J/MNRAS/505/2662 GRB 141220A light curves (Jordana-Mitjans+, 2021)
Coherence scale of magnetic fields generated in early-time forward shocks
of GRBs.
Jordana-Mitjans N., Mundell C.G., Smith R.J., Guidorzi C., Marongiu M.,
Kobayashi S., Gomboc A., Shrestha M., Steele I.A.
<Mon. Not. R. Astron. Soc., 505, 2662-2674 (2021)>
=2021MNRAS.505.2662J 2021MNRAS.505.2662J (SIMBAD/NED BibCode)
ADC_Keywords: Gamma rays ; GRB ; Photometry ; Optical
Keywords: magnetic fields - polarization -
gamma-ray burst: individual: grb 141220A - ISM: jets and outflows
Abstract:
We report the earliest ever detection of optical polarization from a
GRB forward shock (GRB 141220A), measured 129.5-204.3s after the
burst using the multicolour RINGO3 optical polarimeter on the 2-m
fully autonomous robotic Liverpool Telescope. The temporal decay
gradient of the optical light curves from 86 to ∼2200s post-burst is
typical of classical forward shocks with α=1.091±0.008. The
low-optical polarization PBV=2.8-1.6+2.0 per cent (2σ) at
mean time ∼168s post-burst is compatible with being induced by the
host galaxy dust (AV,HG=0.71±0.15mag), leaving low polarization
intrinsic to the GRB emission itself, as theoretically predicted for
forward shocks and consistent with previous detections of low degrees
of optical polarization in GRB afterglows observed hours to days after
the burst. The current sample of early-time polarization data from
forward shocks suggests polarization from (a) the Galactic and host
galaxy dust properties (i.e. P∼1-3 per cent), (b) contribution from a
polarized reverse shock (GRB deceleration time, jet magnetization) or
(c) forward shock intrinsic polarization (i.e. P≤2 per cent), which
depends on the magnetic field coherence length-scale and the size of
the observable emitting region (burst energetics, circumburst
density).
Description:
Here, we present the GRB 141220A optical photometry corresponding to
the Liverpool Telescope (LT) RINGO3 BV,R,I bands, SkyCam-Z
r-equivalent band, and IO:O r band. A bright optical afterglow of
14.84±0.17 mag was detected 86s after BAT trigger at the GRB 141220A
location by the 0.2-m SkyCam-Z telescope, attached to the 2-m LT. At
129.5s post-burst, the LT reacted automatically to Swift alert with
the three-band optical polarimeter and imager RINGO3 and started
observations. LT observations consisted of 3x10min epochs of RINGO3
instrument, followed by 6x10s frames with the r band of the Optical
Wide Field Camera (IO:O) and 7x10min more with RINGO3.
The tabulated data includes the bandpass name, the mean observing time
corrected by T0 (Swift trigger time), the length of the observing
window and the GRB magnitudes and flux density with their
uncertainties. Note that the SkyCam-Z white-band photometry has been
standarized in the r SDSS band. The magnitudes and the flux density
are corrected by Galactic extinction (reddening
E(B-V)=0.0128±0.0005; Schlegel et al., 1998ApJ...500..525S 1998ApJ...500..525S),
but not by host galactic extinction (E(B-V)=0.24±0.06).
Objects:
------------------------------------------------------------
RA (2000) DE Designation(s)
------------------------------------------------------------
13 00 15.76 +32 08 47.0 GRB 141220A = Fermi bn141220252
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 50 908 GRB 141220A optical photometry
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Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 9 A9 --- Band Bandpass name (1)
11- 16 F6.1 s tmid Mean observing time (2)
18- 21 F4.1 s texp/2 Half of the length of the observation window
23- 27 F5.2 mag mag Magnitudes in Band (3)
29- 32 F4.2 mag e_mag Magnitudes uncertainty
34- 41 E8.2 Jy Flux Flux density in Band (3)
43- 50 E8.1 Jy e_Flux Flux density uncertainty
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Note (1): Bands as follows:
BV = Liverpool Telescope RINGO3 BV
r_SkyCamZ = Liverpool Telescope SkyCam-Z r-equivalent band
r_IOO = Liverpool Telescope IO:O r
R = Liverpool Telescope RINGO3 R
I = Liverpool Telescope RINGO3 I
Note (2): tmid corresponds to the mean observing time less T0,
where T0 is BAT trigger time, 20 December 2014 at 06:02:52.7 Universal Time.
Note (3): The photometry is corrected for Galactic extinction, with reddening
E(B-V)= 0.0128±0.0005. The photometry is not corrected for host galaxy
extinction.
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Acknowledgements:
Nuria Jordana-Mitjans, N.Jordana(at)bath.ac.uk
(End) Patricia Vannier [CDS] 21-Nov-2022