[vsnet-grb-info 24756] LIGO/Virgo S200115j: Further Swift-XRT sources

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  26855
SUBJECT: LIGO/Virgo S200115j: Further Swift-XRT sources
DATE:    20/01/22 16:27:48 GMT
FROM:    Phil Evans at U of Leicester  <pae9 at leicester.ac.uk>

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 continued to observe the LIGO/Virgo event S200115j (GCN Circ.
26759). The XRT data cover 36.1 deg^2 covering 16% of the probability in
the 'bayestar' (version 2) skymap (after convolution with 2MPZ
galaxies), but only 10% of the probability from the 'LALInference'
skymap (also after convolution with 2MPZ), due to the shift of
probability towards the southern lobe in the latter map; this lobe is
too close to the Sun to allow Swift observations. Observations were
carried out in two phases; initial observations of 80 s per field, then 
a second pass with 500 s per field. These pointings and associated
metadata have been reported to the Treasure Map (Wyatt et al., arXiv
2001.00588; http://treasuremap.space/alerts?graceids=S200115j).

In total we have detected 146 X-ray sources, which can be viewed at
https://www.swift.ac.uk/LVC/83/. 84 of these are uncatalogued in X-rays
but with fluxes consistent with historical upper limits. 53 are known
X-ray sources whose fluxes are consistent with historical values. 9
further sources were identified as being of potential interest (rank 2;
see https://www.swift.ac.uk/ranks.php). The table below gives the basic
details of the sources; further comments are below.

| Source ID      | RA             | Dec            | Err90   | Note |
| S200115j_X130  | 02h 59m 36.73s | +07d 24' 13.9" |    5.2" | 4    |
| S200115j_X136  | 02h 40m 12.18s | -02d 33' 45.6" |    5.2" | 1    |
| S200115j_X487  | 02h 41m 59.74s | -03d 05' 23.0" |    4.8" | 5    |
| S200115j_X488  | 02h 40m 53.34s | -03d 25' 12.0" |    6.2" | 4    |
| S200115j_X707  | 02h 55m 31.95s | +12d 45' 26.6" |    4.5" | 2    |
| S200115j_X717  | 02h 23m 42.05s | -04d 35' 36.2" |    4.5" | 4    |
| S200115j_X745  | 02h 24m 04.01s | -04d 33' 05.6" |    5.1" | 5    |
| S200115j_X746  | 03h 00m 13.78s | +03d 49' 53.8" |    5.1" | 3    |
| S200115j_X748  | 02h 25m 37.24s | -05d 01' 07.2" |    5.9" | 4    |

Note 1) Of these, sources S200115j_X136, S200115j_X707 and S200115j_X746 
are those we deem most worthy of further investigation.

S200115j_X136 is a known X-ray source, observed several times in the 
past by Swift (see
https://www.swift.ac.uk/2SXPS/2SXPS%20J024012.0-023340). In our GW
follow up observations it is a factor of ~10 brighter than in our
historical dataset, but with no signs of variability between our two
epochs of GW follow up (data collected at ~100 ks and 300ks after the GW
trigger). As reported by Oates et al (GCN Circ. 26808) the UVOT data
also show an increase in flux compared to historical data. This source
is 1.5" from the 2MASS galaxy 2MASX J02401221-0233438, which SIMBAD
reports as having z=0.04; making its distant consistent with the
GW-predicted distance along this line of sight at the ~1.1-sigma level.
If the X-ray source is in this galaxy its observed 0.3-10 keV luminosity
is ~3 x 10^43 erg cm^-2 s^-1. A Vizier search reveals no AGN catalogues
containing this source.


Note 2) S200115j_X707 is uncatalogued in X-rays and in our initial 
detection is ~3.5 sigma above the historical 3-sigma upper limit from 
the RASS. It also faded strongly between observations taken at 160 ks 
after the GW trigger and at 390 ks after the trigger. A Vizier search 
reveals no AGN catalogues containing this source. This source was only 
flagged as "reasonable" by our source detection system, which means the 
probability that it is a spurious detection is ~7%.

Note 3) S200115j_X746 is also uncatalogued and in the first observation 
(7.3 ks after the GW trigger) was 4.7 sigma above the historical 3-sigma 
upper limit from the RASS. A further observation at 510 ks after the GW 
trigger showed the source to have faded by a factor of ~15. A Vizier 
search reveals no AGN catalogues containing this source.

Note 4) Sources S200115j_X130, S200115j_X717 and S200115j_X748 are all 
known X-ray sources whose flux in our GW follow-up observations is 
increased compared to historical data; the latter two have previously 
been observed with Swift and have elevated fluxes compared to those 
previous datasets. However, all of these sources have been identified 
with AGN, and are thus likely to indicate AGN activity although a 
transient within the AGN cannot immediately be ruled out. S200115j_X488 
is similar except that it is not a known X-ray emitter. Its current flux 
is 3.5-sigma above catalogued values; and it shows strong evidence of 
fading, however it corresponds to a source identified as an AGN in the 
WISE AGN candidates catalogs (Assef et al., 2018) and therefore may 
simply be a variable AGN.

Further observations of all of these sources are planned.

Note 5) Sources S200115j_X487 and S200115j_X745 are, upon manual 
investigation, considered unlikely to be counterparts since their 
classification as such depends upon a single, short-exposure data point. 
For S200115j_X487 this datapoint is during a time of very high 
background and is unreliable. S200115j_X745 is coincident with Mkn 1036, 
and only one datapoint from our recent observations is above the 
historical XMM flux.

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