GCN Circular 24273

P.A. Evans (U. Leicester), J.A. Kennea (PSU), A. Tohuvavohu (PSU), S.D.
Barthelmy (NASA/GSFC), A.P. Beardmore (U. Leicester), A.A. Breeveld
(UCL-MSSL), 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), H.A. Krimm (CRESST/GSFC/USRA), 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. Warwick), P.T.
O'Brien (U. Leicester), J.P. Osborne (U. Leicester), C. Pagani (U.
Leicester), K.L. Page (U.Leicester), 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 performed a series of 197 observations, covering 197 separate
locations within the LVC error region for the GW trigger S190426c
convolved with the 2MPZ catalogue (Bilicki et al. 2014, ApJS, 210, 9),
using the 'bayestar1' GW localisation map. As this is a 3D skymap,
galaxy distances were taken into account in selecting which ones to
observe. The observations currently span from 8.6 ks to 43 ks after the
LVC trigger, and cover 21.5 sq degrees on the sky (corrected for
overlaps). 


We have detected 3 X-ray sources, described below. Other sources that
have been distributed as counterpart notices have been identified as
artifacts or diffuse emission and are not described below.

Source 5 is potentially interesting, as its flux is higher (at
2.1-sigma singificance) than in previous XRT observations (see
http://www.swift.ac.uk/1SXPS/1SXPS%20J144850.8-400845 for the
historical dataset in the 1SXPS catalogue). However, it is only 2.2"
away from a know Seyfert 1 at z=0.123 (from SIMBAD), and therefore it
is more likely that this is simply an unrelated flare from that source.
Observations are, nonetheless encouraged.


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
http://www.swift.ac.uk/ranks.php.

We have found:

  * 0 sources of rank 1
  * 0 sources of rank 2
  * 0 sources of rank 3
  * 3 sources of rank 4

For all flux conversions and comparisons with catalogues and upper
limits from other missions, we assumed a power-law spectrum with
NH=3e20 cm^2, and photon index (Gamma)=1.7


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 3:
  =============
    RA: 	 346.9414 ( = 23h 07m 45.94s) J2000
    Dec:	 +83.1619 ( = +83d 09' 42.8") J2000
    Error:	 +24.9 (arcsec, radius, 90% confidence).
    Peak Rate:	 7.1e-02 +/- 3.7e-02 ct/sec (0.3-10 keV)
    Peak Flux:	 3.1e-12 +/- 1.6e-12 erg cm^-2 s^-1 (0.3-10 keV)
    Cat Source:  1RXS J230740.9+830945 in the ROSAT/RASSFSC catalogue
    Separation:  9.4" from the XRT source
    Cat Rate:	 3.4e-02 +/- 8.3e-03 ct/sec 
    Cat Flux:	 9.5e-13 +/- 2.3e-13 erg cm^-2 s^-1 (0.3-10 keV)
       so the source  is 1.3-sigma above the catalogued flux.
    There is no evidence for fading.
    NOTE: this source is NOT within 200 kpc of a GWGC or 2MPZ galaxy
      which is consistent (within 3-sigma) with the distance to the GW
object.
    A SIMBAD object `1RXS J230740.9+830945' is 9.4" away.
    There are 2 2MASS objects within the source's 3-sigma error radius.

  
  Source 5:
  =============
    RA: 	 222.2116 ( = 14h 48m 50.78s) J2000
    Dec:	 -40.1461 ( = -40d 08' 46.0") J2000
    Error:	 +11.5 (arcsec, radius, 90% confidence).
    Peak Rate:	 1.6e-01 +/- 6.3e-02 ct/sec (0.3-10 keV)
    Peak Flux:	 7.0e-12 +/- 2.7e-12 erg cm^-2 s^-1 (0.3-10 keV)
    Cat Source:  1SXPS J144850.8-400845 in the 1SXPS catalogue
    Separation:  0.8" from the XRT source
    Cat Rate:	 3.0e-02 +/- 2.1e-03 ct/sec 
    Cat Flux:	 1.3e-12 +/- 9.0e-14 erg cm^-2 s^-1 (0.3-10 keV)
       so the source  is 2.1-sigma above the catalogued flux.
    There is no evidence for fading.
    NOTE: this source is NOT within 200 kpc of a GWGC or 2MPZ galaxy
      which is consistent (within 3-sigma) with the distance to the GW
object.
    A SIMBAD object `2MASS J14485097-4008456' is 2.2" away.
    There are 3 2MASS objects within the source's 3-sigma error radius.

  
  Source 6:
  =============
    RA: 	 220.9883 ( = 14h 43m 57.19s) J2000
    Dec:	 -39.1433 ( = -39d 08' 35.9") J2000
    Error:	 +5.3 (arcsec, radius, 90% confidence).
    Peak Rate:	 2.1e-01 +/- 5.8e-02 ct/sec (0.3-10 keV)
    Peak Flux:	 8.9e-12 +/- 2.5e-12 erg cm^-2 s^-1 (0.3-10 keV)
    Cat Source:  1SXPS J144357.1-390839 in the 1SXPS catalogue
    Separation:  3.4" from the XRT source
    Cat Rate:	 4.3e-01 +/- 5.5e-03 ct/sec 
    Cat Flux:	 1.9e-11 +/- 2.4e-13 erg cm^-2 s^-1 (0.3-10 keV)
       so the source is not above the catalogued flux.
    There is no evidence for fading.
    There is 1 GWGC or 2MPZ galaxy within 200 kpc of the source.

      and consistent (within 3-sigma) with the distance to the GW
object.
    A SIMBAD object `NAME WISEA J144357.20-390840.0' is 4.3" away.
    There is 1 2MASS object within the source's 3-sigma error radius.

This circular is an official product of the Swift team.