GCN Circular 7145
Subject
GRB 070429B: Possible UVOT Detection of an Optical Afterglow
Date
2007-12-10T15:32:47Z (17 years ago)
From
Stephen Holland at USRA/NASA/GSFC/SSC <sholland@milkyway.gsfc.nasa.gov>
Holland, S. T. (CRESST/USRA/GSFC), de Pasquale, M. (UCL/MSSL), and
Markwardt, C.~B. (CRESST/GSFC/UMD) report on behalf of the Swift/UVOT
team:
The Swift/UVOT observed the field of the short-hard GRB 070429B
starting 230 s after the BAT trigger (Markwardt, et al., GCN
Circ. 6358). We have re-examined this data in light of the redshift
for the Antonelli, et al. (GCN Circ. 6372) source "A" reported by
Perley, et al. (GCN Circ. 7140). We find weak evidence in the UVOT
data for a fading source located inside the refined XRT error circle
(Beardmore, et al., GCN Circ. 6360) at
RA = 21:52:03.68 (J2000)
Dec = -38:49:43.6 (J2000)
with an uncertainty of 0.6 arcsec (90% confidence). This is 0.6
arcsec from the ground-based position of source "A". For a redshift
of z = 0.904, and (H_0,Omega_M,Omega_Lambda) = (70,0.3,0.7) this
corresponds to 4.7 proper kpc. This possible source is detected with
a 3.9-sigma significance in a summed image (summed over all seven
filters) between 591 and 2661 s after the BAT trigger. It is not
detected in subsequent all-filter summed images. This possible UVOT
source has the following magnitudes and upper limits. All magnitudes
are on the UVOT system described in Poole, et al. (2007, MNRAS, in
press).
Filter T_start T_stop Exposure Mag Err Significance
v 2002 2661 94 19.3 0.4 2.5
6805 7004 393 >20.1 3-sigma upper limit
b 1460 2598 156 20.5 0.5 2.2
34,978 35,884 885 >22.0 3-sigma upper limit
u 403 42,285 2260 >21.8 3-sigma upper limit
uvw1 379 41,789 3075 >22.4 3-sigma upper limit
uvm2 354 20,226 2192 >22.0 3-sigma upper limit
uvw2 456 30,222 2378 >22.4 3-sigma upper limit
white 591 977 117 21.6 0.5 2.2
6395 8027 393 >22.1 3-sigma upper limit
The observations where the suspected source is weakly detected
are all at less than one hour after the BAT trigger. The earliest
reported ground-based detection of source "A" is Cucchiara, et
al. (GCN Circ. 6368) who find r = 23.18 +/- 0.15 at approximately 4
hours. Perley, et al. (GCN Circ. 7140) find R = 23.24 +/- 0.05 at
approximately 7 months, which implies that the source did not fade,
and conclude that the "A" source is the host galaxy. They also find g
= 24.29 +/- 0.14 at this time. If we assume that the SDSS g band is
approximately similar to the UVOT v band then the expected v magnitude
of the host at 2332 s (assuming a constant magnitude) is v ~= 24.3,
which is 5 mag fainter than the UVOT detection at that time. This
corresponds to a mean decay index of alpha ~= -2.5.
The observed decay power-law indices from the UVOT data are
alpha_v <= -0.6 +/- 0.3, alpha_b <= -0.5 +/- 0.2, and alpha_white <=
-0.2 +/- 0.2. The shallow decay slope in the white filter, and the
faintness of the White detection at early time (t_mid = 784 s)
relative to the b and v detections at later times (t_mid = 2332 (v),
2029 (b)) suggest that the afterglow candidate may have increased in
luminosity at some point between approximately 784 and 2029 s.
We do not detect any source at the position of source "B"
(Antonelli, et al., GCN Circ. 6372) down to the following limiting
magnitudes.
Filter T_start T_stop Exposure 3-Sigma Upper Limit
v 230 31,793 2011 >21.5
b 427 35,115 1463 >22.3
u 403 42,285 2260 >21.9
uvw1 379 41,789 3075 >22.4
uvm2 354 20,226 2192 >22.0
uvw2 456 30,222 2378 >22.4
white 247 36,269 1088 >22.7
The values quoted above are not corrected for the expected
Galactic extinction corresponding to a reddening of E_{B-V} = 0.03 mag
(Schlegel, et al. 1998).