A study of gamma-ray burst afterglows as they first come into view of the Swift Ultraviolet and Optical Telescope

Abstract

We examine the the emission from optically bright gamma-ray burst (GRB) afterglows as the Ultraviolet and Optical Telescope (UVOT) on the Neil Gehrels Swift Observatory first begins observing, following the slew to target the GRB, while the pointing of the Swift satellite is still settling. We verify the photometric quality of the UVOT settling data using bright stars in the field of view. In the majority of cases we find no problems with the settling exposure photometry, but in one case we excise the first second of the exposure to mitigate a spacecraft attitude reconstruction issue, and in a second case we exclude the first second of the exposure in which the UVOT photocathode voltage appears to be ramping up. Of a sample of 23 afterglows which have peak V magnitudes <16, we find that all are detected in the settling exposures, when Swift arrives on target. For 9 of the GRBs the UVOT settling exposure took place before the conclusion of the prompt gamma-ray emission. Five of these GRBs have well defined optical peaks after the settling exposures, with rises of >0.5 mag in their optical lightcurves, and there is a marginal trend for these GRBs to have long T90. Such a trend is expected for thick-shell afterglows, but the temporal indices of the optical rises and the timing of the optical peaks appear to rule out thick shells.

arXiv commentaccepted for publication in MNRAS

References in corpus (23)

• The Swift X-ray Telescope
• The Swift Ultra-Violet/Optical Telescope
• The Burst Alert Telescope (BAT) on the Swift MIDEX Mission
• Unsteady Outflow Models for Cosmological Gamma-Ray Bursts
• Photometric Calibration of the Swift Ultraviolet/Optical Telescope
• The Internal-Collision-Induced Magnetic Reconnection and Turbulence (ICMART) Model of Gamma-Ray Bursts
• Gamma-Ray Bursts: Progress, Problems & Prospects
• Predictions for The Very Early Afterglow and The Optical Flash
• Dissipative Photosphere Models of Gamma-ray Bursts and X-ray Flashes
• Light Curves of GRB Optical Flashes
• REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination
• Paper II: Calibration of the Swift ultraviolet/optical telescope
• A statistical study of gamma-ray burst afterglows measured by the Swift Ultra-violet Optical Telescope
• Looking Into the Fireball: ROTSE-III and Swift Observations of Early GRB Afterglows
• GRB 061121: Broadband spectral evolution through the prompt and afterglow phases of a bright burst
• The Onset of Gamma-Ray Burst Afterglow
• Optical afterglows of Gamma-Ray Bursts: peaks, plateaus, and possibilities
• The BeppoSAX catalog of GRB X-ray afterglow observations
• A correlation between intrinsic brightness and average decay rate of Swift UVOT GRB optical/UV light curves
• The use and calibration of read-out streaks to increase the dynamic range of the Swift Ultraviolet/Optical Telescope
• The measurement errors in the Swift-UVOT and XMM-OM
• A Large Catalog of Homogeneous Ultra-Violet/Optical GRB Afterglows: Temporal and Spectral Evolution
• Peaks of optical and X-ray afterglow light-curves

Cited by in corpus (4)

• Fires in the deep: The luminosity distribution of early-time gamma-ray-burst afterglows in light of the Gamow Explorer sensitivity requirements
• Swift/UVOT: 18 Years of Long GRB Discoveries and Advances
• Dust around massive stars is agnostic to galactic environment: New insights from PHAT/BEAST
• A two-component jet model for the optical plateau in the afterglow of GRB 191221B