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 (E KE,iso) + Collimation ( � )
Motivation &( !" ) *+, ! Swift -./0)12345,)6 Swift) 178 ! 973/:;70)<=>0 &' !" Prompt Energy (E ! ) [erg] &! !" &" !" #% !" #$ !" ) #$ #% &" &! &' &( !" !" !" !" !" !" Afterglow Energy (E KE ) [erg] Frail et al. 2001 Cenko et al. 2009 Beaming-corrected energetics fundamental to our understanding of progenitors, physics, and cosmological utility
Motivation &( !" ) *+, ! Swift -./0)12345,)6 Swift) 178 ! 973/:;70)<=>0 &' !" Prompt Energy (E ! ) [erg] &! !" &" !" Low-Luminosity #% GRBs !" #$ !" ) #$ #% &" &! &' &( !" !" !" !" !" !" Afterglow Energy (E KE ) [erg] Frail et al. 2001 Cenko et al. 2009 Beaming-corrected energetics fundamental to our understanding of progenitors, physics, and cosmological utility
Motivation &( !" ) *+, ! Swift -./0)12345,)6 Swift) &' 178 ! 973/:;70)<=>0 !" Prompt Energy (E ! ) [erg] &! !" &" !" Magnetar Upper Limit #% !" #$ !" ) #$ #% &" &! &' &( !" !" !" !" !" !" Afterglow Energy (E KE ) [erg] Frail et al. 2001 Cenko et al. 2009 Beaming-corrected energetics fundamental to our understanding of progenitors, physics, and cosmological utility
E � ,iso : Prompt Energy 10 2 N a I 0 GRB 090902B N a I 1 10 0 N a I 2 N a I 9 N a I 10 B GO 0 − 2 10 B GO 1 L A T f r o n t L A T b a c k ! 1 ,- − 4 10 − 6 10 45+ . '(#$%467 − 8 10 ! 2 ,- ! 8 ! − 10 10 6 4 S i g m a 2 0 − 2 ! 3 ,- − 4 . / 0 1 2 3 ,- ,- ,- ,- ,- ,- ,- − 6 !"#$%&'()#*+ 10 2 10 4 10 6 10 8 E n e r gy ( k e V ) Abdo et al., 2009 Broad coverage ⇒ Accurate and precise E � ,iso
Why Fermi I: Spectral Coverage 10 2 N a I 0 GRB 090902B N a I 1 10 0 N a I 2 N a I 9 N a I 10 B GO 0 − 2 10 B GO 1 L A T f r o n t L A T b a c k ! 1 ,- − 4 10 Swift- BAT − 6 10 Swift- BAT 45+ . '(#$%467 − 8 10 ! 2 ,- ! 8 ! − 10 10 6 4 S i g m a 2 0 − 2 ! 3 ,- − 4 . / 0 1 2 3 ,- ,- ,- ,- ,- ,- ,- − 6 !"#$%&'()#*+ 10 2 10 4 10 6 10 8 E n e r gy ( k e V ) Abdo et al., 2009 Broad coverage ⇒ Accurate and precise E � ,iso
E KE,iso : Afterglow Energy Self-similar evolution Synchrotron spectrum Panaitescu & Kumar, 2001 Afterglow energy indirectly inferred from modeling of broadband emission
� : Beaming Angle • To avoid “energy catastrophe”, t jet 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 of outflow Harrison et al. 1999
Why Fermi II: Large E � ,iso && "! 5 &% "! 1 ! 2.,3456*789 &$ "! &# "! &" "! :7* ! ;<./0 ;<./0 =*7>. ! ?@A5B)C, &! "! 5 ! " # $ % & ' ( )*+,-./0 A clean and simple way to target large E � ,iso
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 &( !" ) *+, ! -,+./)0123 456 ! 75./8953)0123 -,+./ ! 7:;)0123 &' !" Prompt Energy (E ! ) [erg] &! !" &" !" #% !" #$ !" ) #$ #% &" &! &' &( !" !" !" !" !" !" Afterglow Energy (E KE ) [erg] After beaming correction, energetic requirements ~ 10 51 - 10 52 erg
Results II: Density && !" ' ()* ! +*),- +*),- ! ./0 123"4"5!67 Afterglow Energy (E KE,iso ) &# !" &$ !" &% !" &! !" ' ! # ! $ ! % ! ! " ! % $ # !" !" !" !" !" !" !" !" !" Circumburst Density (n / A*) Low circumburst densities consistent with expectation of low mass-loss
Results II: Density && !" ' ()* ! +*),- +*),- ! ./0 123"4"5!67 Afterglow Energy (E KE,iso ) &# !" &$ !" &% !" &! !" ' ! # ! $ ! % ! ! " ! % $ # !" !" !" !" !" !" !" !" !" Circumburst Density (n / A*) 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 ~ 10 51 - 10 52 erg • Low circumburst densities (consistent with rapidly rotating progenitors) • Importance of follow-up observations (redshifts)
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