An intrinsic correlation between GRB optical/UV afterglow brightness and decay rate

¹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking Surrey, RH5 6NT, UK ;
e-mail: sro@mssl.ucl.ac.uk
² University of Nevada, Las Vegas, Department of Physics & Astronomy, Box 454002, 4505 Maryland Parkway, Las Vegas, NV 89154-4002, USA
³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
⁴ Space Telescope Science Center, 3700 San Martin Dr., Baltimore, MD 21218, USA
⁵ Astrophysics Science Division, Code 660.1, NASA Goddard Space Flight Centre, 8800 Greenbelt Road, Greenbelt, Maryland 20771, USA

Abstract

We examine 48 Swift/UVOT long Gamma-ray Burst light curves and find a correlation between the logarithmic luminosity at 200 s and average decay rate determined from 200 s onwards, with a Spearman rank coefficient of −0.58 at a significance of 99.998% (4.2σ). We determine the log L_200s − α _> 200s correlation to be intrinsic and discuss two possible causes: there is a property of the central engine, outflow or external medium that effects the rate of energy release so that the bright afterglows release their energy more quickly and decay faster than the fainter afterglows; alternatively, the observers viewing angle may produce the correlation, with observers at large viewing angles observing fainter and slower decaying light curves.