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Very High Energy Emission from Gamma-Ray Bursts: Detectability and Significance

This study discusses the detection and significance of very high-energy emissions, particularly Inverse Compton (IC) components, from Gamma-Ray Bursts using the LAT instrument. It explores the importance of IC and synchrotron emission in different cooling scenarios and analyzes IC emission spectra and detectability in the GeV band. The study also delves into the implications of IC emission from afterglows and highlights different models explaining high-energy emissions, such as the Delayed External Shock scenario.

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Very High Energy Emission from Gamma-Ray Bursts: Detectability and Significance

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  1. GeV emission from Gamma-Ray Bursts Alessandra Galli1,2,3 1: INFN-Trieste, 2:University of Rome “La Sapienza” 3: INAF/IASF-Rome Asi Science Data Center 3 July 2007

  2. Very High Energy emission Hurley et al. 1994 At high energies additional emission process is required, such as Inverse Compton A. Galli-ASDC, 3 July 2007

  3. Galli & Piro 2007 External Shock: Inverse Compton vs Synchrotron Y=LIC/Lsyn FAST COOLING: =1, Y∝(εe/εB)1/2 SLOW COOLING: <1, Y∝(εe/εB)1/2 εB=8·10-2 (E53 n)-1/4 [1+(εe/εB)1/2]-1/2 · ·(εe/εB) -1/2 Td1/4 (1+z)-1/4 Relative importance of IC and synchrotron emission greater in fast cooling than in slow cooling The importance of IC increases with E53 and n A. Galli-ASDC, 3 July 2007

  4. Detectability of the Inverse Compton component in the GeV band with LAT GLAST Galli & Piro 2007 Tint=104 sec Fth(1GeV)~10-10 erg cm-2s-1 Tint=500 sec Fth(1GeV)~2·10-9 erg cm-2s-1 A. Galli-ASDC, 3 July 2007

  5. IC Spectra 50 ksec 500 sec 10 ksec Γ0=120, E53 = 1.0, n = 5, εe= 0.2, p=2.5, and z=1. Green, blue and red spectra are obtained respectively for εB= 10-4, εB= 10-3, and εB= 10-2. A. Galli-ASDC, 3 July 2007

  6. IC emission from afterglow: application to GRB 940217 High energy emission, 30 MeV-30 GeV, up to 5000 sec (Hurley et al. 1994) best fit power law: γ=2.830.64  S~7·10-6 erg cm-2 , F ~2·10-9 erg cm-2 s-1 E53 = 5.0, n = 3.0, εe=0.07, εB= 0.001, p = 2.5, z =1 500 sec 5000 sec F ~t1/3 v < νc,IC F ~t1/8 νc,IC < v < νi,IC F~t-(10-9p)/8νi,IC < ν νc,IC < νi,IC < νobsγ=(p+2)/2 p=2.5  γ=2.25 A. Galli-ASDC, 3 July 2007

  7. Delayed External Shock scenario: thick shell fireballs as a possible explanation for X-ray flares Δ=cteng, thus teng >tdec. Most of the energy is transferred to the surrounding material around the end of the engine activity. Afterglow decay described by a power law only if the time is measured from the instant of the central engine turns off (Lazzati & Begelman, 2005). The flare is produced by an external shock caused by an energy injection lasting until the time of the flare occurrence, i.e. requires long lasting central engine activity A. Galli-ASDC, 3 July 2007

  8. GeV flares in association with X-ray flares -Internal Shock: Low Lorentz factor, low Thompson cross section  no bright GeV flares Different emitting regions  temporal dilatation -External Shock: Higher Lorentz factor  brighter high energy flares Same region and electrons population  similar temporal profiles External Shock Internal Shock 2: IC + 2nd order IC Internal Shock 1: Synchr + self IC A. Galli-ASDC, 3 July 2007

  9. IC emission from a thick shell fireball ISM-Fast Cooling 100 GeV 1 GeV 1 GeV 100 GeV X-ray Galli & Piro, 2007 niIC niIC niIC E53=5,=300, n=1, e=0.1,B=10-4, p=2.5, z=1, t0=500 s A. Galli-ASDC, 3 July 2007

  10. Conclusion • GLAST LAT – and for nearest bursts also AGILE, will be able to detect IC emission from the afterglow of Gamma-Ray Bursts; • Both in the framework of the internal shocks scenario and in that of the external shocks late X-ray flares are related to a long lasting central engine activity; • X-ray flares can be attended by GeV flares produced by IC, that could be detected by GLAST; • IC emission from afterglow can explain also the delayed high energy emission detected by EGRET in GRB 940217; • We expect similar temporal profiles for X-ray and high energy flares in the context of External Shock.

  11. Bonus Slide

  12. GeV flares in association with X-ray flares X-ray flares overlap with the afterglow emission, thus X-ray flares photons can be Inverse Compton scattered in the GeV-TeV band by afterglow electrons. • Late Internal Shock model • Two possible mechanisms (Wang et al. 2006, Fan & Piran 2006): • X-ray flares  synchrotron GeV flares  self IC emission • X-ray flares  IC emission GeV flares  2° order IC on the afterglow electrons Delayed External Shock scenario X-ray flares  synchrotron GeV flares  self-IC emission of flare photons scattered by afterglow electrons A. Galli-ASDC, 3 July 2007

  13. AGILE LAT sensitivity vs. AGILE and EGRET LAT: Fth~ 5·10-7 t-1 erg cm-2 s-1 t < tc, tc~22ksec Fth~3·10-9 t-1/2ergcm-2 s-1 t >tc Sensitivity @ 400 MeV z=0.1 z=1 AGILE: Fth~6·10-6 t-1 erg cm-2 s-1 t < tc , tc~3000 sec Fth~6 ·10-7 t-1/2ergcm-2 s-1 t >tc A. Galli-ASDC, 3 July 2007

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