Diapositiva 1

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The Early Time Properties of GRBs Canonical
Afterglow and the Importance of Prolonged Central
Engine Activity
Andrea Melandri
Astrophysics Research Institute, Liverpool JMU, UK
Collaborators C.G.Mundell, S. Kobayashi, D.
Bersier, I. A. Steele, C.Guidorzi, A. Gomboc, R.
J. Smith, D. Carter, M. F. Bode
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Outline

• Canonical X-ray afterglow
• Early optical light curves canonical?
• X-ray/Optical analysis
• The role of prolonged central engine activity
• - afterglow detections
• - dark bursts
• Beyond the Fireball GRB 070419A
• Conclusions

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Generic X-ray Light curve (Zhang et al. 2006)
-3
-0.5
104 105 s
- 1.2
-2
102 103 s
103 104 s
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Typical X-ray Light curve (Nousek et al. 2006,
OBrien et al. 2006)
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But.

• The X-ray canonical light curve is not
  ubiquitous

• The same mechanisms should produce the optical
  radiation. Optical should track X-ray flux, but
  this is not observed in many GRBs

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Early Optical Light curve
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Optical/X-ray analysis
Simplest explanation in the Fireball model
A no break in the Opt or in the X-ray band
? ?C with respect to the Opt and X-ray bands B
no break in the Opt, break in the X-ray band
? Passage of ?C through the X-ray band C
break in the Opt, no break in the X-ray band ?
Passage of ?C through the Opt band D break
in the Opt and in the X-ray band ?
Cessation of energy injection or jet break
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Optical/X-ray analysis
A possible explanation is the additional X-ray
emission produced by late-time central engine
activity, observed as an enhanced component in
late-time afterglow light curves.
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Optical/X-ray analysis
Dark Bursts

• The majority of the bursts are Dark
  independent of the selected time.
• 10 were identified at NIR, demonstrating a
  small population of bursts in high density
  environments.
• 29 we suggest that if late-time central engine
  activity is responsible for the production of
  early X-ray afterglow emission then the
  additional emission will mask the simultaneous
  (but fainter) FS emission, resulting in a larger
  observed X-ray flux than expected (while the
  optical flux is not suppressed).

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One case..070419A (Melandri et al. 2009)
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One case..070419A (Melandri et al. 2009)

• Clearly too complex to be explained with the
  FS model

• Features are too sharp to be explained as
  density enhancement

• If RS dominating at early times ? tpeak 450
  s, ? 350 ? t 1500 s the passage of ?m,r ?
  ?m,r 3 x 105 Hz ?m,f ??m,r 4 x 1020 Hz ?
  FS should peak t 4 x 106 s !!

• If the fireball is magnetized (with RB 106)
  then ?m,f could be smaller ? the ratio between RS
  and FS luminosity should be 4 x 105 when in
  reality it is only 2 order of magnitude !!

• Refreshed shocks could not be ruled out BUT
  the energy injection rate should be tuned very
  carefully we need a sharp cessation of the
  injection to get a clear break
• Assumption of a finely tuned long-lived
  central engine (consistent with the same
  conditions invoked to explain the plateau phase
  in many X-ray light curves of GRBs)

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Conclusions

• 60 of the detected afterglows in our sample are
  consistent with the standard model
• Few GRBs are not easily explained in the context
  of the simple fireball model even when
  modification are made (i.e. energy injection,
  variation of the density matter, magnetized
  fireball, tail component following the fireball,
  etc)
• The Dark Bursts fraction remains high (50)
  despite deep early-time observations
• Enhanced X-ray emission from late time central
  engine activity plays a big role and may explain
  non-standard light curves (like GRB070419A) and
  Dark Bursts

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