A leptonic-hadronic model for the afterglow of gamma-ray burst 090510

arXiv:1004.3330v3 [astro-ph.HE] 5 Oct 2010 Draft version June 4, 2018 Preprint typeset using LATEX style emulateapj v. 11/10/09 A LEPTONIC-HADRONIC MODEL FOR THE AFTERGLOW OF GAMMA-RAY BURST 090510 Soebur Razzaque1,2 Draft version June 4, 2018 ABSTRACT We model multiwavelength afterglow data from the short Gamma-Ray Burst (GRB) 090510 using a combined leptonic-hadronic model of synchrotron radiation from an adiabatic blast wave. High energy, ≳100 MeV, emission in our model is dominated by proton-synchrotron radiation, while electron-synchrotron radiation dominates in the X ray and ultraviolet wavelengths. The collimation- corrected GRB energy, depending on the jet-break time, in this model could be as low as 3 × 1051 erg but two orders of magnitude larger than the absolute γ ray energy. We also calculated the opacities for electron-positron pair production by γ rays and found that TeV γ rays from proton-synchrotron radiation can escape the blast wave at early time, and their detection can provide evidence of a hadronic emission component dominating at high energies. Subject headings: gamma-ray burst: individual (GRB090510) – relativistic processes – shock waves 1. INTRODUCTION Gamma-Ray Burst science has entered a new era with launch of the Fermi γ-ray space telescope. The main instrument, Large Area Telescope (LAT), is more sen- sitive than any previous instrument in the 20 MeV – 300 GeV range (Atwood et al. 2009), whereas the Fermi Gamma-ray Burst Monitor (GBM) observes the whole unocculted sky in the 8 keV – 40 MeV range (Meegan et al. 2009). With the advent of the Burst Alert Tele- scope (BAT), X-Ray Telescope (XRT) and UV-Optical Telescope (UVOT) onboard the Swift satellite (Gehrels et al. 2004) it is now possible to obtain simultaneous multiwavelength data in the optical to multi-GeV γ-ray energy range from GRBs. GRB 090510 is the first GRB to provide data from si- multaneous observtions by Fermi (Abdo et al. 2010) and Swift (Hoversten et al. 2009), as well as by a couple of other satellites. At a redshift z = 0.903 ± 0.003 (Rau et al. 2009) the isotropic-equivalent γ-ray energy release from this short GRB (T90 ≲2 s) is Eγ,iso = (1.08±0.06)× 1053 erg with a fluence of (5.03 ± 0.25) × 10−5 erg cm−2 in the 10 keV – 30 GeV range (Abdo et al. 2010). Fermi LAT detected long-lived emission up to ∼200 s after trig- ger (T0 = 00:23:00 UT, 2009 May 10) in the ≳100 MeV range. While such high-energy emission, that is tempo- rally extended beyond the keV – MeV emission, was first detected in GRB 940217 by the Compton Gamma-Ray Observatory (Hurley et al. 1994), this feature is common to most GRBs detected with Fermi LAT. Swift XRT and UVOT collected data from GRB 090510 between ∼T0 + 97 s and T0 + 1.9 ks before an Earth Occultation (EO), and again after T0 +5.1 ks (Hoversten et al. 2009). Swift BAT collected most data within T90 = 0.3 ± 0.1 s (15 – 350 keV), and sparsely between T0 + 0.4 s and ∼T0 + 100 s (Hoversten et al. 2009). Smooth temporal evolution of the flux, F ∝t−α, of the long-lived emission in Fermi LAT (αγ = 1.38±0.07), and Swift XRT (αX,1 = 0.74 ± 0.03 before EO) and UVOT 1 Space Science Division, Code 7653, U.S. Naval Research Lab- oratory, 4555 Overlook Ave SW, Washington, DC 20375; sraz- zaque@ssd5.nrl.navy.mil 2 National Research Council Research Associate (αO,1 = −0.50+0.11 −0.13 before EO) observations strongly suggest an afterglow origin (De Pasquale et al. 2010). Synchrotron radiation by shock-accelerated electrons in a decelerating GRB blast wave (M´esz´aros & Rees 1997; Sari et al. 1998) have successfully explained much of the broadband afterglow data at radio, optical, and X-ray frequencies in the pre-Fermi era. However fitting com- bined Fermi and Swift data from GRB 090510 with sim- ple e-synchrotron model results in unusual parameter values, and most importantly it is difficult to reconcile the Fν ∝t−αν−β temporal relations (De Pasquale et al. 2010). More complex scenarios have been proposed to model GRB 090510 data such as a radiative fireball in an e± pair dominated environment (Ghirlanda et al. 2009; Ghisellini et al. 2009), adiabatic fireball in a low den- sity medium and small magnetic field (Kumar & Barniol Duran 2009a,b; Gao et al. 2009), and two component jet (Corsi et al. 2009). Here we present a combined leptonic- and hadronic- afterglow model to fit multiwavelength data from GRB 090510. Inclusion of ion acceleration and radiation in the GRB blast wave is a natural and simple extension of the e-synchrotron blast wave model, and has been discussed by a number of authors (B¨ottcher & Dermer 1998; Totani 1998a; Zhang & M´esz´aros 2001; Wang et al. 2009; Raz- zaque et al. 2010). We show that LAT emission in the ≳100 MeV range is dominated by synchrotron radiation from protons accelerated in the external forward shock of a decelerating blast wave. (Note that Razzaque et al. (2010) considered proton-synchrotron radiation from a coasting blast wave.) The XRT and UVOT light curves can be reaso

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