1. High Energy Emission from Gamma-Ray Bursts
The 3rd International Workshop
on the High Energy cosmic-Radiation Detection Facility
High Energy Emission from Gamma-Ray Bursts
Xuefeng Wu
Purple Mountain Observatory
, Xi’an

2. Basic Features of GRBs -- detection rate: -- temporal features
1-2 events per day
by CGRO/BATSE
-- detection rate:
-- temporal features
-- spectral features
Profiles
Complicated
Durations
5 ms ─ 10 s ─ 5×103 s
Variability
0.1ms ─1ms ,
No repetition
-- spatial features
Photon Energy
1 eV ─ 1 MeV ─ 30 GeV
Non-thermal:
N(E) ∝ E-α
High Energy Tail:
no cutoff above 1 MeV
Fluence:
10-10 ─ J /m2
log
Isotropic
Lack of weak GRBs
GRB :
低能波段只持续了约180秒，其间观测到约10个高能光子，高至4GeV
但1.6 小时之后仍观测到高能光子（200MeV －20GeV）
图中的粗黑线表示地球遮挡无法观测的时间
GRB : 观测到高达200MeV的幂律谱成份（Gonzalez et al. 2003, Nature, 424, 749）
GRB A: 观测到有0.1TeV的光子 （Atkins et al. 2000, ApJ, 533, L119

3. Gehrels, Piro & Leonard 2002, Scientific American

4. Gehrels, Piro & Leonard 2002, Scientific American

5. Why GeV-TeV emission so important?
10-4
10-5
10-6
10-7
10-8
10-9
10-10
Zhang, S. N., et al., arXiv:
radiation mechanism for >GeV emission
particle acceleration responsible for the >GeV emission
GRB physics (Lorentz factor, jet composition, emission site)

6. High energy emission from GRBs: Pre-Fermi era
Little is known about GRB emission above ~100 MeV
EGRET detections from a few GRBs:
GRB941017: HE extra component (up to 200 MeV), with different temporal evolution in. Inconsistent with the synchrotron model !
GRB940217‏: GeV photons detected up to 90 minutes after trigger
Pre-Fermi Record holders:
GRB990123: highest measured Eiso ~ 2x1054 ergs
GRB971214: highest lower limit on bulk Lorentz factor: >410
AGILE observation of GRB080514B
Need for more data…
GRB940217
(Hurley et al. 94)
GRB941017
(Gonzalez et al. 03)
GRB080514B, AGILE
(Giuliani et al. 08)
-18 to 14 sec
14 to 47 sec
47 to 80 sec
sec
sec
BATSE - LAD EGRET - TASC

7. The Fermi Observatory Spacecraft : Large Area Telescope (LAT) :
Low-Earth Circular Orbit (altitude 550 km) with 28.5° inclination
Large Area Telescope (LAT) :
Energy range : 20 MeV to >300 GeV
Large field of view : ~2.4 sr at 1 GeV
Full sky coverage every 3 hours
Localization, spectroscopy and GRB trigger capabilities (on board and ground)
Gamma-ray Burst Monitor (GBM):
Full unocculted sky coverage : >9.5 sr
On board triggers
8 keV to 40 MeV
12 NaI (8 keV to 1 MeV)
Localization (on board & ground)
Spectroscopy
2 BGO (200 keV to 40 MeV)
Spectroscopy

8. GRBs detected with Fermi (2rd GRB catalogue by GBM, 2008-2012)
GBM: 250 GRB/yr LAT: 10 GRB/yr

9. I. Delay High-Energy Emission onset (>100 MeV)
Case of the long GRB C
Abdo et al. 2009, Science 323, 1688
Case of the short GRB
Abdo et al. 2009, Nature 462, 331
The first LAT peak coincides with the
second GBM peak
Delay in HE onset: ~4-5 s
The first few GBM peaks are missing but later peaks coincide
Delay in HE onset: ~ s

10. LAT Fluence vs GBM Fluence
Comparable LE and HE gamma-ray outputs for short GRBs
Long GRBs seem to emit ~5-20 times less at HE than at LE w.r.t. short GRBs 10

11. II. Long Lived GeV Emission
Case of GRB A
Case of GRB
t1.690.0
5σ Detection up to 4800s
5σ Detection up to 1600s

12. II. Long Lived GeV Emission
Case of GRB (De Pasquale et al 2010, ApJL 709, 146)
t1.380.07
LAT emission until 200 s
No spectral evolution
(photon index -2.1 ± 0.1)
Fit of the SED at T s
>4.5 σ break at HE‏
beta2 =
beta3 =
Eb2 in [10 – 130] MeV
Forward shock model can reproduce the spectrum from the optical up to GeV energies! (non thermal synchrotron emission from the decelerating blast wave)
Extensions needed to arrange the temporal properties

13. Synchrotron LAT afterglow scenario ? (Kumar & Barniol Duran 09, 10)
Pros
Can easily explain the simple decay
Can explain the delayed onset as the onset of the HE afterglow
The flux level matches the observations
Cons
Maximum synchrotron energy is ~50 MeV in the shock rest frame (Bohm acceleration approximation)
Observer frame： 50MeV*Γ/(1+z), where
>10 GeV photons strongly challenge the synchrotron scenario (e.g. Piran & Nakar 10; Wang et al. 13)

14. LAT afterglow by inverse Compoton scattering?
Liu, Wang, Wu 2013, ApJL

15. Hadronic model?
Hadronic model: require very high isotropic-equivalent proton energies >10^55 erg/s (Asano & Meszaros 09)

16. III. Spectral Evolution
Case of GRB C (Abdo et al. 2009, Science 323, 1688)
Alpha /-0.02
Beta /-0.03
Epeak 1170+/-142 keV
Amp / photons/s-cm2-keV
REDUCED CHISQ = 0.963, PROB = 0.698
Consistent with a single Band function from 10 keV to 10 GeV.
Global soft-hard-soft evolution.

17. IV. Additional Spectral Component
Case of the short GRB
Ackermann et al. 2010, ApJ 716, 1178
Case of the long GRB B
Abdo et al. 2009, ApJL 706, 138
Spectral deviation from the standard Band function adequately fit with an additional power law in long and short GRBs.
The Extra-PL is usually not present during the all burst duration.
Usually, the extra-PL over-power the Band function at low (< ~tens of keV) and high (> ~tens of MeV) energy.
Possible PL break in a time resolved spectrum of GRB B.

18. Additional Spectral Component in GBM Data Only
Case of GRB B (Guiriec et al, ApJ)
Band (Cstat: 699/607 dof)
Cutoff PL + PL (Cstat: 689/606 dof)
νFν (ph/keV/cm2/s) α β
Epeak σ
10000
1000
100 10 6 -6
Energy (keV) PL
Cutoff PL
Cutoff PL+PL prefered over the standard Band function usually used
The additional component dominates the standard “Band function” at both low and high Energy 18

19. High-Energy Spectral Cutoff in the Extra-PL Component of GRB 090926A
The extra-PL overpower the standard Band function above ~10 MeV.
Existence of a 6σ spectral cutoff at ~1.4 GeV in the extra-PL.
Break shape not constraint.

20. Lower Limit on the Jet Lorentz-Factor : Γmin
Compactness problem :
GRBs have a high luminosity in the gamma-ray energy range (Liso ~ erg/s).
Small emission region (computed from the variability time-scale observed in the light curves).
Non thermal gamma-ray emission should be blocked due to the γ- γ pair production opacity.
A relativistic jet reduces the photon seed population above the pair production threshold (compatible with the observations)
Estimation of the jet Lorentz-Factor lower-limit : -
Hypothesis : uniform, isotropic and time-independent seed photon field.
More realistic models (i.e. Granot, 2008) give significantly lower values (~3 times).
Sylvain Guiriec – Workshop GRBs,- Toulouse, 2010

21. Lower Limit on the Jet Lorentz-Factor : Γmin
Case of the short GRB
Result Summary
Lower limit of the bulk Lorentz factor >900.
GRB A : it is the first direct measurement of the bulk Lorentz factor if the high-energy power-law cutoff is due to the γ-γ pair production opacity => Γ ~ (model dependent).

22. Contraints on the Lorentz Invariance Violation
Case of GRB (Abdo et al. 2009, Nature 462, 331)
Some QG models predict a violation of the Lorentz invariance (Vph(E) ≠ c)
The LAT is mostly sensitive to linear variation (n=1) and maybe quadratic variations (n=2)
Sylvain Guiriec – Workshop GRBs,- Toulouse, 2010

23. Contraints on the Lorentz Invariance Violation
Case of GRB (Abdo et al. 2009, Nature 462, 331)
Some QG models predict a violation of the Lorentz invariance (Vph(E) ≠ c)
HERD is more sensitive
than Fermi/LAT at >20 GeV band
The LAT is mostly sensitive to linear variation (n=1) and maybe quadratic variations (n=2)
Sylvain Guiriec – Workshop GRBs,- Toulouse, 2010

24. Contraints on the EBL models
HERD is more
sensitive than
Fermi
Most models are optically thin for the 33 GeV photon from GRB B.
“Baseline” and “fast evolution” models rejected at 3.6 σ.

25. Polarization of GRBs -high polarization degree
pros and cons：
Rutledge & Fox (2004) – con:
a) RHESSI---low signal /noise，
b) CB03 overestimated the scattered photon numbers by 10 times,
c) CB03 did not considered systematic errors。
Wiggle et al. (2004) :
P= %，
data quality is not enough good。
GRB
P =80+/-20%
Addition: Willis et al. (2005) ,
CGRO/BATSE detections，
GRB : P >35%,
GRB : P >50%,
however, systematic errors are big。
Coburn & Boggs, 2003, Nature, 423, 415

26. Polarization of GRBs -high polarization degree
GRB A: INTEGRAL detection
McGlynn et al., 2007, A&A, 466, 895

27. Polarization of GRBs -high polarization degree
GRB A: GAP ( keV) onboard IKAROS
P=27+/-11%（2.9σ），
evidence for evolution of P. A. (Polarization Angle)
Yonetoku et al., 2011, arXiv:

28. Polarization of GRBs -high polarization degree
GRB A: GAP ( keV) onboard IKAROS
P=70+/-22% (3.7σ)，
no evidence for P.A. evolution
Yonetoku et al., 2012, arXiv:

29. Polarization of GRBs -high polarization degree
GRB A: GAP ( keV) onboard IKAROS
P=84+16/-28% (3.3σ)，
no evidence for P. A evolution
Yonetoku et al., 2012, arXiv:

30. LIV birefringence constraint with GRB prompt emission polarization
3 GRBs with poln detection with GAP (Toma et al., 2013, PRL)
GRB A:
HERD is more sensitive than
Fermi/LAT at >20 GeV band!

31. Why HERD so important for GRB emission beyond 10 GeV?
10-4
10-5
10-6
10-7
10-8
10-9
10-10
Zhang, S. N., et al., arXiv:
radiation mechanism for >GeV emission
particle acceleration responsible for the >GeV emission
GRB physics (Lorentz factor, jet composition, emission site)
with >10 GeV or even >100 GeV photons
Constraining EBL much severely
Constraining LIV (time of flight, birefringence) at least better than previous

32. Thank You