GCN Circular 24767

N. P. M. Kuin (UCL-MSSL), A. A. Breeveld (MSSL-UCL),  C. Gronwall (PSU),
F. E. Marshall (NASA/GSFC), S. R. Oates (Uni. of Warwick), M.J. Page
(UCL-MSSL),
M. de Pasquale (U Istambul), M. H. Siegel (PSU), A. D'Ai (INAF-IASFPA),
P. D'Avanzo (INAF-OAB), S. D. Barthelmy (NASA/GSFC), A. P. Beardmore (U.
Leicester),
D. N. Burrows (PSU), S. Campana (INAF-OAB), S. B. Cenko (NASA/GSFC),
G. Cusumano (INAF-IASF PA), V. D'Elia(ASDC), P. A. Evans (U. Leicester),
P. Giommi (ASI), D. Hartmann (Clemson U.), J. A. Kennea (PSU),
H. A. Krimm (NSF), A. Y. Lien (GSFC/UMBC), D. B. Malesani (DARK/NBI),
A. Melandri (INAF-OAB), J. A.  Nousek (PSU), P. T. O'Brien (U. Leicester),
J. P. Osborne (U. Leicester), C. Pagani (U. Leicester), K. L. Page
(U.Leicester),
D.M. Palmer (LANL), M. Perri (ASDC), J. L. Racusin (NASA/GSFC), T. Sakamoto
(AGU),
B. Sbarufatti (INAF-OAB/PSU), G. Tagliaferri (INAF-OAB), A. Tohuvavohu
(PSU),
and E. Troja (NASA/GSFC/UMCP) report on behalf of the Swift team:

The Neil Gehrels Swift Observatory UVOT and XRT instruments began pointed
galaxy-targeted followup of the LIGO/Virgo detected S190425z (LVC GCN.
24168)
at 2019-04-25 12:53 UT (T0+274 min), delayed due to a commanding gap.
The observations continued until 2019-04-26 20:15 UT, when they were aborted
to begin followup of S190426c (LVC GCN. 24237).

The UVOT points at the same sky area as the Swift XRT but with a slightly
smaller FOV (17'x17'). As announced in Tohuvavohu et al.
(GCN Circ. 24353
Not found
 
 
 ), the Swift GW follow up can be seen at
http://www.swift.ac.uk/LVC/.

The Swift UVOT exposures of around 70s are usually in the u-band (central
wavelength 350nm) or a UV band when there are bright stars in the FOV. The
nominal exposure time varies depending on several factors like the slew
time.
The limiting magnitude can vary but typically is 18.6th magnitude.

Each field is automatically processed by detecting sources in the image,
matching them to several catalogs and screening the remaining sources to
remove artifacts.  A check is made for any remaining candidates against the
Minor Planet Center list of solar system objects. Candidates are ranked for
further examination. In addition, for each galaxy detected in the UVOT image
which is in the GLADE catalog, a "postage stamp" image of a small region
around
the galaxy is produced as well as a comparison "postage stamp" from the
DSS to be used for manual inspection.

After the automated processing, potential candidates are all examined
manually.
This allows for the removal of image artifacts from our candidate lists
that
sometimes get flagged as potential sources. For S190425z the automated
processing
identified 21 source candidates which all, except for one, were either
image
artifacts or fast moving objects that passed the screening.

A further examination of the "postage stamps" with galaxies is then made as
effort allows. This is done mainly because the source-finding algorithm
has difficulty finding point sources in extended sources such as galaxies.

For the fields observed for S190425z, 2298 galaxy postage stamp images were
generated. No credible sources were found.

The candidate source reported in Breeveld et al. (GCN Circ. 24296
Not found
 
 
) was
identified
initially by the automated processing, and is likely a very red flaring
object
(see also Lipunov et al. GCN Circ. 24326
Not found
 
 
).