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QUICK DESIGN GUIDE (--THIS SECTION DOES NOT PRINT--) This PowerPoint 2007 template produces a 91cm x 122cm professional poster. You can use it to create your research poster and save valuable time placing titles, subtitles, text, and graphics. We provide a series of online tutorials that will guide you through the poster design process and answer your poster production questions. To view our template tutorials, go online to PosterPresentations.com and click on HELP DESK. When you are ready to print your poster, go online to PosterPresentations.com. Need Assistance? Call us at 1.866.649.3004 Object Placeholders Using the placeholders To add text, click inside a placeholder on the poster and type or paste your text. To move a placeholder, click it once (to select it). Place your cursor on its frame, and your cursor will change to this symbol. Click once and drag it to a new location where you can resize it. Section Header placeholder Click and drag this preformatted section header placeholder to the poster area to add another section header. Use section headers to separate topics or concepts within your presentation. Text placeholder Move this preformatted text placeholder to the poster to add a new body of text. Picture placeholder Move this graphic placeholder onto your poster, size it first, and then click it to add a picture to the poster. RESEARCH POSTER PRESENTATION DESIGN © 2012 www.PosterPresentations.com © 2013 PosterPresentations.com 2117 Fourth Street, Unit C Berkeley CA 94710 posterpresenter@gmail.com Student discounts are available on our Facebook page. Go to PosterPresentations.com and click on the FB icon. ―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. Introduction Models Application to GRBs 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). a.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: The picture above shows the evolution of characteristic frequencies and the afterglows in our model (left panel for ISM case and right panel for Wind case). As is seen from this figure, the giant optical bump in the later time can be reproduced as expected. In our calculations, the effect of equal time arrival surface and synchrotron self- Compton process has been considered. For the broken-power-law mode, we have assumed that the late energy release is proportional to the mass accretion rate for simplicity. 1 Department of Astronomy, Nanjing University, Nanjing 210093, hyf@nju.edu.cn ; 2 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210080, xfwu@pmo.ac.cn. Jin Jun Geng 1, X. F. Wu 2, Y. F. Huang 1 & Y. B. Yu 1 Delayed energy injection model for Gamma-Ray Burst afterglows References Huang, Y. F., Dai, Z. G., & Lu, T. 2000a, MNRAS, 316, 943 Kumar, P., Narayan, R., & Johnson, J. L. 2008b, MNRAS, 388, 1729 Sari, R., & Piran, T. 1999a, ApJ, 517, L109 Sari, R., Piran, T., & Narayan, R. 1998, ApJ, 497, L17 Xue-Wen, L., Xue-Feng, W., & Tan, L. 2009, arXiv:0907.1767