HAPPY BIRTHDAYSWIFT: ULTRA-LONG GRB 141121A AND ITS BROADBAND AFTERGLOW

Abstract

We present our extensive observational campaign on the Swift-discovered GRB 141121A, almost ten years after its launch. Our observations covers radio through X-rays, and extends for more than 30 days after discovery. The prompt phase of GRB 141121A lasted 1410 s and, at the derived redshift of z = 1.469, the isotropic energy is E γ,iso = 8.0 × 10 52 erg. Due to the long prompt duration, GRB 141121A falls into the recently discovered class of UL-GRBs. Peculiar features of this burst are a flat early-time optical light curve and a radio-to-X-ray rebrightening around 3 days after the burst. The latter is followed by a steep optical-to-X-ray decay and a much shallower radio fading. We analyze GRB 141121A in the context of the standard forward-reverse shock (FS,RS) scenario and we disentangle the FS and RS contributions. Finally, we comment on the puzzling early-time (t 3 d) behavior of GRB 141121A, and suggest that its interpretation may require a two-component jet model. Overall, our analysis confirms that the class of UL-GRBs represents our best opportunity to firmly establish the prominent emission mechanisms in action during powerful GRB explosions, and future missions (like SVOM, XTiDE, or ISS-Lobster) will provide many more of such objects.

The Arcminute Microkelvin Imager catalogue of gamma-ray burst afterglows at 15.7 GHz

“…The flatness of the AMI light curve up to 12 days post-burst, followed by evidence for late time rebrightening, may support the detection of multiple peaks from the forward-and reverse-shock at 15.7 GHz. Energy injections, which is supported by the detection of flares and plateaus in the XRB light curve of GRB 141121A (Cucchiara et al 2015), could also cause radio lightcurve flattening (Frail et al 2004). However, this flatness may also be an artefact of the radio counterpart being close to the AMI detection limit.…”

“…The 4σs upper limits on the non-detections are also illustrated (red triangles). The light-curve of GRB 141121A is plotted twice, the second plot zooming in at late times ( 1 day post-burst) so as to better discern the light-curve structure, while also including VLA 15 GHz detections (Cucchiara et al 2015). All error-bars are 1σs.…”

“…Figure 1 shows the AMI light curve of GRB 141121A. A multi-wavelength analysis of GRB 141121A has also been conducted by Cucchiara et al (2015).…”

“…A zoomed in light curve beginning ∼ 1 days post-burst that includes the 15.7 GHz VLA detections (Cucchiara et al 2015) can be found in Figure 1. The AMI detections, forced fits and upper limits are consistent with the VLA detections but the AMI observations also show evidence for radio re-brightening at ∼ 12 and ∼ 39 days post-burst (note that the AMI detection at 39 days is within 3σs of the VLA detection at 33 days but both detections show the 15.7 GHz flux is increasing at late times).…”

“…The AMI detections, forced fits and upper limits are consistent with the VLA detections but the AMI observations also show evidence for radio re-brightening at ∼ 12 and ∼ 39 days post-burst (note that the AMI detection at 39 days is within 3σs of the VLA detection at 33 days but both detections show the 15.7 GHz flux is increasing at late times). Cucchiara et al (2015) demonstrated that around 3 days, when the 15.7 GHz flux is near its peak, at least 50% of the radio flux is being contributed by the reverse-shock. The flatness of the AMI light curve up to 12 days post-burst, followed by evidence for late time rebrightening, may support the detection of multiple peaks from the forward-and reverse-shock at 15.7 GHz.…”