The Highest Energy Emission from
Short Gamma-Ray Bursts
Pablo Saz Parkinson
Santa Cruz Institute for Particle Physics, UCSC
SCIPP Seminar, 9 March 2007
Outline
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)
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, …
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Norris et al. (1984)
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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 …
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A word about SGRs
Boggs et al. 2006
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)
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Another distinguishing feature
(Norris et al. 2006)
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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 GRBs
Shorter duration
Harder spectrum
Narrower pulses
Good for testing QG
No spectral lag
(Amelino-Camelia 2005
Scargle et al. 2006)
Less luminous
Lower redshift Less absorption by EBL
No associated supernova
Location in galaxies with low SFR
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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 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 Spectrum
dN/dE ~ E-1.95
Dingus (2003)
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High Energy emission from GRB
GRB 941017
GRB 940217
-18-14s
18 GeV!
14-47s
47-80s
80-113s
113-211s
Hurley et al., Nature 372, 652 (1994)
Gonzalez et al., Nature 424, 749 (2003)
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Short GRB 930131?
Note: EGRET deadtime ~ 100 ms
T90=14 s, fluence = 1.2x10-5 erg cm-2
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Credit: J. Norris
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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.
Pe’er & Waxman (ApJL 603,1, L1-L4, 2004)
constrain source parameters for
Inverse Compton emission
of GRB941017
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z=0.5
z=0.2
z=0.02
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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.
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Razzaque and Meszaros model
(Razzaque & Meszaros 2006)
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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
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MAGIC response to GRBs
Albert et al. (2006)
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The Milagro TeV observatory
•
•
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2630 m above sea level in the Jemez
Mountains, Los Alamos, New Mexico
Operational since 2000 (with
8” PMTs with “baffles”
outriggers since 2003)
2.8 x 2.8 m spacing
Duty cycle greater than 90%
~ 2sr field of view
Top Layer: 450 PMTs, 1.5 m deep
Trigger rate 1.5-2 kHz
Bottom Layer: 273 PMTs, 6.5 m deep
Angular resolution of 0.45 degrees
Outriggers: 175 black plastic tanks each
Energy: ~ 100 GeV – 100 TeV
with a PMT, spread over 20,000 m2
(median ~ 2.5 TeV)
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Event Reconstruction
Real air shower event
Monte Carlo gamma-ray shower
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Milagro Effective Area
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Why is GRB VHE emission elusive?
Atmospheric Cerenkov Telescopes cannot search for
prompt emission
Primack et al. 05
Extragalactic Background
Light (EBL) absorption
High Energy+EBL –> e+ eI=I0e-t
t=1 => ~ 0.37
t=10 => ~ 4.5 x 10-5
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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”
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: outputs
Probability histograms
No significant emission
detected
0.0251s
0.0398s
0.1s
0.158s
-20
-10 log(P) -20
-10 log(P)
Milagro can set model-dependent
upper limits on VHE emission from
GRBs.
D. Noyes, PhD Thesis, 2005
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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.
Milagrito evidence for TeV emission
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)
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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)
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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)
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Significances
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Milagro 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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Results
Submitted to ApJ
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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: 22,500m2
1 year survey point source sensitivity of ~60mCrab
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Future prospects: HAWC
Milagro
HAWC
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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. 35
Pablo Saz Parkinson. 9 March 2007
Thank You
Pablo Saz Parkinson. 9 March 2007
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