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

2. 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

3. 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 with no cutoff up to 10 GeV

4. 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

5. 2nd Higher Energy component
Gonzalez et al, Nature, 424, 749 (2003) s s s s s
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 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

6. 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
3.5E-5
920622
4.5E-3
950425
42-75
1.1E-4
981021
1-3
2.6E-4
980923
13-48
3.4E-7

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 GRB 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)

8. 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 o 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

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

10. 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

11. 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

12. 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

13. Milagrito Evidence for TeV GRB
Milagrito operated ~ 1 year during the BATSE era
54 GRBs within fov
GRB a
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.

14. 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

15. 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 :
long (520s)
bright (1x10-4 erg cm-2)
Gal Lat=0
If z= => Eiso(TeV)/Eiso(keV) <
GRB b
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

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

17. Summary of Milagro’s Search for GRBs
No GRBs detected yet
Satellite detections have been rare
Milagrito: ~ 54 GRB/year
: ~ 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 A&A
Pre Swift <z> = 1.2
Swift <z> = 2.5

18. 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)
PMTs in 1-2 layers
~$20-30M depending on site
Detect Crab in < ½ hr

19. 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

20. miniHAWC Detector Performance
50 PMT Trigger 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

21. Effective Area vs Energy

22. 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

23. 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)

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

25. 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)

26. 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

27. 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)

28. 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