1. The Highest Energy Emission from Short Gamma-Ray Bursts Pablo Saz Parkinson Santa Cruz Institute for Particle Physics, UCSC SCIPP Seminar, 9 March 2007

2. Pablo Saz Parkinson. 9 March 2007 2Outline Introduction: What is a short GRB? Motivation: Why search for HE emission? Milagro Search for VHE emission Future prospects

3. Pablo Saz Parkinson. 9 March 2007 3 Gamma-Ray Bursts (GRBs) Large explosions of gamma rays discovered in late 60’s. First afterglow (and redshift) late 90’s. First short burst afterglow detected May 2005. Two types of GRBs: short ( 2s). Long bursts related to death of massive stars. Short bursts related to binary mergers. ‘Swift’ surprises: Bright X-ray flares, steep decays, shallow decays, …

4. Pablo Saz Parkinson. 9 March 2007 4 Norris et al. (1984)

5. Pablo Saz Parkinson. 9 March 2007 5 Kouveliotou et al. (1993) Distributions “overlap”. Duration alone cannot distinguish the two populations. In addition, bursts may have Extended emission (e.g. Lazzati et al. 2001, Norris et al. 2006) The first 2 s of a long burst is spectrally similar to short bursts (Ghirlanda et al 2004). Some bursts may look long but be “short”, and vice versa. There may be more than two populations …

6. Pablo Saz Parkinson. 9 March 2007 6 A word about SGRs The flare from SGR 1806-20 was the brightest explosion ever detected Maybe some short GRBs are SGRs Estimates vary a great deal but can at most account for 20% This SGR outburst was at high zenith angle for Milagro (almost 70 degrees) Boggs et al. 2006

7. Pablo Saz Parkinson. 9 March 2007 7 Another distinguishing feature (Norris et al. 2006)

8. Pablo Saz Parkinson. 9 March 2007 8 Donaghy et al. (2006) Conclusion: The duration at which a burst is equally likely to be in the SPB class and the LPB class is found to be 5 seconds.

9. Pablo Saz Parkinson. 9 March 2007 9 Characteristics of Short GRBs Shorter duration Harder spectrum Narrower pulses No spectral lag Less luminous Lower redshift No associated supernova Location in galaxies with low SFR Good for testing QG (Amelino-Camelia 2005 Scargle et al. 2006) Less absorption by EBL

10. Pablo Saz Parkinson. 9 March 2007 10 So what causes short GRBs? Favorite model: Binary merger - Energetics is the right order of magnitude - Most have been found in low SFR regions - Time scales are consistent - No apparent SN association No conclusive evidence (waiting for LIGO)

11. Pablo Saz Parkinson. 9 March 2007 11 Search for VHE emission from GRBs Experimental Motivation –EGRET (e.g. GRB 940217) –GRB 941017 (High Energy component) –Milagrito Burst (GRB 970417a) Theoretical –Many 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

12. Pablo Saz Parkinson. 9 March 2007 12 EGRET GRB Spectrum Dingus (2003) dN/dE ~ E -1.95

13. Pablo Saz Parkinson. 9 March 2007 13 High Energy emission from GRB Hurley et al., Nature 372, 652 (1994) 18 GeV! Gonzalez et al., Nature 424, 749 (2003) GRB 941017 GRB 940217 -18-14s 14-47s 47-80s 80-113s 113-211s

14. Pablo Saz Parkinson. 9 March 2007 14 Short GRB 930131? Credit: J. Norris T90=14 s, fluence = 1.2x10 -5 erg cm -2 Note: EGRET deadtime ~ 100 ms

15. Pablo Saz Parkinson. 9 March 2007 15 Pe’er & Waxman (ApJL 603,1, L1-L4, 2004) constrain source parameters for Inverse Compton emission of GRB941017 z=0.2 z=0.02 Theory of the high E component Shape of high energy component applies 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

16. Pablo Saz Parkinson. 9 March 2007 16 Dermer et al. 1999 TeV emission mirrors MeV Measurement of time dependence Of the high energy emission can test the SSC model and the external shock scenario.

17. Pablo Saz Parkinson. 9 March 2007 17 Razzaque and Meszaros model (Razzaque & Meszaros 2006)

18. Pablo Saz Parkinson. 9 March 2007 18 Detecting Gamma Rays High Sensitivity HESS, MAGIC, CANGAROO, VERITAS Large Aperture/High Duty Cycle Milagro, Tibet, ARGO, HAWC? Low Energy Threshold EGRET/GLAST 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

19. Pablo Saz Parkinson. 9 March 2007 19 MAGIC response to GRBs Albert et al. (2006)

20. Pablo Saz Parkinson. 9 March 2007 20 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 m 2

21. Pablo Saz Parkinson. 9 March 2007 21 Event Reconstruction Real air shower event Monte Carlo gamma-ray shower

22. Pablo Saz Parkinson. 9 March 2007 22 Milagro Effective Area

23. Pablo Saz Parkinson. 9 March 2007 23 Why is GRB VHE emission elusive? Primack et al. 05 I=I 0 e   =1 => ~ 0.37  =10 => ~ 4.5 x 10 -5 Atmospheric Cerenkov Telescopes cannot search for prompt emission Extragalactic Background Light (EBL) absorption  High Energy +  EBL –> e + e -

24. Pablo Saz Parkinson. 9 March 2007 24 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

25. Pablo Saz Parkinson. 9 March 2007 25 “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

26. Pablo Saz Parkinson. 9 March 2007 26 The untriggered search: outputs Probability histograms No significant emission detected Milagro can set model-dependent upper limits on VHE emission from GRBs. 0.0398s 0.158s0.1s 0.0251s -20 -10 log(P) D. Noyes, PhD Thesis, 2005

27. Pablo Saz Parkinson. 9 March 2007 27 The triggered search 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) Milagrito evidence for TeV emission

28. Pablo Saz Parkinson. 9 March 2007 28 Short GRBs in Milagro’s FOV We define “short” to be 5 s 2000-2007: 17 GRBs (15 well localized) 6 Swift GRBs 6 Inter-Planetary Network (IPN) 4 BATSE 1 HETE 3 firm redshifts (0.55,0.86,3.91) 3 questionable redshifts (0.001,0.225,0.41)

29. Pablo Saz Parkinson. 9 March 2007 29 Search for a TeV signal 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 Light curve (T=0 trigger time) Number of events in 1.6 degree bin Number of events expected from background Significance (GRB location at center)

30. Pablo Saz Parkinson. 9 March 2007 30Significances

31. Pablo Saz Parkinson. 9 March 2007 31 Milagro Limits for Some Bursts GRB 050509b: A short/hard burst (z=0.225?) – E iso (keV) = 2 x 10 -8 ergs/cm 2 – E iso (TeV)/E iso (keV) < 10 – 20 (GCN Circular 3411) –Razzaque et al. model would give ~0.02 s -1 GRB 051103: A short/hard (0.17 s) burst detected by the IPN – E iso (keV) = 2.34 x 10 -5 ergs/cm 2 – E iso (TeV)/E iso (keV) M81 < 4 Mpc) (GCN Circular 4249) GRB 060427b: Another short (0.2 s) IPN burst, z=?, 16 o zenith –E iso (TeV)/E iso (keV) < 4 (for z=0.5) (GCN Circular 5061) –E iso (TeV)/E iso (keV) ~ 0.1 for z=0

32. Pablo Saz Parkinson. 9 March 2007 32Results Submitted to ApJ

33. Pablo Saz Parkinson. 9 March 2007 33 Future prospects: HAWC 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: 22,500m 2 1 year survey point source sensitivity of ~60mCrab

34. Pablo Saz Parkinson. 9 March 2007 34 Future prospects: HAWC Milagro HAWC

35. Pablo Saz Parkinson. 9 March 2007 35 Summary and Conclusions - Our knowledge of short GRBs is still in its infancy. - Short GRBs are good candidates for VHE emission. - Detection of VHE emission should constrain the numerous models and can also be used to probe deeper physics questions (e.g. QG) - No VHE emission from GRBs has been detected to date, but it cannot be definitely ruled out. Swift will continue to provide a number of potential candidates and blind searches will help to constrain such emission. - A future detector, HAWC, larger and at higher altitude (~4500 m) would significantly improve the prospects for detecting VHE emission from short GRBs. - GLAST, in conjunction with the ground-based TeV detectors will put severe constraints on emission models.

36. Pablo Saz Parkinson. 9 March 2007 36 Thank You