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 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.