[vsnet-grb-info 24592] Swift follow-up observations of IceCube-200107A: Identification of X-ray high state for 4FGL J0955.1+3551

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  26691
SUBJECT: Swift follow-up observations of IceCube-200107A: Identification of X-ray high state for 4FGL J0955.1+3551
DATE:    20/01/09 19:51:46 GMT
FROM:    Derek B. Fox at Penn State  <dbf11 at psu.edu>

F. Krauss, T. Gregoire, D.B. Fox, J. Kennea (PSU) and P. Evans
(U. Leicester) report for AMON (https://amon.psu.edu/):

"We have observed the positions of the two Fermi/LAT sources (Garrappa
et al., GCN Circ. 26669), positionally coincident with the IceCube
neutrino event IceCube-200107A (GCN Circ. 26655) with the Neil Gehrels
Swift Observatory X-ray Telescope (XRT). We detect the blazar 4FGL
J0955.1+3551 with an observed 2-10 keV flux of 4.7 (+/- 0.8) x 10^-12
erg cm-2 s-1 (2-10 keV). A spectrum constructed from these data can be 
well described by an absorbed power law with N_H = 5.3 x 10^20 cm-2
(wilm abundances, vern cross-sections), and a photon index of 1.8
(+0.23, -0.13). These data have also been reported in ATel #13394. 

This object has been previously observed by Swift/XRT, and its
historical behavior can be seen in the 2SXPS catalogue (Evans et al.,
2019):  https://www.swift.ac.uk/2SXPS/2SXPS%20J095507.8%2B355100
Our recent observations are significantly brighter than the average
flux in archival data: 1.78 (+0.14, -0.13) x 10-12 erg cm-2 s-1
(observed 2-10 keV flux), although the historical light curve does
show that the source flux was gradually increasing in 2012 to
2013. The catalogued ROSAT flux of this source (as 1RXS
J095508.2+355054) is 0.11 ct/sec which, assuming the above spectrum,
corresponds to a 2-10 keV observed flux of ~1.9 x 10-12 erg cm-2 s-1;
therefore we suggest that this source is currently in a high
state. Further observations of this source are strongly recommended. 

The other source reported in GCN Circ. 26669 (4FGL J0957.8+3423) was
not detected in our observations, with a 3-sigma upper limit of 0.010
ct/sec (0.3-10 keV). Assuming a standard spectrum (N_H = 3 x 10^20
cm-2, Gamma=1.7) this corresponds to a observed 2-10 keV flux upper
limit of 2.7 x 10-13 erg cm-2 s-1. This source has not been previously
covered by Swift/XRT observations."