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Searching for the highest energy emission from Gamma-Ray Bursts

Searching for the highest energy emission from Gamma-Ray Bursts. Pablo Saz Parkinson Santa Cruz Institute for Particle Physics, UCSC. SSL, Berkeley, 26 May 2006. Outline. Introduction: The Milagro Gamma-ray Observatory Some Recent Results - VHE emission from the Galactic Plane

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Searching for the highest energy emission from Gamma-Ray Bursts

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  1. Searching for the highest energy emission from Gamma-Ray Bursts Pablo Saz Parkinson Santa Cruz Institute for Particle Physics, UCSC SSL, Berkeley, 26 May 2006

  2. Outline • Introduction: The Milagro Gamma-ray Observatory • Some Recent Results • - VHE emission from the Galactic Plane • Search for VHE emission from GRBs • Future prospects Pablo Saz Parkinson. 26 May 2006

  3. Milagro Collaboration List A. Abdo1, B. Allen2, D. Berley3, E. Blaufuss3, S. Casanova4, D.G. Coyne5, S. DeLay2, B.L. Dingus4, R.W. Ellsworth6, L. Fleysher7, R. Fleysher7, M.M. Gonzalez8, J.A. Goodman3, E. Hays3, C. M. Hoffman4, L.A. Kelley5, C.P. Lansdell3, J.T. Linnemann1, J.E. McEnery9, A.I. Mincer7, P. Nemethy7, D. Noyes3, J.M. Ryan10, F. W. Samuelson4, P. Saz Parkinson5, M. Schneider5, A. Shoup2, G. Sinnis4, A.J. Smith3, G.W. Sullivan3, V. Vasileiou3, G. Walker4, D.A. Williams5, G.B. Yodh2 • Department of Physics and Astronomy, Michigan State University. • Department of Physics and Astronomy, University of California, Irvine. • Department of Physics, University of Maryland. • Group P-23, Los Alamos National Laboratory • Santa Cruz Institute for Particle Physics, University of California, Santa Cruz. • Department of Physics and Astronomy, George Mason University. • Department of Physics, New York University. • Department of Physics, University of Wisconsin. • NASA Goddard Space Flight Center. • Department of Physics, University of New Hampshire. Pablo Saz Parkinson. 26 May 2006

  4. Detecting Gamma Rays High Sensitivity HESS, MAGIC, CANGAROO, VERITAS Low Energy Threshold EGRET/GLAST Large Aperture/High Duty Cycle Milagro, Tibet, ARGO, HAWC? Large Effective Area Excellent Background Rejection (>99%) Low Duty Cycle/Small Aperture Space-based (small area) “Background Free” Large Duty Cycle/Large Aperture Moderate Area/Large Area (HAWC) Good Background Rejection Large Duty Cycle/Large Aperture High Resolution Energy Spectra Studies of known sources Surveys of limited regions of sky Point source sensitivity Unbiased Sky Survey (<300 GeV) AGN Physics Transients (GRBs) (<100 GeV) Unbiased Sky Survey Extended sources Transients (GRB’s) Solar physics/space weather Pablo Saz Parkinson. 26 May 2006

  5. The Milagro TeV observatory • 2630 m above sea level in the Jemez Mountains, Los Alamos, New Mexico • Operational since 2000 (with outriggers since 2003) • Duty cycle greater than 90% • ~ 2sr field of view • Trigger rate 1.5-2 kHz • Angular resolution of 0.45 degrees • Energy: ~ 100 GeV – 100 TeV • (median ~ 2.5 TeV) • 8” PMTs with “baffles” • 2.8 x 2.8 m spacing • Top Layer: 450 PMTs, 1.5 m deep • Bottom Layer: 273 PMTs, 6.5 m deep • Outriggers: 175 black plastic tanks each with a PMT, spread over 20,000 m2 Pablo Saz Parkinson. 26 May 2006

  6. Milagro Outriggers • 175 black plastic 4000 liter tanks deployed over 20,000 m2 • Each lined with Tyvek and filled with filtered water • A single 8” PMT in each • Operating since 2003 • Improve angular resolution and background rejection. Pablo Saz Parkinson. 26 May 2006

  7. Event Reconstruction Real air shower event Monte Carlo gamma-ray shower Pablo Saz Parkinson. 26 May 2006

  8. Background rejection Proton Proton Gamma Gamma • Cosmic rays outnumber gamma rays by more than 1000 to 1 • Differences between Hadron- and Gamma-ray- initiated showers • Hadronic showers contain more muons than gamma ray showers • Hadronic showers form bright, compact clusters in the muon layer • Gamma showers illuminate muon layer uniformly, with small hits Pablo Saz Parkinson. 26 May 2006

  9. Background rejection (cont’d) • Parameterize “clumpiness” of the bottom layer hits • Compactness • Require C > 2.5 • 50% gammas & 10% hadrons • Sensitivity improved by 1.6 • Require A4 > 3.0 • 20% gammas & 1% hadrons • Sensitivity further improved by 1.4 mxPE: maximum # PEs in bottom layer PMT nb2: # bottom layer PMTs with 2 PEs or more fTop: # fraction of hit PMTs in Top layer fOut: # fraction of hit PMTs in Outriggers nFit: # PMTs used in the angle reconstruction Pablo Saz Parkinson. 26 May 2006

  10. Milagro’s Science Goals: • Survey the Northern Hemisphere TeV Sky • Transient phenomena • Gamma ray bursts • SGR outbursts • Flares from active galaxies • Solar events (coronal mass ejections) • Year-round observation of all sources • Extended sources • Diffuse emission from the Galactic plane • cosmic ray generation and propagation • Molecular clouds • Supernova remnants • Galaxy clusters Pablo Saz Parkinson. 26 May 2006

  11. Galactic Plane EGRET All-sky survey above 100 MeV EGRET diffuse GeV flux (in black) Milagro exposure (in red) Pablo Saz Parkinson. 26 May 2006

  12. TeV Gamma Rays from Galactic Plane Preliminary 5 Years of data, 2 with outriggers used in event reconstruction providing much better sensitivity. Pablo Saz Parkinson. 26 May 2006

  13. The Cygnus Region in TeV Preliminary • GP diffuse excess clearly visible from l=25O to l=90O. • Cygnus Region shows extended excess • FCygnus ~ 2 x Fcrab • 120 square degrees l (65,85), b (-3,3) Pablo Saz Parkinson. 26 May 2006

  14. Gamma-Ray Bursts (GRBs) • Discovered in late 60’s. • First afterglow/z late 90’s. • Two types: short (< 2s) and long (> 2s). • Long bursts related to death of massive stars. • First short burst afterglow: 2005. • ‘Swift’ surprises: Bright X-ray flares, steep decays, shallow decays, … Kouveliotou et al, 1993 Nousek et al, 2006 Pablo Saz Parkinson. 26 May 2006

  15. Search for VHE emission from GRBs • Experimental Motivation • EGRET (e.g. GRB 940217) • GRB 941017 (High Energy component) • Milagrito Burst (GRB 970417a) • Theoretical • Models predict VHE emission (e.g. SSC) • Why Milagro? • Large (1/6 sky) field of view and > 90% duty cycle • No need to point: search for prompt emission • Best current instrument for this type of search Pablo Saz Parkinson. 26 May 2006

  16. High Energy emission from GRB GRB 941017 GRB 940217 -18-14s 14-47s 47-80s 80-113s 113-211s 18 GeV! Hurley et al., Nature 372, 652 (1994) Gonzalez et al., Nature 424, 749 (2003) Pablo Saz Parkinson. 26 May 2006

  17. Theory of the high E component Pe’er & Waxman (ApJL 603,1, L1-L4, 2004) constrain source parameters for Inverse Compton emission of GRB941017 z=0.2 z=0.02 • Shape of high energy component applies tight constraints to ambient densities and magnetic fields. • Milagro has the sensitivity to observe the predicted emission or rule out the model. • More GRBs with low redshift are needed. z=0.5 Pablo Saz Parkinson. 26 May 2006

  18. Atmospheric Cerenkov Telescopes cannot search for prompt emission Extragalactic Background Light (EBL) absorption High Energy+EBL –> e+ e- Why is GRB VHE emission elusive? Primack et al. 04 I=I0e-t t=1 => ~ 0.37 t=10 => ~ 4.5 x 10-5 Pablo Saz Parkinson. 26 May 2006

  19. Why VHE emission is elusive (Cont’d) • Most bursts are at high z • ~ 20% of bursts with measured z have z < 0.5 • Milagro expects ~ 1/year in its FOV with z < 0.5 Pablo Saz Parkinson. 26 May 2006

  20. “triggered” vs “untriggered” • Untriggered Search: • Real-time, all locations, instant notification • Many time scales (0.25 msec to > 2hr) • Drawback: LARGE number of trials • Triggered Search: • Satellites provide time, location, and duration of burst -> more sensitive • Even limits on bursts with redshifts are important • Swift is greatly increasing our sample • Drawback: small number of bursts Pablo Saz Parkinson. 26 May 2006

  21. The untriggered search: outputs 0.0251s 0.0398s • Probability histograms • GRB alerts • No significant emission detected 0.1s 0.158s -20 -10 log(P) -20 -10 log(P) The number of trials is optimized to achieve the best sensitivity with the available computing power. Pablo Saz Parkinson. 26 May 2006

  22. Constraining GRB models redshift Eiso T90 • Redshift dependence • EBL model dependence • Fluence dependence Conclusion: Milagro sets model-dependent upper limits on the VHE emission from GRBs. D. Noyes, PhD Thesis, 2005 Pablo Saz Parkinson. 26 May 2006

  23. The triggered search Milagrito evidence for TeV emission • More sensitive than untriggered search (know location and duration) • Ideal GRB: bright, nearby, at a good zenith angle. Have not had such a burst. Swift could change this. This was 1 of 54 bursts searched. The Milagro sample of bursts has only recently surpassed this number. GRB 970417a had a post-trial probability of 1.7x10-3 (including the 54 bursts searched) Pablo Saz Parkinson. 26 May 2006

  24. GRB Sample in Milagro • 2000-2001: 25, 3 with z, ApJ 630, 996 (2005) • 2002-2004: 12, 2 with z • 2005: 20, 7-8 with z • 2006: 17, 2 with z (through May 15) • Total: 74 • 14-15 with measured z • 2-3 with z< 0.5 GRB 050509b • 4-5 with z<1 tentatively 0.225 Pablo Saz Parkinson. 26 May 2006

  25. Search for a TeV signal Light curve (T=0 trigger time) Number of events in 1.6 degree bin Look at number of events in a given bin during the relevant time (e.g. T90) Compute estimated Background in that bin using 2 hours of data around the burst Calculate significance Number of events expected from background Significance (GRB location at center) Pablo Saz Parkinson. 26 May 2006

  26. Early results (pre-Swift) Atkins et al, Astrophysical Journal 630 (2005) 996-1002 Pablo Saz Parkinson. 26 May 2006

  27. Early results (cont’d) • GRB 010921: A relatively nearby burst • z=0.45, 10 degrees zenith (ApJ, 2005 ) GRB010921, z=0.45 Eiso(TeV)/Eiso(keV) < 1-4 Pablo Saz Parkinson. 26 May 2006

  28. 2002-2004: ~ 30 GRBs per year ~ 5 redshifts per year 12 GRBs (~4 /yr) in Milagro FOV, 2 with redshift The ‘Swift’ Era Swift was launched on 20 November 2004. The first reported GRB was 041217. Normal operations began on 5 April 2005. • SINCE SWIFT: • ~ 90 GRBs more per year • ~ 25 redshifts per year • 05-06 (through 5/15): 37 GRBs • in Milagro FOV, 10 with redshift Pablo Saz Parkinson. 26 May 2006

  29. Milagro Limits for Some Bursts • GRB 041219: A very long, bright INTEGRAL/Swift burst • If z = 0.1–0.5, Eiso(TeV)/Eiso(keV)<0.3–7 • GRB 050509b: A short/hard burst (z=0.225?) • Eiso(keV) = 2 x 10-8 ergs/cm2 • Eiso(TeV)/Eiso(keV) < 10 – 20 (GCN Circular 3411) • GRB 051103: A short/hard (0.17 s) burst detected by the IPN • Eiso(keV) = 2.34 x 10-5 ergs/cm2 • Eiso(TeV)/Eiso(keV) < 1 (if z~0 -> M81 < 4 Mpc) • Sent GCN Circular 4249 • GRB 060218: T90=2100 s, z=0.03, 43.5o from zenith and setting • For 10 s hard spike within burst Eiso(TeV)/Eiso(keV) < 4 • GRB 060427b: Another short (0.2 s) IPN burst, z=?, 16o zenith • Eiso(TeV)/Eiso(keV) < 4 (for z=0.5) (GCN Circular 5061) Pablo Saz Parkinson. 26 May 2006

  30. Short/Hard GRBs in Milagro FOV • Milagro is uniquely capable of searching for prompt • emission from Short Gamma-ray bursts. • A significant fraction of well-localized short GRBs have falllen in Milagro’s field of view: • GRB 040924 : 0.6s, 43o, z=0.859, 2.6x10-6 erg cm-2 • GRB 051103: 0.17s, 50o, z=M81?,2.3x10-5 erg cm-2 • GRB 051221: 0.2s, 42o, z=0.55, 2.4x10-6 erg cm-2 • GRB 050509b: 0.128s, 10o, z=0.226?, 2.3x10-8 erg cm-2 • GRB 060313: 0.8s, 47o,z=?, 1.4x10-5 erg cm-2 • GRB 060427b: 0.2s, 16o, z=?,4.95x10-6 erg cm-2 No VHE emission detected. Milagro limits could test/rule out certain models (e.g. Razzaque & Meszaros) Pablo Saz Parkinson. 26 May 2006

  31. Emission from X-ray flares? • Some GRBs observed by Swift display very bright X-ray flares at late times, sometimes as bright as the GRB itself. Some models predict that these could be detectable in the GeV-TeV range GRB Fluence (15-350 keV): 8e-7 erg cm-2 Flare Fluence (15-350 keV): 14e-7 erg cm-2 Comes into view for Milagro GRB 050502B (Falcone et al, 2005) Pablo Saz Parkinson. 26 May 2006

  32. TeV Emission from Flares? Upper limits 1-2 orders of magnitude higher than measured X-ray fluence. Pablo Saz Parkinson. 26 May 2006

  33. Future prospects: miniHAWC A low-cost successor to Milagro, reusing the PMTs and much of the instrumentation, optimized layout, at high altitude (~4500 m), with a potential increase in sensitivity of > 15. 841 PMTs (29x29) in one layer 5.0m spacing Single layer with 4m depth Instrumented Area: 22,500m2 1 year survey point source sensitivity of ~60mCrab Pablo Saz Parkinson. 26 May 2006

  34. Future prospects: miniHAWC miniHAWC Milagro Pablo Saz Parkinson. 26 May 2006

  35. Summary and Conclusions • Milagro has been operational since 2000. • Operating with “outriggers” since 2003 and currently in its most sensitive configuration. • No VHE emission from GRBs has been detected, but it is early still to rule out. Swift is providing a large number of potential candidates. • A future detector, miniHAWC, larger and at higher altitude (~4500 m) could significantly improve the prospects for detecting GRBs in the near future. Pablo Saz Parkinson. 26 May 2006

  36. Thank You Pablo Saz Parkinson. 26 May 2006

  37. Energy Spectrum • Consistent with extrapolation from EGRET • Any rapidly rising component to explain >1 GeV excess cannot continue to 1 TeV • Previous upper limits in this energy range well above extrapolation • Atkins et al, Physical Review Letters 95 (2005) 251103 (astro-ph/0502303) Pablo Saz Parkinson. 26 May 2006

  38. Bkg Rejection – “Compactness” PMTs > 2 PE Gammas (MC) Maximum PE Data Proton (MC) Compactness C = using just bottom layer PMTs C > 2.5 removes 90% of the protons and keeps 50% of the gammas Astrophysical Journal595, 803–811 (2003) Pablo Saz Parkinson. 26 May 2006

  39. GRB 060427B Duration = 0.2 s 16 degrees zenith 4.95x10-6 erg cm-2 (20keV-10 MeV) No absorption: 5.0x10-7 erg cm-2 (median 2 TeV) Absorption (z=0.5): 1.9x10-5 erg cm-2 (median 150 GeV) Pablo Saz Parkinson. 26 May 2006

  40. Milagro Effective Area Pablo Saz Parkinson. 26 May 2006

  41. X-ray Flares • Bright X-ray Flares during GRB Afterglow • PRELIMINARY results from Swift (courtesy A. Falcone) • GRB 050607 • Flare 1: peak ~ 8 c/s, 4x afterglow; ~ 2.8e-8 erg cm-2 • Flare 2: peak ~ 60 c/s, 40x afterglow; ~ 1.4e-7 erg cm-2 • GRB 050712 • Flare 1: peak ~ 16 c/s, 4x afterglow • Flare 2: peak ~ 5 c/s, 4x afterglow • GRB 050716 • Flare 1: peak ~ 35 c/s, 2x afterglow • Flare 2: peak ~ 12 c/s, 2x afterglow Pablo Saz Parkinson. 26 May 2006

  42. SGR Outbursts • SGR 1806-20, Giant Flare of 27 December 2004 • Very close (z=0) and Very bright(0.3-0.8 erg cm-2), but very bad zenith angle (68 degrees) • Brightest transient event ever recorded • Effective area of Milagro is ~ 0.5 m2 • Our preliminary upper limit is ~ 6 x 10-4 erg cm-2 • More recent outburst from SGR 1900+14 on 25 March 2006 through 29 March 2006 • No significant detection for 4 mini-bursts that fell in Milagro field of view. Working on correlation of Milagro-Swift data. Pablo Saz Parkinson. 26 May 2006

  43. Milagro TeV Survey 3 Years: 2000 December – 2003 November 95% UL 275-600 mCrab Pablo Saz Parkinson. 26 May 2006

  44. VME Trigger Jan 1999 – Mar 2002 Multiplicity trigger: ~ 55 Tubes, 200 ns Pablo Saz Parkinson. 26 May 2006

  45. Razzaque and Meszaros model Pablo Saz Parkinson. 26 May 2006

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