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High Energy Gamma-ray Emission from GRBs

High Energy Gamma-ray Emission from GRBs. Masanori Ohno(ISAS/JAXA) on behalf of Fermi LAT/GBM collaborations. ※MeV-GeV での話です。. Outline. Introduction Fermi Gamma-ray Space Telescope - Instruments - How does Fermi observe GRBs ? Science Result by Fermi

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High Energy Gamma-ray Emission from GRBs

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  1. High EnergyGamma-ray Emissionfrom GRBs Masanori Ohno(ISAS/JAXA) on behalf of Fermi LAT/GBM collaborations ※MeV-GeVでの話です。 ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  2. Outline • Introduction • Fermi Gamma-ray Space Telescope - Instruments - How does Fermi observe GRBs ? • Science Result by Fermi - Extra component, delayed/Extended emission - Constraint on jet dynamics - other GRB topics (EBL, QG) - Upper limit on HE emission by Fermi data • Summary ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  3. Gamma-Ray Bursts Bright gamma-ray pulse in gamma-ray band is discovered in 1967 BATSE (1991-) Light curve • GRBs originate from All-sky (~1GRBs/day) • Bimodal duration distribution: • non-thermal spectrum (Band function; synchrotron ?) • Short (<2s) and Long (>2s) GRB 20s counts BeppoSAX(1996-) • discovery of the X-ray afterglow This leads a redshift measurement. cosmological origin for long GRBs(z=0.1-8) most energetic explosion in the Universe (Eiso~1052 erg) Duration distribution relativistic jet is required (compactness problem) HETE-2 (2002-) Swift (2004-) short long • Leads many afterglow observations • Association with SN and long GRBs • Discovery of afterglow from short GRBs 2s ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  4. Gamma-Ray Bursts Bright gamma-ray pulse in gamma-ray band is discovered in 1967 BATSE (1991-) Fv Light curve • GRBs originate from All-sky (~1GRBs/day) • Bimodal duration distribution: • non-thermal spectrum (Band function; synchrotron ?) • Short (<2s) and Long (>2s) GRB 20s counts vFv β BeppoSAX(1996-) α • discovery of the X-ray afterglow Epeak This leads a redshift measurement. cosmological origin for long GRBs(z=0.1-8) 10 100 1000 Energy(keV) most energetic explosion in the Universe (Eiso~1052 erg) Duration distribution relativistic jet is required (compactness problem) HETE-2 (2002-) Swift (2004-) short long • Leads many afterglow observations • Association with SN and long GRBs • Discovery of afterglow from short GRBs 2s ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  5. Standard Model Afterglow Prompt emission Piran 2003 ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  6. まだ謎は多い emission mechanism, progenitor, short GRB…. etc GRB 090709A Band function fit Iwakiri et al. in prep (4月京都会議より） vFv 100 1000 keV BB + PL fit vFv 8秒の周期性? SGR と関係? 熱的放射 ? (Ryde & Pe’er 2009) 高エネルギーガンマ線(>100MeV)の起源は特によく分かっていない。 ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  7. GRB940217v(Hurley et al. 94) GRB941017 (Gonzaletz et al. 03) -18 to 14 sec 14 to 47 sec 47 to 80 sec 80-113 sec 113-211 sec HE emission from GRBs : Pre-Fermi Era GeV photons up to 90min after the trigger GRB080514B AGILE Giuliani et al. 08 Long-lived HE emission Temporary distinct HE spectral component ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  8. Composite spectrum of 5 EGRET Bursts Dingus et al. 1997 No evidence of cutoff HE emission from GRBs : Pre-Fermi Era • 5 EGRET bursts with >50 MeV observations in 7 years • No evidence of cutoff in the summed spectrum • Evidence for extended emission • >hour (afterglow?) GeV emission • Extra component at 100 s ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  9. What can we get from HE emission of GRBs? • Extra component of the prompt emission ? Different emission mechanism: Synchrotron self Compton ? Hadronic origin ? Only GRB941017 shows the sign of extra component • What is the maximum energy of high energy photon? Constrain the bulk Lorentz factor of the relativistic jet No evidence of the cut-off so far. • Delayed or long-lived high energy emission ? Suggests another emission mechanism Time delay of high energy photon  Limit on the quantum gravity mass :MQG A few GRBs show delayed high energy emission (GRB940217, GRB080714) Need more sensitivity and larger FoV ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  10. Fermi Gamma-ray Space Telescope LAT Gamma-ray BurstMonitor (GBM) • 12 NaI detectors (8keV-1MeV) • - onboard trigger , localization • - spectroscopy • 2 BGO detectors (150keV-40MeV) • - spectroscopy (overlapping LAT band) Silicon-Strip detectors - Identification &direction measurement of γ-rays CsI calolimetor - Energy measurement ACD (plastic scintillators) - background rejection ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  11. LAT EGRET Energy range 20 MeV to >300 GeV 20 MeV – 30 GeV Energy resolution (on axis, 100 MeV – 10 GeV)‏ <10% 10% Peak effective area 9000 cm2 1500 cm2 Angular resolution (single photon, 10 GeV)‏ 0.15 0.54 Field of view >2.2 sr 0.4 sr Deadtime per event 27 us 100 ms Many GRBs More photons detected from each GRB Good GRB locations LAT Performance • Major improvements in capabilities for GRB observation • Efficient observing mode (don’t look at Earth)‏ • Wide FoV • Low deadtime (exploring dt’s down to µsec)‏ • Studies of short bursts possible • Large effective area • Good angular resolution • Increased energy coverage (to hundreds of GeV)‏ ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  12. Fermi GRB spectrum GRB 080916C (Abdo et al. 2009, Science 323, 1688) GBM/NaI GBM/BGO Alpha -1.02 +/- 0.02 Beta -2.21 +/- 0.03 Epeak 1170 +/- 142 keV Amp. 0.0354 +/- 0.001 photons/s-cm2-keV REDUCED CHISQ = 0.963, PROB = 0.698 LAT • Consistent with Band function from 10 keV to 10 GeV • No evidence for any other component • No evidence for any roll-off 10 103 105 107 Energy (keV) ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  13. Onboard Alert and Notices • GBM/LAT on-board processing (10—15 s): • GCN alert within 10—15 s from the trigger time through TDRSS (alert, location). • GBM ground processing of prompt data (few minutes): • Updated GBM position, preliminary light curve. • LAT ground processing (a few hours after data downlink) • Final location, spectrum (1st circular). • Final location, high-energy flux and spectrum, afterglow search results (2nd circular). ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  14. Autonomous Repoint Request Fermi S/C can be repointed for LAT observations of extended emission from bright GRBs Track the target direction while above the horizon by at least the Earth Avoidance Angle (20. nominal), then slew at constant angle from the Earth limb until the the target rises on the other side. ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  15. GRB 090902B ARR ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  16. Burst Advocate Fermi or other satellites detected GRB. Alert (E-mail) LAT onboard alert ? No Yes Notify analysis results to the team when the data is available. Within 30 min Significant detection !! ~ a half of day (typically) 1st GCN Circulars !! BA covers 24 hrs/day and 7day/week by an international collaboration. Japanesemembers : ISAS/JAXA, Hiroshima U. and TITEC ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  17. GRB 090902B: a successful observation sequence • 11:05:15 UT = T0 – GBM GCN alert – ARR initiated • 15:05 UT – GBM human-in-the-loop localization • LAT data monitoring and processing • 14:44 UT – GRB is seen in the telemetry • 18:24 UT – data ingest • 19:54 UT – GRB is seen in datamon plots • 20:59:48 UT – FT1 file available [T0 + 10 hr] • ASP results ~20 min later, human-in-the-loop localization • Swift ToO request issued at ~21:30 UT, begins at 23:36 UT [T0 + 12.5 hr]‏ • 21:19:03 UT – 1st GBM circular (GCN 9866)‏ • 22:48:18 UT – 1st LAT circular (GCN 9867)‏ • (RA,Dec=265.00, 27.33) with a 90% containment radius of 0.06 deg (statistical; 68% • containment radius: 0.04 deg, systematic error <0.1 deg) • 03:00:57 UT – Swift/XRT afterglow candidate (GCN 9868)‏[T0 + 16hr] • Estimated uncertainty of 4.2 arcseconds radius (90% confidence) • XRT position 3.2 arcmin from LAT position, inside error radius • 04:57:44 UT – Swift/UVOT observations, no afterglow confirmation (GCN 9869)‏ • 04:57:44 UT – enhanced Swift/XRT position (GCN 9871)‏ • 07:36:42 UT – Fermi LAT and GBM refined analysis (GCN 9872)‏ • 08:23:17 UT – Gemini-N absorption redshift (GCN 9873) z=1.822 (GMOS spectro) [T0 + 21.3 hr] • 09:14:50 UT – GROND ‏localization 3.3 arcmin from LAT position Easily seen in 1 day counts maps ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  18. Fermi GRBs Detections as of 090904 • The GBM detects ~250 GRBs/year (~400 total) • ~18% short • ~50% in the LAT FoV • The LAT detects ~10 GRBs/year • 19 total as of today (recent detection : GRB 100724A) • ~10% of GBM GRBs observed • Only one case, LAT onboard notice has been issued (GRB090510) •  onboard algolythm has been updated, now we can expect 5 notices/year ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  19. ~9.3 GRBs/year with >10 photons >100 MeV ~2.7 GRBs/year with >10 (100) photons >1 (0.1) GeV Comparable to estimates based on Band spectrum fits to bright BATSE GRBs Suggests that on average, GRBs don't have much excess (HE component) or deficit (cutoff) in the LAT energy range w.r.t. the extrapolated Band spectrum from <2 MeV LAT GRB detection rate Band et al., ApJ 701, 1673 (2009)

  20. What can we get from HE emission of GRBs? • Extra component of the prompt emission ? Different emission mechanism: Synchrotron self Compton ? Hadronic origin ? Only GRB941017 shows the sign of extra component • What is the maximum energy of high energy photon? Constrain the bulk Lorentz factor of the relativistic jet No evidence of the cut-off so far. • Delayed or long-lived high energy emission ? Suggests another emission mechanism Time delay of high energy photon  Limit on the quantum gravity mass :MQG A few GRBs show delayed high energy emission (GRB940217, GRB080714) ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  21. What can we get from HE emission of GRBs? • Extra component of the prompt emission ? Different emission mechanism: Synchrotron self Compton ? Hadronic origin ? Only GRB941017 shows the sign of extra component • What is the maximum energy of high energy photon? Constrain the bulk Lorentz factor of the relativistic jet No evidence of the cut-off so far. • Delayed or long-lived high energy emission ? Suggests another emission mechanism Time delay of high energy photon  Limit on the quantum gravity mass :MQG A few GRBs show delayed high energy emission (GRB940217, GRB080714) ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  22. Extra PL component in short and long GRBs GRB 090510 (short) GRB 090902B (long) Abdo, A. A. et al., ApJL 706, 138 (2009) Abdo, A. A. et al., ApJ submitted T0+4.6s to T0+9.6s • First time a low-energy extension of the • PL componenthas been seen First extra component by Fermi At > 5 sigma level 3 LAT GRBs shows extra PL component (090510, 090902B, 090926A) ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  23. What can we get from HE emission of GRBs? • Extra component of the prompt emission ? Different emission mechanism: Synchrotron self Compton ? Hadronic origin ? Only GRB941017 shows the sign of extra component • What is the maximum energy of high energy photon? Constrain the bulk Lorentz factor of the relativistic jet No evidence of the cut-off so far. • Delayed or long-lived high energy emission ? Suggests another emission mechanism Time delay of high energy photon  Limit on the quantum gravity mass :MQG A few GRBs show delayed high energy emission (GRB940217, GRB080714) ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  24. Limit on bulk Lorentz factor Due to large luminosity and small emitting region, optical depth for the γ-γ -> e+e- pair production is too large to observe the non-thermal emission from GRB  compactness problem. Relativistic motion (Γ>>1) could avoid this compactness problem Γmin Γmin can be derived using observed highest energy photon 090510 E=31 GeV 080916C E=3 GeV 090902B E=33 GeV z Γmin~1000 for short and long GRBs ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  25. GRB 090926A: the first HE spectral cutoff Preliminary ! 8-14.3keV Time-integrated photon spectrum(3.3-21.6s) 14.3-260 keV νFν(erg/cm2/s) 0.26-5 MeV LAT all event 10 102 103 104 105 106 Energy (keV) - Delay in HE onset: ~3 s - The extra component shows at >4 σ spectral break at ~1.4 GeV - First direct measurement of Γ ~ 720 (if cutoff due to γ-γabsorption) >100 MeV >1GeV ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  26. What can we get from HE emission of GRBs? • Extra component of the prompt emission ? Different emission mechanism: Synchrotron self Compton ? Hadronic origin ? Only GRB941017 shows the sign of extra component • What is the maximum energy of high energy photon? Constrain the bulk Lorentz factor of the relativistic jet No evidence of the cut-off so far. • Delayed or long-lived high energy emission ? Suggests another emission mechanism Time delay of high energy photon  Limit on the quantum gravity mass :MQG A few GRBs show delayed high energy emission (GRB940217, GRB080714) ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  27. Long-lived GeV emission ~ Swift and Fermi view of GRB 090510 ~ UVOT XRT Fermi/LAT GRB 090510 (short GRB) De Pasquale et al., ApJL 709, 146 (2010) t1.380.07 LAT emission until 200 s No spectral evolution (photon index -2.1 ± 0.1) Simultaneous fit of the SED at 5 different times • Forward shock model can • reproduce the spectrum from • the optical up to GeV energies • Extensions needed to arrange • the temporal properties ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  28. The first few GBM peaks are missing in the LAT but later peaks coincide Delay in HE onset: 0.1-0.2 s HE delayed onset in short and long GRBs GRB 080916C (long) GRB 090510 (short) Abdo et al. 2009, Science 323, 1688 Abdo et al. 2009, Nature 462, 331 8-260keV 0.26-5MeV LAT all events >100 MeV >1GeV The first LAT peak coincides with the second GBM peak Delay in HE onset: ~4-5 s HE delayed onset can be seen from almost all LAT GRBs ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  29. Constraint on QG and EBL models GRB 090510 GRB 090902B Abdo et al. 2009, Nature 462, 331 Abdo, A. A. et al., ApJL 706, 138 (2009) GBM NaI GBM BGO LAT (>1MeV) 31 GeV 0.83 s Constraints on the quantum gravity mass (MQG) by direct measurement of photon arrival time Most models are optically thin for 33 GeV photon from GRB 090902B (z=1.822) “baseline” and “fast evolution” models are rejected at 3.6 σ level MQG,1/Mplank > 1.19 Disfavors quantum gravity models which linearly alters the speed of light (n=1) ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  30. Models for HE delayed onset and extra-PL • Leptonic models (inverse-Compton or SSC) (Toma et al., 2009) • Hard to produce a delayed onset longer than spike widths • Hard to produce a low-energy (<50 keV) power-law excess • Hard to account for the different photon index values of the Band spectrum at low energie (but photospheric models can) and of the HE component • But, photospheric models could explain these properties (Toma et al. 2010) • Hadronic models (pair cascades, proton synchrotron) (Asano et al., 2009) • GRBs as possible sources of Ultra-High Energy Cosmic Rays • Late onset: time to accelerate protons & develop cascades? • Proton synchrotron radiation (requires large B-fields) • Synchrotron emission from secondary e± pairs produced via photo-hadron interactions • can naturally explain the power-law at low energies • require substantially more energy than observed (GRB 090510: Etotal / Eiso ~ 100-1000) • Hard to produce correlated variability at low- and high-energies (e.g. spikes of GRB 090926A) ? • Early Afterglow (e+e- synchrotron from external shock)(Kumar et al, 2009) • Can account for possible delayed (~9 s) onset of power-law component in GRB 090902B • Short variability time scales in LAT data (e.g. GRB 090926A) argues against external shock • Requires larger bulk Lorentz factor than measured for GRB 090926A ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  31. Summary of LAT GRBs Detections as of 090904 Detections as of 090904 ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  32. Long vs Short GRBs (1) Similar high energy properties between short and long GRBs ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  33. Long vs Short GRBs (2) Abdo, A. A. et al., ApJ 712, 558 (2010) Preliminary ! short short short • 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 ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  34. Upper Limit on LAT GRB Flux Fermi GRB statistics up to April 2010 GBM Detections : 427 events - Within LAT FoV (<65 deg from LAT boresight) 213 events (~50 %) - LAT Detection 16 events (~7.5 %) What can we say about remaining 197 events (92.5%)? (1) LAT flux upper limit (2) Expected LAT flux by GBM extrapolation  Possible Implication.. • Intrinsic spectral break ? • EBL or γ-γ absorption ? ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  35. GBM Extrapolation • Bright GBM sample : • GBM events with > 70 BGO cts • But no LAT detections • Performed spectral fitting • 16 events in “Gold” sample • beta can be constrained 53 sample • Obtain expected LAT flux from GBM extrapolation • Compare LAT flux upper limit with expected LAT flux ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  36. Example: GRB 090620.400 ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  37. Example: GRB 081207.680 ガンマ線バーストによるダークな宇宙の観測に向けたワークショップ

  38. Summary • Fermi detected ~400 GRBs including 19 LAT GRBs in ~1.5 years • => 250 GRBs/year for GBM and ~10 GRBs/year for LAT • Extra component of the prompt emission ? -Clear evidence of extra PL component from 3 LAT GRBs -Low-energy excess is also seen • What is the maximum energy of HE emission ? -Constraint lower limit of bulk Lorentz factor: Γmin ~1000 -GRB 090926A, first detection of HE spectral cutoff : Γ ~ 720 • HE emission is delayed and/or long-lived ? -Many LAT GRBs show delayed and long-lived high energy emission • Many leptonic or hadronic models are proposed for LAT high energy emission • No difference of high energy properties between short and long GRBs • Constraint on QG and EBL models Deciphering the Ancient Universe with Gamma-Ray Bursts

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