The Highest Energy Emission from Short Gamma-Ray Bursts Pablo Saz Parkinson Santa Cruz Institute for...

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, …

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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