GCN Circular 40130

P.A. Evans (U. Leicester) and J.A. Kennea (PSU) report on behalf of the Swift-XRT team,

Swift-XRT observed the field of IceCube-250406A (IceCube Collaboration, GCN 40103
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 ) between 22:21 UT on 2025 April 7 and 03:36 UT on 2025 April 8 (85 to 104 ks after the trigger), gathering 4.1 ks over 7 pointings centred on the position from the refined position notice.

Three X-ray sources are detected, all of which have counterparts in the 1RXS Rosat catalogue (Voges et al., 1999) and show no sign of elecated flux compared to those data. The sources are:

Source 1: Swift J052456.1+170357 at RA, Dec = 81.23375, +17.0660 which is equivalent to: 

RA (J2000):   05h 24m 56.10s
Dec (J2000): +17d 03' 57.8" 

with an uncertainty of 4.7 arcsec (radius, 90% confidence), and consistent with the position of 1RXS J052456.3+170400. The mean 0.3-10 keV count-rate is 0.030 (+/-0.007)  ct/sec, which corresponds to 1.3 (+/-0.3) x 10^-12 erg/cm^2/s.


Source 2: Swift J052427.1+172323 at RA, Dec = 81.11301, +17.3898 which is equivalent to: 

RA (J2000):   05h 24m 27.12s
Dec (J2000): +17d 23' 23.2" 

with an uncertainty of 5.7 arcsec (radius, 90% confidence), and consistent with the position of 1RXS J052425.7+172301. The mean 0.3-10 keV count-rate is 0.051 (+0.016, -0.013) ct/sec, which corresponds to 2.2 (+0.7, -0.6) x 10^-12 erg/cm^2/s.

Source 3: Swift J052425.1+172240 at RA, Dec = 81.10483, +17.3780 which is equivalent to: 

RA (J2000):   05h 24m 25.16s
Dec (J2000): +17d 22' 40.9" 

with an uncertainty of 8.0 arcsec (radius, 90% confidence), and consistent with the position of 1RXS J052425.7+172301 . The mean 0.3-10 keV count-rate is 0.042 (+0.015, -0.012) ct/sec, which corresponds to 1.8 (+0.6, -0.5) x 10^-12 erg/cm^2/s.


Note that sources 2 and 3 correspond to the same Rosat source (due to the comparitively large Rosat position errors). In fact,  neither source looks particularly point-like and both are within 30" of * 111 Tau -- a fifth magnitude BY Dra star. At this brightness the star will cause optical loading -- the detection of spurious 'X-rays' which are actually an accumulation of optical photons. We therefore view both of these sources with scepticism.

A fourth source is also detected, Swift J052440.8+163236 at RA, Dec = 81.17009, 16.5435 degrees and an uncertainty of 8.4 arcsec (radius, 90% confidence). However, this is classified "poor" which corresponds approximately to a 1-sigma detection and is thus likely spurious.

The full XRT analysis can be viewed online at https://www.swift.ac.uk/neutrino/NEUTRINO_FIELD00043/.