High Energy Emissions from Gamma-ray Bursts (GRBs)

1. High Energy Emissions from Gamma-ray Bursts (GRBs) Soeb Razzaque Penn State University

2. Gamma Ray Burst Most violent explosion in the Universe! Bright flash of -rays outshining the entire universe for seconds • Total energy output in -rays ~1049-1051 erg Credit: Tyce DeYoung • Peak photon energy ~0.1-1 MeV • Non-thermal -ray spectrum • Isotropic distribution • Rate ~1000/year • Extra-galactic (redshift~1-2)

3. Long bursts Short bursts GRB Prompt Emission Highly variable -ray emission (down to milliseconds)  Compact source Time (s) Bi-modal distribution of burst duration  Different origins

4. GRB Afterglow Late time (hours-days) emission of X-ray, UV, optical light BeppoSAX Feb 28 GRB 970228 Mar2 • Identify host galaxy  redshift

5. X ISM Core collapse UV O Afterglow • Isotropic-equivalent • total energy outflow GRB Relativistic jetted outflow • Initial fireball radius Accretion disk • Initial temperature Binary mergers

6. Gamma-ray Spectrum • Time-averaged spectrum fitted by • broken power-laws (Band fit) •  Non-thermal Break energy ~0.1-1 MeV • Origin: Internal shocks •  e-synchrotron radiation (low energy) •  Inverse Compton scattering (high energy) Observation: =2 for strong shock • Theoretical model: •  e - shock acceleration  Synch/IC spectrum • Fast cooling: •  shock accelerated e - population lose energy • completely (e to ) within dynamic time  ~0.1 model parameter

7. Afterglow Spectrum Ambient medium e -synchrotron cooling time longer than dynamic time Reverse | Forward shocks Break frequency decreases in time at rate depending on constant (ISM) or wind (density  r-2 ) ambient medium Sari, Piran & Narayan ’98

8. TeV -ray Detection Status Milagro • Milagrito: GRB 970417a • Tentative 3 detection • Unknown redshift (less than 100 Mpc?) • Atkins et al. ‘00 • Tibet Array: • 50-60 GRB stacked in time coincidence with MeV • 6 significance • Amenomori et al. ‘96 • GRAND: GRB 971110 • Reported significance 2.7 • Poirier et al. ’03 • MAGIC: GRB050713a • Flux upper limits • Albert et al. ‘06 Tibet Array GRAND Array MAGIC

9. GeV -ray Detection Hurley et al. ‘94 GRB 970217 GRB 941017 t<14 s t <47 s t < 80 s • Handful of GRB detection at ~GeV by EGRET • Hard spectra and delayed emission • More energy in HE component? • Need more data! GLAST t < 113 s Future detector t < 211 s Gonzalez et al. ‘03

10. High Energy -rays from GRBs • Electromagnetic process: Inverse Compton (IC) • Maximum electron energy ~100 TeV • Maximum -ray energy ~TeV • Inefficient in the Klein-Nishina limit • Hadronic Process: Photomeson  0 decay • Maximum proton energy ~1020 eV • Maximum -ray energy ~EeV • In general inefficient: opacity~1 (long) <1 (short) • Single or multi (internal-external shocks) zone(s) emission? • High energy -rays may attenuate at the source • -rays with energy >100 GeV are attenuated in background radiation fields (IR/CMB)

11. Which Model? One zone model for MeV and HE  Time delay by slower p cascade and secondary radiation Early Afterglow: >100 MeV IC e-sync Boettcher & Dermer ‘98 p-sync tdec ~2 Internal shock  MeV -rays External shock  high energy  Insignificant proton contribution Zhang & Meszaros ’01 Granot & Guetta ‘03

12. -ray Opacity of the Universe >100 GeV -rays from GRBs suffer attenuation in IR & CMB background   e   Coppi & Aharonian ‘97 Baring ‘99 High energy -ray attenuation from GRBs may probe astrophysical model(s)

13. HE Photon Opacity in GRBs Optical depth Internal shock radius Razzaque, Meszaros & Zhang ‘04

14. GRB Prompt and Delayed Spectra Re-processed high energy -ray 10-17 G IG B-field 10-20 G Razzaque, Meszaros & Zhang ‘04

15. Diffuse <TeV -rays from GRBs Casanova, Dingus & Zhang ‘06

16. >TeV -ray from UHE Cosmic-ray • Shock-acceleration in GRB • ≥1020 eV cosmic-rays >1 TeV -ray fluence 1051 erg GRB energy at 100 Mpc Cascades on IR/CMB background radiation  Delayed emission ~day Patchy IGM (80% voids w. B10-15 G, 20% w. B~10-11 G) TeV Fluence ~2% of energy in GZK protons Waxman & Coppi ’96 Dermer ’02 Armengaud, Sigl & Miniati ‘06

17. Compton Coulomb nuclear Initial fireball Inelastic p-n scattering Initial fireball n-p decouples GRB Fireball Evolution Baryon loading coasting fireball Derishev, Kocharovsky & Kocharovsky ‘99

18. n-p Decoupling in Short GRB n-p Decoupling Radius Rnp~RTh Razzaque & Meszaros ‘06

19. n-p Decoupling Gamma-rays • Only photons produced at photosphere may escape un-attenuated Bahcall & Meszaros ‘00 (LGRB) • 0 decay photon energy (SGRB) Razzaque & Meszaros ‘06 Probability • Flux from an SGRB at z=0.1 MILAGRO • GLAST : Too small effective area • MILAGRO Energy below threshold?

20. Short GRB Model Flux Predictions Model parameters Data credits: Pablo Saz Parkinson Predictions • These are still below detection • Need bigger detectors with lower threshold

21. GeV Gamma-rays from Short GRB • IC scattering Razzaque & Meszaros ‘06

22. Late X-ray Flares in GRB • Various models: • Refreshed shocks • IC from reverse shock • External density bumps • Multiple component jet • Late central engine activity • Main constraints: • sharp rise and decline • GeV-TeV  rays: • IC scattering of x-ray photons by external forward shocked electron GRB X-ray flare Underlying afterglow light curve t -0.8 Burrows et al. ’05, Zhang et al. ‘05 Wang, Li & Meszaros ‘06

23. HE  from Old GRB Remnants HESS J1301-631 Age: 1.5×104 yr ; Distance: 12 kpc 0decay model 25’≤≤1o ≤10’ 10’≤≤25’ Atoyan, Buckley & Krawczynski ‘06

24. HE  from Old GRB Remnants GRB jet: p   +n  neutron decay: n  e - W49B e - CMB  e - HE TeV Ioka, Kobayashi & Meszaros ‘04

25. Conclusion • GRBs are the brightest MeV -ray transient sources in the universe • GeV and TeV (tentative) -rays have been observed from a few bursts • Both Leptonic and Hadronic models may account for GeV data  Need more data! • Short GRBs may produce ~100 GeV -rays • Less luminous than long GRBs but much nearer • Less attenuation in background radiation • TeV detection in current detectors requires luminous and nearby GRBs • Need more GeV-TeV data  need bigger detector!