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Early-time colour variations in the GRB 201015A afterglow

Presentation #109.02 in the session Gamma Ray Bursts.

Published onJun 29, 2022
Early-time colour variations in the GRB 201015A afterglow

Instruments such as the ROTSE, TORTORA, Pi of the Sky, MASTER-net, and others have recorded single-band optical flux measurements starting as early as ~ 10 s after the gamma-ray trigger. The earliest measurements of optical spectral shape have been made only much later, on hour time scales, never on the time scale approaching a few minutes or less, until now. We began observations of GRB201015A beginning only 58 seconds after the Swift BAT trigger, measuring simultaneously in three filter bands, g', r', and i', using the Burst Simultaneous Three-Channel Imager (BSTI, see Grossan & Maksut 2020) instrument on the Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTelA-TAO). We observed the afterglow peak and a power law temporal decay, t , with α1 =1.2±0.07 bewteen 500 and 1600 seconds, but later this changed to a value of α2 =0.8±0.1 between 1900 and 4000 seconds. The spectrum between the three filters was consistent with a power law fν ∝ ν with a value of β of 0.73±0.08 at the peak, 130 to 500 seconds, and 0.42±0.14 during the ”early afterglow” phase, 500 to 1600 seconds, and 0.01±0.11 during the later afterglow phase, 1900 to 4000 seconds (we assumed a galactic reddening of 1.12, 0.77 and 0.58 mags, and source reddening of 0.19, 0.14 and 0.11 mags for the sloan g', r' and i' filters, respectively). We measured the X-ray temporal decay index of 0.8 ±0.1 which is consistent with our late optical temporal decay index and Chandra X-ray decay index reported by Gompertz et al. (2020). Changes from one power law decay time and spectral index to another in this phase of the afterglow have been attributed to jet components of different angular extent dominating emission at different times (e.g. Racusin et al. 2008), or the transition from reverse shock emission dominance to forward shock dominance (e.g. Vestrand et al. 2014, Racusin et al. 2008, Wozniak et al. 2009, Bloom et al. 2009); we discuss the relative merits of these scenarios in the body of this work. The changes we see in spectral shape in early afterglows suggest that significant information about this emission phase (and possibly prompt emission, if observed early enough) is being missed without such fast (sub-minute) response observations with simultaneous multi-color instruments.

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