Hypernova and Gamma-Ray Burst Remnants as TeV Unidentified Sources

📝 Abstract

We investigate hypernova (hyper-energetic supernova) and gamma-ray burst (GRB) remnants in our Galaxy as TeV gamma-ray sources, particularly in the role of potential TeV unidentified sources, which have no clear counterpart at other wavelengths. We show that the observed bright sources in the TeV sky could be dominated by GRB/hypernova remnants, even though they are fewer than supernova remnants (SNRs). If this is the case, TeV SNRs are more extended (and more numerous) than deduced from current observations. In keeping with their role as cosmic ray accelerators, we discuss hadronic gamma-ray emission from pi^0 decay, from beta decay followed by inverse Compton emission, and propose a third, novel process of TeV gamma-ray emission arising from the decay of accelerated radioactive isotopes such as 56Co entrained by relativistic or semi-relativistic jets in GRBs/hypernovae. We discuss the relevant observational signatures which could discriminate between these three mechanisms.

💡 Analysis

We investigate hypernova (hyper-energetic supernova) and gamma-ray burst (GRB) remnants in our Galaxy as TeV gamma-ray sources, particularly in the role of potential TeV unidentified sources, which have no clear counterpart at other wavelengths. We show that the observed bright sources in the TeV sky could be dominated by GRB/hypernova remnants, even though they are fewer than supernova remnants (SNRs). If this is the case, TeV SNRs are more extended (and more numerous) than deduced from current observations. In keeping with their role as cosmic ray accelerators, we discuss hadronic gamma-ray emission from pi^0 decay, from beta decay followed by inverse Compton emission, and propose a third, novel process of TeV gamma-ray emission arising from the decay of accelerated radioactive isotopes such as 56Co entrained by relativistic or semi-relativistic jets in GRBs/hypernovae. We discuss the relevant observational signatures which could discriminate between these three mechanisms.

📄 Content

arXiv:0901.0744v2 [astro-ph.HE] 14 Dec 2009 Preprint typeset using LATEX style emulateapj v. 08/22/09 HYPERNOVA AND GAMMA-RAY BURST REMNANTS AS TEV UNIDENTIFIED SOURCES KUNIHITO IOKA1, AND PETER MÉSZÁROS2 ABSTRACT We investigate hypernova (hyper-energetic supernova) and gamma-ray burst (GRB) remnants in our Galaxy as TeV gamma-ray sources, particularly in the role of potential TeV unidentified sources, which have no clear counterpart at other wavelengths. We show that the observed bright sources in the TeV sky could be dominated by GRB/hypernova remnants, even though they are fewer than supernova remnants (SNRs). If this is the case, TeV SNRs are more extended (and more numerous) than deduced from current observations. In keeping with their role as cosmic ray accelerators, we discuss hadronic gamma-ray emission from π0 decay, from β decay followed by inverse Compton emission, and propose a third, novel process of TeV gamma-ray emission arising from the decay of accelerated radioactive isotopes such as 56Co entrained by relativistic or semi-relativistic jets in GRBs/hypernovae. We discuss the relevant observational signatures which could discriminate between these three mechanisms. Subject headings: cosmic rays — gamma rays: bursts — gamma rays: theory — radiation mechanism: non- thermal — supernova remnants

1. INTRODUCTION Observations at a new wavelengths have always led in as- tronomy to new discoveries, such as radio pulsars, gamma- ray bursts (GRBs), etc. The TeV gamma-ray sky is likely to yield similar surprises, since among the rapidly growing num- ber of TeV sources (Aharonian et al. 2006) the most abun- dant category is of a mysterious nature, the so-called TeV unidentified sources (TeV unIDs), which have no clear coun- terpart at other wavelengths (Aharonian et al. 2005, 2008; Mukherjee and Halpern 2005). Our knowledge of TeV unIDs is very limited: (1) So far NunID ∼10-30 TeV unIDs have been observed. (2) They generally lie close to the Galactic plane, suggest- ing a Galactic origin. (3) They are extended, ∆Ω∼0.05-0.3◦. (4) The flux is εγFεγ ∼10−12-10−11 erg s−1 cm−2 at εγ ∼0.2 TeV. (5) They have a power-law spectrum with index of 2.1-2.5. (6) Some TeV unIDs have strong upper limits in X-rays with a TeV to X-ray flux ratio of FTeV/FX ≳50 from Suzaku (Matsumoto et al. 2007; Bamba et al. 2007) and in radio with FTeV/Fradio ∼103 (Atoyan et al. 2006; Tian et al. 2008). The high energy nature of TeV unIDs naturally leads us to consider the cosmic ray (CR) accelerators as their possible origin. Since the dominant CR sources are supernova rem- nants (SNRs), TeV unIDs may be related to SNRs, in partic- ular old ones, which are expected to be less luminous at other wavelengths (Yamazaki et al. 2006). However, the required galactic energy budget of TeV unIDs is only 4πd2εγFεγNunID ∼1034−35  d 10kpc 2 erg s−1, (1) 1 KEK Theory Center and the Graduate University for Advanced Studies (Sokendai), 1-1 Oho, Tsukuba 305-0801, Japan 2 Center for Particle Astrophysics, Dept. of Astronomy and Astrophysics, Dept. of Physics, Pennsylvania State University, University Park, PA 16802 which is much less than that of SNe, 1050erg 100yr ∼1041erg s−1, (2) implying a rarer type of source. Long GRBs are actually rare SNe endowed with rel- ativistic jets, which also are expected to leave SNR-like remnants (Perna et al. 2000; Ayal & Piran 2001). Even so, the expected number of GRB remnants is so small that previous works have addressed only specific sources (Ioka et al. 2004; Atoyan et al. 2006). However, recent ob- servations suggest that hyper-energetic SNe, the so-called hypernovae which are sometimes associated with GRBs such as SN1998bw/GRB980425, SN2003dh/GRB030329 and SN2003lw/GRB031203 (Maeda and Nomoto 2002), oc- cur more frequently than GRBs (Guetta & Della Valle 2007). The hypernova rate may be even higher if we are miss- ing GRB-unassociated hypernovae like SN1997ef. On the other hand, a larger fraction of SNe may be endowed with slower or semi-relativistic jets, causing only low-luminosity (LL) GRBs. Actually, a significant energy ∼1050 erg was released in the form of a mildly relativistic ejecta in the very faint GRB980425/SN1998bw. The recently dis- covered LL GRB060218/SN2006aj could be also be pro- duced by a slower or semi-relativistic jet (Toma et al. 2007; Waxman et al. 2007), occurring at a ∼10 times higher rate than GRBs (Guetta & Della Valle 2007; Liang et al. 2007; Soderberg et al. 2006c). Such semi-relativistic jets could be intermediate between choked and breaking-out jets emerging through the progenitor’s outer envelope. An interesting point in TeV unIDs is that their emission may be related with hadronic processes, since a simple lep- tonic process for TeV gamma-rays by inverse Compton (IC) would require too much synchrotron emission than what is observed in TeV unIDs. Therefore we consider the hadronic processes associated with the π0 decay (§ 2.1) for the hyper- nova shocks, and with the β decay mechan

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