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Exploring GRBs through Optical, X-ray, and Gamma-ray Observations

This paper discusses multiwavelength observations of Gamma-ray Bursts (GRBs) using Fermi LAT and GBM instruments, as well as Suzaku WAM and Kanata Optical Telescope. It also highlights GRBs detected by the Fermi Observatory and the preliminary analysis of the GRB081024B burst.

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Exploring GRBs through Optical, X-ray, and Gamma-ray Observations

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  1. 可視・X線・γ線同時観測で探るγ線バースト (Exploring GRBs through Optical, X-ray and Gamma-ray Observations) November 27, 2008 Tsunefumi Mizuno (Hiroshima Univ.) mizuno@hep01.hepl.hiroshima-u.ac.jp Yasushi Fukazawa, Koji Kawabata, Hideaki Katagiri , Yoshitaka Hanabata (Hiroshima Univ.), Goro Sato, Masanori Ohno (ISAS/JAXA) Hiroyasu Tajima (SLAC) on behalf of the Fermi GBM/LAT Collaboration Contribution to the Conference “Deciphering the Ancient Universe with Gamma-Ray Bursts” at Mitaka, Tokyo, Japan (Nov 27-29, 2008)

  2. Introduction • Multiwavelength Observation to explorer GRB phenomena • Fermi LAT and GBM • 7 decades of energy coverage (8 keV-300 GeV) • High sensitivity imaging in GeV band (>100 MeV) • Spectral cutoff (G-factor), GeV excess (hadronic origin?), etc. • Suzaku WAM • Largest Aeff in 300-5000 keV, large FOV (~2p) • “best friend” to Fermi LAT/GBM; better constrain of the spectral parameter, spectral-lag analysis, time-resolved spectroscoy. • Kanata Optical Telescope • 1.5 m diameter & very fast driving speed (5deg/sec) • HOWPol is about to ready • Polarimetry of optical afterglow to constrain the magnetic field structure.

  3. The Fermi Observatory LAT+GBM=> more than 7 energy decades! (For detail, see talk by K. Asano’s in this afternoon) Large Area Telescope (LAT) • Large Field of View (>2.4 sr) • views entire sky every 3 hrs in wide energy range (20 MeV - 300 GeV) Gamma-ray Burst Monitor (GBM) • Views entire unocculted sky • 12 NaI detectors: • 8 keV - 1 MeV • 2 BGO detectors: • 150 keV - 30 MeV ~200 GRBs/year expected by the GBM

  4. Fermi Launch !! • Launch from Cape Canaveral Air Station 11 June 2008 at 12:05 PM EDT • Circular orbit, 565 km altitude (96 min period), 25.6 deg inclination. • Operation • Onboard processing (GBM): alerts and updates to GCN with localization. • LAT ground processing (5-12 hours): updates location and the spectrum/flux. • Scientists serve as “Burst Advocator” (for GBM and LAT individually) • LAT BAs cover 24hrs/day and 7days/week by an international collaboration. They continuously check the GCN Notice and perform the analysis of LAT data. • In Japan, ISAS/JAXA, Tokyo Institute of Technology and Hiroshima Univ. are involved in BA. See talk by R. Sato for detail. (photo taken by M. Ohno)

  5. GRBs Detected by Fermi • Sep. 18, 2008 “Fermi/GBM Data Now Available” (http://heasarc.gsfc.nasa.gov/cgi-bin//W3Browse/w3table.pl?Action=Detailed%20Mission&Observatory=fermi) • 32 GRBs from Sep. 12th to Nov. 19th (~0.5/day, as expected before the launch) • 11 GRBs are also detected by Suzaku WAM • 3 LAT GRBs • GRB080825C (<35sec, 14 photons in 100MeV-1GeV) • GRB080916C (<100sec, 14 photons above GeV) see K. Asano’s talk • GRB081024B (short GRB, up to 3 GeV. GCN Circular #8407)

  6. GRB081024B by Fermi LAT GCN Circular #8407 • Nicola Omodei On behalf of the Fermi LAT Collaboration • The Fermi LAT telescope detected an increased count rate at 2008/10/24 (21:22:41), associated with a GRB detected also by the Fermi GBM (Trigger number:246576161). The location of the GRB reported by Fermi LAT data ground analysis is: Ra=322.9, Dec=21.204 which corresponds to Ra (J2000) = 21:31:36.00 Dec (J200) = 21:12:14.4 within a statistical uncertainty of 0.16 degrees (68% containment radius). The emission from this point source was seen up to 3 GeV, in the first 5 seconds after the trigger. The Fermi LAT point of contact for this burst is Nicola Omodei (nicola.omodei@pi.infn.it). The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden. This message can be cited. • 13 LAT photos above 100 MeV • The first short GRB with emission above GeV (possible exception of GRB930131; Sommer et al. 1994). • Joint analysis of GBM/LAT data is important and is underway by Fermi GBM/LAT team.

  7. Detailed GBM Light Curve NaI (30-110 keV) • Background subtracted light curves, with 0.09375s bin width (to make them match with WAM light curves) • 3 time regions could be specified (colored areas) • Only below ~100 keV, two pulses are clearly separated. • possible soft lag of the 2nd pulse. • 2nd pulse is shorter in high energy band. • possible spectral evolution 2 (s) 0 -2 NaI (110-240 keV) NaI (240-520 keV) BGO (520-960 keV)

  8. GBM Spectra of GRB081024B Interval1 (0-0.094 s) Interval2 (0.094-0.28 s) NaI & BGO fit by a single power-law model: index ~ 1.0 +/- 0.1 single PL-fit: Index ~ 0.9 +/- 0.2 Preliminary Interval3 (0.28-0.75 s) • Apparent cutoff not seen up to 1 MeV • Possible spectral evolution • In interval 3, spectrum softens and shows deficit of photons below 30 keV. • Statistics-limited analysis with preliminary detector responses. Further analysis using more accurate responses is underway. single PL-fit: Index~ 1.35 +/- 0.15

  9. Suzaku WAM also Detected the GRB WAM TH0(50-110 keV) • Barycentric correction not applied yet • counts/significance in 0-0.75s (stat error only) • TH0: 75(3.3s) • TH1: 178(7.6s) • TH2: 85(3.2s) • TH3:139(3.9s) • Above 100 keV, factor of ~2 higher counts and slightly better significance than GBM => WAM/GBM joint analysis could improve the spectral fit. 2 (s) 0 -2 WAM TH1(110-240 keV) WAM TH2(240-520 keV) WAM TH3(520-5000 keV)

  10. HOWPol on Kanata Telescope(1) (Hiroshima One-shot Wide-field Polarimeter) １露出型偏光撮像（広視野 7’×7’、狭視野 1’×15’） Filter exchanger Prism exchanger Focus adj. stage Focal mask exchanger CCD dewer Collimator lens Shutter and pupil Camera lens Polarmetric unit exchanger • 2008年7月28-31日 • ファーストライト • 効率測定： B: 7%, V: 19%, R: 27%, I: 25% (大気含む) • 2008年11月4日～ • 広視野用偏光プリズム導入 • 定常試験観測開始 • 　器械偏光 ~4%　（第三鏡による偏光面回転のみで0.4%精度での補正が可能、他の依存性調査中）

  11. HOWPol on Kanata Telescope (2) • 2008年12月～　ＧＲＢアラート対応観測開始 • ⇒バーストから100秒以内の偏光観測開始めざす • Akerlof & Heather (2007) Statistics of 108 Swift GRBs • 40%で光学残光　うちt=1000秒換算での等級が14等,16等,18等より明るいもの0.9%,4.6%,16.7% • 4年間観測を続けると • 1000秒後でも16等より明るいサンプルを１個/4年(100-1000sの間で偏光の変化を追う) • 200sで16等より明るいサンプルを3-4個(10分露出で偏光測定) • =>年1個程度の偏光観測を期待 200秒 1000秒 Δp=0.2% 偏光測定 限界 1分露出時 10分露出時 仮定： 　発見レート100個/年 　高度係数 0.33 (>20度） 　夜間係数0.4 　晴天係数0.4

  12. Summary • Fermi GBM detects ~0.5GRB/day, as expected before the launch. Now the GBM data is publicly available. • Three LAT GRBs above 100 MeV • Two long (GRB080825C, GRB080916C) and one short (GRB081024B). • GRB081024B is the first short GRB with photons above 1 GeV. • LAT detected photons up to 3 GeV • In GBM band, we observed a “hint” of the soft lag below 100 keV and the spectral evolution in the 2nd pulse. (preliminary statement) • Detailed analysis is underway by GBM/LAT team. • Suzaku-WAM has largest Aeff in 300-5000 keV and is the best friend of Fermi GBM/LAT • Simultaneous detection of GRB081024B. Better statistics above 100 keV. • HOWPol on Kanata Telescope is about to ready • Study the magnetic filed of external shock. ~1GRB/year (<16mag@200s) is expected.

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