TITLE:   GCN CIRCULAR
NUMBER:  20773
SUBJECT: LIGO/Virgo G275697: Swift-XRT sources
DATE:    17/02/28 08:59:52 GMT
FROM:    Phil Evans at U of Leicester  <pae9@leicester.ac.uk>

P.A. Evans (U. Leicester), J.A. Kennea (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), P. Giommi (ASI), C. Gronwall (PSU), H.A. Krimm
(CRESST/GSFC/USRA), N.P.M. Kuin (UCL-MSSL), A.Y. Lien (GSFC/UMBC), D.
Malesani (DARK/NBI), F.E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
B. Mingo (U. Leicester),  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 62 observations of galaxies (from the
2MPZ catalogue) within the LVC error region for the GW trigger G275697,
using the 'bayestar' 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 15 ks to 24 ks after the
LVC trigger, and cover 7.6 sq degrees on the sky (corrected for
overlaps). This covers 0.011 of the probability in the LVC skymap, and
0.022 of the probability in the LVC map after convolving with the 2MPZ
galaxy catalogue, as described by Evans et al., (2016, MNRAS,  462,
1591).

We have detected a 1 X-ray source. 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.

The detected source is currently given "rank 3" since it is only
1.6-sigma above the historical upper limit at this location from the
Rosat All Sky Survey, and therefore cannot be identified as a new
source with a significant degree of confidence. We only have a single
data point so cannot comment on variability. We note, however, that
this uncatalogued source is consistent with a galaxy in 2MPZ with a
distance (based on a photometric redshift) of at 291 ± 61 Mpc. The
LIGO distance probability on the line of sight to this has mean 160 and
sigma 57 Mpc. 96% of the integral of the LIGO distance function (Singer
et al., 2016, ApJL, 829, 15) along this line of sight lies within the
3-sigma confidence interval on this galaxy's position. Follow up
observations of this source are encouraged. 

We have found:

  * 0 sources of rank 1
  * 0 sources of rank 2
  * 1 source of rank 3
  * 0 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 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 1:
  =============
    RA: 	 324.9781 ( = 21h 39m 54.74s) J2000
    Dec:	 +44.7641 ( = +44d 45' 50.8") J2000
    Error:	 +5.9 (arcsec, radius, 90% confidence).
    Peak Rate:	 1.1e-01 +/- 5.0e-02 ct/sec (0.3-10 keV)
    Peak Flux:	 4.7e-12 +/- 2.1e-12 erg cm^-2 s^-1 (0.3-10 keV)
    1XMM UL:	 7.4e-12 ct/sec, (0.3-10 keV)
		 So the source is 1.3-sigma above the 
		 XMM 3-sigma upper limit.
    RASS UL:	 3.0e-02 ct/sec, 3-sigma, converted to XRT (0.3-10 keV)

		 So the source is not above the RASS 3-sigma upper 
		 limit.
    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.

    There is 1 2MASS object within the source's 3-sigma error radius.

This circular is an official product of the Swift team.