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Long term X-ray and GeV flares in GRB light curves

Long term X-ray and GeV flares in GRB light curves. Alessandra Galli 1,2,3 & L. Piro 2 , F. Longo 3,4 , N. Omodei 5 , G. Barbiellini 5. 1 : University of Rome “La Sapienza”, 2 : INAF/IASF-Rome, 3 : INFN-Trieste, 4 : University of Trieste, 5 : INFN-Pisa.

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Long term X-ray and GeV flares in GRB light curves

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  1. Long term X-ray and GeV flares in GRB light curves Alessandra Galli 1,2,3 & L. Piro 2, F. Longo 3,4, N. Omodei 5, G. Barbiellini 5 1: University of Rome “La Sapienza”, 2: INAF/IASF-Rome, 3: INFN-Trieste, 4: University of Trieste, 5: INFN-Pisa 4th Workshop on Science with the New generation of High Energy Gamma-ray Experiments Portoferraio, Isola d’Elba (Italy) 20-21-22 June, 2006

  2. Outline • The X-ray flares phenomenon • X-ray flares models • Application to some X-ray flares • GeV flares • Prediction for high energy flares in the LAT GLAST energy band A. Galli - Elba, 20-22 June 2006

  3. GRB Temporal-Spectral Evolution Prompt-to-afterglow transition is characterized by a variety of behaviours, likely due to the contribution of both prompt and afterglow emission. Most intriguing is the presence of X-ray flares. X-ray flares are likely to trace the activity of the central engine, thus they can give important information about the physics of the progenitor during the first phases of the burst. BeppoSAX Era - appear minutes after the burst: GRB 011121, XRR 011211 (Piro et al. 2005) and XRF 011030(Galli & Piro 2005); - connect to the late afterglow only if the origin of the time shifted to the instant of the flare appearance. These flares have a soft spectrum consistent with that of the afterglow. A. Galli - Elba, 20-22 June 2006

  4. Swift Era -X-ray flares detected in about one half of the sample (O'Brien et al. 2006); -Several bursts show multiple flares, e.g. GRB 050607, GRB 050730 ... ; -X-ray flares are present also in XRF, e.g. XRF 050406(Romano et al. 2006) and in short GRB, e.g. GRB 050724(Barthelmy et al. 2005); -X-ray flares are globally softer than the prompt phase (Falcone et al. 2005); -In several bursts X-ray flares show hard-to-soft spectral evolution resembling that of the prompt emission (Burrows et al. 2005); -Other bursts, e.g., GRB 050126 and GRB 050219A (Tagliaferri et al. 2005), GRB 050712 (De Pasquale et al. 2006), GRB 050904(Gendre et al. 2006) do not show spectral evolution and have a spectrum consistent with that of the afterglow; Two families of flares could exist A. Galli- Elba, 20-22 June 2006

  5. Possible Scenarios I • Does not require long duration engine activity • -Two components jet: the flare is due to the deceleration of a moderately relativistic jet component as it interacts with the ambient (Zhang et al. 2004); • Patchy jets: have large energy fluctuations in the angular direction and behave as multi-component jets (Kumar & Piran 2000); • Forward-reverse shock (FS-RS):for appropriate FS and RS shock parameters the RS can dominate in the X-ray (Fan & Wei 2005); • External shock on a clumpy medium: external shock on small radii clouds can produce high variable GRB light curves (Dermer & Mittman 1999, Dermer 2006) A. Galli- Elba, 20-22 June 2006

  6. Possible Scenarios II • Long activity/reactivation of the central engine • Post energy injection into the blast wave: the faster shell is decelerated and the slower part of the outflow can catch up with it at later time injecting energy in the blast wave. This collision can generate a bump (Rees & Meszaros 1998, Kumar & Piran 2000); • Internal shocks: late internal shocks produce a long duration prompt emission (Burrows et al. 2005); • External shock from thick shell fireballs:X-ray flares produced in external shocks by a thick shell fireball; the flare mark the beginning of the afterglow emission (Piro et al. 2005); We focus on the External shock scenario by thick shell fireballs. Spectral similarities are straightforwardly accounted for in this scenario. A. Galli- Elba, 20-22 June 2006

  7. Theoretical ground The onset of the external shock depends on the dynamical regime of the fireball, i.e.on its Lorentz factor G0 andon thickness=cteng. In thick shells the Reverse Shock ends crossing the shell after the fireball starts to decelerate, thus teng >tdec. The crossing of the RS increases the emitting volume of the shell, thus the most of the energy is transferred to the surrounding material at teng, around the end of the engine activity. If the engine releases most of the energy at the end of its activity, the afterglow decay is 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 as late internal shocks models. A. Galli- Elba, 20-22 June 2006

  8. This suggests that the flare is the beginning of the afterglow emission Application of the External Shock Model: GRB 011121 Flare at about 240s; its spectrum is softer than the main pulseand is consistent with the afterglow spectrum at 1 day (Piro et al. 2005) The light curve from the decay part of the flare is nicely reproduced by a power law if the origin of the time is shifted to the time of the flare. Piro et al. 2005 Galli & Piro 2006 Wind E53=0.28,130, A*=0.003, e=0.01, B=0.5, p=2.5, t0=250 s A. Galli- Elba, 20-22 June 2006

  9. Application of the External Shock Model: XRF 011030 A flare, about 200 s long, appears about 1300 s after the burst. The flare spectrum is marginally softer than the main pulse and consistent with the afterglow. Galli & Piro 2006 When t0 is shifted to the onset of the flarethe calculated light curves can describe the flare, both in a uniform interstellar medium and in a wind profile environment (Galli & Piro 2006). A. Galli- Elba, 20-22 June 2006

  10. Blue curve: E53=0.03,130, n=5, e=0.29,B=8·10-5,p=2.1, Tb=8·105 s. XRF 011030 broad-band analysis: X, optical and radio ISM case Radio X Radio X Optical • Break ascribed to a jetted fireball • -Similar solution in a wind A. Galli- Elba, 20-22 June 2006

  11. Application of the External Shock Model: GRB 050712 The third flares, about 400 sec after the burst, has spectrum consistent with the following afterglow emission; The spectral (1.16) and the temporal (0.86) indexes are not consistent with high latitude emission from internal shock, temporal-spectral relation +=2 . A thick shell fireball is required. De Pasquale et al. 2006 A. Galli- Elba, 20-22 June 2006

  12. Application of the External Shock Model: GRB 050904 The flare spectrum is consistent with the afterglow emission,Γ~1.6. The thick shell model nicely describe the X-ray flare and the early X-ray light curve, but not the simultaneous optical flare detected by TAROT. The extrapolation of the X-ray flare to the optical band is well below the TAROT data. Two components models could solve this problem: - X-ray flare could be IC emission associated with the optical flare produced by the Reverse Shock. The predicted X-ray flare temporal decay is too shallow (Kobayashy et al. 2006); - The optical flare comes from late internal shock synchrotron emission and X-ray flare by late internal shock IC emission (Wei et al. 2006).The optical flare and the overall X-ray behaviour need to be modeled in detail. Gendre, Galli, Corsi et al. 2006 E53=10,400, A*=0.01, e=0.01,B=10-4, p=2.2, t0=450 s A. Galli- Elba, 20-22 June 2006

  13. GRB 050713A: another variant of the external shock model, refreshed shocks The first flare in the XRT light curve can be ascribed to energy injections in refreshed shock as slower shells emitted during the prompt phase catch up with the afterglow. The rise and the decrease of the flare are fitted by a simple power law with spectral index 1=0.5 and 2=1.6, respectively. X-ray flare spectral variability explained with the fireball emitting in the fast cooling regime and requiring the passage of the injection frequency below the X-ray band during the flare. Above the X-ray band the spectral index is 1= 0.5, below is 2=p/2. Guetta et al. 2006 • Observed by MAGIC above 175 GeV • during the prompt phase (Albert et al. 2006) A. Galli- Elba, 20-22 June 2006

  14. 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 by afterglow electrons producing flares in the GeV-TeV band. • Late Internal Shock model • The mechanism for X-ray flares is not clear (Wang & Meszaros 2006): • X-ray flares produced by synchrotron and GeV flares produced by self IC emission on the same electrons producing the flare • X-ray flares produced by IC and GeV flares produced by 2° order IC on the afterglow electrons External Shock model-Thick shell fireballs X-ray flares produced by synchrotron and GeV flares produced by self-IC emission of flare photons scattered by afterglow electrons A. Galli- Elba, 20-22 June 2006

  15. Late internal shock Flare X by synchrotron emission and high energy flare by self IC: • - Low Lorentz factor for the shell producing the flare and thus also a low contrast between shell’s Lorentz factors (Falcone et al 2005). • Low contrast means low IC peak energies (Wang & Meszaros 2006). Flare X by IC emission and high energy flare by 2nd order IC: • - X-ray flare photons need of several time to reach and scatter with the forward shock electrons and during this time the beam spread out.In the forward shock electrons rest frame the flare photons are anisotropic. The Thompson cross section decreases respect to the isotropic case, and the high energy photon emission is suppressed by a factor of several (Fan & Piran 2006). • The high energy flare could last much longer than the X-ray flare because of the angular dispersion of the up-scattered photons • ( Beloborodov 2005) A. Galli- Elba, 20-22 June 2006

  16. External shock • Stronger high energy emission is expected due to higher Lorentz factor. • X-ray and high energy flares are produced by the same region and electrons population, similar duration and temporal profiles are expected • The integrated emission is simultaneous with the X-ray flare. This can be tested in the GLAST/Swift era. External Shock Internal Shock: Synchr +self IC Internal Shock: IC + 2nd order IC A. Galli- Elba, 20-22 June 2006

  17. GRB 011121 GRB 050502B, GRB 050904 GRB 050712, XRF 011030 Predictions in the context of the External Shock ISM-Fast Cooling 1 GeV 100 GeV niIC niIC niIC E53=5,300, n=1, e=0.1,B=10-4, p=2.5, z=1, t0=500 s Between 500 s and 5000 s the transition from fast cooling to slow cooling occurs. A. Galli- Elba, 20-22 June 2006

  18. Predictions in the contest of the External Shock ISM-Slow Cooling 1 GeV 100 GeV ncIC ncIC E53=0.1,150, n=1, e=0.1,B=10-4, p=2.5, z=1, t0=500 s For certain electron population indexes the IC cooling frequency can increase with the time. A. Galli- Elba, 20-22 June 2006

  19. Flare simulation with GLAST LAT software PRELIMINARY! PRELIMINARY! Simulated spectral analysis of a moderate-intense flare with a mean of 1 ph s-1 m-2 in LAT band, in 1000 s Simulated counts map of moderate-intense flare with a mean of 1 ph s-1 m-2 in LAT band, in 1000 s Simulated spectrumDetected spectrum Ebreak= 2 GeVEbreak=1.60.2 a=-1.5a=-1.40.1 b=-2.25b=-2.40.2 A. Galli- Elba, 20-22 June 2006

  20. Conclusion • The present data suggest the existence of two categories of X-ray flares well differentiated by their spectral behaviour; • In some cases spectral variations can be explained also in the context of the external shock; • 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, that could be detected by GLAST; • In the framework of the external shock we expect similar temporal profiles for X-ray and high energy flares. This is a strong prediction that will permit to discriminate between different models; A. Galli- Elba, 20-22 June 2006

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