1. High Energy Emissions from Gamma-ray Bursts (GRBs)
Soeb Razzaque
Penn State University
TeV 06

2. Bright flash of -rays outshining the entire universe for seconds
Gamma Ray Burst
Most violent explosion in the Universe!
Bright flash of -rays outshining the entire universe for seconds
Total energy output in -rays ~ 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)
TeV 06

3. GRB Prompt Emission Highly variable -ray emission
(down to milliseconds)
 Compact source
Long bursts
Short bursts
Time (s)
Bi-modal distribution
of burst duration
 Different origins
TeV 06

4. GRB Afterglow BeppoSAX
Late time (hours-days) emission of X-ray, UV, optical light
BeppoSAX
Feb GRB Mar2
Identify host galaxy  redshift
TeV 06

5. Core collapse Binary mergers X ISM UV O Afterglow Isotropic-equivalent
total energy outflow
GRB
Relativistic
jetted outflow
Initial fireball radius
Accretion disk
Initial temperature
Binary mergers
TeV 06

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:
Theoretical model:
 e - shock acceleration
=2 for strong shock
 Synch/IC spectrum
Fast cooling:
 shock accelerated e - population lose energy
completely (e to ) within dynamic time 
~0.1 model parameter
TeV 06

7. Afterglow Spectrum Ambient medium Reverse | Forward shocks
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
TeV 06

8. TeV -ray Detection Status
Milagro
Milagrito: GRB a
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
Reported significance 2.7
Poirier et al. ’03
MAGIC: GRB050713a
Flux upper limits
Albert et al. ‘06
Tibet Array
GRAND Array
MAGIC
TeV 06

9. GeV -ray Detection GLAST Future detector GRB 970217 GRB 941017
Hurley et al. ‘94
GRB
GRB
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
TeV 06
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)
TeV 06

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
TeV 06

12. -ray Opacity of the Universe
>100 GeV -rays from GRBs suffer
attenuation in IR & CMB background
  e  
Coppi & Aharonian ‘97
High energy -ray attenuation from
GRBs may probe astrophysical model(s)
Baring ‘99
TeV 06

13. HE Photon Opacity in GRBs
Optical depth
Internal shock radius
Razzaque, Meszaros & Zhang ‘04
TeV 06

14. GRB Prompt and Delayed Spectra
Re-processed high energy -ray
10-17 G
IG B-field
10-20 G
Razzaque, Meszaros & Zhang ‘04
TeV 06

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

17. GRB Fireball Evolution
Initial fireball
Coulomb
Compton
nuclear
Baryon loading
coasting fireball
Initial fireball
Inelastic p-n
scattering
n-p decouples
TeV 06
Derishev, Kocharovsky & Kocharovsky ‘99

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

20. Short GRB Model Flux Predictions
Model parameters
GRB
Distance
(z)
L_iso
(erg/s)
Duration
(s) E
(GeV)
Flux
(/cm2/s)
040924
050509b
051103
051221
0.859
0.225
0.001(?)
0.547
1.48E52
8.6E48
2.6E47
1.7E51
0.6
0.128
0.17
1.4 22 59 36
9.7E-6
2.3E-7
8.6E-4
2.3E-6
Data credits: Pablo Saz Parkinson
Predictions
These are still below detection
Need bigger detectors with lower threshold
TeV 06

21. GeV Gamma-rays from Short GRBIC
scattering
Razzaque & Meszaros ‘06
TeV 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
TeV 06

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

24. HE  from Old GRB Remnants
GRB jet: p   +n  neutron decay: n   e -
e - CMB  e - HE
W49B
TeV
Ioka, Kobayashi & Meszaros ‘04
TeV 06

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!
TeV 06