Gamma-ray Burst Prompt Emissions: Fermi GBM Time-resolved - PowerPoint PPT Presentation

Gamma-ray Burst Prompt Emissions: Fermi GBM Time-resolved Spectroscopy Hoi-Fung Yu 1 Collaborators: Jochen Greiner 1 *, William S. Paciesas 2 *, Robert D. Preece 3 , Charles A. Meegan 3 , Michael S. Briggs 3 , David Gruber 1 , Andreas von Kienlin 1 1 Max-Planck-Institut für extraterrestrische Physik, Germany 2 Universities Space Research Association, USA 3 University of Huntsville in Alabama, USA * Fermi GBM Principal Investigator 4 th FAN Workshop, University of Hong Kong, Hong Kong - 08.07.2013

Fermi GBM These detectors and the data read-out systems were actually built at MPE! • The Gamma-ray Burst Monitor (GBM) is the second payload onboard Fermi , which is a joint project of MPE and UAH • GBM consists of 12 thallium activated sodium iodide NaI detector (Meegan et al. 2009) scintillation detectors (NaIs) and 2 bismuth germanate scintillation detectors (BGOs) • NaIs cover lower energy spectrum (8 keV - 1 MeV), BGOs cover higher energy spectrum (200 keV - 40 MeV) BGO detector (Meegan et al. 2009) Hoi-Fung Yu: Fermi GBM Time-resolved Spectroscopy - 08.07.2013

• The axes of the NaI detectors are oriented such that the positions of GRBs can be derived from the measured relative counting rates, a technique previously employed by CGRO BATSE (Meegan et al. 2009) • Joint analysis of spectra and time histories of Gamma-ray Bursts (GRBs) observed by both the GBM and LAT is possible for bright and hard GRBs • Provide near-realtime burst locations onboard to permit repointing of the spacecraft to obtain LAT observations of delayed emission from bursts, and to disseminate burst locations rapidly to the GRB community • Provide excellent quality of spectral (128 energy channels CSPEC and TTE data) and temporal (up to 2 us TTE data) resolved Locations and orientations of the GBM data! detectors (Meegan et al. 2009) Hoi-Fung Yu: Fermi GBM Time-resolved Spectroscopy - 08.07.2013

GRBs • Gamma-ray burst was first discovered in 1967 by Vela 3 & 4 satellites. In 1973 Klebesadel et al. estimated the rough sky positions of 16 bursts and ruled out solar system origin, thus confirming the cosmological nature of GRBs • After ~ 40 years of research, we still don’t Paciesas et al. (2012) showing the sky positions of 491 GRBs detected have a consensus about the underlying by Fermi GBM in the first 2 years of mission physical processes producing GRBs • There are two empirical types of GRBs, namely the long and short GRBs. Usually the GRB research community adopts the definition that for short GRBs T 90 < 2 s and for long GRBs T 90 > 2 s (Kouveliotou et al. 1993) • T 90 is defined to be the time interval in the observer’s frame such that the 5% - 95% counts of the prompt gamma-ray emission are observed within a certain Kouveliotou et al. (1993) showing the bimodality of energy range (usually 50-300 keV) GRBs detected in the BATSE first year catalog Hoi-Fung Yu: Fermi GBM Time-resolved Spectroscopy - 08.07.2013

• Using dynamical arguments, the prevailing physical explanation for GRBs is that the long GRBs are related to core-collapse massive stars, while the short GRBs are related to compact binary mergers (see, e.g., Kouveliotou, Woosley, & Wijers 2002; Woosley & Bloom 2006; Nakar 2007 for reviews) • GRBs can release in gamma-ray energies as high as E iso ~ 10 53 erg s -1 in the rest frame (Gruber et al. 2011) Gruber et al. (2011) showing the distribution of rest frame isotropic energies of 32 Fermi GBM bursts • However, the emission process of high energy photons is still poorly understood today Hoi-Fung Yu: Fermi GBM Time-resolved Spectroscopy - 08.07.2013

• The popular model among the GRB community is the so-called “fireball model” (Cavallo & Rees 1978; Goodman 1986; Paczy ń ski 1986) which assumes a large amount of energy is released in a small area resulting in high energy emissions in the gamma-ray energy band • The prevailing explanation is that relativistic expanding shells of baryons colliding with each other and with the interstellar medium (indeed not necessarily ISM, can also be the remaining materials from the progenitor), creating shock waves and converting kinetic and thermal energy into gamma radiation (see, e.g., Cohen et al. 1997) Schematic drawing of the internal-external shock model of GRBs (Mészáros 2001) • Observations suggest that the gamma-ray prompt emission is due to “internal shocks” between expanding shells (for reviews see e.g. Piran 1999; Mészáros 2001) while the X-ray/optical/infrared/radio afterglow emission is due to “external shocks” between the shells and the ISM Hoi-Fung Yu: Fermi GBM Time-resolved Spectroscopy - 08.07.2013

GRB Spectral Properties • The most frequently used model is the Band’s GRB function, first introduced by Band et al. (1993): • If beta < -2 then E peak represents the peak energy in the vF v spectrum. The Band function has been successfully fitting most of the GRB spectra (both time-averaged and time-resolved, e.g. Preece et al. 1998; Kaneko et al. 2006; Goldstein et al. 2012; Gruber et al. 2013, in prep.). However, it bears no physical origin and is completely empirical • The simplest and most intuitive physical emission mechanism of the fireball model is the synchrotron emission by electrons in the shocked materials, which provides theoretical predictions for the power-law indices and the break frequencies of the spectrum (Rees & Mészáros 1992, 1994; Mészáros & Rees 1993; Katz 1994; Tavani 1996) • With certain physical assumptions, it predicts the value of alpha to be within -2/3 and -3/2 (the so-called “synchrotron line-of-death” and the “second line-of-death” respectively, see Preece et al. 2002). The difference in alpha and beta can also be used to constrain some physical parameters (Preece et al. 2002). There are also works done using physical models to fitting the data (e.g. the synchrotron cooling + blackbody model used in Burgess et al. 2011, 2013, in prep.) Hoi-Fung Yu: Fermi GBM Time-resolved Spectroscopy - 08.07.2013

• Two approaches to distinguish among various physical models: • to fit multi-wavelength data simultaneously in order to obtain widest spectral coverage (e.g. Kopac et al. 2013; Elliott, Yu et al. 2013, in prep.) • to fit the data using time-resolved instead of time-averaged spectroscopy in order to separate contributions from different emission episodes and to identify spectral evolutions, e.g. Burgess et al. 2011; 2013, in prep.; Yu et al. 2013, in prep.) • However, the progress of researches was slow until recently years, due to the difficulty in obtaining multi-wavelength and high temporal resolution data for large GRB samples preliminary Joint gamma-ray, X-ray, optical and infrared spectral fit of the recent GRB 121217A (Elliott, Yu et al. 2013, in prep.). This burst is unfortunately not bright enough to provide enough counts for a good time-resolved spectral analysis. Hoi-Fung Yu: Fermi GBM Time-resolved Spectroscopy - 08.07.2013

Current Work: Fermi Time-resolved Spectral Catalog • In this study we will give the most detailed time-resolved spectral analysis of Fermi GBM GRBs, thanks to the high temporal (64 ms for TTE data) and spectral resolution (128 energy channels for both the NaIs and BGOs) of the Fermi GBM • We selected bursts with 10 - 1000 keV fluence > 4 x 10 -5 erg cm -2 and 50 - 300 keV peak flux > 55 ph s -1 cm -2 , which consists the brightest Fermi GRBs for high quality time-resolved spectroscopy • In the cases of weak BGO detections (i.e. relatively soft bursts), we include at least one of the BGOs to give an upper limit in the > 1 MeV counts, which can constraint the high-energy index beta • 54 out of the 954 GRBs in the 2 nd Fermi GBM GRB catalog (von Kienlin et al. 2013, in prep.) match the above selection criteria preliminary preliminary > 4 x 10 -5 erg cm -2 > 55 ph s -1 cm -2 Histogram plots of the energy fluence and peak photon flux of 954 GRBs detected in the first 4 years of Fermi mission (Yu et al. 2013, in prep.) Hoi-Fung Yu: Fermi GBM Time-resolved Spectroscopy - 08.07.2013

GRB 130606B preliminary • We use the bright and hard burst 130606B to illustrate the works that could be showing in the time-resolved spectral catalog (Yu et al. 2013) • We fitted individual time bins with the following 3 spectral models and choose the best one with the lowest Casher C- statistics (C-stat, Cash 1979): • Power-law preliminary • Comptonized (high-energy cutoff) • Band function • E peak traces the spectral hardness of the burst. There are basically two empirical types of E peak evolution: (1) power-law decay and (2) pulse-tracking behavior (see e.g. Preece et al. 2000; Kaneko et al. 2006) • In this burst, the values alpha is consistent with the -2/3 and -3/2 lines-of-death Spectral evolution of GRB 130606B with the 1 s binning CSPEC throughout the whole prompt emission lightcurve overlaid Hoi-Fung Yu: Fermi GBM Time-resolved Spectroscopy - 08.07.2013

• The temporal evolution of beta can be used to trace the bulk preliminary lorentz factors of colliding shells (Lithwich & Sari 2001) • In this burst the redshift is not available, so we computed the minimum initial lorentz factor as a function of redshift, as described in Gruber et al. 2010, according to the formulae given preliminary in Lithwich & Sari 2001: Estimation of the minimum (initial) lorentz factors of different emission episodes of GRB 130606B as a function of redshift Hoi-Fung Yu: Fermi GBM Time-resolved Spectroscopy - 08.07.2013