GRB Physics in the Swift Era

1
GRB Physics in the Swift Era

• Bing Zhang
• Physics Department
• University of Nevada, Las Vegas
• May 19, 2006, Hangzhou, China

2
Collaborators

• E. W. Liang, J. Dyks, D. Proga (UNLV)
• S. Kobayashi (Liverpool JMU)
• P. Meszaros (PSU)
• R. Perna, P. Armitage (UC Boulder)
• Z. G. Dai, X. Y. Wang, X. F. Wu (Nanjing U)
• Y. Z. Fan, D. M. Wei (PMO)
• Swift collaboration (N. Gehrels, D. Burrows, P.
  Roming, J. Nousek, P. OBrien, G. Chincarini et
  al)

3
Gamma-ray bursts the most violent explosions in
the universe!
4
Milestone 1 1969-1973(discovery)
5
Milestone 2 1991-1996(CRGO era)
6
Milestone 3 1997-2004 (BeppoSAX-HETE era)
7
Milestone 4 2005(Swift era)
Launched on Nov. 20, 2004
Prime institution NASA/GSFC Leading university
partner PSU Country involved USA, Italy, UK
8
Swift discoveries - highlights

• Mystery of short-hard GRBs partially solved
• GRB 050904 z6.3 GRB
• GRB 060218/SN 2006aj nearby low-lum GRB
• Extensive study of early afterglows
• Bridge between the prompt emission and the
  afterglow
• X-ray flares
• Tight UVOT early upper limits

9
Purpose of the talk

• How Swift revolutionized our understanding of GRB
  physics?
• Short GRB highlight
• High-z GRB highlight
• A canonical X-ray afterglow lightcurve
• Origin of the steep decay
• Origin of X-ray flares
• Origin of the shallow decay phase
• Comments on the early dimness of optical
  afterglow
• Nearby GRB 060218/SN 2006aj

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Short GRBsMystery solved?
11
Short GRB 050509B(Gehrels et al. 2005 Bloom et
al. 2005)
Z0.225
12
Short GRB 050709(Fox et al. 2005 Covino et al.
2005)
Z0.16
13
Short GRB 050724(Barthelmy et al. 2005 Berger
et al. 2005)
Z0.258
14
Origin or short GRBs Compact star mergers?
15
High-z GRBsNew frontier
16
High-z (6.3) GRB 050904(Haislip et al. 2005
Cusumano et al. 2005)
17
GRB physicsNew windows
18
A generic GRB fireball
UV/opt/IR/radio
gamma-ray

central photosphere internal
external shocks engine
(shocks)
(reverse) (forward)
19
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?)

20
Typical XRT afterglow(Nousek et al. 2006, ApJ
OBrien et al., 2006, ApJ)
Steep decline common
Temporal break around 500-1000 s
21
The oddball cases(Nousek et al. 2006, ApJ
OBrien et al. 2006, ApJ)
22
A Generic X-ray Lightcurve(Zhang, Fan, Dyks,
Kobayashi, Meszaros, Burrows, Nousek Gehrels
2006, ApJ)
-3
prompt emission
-0.5
104 105 s
- 1.2
-2
102 103 s
103 104 s
23
Five components

• A steep decay - GRB tail emission
• A shallow-than-normal decay - refreshed shock
• A normal decay - a fireball with constant energy
  running into a medium with a constant density
• A possible jet break
• One or more X-ray flares
• Although the 3rd and 4th components are
  expected, the other three components are
  surprises to GRB workers.

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Rosetta Stone (the burst that has it all!)
GRB 050315 Vaughn et al. 2005
0
t -5.2 ? -1.9?0.9
1
2
3
t -0.4
4
t -0.7 ? -0.73?0.11
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Surprise 1 Rapid Early Declines
GRB050219a
050117 050126 050219A 050315 050319 050412? 050416
A 050422 050713B 050721 050803 050813? 050814 0508
19
GRB050319
26
A Generic X-ray Lightcurve(Zhang et al. 2006)
-3
-0.5
104 105 s
- 1.2
-2
102 103 s
103 104 s
27
Interpretation

• Tail of prompt GRB emission or the late central
  engine emission curvature effect (Kumar
  Panaitescu 2000 Zhang et al. 2006 Dyks et al.
  2006)
• Important implication GRBs and afterglows come
  from different locations!

-?
-?
? ? 2, F t ?
tail
afterglow
GRB
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A generic GRB fireball
UV/opt/IR/radio
gamma-ray

central photosphere internal
external shocks engine
(shocks)
(reverse) (forward)
29
Surprise 2 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
30
A Generic X-ray Lightcurve(Zhang et al. 2006)
-3
-0.5
104 105 s
- 1.2
-2
102 103 s
103 104 s
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.
2006, ApJ
32
Flares in short GRB 050724(Barthelmy et al. 2005)
33
Flares in the z6.29 GRB 050904(Cusumano et al.
2005)
34
Properties of X-ray flares

• Rapid rise and fall - not external-related events
• Evidence of underlying afterglow component with
  the same slope - an independent emission
  component
• Multiple early flares exist in a same burst -
  rule out one-time models (blastwave deceleration,
  reverse shock, companion, etc)
• Sometimes (GRB 050502B) flux increases by a
  factor of 500 - cannot be due to density clumps
• Softens as they progress
• Durations positively correlated to epochs
• Sometimes late flares at days
• No difference between long and short GRBs, hard
  GRBs and X-ray flashes

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RS SSC bump? - No!
Can interpret 050406, but difficult to interpret
050502b Kobayashi et al., 2005
36
Density Bump? - No!
37
Two Component Jets? - No!
38
Patchy Shells? - No!
39
Late central engine activity- late internal
shocks (YES!)(Burrows et al 2005 Zhang et al.
2006 Fan Wei 2005 Wu et al. 2006)

• 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)

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Clue again 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. 2006 Fan Wei 2005 Panaitescu et al. 2006
  Dyks et al. 2006)

-?
-?
? ? 2, F t ?
?
tail
afterglow
GRB
41
Ubiquitous ? ? 2
Valid regardless of the history of the
shell Zhang et al. (2005)
42
Ubiquitous ? ? 2
Insensitive to jet structure, Dyks, Zhang Fan
(2006)
43
Ubiquitous ? ? 2
Insensitive to jet structure, Dyks, Zhang Fan
(2006)
44
Complications (1) T0
Zhang et al. (2005)
45
Complications (2) superposition
Observed
GRB flare tail emission
Underlying forward shock emission
Zhang et al. (2005)
46
Testing curvature effect interpretation
Liang et al. (2006)

• 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?

47
Testing curvature effect interpretation
Liang et al. (2005)
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Testing curvature effect interpretation
Liang et al. (2005)
49
Testing curvature effect interpretation
Liang et al. (2005)
The long GRB 050502B and the short GRB 050724
have similar observational properties!
50
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

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ChallengeHow to restart the central engine?
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GRB central engine
Most likely Black Hole Torus Another
possibility A rapidly Rotating Neutron Star (
Torus)
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Idea 1 Fragment the Star(King et al. 2005, ApJL)
Problem Flares occur in both long and short GRBs!
54
Idea 2 Fragment the Disk(Perna, Armitage
Zhang, 2006, ApJL)

• Strength
• Both long and short GRBs have disks
• Duration - epoch correlation luminosity - epoch
  anti-correlation
• Uncertainty
• Fragmentation mechanism unclear (some
  speculations)

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A Generic ConstraintMagnetic 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

?
56
Idea 3 Magnetic barrier modulated accretion
flow(Proga Zhang, 2006, MNRAS Letters)

• Strength
• Disk does not have to be chopped at larger radii
• Duration - epoch correlation luminosity - epoch
  anti-correlation
• Seen in numerical simulations
• Limitation
• Difficult to fully simulate numerically

57
Idea 4 Post-merger millisecond pulsar(Dai,
Wang, Wu Zhang, 2006, Science)

• Strength
• Save the merger model
• NS EOS is stiff - Seen in numerical simulations
• Questions
• Are long GRB engines also NS?
• What happens when the NS further collapses to a
  BH?

58
Surprise 3 Shallow-than-normal decay
050319 050416A 050713B 050721 and more
59
A Generic X-ray Lightcurve(Zhang et al. 2006)
-3
-0.5
104 105 s
- 1.2
-2
102 103 s
103 104 s
60
Interpretations

• Hydro-dynamical
• Continuous energy injection from a long-live
  central engine
• Continuous energy injection from promptly ejected
  slow materials
• Delayed energy transfer (slowness to enter
  Blandford-Mckee phase Poynting-flux-dominated
  flow)
• Precursor energy injection
• Geometrical
• Off-beam jets (combined with the steep decay)
• Patchy jets

This Is Still A Major Mystery.
61
Disappointment in Optical Band

• Reverse Shock Emission
• Fireball Composition

62
A generic GRB fireball
UV/opt/IR/radio
gamma-ray

central photosphere internal
external shocks engine
(shocks)
(reverse) (forward)
63
Early optical afterglow lightcurves(Zhang,
Kobayashi Meszaros 2003)
64
GRB 990123(Akerlof et al. 1999)
RB Br / Bf 15 Zhang, Kobayashi Meszaros,
2003 Fan et al. 2002
65
GRB 021211(Fox et al. 2003 Li et al. 2003)
RB Br / Bf gtgt 1 Zhang, Kobayashi Meszaros,
2003 Kumar Panaitescu 2003
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GRB 041219A(Vestrand et al. 2005 Blake et al.
2005)
RB Br / Bf 3 Fan, Zhang Wei 2005
67
UVOT Dark Bursts
Lack of reverse shock Highly magnetized
flow? Roming et al., 2005
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An analytic MHD shock Solution for GRB reverse
shocks (Zhang Kobayashi 2004) Two free
parameters ?, ?34
? 0 Blandford-McKee (1976)
?34 ? Kennel-Coroniti (1984)
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t-1
t1/2
Optical, forward shock emission
70
t-2
t1/2
t?
t-1
Optical, forward reverse shock emission s 0
71
t-2
t?
t1/2
t-1
Optical, forward reverse shock emission s
0.01
72
t-2
t?
t1/2
t-1
Optical, forward reverse shock emission s 1
73
t-2
t?
Optical, forward reverse shock emission s 10
74
Optical, forward reverse shock emission s
100
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Implications

• The early dimness of the optical afterglow could
  be due to that
• the ejecta is completely not magnetized
• the ejecta is highly magnetized
• Are GRBs Poynting-flux-dominated flow?
• early dimness
• delayed energy transfer
• X-ray flares

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GRB 060218/SN 2006aj

• Prompt emission mechanism
• New GRB population

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GRB 060218(Campana et al. 2006, Nature)

• Nearby z0.0335 (2nd nearest after GRB 980425/SN
  1998bw, z0.0085)
• SN association (SN 2006aj)
• Low luminosity (1047 ergs/s), low energy (1049
  ergs)
• Long duration (900 s in gamma-rays, 2600 s in
  X-rays)
• A thermal component identified in early X-rays
  and late UV-optical band

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GRB 060218 prompt emission(Dai, Zhang Liang
2006)

• Dim early UVOT emission - can not be synchrotron
  emission
• The existence of thermal X-ray component -
  inverse Compton?
• Sharp decay following both gamma-rays and X-rays
  - must be of internal origin
• Non-thermal spectrum - must be above the
  photosphere

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GRB 060218 prompt emission(Dai, Zhang Liang
2006)
Thermal X-ray Shock breakout
reverse shock prompt gamma-rays
forward shock prompt non- thermal X-rays
External shock late X-rays
Internal shock
80
GRB 060218 prompt emission(Dai, Zhang Liang
2006)
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GRB 060218 a distinct new low-luminosity
population(Liang, Zhang Dai 2006)

• In order to let Swift detect one event in a year,
  there must be a second low-luminosity component
  with high event rate at luminosities below 1049
  ergs
• Compared with the high-luminosity population,
  this component has a local rate higher by a
  factor of 500
• Diverse populations of GRB/SN associations
• New progenitor system?

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GRB 060218 a distinct new low-luminosity
population(Liang, Zhang Dai 2006)
83
Conclusions

• Swift answers the mystery of short GRBs, bring
  major observational breakthroughs to study high-z
  GRBs and low-z low luminosity GRBs
• Swift is revolutionizing our understanding of
  GRBs - open the window to study the central
  engine and the ejecta itself
• Prompt emission is originated from a different
  site from the afterglow
• The central engine is still active after the GRB
  phase
• There is tentative evidence of the magnetic
  nature of GRB central engine and outflow
• Low luminosity GRBs form a distinct new
  population, may have different progenitor and
  prompt emission mechanism