Fast Optical Variability of a Naked-Eye Burst—manifestation of the Periodic Activity of an Internal Engine

Abstract

We imaged the position of the naked-eye burst, GRB080319B, before, during, and after its gamma-ray activity with sub-second temporal resolution using the TORTORA wide-field camera. The burst optical prompt emission, which reached 5.3 mag, has been detected, and its periodic optical variability has been discovered in the form of four equidistant flashes with a duration of several seconds. We also detected a strong correlation (r ≈ 0.82) between optical and gamma-ray light curves with a 2 s delay of the optical emission with respect to the gamma-ray emission. The revealed temporal structure of the optical light curve in comparison with the gamma-ray light curve can be interpreted in the framework of the model of shell collisions in the ejecta containing a significant neutron component. All observed emission features reflect the non-stationary behavior of the burst internal engine-supposedly, a hyperaccreting solar-mass black hole formed in the collapse of a massive stellar core.

THE AFTERGLOWS OFSWIFT-ERA GAMMA-RAY BURSTS. I. COMPARING PRE-SWIFTANDSWIFT-ERA LONG/SOFT (TYPE II) GRB OPTICAL AFTERGLOWS

“…At late times, the non-breaking afterglow of GRB 060729 (Grupe et al 2007) is brighter than any other except for GRB 030329. At very early times, the prompt flash of the "naked-eye" GRB 080319B (Racusin et al 2008;Woźniak et al 2009;Beskin et al 2010) reaches 4 mag brighter than the previous record holder, GRB 990123. GRB 061007 comes close to the magnitude of the optical flash of GRB 990123, making it the third-brightest afterglow ever detected.…”

“…Only a few GRBs of the Swift era are observationally as bright as the brightest pre-Swift afterglows. At early times, the prompt optical flash of the "naked-eye" GRB 080319B (Racusin et al 2008;Bloom et al 2009;Woźniak et al 2009;Beskin et al 2010) lies several magnitudes above all other afterglows. Otherwise, only the afterglow of GRB 061007 is comparable 64 to the optical flash of GRB 990123 (Akerlof et al 1999).…”

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

“…The last prompt gamma-ray spike detected by BAT 150 s after the trigger, together with the very hard XRT spectrum (photon index from the XRT Γ X ≈ 1 at 150 s) 4 in this phase, suggests that all Swift data from UVOT to BAT before 250 s are still dominated by the prompt emission. This idea was previously suggested for other promptly detected optical-UV GRBs (see, e.g., the case of the naked-eye burst Racusin et al 2008;Beskin et al 2010), but in this case the data sample is not dense enough to constrain a possible delay between gamma-ray and UVOT peak.…”

Prompt emission of GRB 121217A from gamma-rays to the near-infrared

“…Fortunately, however, there exist tens of fortuitous cases in which both the gamma-ray emission and optical emission have been detected during the prompt period. These can be divided into three possible scenarios: (i) a wide-field camera is observing the same field position as a satellite and so catches the optical emission simultaneously (e.g., 080319B, 130427A; Racusin et al 2008;Bloom et al 2009;Beskin et al 2010;Wren et al 2013); (ii) the prompt period is long enough that optical instruments slew in time to observe the prompt period (e.g., 990123, 080928, 110205A, 091024; Akerlof et al 1999;Rossi et al 2011;Cucchiara et al 2011;Gruber et al 2011;Gendre et al 2012;Zheng et al 2012;Virgili et al 2013);and (iii) there is a precursor to the main event so that optical instruments can slew in time (e.g., 041219A, 050820A, 061121; Blake et al 2005;Vestrand et al 2005Vestrand et al , 2006Page et al 2007). Only recently has it become possible to compile samples of bursts that exhibit optical emission during the prompt phase (Kopač et al 2013).…”