Gamma-Ray Burst Early Afterglows

1
Gamma-Ray Burst Early Afterglows

• Bing Zhang
• Physics Department
• University of Nevada, Las Vegas
• Dec. 11, 2005, Chicago, IL

2
Collaborators

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

3
A generic GRB fireball
UV/opt/IR/radio
gamma-ray

central photosphere internal
external shocks engine
(shocks)
(reverse) (forward)
4
Why is the early afterglow essential?

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

5
Optical BandExpectations

• Reverse Shock Emission
• Fireball Composition

6
GRB Composition

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

7
Early optical afterglow lightcurves(Zhang,
Kobayashi Meszaros 2003)
8
An analytic MHD shock Solution for GRB reverse
shocks (Zhang Kobayashi 2004) Two free
parameters ?, ?34
? 0 Blandford-McKee (1976)
?34 ? Kennel-Coroniti (1984)
9
Zhang Kobayashi 2005
t-1
t1/2
Optical, forward shock emission
10
Zhang Kobayashi 2005
t-2
t1/2
t?
t-1
Optical, forward reverse shock emission s 0
11
Zhang Kobayashi 2005
t-2
t?
t1/2
t-1
Optical, forward reverse shock emission s
0.01
12
Zhang Kobayashi 2005
t-2
t?
t1/2
t-1
Optical, forward reverse shock emission s 1
13
Zhang Kobayashi 2005
t-2
t?
Optical, forward reverse shock emission s 10
14
Zhang Kobayashi 2005
Optical, forward reverse shock emission s
100
15
GRB 990123(Akerlof et al. 1999)

• RB Br / Bf 15
• Zhang, Kobayashi
• Meszaros, 2003
• Fan et al. 2002
• 1
• Zhang Kobayashi 2005

16
GRB 021211(Fox et al. 2003 Li et al. 2003)

• RB Br / Bf gtgt 1
• Zhang, Kobayashi
• Meszaros, 2003
• Kumar Panaitescu 2003
• 1?
• Zhang Kobayashi 2005

17
Launched on Nov. 20, 2004
Prime institution NASA/GSFC Leading university
partner PSU Country involved USA, Italy, UK
18
GRB 041219A(Vestrand et al. 2005 Blake et al.
2005)

• RB Br / Bf 3
• Fan, Zhang Wei 2005
• ltlt 1
• Zhang Kobayashi 2005

19
GRB 050525a

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

20
UVOT Dark Bursts
Lack of reverse shock Highly magnetized
flow? Roming et al., 2005
21
X-Ray BandSurprises

• Emission Site
• Central Engine

22
Launched on Nov. 20, 2004
Prime institution NASA/GSFC Leading university
partner PSU Country involved USA, Italy, UK
23
Typical XRT afterglow(Nousek et al. 2005, ApJ,
submitted)
Steep decline common
Temporal break around 500-1000 s
24
The oddball cases(Nousek et al. 2005, ApJ,
submitted)
25
A Generic X-ray Lightcurve?(Zhang et al. 2005)
-3
-0.5
104 105 s
- 1.2
-2
102 103 s
103 104 s
26
Surprise 1 Rapid Early Declines
GRB050219a
050117 050126 050219A 050315 050319 050412? 050416
A 050422 050713B 050721 050803 050813? 050814 0508
19
GRB050319
27
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)
• Important implication GRBs and afterglows come
  from different locations!

tail
afterglow
GRB
28
A generic GRB fireball
UV/opt/IR/radio
gamma-ray

central photosphere internal
external shocks engine
(shocks)
(reverse) (forward)
29
Surprise 2 Shallow-than-normal decay
050319 050416A 050713B 050721 and more
30
Surprise 3 X-ray Flares
050219A? 050406 050421 050502B 050607 050712 05071
3A 050714B 050716 050724? 050726 050730 050820A 05
0822 050904 050908
GRB 050726
GRB 050730
31
Giant X-ray Flare GRB050502b
500x increase!
GRB Fluence 8E-7 ergs/cm2 Flare Fluence 9E-7
ergs/cm2
Burrows et al. 2005, Science Falcone et al.
2005, ApJ
32
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.
• A much smaller energy budget is needed.

central photosphere internal
external shocks engine
(shocks)
(reverse) (forward)

33
Clue rapid decay

• 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
• (Kumar Panaitescu 2000 Dermer 2004 Zhang et
  al. 2005 Dyks et al. 2005)

-?
-?
? ? 2, F t ?
?
tail
afterglow
GRB
34
Complications (1) T0
Zhang et al. (2005)
35
Complications (2) superposition
Observed
GRB flare tail emission
Underlying forward shock emission
Zhang et al. (2005)
36
Testing curvature effect interpretation
Liang et al. (2005)

• Assume the rapid decay is the superposition of
  tail emission (? ? 2) and the underlying
  forward shock emission
• Search for T0.
• Is T0 consistent with the expectation, i.e. the
  beginning of the flare or last pulse in the
  prompt emission?

37
Testing curvature effect interpretation
Liang et al. (2005)
38
Testing curvature effect interpretation
Liang et al. (2005)
39
Testing curvature effect interpretation
Liang et al. (2005)
40
Testing curvature effect interpretation
Liang et al. (2005)
The long GRB 050502B and the short GRB 050724
have similar observational properties!
41
Conclusions from data

• 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
• What can we infer about the central engine?

42
Inference 1 Magnetic Central Engine
Fan, Zhang Proga, 2005, ApJL

• Short GRB 050724
• accretion rate at most 0.01 M / s
• luminosity 1048-1049 ergs / s
• neutrino annihilation mechanism too inefficient
• flares must be launched via magntic processes
• flares should be linearly polarized
• Long GRBs
• Similar arguments may apply
• Especially in view of the close analogy between
  050724 and 050502B

?
43
Inference 2 Viscous disk evolution
Perna, Armitage Zhang, 2005, ApJL

• Observation properties
• duration - time scale correlation
• duration - peak luminosity anti-correlation
• Model
• initial disk fragmentation or large-amplitude
  variability (gravitational instability?)
• viscous time defines duration, the larger the
  longer
• common origin for flares following both long and
  short GRBs.

44
Inference 3 Magnetic barrier accretion

• 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.
• Consistent with the magnetic origin of X-ray
  flares

Proga Begelman 2003
45
Conclusions

• 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
• The GRB outflows are likely magnetized
• 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.