Detection of an oscillatory phenomenon in optical transient counterpart of GRB090522C from observations on Peak Terskol

Abstract

22 Sep 2005 Swift-BAT triggered and located GRB050922C. The light curve shows the intense broad peak with $T_{90}$ of $(5 \pm 1)$ s. The Nordic Optical Telescope has obtained spectra of the afterglow with several absorption features corresponding to a redshift of $z = 2.17 \pm 0.03$. Observation of optical transient of GRB050922C was carried out in the R-band with the 60-cm telescope equipped with a CCD on Peak Terskol (North Caucasus). The OT magnitude was fading from R $\approx 16$ to $\approx 17.5$. Detection of an oscillatory phenomenon in the R post-burst light curve is described in this work. Analysis of the R data reveals coherent harmonic with a period of $0.0050 \pm 0.0003$ days (7.2 min) during observing run of about 0.05 days ($\sim 70$ min). Amplitude of oscillations is about 0.05 magnitude. The simplest model suggests that GRB050922C may result from tidal disruption of a white dwarf star by a black hole of about one thousand solar mass. The periodicity in the light curve can be identified with relativistic precession of an accretion disc.