Thesis Defense

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The Highest Energy Emission from Short Gamma-Ray
Bursts
Pablo Saz Parkinson Santa Cruz Institute for
Particle Physics, UCSC
SCIPP Seminar, 9 March 2007
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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
  60s.
• First afterglow (and redshift) late 90s.
• First short burst afterglow detected May 2005.
• Two types of GRBs short (lt 2s) and long (gt 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
• No spectral lag
• Less luminous
• Lower redshift
• 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 gt 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 14-47s 47-80s 80-113s 113-211s
18 GeV!
Hurley et al., Nature 372, 652 (1994)
Gonzalez et al., Nature 424, 749 (2003)
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Short GRB 930131?
Credit J. Norris
Note EGRET deadtime 100 ms
T9014 s, fluence 1.2x10-5 erg cm-2
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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.

z0.5
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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
Large Aperture/High Duty Cycle Milagro, Tibet,
ARGO, HAWC?
Low Energy Threshold EGRET/GLAST
Large Effective Area Excellent Background
Rejection (gt99) 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 (lt300 GeV) AGN
Physics Transients (GRBs) (lt100 GeV)
Unbiased Sky Survey Extended sources Transients
(GRBs) Solar physics/space weather
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MAGIC response to GRBs
Albert et al. (2006)
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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 m2

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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
• Extragalactic Background
• Light (EBL) absorption
• ?High Energy?EBL gt e e-

II0e-t t1 gt 0.37 t10 gt 4.5 x 10-5
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Why VHE emission is elusive (Contd)
Most bursts are at high z 20 of bursts with
measured z have z lt 0.5 Milagro expects 1/year
in its FOV with z lt 0.5
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triggered vs untriggered

• Untriggered Search
• Real-time, all locations, instant notification
• Many time scales (0.25 msec to gt 2hr)
• Drawback LARGE number of trials
• Triggered Search
• Satellites provide time, location, and duration
  of burst -gt 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
0.0398s
0.0251s

• Probability histograms
• No significant emission detected

0.158s
0.1s
-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
Milagrito evidence for TeV emission

• 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)
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Short GRBs in Milagros 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 (T0 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 (z0.225?)
• Eiso(keV) 2 x 10-8 ergs/cm2
• Eiso(TeV)/Eiso(keV) lt 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) lt 1 (if z0 -gt M81 lt 4 Mpc)
• (GCN Circular 4249)
• GRB 060427b Another short (0.2 s) IPN burst,
  z?, 16o zenith
• Eiso(TeV)/Eiso(keV) lt 4 (for z0.5) (GCN Circular
  5061)
• Eiso(TeV)/Eiso(keV) 0.1 for z0

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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 gt 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
HAWC
Milagro
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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.

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• Thank You