Measuring the Pulse of GRB 090618: A Simultaneous Spectral and Timing Analysis of the Prompt Emission

Abstract

We develop a new method for simultaneous timing and spectral studies of Gamma Ray Burst (GRB) prompt emission and apply it to make a pulse-wise description of the prompt emission of GRB 090618, the brightest GRB detected in the Fermi era. We exploit the large area (and sensitivity) of Swift /BAT and the wide band width of Fermi/GBM to derive the parameters for a complete spectral and timing description of the individual pulses of this GRB, based on the various empirical relations suggested in the literature. We demonstrate that this empirical model correctly describes the other observed properties of the burst like the variation of the lag with energy and the pulse width with energy. The measurements also show an indication of an increase in pulse width as a function of energy at low energies for some of the pulses, which is naturally explained as an off-shoot of some particular combination of the model parameters. We argue that these model parameters, particularly the peak energy at the beginning of the pulse, are the natural choices to be used for correlation with luminosity. The implications of these results for the use of GRBs as standard candles are briefly described.

Discovery of Smoothly Evolving Blackbodies in the Early Afterglow of GRB 090618: Evidence for a Spine–sheath Jet?

“…Moreover, the correct shape of the thermal component is debatable. For example, the timeresolved spectra of GRB 090902B are consistent with a variety of models -a single blackbody with a powerlaw (BBPL; Zhang et al 2011), a multicolour blackbody along with a power-law (mBBPL; Ryde et al 2010), and two blackbodies with a power-law (2BBPL; Basak & Rao 2013a;Rao et al 2014). Such a diversity in the spectral models is perhaps due to the limited energy resolution and/or bandwidth of the instruments used for GRB spectral analysis.…”

“…The value of α, however, is not always compatible with this non-thermal model, and sometimes is found to be even greater than the maximum value allowed in a slow cooling regime, -2/3 (Crider et al 1997(Crider et al , 1999Preece et al 1998). In addition, evidence for a thermal component has been seen in a few bright GRBs (e.g., Ghirlanda et al 2003;Ryde 2004Ryde , 2005Shirasaki et al 2008;Ryde & Pe'er 2009;Guiriec et al 2011Guiriec et al , 2013Axelsson et al 2012;Basak & Rao 2013b;Rao et al 2014). The spectrum contains either only a thermal component, or a combination of a thermal and a non-thermal component.…”

GRB as luminosity indicator

“…Pulse-wise analysis not only restores the correlation, it improves that (0.89). They used the pulse description of Basak & Rao (2012a) and found that replacement of E peak with a new quantity of their model, namely the peak energy at zero fluence (E peak,0 ) improves the correlation even further (0.96).…”

Pulse-wise GRB correlation: Implication as a cosmological tool

“…4 For this we use the spectral model as Band function 9 , lightcurve as Norris model 10 and spectral evolution as Liang-Kargatis model…”

“…In recent works, 4,5 we have given particular attention to the individual GRB pulses and obtained the timing and spectral informations. We have then used the GRB pulses to study GRB correlations 6,7 similar to the Amati correlation 8 .…”