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GCN Circular 20841

LIGO/Virgo G275697: Swift-XRT source 23 is fading
2017-03-09T14:26:52Z (7 years ago)
Phil Evans at U of Leicester <>
P.A. Evans (U. Leicester), A.A. Breeveld (UCL-MSSL), J.A. Kennea (PSU), 
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 
V.D'Elia(ASDC), P. Giommi (ASI), C. Gronwall (PSU), H.A. Krimm 
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:

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.

Source 23 (Evans et al, LVC Circ. 20812) appears to be fading. This 
source has now been observed three times, and a power-law fit to the 
light curves gives a slope of alpha=0.87 +/- 0.26 (1-sigma errors), i.e. 
the fading is confirmed with 3.5-sigma significance. Follow-up
observations of this source at other wavelengths are strongly 
encouraged. The UVOT-enhanced position of this source is:

RA (J2000.0):  21h 33m 14.96s = 323.31234 degrees
Dec (J2000.0): +39d 41' 09.2" = 39.68590 degrees

with an uncertainty of 4.4 arcsec (radius, 90% confidence).

Swift-UVOT observed this field in the u and uvw1 filters. No new source 
is detected to 3-sigma upper limits of 19.3 mag (u) and 19.9 mag (uvw1), 
but the position is just 10.7 arcsec from the B=12.3 mag source 
USNO-B1.0 1296-047182, so these upper limits are approximate.

X-ray Sources 1,8,14 and 18 have also been re-observed and are found to 
be ~constant and are therefore unlikely to be related to the GW event. 
Source 19 has been re-observed, but at present we cannot ascertain 
whether or not it is fading.

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