Central Engines of GammaRay Bursts

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Central Engines of Gamma-Ray Bursts

• S. R. Kulkarni
• California Institute of Technology
• http//www.astro.caltech.edu/srk

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My summary of what we know about GRBs

• GRBs are highly collimated explosions and possess
  central engines which drive the explosion
• Long duration GRBs are deaths of massive stars
  (SN Ib/c connection)
• There is growing evidence of underenergetic GRBs
  (e.g. 980425, 030329, 031203) with engines
  outputing a mix of ejecta ultra-relativistic (?
  gt100), relativistic(? gt10) and mildly
  relativistic (? gt2) ejecta
• The fraction of nearby Ib/c supernovae with
  features indicative of a central engine is small,
  less than 10.

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SNe versus GRBs

• E0 1051 erg
• Mej 1-10 Msun
• n(r) r-2
• Reverse Shock (X-ray)
• Forward Shock (Thermal and non-thermal)
• EXPLOSION

• E0 1051 to 1052 erg
• Mej 10-5 Msun
• n(r) r0 or r-2 or
• Reverse Shock Radio or Optical
• Forward Shock (synchrotron/IC)
• ENGINE DRIVEN

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Examples of Broadband (Radio-X-ray) Fits
Kumar Panaitescu Harrison Yost
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Geometry can be inferred from light curves
t lt tjet high ?
log f
tjet
log t
t gt tjet low ?
log f
log t
tjet
Rhoads
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GRB Energetics Tiger becomes Lamb
Before the beaming correction
(isotropic)
After the beaming correction
(Frail et al.)
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Radio Light Curves at 8.5 GHz
Radio Afterglows Angular Size and Calorimetry
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970508 ISS

• Rapid (hrs), narrow-band (GHz) flux variations
• diffractive scintillation
• quenches at 1 month
• angular size 3 uas
• superluminal expansion
• Low frequency turnover
• synchrotron self-absorption
• size at 1 month 3 uas
• Frail et al.

Flux variations /-50
Narrow-band ltfew GHz
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and the latest .

• GRB 030329, 24 days after the burst
• VLBABonn at 22 GHz
• Marginally resolved at 0.08 milliarcsec
• In line with expectations from the fireball model
• superluminal expansion (5c)

0.45 x 0.18 mas
Taylor et al.
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GRB 030329 No proper motion
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ISS and GRBs

• ISS has enabled us to directly demonstrate that
  GRBs are relativistic explosions
• BUT need concerted effort to exploit ISS as a
  quantitive tool (logistical problems)

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Calorimetry

• Afterglow estimates sensitive to jet opening
  angles
• At late times the blast wave becomes
  non-relativistic and rapidly becomes spherical.
  Thus one can apply minimum energy method (or
  variations) with confidence.
• Radio observations have confirmed that the
  overall energetics scale is correct and in some
  cases evidence for copious amount of mildly
  relativistic ejecta.

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GRB 980703 Non-relativistic Transition
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GRB 030329 Non-relativistic Transition
Scaled to nu-0.6
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Gray Clouds I Adjustable Microphysics?

• Microphysics of relativistic acceleration and
  B-field is parametric
• characterized by constant but freely
  variable, ?B and ?e
• However, other prescriptions e.g. ?B??-1
  can improve fit Rossi
• Energy index of ultra-relativistic electrons is
  also a free variable
• p2.2 is favored by theory
• some bursts are best described by p lt 2
  Bhattacharya

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Gray Clouds II Energy Injection
Density Variations

• Fireball usually described by a single ejecta
  with a fixed Lorentz factor
• Circumburst medium is assumed to be a power law
  distribution (constant or r-2)
• Both assumptions can be and appear to be violated
  with the best studied afterglows

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Gray Clouds III Two types of Engines?
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Clue 1 The nearest GRB 030329 is peculiar
Puzzle A single fireball does not account for
radio X-ray emission

• A possible solution
• a narrow, ultra-relativistic jet with low energy
  which produces X-ray optical
• a wide, mildly relativistic jet carrying the bulk
  of the energy and powering the radio

Berger et al in prep.
Berger et al. 2003
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Clue 2 X-ray Flashes
Heise
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Radio Afterglow from XRF (ISS)
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Clue 2 X-Ray Flashes (XRFs)
XRFs were identified in the 1990s by the BeppoSAX
satellite with properties similar to those of
GRBs (HETE-2 XRFs are 1/3 of event rate)

• Prompt emission duration distribution (10-200
  sec)
• Bright X-ray sources, 10-8 10-7 erg/s/cm2
  (2-25 keV)
• Energy spectrum can be fit with a power law (?
  0.5-3)
• X-ray and radio afterglows similar to GRB
  afterglows
• BUT Peak energy of 25 keV (c.f. GRBs 250 keV)
  

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Intrinsic vs Extrinsic Differences
Extrinsic XRFs are simply GRBs viewed away from
the jet collimation axis where there is less
?-ray emission. Intrinsic XRFs are similar
in energetics to GRBs, but have less relativistic
ejecta possibly due to a heavier baryonic load.
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XRF 020903 First redshift (z0.251)
Soderberg et al
Energy in the Explosion (Prompt) 1049 erg (low
compared to GRBs)
No evidence for off-axis model (optical flux
declines) However, evidence for mildly
relativistic ejecta from radio afterglow
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Clue 3 SN 1998bw/GRB 980425
Galama et al.
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Radio Emission from SN 1998bw
Kulkarni et al
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Clue 3 Mildly Relativistic Ejecta in SN 1998bw
Kulkarni et al
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What is SN 1998bw?

• An off-axis cosmological GRBf Nakamura
• A new beast, an under-energetic engine explosion
  Kulkarni, Chevalier
  Li
• Developments
• There is no evidence for energy addition on
  timescales of months to years
• SN 1998bw is rare in the local population of
  Ibc SNe (based on Ibc VLA survey of Berger et al)

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Clue 4 GRB 031203, another underenergetic event

• Localized by IBIS (Gotz et al)
• XMM TOO observations (Watson)
• Plethora of ground-based optical
• Radio afterglow candidate identified (1 arcsec)
• Putative host galaxy coincident with radio source
  at z0.1 identified (Bloom)
• Discovery of X-ray scattered halo from XMM
  observations (Vaughn)
• Continued VLA monitoring shows event is weak

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Putting it altogether Engine
Soderberg in prep Please do not distribute
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Putting it altogether Nucleosynthesis
HST proposal approved!
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Summing up

• A number of events are sub-energetic in the
  gamma-ray/X-ray band but more energy in the radio
  afterglow (by x10)
• Curiously these are the nearest events
• In only a small fraction of local Ib/c (100 Mpc)
  is there evidence for energy addition over
  extended time
• gt Superonovae explosions are two-parameter
  family nucleosynthesis and engine

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The Future is Bright

• HETE, Integral, IPN in operation
• Imminent launch of SWIFT (Sep 2004)
• Dedicated ground-based experiments
• ROTSE, TAROT, BOOTES, REM, NGAT
• Rapid Response by Premier
  Facilities (VLT, HST, Chandra..)
• At Palomar we have robotocized
  the 60-inch telescope and ready to go!

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New Missions

• AGILE, GLAST (GeV Missions)
• Milagro (TeV Telescope)
• ICECUBE (neutrino)
• LIGO (gravitational wave)
• AUGER (ultra-high energy cosmic rays)