The Collimation and Energetics of Fermi- LAT Gamma-Ray Bursts S. - - PowerPoint PPT Presentation

The Collimation and Energetics of Fermi-LAT Gamma-Ray Bursts

• S. Bradley Cenko, Fiona Harrison, Dale Frail,

Poonam Chandra, Josh Bloom, Derek Fox, Nat Butler, Eran Ofek, Shri Kulkarni, Bethany Cobb, Dan Perley, Alex Filippenko

GRB Overview

Meszaros, 2001

Prompt Energy (E,iso ) + Afterglow Energy (EKE,iso) + Collimation ()

Motivation

Frail et al. 2001 Cenko et al. 2009

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Beaming-corrected energetics fundamental to our understanding of progenitors, physics, and cosmological utility

Motivation

Frail et al. 2001 Cenko et al. 2009

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Beaming-corrected energetics fundamental to our understanding of progenitors, physics, and cosmological utility

Low-Luminosity GRBs

Motivation

Frail et al. 2001 Cenko et al. 2009

Beaming-corrected energetics fundamental to our understanding of progenitors, physics, and cosmological utility

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Magnetar Upper Limit

E,iso: Prompt Energy

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Broad coverage ⇒ Accurate and precise E,iso

Abdo et al., 2009

GRB 090902B

Why Fermi I: Spectral Coverage

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GRB 090902B Swift-BAT Swift-BAT

EKE,iso: Afterglow Energy

Synchrotron spectrum Self-similar evolution

Afterglow energy indirectly inferred from modeling of broadband emission

Panaitescu & Kumar, 2001

: Beaming Angle

• To avoid “energy catastrophe”,

GRB ejecta must be highly beamed ( ~ 1-10 degrees)

• Relativistic beaming effects

cause achromatic steepening in light curves when ~ -1

• By measuring time of “jet

break”, infer collimation angle

• f outflow

Harrison et al. 1999

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Why Fermi II: Large E,iso

A clean and simple way to target large E,iso

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Our Fermi Energetics Campaign

• Response to joint Fermi / VLA

announcement

• Broadband (radio, optical, and X-ray)

follow-up of LAT GRBs to constrain collimation and energetics

• Cycle 1 GRBs: 090323, 090328, 090902B,

and 090926A (no radio)

Results I: Energetics

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Afterglow Energy (EKE) [erg]

Prompt Energy (E!) [erg]

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After beaming correction, energetic requirements ~ 1051 - 1052 erg

Results II: Density

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Circumburst Density (n / A*) Afterglow Energy (EKE,iso)

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Low circumburst densities consistent with expectation of low mass-loss

Results II: Density

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Circumburst Density (n / A*) Afterglow Energy (EKE,iso)

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Low circumburst densities consistent with expectation of low mass-loss

Conclusions

• Use broadband afterglow observations to

constrain collimation and energetics from 4 Fermi LAT GRBs

• All 4 tightly collimated ( <~ 10 deg)
• Energy release ~ 1051 - 1052 erg
• Low circumburst densities (consistent with rapidly

rotating progenitors)

• Importance of follow-up observations (redshifts)