The unusual afterglow of the gamma-ray burst 100621A

Abstract

Aims. With the afterglow of GRB 100621A being the brightest detected so far in X-rays, and superb GROND coverage in the optical/near-infrared during the first few hours, an observational verification of basic fireball predictions seemed possible. Methods. In order to constrain the broad-band spectral energy distribution of the afterglow of GRB 100621A, dedicated observations were performed in the optical/near-infrared with the 7-channel Gamma-Ray Burst Optical and Near-infrared Detector (GROND) at the 2.2 m MPG/ESO telescope, in the sub-millimeter band with the large bolometer array LABOCA at APEX, and at radio frequencies with ATCA. Utilizing also Swift X-ray observations, we attempt an interpretation of the observational data within the fireball scenario. Results. The afterglow of GRB 100621A shows a very complex temporal and spectral evolution. We identify three different emission components, the most spectacular one causing a sudden intensity jump about one hour after the prompt emission. The spectrum of this component is much steeper than the canonical afterglow. We interpret this component using a two-shell collision prescription after the first shell has been decelerated by the circumburst medium. We use the fireball scenario to derive constraints on the microphysical parameters of the first shell. Long-term energy injection into a narrow jet seems to provide an adequate description. Another noteworthy result is the large (A V = 3.6 mag) line-of-sight host extinction of the afterglow in an otherwise extremely blue host galaxy. Conclusions. Some GRB afterglows have shown complex features, and that of GRB 100621A is another good example. Yet, detailed observational campaigns of the brightest afterglows promise to deepen our understanding of the formation of afterglows and the subsequent interaction with the circumburst medium.

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

“…3.4, we find the best-fit optical rise between 20 ks and 100 ks after the GRB trigger to be α reb ∼ −2.9. This high value is less extreme than α reb ∼ −8.2 and α reb ∼ −14 measured for GRB 081029 (Nardini et al 2011) and GRB 100621A (Nardini et al 2012;Greiner et al 2013), respectively, and agrees with both the typical values for FROAs reported (PV08) and with the late-bump rise steepnesses derived by Liang et al (2013) in their sample. Liang et al (2013) also found a good correlation between the optical luminosity at the rebrightening peak and the host frame peak time.…”

“…Moreover, thanks to the rich coverage and the small uncertainties in the v, and u bands at ∼60 ks, we can identify the smaller-scale fluctuations around the rebrightening peak. Those wiggles are similar to the ones observed around the peak of the prominent optical rebrightenings of GRB 081029 and GRB 100621A discussed in Nardini et al (2011) and Greiner et al (2013), respectively. The late fading in the UVOT bands tracks the power-law decay observed in the bluer GROND bands well.…”

“…The early light curve is dominated by the emission of a faster and narrower jet, while a slower and wider second jet becomes dominant at later time because its afterglow onset peaks while the first jet is already decaying fast in its post-jetbreak phase. Unlike the extreme cases of GRB 081029 (Nardini et al 2011) and GRB 100621A (Greiner et al 2013), the value of α reb is consistent with the pre-onset rise of a standard afterglow in a homogeneous external medium viewed off-axis and is also marginally consistent with the on-axis case (α = −2) when the uncertainties on the jet-break time of the first component are taken into account. If this second jet is considered independently from the first one and is assumed to be expanding in an unshocked ISM medium, we can estimate the Lorentz factor of this second jet to be Γ 0 = 16 following Eq.…”

“…Unlike for GRB 081029 (Nardini et al 2011) or 100621A (Greiner et al 2013), GROND was unable to follow the full rising phase since it occurred during daytime in Chile. This temporal window was in part covered by UVOT.…”

“…Combining the simultaneous g , r , i , z , J, H, K s GROND observations with UVOT and XRT data gives an extensive and densely sampled picture of the afterglow emission from the NIR to the X-ray bands. This synergy recently allowed us to discover a particularly interesting family of long GRBs, whose light curves are characterised by the presence of prominent chromatic A&A 562, A29 (2014) rebrightenings in the GROND bands, for example, GRB 081029 (Nardini et al 2011) and GRB 100621A (Greiner et al 2013). These extremely fast rebrightenings cannot be explained by any of the commonly proposed emission models.…”