GCN Circular 37527

P.A. Evans (U. Leicester), J.A. Kennea (PSU), A. Tohuvavohu (U.
Toronto), S.B. Cenko (NASA/GSFC), R.A.J. Eyles-Ferris (U. Leicester),
K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), M.G.
Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL), S. Campana (INAF-OAB),
J.J. Delaunay (PSU), M. De Pasquale (University of Messina), S.
Dichiara (PSU), P. D’Avanzo (INAF-OAB), A. D’Aì (INAF-IASFPA) , V.
D’Elia (ASI-SSDC & INAF-OAR), C. Gronwall (PSU), D. Hartmann (Clemson
University), N. Klingler (NASA-GSFC / UMBC / CRESST II), N.P.M. Kuin
(UCL-MSSL), S. Laha (NASA/GSFC), S.R. Oates (U. Birmingham), J.P.
Osborne (U. Leicester), P. O’Brien (U. Leicester), M.J. Page
(UCL-MSSL), G. Raman (PSU) S. Ronchini (PSU), T. Sbarrato (INAF-OAB),
B. Sbarufatti (INAF-OAB), M.H. Siegel (PSU), G. Tagliaferri (INAF-OAB),
E. Troja (U Tor Vergata, INAF) report on behalf of the Swift team:

Swift has carried out 309 observations of the LVC error region for the
GW trigger S240915b convolved with the 2MPZ catalogue (Bilicki et al.
2014, ApJS, 210, 9), using the 'ligo-skymap-from-samples' (version
ligo-skymap-from-samples.fits.gz) GW localisation map. As this is a 3D
skymap, galaxy distances were taken into account in selecting which
ones to observe. The observations currently span from 22 ks to 82 ks
after the LVC trigger, and the XRT has covered 17.7 deg^2 on the sky
(corrected for overlaps). This covers 71% of the probability in the
'ligo-skymap-from-samples' (version ligo-skymap-from-samples.fits.gz)
skymap, and 71% after convolving with the 2MPZ galaxy catalogue, as
described by Evans et al. (2016, MNRAS,  462, 1591).

We have detected 8 X-ray sources. Each source is assigned a rank of 1-4
which describes how likely it is to be related to the GW trigger, with
1 being the most likely and 4 being the least likely. The ranks are
described at https://www.swift.ac.uk/ranks.php.

We have found:

  * 0 sources of rank 1
  * 0 sources of rank 2
  * 4 sources of rank 3
  * 4 sources of rank 4


RANK 3 sources
==============

These are uncatalogued X-ray sources, however they are not brighter
than previous upper limits, so do not stand out as likely counterparts
to the GW trigger.

| Source ID	 | RA		  | Dec 	   | Err90   |
|   S240915b_X1  | 23h 02m 37.75s | +71d 35' 58.4" |	7.5" |
|   S240915b_X4  | 07h 31m 44.23s | -50d 15' 10.9" |	9.4" |
|   S240915b_X5  | 19h 12m 55.80s | +69d 05' 04.2" |	7.5" |
|  S240915b_X10  | 07h 30m 08.97s | -50d 39' 29.4" |	7.6" |


RANK 4 sources
==============

These are catalogued X-ray sources, showing no signs of outburst
compared to previous observations, so they are not likely to be related
to the GW trigger.

| Source ID	 | RA		  | Dec 	   | Err90   |
|   S240915b_X3  | 07h 39m 54.54s | -49d 25' 34.3" |	6.4" |
|   S240915b_X6  | 19h 39m 01.84s | +69d 50' 19.6" |	6.3" |
|   S240915b_X9  | 07h 35m 52.77s | -51d 07' 12.8" |	5.6" |
|  S240915b_X11  | 19h 46m 36.70s | +70d 46' 02.5" |	8.1" |

For all flux conversions and comparisons with catalogues and upper
limits from other missions, we assumed a power-law spectrum with
NH=3x10^20 cm^-2, and photon index (Gamma)=1.7, unless otherwise
stated.

The results of the XRT automated analysis, including details of the
sources listed above, are online at
https://www.swift.ac.uk/LVC/S240915b

This circular is an official product of the Swift XRT team.