High-Energy Emission from GRBs: expectations & first results from the Fermi Gamma-ray Space Telescope Jonathan Granot University of Hertfordshire (Royal Society Wolfson Research Merit Award Holder) on behalf of the Fermi LAT & GMB Collaborations Nordita, Stockholm, Sweden, May 25, 2009

Outline of the Talk: New Space missions help drive progress in GRBs GRB: some basic properties, theoretical framework High-energy emission processes & pre-Fermi obs. Some expectations & hopes from Fermi Fermi: initial results GRB081024B: 1st clearly short GRB above 1 GeV GRB080916C: very bright – a lot of interesting data Lower limit on the Bulk Lorentz factor Limit on Lorentz invariance violation Possible causes of HE delayed onset & longer duration Conclusions

GRBs: Brief Historical Overview 1967: 1st detection of a GRB (published in 1973) In the early years there were many theories, most of which invoked a Galactic (neutron star) origin 1991: the launch of CGRO with BATSE lead to significant progress in our understanding of GRBs BATSE: ~ 30 keV – 2 MeV, full sky (~ ½ Earth occ.) Isotropic dist. on sky: favors a cosmological origin Bimodal duration distribution: short vs. long GRBs EGRET: ~ 30 MeV – 30 GeV, FoV ~ 0.6 sr ~2 s

BeppoSAX (1996 - 2002): lead to discovery of afterglow (1997) in X-rays, optical, radio (for long GRBs) This led to redshift measurements: clear cut determination of the distance/energy (long GRBs) Eγ,iso ~ 1052 -1054 erg Afterglow observations provided information on beaming (narrow jets: Eγ ~ 1051 erg), event rate, external density, supernova connection ( long GRB progenitors) Swift (2004 - ?): autonomously localizes GRBs slews in ~1-2 min) and observed in X-ray + optical/UV Discovered unexpected behavior of early afterglow Led to the discovery of afterglow from short GRBs  host galaxies, redshifts, energy, rate, clues for progenitors Fermi (2008 - ?): may also lead to significant progress in GRB research, like previous major space missions

Fermi Gamma-ray Space Telescope (Fermi Era; launched on June 11, 2008): Fermi GRB Monitor (GBM): 8 keV – 40 MeV (12×NaI 8 – 103 keV, 2×BGO 0.15-30 MeV), full sky Slightly less sensitive than BATSE: expected to detect ~ 200 GRB/yr (≳ 60 in the LAT FoV) Large Area Telescope (LAT): 20 MeV – 300 GeV FoV ~ 2.4 sr ; up to 40 times the EGRET sensitivity Its new capabilities are expected to lead to progress in the field, similar to previous major new space missions

Prompt GRB Observations (≲ MeV) Time Flux Variable light curve down to millisecond timescales Duration: ~10 −2 – 103 sec Spectrum: non-thermal F peaks at ~ 0.1-1 MeV (well fit by a Band function) Rapid variability, non thermal spectrum & z ~ 1  relativistic source ( ≳ 100) (compactness problem: Schmidt 1978; Fenimore et al. 1993; Woods & Loeb 1995;…)  F

GRB Theory: Fireball vs. Poynting Flux Afterglow *Meszaros & Rees 92, Katz 94, Sari & Piran 95 Internal Shocks R ~ 1013-1015 cm Prompt GRB X-rays Optical Radio † Shemi & Piran 90, Goodman 86, Paczynski 86,… Optical Radio Matter dominated† outflow Ekin ≳ EEM ejecta Compact Source Reverse shock* External medium Forward Shock (Rees & Meszaros 92) Particle acceleration  -rays (synchrotron ?) Poynting flux†† dominated flow EEM ≫ Ekin X-rays Optical Radio reconnection (or other EM instability) R ~ 1016-1017 cm Magnetic bubble †Thompson 94, Usov 94, Meszaros & Rees 97, Katz 97,… Lyutikov & Blandford 02,03

GRB: High Energy Emission Processes Inverse-Compton or Synchrotron-Self Compton (SSC): Ep,SSC / Ep,syn ~ γe2 , LSSC / Lsyn=Y , Y(1+Y) ~ εradεe /εB Hadronic processes: photopair production (p + γ  p + e e), proton synchrotron, pion production via p –γ (photopion) interaction or p-p collisions The neutral pions decay into high energy photons π0  γγ that can pair produce with lower energy photons γγ  e e− - producing a pair cascade Fermi may help determine the identity of the dominant emission mechanism at high & low energies Most of the radiated energy can be in the LAT range (energetics) ν Fν Ep,syn Ep,SSC γe2 Y SSC synchrotron ν

High energy emission from GRBs: Pre-Fermi era Little known about GRB emission above ~100 MeV EGRET detected only a few GRBs, most notably: GRB940217: GeV photons were detected up to 90 minutes after the GRB trigger GRB941017: distinct high- energy spectral component (up to 200 MeV), with a different temporal evolution & at least 3 times more energy AGILE recently observed GRB080514B and detected photons up to a few 100 MeV lasting somewhat longer than the soft gamma-rays GRB940217 (Hurley et al. 94) EGRET GRB941017 (Gonzalez et al. 03) GRB080514B (Giuliani et al. 08) -18 to 14 sec 14 to 47 sec 47 to 80 sec GRB940217: long lived emission GRB941017: search for extra component (could help distinguishing leptonic / hadronic model) AGILE 80-113 sec 113-211 sec BATSE - LAD EGRET - TASC

High-Energy emission from GRBs: Expectations & Hopes for Fermi Prompt GRB emission: Detecting a distinct high-energy spectral component  teach us about emission mechanism & energetics Detecting  opacity signatures (internal / external-EBL) Constraining Lorentz Invariance Violation (LIV) First several hours after the GRB: HE afterglow emission (SSC from the forward shock) IC from the forward-reverse shock system near ~ tdec Pair echo: TeV (GRB) + CIB γγ  e e− EC on CMB IC involving the external shock & late source activity Hadronic induced cascades in the early afterglow

Fermi GRB detections: GBM: 160 GRBs so far (18% are short) Detection rate: ~200-250 GRB/yr A fair fraction are in LAT FoV Automated repoint enabled LAT detections: 8 in 1st 10 months GRB080825C: >10 events above 100 MeV GRB080916C: >10 events above 1 GeV and >140 events above 100 MeV GRB081024B: first short GRB with >1 GeV emission (090510) 8 + 2 more possible detections 080916C 081215A 080825C 090217 081024B LAT FoV cosθ

Breaking News: GRB 090510 Short-hard GRB (T90 ~ 0.5 s, Epeak ~ 4-5 MeV) Detected by Swift, Fermi, Suzaku, AGILE, K-W AGILE: >5 σ detection of >100 MeV γ-rays, during the prompt phase & a “delayed” phase (GCN 9343) Fermi LAT: >50 γ’s above 100 MeV (>10 γ’s above 1 GeV) in 1st sec.; >150 γ’s above 100 MeV (>20 above 1 GeV) in 1st min. (GCN 9350) X-ray (Swift XRT) & optical afterglow Spectroscopic redshift: z = 0.903  Eγ,iso ~ 4×1052 erg (GCN 9350) Very interesting GRB: I can’t say much more now

GRB080825C: the 1st LAT GRB The 1st LAT events coincide with the 2nd GBM peak The high-energy emission lasts longer: highest energy photon arrives when the GBM emission is very weak PRELIMINARY! Fluence(50-300): 2.5x10^-5 ergs.cm-2

GRB081024B: 1st short GRB >1 GeV 1st LAT events coincide with 2nd GBM peak (delayed HE onset) The HE emission lasts longer than low-energies Single Band spectrum! Lower limit on Γ from pair opacity constraints: Γmin(z = 0.1) ≈ 150 Γmin(z = 3.0) ≈ 900 (best so far for a short GRB, but z is uncertain) Fluence (50-300): 3.4x10^-7 ergs.cm-2 PRELIMINARY!

Γmin : The Compactness Problem: (Schmidt 1978; Fenimore et al Γmin : The Compactness Problem: (Schmidt 1978; Fenimore et al. 1993; Woods & Loeb 1995;…) The large γ-ray flux implies huge luminosities for cosmological GRBs, Liso ~ 1050 - 1053 erg/s For sources at rest: short variability time Δt  small source R < cΔt & ε = Eph /mec2 ~ 1  large fraction of ’s can pair produce (  ee−) (ε) ~ σTnph(1/ε)R, nph(1/ε) ~ L1/ε/4πR2mec3  (ε) ~ σTL1/ε /4πmec3R ≳ 1014 L1/ε,51(Δt / 1 ms)−1 Such a huge  would produce a thermal spectrum  inconsistent with the observed high energy tail

Solution: Relativistic Motion  ≫1 Source can be larger: R ≲ Γ2cΔt (factor Γ−2 in ) angular time from a thin spherical shell (also the radial time For emission over ΔR ~ R) Factor of 1− cosθ12 ~ Γ−2 in  expression (Γ−2)   ee− threshold: ε1ε2 ≳ Γ2 (Γ2(1+β), Lε ε1+β) Altogether  is reduced by a factor of Γ2(1−β) and since -β ~ 2-3,  < 1 typically implies Γ > Γmin ~ 100  ~ σTΓ2βL1/ε/4πmec3R ≳ Γ−2(1−β)σTL1/ε/4πmec4Δt R R(1− cos θ) ≈ R/2Γ2 ~ cΔt 1/Γ R 1/Γ photon observer R

GRB080916C: detection, localization, follow-up obs. GBM on-ground position error: ±1° (68% stat.) + 2-3° syst. T90 = 66 s, several peaks in lightcurve LAT on-ground position: statistical error: 0.09° (0.13°) at 68% (90%) + systematic error < 0.1° (preliminary) GROND optical/NIR follow-up obs. Resulted in a positional accuracy of 0.5” & a redshift of z = 4.35 ± 0.15 Swift X-ray afterglow from T0+17 hr GROND position (Greiner et al. 08, submitted) GROND SED of the GRB afterglow (GCN report 166.1) Swift XRT lightcurve

GRB080916C: multi-detector light curve 8 keV – 260 keV 260 keV – 5 MeV LAT raw LAT > 100 MeV LAT > 1 GeV T0 First 3 lightcurves are background subtracted The LAT can be used as a counter to maximize the rate and to study time structures > tens of MeV >3000 LAT photons in the 1st 100 seconds The first low-energy peak is not observed at LAT energies Spectroscopy: LAT event selection >100 MeV  145 events made it 5 time bins (a to e) 14 events > 1 GeV

GRB080916C: multi-detector light curve Most of the emission in the 2nd peak occurs later at higher energies This is clear evidence of spectral evolution The delay of the HE emission seems to be a common feature of the GRBs observed by the LAT so far.

GRB080916C: time resolved spectroscopy Main LAT peak (time bin ‘b’) Time resolved spectroscopy over 6 decades in energy!!! (10 keV – 10 GeV) Consistent with a Band function: a single dominant spectral component No strong evidence for an additional spectral component Alpha -1.02 ± 0.02 Beta -2.21 ± 0.03 Epeak 1170 ± 142 keV Amp. 0.035 ± 0.001 photons/s-cm2-keV REDUCED CHISQ = 0.963, PROB = 0.698 time bin ‘d’ A likelihood ratio test in bin ‘d’: the probability of having no additional HE spectral component is 1% (5 bins/trials) Possible pair production (  ee−) of HE photons with the EBL leaves this probability from unchanged to 0.03% depending on the model chosen. Band + PL

GRB080916C: time resolved spectroscopy The Stecker et al. model/s would imply  ~ 3-4  > 3 σ for distinct HE spectral component that carries significant energy For other EBL models  ≪ 1: weak evidence for an extra HE spectral component (5% chance probability for no HE- component accounting for the 5 trials / bins) EBL model predictions for z = 4.35 time bin ‘d’ A likelihood ratio test in bin ‘d’: the probability of having no additional HE spectral component is 1% (5 bins/trials) Possible pair production (  ee−) of HE photons with the EBL leaves this probability from unchanged to 0.03% depending on the model chosen. Band + PL

GRB080916C: Spectral Evolution Band function fits Soft  hard  soft Epeak evolution Low (α) & high (β) energy photon indexes change significantly only between time bins ‘a’ and ‘b’ a b c d e

GRB080916C: Bulk Lorentz factor ≳ 900 Robust + highest lower limit on Γ from opacity constraints: Γmin≈ 890 ± 20 (bin ‘b’, for t = 2 s) & Γmin ≈ 600 (bin ‘d’) Δt = 0.5 s bin-b INTEGRAL light curve (Greiner et al. 09)) GBM NaI Δt = 2 s bin-d Δt = 20 s 1st HE (> 100 MeV) detection of a GRB with known redshift Our Γmin is more robust than before: it doesn’t assume the spectrum extends beyond highest energy detected photon Under this conservative assumption: Γmin ≲ (1+z)Eph,max/mec2 ≈ 200(1+z)(Eph,max / 100 MeV) so that a high Γmin requires the observed spectrum to reach a sufficiently high energy Eph,max

Other Interesting Results for GRB080916C: Large fluence (2.4×10-4 erg/cm2) & redshift (z = 4.35 ± 0.15)  record breaking apparent isotropic energy release Eγ,iso ≈ 8.8×1054 erg ≈ 4.9 Mc2  suggests strong beaming (jet) Single dominant emission mechanism: if synchrotron, SSC is expected, and can avoid detection if Epeak,SSC ≫ 10 GeV (γe ≫ 100), or if Y ≈ εe/εB ≲ 0.1 (constrains other emission mechanisms as well) Epeak,syn γe2 Epeak,SSC ν Fν Y possible SSC synchrotron (?) ~10 GeV ν

Limits on Lorentz Invariance Violation Some QG models violate Lorentz invariance: vph(Eph) ≠ c A high-energy photon Eh would arrive after (or possibly before in some models) a low-energy photon El emitted together GRB080916C: highest energy photon (13 GeV) arrived 16.5 s after low-energy photons started arriving (= the GRB trigger)  a conservative lower limit: MQG,1 > (1.50 ± 0.20)×1018 GeV/c2 min MQG (GeV/c2) 1016 1017 1018 1015 1.8x1015 Pulsar (Kaaret 99) 0.9x1016 1.8x1017 0.2x1018 4x1016 GRB (Ellis 06) (Boggs 04) AGN (Biller 98) (Albert 08) GRB080916C Planck mass 1019 1.5x1018 1.2x1019 (Jacob & Piran 2008) n = 1,2 for linear and quadratic Lorentz invariance violation, respectively

GRB080916C: temporally extended emission Most LAT detected GRBs show significant HE emission lasting after the low-energy emission becomes (almost) undetectable (originally detected by EGRET; Hurley et al. 94) GRB080916C: the HE emission that extends ~103 sec beyond the detectable keV-MeV emission Possible origins: Afterglow SSC emission (though no spectral hardening, time gap, or synchrotron/SSC valley in the spectrum are observed) IC scattering of late X-ray flare photons by afterglow electrons Long lived cascade induced by ultra-relativistic ions Pair echo: TeV + EBL   ee−, & the ee− IC scatter the CMB

Delayed onset of HE emission: Possible Causes 1. The 1st and 2nd peaks are emitted from distinct physical regions (e.g. different colliding shell in the internal shocks model) Unclear why a similar behaviour occurs in all 3 LAT GRBs (if it is random then some GRBs should have a reverse order) 2. opacity effects don’t work well as there is no cutoff or steepening of the spectrum at high-energies 3. Hadronic origin: time to accelerate protons & develop pair cascade, if the high-energy emission is of hadronic origin Two distinct spectral components (leptonic at low-energies & hadronic at high-energies) & hard to soft evolution are expected (but not seen); hard to explain sharpness of 1st LAT peak + coincidence with 2nd GBM peak

Fermi LAT GRB detection rate ~ 8-9 GRB/yr with >10 photons above 100 MeV ~ 2-2.5 GRB/yr with >10 photons above above 1 GeV Somewhat below estimates based on a Band spectrum for a bright GRB BATSE sample Suggests: most GRBs don’t have significant excess (HE component) or deficit (cutoff) in the LAT energy range w.r.t the extrapolation of a Band spectrum from lower energies

Conclusions: Expectations: First results: Like previous major relevant space missions, Fermi is expected to significantly advance the GRB field Particularly: prompt emission mechanism & region, energy budget, early afterglow, opacity effects First results: ~ 9-10 LAT GRBs/yr suggest that most GRBs do not strongly deviate from a Band spectrum in LAT range Spectra: consistent with single dominant component 1st 3 LAT GRBs show later onset & longer duration of the high-energy emission, relative to low energies short & long GRBs seem to have similar HE spectra GRB080916C:  ≳ 900, MQG,1 > 1.3×1018 GeV/c2