Searching for VHE Gamma-Ray Bursts with Milagro Brenda Dingus dingus@lanl.gov OUTLINE Past: EGRET Observations Now: Milagro’s Search Satellite Triggered Notifications Untriggered Search Future: miniHAWC (High Altitude Water Cherenkov)

EGRET Observations > 30 MeV 4 GRBs detected in BATSE T90 time interval. Due to large deadtime of EGRET, only a lower limit on the fluence is known. EGRET Fluence assumes E-2.1 differential power law. E-2.4 lowers the flux estimate by factor of 2-3. From the other ~100 GRBs, only 7 gamma rays are detected in 6 GRBs >30 MeV

Average High Energy Spectrum The 4 detected GRBs have 45 g-rays above 30 MeV and 4 above 1 GeV within 200 sec of the BATSE trigger Average spectrum fits dN/dE a E-1.95 +- 0.25 with no cutoff up to 10 GeV

EGRET Light Curves Prompt and Afterglow Detections > 50 keV counts/sec Sommer et al., 1994 ApJ 422 L63 Prompt and Afterglow Detections Prompt Flux Unknown due to High Deadtime Afterglow Flux has large statistical uncertainty Hurley et al., 1994 Nature 372, 652

2nd Higher Energy component Gonzalez et al, Nature, 424, 749 (2003) -18 - 14s 14 - 47s 47 - 80s 80 - 113s 113 - 211s Classic sub-MeV component observed in BATSE data which decays by factor of 1000 and Epeak moves to lower energies Higher Energy component observed within 14-47 seconds by EGRET and at later times by both BATSE and EGRET detectors Higher Energy Component has dNg/dE = kE-1 lasts ~200 seconds Increases total energy flux by factor of 3

Other GRBs with Similar Higher Energy Components 6/56 spectra with probability < 10-3 that addition of higher energy power law improves fit by chance See Ph.D. thesis by Magda Gonzalez GRB Seconds From Trig Amplitude (10-7/cm2seckeV) Probability 910503 3-7 4.6-7.3 3.5E-5 920622 2.8-5.8 4.5E-3 950425 42-75 1.6-2.7 1.1E-4 981021 1-3 1.1-9.9 2.6E-4 980923 13-48 4.2-5.6 3.4E-7

Higher Energy Component at Even Higher Energies? EGRET observation extends to ~200 MeV with dNg/dE = kE-1 can’t extend forever Inverse Compton Emission in early afterglow can explain hard spectrum but tightly constrains ambient densities and magnetic fields Or evidence of origin of Ultra High Energy Cosmic Rays (Dermer & Atoyan, 2004) Or ??? Pe’er & Waxman ApJ 2004 constrain source parameters for Inverse Compton emission of GRB941017 in “transition” phase to afterglow Milagro Sensitivity z=0.2 z=0.02 Increases total fluence from 6x10-4 to (2-6)x10-3 ergs/cm2 (very high but z unknown)

Water Cherenkov Extensive Air Shower Detectors Detect Particles in Extensive Air Showers from Cherenkov light created in a covered pond containing filtered water. Reconstruct shower direction to 0.3-0.7o from the time different photodetectors are hit. Multi-kHz trigger rate mostly due to Extensive Air Showers created by cosmic rays Field of view is ~2 sr and the average duty factor is nearly 100% 8 meters e m g 80 meters 50 meters Milagro Cross Section Schematic

Sensitivity vs Energy Milagro Effective Area for 3 Zenith Angles E2 GLAST on-axis area Milagro Effective Area for 3 Zenith Angles 100 GeV 1 TeV 10 TeV 100TeV

Milagro GRB Sensitivity 40 second GRB duration at unknown location dN/dE a E-2 Atkins, et al. ApJ Lett. 604, 25, 2004 Emax = 300 GeV Emax = 1 TeV Emax = 20TeV

Gamma Ray Bursts & Other Transients Milagro data searched within 4 seconds for transients. Time intervals searched vary from milliseconds to weeks. No significant excess has been detected. Constrains TeV emission from Gamma-Ray Bursts for an assumption of the redshift distribution of sources and the luminosity distribution Distribution of probabilities for a typical day’s observations of one time interval Log10 (Probability of Excess) # of Occurrences with < Probability

Implications of Milagro’s Search for GRBs Milagro data searched within 4 seconds for transients Model dependent limit on VHE fluence from GRBs (1) Assumptions: redshift T90 Eiso (3) Upper Limit on VHE Emission: (2) Predictions: See David Noyes Ph.D. Thesis for details

Milagrito Evidence for TeV GRB Milagrito operated ~ 1 year during the BATSE era 54 GRBs within fov GRB 970417a Weak BATSE GRB with poor localization Milagrito detection had a post-trial chance probability of 1.7x10-3 (including the 54 bursts searched and the large location uncertainty). TeV fluence > 10x keV fluence GRB970417a Atkins et al., 2003, Ap J 583, 824 Redshift unknown so TeV flux emitted is uncertain.

Milagro U.L. from Satellite Triggered GRBs E2 dNg/dE (ergs/cm2) assuming dNg/dE=kE-2.4 at the median detection energy (3 TeV for unabsorbed spectra) Fluence UL accounts for pair absorption in transit due to z and is given at median detection energy of few hundred GeV Short Bursts Pablo Saz Parkinson, GRB’s in SWIFT Era Conference, 2005

Interesting GRBs in Milagro’s f.o.v. GRB010921, z=0.45 Eiso(TeV)/Eiso(keV) < 1-4 (depending on EBL model) Atkins et al 2005 ApJ GRB 041219: long (520s) bright (1x10-4 erg cm-2) Gal Lat=0 If z=0.1-0.5 => Eiso(TeV)/Eiso(keV) < 0.3 - 7 GRB 050509b short/hard burst Weak (2x10-8 erg cm-2) z=0.225? GCN Circular 3411 Eiso(TeV)/Eiso(keV) < 10 – 20 (depending on EBL model) GRB010921, z=0.45 Eiso(TeV)/Eiso(keV) < 1-4 (depending on IR model) Milagro data Satellite data

X-ray Flares GRB050607 z=? Milagro 99% upper limits @ 3 TeV median energy SWIFT lightcurve from Burrows astroph/05110329

Summary of Milagro’s Search for GRBs No GRBs detected yet Satellite detections have been rare Milagrito: ~ 54 GRB/year 2002-2004: ~ 4 GRB/year Since SWIFT launch ~ 50 GRBs, but average redshift > 2 TeV g-rays are absorbed by extragalactic IR photons z<0.3 is needed to make constraining measurement with Milagro Optical Depth Kneiske, et al. 2004 A&A Pre Swift <z> = 1.2 Swift <z> = 2.5

Beyond Milagro: HAWC (High Altitude Water Cherenkov) 2600 m elevation 898 PMTs in 2 layers and 175 outriggers $3.4M construction cost + ~$1M LANL/UC infrastructure Detect Crab in ½ year miniHAWC: > 4000 m elevation 841 PMTs in single deep layer <$5M depending on site Investigating sites in Tibet, Bolivia and Mexico Detect Crab in < 2 days HAWC: 5200 m elevation (e.g. Atacama, Chile) 5000-10000 PMTs in 1-2 layers ~$20-30M depending on site Detect Crab in < ½ hr

Gamma/Hadron Separation Gammas Protons 30 GeV 70 GeV 230 GeV 20 GeV 270 GeV Size of HAWC Size of miniHAWC Background rejection improves with the size of the detector because the background cosmic-rays have a flat lateral distribution of penetrating muons. Size of Milagro deep layer

miniHAWC Detector Performance 50 PMT Trigger 200 PMT Trigger Angular Resolution s = ~0.4 deg s = ~0.25 deg Cut: nTop/cxPE>5.0 Eff g = 34% Eff CR= 3% Cut: nTop/cxPE>5.0 Eff g = 56% Eff CR= 1.5% g/hadron Separation

Effective Area vs Energy

miniHAWC e m g Build pond at extreme altitude (Tibet 4300m, Bolivia 4800m, Mexico 4030m) Incorporate new design Optical isolation between PMTs Larger PMT spacing Single Deep Layer (4 m vs Milagro’s 1.5 m and 6.0 m) Reuse Milagro PMTs and electronics e m g 150 meters 4 meters

Point Source Sensitivity Crab Nebula Standard Candle Space Det. (1-3 sr fov) Ground Det. (mixed fov) Atmospheric Cherenkov Telescopes with few sq deg fov (50 hr obs.) Extensive Air Shower Observatories with ~2 sr fov (1 yr obs)

Point Source Survey Sensitivity 4 min/fov* 7 min/fov* 1500 hrs/fov 1500 hrs/fov *Assuming 800 hrs/yr for Atmospheric Cherenkov Telescopes

Gamma Ray Burst Sensitivity Milagro miniHAWC Fluence Sensitivity to 100 second long Gamma Ray Burst. Both Milagro and miniHAWC can “self trigger” and generate alerts in real time. Gamma Ray Burst rate in FOV ~100 GRB/year (BATSE rate)

Site Exploration Bolivia Mexico Tibet 1/2 hr drive from LaPaz airport 4800 m elevation Near Mt. Chacaltaya Cosmic Ray Laboratory Mexico 2 hr drive from Puebla 4000 m elevation Site of the US/Mexico Large Millimeter Telescope Tibet 4300 m elevation Site of the Chinese/Italian ARGO detector

miniHAWC Proposal in Preparation Additional Collaborators – University of New Mexico (John Matthews), University of Utah (Dave Kieda, Stephan LeBohec, Miguel Mustafa), and International Partners Costs Facility ~3M$ Excavation, Liner, Building, Roads etc. Water Recirculation System ~50k$ Cabling & DAQ Upgrade ~200k$ Other costs: ~300k$? Computing, Archiving, Monitoring, Cooling, Shipping… Getting the Water (site dependent) Electrical (site dependent) Communications (site dependent) Total ~5M$ (assuming 20% contingency)

Conclusion EGRET detected a few GRBs Some GRBs extend up to 20 GeV Some have 2nd high energy component up to 0.1 GeV Milagro observations limit the high energy emission Searches without a satellite trigger are very model dependent Need low redshift GRB within Milagro’s field of view Water Cherenkov Extensive Air Shower technique can be improved to increase sensitivity and decrease the low energy threshold miniHAWC >10x Milagro sensitivity ~ 5x lower energy threshold