GCN Circular 27288

A. A. Breeveld (MSSL-UCL), S. R. Oates (U. Birmingham), F. E. Marshall (NASA/GSFC), N. P. M. Kuin 
(UCL-MSSL), P. Brown (TAMU), C. Gronwall (PSU), M.J. Page (UCL-MSSL), M. De Pasquale (Istanbul U.), 
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), N. J. Klingler (PSU), H. A. Krimm (NSF), A. Y. Lien (GSFC/UMBC), 
D. B. Malesani (DTU Space), 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 (U Toronto), and E. Troja (NASA/GSFC/UMCP) 
report on behalf of the Swift team:

We report a transient found in the UVOT search results of the LVC event S200224ca (LIGO/VIRGO 
Collaboration GCN Circ. 27184
Not found
 
 
).

On 2020-02-25 at 16:05:25 UT Swift UVOT took a 75s exposure, 63.7ks after the GW trigger, in the u 
band (exposure ID uu604339530, target ID 07031750) which showed a source that had brightened 
significantly compared with archival images/catalogues. The source is not seen in the DSS, is not 
listed in the Gaia DR2, 2MASS or GSC2.3 catalogues and is not listed as a minor planet. However, a 
source is seen at this position in the Pan-STARRS catalogue (id 94241765879572233) with imag=21.6, 
and in VISTA (id 473452322969).

The position is:
RA = 176.58794 deg
Dec = -11.46508 deg
which is RA=11:46:21.11, Dec=-11:27:54.3 (J2000).

The magnitude using the UVOT photometric system  (Breeveld et al. 2011, AIP  Conf. Proc. 1358, 373) 
is u=18.6 +/- 0.2 mag (Vega). In a followup observation with all filters, 275ks after the GW 
trigger, the object had faded to u>19.6 mag, with upper limits in all filters.

Filter    Date-obs              Exp(s)       Mag

u         2020-02-25T16:05:25    75           18.6 +/- 0.2
v         2020-02-28T02:44:02   334          >19.5
b         2020-02-28T02:48:35   122          >19.9
u         2020-02-28T02:47:27   143          >19.6
w1        2020-02-28T02:46:19   307          >19.7
m2        2020-02-28T02:45:10   333          >19.7
w2        2020-02-28T02:42:54   333          >19.9

No source is found at this position in the XRT, with a 3-sigma upper limit of  4.75 x 10-3 s^-1 
which corresponds to a 0.3-10 keV observed flux of 1.9e-13 erg cm^-2 s^-1.

At this position, the distance estimate from the LALInference sky map has a mean of 1556.46 Mpc with 
a sigma of 320.357 Mpc.

This circular is an official product of the Swift team.