GCN Circular 20884

P.A. Evans (U. Leicester), J.A. Kennea (PSU), S.D. Barthelmy
(NASA/GSFC), A.P. Beardmore (U. Leicester), A.A. Breeveld (UCL-MSSL),
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), P. Giommi (ASI), C. Gronwall (PSU), H.A. Krimm
(CRESST/GSFC/USRA), N.P.M. Kuin (UCL-MSSL), A.Y. Lien (GSFC/UMBC), D.
Malesani (DARK/NBI), F.E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
B. Mingo (U. Leicester),  J.A. Nousek (PSU), S.R. Oates (U. Warwick),
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), B. Sbarufatti (INAF-OAB/PSU), M.H.
Siegel (PSU), G. Tagliaferri (INAF-OAB), E. Troja (NASA/GSFC/UMCP)
report on behalf of the Swift team:

As discussed in LVC Circular 20841, Swift is conducting further
observations of the 'rank 3' X-ray sources detected in our initial search
of the LIGO/VIRGO error region. These are uncatalogued X-ray sources which
are below existing catalogue limits and therefore cannot be distinguished
as new or pre-existing sources.

Previously, Source 23 (Evans et al, LVC Circ. 20812, 20841) appeared to be
fading with a significance of 3.5 sigma. Swift subsequently performed a
series of daily observations, between 2017-03-13 and 2017-03-16, which
have shown that the source flux is remaining approximately constant, so is
unlikely to be related to the GW event and is therefore no longer
considered a source of interest.

In addition, sources 30, 88, 100 and 108 also show no signs of
variability. Source 89 matches a catalogued object which appears to be a
flare star, so the X-ray detection was most likely simply a stellar flare.
Observations of the other 'rank 3' sources are ongoing.

This circular is an official product of the Swift-XRT team.