TITLE: GCN CIRCULAR NUMBER: 24767 SUBJECT: LIGO/Virgo S190425z: Swift UVOT - no new sources identified, and a summary of the Swift UVOT processing of GW triggers. DATE: 19/06/06 22:47:28 GMT FROM: Paul Kuin at MSSL 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 ), 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) was identified initially by the automated processing, and is likely a very red flaring object (see also Lipunov et al. GCN Circ. 24326).