GCN Circular 26798

P.A. Evans (U. Leicester),  K.L. Page (U.Leicester), J.A. Kennea (PSU),
A. Tohuvavohu (U. Toronto), S.D. Barthelmy (NASA/GSFC), A.P. Beardmore
(U. Leicester), M.G. Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL),
P. Brown (TAMU), D.N. Burrows (PSU), S. Campana (INAF-OAB), S.B. Cenko
(NASA/GSFC), G. Cusumano (INAF-IASF PA), A. D'Ai (INAF-IASFPA), P.
D'Avanzo (INAF-OAB), V. D'Elia(ASDC), S.W.K. Emery (UCL-MSSL), P.
Giommi (ASI), C. Gronwall (PSU), D. Hartmann (U. Clemson), H.A. Krimm
(CRESST/GSFC/USRA), N.J. Klingler (PSU), N.P.M. Kuin (UCL-MSSL), A.Y.
Lien (GSFC/UMBC), F.E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
J.A. Nousek (PSU), S.R. Oates (U. Birmingham), P.T. O'Brien (U.
Leicester), J.P. Osborne (U. Leicester), C. Pagani (U. Leicester),
M.J.Page (UCL-MSSL), D.M. Palmer (LANL), M. Perri (ASDC), J.L. Racusin
(NASA/GSFC), B. Sbarufatti (INAF-OAB/PSU), M.H. Siegel (PSU), G.
Tagliaferri (INAF-OAB), E. Troja (NASA/GSFC/UMCP) report on behalf of
the Swift team:

Swift has carried out 456 observations of the LVC error region for the
GW trigger S200115j convolved with the 2MPZ catalogue (Bilicki et al.
2014, ApJS, 210, 9), using 23 fields from the 'bayestar' (version 0) GW
localisation map and 433 fields from the 'bayestar' (version 2) GW
localisation map. As these are 3D skymaps, galaxy distances were taken
into account in selecting which ones to observe. The observations
currently span from 7.1 ks to 164 ks after the LVC trigger, and the XRT
has covered 28.5 deg^2 on the sky (corrected for overlaps). This covers
12% of the probability in the 'bayestar' (version 2) skymap, and 24%
after convolving with the 2MPZ galaxy catalogue, as described by Evans
et al. (2016, MNRAS,  462, 1591). Using the earlier  'bayestar'
(version 0) skymap our observations cover 7.8% of the probability (16%
when convolved). These pointings and associated metadata have been
reported to the Treasure Map (Wyatt et al., arXiv 2001.00588;
http://treasuremap.space/alerts?graceids=S200115j).

2 rank 2 sources have been found. These are candidates to be the
counterpart to the GW trigger. Details of these sources are:
  
  Source S200115j_X130:
  =============
    RA: 	 44.9028 ( = 02h 59m 36.67s) J2000
    Dec:	 +7.4042 ( = +07d 24' 15.1") J2000
    Error:	 4.7 (arcsec, radius, 90% confidence).
    Peak Rate:	 2.7e-02 +/- 6.9e-03 ct/sec (0.3-10 keV)
    Peak Flux:	 1.2e-12 +/- 3.0e-13 erg cm^-2 s^-1 (0.3-10 keV)
    Catalogued:  Yes
    Cat Source:  1WGA J0259.6+0724 in the ROSAT/WGACAT catalogue
    Separation:  6.5" from the XRT source
    Cat Rate:	 4.9e-03 +/- 1.8e-03 ct/sec 
    Cat Flux:	 1.5e-13 +/- 5.5e-14 erg cm^-2 s^-1 (0.3-10 keV)
		 So the source	is 3.4-sigma above the catalogued flux.

    There is 1 GWGC or 2MPZ galaxy within 200 kpc of the source.
    
The determination that this source has brightened compared to the ROSAT
data is of course dependent on the assumed X-ray spectrum: if the
source is heavily absorbed then the brightness increase becomes less
noteworthy. Unfortunately, we have insufficient data to investigate the
spectrum at the present time. Further observations are expected.
    
  
  Source S200115j_X136:
  =============
    RA: 	 40.0508 ( = 02h 40m 12.19s) J2000
    Dec:	 -2.5626 ( = -02d 33' 45.4") J2000
    Error:	 4.7 (arcsec, radius, 90% confidence).
    Peak Rate:	 8.6e-02 +/- 1.7e-02 ct/sec (0.3-10 keV)
    Peak Flux:	 3.7e-12 +/- 7.2e-13 erg cm^-2 s^-1 (0.3-10 keV)
    Catalogued:  Yes
    Cat Source:  2SXPS J024012.0-023340 in the 2SXPS catalogue
    Separation:  4.9" from the XRT source
    Cat Rate:	 6.7e-03 +/- 1.6e-03 ct/sec 
    Cat Flux:	 2.9e-13 +/- 6.9e-14 erg cm^-2 s^-1 (0.3-10 keV)
       so the source  is 4.7-sigma above the catalogued flux.

This source has been observed multiple times by Swift-XRT before, and
its historical light curve can be seen at
https://www.swift.ac.uk/2SXPS/2SXPS%20J024012.0-023340. This source is
coincident with a 2MASS galaxy, and given the historical data, we
assume that this galaxy is an AGN. Therefore the flux increase compared
to previous Swift observations may simply reflect AGN activity, or may
indicate a transient within that galaxy (as was the case for GW 170817,
whose host galaxy was an X-ray emitter). Further observations are
expected.


In total, we have detected 14 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
  * 2 sources of rank 2
  * 3 sources of rank 3
  * 9 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   |
|  S200115j_X62  | 02h 48m 23.89s | +05d 34' 46.4" |	4.5" |
|  S200115j_X63  | 02h 47m 25.18s | +05d 29' 41.7" |	7.0" |
| S200115j_X135  | 02h 42m 11.50s | -03d 08' 13.8" |   28.4" |


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   |
|  S200115j_X40  | 02h 46m 14.64s | +05d 35' 29.9" |	4.5" |
|  S200115j_X73  |  02h 27m 0.61s | -04d 20' 20.3" |	6.9" |
|  S200115j_X74  | 02h 47m 27.22s | -00d 12' 21.5" |	3.9" |
|  S200115j_X75  | 02h 55m 14.42s | +09d 06' 26.6" |	8.3" |
|  S200115j_X76  | 02h 38m 27.56s | +01d 54' 29.5" |	4.8" |
| S200115j_X105  | 03h 57m 45.52s | +41d 55' 07.9" |	5.0" |
| S200115j_X106  | 03h 56m 57.58s | +42d 55' 44.3" |	4.4" |
| S200115j_X127  | 02h 58m 51.32s | +06d 22' 26.4" |	5.2" |
| S200115j_X134  | 02h 42m 40.74s | +00d 57' 31.5" |	4.8" |

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

The results of the XRT automated analysis, including details of the
sources listed above, are online at
https://www.swift.ac.uk/LVC/S200115j

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