A Comprehensive Study of Gamma-Ray Burst Optical Emission. Ii. Afterglow Onset and Late Re-Brightening Components

Abstract

We continue our systematic statistical study of various components of gamma-ray burst (GRB) optical light curves. We decompose the early onset bump and the late re-brightening bump with empirical fits and analyze their statistical properties. Among the 146 GRBs that have well-sampled optical light curves, the onset and re-brightening bumps are observed in 38 and 26 GRBs, respectively. It is found that the typical rising and decaying slopes for both the onset and re-brightening bumps are ∼1.5 and ∼−1.15, respectively. No early onset bumps in the X-ray band are detected to be associated with the optical onset bumps, while an X-ray re-brightening bump is detected for half of the re-brightening optical bumps. The peak luminosity is anti-correlated with the peak time L p ∝ t −1.81±0.32 p for the onset bumps and L p ∝ t −0.83±0.17 p for the re-brightening bumps. Both L p and the isotropic energy release of the onset bumps are correlated with E γ,iso , whereas no similar correlation is found for the re-brightening bumps. These results suggest that the afterglow onset bumps are likely due to the deceleration of the GRB fireballs. Taking the onset bumps as probes for the properties of the fireballs and their ambient medium, we find that the typical power-law index of the relativistic electrons is 2.5 and the medium density profile behaves as n ∝ r −1 within the framework of the synchrotron external shock models. With the medium density profile obtained from our analysis, we also confirm the correlation between the initial Lorentz factor (Γ 0 ) and E iso,γ in our previous work. The jet component that produces the re-brightening bump seems to be on-axis and independent of the prompt emission jet component. Its typical kinetic energy budget would be about one order of magnitude larger than the prompt emission component, but with a lower Γ 0 , typically several tens.

Afterglow rebrightenings as a signature of a long-lasting central engine activity?

“…(15) from Ghirlanda et al (2011). This is the preferred model chosen by Liang et al (2013) to explain the afterglow rebrightenings in their sample.…”

Exploring the canonical behaviour of long gamma-ray bursts using an intrinsic multiwavelength afterglow correlation

“…However, statistical investigations of large samples of GRBs aim to find common characteristics and correlations that link individual events and therefore provide insight into the mechanisms common to GRBs. Statistical investigations performed so far have found a number of trends and correlations within and linking the prompt gamma-ray emission and the afterglow emission (e.g., Amati et al 2002;Ghirlanda, Ghisellini & Lazzati 2004;Dainotti, Cardone & Capozziello 2008;Panaitescu & Vestrand 2008;Bernardini et al 2012;D'Avanzo et al 2012;Li et al 2012;Liang et al 2013;Zaninoni et al 2013;Panaitescu, Vestrand & Woźniak 2013). Within the prompt gamma-ray emission, the most renowned correlation discovered is the Amati relation, a correlation between the isotropic γ-ray energy Eiso and the restframe γ-ray peak energy E peak (see Amati et al 2002, and references therein).…”

Optical and X-ray rest-frame light curves of the BAT6 sample

“…In the past few years, the advances in detecting, fast re-pointing and observing gamma-ray bursts (GRBs) by the Swift satellite (Gehrels et al 2004) has allowed the creation of uniform collections of events for statistical purposes (Nousek et al 2006;Zhang et al 2006;Gehrels et al 2008;Evans et al 2009;Roming et al 2009;Oates et al 2009;Kann et al 2006Kann et al , 2011Rykoff et al 2009;Nysewander et al 2009;Melandri et al 2008;Liang et al 2012;Salvaterra et al 2012;Margutti et al 2013). Depending on the selection criteria defining the sample, these collections of GRBs probe the parameter space of rest-frame time-brightness in a different manner.…”