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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. Outline. Introduction: What is a short GRB? Motivation: Why search for HE emission? Milagro Search for VHE emission Future prospects.

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slide1

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
Outline
  • Introduction: What is a short GRB?
  • Motivation: Why search for HE emission?
  • Milagro Search for VHE emission
  • Future prospects

Pablo Saz Parkinson. 9 March 2007

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 2007

norris et al 1984
Norris et al. (1984)

Pablo Saz Parkinson. 9 March 2007

kouveliotou et al 1993
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 …

Pablo Saz Parkinson. 9 March 2007

a word about sgrs
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)

Pablo Saz Parkinson. 9 March 2007

another distinguishing feature
Another distinguishing feature

(Norris et al. 2006)

Pablo Saz Parkinson. 9 March 2007

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.

Pablo Saz Parkinson. 9 March 2007

characteristics of short grbs
Characteristics of Short GRBs

Good for testing QG

(Amelino-Camelia 2005

Scargle et al. 2006)

Less absorption by EBL

  • Shorter duration
  • Harder spectrum
  • Narrower pulses
  • No spectral lag
  • Less luminous
  • Lower redshift
  • No associated supernova
  • Location in galaxies with low SFR

Pablo Saz Parkinson. 9 March 2007

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)

Pablo Saz Parkinson. 9 March 2007

search for vhe emission from grbs
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

Pablo Saz Parkinson. 9 March 2007

egret grb spectrum
EGRET GRB Spectrum

dN/dE ~ E-1.95

Dingus (2003)

Pablo Saz Parkinson. 9 March 2007

high energy emission from grb
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)

Pablo Saz Parkinson. 9 March 2007

short grb 930131
Short GRB 930131?

Note: EGRET deadtime ~ 100 ms

Credit: J. Norris

T90=14 s, fluence = 1.2x10-5 erg cm-2

Pablo Saz Parkinson. 9 March 2007

theory of the high e component
Theory of the high E component

Pe’er & Waxman (ApJL 603,1, L1-L4, 2004)

constrain source parameters for

Inverse Compton emission

of GRB941017

z=0.2

z=0.02

  • 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

Pablo Saz Parkinson. 9 March 2007

dermer et al 1999
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.

Pablo Saz Parkinson. 9 March 2007

razzaque and meszaros model
Razzaque and Meszaros model

(Razzaque & Meszaros 2006)

Pablo Saz Parkinson. 9 March 2007

detecting gamma rays
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

Pablo Saz Parkinson. 9 March 2007

magic response to grbs
MAGIC response to GRBs

Albert et al. (2006)

Pablo Saz Parkinson. 9 March 2007

the milagro tev observatory
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

Pablo Saz Parkinson. 9 March 2007

event reconstruction
Event Reconstruction

Real air shower event

Monte Carlo gamma-ray shower

Pablo Saz Parkinson. 9 March 2007

milagro effective area
Milagro Effective Area

Pablo Saz Parkinson. 9 March 2007

why is grb vhe emission elusive
Atmospheric Cerenkov Telescopes cannot search for prompt emission

Extragalactic Background

Light (EBL) absorption

High Energy+EBL –> e+ e-

Why is GRB VHE emission elusive?

Primack et al. 05

I=I0e-t

t=1 => ~ 0.37

t=10 => ~ 4.5 x 10-5

Pablo Saz Parkinson. 9 March 2007

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

Pablo Saz Parkinson. 9 March 2007

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

Pablo Saz Parkinson. 9 March 2007

the untriggered search outputs
The untriggered search: outputs

0.0251s

0.0398s

  • Probability histograms
  • No significant emission detected

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

Pablo Saz Parkinson. 9 March 2007

the triggered search
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)

Pablo Saz Parkinson. 9 March 2007

short grbs in milagro s fov
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)

Pablo Saz Parkinson. 9 March 2007

search for a tev signal
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)

Pablo Saz Parkinson. 9 March 2007

significances
Significances

Pablo Saz Parkinson. 9 March 2007

milagro limits for some bursts
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

Pablo Saz Parkinson. 9 March 2007

results
Results

Submitted to ApJ

Pablo Saz Parkinson. 9 March 2007

future prospects hawc
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

Pablo Saz Parkinson. 9 March 2007

future prospects hawc34
Future prospects: HAWC

HAWC

Milagro

Pablo Saz Parkinson. 9 March 2007

summary and conclusions
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.

Pablo Saz Parkinson. 9 March 2007

slide36
Thank You

Pablo Saz Parkinson. 9 March 2007