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GCN Circular 17139

Subject
GRB 141202A: Swift/UVOT Detection of a Variable Source Near the IPN Error Box
Date
2014-12-05T15:43:31Z (9 years ago)
From
Frank Marshall at GSFC <femarsha@khamseen.gsfc.nasa.gov>
F. E. Marshall (NASA/GSFC) and S. B. Cenko (NASA/GSFC) 
report on behalf of the Swift/UVOT team:

The short/hard GRB 141202A was localized to a 1439 sq. arcmin
error box with the IPN (Golenetskii et al. GCN Circ. 17131), and
Swift tiled the error box with 7 fields starting
93.5 ks after the trigger. No credible afterglow was detected
with the Swift/XRT (Pagani et al. GCN Circ. 17134).

In two exposures with the U filter starting 128 ks after the trigger, 
Swift/UVOT finds a bright source just outside the IPN error region
that is not seen in the DSS or the SDSS. In addition, no source at this position
was found with the Minor Planet Checker. There is a dim SDSS
galaxy with a g' magnitude (AB) of 23.0 +/- 0.2 coincident with the UVOT source.
A 3-sigma upper limit for this galaxy in the u' band is 22.4 mag.

The preliminary UVOT position is:
   RA  (J2000) =  09:32:29.78 = 143.12408 (deg.)
   Dec (J2000) =  55:19:44.4  =  55.32899 (deg.)
with an estimated uncertainty of 0.5 arcsec. (radius, 90% confidence).

There is very marginal evidence (1.8-sigma) for variability between
the two UVOT observations. The preliminary 
AB magnitudes using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) is:

Filter         T_start(ks)   T_stop(ks)    Exp(ks)        Mag (AB)

u                127.9        128.6         0.69       20.43 +/- 0.09
u                129.4        130.3         0.88       20.69 +/- 0.13

No correction is made for the Galactic extinction
due to the reddening of E(B-V) = 0.02 in the direction of the source
(Schlegel et al. 1998).

The 3-sigma upper limit for the XRT count rate at the position of the
UVOT source is 9.6e-3 counts/s in the 0.3-10 keV band.

It would be very unusual for UVOT to find a bright optical afterglow for
a short/hard GRB so long after the trigger, especially since
XRT did not find an afterglow. The long-term increase in the optical flux by
a factor of at least 6 indicates that this may nevertheless be an 
interesting object.
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