the first GRB detected by Astrosat CZTI and has a polarization measurement (Rao et al 2016).…”

Observing Compact Stars with AstroSat

“…The combined effective area of the three units is ∼ 6000 cm 2 in 5−20 keV band, dropping to ∼ 5000 cm 2 at 50 keV and ∼ 1000 cm 2 at 100 keV. The instrument records photon events with a time resolution of 10 µs.…”

“…Two modes of readout are available to the user, a Photon Counting (PC) mode of the full CCD frame with a time resolution of 2.4 s, and a Fast Window (FW) mode that reads only the central 150×150 pixels with a time resolution of 0.278 s, which is the highest time resolution available with the SXT. The peak effective area is ∼ 120 cm 2 in 0.8-2 keV range; over 2.5-5 keV the area is ∼ 60 cm 2 and drops gradually to ∼ 7 cm 2 at 8 keV. Details of the SXT and its operations are described in Singh et al (2017).…”

“…This instrument consists of three position sensitive proportional counter detectors with one-dimensional coded masks, akin to the All Sky Monitor aboard NASA's Rossi X-ray Timing Explorer mission (Levine et al 1996). Each detector has a geometric area of ∼ 60 cm 2 , and an operating energy range of 2-10 keV. In one rotation of the platform the field of view of these cameras sweep through a full hemisphere of the sky.…”