A statistical study of gamma-ray burst afterglows measured by the Swift Ultra-violet Optical Telescope

Abstract

We present the first statistical analysis of 27 UVOT optical/ultra-violet lightcurves of GRB afterglows. We have found, through analysis of the lightcurves in the observer's frame, that a significant fraction rise in the first 500s after the GRB trigger, that all lightcurves decay after 500s, typically as a power-law with a relatively narrow distribution of decay indices, and that the brightest optical afterglows tend to decay the quickest. We find that the rise could either be produced physically by the start of the forward shock, when the jet begins to plough into the external medium, or geometrically where an off-axis observer sees a rising lightcurve as an increasing amount of emission enters the observers line of sight, which occurs as the jet slows. We find that at 99.8% confidence, there is a correlation, in the observed frame, between the apparent magnitude of the lightcurves at 400s and the rate of decay after 500s. However, in the rest frame a Spearman Rank test shows only a weak correlation of low statistical significance between luminosity and decay rate. A correlation should be expected if the afterglows were produced by off-axis jets, suggesting that the jet is viewed from within the half-opening angle theta or within a core of uniform energy density theta_c. We also produced logarithmic luminosity distributions for three rest frame epochs. We find no evidence for bimodality in any of the distributions. Finally, we compare our sample of UVOT lightcurves with the XRT lightcurve canonical model. The range in decay indices seen in UVOT lightcurves at any epoch is most similar to the range in decay of the shallow decay segment of the XRT canonical model. However, in the XRT canonical model there is no indication of the rising behaviour observed in the UVOT lightcurves.

16 pages, 9 figures, accepted MNRAS

References in corpus (12)

• A Comprehensive Analysis of the Swift/XRT Data: II. Diverse Physical Origins of the Shallow Decay Segment
• REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination
• The First Swift BAT Gamma-Ray Burst Catalog
• Taxonomy of GRB optical light-curves: identification of a salient class of early afterglows
• The Remarkable Afterglow of GRB 061007: Implications for Optical Flashes and GRB Fireballs
• Global Properties of X-Ray Flashes and X-Ray-Rich Gamma-Ray Bursts Observed by Swift
• The Early-time Optical Properties of Gamma-Ray Burst Afterglows
• On the existence of a reverse shock in magnetized GRB ejecta
• Jet breaks at the end of the slow decline phase of Swift GRB lightcurves
• The Two-Component Afterglow of Swift GRB 050802
• Early emission of rising optical afterglows: The case of GRB 060904B and GRB 070420
• Optical afterglow luminosities in the Swift epoch: confirming clustering and bimodality

Cited by in corpus (15)

• The optically unbiased GRB host (TOUGH) survey. I. Survey design and catalogs
• Astrophysical Ionizing Radiation and the Earth: A Brief Review and Census of Intermittent Intense Sources
• Multiwavelength observations of GRB 110731A: GeV emission from onset to afterglow
• Phenomenology of Reverse-Shock Emission in the Optical Afterglows of Gamma Ray Bursts
• A Correlated Study of Optical and X-ray Afterglows of GRBs
• GRB 090902B: afterglow observations and implications
• Optical and X-ray Rest-frame Light Curves of the BAT6 sample
• A detailed study of the optical attenuation of gamma-ray bursts in the Swift era
• GROND coverage of the main peak of Gamma-Ray Burst 130925A
• Challenging GRB models through the broadband dataset of GRB060908
• Very Bright Prompt and Reverse Shock Emission of GRB 140512A
• A Large Catalog of Homogeneous Ultra-Violet/Optical GRB Afterglows: Temporal and Spectral Evolution
• Smooth optical self similar emission of gamma-ray bursts
• A study of gamma-ray burst afterglows as they first come into view of the Swift Ultraviolet and Optical Telescope
• Constraining External Reverse Shock Physics of GRBs from ROTSE-III Limits