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Searching for the highest energy emission from Gamma-Ray Bursts. Pablo Saz Parkinson Santa Cruz Institute for Particle Physics, UCSC. SSL, Berkeley, 26 May 2006. Outline. Introduction: The Milagro Gamma-ray Observatory Some Recent Results - VHE emission from the Galactic Plane

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Searching for the highest energy emission from Gamma-Ray Bursts

Pablo Saz Parkinson

Santa Cruz Institute for Particle Physics, UCSC

SSL, Berkeley, 26 May 2006


Outline l.jpg

Outline

  • Introduction: The Milagro Gamma-ray Observatory

  • Some Recent Results

  • - VHE emission from the Galactic Plane

  • Search for VHE emission from GRBs

  • Future prospects

Pablo Saz Parkinson. 26 May 2006


Milagro collaboration list l.jpg

Milagro Collaboration List

A. Abdo1, B. Allen2, D. Berley3, E. Blaufuss3, S. Casanova4, D.G. Coyne5,

S. DeLay2, B.L. Dingus4, R.W. Ellsworth6, L. Fleysher7, R. Fleysher7, M.M. Gonzalez8, J.A. Goodman3, E. Hays3, C. M. Hoffman4, L.A. Kelley5, C.P. Lansdell3,

J.T. Linnemann1, J.E. McEnery9, A.I. Mincer7, P. Nemethy7, D. Noyes3, J.M. Ryan10, F. W. Samuelson4, P. Saz Parkinson5, M. Schneider5, A. Shoup2, G. Sinnis4, A.J. Smith3, G.W. Sullivan3, V. Vasileiou3, G. Walker4, D.A. Williams5, G.B. Yodh2

  • Department of Physics and Astronomy, Michigan State University.

  • Department of Physics and Astronomy, University of California, Irvine.

  • Department of Physics, University of Maryland.

  • Group P-23, Los Alamos National Laboratory

  • Santa Cruz Institute for Particle Physics, University of California, Santa Cruz.

  • Department of Physics and Astronomy, George Mason University.

  • Department of Physics, New York University.

  • Department of Physics, University of Wisconsin.

  • NASA Goddard Space Flight Center.

  • Department of Physics, University of New Hampshire.

Pablo Saz Parkinson. 26 May 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

Pablo Saz Parkinson. 26 May 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

Pablo Saz Parkinson. 26 May 2006


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Milagro Outriggers

  • 175 black plastic 4000 liter tanks deployed over 20,000 m2

  • Each lined with Tyvek and filled with filtered water

  • A single 8” PMT in each

  • Operating since 2003

  • Improve angular resolution and background rejection.

Pablo Saz Parkinson. 26 May 2006


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Event Reconstruction

Real air shower event

Monte Carlo gamma-ray shower

Pablo Saz Parkinson. 26 May 2006


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Background rejection

Proton

Proton

Gamma

Gamma

  • Cosmic rays outnumber gamma rays by more than 1000 to 1

  • Differences between Hadron- and Gamma-ray- initiated showers

    • Hadronic showers contain more muons than gamma ray showers

    • Hadronic showers form bright, compact clusters in the muon layer

    • Gamma showers illuminate muon layer uniformly, with small hits

Pablo Saz Parkinson. 26 May 2006


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Background rejection (cont’d)

  • Parameterize “clumpiness” of the bottom layer hits

    • Compactness

      • Require C > 2.5

      • 50% gammas & 10% hadrons

        • Sensitivity improved by 1.6

    • Require A4 > 3.0

    • 20% gammas & 1% hadrons

    • Sensitivity further improved by 1.4

mxPE:maximum # PEs in bottom layer PMT

nb2:# bottom layer PMTs with 2 PEs or more

fTop:# fraction of hit PMTs in Top layer

fOut:# fraction of hit PMTs in Outriggers

nFit:# PMTs used in the angle reconstruction

Pablo Saz Parkinson. 26 May 2006


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Milagro’s Science Goals:

  • Survey the Northern Hemisphere TeV Sky

  • Transient phenomena

    • Gamma ray bursts

    • SGR outbursts

    • Flares from active galaxies

    • Solar events (coronal mass ejections)

  • Year-round observation of all sources

  • Extended sources

    • Diffuse emission from the Galactic plane

      • cosmic ray generation and propagation

    • Molecular clouds

    • Supernova remnants

    • Galaxy clusters

Pablo Saz Parkinson. 26 May 2006


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Galactic Plane

EGRET All-sky survey above 100 MeV

EGRET diffuse GeV flux (in black)

Milagro exposure (in red)

Pablo Saz Parkinson. 26 May 2006


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TeV Gamma Rays from Galactic Plane

Preliminary

5 Years of data, 2 with outriggers used in event reconstruction providing much

better sensitivity.

Pablo Saz Parkinson. 26 May 2006


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The Cygnus Region in TeV

Preliminary

  • GP diffuse excess clearly

    visible from l=25O to l=90O.

  • Cygnus Region shows

    extended excess

  • FCygnus ~ 2 x Fcrab

  • 120 square degrees l (65,85), b (-3,3)

Pablo Saz Parkinson. 26 May 2006


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Gamma-Ray Bursts (GRBs)

  • Discovered in late 60’s.

  • First afterglow/z late 90’s.

  • Two types: short (< 2s) and long (> 2s).

  • Long bursts related to death of massive stars.

  • First short burst afterglow: 2005.

  • ‘Swift’ surprises: Bright X-ray flares, steep decays, shallow decays, …

Kouveliotou et al, 1993

Nousek et al, 2006

Pablo Saz Parkinson. 26 May 2006


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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

    • 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. 26 May 2006


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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)

Pablo Saz Parkinson. 26 May 2006


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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 tight 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. 26 May 2006


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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. 04

I=I0e-t

t=1 => ~ 0.37

t=10 => ~ 4.5 x 10-5

Pablo Saz Parkinson. 26 May 2006


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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

Pablo Saz Parkinson. 26 May 2006


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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

Pablo Saz Parkinson. 26 May 2006


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The untriggered search: outputs

0.0251s

0.0398s

  • Probability histograms

  • GRB alerts

  • No significant emission detected

0.1s

0.158s

-20 -10 log(P)

-20 -10 log(P)

The number of trials is optimized

to achieve the best sensitivity with

the available computing power.

Pablo Saz Parkinson. 26 May 2006


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Constraining GRB models

redshift

Eiso

T90

  • Redshift dependence

  • EBL model dependence

  • Fluence dependence

Conclusion: Milagro sets model-dependent upper limits on the VHE emission from GRBs.

D. Noyes, PhD Thesis, 2005

Pablo Saz Parkinson. 26 May 2006


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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)

Pablo Saz Parkinson. 26 May 2006


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GRB Sample in Milagro

  • 2000-2001: 25, 3 with z, ApJ 630, 996 (2005)

  • 2002-2004: 12, 2 with z

  • 2005: 20, 7-8 with z

  • 2006: 17, 2 with z (through May 15)

  • Total: 74

    • 14-15 with measured z

    • 2-3 with z< 0.5 GRB 050509b

    • 4-5 with z<1 tentatively 0.225

Pablo Saz Parkinson. 26 May 2006


Search for a tev signal l.jpg

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. 26 May 2006


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Early results (pre-Swift)

Atkins et al, Astrophysical Journal 630 (2005) 996-1002

Pablo Saz Parkinson. 26 May 2006


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Early results (cont’d)

  • GRB 010921: A relatively nearby burst

    • z=0.45, 10 degrees zenith (ApJ, 2005 )

GRB010921, z=0.45

Eiso(TeV)/Eiso(keV) < 1-4

Pablo Saz Parkinson. 26 May 2006


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2002-2004:

~ 30 GRBs per year

~ 5 redshifts per year

12 GRBs (~4 /yr) in Milagro FOV, 2 with redshift

The ‘Swift’ Era

Swift was launched on 20 November 2004. The first reported

GRB was 041217. Normal operations began on 5 April 2005.

  • SINCE SWIFT:

  • ~ 90 GRBs more per year

  • ~ 25 redshifts per year

  • 05-06 (through 5/15): 37 GRBs

  • in Milagro FOV, 10 with redshift

Pablo Saz Parkinson. 26 May 2006


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Milagro Limits for Some Bursts

  • GRB 041219: A very long, bright INTEGRAL/Swift burst

    • If z = 0.1–0.5, Eiso(TeV)/Eiso(keV)<0.3–7

  • 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)

  • 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)

    • Sent GCN Circular 4249

  • GRB 060218: T90=2100 s, z=0.03, 43.5o from zenith and setting

    • For 10 s hard spike within burst Eiso(TeV)/Eiso(keV) < 4

  • GRB 060427b: Another short (0.2 s) IPN burst, z=?, 16o zenith

    • Eiso(TeV)/Eiso(keV) < 4 (for z=0.5) (GCN Circular 5061)

Pablo Saz Parkinson. 26 May 2006


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Short/Hard GRBs in Milagro FOV

  • Milagro is uniquely capable of searching for prompt

  • emission from Short Gamma-ray bursts.

  • A significant fraction of well-localized short GRBs have falllen in Milagro’s field of view:

  • GRB 040924 : 0.6s, 43o, z=0.859, 2.6x10-6 erg cm-2

  • GRB 051103: 0.17s, 50o, z=M81?,2.3x10-5 erg cm-2

  • GRB 051221: 0.2s, 42o, z=0.55, 2.4x10-6 erg cm-2

  • GRB 050509b: 0.128s, 10o, z=0.226?, 2.3x10-8 erg cm-2

  • GRB 060313: 0.8s, 47o,z=?, 1.4x10-5 erg cm-2

  • GRB 060427b: 0.2s, 16o, z=?,4.95x10-6 erg cm-2

No VHE emission detected. Milagro limits could

test/rule out certain models (e.g. Razzaque & Meszaros)

Pablo Saz Parkinson. 26 May 2006


Emission from x ray flares l.jpg

Emission from X-ray flares?

  • Some GRBs observed by Swift display very bright X-ray flares at late times, sometimes as bright as the GRB itself. Some models predict that these could be detectable in the GeV-TeV range

GRB Fluence (15-350 keV): 8e-7 erg cm-2

Flare Fluence (15-350 keV): 14e-7 erg cm-2

Comes into

view for Milagro

GRB 050502B

(Falcone et al, 2005)

Pablo Saz Parkinson. 26 May 2006


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TeV Emission from Flares?

Upper limits 1-2 orders

of magnitude higher

than measured X-ray

fluence.

Pablo Saz Parkinson. 26 May 2006


Future prospects minihawc l.jpg

Future prospects: miniHAWC

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. 26 May 2006


Future prospects minihawc34 l.jpg

Future prospects: miniHAWC

miniHAWC

Milagro

Pablo Saz Parkinson. 26 May 2006


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Summary and Conclusions

  • Milagro has been operational since 2000.

  • Operating with “outriggers” since 2003 and currently in its most sensitive configuration.

  • No VHE emission from GRBs has been detected, but it is early still to rule out. Swift is providing a large number of potential candidates.

  • A future detector, miniHAWC, larger and at higher altitude (~4500 m) could significantly improve the prospects for detecting GRBs in the near future.

Pablo Saz Parkinson. 26 May 2006


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  • Thank You

Pablo Saz Parkinson. 26 May 2006


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Energy Spectrum

  • Consistent with extrapolation from EGRET

    • Any rapidly rising component to explain >1 GeV excess cannot continue to 1 TeV

  • Previous upper limits in this energy range well above extrapolation

  • Atkins et al, Physical Review Letters 95 (2005) 251103 (astro-ph/0502303)

Pablo Saz Parkinson. 26 May 2006


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Bkg Rejection – “Compactness”

PMTs > 2 PE

Gammas (MC)

Maximum PE

Data

Proton (MC)

Compactness

C =

using just bottom layer PMTs

C > 2.5 removes 90% of the protons and keeps 50% of the gammas

Astrophysical Journal595, 803–811 (2003)

Pablo Saz Parkinson. 26 May 2006


Grb 060427b l.jpg

GRB 060427B

Duration = 0.2 s

16 degrees zenith

4.95x10-6 erg cm-2

(20keV-10 MeV)

No absorption:

5.0x10-7 erg cm-2

(median 2 TeV)

Absorption (z=0.5):

1.9x10-5 erg cm-2

(median 150 GeV)

Pablo Saz Parkinson. 26 May 2006


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Milagro Effective Area

Pablo Saz Parkinson. 26 May 2006


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X-ray Flares

  • Bright X-ray Flares during GRB Afterglow

  • PRELIMINARY results from Swift (courtesy A. Falcone)

  • GRB 050607

  • Flare 1: peak ~ 8 c/s, 4x afterglow; ~ 2.8e-8 erg cm-2

  • Flare 2: peak ~ 60 c/s, 40x afterglow; ~ 1.4e-7 erg cm-2

  • GRB 050712

  • Flare 1: peak ~ 16 c/s, 4x afterglow

  • Flare 2: peak ~ 5 c/s, 4x afterglow

  • GRB 050716

  • Flare 1: peak ~ 35 c/s, 2x afterglow

  • Flare 2: peak ~ 12 c/s, 2x afterglow

Pablo Saz Parkinson. 26 May 2006


Sgr outbursts l.jpg

SGR Outbursts

  • SGR 1806-20, Giant Flare of 27 December 2004

  • Very close (z=0) and Very bright(0.3-0.8 erg cm-2), but very bad zenith angle (68 degrees)

  • Brightest transient event ever recorded

  • Effective area of Milagro is ~ 0.5 m2

  • Our preliminary upper limit is ~ 6 x 10-4 erg cm-2

  • More recent outburst from SGR 1900+14 on 25 March 2006 through 29 March 2006

  • No significant detection for 4 mini-bursts that fell in Milagro field of view. Working on correlation of Milagro-Swift data.

Pablo Saz Parkinson. 26 May 2006


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Milagro TeV Survey

3 Years: 2000 December – 2003 November

95% UL 275-600 mCrab

Pablo Saz Parkinson. 26 May 2006


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VME Trigger

Jan 1999 – Mar 2002

Multiplicity trigger:

~ 55 Tubes, 200 ns

Pablo Saz Parkinson. 26 May 2006


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Razzaque and Meszaros model

Pablo Saz Parkinson. 26 May 2006


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