{
  "bibcode": "2020GCN.26855....1E",
  "body": "P.A. Evans (U. Leicester),  K.L. Page (U.Leicester), J.A. Kennea (PSU),\nA. Tohuvavohu (U. Toronto), S.D. Barthelmy (NASA/GSFC), A.P. Beardmore\n(U. Leicester), M.G. Bernardini (INAF-OAB), A.A. Breeveld (UCL-MSSL),\nP. Brown (TAMU), D.N. Burrows (PSU), S. Campana (INAF-OAB), S.B. Cenko\n(NASA/GSFC), G. Cusumano (INAF-IASF PA), A. D'Ai (INAF-IASFPA), P.\nD'Avanzo (INAF-OAB), V. D'Elia(ASDC), S.W.K. Emery (UCL-MSSL), P.\nGiommi (ASI), C. Gronwall (PSU), D. Hartmann (U. Clemson), H.A. Krimm\n(CRESST/GSFC/USRA), N.J. Klingler (PSU), N.P.M. Kuin (UCL-MSSL), A.Y.\nLien (GSFC/UMBC), F.E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),\nJ.A. Nousek (PSU), S.R. Oates (U. Birmingham), P.T. O'Brien (U.\nLeicester), J.P. Osborne (U. Leicester), C. Pagani (U. Leicester),\nM.J.Page (UCL-MSSL), D.M. Palmer (LANL), M. Perri (ASDC), J.L. Racusin\n(NASA/GSFC), B. Sbarufatti (INAF-OAB/PSU), M.H. Siegel (PSU), G.\nTagliaferri (INAF-OAB), E. Troja (NASA/GSFC/UMCP) report on behalf of\nthe Swift team:\n\nSwift has continued to observe the LIGO/Virgo event S200115j (GCN Circ.\n26759). The XRT data cover 36.1 deg^2 covering 16% of the probability in\nthe 'bayestar' (version 2) skymap (after convolution with 2MPZ\ngalaxies), but only 10% of the probability from the 'LALInference'\nskymap (also after convolution with 2MPZ), due to the shift of\nprobability towards the southern lobe in the latter map; this lobe is\ntoo close to the Sun to allow Swift observations. Observations were\ncarried out in two phases; initial observations of 80 s per field, then \na second pass with 500 s per field. These pointings and associated\nmetadata have been reported to the Treasure Map (Wyatt et al., arXiv\n2001.00588; http://treasuremap.space/alerts?graceids=S200115j).\n\nIn total we have detected 146 X-ray sources, which can be viewed at\nhttps://www.swift.ac.uk/LVC/83/. 84 of these are uncatalogued in X-rays\nbut with fluxes consistent with historical upper limits. 53 are known\nX-ray sources whose fluxes are consistent with historical values. 9\nfurther sources were identified as being of potential interest (rank 2;\nsee https://www.swift.ac.uk/ranks.php). The table below gives the basic\ndetails of the sources; further comments are below.\n\n| Source ID      | RA             | Dec            | Err90   | Note |\n| S200115j_X130  | 02h 59m 36.73s | +07d 24' 13.9\" |    5.2\" | 4    |\n| S200115j_X136  | 02h 40m 12.18s | -02d 33' 45.6\" |    5.2\" | 1    |\n| S200115j_X487  | 02h 41m 59.74s | -03d 05' 23.0\" |    4.8\" | 5    |\n| S200115j_X488  | 02h 40m 53.34s | -03d 25' 12.0\" |    6.2\" | 4    |\n| S200115j_X707  | 02h 55m 31.95s | +12d 45' 26.6\" |    4.5\" | 2    |\n| S200115j_X717  | 02h 23m 42.05s | -04d 35' 36.2\" |    4.5\" | 4    |\n| S200115j_X745  | 02h 24m 04.01s | -04d 33' 05.6\" |    5.1\" | 5    |\n| S200115j_X746  | 03h 00m 13.78s | +03d 49' 53.8\" |    5.1\" | 3    |\n| S200115j_X748  | 02h 25m 37.24s | -05d 01' 07.2\" |    5.9\" | 4    |\n\nNote 1) Of these, sources S200115j_X136, S200115j_X707 and S200115j_X746 \nare those we deem most worthy of further investigation.\n\nS200115j_X136 is a known X-ray source, observed several times in the \npast by Swift (see\nhttps://www.swift.ac.uk/2SXPS/2SXPS%20J024012.0-023340). In our GW\nfollow up observations it is a factor of ~10 brighter than in our\nhistorical dataset, but with no signs of variability between our two\nepochs of GW follow up (data collected at ~100 ks and 300ks after the GW\ntrigger). As reported by Oates et al (GCN Circ. 26808) the UVOT data\nalso show an increase in flux compared to historical data. This source\nis 1.5\" from the 2MASS galaxy 2MASX J02401221-0233438, which SIMBAD\nreports as having z=0.04; making its distant consistent with the\nGW-predicted distance along this line of sight at the ~1.1-sigma level.\nIf the X-ray source is in this galaxy its observed 0.3-10 keV luminosity\nis ~3 x 10^43 erg cm^-2 s^-1. A Vizier search reveals no AGN catalogues\ncontaining this source.\n\n\nNote 2) S200115j_X707 is uncatalogued in X-rays and in our initial \ndetection is ~3.5 sigma above the historical 3-sigma upper limit from \nthe RASS. It also faded strongly between observations taken at 160 ks \nafter the GW trigger and at 390 ks after the trigger. A Vizier search \nreveals no AGN catalogues containing this source. This source was only \nflagged as \"reasonable\" by our source detection system, which means the \nprobability that it is a spurious detection is ~7%.\n\nNote 3) S200115j_X746 is also uncatalogued and in the first observation \n(7.3 ks after the GW trigger) was 4.7 sigma above the historical 3-sigma \nupper limit from the RASS. A further observation at 510 ks after the GW \ntrigger showed the source to have faded by a factor of ~15. A Vizier \nsearch reveals no AGN catalogues containing this source.\n\nNote 4) Sources S200115j_X130, S200115j_X717 and S200115j_X748 are all \nknown X-ray sources whose flux in our GW follow-up observations is \nincreased compared to historical data; the latter two have previously \nbeen observed with Swift and have elevated fluxes compared to those \nprevious datasets. However, all of these sources have been identified \nwith AGN, and are thus likely to indicate AGN activity although a \ntransient within the AGN cannot immediately be ruled out. S200115j_X488 \nis similar except that it is not a known X-ray emitter. Its current flux \nis 3.5-sigma above catalogued values; and it shows strong evidence of \nfading, however it corresponds to a source identified as an AGN in the \nWISE AGN candidates catalogs (Assef et al., 2018) and therefore may \nsimply be a variable AGN.\n\nFurther observations of all of these sources are planned.\n\nNote 5) Sources S200115j_X487 and S200115j_X745 are, upon manual \ninvestigation, considered unlikely to be counterparts since their \nclassification as such depends upon a single, short-exposure data point. \nFor S200115j_X487 this datapoint is during a time of very high \nbackground and is unreliable. S200115j_X745 is coincident with Mkn 1036, \nand only one datapoint from our recent observations is above the \nhistorical XMM flux.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 26855,
  "createdOn": 1579710468000,
  "email": "pae9@leicester.ac.uk",
  "subject": "LIGO/Virgo S200115j: Further Swift-XRT sources",
  "submitter": "Phil Evans at U of Leicester  <pae9@leicester.ac.uk>",
  "eventId": "LIGO/Virgo S200115j"
}