Long-term Continuous Energy Injection in the Afterglow of GRB 060729

📝 Abstract

A long plateau phase and an amazing brightness have been observed in the Xray afterglow of GRB 060729. This peculiar light curve is likely due to long-term energy injection in external shock. Here we present a detailed numerical study on the energy injection process of magnetic dipole radiation from a strongly magnetized millisecond pulsar and model the multi-band afterglow observations. It is found that this model can successfully explain the long plateaus in the observed X-ray and optical afterglow light curves. The sharp break following the plateaus should be due to the rapid decline of the emission power of the central pulsar. At an even late time (5*10^6s), an obvious jet break appears, which implies a relatively large half opening angle of theta0.3 for the GRB ejecta. Due to the energy injection, the Lorentz factor of the outflow is still larger than two 10^7s post the GRB trigger, making the X-ray afterglow of this burst detectable by Chandra even 642 days after the burst.

💡 Analysis

A long plateau phase and an amazing brightness have been observed in the Xray afterglow of GRB 060729. This peculiar light curve is likely due to long-term energy injection in external shock. Here we present a detailed numerical study on the energy injection process of magnetic dipole radiation from a strongly magnetized millisecond pulsar and model the multi-band afterglow observations. It is found that this model can successfully explain the long plateaus in the observed X-ray and optical afterglow light curves. The sharp break following the plateaus should be due to the rapid decline of the emission power of the central pulsar. At an even late time (5*10^6s), an obvious jet break appears, which implies a relatively large half opening angle of theta0.3 for the GRB ejecta. Due to the energy injection, the Lorentz factor of the outflow is still larger than two 10^7s post the GRB trigger, making the X-ray afterglow of this burst detectable by Chandra even 642 days after the burst.

📄 Content

arXiv:0909.5318v1 [astro-ph.HE] 29 Sep 2009 Research in Astron. Astrophys. 2009 Vol. 9 No. XX, 000–000 http://www.raa-journal.org http://www.iop.org/journals/raa Research in Astronomy and Astrophysics Long-term Continuous Energy Injection in the Afterglow of GRB 060729 M. Xu1, Y.-F. Huang1 and T. Lu2 1 Department of Astronomy, Nanjing University, Nanjing 210093, China; hyf@nju.edu.cn 2 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China Received [year] [month] [day]; accepted [year] [month] [day] Abstract A long plateau phase and an amazing brightness have been observed in the X- ray afterglow of GRB 060729. This peculiar light curve is likely due to long-term energy injection in external shock. Here we present a detailed numerical study on the energy injection process of magnetic dipole radiation from a strongly magnetized millisecond pulsar and model the multi-band afterglow observations. It is found that this model can successfully explain the long plateaus in the observed X-ray and optical afterglow light curves. The sharp break following the plateaus should be due to the rapid decline of the emission power of the central pulsar. At an even late time (∼5 × 106s), an obvious jet break appears, which implies a relatively large half opening angle of θ ∼0.3 for the GRB ejecta. Due to the energy injection, the Lorentz factor of the outflow is still larger than two 107s post the GRB trigger, making the X-ray afterglow of this burst detectable by Chandra even 642 days after the burst. Key words: gamma rays: bursts -ISM: jets and outflows 1 INTRODUCTION GRB 970228 is the first gamma-ray burst (GRB) with an X-ray afterglow detected (Costa et al. 1997). Optical (van Paradijs et al. 1997) and radio afterglow (Frail et al. 1997) has also been unprecedently detected from this event. The relativistic internal and external shock model is the most successful model to explain these violent events ( Rees & M´esz´aros 1994; Piran 1999; Zhang 2007). It is also widely believed that long GRBs should be due to the collapse of massive stars (Woosley 1993; Paczy´eski 1998; MacFadyen & Woosley 1999), and short GRBs should be connected with the coalescence of two compact objects (Eichler et al. 1989; Narayan et al. 1992; Gehrels et al. 2005; Nakar 2007). The X-ray telescope (XRT) on board Swift reveals that the X-ray afterglows of GRBs generally show a canonical behavior, with five components in the observed X-ray afterglow light curves, i.e., steep decay phase, shallow decay phase, normal decay phase, post jet break phase and X-ray flares (Zhang et al. 2006; Nousek et al. 2006). The conventional models for shallow decay phase are energy injection from strongly magnetized millisecond pulsar (Dai & Lu 1998; Zhang & M´esz´aros 2001; Liang et al. 2007; Lyons et al. 2009) or from ejecta with a highly dispersed Lorentz factor distribution (Rees & M´esz´aros 1998; Sari & M´esz´aros 2000). At 19 : 12 : 29 UT of July 29, 2006, GRB 060729 triggered the Swift Burst Alert Telescope (BAT) and was quickly located (Grupe et al. 2006). This event has a duration of T90 = 116 ± 10s (Parsons et al. 2006) and a redshift of z = 0.54 (Thoene et al. 2006). The isotropic energy release in the rest-frame in 1keV −10MeV band was Eiso = 1.6×1052ergs for a standard cosmology model with ΩM = 0.27, ΩΛ = 0.73 and a Hubble constant of H0 = 71km · s−1 · Mpc−1. 2 M. Xu, Y.-F. Huang & T. Lu One of the distinguished properties of GRB 060729 is that it has a long flat phase in the X-ray afterglow light curve (Grupe et al. 2007). Another prominent character of GRB 060729 is its brightness. It can be observed by Chandra even 642 days after the burst trigger (Grupe et al. 2009). Grupe et al. (2009) compared the X-ray afterglow of GRB 060729 with other bright X-ray afterglows and concluded that GRB 060729 was an exceptionally long-lasting event. Actually, the brightness of the X-ray after- glow of GRB 060729 is not extraordinary at early time (t < 30000s), but it becomes the brightest one among all GRBs after 30000s since the trigger. In view of the long plateau phase (500s −30000s) and the late time (> 30000s) brightness of GRB 060729, a strong and long-term continuous energy injection is implied (Liang et al. 2007; Grupe et al. 2007, 2009). Grupe et al. (2007) presented an extensive study on this peculiar event and made a detailed analysis on the pulsar-type energy injection for this plateau. But at that time there was only 125 days of data and the jet break still did not appear. In this paper, we use the energy injection model that involves the dipole radiation from a strongly magnetized millisecond pulsar to explain the special behavior of the multi-band afterglow of GRB 060729. The new data observed by Chandra (Grupe et al. 2009) will be incorporated. We detailedly calculate the X-ray and optical (U-band, B-band and V-band) afterglow light curves, and compare them with the observations. In Section 2, we briefly describe the energy injection

This content is AI-processed based on ArXiv data.