X-ray/Optical flares in Gamma-Ray Bursts

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X-ray/Optical flares in Gamma-Ray Bursts

• Daming Wei
• ( Purple Mountain Observatory, China)

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Bright X-ray flares
GRB011121
Z0.36 Eiso2.81052erg It is a normal long GRB.
Afterglow
prompt emission
Piro et al. 2005 suggested that the flares
were the signatures of the onset of the forward
shock.
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Implication of X-ray flare light curve(Fenimore
et al. 1996 Kumar Panaitescu 2000 Nakar
Piran 2003 Fan Wei 2005 Zhang et al. 2006
Wu et al. 2006)
External shock
internal shock
their duration to their time of occurrence
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Observations show that for many XRFs they
have dt /t ltlt 1, and the decay index agtgt 2ß.
Therefore we proposed that the flare should be
attributed to the re-activity of the central
engine, i.e., they were the extension of the
prompt emission but in a lower energy band (see
also Zhang et al. 2006), namely, the late
internal shock.
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X-ray flares more cases
GRB 050724
GRB 050904

• X-ray flares following long, short, and high
  redshift GRBs
• (Burrows et al.
  2005 Barthelmy et al. 2005 Watson et al. 2006)

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Internal shock
Internal shock
evidences for X-ray flares arising from internal
shocks (From
Chincarini et al. 2007)
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Optical flare uncorrelated with gamma-ray
emission
GRB990123
RS emission
FS emission
(Akerlof, et al., 1999 Sari Piran 1999
Meszaros Rees 1999 Fan et al. 2002)
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GRB021211
FS
RS
(Wei 2003 Zhang, et al. 2003 Kumar Panaitescu
2003)
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Optical flare correlated with gamma ray emission
The prompt optical emission is the low energy
tail of the gamma-ray emission. (Vestrand et al.
2005 Wei 2007) Why not SSC? Y105 !
Unreasonable large !

(Vestrand et al., 2005)
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Optical flare correlated with X-ray flare(The
most distant GRB050904 at z6.29)
The optical flare was temporally coincident with
the X-ray flare. In the late internal shock
model, The optical flare was produced by the
synchrotron radiation, while the X-ray flare was
produced by the SSC process. (Wei et al. 2006)
X-ray flare
Optical flare
(Boër et al. 2006)
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GRB080319B
SynSSC
(Racusin, et al., 2008 Kumar Panaitescu 2008
Fan Piran 2008)
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Optical uncorrelated with gamma-ray arising
from different internal shocks?

• Mészáros Rees (1999) argued that the internal
  shock model can well explain the temporal
  behavior of the optical flare.
• We need to calculate the emission features of the
  internal shock.

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Emission of internal shock
For GRB990123, G800, dt 2s, which is quite
similar to that of GRB080319B (Racusin, et al.,
2008 Kumar Panaitescu 2008) , implying to
large emission site and small synchrotron-self-abs
orption frequency. (Wei 2007)
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Optical uncorrelated with gamma-ray arising
from different internal shocks? (Wei 2007)
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Neutron-rich internal shock

• The central engine of GRBs, usually believed to
  be a newly formed black hole with an accreting
  torus, is very compact and hot, with a
  temperature no less than several MeV. Such a high
  temperature exceeds the threshold value for
  nuclear dissociation, therefore GRB outflows are
  very likely to carry free neutrons unless they
  are dominated by Poynting flux (Derishev et al.
  99 Pruet et al. 03 Beloborodov 03).
• If it is the case, then the dynamics and the
  emission features may be very different from the
  normal fireball model (Derishev et al. 99
  Bahcall Meszaros 00).

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Decoupling of protons and neutrons
In the beginning the n and p are coupled
together. When the scattering optical depth is
smaller than 1, the n, p will decouple and the
neutrons can not be accelerated any more . The
decoupling may occur in the coasting phase or in
the accelerating phase which depends
on whether the dimensionless entropy ? is below
or above the critical value
(Bahcall Meszaros 2000)
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Neutron-rich internal shocks(Fan Wei 04, ApJL)
The beta-decay products of the early neutron
shells
Proton shell
Regular internal shocks at 1013 cm powering
gamma-ray emission
Secondary internal shocks at 1016 cm powering
UV/optical emission
The beta-decay radius
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The secondary internal shocks are more likely to
produce UV/optical flare rather than X-ray
(gamma-ray) emissions for the following reasons

• They are generated at a radius much larger than
  that of the regular internal shocks (Rint
  1013cm), so that the magnetic fields are much
  weaker than those in the regular internal shocks.
  
• The Lorentz contrast between the merged proton
  shell and the neutron shell is smaller than that
  of the regular internal shocks. So the
  accelerated electron energy and the emission
  frequency are smaller.

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The naked-eye optical flash of GRB 080319B
Tracing the decaying neutrons of the outflow?
(Fan, Zhang Wei 2008)

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Comparing with SSC model

• In the SSC model, one predicts very bright prompt
  GeV emission.
• In neutron-rich model, the prompt GeV emission is
  weaker.
• Being at a larger emission radius, the optical
  variability is smoothed by the geometric effect.
  This is consistent with the fact that the optical
  light curve is smoother than the gamma-ray light
  curve.
• The time delay between the optical and gamma-ray
  peaks is 1s.

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Extinction of early optical emission

• GRBs lie in star-forming region, large amounts of
  dust may exist, so early optical emission may
  subject to severe extinction.
• The prompt emission and the afterglow emission
  may destroy the dust grains, so the dust
  extinction may decrease with time. So detailed
  calculation on dust destruction and extinction is
  important when considering early optical
  emission.
• (Perna Lazzati 2002 Jin Wei 2008)

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GRB030418
Rykoff et al. 2004 Jin Wei 2008
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Absence of optical flare

• For many GRBs we have not detected the
  optical flares, there are several reasons, such
  as
• High self-absorption frequency ?a. In the
  neutron-rich internal shock model, the
  self-absorption frequency is usually much smaller
  than that in the standard internal shock.
• Dust extinction. We found at least in some GRBs
  the dust extinction is large. (Chen, Li Wei
  2006 Li, Li Wei 2008)
• The reverse shock region might be highly
  magnetized.
• (Fan et al. 2004 Zhang kobayashi 2005)

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Conclusion

• Internal shock not only produce the prompt
  gamma-ray emission, but also can produce X-ray
  flares and optical flares.
• Prompt optical emission may include both internal
  shock and external shock components.
• The absence of optical flares may be due to dust
  extinction, large self-absorption frequency,
  etc..
• Inverse Compton scattering of internal shock can
  produce high energy (MeV GeV) emission.

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(Vestrand, et al., 2006)
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Internal-External shock
IC??-rays
may be neutron shell
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Peak in optical, SSC?gamma-ray
Peak in gamma-ray, Low energy extension-optical
flux
Produced by different shells
External shock
optical
gamma-rays
time
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• Thank You !