[vsnet-grb-info 19863] Further Swift-XRT observations of IceCube 170922A

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  21941
SUBJECT: Further Swift-XRT observations of IceCube 170922A
DATE:    17/09/28 11:58:48 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

P.A. Evans (U. Leicester) A. Keivani (PSU), J.A. Kennea (PSU),  D.B. Fox
(PSU), D.F. Cowen (PSU), J.P. Osborne (U. Leicester), and F.E.
Marshall (GSFC) report on behalf of the Swift-IceCube collaboration:

Fermi-LAT has reported a gamma-ray source (blazar), TXS 0506+056 (3FGL
J0509.4+0541 / 3FHL J0509.4+0542) which is located inside the IceCube-170922A
event error region (Kopper & Blaufuss, GCN Circ. 21916) and is flaring above 800 MeV
(Tanaka et al., ATEL #10791). This source is
also observed in our Swift-XRT follow-up of IceCube-170922A (Source 2, 1SXPS
J050925.9+054134 in the 1SXPS catalogue), reported previously by Keivani et al. (GCN #21930)

We conducted a further 5 ks observation of this Source with Swift, beginning at
2017 Sep 27 at 18:52 UT (4.95 d after the neutrino event). In these data the
X-ray source has brightened since we the original observations. The current
spectral photon index (gamma) is 2.50 [+0.23, -0.12], similar to the historical
value in 1SXPS: gamma = 2.32 [+0.33, -0.29]
(http://www.swift.ac.uk/1SXPS/1SXPS%20J050925.9%2B054134; Evans et al. 2014). In
our initial observations following the neutrino trigger, gamma was marginally harder
but with large uncertainty: 1.9 [+0.8, -0.7]. The hardness ratio light curve of
the observations taken since the neutrino trigger also shows evidence for
spectral softening between the two epochs, suggesting that the source is
undergoing spectral evolution.