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The Highest Energy Emission from The Highest Energy Emission from Short Gamma-Ray BurstsShort Gamma-Ray Bursts
Pablo Saz ParkinsonPablo Saz ParkinsonSanta Cruz Institute for Particle Physics, UCSCSanta Cruz Institute for Particle Physics, UCSC
SCIPP Seminar, 9 March 2007
Pablo Saz Parkinson. 9 March 20072
OutlineOutline
Introduction: What is a short GRB?
Motivation: Why search for HE emission?
Milagro Search for VHE emission
Future prospects
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Gamma-Ray Bursts (GRBs)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) and long (> 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, …
Pablo Saz Parkinson. 9 March 20074
Norris et al. (1984)Norris et al. (1984)
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Kouveliotou et al. (1993)Kouveliotou et al. (1993)Distributions “overlap”. Durationalone cannot distinguish the twopopulations.
In addition, bursts may haveExtended 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 butbe “short”, and vice versa.
There may be more than two populations …
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A word about SGRsA word about SGRs
The flare from SGR 1806-20 was the brightest explosion ever detectedMaybe some short GRBs are SGRsEstimates 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
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Another distinguishing featureAnother distinguishing feature
(Norris et al. 2006)
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Donaghy et al. (2006)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.
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Characteristics of Short GRBsCharacteristics 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 2005Scargle et al. 2006)
Less absorption by EBL
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So what causes short GRBs?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)
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Search for VHE emission from GRBsSearch 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
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EGRET GRB SpectrumEGRET GRB Spectrum
Dingus (2003)dN/dE ~ E-1.95
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High Energy emission from GRBHigh 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
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Short GRB 930131?Short GRB 930131?
Credit: J. Norris
T90=14 s, fluence = 1.2x10-5 erg cm-2
Note: EGRET deadtime ~ 100 ms
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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 componentTheory 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
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Dermer et al. 1999Dermer et al. 1999
TeV emission mirrors MeV
Measurement of time dependenceOf the high energy emission can test the SSC model and the external shock scenario.
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Razzaque and Meszaros modelRazzaque and Meszaros model
(Razzaque & Meszaros 2006)
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Detecting Gamma RaysDetecting Gamma Rays
High SensitivityHESS, MAGIC, CANGAROO,
VERITAS
Large Aperture/High Duty CycleMilagro, Tibet, ARGO, HAWC?
Low Energy ThresholdEGRET/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
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MAGIC response to GRBsMAGIC response to GRBs
Albert et al. (2006)
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The Milagro TeV observatoryThe 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 kHzAngular 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
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Event ReconstructionEvent Reconstruction
Real air shower event Monte Carlo gamma-ray shower
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Milagro Effective Area Milagro Effective Area
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Why is GRB VHE emission elusive?Why is GRB VHE emission elusive?
Primack et al. 05
I=I0e
=1 => ~ 0.37=10 => ~ 4.5 x 10-5
Atmospheric Cerenkov Telescopes cannot search for prompt emission
Extragalactic BackgroundLight (EBL) absorption
High Energy+EBL –> e+ e-
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Why VHE emission is elusive (Cont’d)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
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““triggered” vs “untriggered”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
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The untriggered search: outputsThe untriggered search: outputs
Probability histograms
No significant emission detected
Milagro can set model-dependentupper limits on VHE emission fromGRBs.
0.0398s
0.158s0.1s
0.0251s
-20 -10 log(P)-20 -10 log(P)
D. Noyes, PhD Thesis, 2005
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The triggered searchThe 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
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Short GRBs in Milagro’s FOVShort GRBs in Milagro’s FOV
We define “short” to be 5 s2000-2007: 17 GRBs (15 well localized)
6 Swift GRBs6 Inter-Planetary Network (IPN)4 BATSE1 HETE
3 firm redshifts (0.55,0.86,3.91)3 questionable redshifts (0.001,0.225,0.41)
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Search for a TeV signalSearch for a TeV signal
Look at number of eventsin a given bin during therelevant time (e.g. T90)
Compute estimatedBackground in that binusing 2 hours of dataaround the burst
Calculate significance
Light curve (T=0 trigger time) Number of events in 1.6 degree bin
Number of events expectedfrom background
Significance (GRB location at center)
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SignificancesSignificances
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Milagro Limits for Some BurstsMilagro Limits for Some Bursts
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)– Razzaque et al. model would give ~0.02 s-1
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) (GCN Circular 4249)
GRB 060427b: Another short (0.2 s) IPN burst, z=?, 16o zenith– Eiso(TeV)/Eiso(keV) < 4 (for z=0.5) (GCN Circular 5061)– Eiso(TeV)/Eiso(keV) ~ 0.1 for z=0
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ResultsResults
Submitted to ApJ
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Future prospects: HAWCFuture 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 layer5.0m spacingSingle layer with 4m depthInstrumented Area: 22,500m2
1 year survey point source sensitivity of ~60mCrab
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Future prospects: HAWCFuture prospects: HAWC
Milagro HAWC
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Summary and ConclusionsSummary 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.
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Thank You