GCN Circular 18231

D. Dornic (CPPM), S. Basa (LAM), P.A. Evans (U. Leicester), J.A. Kennea 
(PSU), J.P. Osborne (U. Leicester) and V. Lipunov (MSU) report on behalf 
of the ANTARES Collaboration, the Swift-XRT team and MASTER team:

September 1st, 2015, at 07:38:25 UT, ANTARES has detected a bright 
neutrino at a location of:
   RA(J2000) = 16h 25m 42s
   DEC (J2000) = -27d 23m 24s
with an uncertainty of 18 arcmin (radius, 50% containment)

A target of opportunity alert has been sent immediately to Swift. The 
XRT onboard Swift followed the ANTARES error box 10 hours after the 
neutrino detection. An uncatalogued X-ray source has been detected above 
the limit of RASS, with the flux varying between 5e-13 and 1.4e-12 erg 
cm^-2 s^-1 (0.3-10 keV), at location:
   RA(J2000) = 16h 26m 2.12s
   DEC (J2000) = -27d 18m 14.8s
with an uncertainty of 2.4 arcsec (radius, 90% containment).

The detected X-ray source seems to be variable. By contrast no 
transient source in the visible domain with MASTER SAAO has been 
observed so far until the magnitude 18.5 with a galactic extinction of 
2 (Schlegel et al). Further Swift observations have been planned.

We encourage strongly further multi-wavelength observations to identify 
this X-ray source.