Very High Energy Transient Extragalactic Sources: GRBs

Very High Very High Energy Energy Transient Transient Extragalactic Extragalactic Sources: GRBsSources: GRBs

David A. WilliamsDavid A. Williams

Santa Cruz Institute for Particle PhysicsSanta Cruz Institute for Particle Physics

University of California, Santa CruzUniversity of California, Santa CruzOctober 20, 2005

October 20, 2005"Towards the Future" Workshop — DAW

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Two Classes of BurstsTwo Classes of Bursts

Short, Hard Bursts•19 sr-1 yr-1

•Prompt emission too short to catch with slewed response•May be “nearby”

z = 0.16, 0.225?, 0.258•Delayed X-ray flare from 050724 ~200 seconds later

Long, Soft Bursts•38 sr-1 yr-1

•14% longer than 100 s•28% longer than 63 s•z ~ 1, but wide range

z = 0.0085 to 6.3•Several types of delayed emission

Milagro/HAWC 2 sr yr annuallyVERITAS 3 x 10-4 sr yr annually


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Redshift DistributionRedshift Distribution

Klose, http://www.mpe.mpg.de/~jcg/grbrsh.html

Redshifts established for 32 bursts (as of 2004)

1.3 < z < 2.5 difficult–

spectra lack suitable lines

These are all long duration (>2 s) bursts


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High-Energy High-Energy -Rays-Rays

Hurley et al., Nature 372, 652 (1994)

Ulyssesdata

BATSEdata

EGRETdata

GRB 940217 18 GeV!


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High-Energy High-Energy -Rays II-Rays II

•EGRET saw >100 MeV emission from three other bursts•dN/dE ~ E-1.95 up to 10 GeV•Limited exposure so such emission may be typical•Longer duration, hard component in GRB 941017 (Gonzalez et al 2003) and 980923 (Gonzalez 2004)


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VHE VHE -Rays from GRB 970417a?-Rays from GRB 970417a?

Astrophysical Journal Letters 533, L119 (2000)

Astrophysical Journal 583, 824 (2003)

•Evidence for TeV emission from GRB 970417a seen by Milagrito•Probability of background fluctuation 1.5 x 10-3 (3)•More luminosity at TeV energies than MeV•(Lack of) EBL absorption implies GRB must be close, so total energy released is not unusually large

–z~0.1, then E < 700 GeV, so L < 5 x 1051 ergs–z~0.03, then E < 10 TeV, so L < 1 x 1049 ergs


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VHE VHE -Rays from GRB?-Rays from GRB?

•Amenomori et al. 1996 (Tibet AS-–6 excess at ~10 TeV in stacked analysis of 57 BATSE GRB

•Padilla et al. 1998 (HEGRA)–2.7 excess above 20 TeV from GRB 920925c

•Atkins et al. 2005; Saz Parkinson 2005 (Milagro)–Limits >100 GeV from 33 GRB during 2000–2003

•Connaughton et al. 1997 (Whipple)–Limits >250 GeV from follow-up observations of 9 BATSE bursts

•Horan 2005 (Whipple)–Limits >400 GeV from follow-up observations of 10 GRB 2002–2004

•Jarvis et al. 2005 (STACEE)–No detection in follow-up observations of 8 GRB 2002–2005

•Bastieri et al. 2005 (MAGIC)–Upper limit starting 40 s after onset, overlapping T90 for 30 s, of GRB 050713a


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The Fireball ModelThe Fireball Model

Zhang & Meszaros, Intl. J. Mod. Phys. A. 19, 2385 (2004)

107 cm

Central engine injects 1051 ergs over 1 secondExpanding ,e+,e- plasma at 1010 KVariability can produce multiple thin shells


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1013 cm

Baryons are accelerated by radiation pressure until they share a common bulk Lorentz factor ~300 with the plasmaFireball coasts

1000 s / 5 ms


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Typical radius at which shells with different overtake each other, creating “internal” shocks

5 x 1014 cm104 s / 0.05 s


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Deceleration radius – the fireball is decelerated by the ambient medium“External” shock propagates into the ambient medium“Reverse” shock propagates into fireball shell

3 x 1016 cm105 s / 5 s


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External Shock Model PredictionsExternal Shock Model Predictions

Dermer & Chiang, AIP Conf. Proc. 515, 225 (2000)

For burst at z ~ 1, dL ~ 1028 cm, log10[F] = 47.4 corresponds to Crab flux (c) at log10[] = 25.4 (100 GeV)

c c


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Internal Shocks–The Bad NewsInternal Shocks–The Bad News

Razzaque, Meszaros & Zhang, Astrophys. J. 613, 1072 (2004)

Emission is at smaller radiusFireball is denser—

Typically opaque above 10–100 GeV


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Flux for burst at z = 1 Includes EBL absorption a la Malkan & Stecker 2001 for z = 1

BIG = 10-20 GBIG = 10-17 G

Reprocessing by EBL–Better NewsReprocessing by EBL–Better News

Razzaque, Meszaros & Zhang, Astrophys. J. 613, 1072 (2004)

c c

104 s

106 s102 s

50 s


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Fluxes for burst at z = 1 (But no EBL absorption)

Different values ofDensity of surrounding mediumMagnetic fieldFraction of energy in e±

Afterglow EmissionAfterglow Emission

Zhang & Meszaros,Astrophys. J. 559, 110 (2001)

c c

c

T01 m

1 h1 d

1 mo


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c

Color curves are IC from afterglow (external shock) electrons in scenarios with different•Total energy•Density of surrounding medium•Bulk Lorentz factor•Magnetic fieldconstrained to fit the EGRET data

Black curve is model based on rapid variability in prompt emission

Modeling GRB 941017Modeling GRB 941017

Pe’er & Waxman, Astrophys. J. Lett. 603, L1 (2004)

Spectrum between 100 and 200 s after start of burst


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SummarySummary

Solid evidence for prompt and afterglow emission from EGRET up to 10 GeV

No firm VHE signal from a burst yetLow threshold desirable

– Elude EBL absorption in transit– Elude self-absorption in source

Prompt emission– Potentially quit strong (many Crab) once below absorption cutoffs– Can determine burst parameters, e.g. B from intrinsic cutoff– Short bursts can only be caught with large (~sr) field of view– Tail of longer bursts can be caught with slewed responses of ≤1 minute

Afterglow emission– Can be quit strong (~Crab or more) initially. How long is it interesting to

follow?– Can be as valuable for determining burst properties, e.g. GRB 941017– Accessible to slewed instruments

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Very High Energy Transient Extragalactic Sources: GRBs