?The shallow decay phase and the flares in the afterglows of gamma-ray bursts (GRBs) is widely believed to be associated with the later activation of central engine.

1

• Delayed energy injection model for Gamma-Ray
  Burst afterglows

• Jin Jun Geng1, X. F. Wu2, Y. F. Huang1 Y. B.
  Yu1

• 1Department of Astronomy, Nanjing University,
  Nanjing 210093, hyf_at_nju.edu.cn
• 2Purple Mountain Observatory, Chinese Academy of
  Sciences, Nanjing 210080, xfwu_at_pmo.ac.cn.

• Introduction

• Application to GRBs

• ?The shallow decay phase and the flares in the
  afterglows of gamma-ray bursts (GRBs) is widely
  believed to be associated with the later
  activation of central engine.
• ?Some GRBs (GRB 081029, GRB 100621A) have shown
  steep giant bumps in their optical afterglow.
• Goals Used a delayed energy injection model to
  fit the unusual multiple-band afterglows.
• Assuming In the scenario involving with a black
  hole accretion system, the energy flow from the
  fall-back accretion may be delayed for a
  fall-back time.

• Models

• Shock dynamics When Poynting-flux energy
  injection is taken into account, the basic
  equation for GRB outflow dynamics during the
  afterglow phase can be modified to be (Huang et
  al. 2000 Xue et al. 2009)
• b. External shock radiation The main radiation
  mechanism for the outflow is the synchrotron
  radiation from electrons (Sari et al. 1998 Sari
  Piran 1999).
• c. The luminosity profile of the fall-back power
• d. Numerical results

• Left panel Our best fit to the multi-band
  afterglow of GRB 081029. Right panel Our best
  fit to the multi-band afterglow of GRB 100621A.

• Discussion and Conclusions

• ?The bumps in the later multiple-wavelength
  observations indicate the reactivity of the
  central engine. If this bump is truly due to the
  mass fall-back, the re-brightening time should
  correspond to the fall-back time 680 s and 2330
  s (in the cosmological local frame) for GRB
  081029 and GRB 100621A respectively.
• ?The total fall-back mass needed to support the
  injected energy should be 3.5 and 1.0 Solar mass
  if only potential energy considered.
• ?GRB 081029 and GRB 100621A might be accompanied
  with a low energy supernova, or even a failed
  supernova.
• ?However, we have only considered the simplified
  case. Some physical processes for photons
  streaming through the external shock may be
  considered later. Besides, the jet power from the
  fall-back accretion should depend on the spin
  evolution of the BH and the magnetic field around
  the BH.

• Acknowledgements

• We thank Shanqin Wang, Shujing Hou, Kai Wang and
  Yuanpei Yang for helpful discussion. We also
  thank Nardini et al. and Greiner at al. for
  kindly sharing the data of GRB 081029 and GRB
  100621A obtained with the seven-channel Gamma-Ray
  burst Optical and Near-infrared Detector (GROND).
  Our study made use of data supplied by the UK
  Swift Science Data Centre at the University of
  Leicester. This work was supported by the
  National Basic Research Program of China (973
  Program, Grant No. 2009CB824800) and the National
  Natural Science Foundation of China (Grant Nos.
  11033002).

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