J/MNRAS/488/2855 SWIFT observations of gamma-ray burst afterglows (Page+, 2019)
A study of gamma-ray burst afterglows as they first come into view of the
SwiftUltraviolet and Optical Telescope.
Page M.J., Oates S.R., De Pasquale M., Breeveld A.A., Emery S.W.K.,
Kuin N.P.M., Marshall F.E., Siegel M.H., Roming P.W.A.
<Mon. Not. R. Astron. Soc., 488, 2855-2863 (2019)>
=2019MNRAS.488.2855P 2019MNRAS.488.2855P (SIMBAD/NED BibCode)
ADC_Keywords: GRB ; Positional data ; Photometry, ultraviolet ; Optical
Keywords: gamma-ray burst: general
Abstract:
We examine 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
nine 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
>0.5mag in their optical light curves, 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.
Description:
Our objective is to examine the early periods of GRB optical afterglow
light curves to determine what fraction of the sample shows
significant afterglow emission when Swift UVOT first arrives on
target. Therefore, we require a sample of GRB afterglows that (a) are
optically bright enough that we can expect to detect the afterglow in
settling observations at a significant fraction of the peak flux, (b)
were observed soon after the trigger, and (c) have redshifts so that
their observed properties can be transformed to the rest frame.
With these criteria in mind, we have started with the sample of GRBs
studied by Oates et al. (2012MNRAS.426L..86O 2012MNRAS.426L..86O). The sample of Oates et
al. (2012MNRAS.426L..86O 2012MNRAS.426L..86O) was drawn from the second UVOT GRB catalogue
(Roming et al. 2017ApJS..228...13R 2017ApJS..228...13R), which contains all of the GRBs
observed with Swift UVOT from the launch of Swift until 2010 December
25. For our study we have included only objects which have a peak V
magnitude of =<16.0, which corresponds to a sample of 23 GRBs.
All of the GRBs in the sample were observed with Swift UVOT in
automated target (AT) mode, and the analysis is restricted to those
observations obtained in the initial observing segment, roughly the
first 24h from the detection of the burst. For each GRB the UVOT
V-band sky images and event data were retrieved from the UK Swift
Science Data Centre (http://www.swift.ac.uk).
Table A1 gives the photometry for the stars in each GRB field of view.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
tablea1.dat 68 62 Photometry of bright stars within the UVOT
fields of view for the GRB afterglows used in
the study
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Byte-by-byte Description of file: tablea1.dat
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Bytes Format Units Label Explanations
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1- 11 A11 --- GRB GRB name (GRB YYMMDDA)
13- 14 I2 h RAh Right ascension (J2000)
16- 17 I2 min RAm Right ascension (J2000)
19- 23 F5.2 s RAs Right ascension (J2000)
25 A1 --- DE- Declination sign (J2000)
26- 27 I2 deg DEd Declination (J2000)
29- 30 I2 arcmin DEm Declination (J2000)
32- 35 F4.1 arcsec DEs Declination (J2000)
37- 41 F5.2 mag Vsetmag V magnitude in the settling exposure
43- 46 F4.2 mag e_Vsetmag Error on Vsetmag
48- 52 F5.2 mag Vsubmag V magnitude in the subsequent settled
exposures
54- 57 F4.2 mag e_Vsubmag Error on Vsubmag
59- 63 F5.2 mag DeltaV Magnitude difference Vsubmag-Vsetmag (1)
65- 68 F4.2 mag e_DeltaV Error on DeltaV
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Note (1): Negative values of DeltaV (ΔV) correspond to the reference star
appearing fainter in the settling image
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History:
From electronic version of the journal
(End) Ana Fiallos [CDS] 09-Dec-2022