LIGO/Virgo S240919bn

K.L. Page (U. Leicester), 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), 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 217 observations of the LVC error region for the
GW trigger S240919bn convolved with the 2MPZ catalogue (Bilicki et al.
2014, ApJS, 210, 9), using 139 fields from the 'BAYESTAR' GW
localisation map and 78 fields from the 'ligo-skymap-from-samples' GW
localisation map. As these are 3D skymaps, galaxy distances were taken
into account in selecting which ones to observe. The observations span
from 3.2 ks to 92 ks after the LVC trigger, and the XRT has covered 6.6
deg^2 on the sky (corrected for overlaps). This covers 55% of the
probability in the 'ligo-skymap-from-samples' (version
ligo-skymap-from-samples.fits.gz) skymap, and 55% after convolving with
the 2MPZ galaxy catalogue, as described by Evans et al. (2016, MNRAS, 
462, 1591). Using the earlier  'BAYESTAR' (version BAYESTAR.fits.gz)
skymap our observations cover 46% of the probability (45% when
convolved). 

We have detected 10 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
  * 6 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   |
|  S240919bn_X1  | 10h 26m 54.08s | -79d 50' 18.5" |	5.3" |
|  S240919bn_X3  | 10h 25m 18.06s | -79d 58' 45.0" |   43.1" |
|  S240919bn_X4  | 10h 37m 19.30s | -79d 30' 10.3" |	6.9" |
|  S240919bn_X9  | 10h 24m 36.66s | -79d 58' 08.5" |	7.3" |
| S240919bn_X10  | 03h 33m 11.93s | +37d 44' 32.6" |	8.3" |
| S240919bn_X11  | 03h 35m 10.00s | +36d 46' 56.8" |   41.6" |


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   |
|  S240919bn_X2  | 03h 33m 18.93s | +37d 18' 10.8" |	4.5" |
|  S240919bn_X5  | 03h 34m 17.94s | +35d 46' 04.0" |	8.2" |
|  S240919bn_X6  | 03h 33m 10.29s | +36d 33' 47.7" |	7.9" |
|  S240919bn_X7  | 03h 34m 29.02s | +37d 14' 55.3" |	6.4" |

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/S240919bn

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

S. Mandhai, A. Kumar, D. O'Neill, B. P. Gompertz, G. Ramsay, R. Starling, K. Ackley; M. J. Dyer; J. Lyman; K. Ulaczyk; F. Jimenez-Ibarra; D. Steeghs; D. K. Galloway; V. Dhillon; P. O'Brien; K. Noysena; R. Kotak; R. P. Breton; L. K. Nuttall; E. Palle and D. Pollacco report on behalf of the GOTO collaboration:

The Gravitational-wave Optical Transient Observer (GOTO; Steeghs et al. 2022; Dyer et al 2024) serendipitously covered a part of the localisation region of the  GW trigger S240919bn (LIGO Scientific Collaboration, GCN 37552
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 , GCN 37559
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 ) in survey mode on 2024-09-20 from 02:03:57 UT. The observations covered the location of 6 out of the 10 X-ray sources detected by the Swift XRT (Page et al. GCN 37589
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 ).  Each observation consisted of 4x45s exposures in the GOTO L-band (400-700 nm).

Images were processed immediately after acquisition using the GOTO pipeline. Difference imaging was performed using deeper template observations of the same pointings.

No new candidate optical counterparts are identified in the difference images at the locations of the below-mentioned X-ray sources detected by the Swift-XRT (Page et al. GCN 37589
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 ). The upper limits are as follows:

Event                 Date           UT          t-t0 (hours)    3-sigma UL (mag)
========================================================================================

Rank 3 sources
-------------------
S240919bn_X10          2024-09-20    02:06:52      19.85            >19.26
S240919bn_X11          2024-09-20    02:03:57      19.80            >19.23

Rank 4 sources
-------------------
S240919bn_X5           2024-09-20    02:03:57      19.80            >19.24
S240919bn_X6           2024-09-20    02:03:57      19.80            >19.24	
S240919bn_X7           2024-09-20    02:06:52      19.85            >19.20
S240919bn_X2           2024-09-20    02:06:52      19.85            A source in the difference images but pre-gw event detections. Not related to S240919bn.

Magnitudes were calibrated using ATLAS-REFCAT2 (Tonry et al. 2018) and were not corrected for Galactic extinction.

GOTO (https://goto-observatory.org) is a network of telescopes that is principally funded by the STFC and operated at the Roque de los Muchachos Observatory on La Palma, Spain, and Siding Spring Observatory in NSW, Australia, on behalf of a consortium including the University of Warwick, Monash University, Armagh Observatory & Planetarium, the University of Leicester, the University of Sheffield, the National Astronomical Research Institute of Thailand (NARIT), the University of Turku, the University of Portsmouth, the University of Manchester and the Instituto de Astrofisica de Canarias (IAC).