Gamma-Ray Burst Early Afterglows Bing Zhang Physics Department University of Nevada, Las Vegas Dec. 11, 2005, Chicago, IL

Collaborators E. W. Liang, Y. Z. Fan, J. Dyks & D. Proga (UNLV) E. W. Liang, Y. Z. Fan, J. Dyks & D. Proga (UNLV) S. Kobayashi (Livermore JMU) & P. Meszaros (PSU) S. Kobayashi (Livermore JMU) & P. Meszaros (PSU) R. Perna & P. Armitage (UC Boulder) R. Perna & P. Armitage (UC Boulder) D. Burrows, J. Nousek, N. Gehrels (Swift collaboration) D. Burrows, J. Nousek, N. Gehrels (Swift collaboration)

A generic GRB fireball central photosphere internal external shocks engine (shocks) (reverse) (forward) gamma-ray UV/opt/IR/radio

Why is the early afterglow essential? Diagnose the composition of the fireball ejecta (Baryonic or magnetic?) Diagnose the composition of the fireball ejecta (Baryonic or magnetic?) late afterglow is the emission from the medium) late afterglow is the emission from the medium) Diagnose the immediate environment of GRBs (ISM or wind? Density clumps?) Diagnose the immediate environment of GRBs (ISM or wind? Density clumps?) Diagnose the site of GRB emission (external or internal?) Diagnose the site of GRB emission (external or internal?) Diagnose the central engine activity (any long-lasting injection?) Diagnose the central engine activity (any long-lasting injection?)

Optical Band: Expectations Reverse Shock Emission & Fireball Composition

GRB Composition Baryonic component Baryonic component Protons and electrons Protons and electrons Neutrons Neutrons Magnetic fields (Poynting flux) Magnetic fields (Poynting flux) Answer: from Early Afterglows!

Early optical afterglow lightcurves (Zhang, Kobayashi & Meszaros 2003)

An analytic MHD shock Solution for GRB reverse shocks (Zhang & Kobayashi 2004) Two free parameters: ,  34  = 0 Blandford-McKee (1976)  34 =  Kennel-Coroniti (1984)

t -1 t 1/2 Optical, forward shock emission Zhang & Kobayashi 2005

t -1 t 1/2 Optical, forward + reverse shock emission σ ~ 0 t -2 t?t? Zhang & Kobayashi 2005

t -1 t 1/2 Optical, forward + reverse shock emission σ ~ 0.01 t -2 t?t? Zhang & Kobayashi 2005

t -1 t 1/2 Optical, forward + reverse shock emission σ ~ 1 t -2 t?t? Zhang & Kobayashi 2005

Optical, forward + reverse shock emission σ ~ 10 t -2 t?t? Zhang & Kobayashi 2005

Optical, forward + reverse shock emission σ ~ 100 Zhang & Kobayashi 2005

GRB (Akerlof et al. 1999) R B = B r / B f ~ 15 Zhang, Kobayashi & Meszaros, 2003 Fan et al  ~ 1 Zhang & Kobayashi 2005

GRB (Fox et al. 2003; Li et al. 2003) R B = B r / B f >> 1 Zhang, Kobayashi & Meszaros, 2003 Kumar & Panaitescu 2003  ~ 1? Zhang & Kobayashi 2005

Prime institution: NASA/GSFC Leading university partner: PSU Country involved: USA, Italy, UK The GRB Explorer Mission:. A Multiwavelength Observatory for Rapid-Response Observations of Transient Targets Launched on Nov. 20, 2004

GRB A (Vestrand et al. 2005; Blake et al. 2005) R B = B r / B f ~ 3 Fan, Zhang & Wei 2005  << 1 Zhang & Kobayashi 2005

GRB a Early reverse shock Transition to forward shock Re-brightening Jet break Blustin et al., 2005

UVOT Dark Bursts Lack of reverse shock Highly magnetized flow? Roming et al., 2005

X-Ray Band: Surprises Emission Site & Central Engine

Prime institution: NASA/GSFC Leading university partner: PSU Country involved: USA, Italy, UK The GRB Explorer Mission:. A Multiwavelength Observatory for Rapid-Response Observations of Transient Targets Launched on Nov. 20, 2004

Typical XRT afterglow (Nousek et al. 2005, ApJ, submitted) Steep decline common Temporal break around s

The “oddball” cases (Nousek et al. 2005, ApJ, submitted)

A Generic X-ray Lightcurve? (Zhang et al. 2005) ~ -3 ~ -0.5 ~ ~ -2 10^2 – 10^3 s10^3 – 10^4 s 10^4 – 10^5 s

Surprise 1: Rapid Early Declines A ? A B ? GRB050219a GRB GRB GRB A GRB GRB050117a

Interpretation Tail of prompt GRB emission or the late central engine emission – curvature effect (Kumar & Panaitescu 2000; Zhang et al. 2005; Dyks et al. 2005) Tail of prompt GRB emission or the late central engine emission – curvature effect (Kumar & Panaitescu 2000; Zhang et al. 2005; Dyks et al. 2005) Important implication: GRBs and afterglows come from different locations! Important implication: GRBs and afterglows come from different locations! GRB tail afterglow

A generic GRB fireball central photosphere internal external shocks engine (shocks) (reverse) (forward) gamma-ray UV/opt/IR/radio

Surprise 2: Shallow-than-normal decay A B and more GRB A GRB

Surprise 3: X-ray Flares A? B A B ? A GRB GRB GRB B GRB GRB GRB A GRB B GRB GRB GRB

Giant X-ray Flare: GRB050502b 500x increase! GRB Fluence: 8E-7 ergs/cm 2 Flare Fluence: 9E-7 ergs/cm 2 Burrows et al. 2005, Science; Falcone et al. 2005, ApJ

Late central engine activity - late internal dissipations (Burrows et al. 2005, Zhang et al. 2005) Can naturally interpret rapid rise and rapid fall of the lightcurves. Can naturally interpret rapid rise and rapid fall of the lightcurves. A much smaller energy budget is needed. A much smaller energy budget is needed. central photosphere internal external shocks engine (shocks) (reverse) (forward)

Clue: rapid decay Rapid decays are following both prompt emission and X- ray flares Rapid decays are following both prompt emission and X- ray flares Very likely it is due to high-latitude emission upon sudden cessation of emission – curvature effect Very likely it is due to high-latitude emission upon sudden cessation of emission – curvature effect (Kumar & Panaitescu 2000; Dermer 2004; Zhang et al. 2005; Dyks et al. 2005) GRB tail afterglow  =  + 2, F = t -- --

Complications (1): T0 Zhang et al. (2005)

Complications (2): superposition Zhang et al. (2005) GRB & flare tail emission Underlying forward shock emission Observed

Testing curvature effect interpretation Assume the rapid decay is the superposition of tail emission () and the underlying forward shock emission Assume the rapid decay is the superposition of tail emission (  =  + 2 ) and the underlying forward shock emission Search for T0. Search for T0. Is T0 consistent with the expectation, i.e. the beginning of the flare or last pulse in the prompt emission? Is T0 consistent with the expectation, i.e. the beginning of the flare or last pulse in the prompt emission? Liang et al. (2005)

Testing curvature effect interpretation Liang et al. (2005)

Testing curvature effect interpretation Liang et al. (2005)

Testing curvature effect interpretation Liang et al. (2005)

Testing curvature effect interpretation Liang et al. (2005) The long GRB B and the short GRB have similar observational properties!

Conclusions from data The rapid decay following X-ray flares is consistent with the curvature effect interpretation The rapid decay following X-ray flares is consistent with the curvature effect interpretation X-ray flares therefore are of internal origin, caused by late central engine activity X-ray flares therefore are of internal origin, caused by late central engine activity What can we infer about the central engine?

Inference 1: Magnetic Central Engine Short GRB Short GRB accretion rate at most 0.01 M / s accretion rate at most 0.01 M / s luminosity ergs / s luminosity ergs / s neutrino annihilation mechanism too inefficient neutrino annihilation mechanism too inefficient flares must be launched via magntic processes flares must be launched via magntic processes flares should be linearly polarized flares should be linearly polarized Long GRBs Long GRBs Similar arguments may apply Similar arguments may apply Especially in view of the close analogy between and B Especially in view of the close analogy between and B Fan, Zhang & Proga, 2005, ApJL 

Inference 2: Viscous disk evolution Observation properties Observation properties duration - time scale correlation duration - time scale correlation duration - peak luminosity anti-correlation duration - peak luminosity anti-correlation Model Model initial disk fragmentation or large-amplitude variability (gravitational instability?) initial disk fragmentation or large-amplitude variability (gravitational instability?) viscous time defines duration, the larger the longer viscous time defines duration, the larger the longer common origin for flares following both long and short GRBs. common origin for flares following both long and short GRBs. Perna, Armitage & Zhang, 2005, ApJL

Inference 3: Magnetic barrier & accretion Disk is not necessarily chopped from very beginning (e.g. gravitational instability; fragmentation) Disk is not necessarily chopped from very beginning (e.g. gravitational instability; fragmentation) It could be chopped into pieces during the accretion due to the interplay between the magnetic barrier and accretion flow. It could be chopped into pieces during the accretion due to the interplay between the magnetic barrier and accretion flow. Consistent with the magnetic origin of X-ray flares Consistent with the magnetic origin of X-ray flares Proga & Begelman 2003

Conclusions Swift is revolutionizing our understanding of GRBs; Swift is revolutionizing our understanding of GRBs; Early UV/optical/IR observations may be interpreted within the simplest reverse shock + forward shock framework, but the case is inconclusive; Early UV/optical/IR observations may be interpreted within the simplest reverse shock + forward shock framework, but the case is inconclusive; The GRB outflows are likely magnetized; The GRB outflows are likely magnetized; Prompt emission is originated from a different site from the afterglow; Prompt emission is originated from a different site from the afterglow; GRB central engine is still alive after the GRB ceases. This challenges the traditional central engine models. GRB central engine is still alive after the GRB ceases. This challenges the traditional central engine models.