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Further Swift-XRT observations of IceCube 170922A

ATel #10792; 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:
on 28 Sep 2017; 11:57 UT
Credential Certification: Phil Evans (pae9@star.le.ac.uk)

Subjects: X-ray, Quasar, Variables

Referred to by ATel #: 10794, 10799, 10817, 10830, 10838, 10840, 10844, 10861

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 #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 (Γ) is 2.50 [+0.23, -0.12], similar to the historical value in 1SXPS: Γ = 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, Γ 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.