Difference: GCNCIRCULAR20773 (1 vs. 2)

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.

-- ThomasKupfer - 28 Feb 2017