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GCN Circular 24296

Subject
LIGO/Virgo S190425z: Transient found in Swift/UVOT counterpart search
Date
2019-04-27T18:38:54Z (5 years ago)
From
Mike Siegel at PSU/Swift MOC <mhs18@psu.edu>
LIGO/Virgo S190425z: Transient found in Swift/UVOT counterpart search

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

We report a candidate transient found in the UVOT search results of the
LVC event S190425z (LIGO/VIRGO Collaboration GCN Circ. 24168). 

On 2019-04-26 at 18:49:22 UT Swift UVOT took a 66.8 s exposure in the u band with 
exposure ID uu577997367I (target ID 07016575) which shows a new source when compared with
archived DSS images. The new source is not listed in the Gaia DR1, 2MASS or GSC2.3 
catalogs or Pan-STARRS and is not listed as a minor planet. 

The position is: 
RA = 255.5800 deg
Dec = -12.48562 deg
which is RA=17:02:19.2, Dec=-12:29:08.2 (J2000).

The magnitude using the UVOT photometric system  (Breeveld et al. 2011, AIP 
Conf. Proc. 1358, 373) is u=17.74 +/- 0.18 mag (Vega).

No source is found at this position in the XRT, with a 3-sigma upper limit of  
5.8 x 10^-2 ct s^-1, which corresponds to a 0.3-10 keV observed flux of 
2.5 x 10^-11 erg cm^-2 s^-1.

At this position, the distance estimate from the LALInference sky map has a mean 
of 132 Mpc with a sigma of 35 Mpc. 

There is a bright galaxy in the 2MPZ catalogue (Bilicki et al., 2014) 2MPZ
J17021587-1228210 at 68 arcsec (z = 0.022, D = 99 Mpc, abs K magnitude = 12.4),
which is 33 kpc away in projection. A second galaxy, 2MPZ J17020235-1229511 lies
250 arcsec away from the UVOT position. This galaxy has a distance of 106 Mpc in
the 2MPZ catalogue so the projected offset is 128 kpc. At the distance of these
galaxies, the absolute U magnitude would be -17.3, which is brighter than AT 2017gfo at
t ~ 2 days.

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