Klein-Nishina effect on high-energy gamma-ray emission of GRBs Xiang-Yu Wang ( 王祥玉） Nanjing University, China （南京大學） Co-authors: Hao-Ning He (NJU), Zhuo Li (PKU), Zi-Gao Dai (NJU), Xue-Feng Wu (PennState), Peter Meszaros (PennState), Deciphering the Ancient Universe with Gamma-Ray Bursts April 2010, Kyoto, Japan

Xiang-Yu Wang Nanjing Univ. Fermi observations of GRB080916c Abdo et al. 09

Short GRB: T_90=~1 s Significant high-energy emission up to T0+200s Extended high-energy emission of short GRB GRB De Pasquale et al. 09

GRB090902B—a long GRB t^-1.5 Abdo et al. 09

Klein-Nishina (KN) effect may be important in Prompt high-energy gamma-ray emission ( if it is due to synchrotron emission) Prompt high-energy gamma-ray emission ( if it is due to synchrotron emission) Temporal Extended High Energy Emission Temporal Extended High Energy Emission

Klein-Nishina IC scattering Thomson IC scattering KN IC scattering Xiang-Yu Wang Nanjing Univ.

KN effect may be important in Prompt high-energy gamma-ray emission Prompt high-energy gamma-ray emission Temporal Extended High Energy Emission Temporal Extended High Energy Emission

Xiang-Yu Wang Nanjing Univ. Prompt spectrum of GRB080916C ab c d 1.Band function fits the KeV-GeV data 2. No bump is seen at high energies

Some other bursts: high-energy emission consistent with the extrapolation GRB080825C, GRB GRB080825C, GRB GRB090217

Xiang-Yu Wang Nanjing Univ. The synchrotron scenario 1) Can the maximum syn. energy reach 70 GeV ? 1) Can the maximum syn. energy reach 70 GeV ? Yes, only when Bohm diffusive shock acceleration ( ) ---a parameter describing the efficiency of the shock acceleration ( Wang, Li, Dai & Meszaros 2009 ) (cf Ioka’talk)

Xiang-Yu Wang Nanjing Univ. The synchrotron scenario two assumptions: 1)equipartition magnetic field: 2) causality constraint: Inverse Compton must be in the Klein-Nishina regime, which leads naturally to a low, invisible IC component 2) Why no visible IC bump?

Xiang-Yu Wang Nanjing Univ. The synchrotron scenario KN effect on the low-energy spectrum Can makes the low-energy spectrum harder (α= -1.02±0.02) in GRB080916C () Can makes the low-energy spectrum harder (α= -1.02±0.02) in GRB080916C ( also see Derishev et al. 2003; Nakar et al. 09 ) Low-energy spectral index ? The ratio of IC cooling efficiency to syn cooling efficiency is not a constant anymore, but depends on γ

KN effect may be important in Prompt high-energy gamma-ray emission (some bursts Band function fit well, some deviate from Band function) Prompt high-energy gamma-ray emission (some bursts Band function fit well, some deviate from Band function) Temporal Extended High Energy Emission Temporal Extended High Energy Emission

Xiang-Yu Wang Nanjing Univ. Models for the extended emission Hadronic cascade process (Dermer & Atoyan 04) Hadronic cascade process (Dermer & Atoyan 04) Forward shock — long lived Forward shock — long lived  synchrotron: slow decay (Kumar & Barniol Duran09)  IC: rise initially and slow decay (Zhang & Meszaros 01; Fan et al. 08) Reverse shock --- short lived, fast decay (Wang et al. 2001) Reverse shock --- short lived, fast decay (Wang et al. 2001)

Xiang-Yu Wang Nanjing Univ. Forward shock IC emission Forward shock IC emission Zhang & Meszaros 01 First rise, then decay

Forward shock synchrotron scenario Can easily explain the simple decay Can easily explain the simple decay The flux level matches the observations (Kumar &Barniol Duran 09) The flux level matches the observations (Kumar &Barniol Duran 09) Possible problems: Possible problems: 1) maximum photon energy (Abdo et al. arXiv: ; Piran & Nakar 10) 2) too steep (Ghisellini et al. 09), see also Poster #95 (T. Uehara) on A Barniol Duran & Kumar 09

If afterglow emission, KN effect should be taken into account For afterglow electrons in the Thomson scattering, For afterglow electrons in the Thomson scattering, Y Y For high-energy afterglow emission, For high-energy afterglow emission, ( ) is large, inverse Compton scattering with synchrotron peak photons should be in Klein-Nishina regime ( ) is large, inverse Compton scattering with synchrotron peak photons should be in Klein-Nishina regime Sari & Esin 2001: < Compton Y parameter depends on γ, therefore depends on ν !

One example: the slow-cooling case

i) Values of compton Y parameters (t=1 s) Wang, He, et al. 2010, ApJ

Compton Y parameters (t=10 s)

KN effects -summary (1) For a wide range of parameters, Y(100MeV) is initially small, typically smaller than 1 For a wide range of parameters, Y(100MeV) is initially small, typically smaller than 1 Leading to a high synchrotron luminosity at early time Leading to a high synchrotron luminosity at early time Xiang-Yu Wang Nanjing Univ. Y here is dependent of ν Kumar & Barniol Duran 09:

ii) KN effect on the spectrum Slow-cooling caseFast-cooling case

Xiang-Yu Wang Nanjing Univ. The short GRB case (He & Wang 09) Spectra and Light curves under typical parameters for short GRBs Spectra and Light curves under typical parameters for short GRBs

KN effects –summary (2) At very early time, synchrotron emission is usually dominant in the LAT energy band (30MeV to tens of GeV) At very early time, synchrotron emission is usually dominant in the LAT energy band (30MeV to tens of GeV) SSC dominates only above the maximum synchrotron energy SSC dominates only above the maximum synchrotron energy Xiang-Yu Wang Nanjing Univ.

iii) Evolution of Y parameters with time Slow-cooling case Slow-cooling case Wang, He, et al. 2010

The fast-cooling case

KN effects –summary (3) For certain parameter space, Y(100MeV) increases with time--- the KN suppression effect of high-energy electrons weakens with time, so that the IC loss increases with time. For certain parameter space, Y(100MeV) increases with time--- the KN suppression effect of high-energy electrons weakens with time, so that the IC loss increases with time. If Y(100MeV) >1 as well, leading to a steeper decay than predicted by the standard synchrotron theory If Y(100MeV) >1 as well, leading to a steeper decay than predicted by the standard synchrotron theory A testable prediction for this scenario: the spectrum becomes harder meanwhile A testable prediction for this scenario: the spectrum becomes harder meanwhile Xiang-Yu Wang Nanjing Univ.

GRB De Pasquale et al Ghirlanda et al Standard syn model: LAT should decay as

GRB090902B t^-1.5

Xiang-Yu Wang Nanjing Univ. Conclusions If the prompt high-energy emission is of synchrotron origin, KN is important and may affect the prompt low-energy spectrum If the prompt high-energy emission is of synchrotron origin, KN is important and may affect the prompt low-energy spectrum KN effect is important in estimating the afterglow synchrotron emission, which leads to KN effect is important in estimating the afterglow synchrotron emission, which leads to  Early high synchrotron luminosity  Faster temporal decay in some parameter space When modeling the high-energy afterglow emission, treat carefully the KN scattering effect on the electron radiation When modeling the high-energy afterglow emission, treat carefully the KN scattering effect on the electron radiation

Backup slide KN effect on electron distribution How the synchrotron luminosity is affected depends on the electron distribution (Nakar et al. 09; Wang et al. 10) How the synchrotron luminosity is affected depends on the electron distribution (Nakar et al. 09; Wang et al. 10) 1) slow cooling 2) fast cooling