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GCN Circular 20390

Subject
LIGO/Virgo G268556: Swift-XRT detection of ATLAS17aeu
Date
2017-01-07T13:33:36Z (7 years ago)
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), N. Gehrels (NASA/GSFC), 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 targetted observation of the interesting
transient ATLAS17aeu (Tonry et al., LVC Circ. 20382). The observations
ran from 00:00 UT to 01:37 UT on 2017 Jan 7, and contained 1.4 ks of
cleaned PC mode data.

A faint, uncatalouged X-ray source is found at RA, Dec = 138.3059,
+61.0919 degrees, which corresponds to:

RA (J2000):  09h 13m 13.42s
Dec (J2000): +61d 05' 30.8") 

with an uncertainty of 6.3 arcsec (radius, 90% confidence). This is
4.1" from the position of ATLAS17aeu, consitent with that object.

This source ("source5" of the XRT follow up of G268556) has a 0.3-10
keV count rate of 7.2 (+/-2.9) ��10^-3 ct s^-1, corresponding to a
0.3-10 keV flux of 3.1 (+/-1.2) ��10^-13 erg cm^-2 s^-1, assuming a
power-law spectrum with Gamma=1.7 and NH=3e20 cm^-2. This is below the
upper limit derived from the RASS at the source location. This source
has not been previously observed by public XMM data, or Swift
observations.

At the present time we cannot ascertain whether the source is variable,
but follow up in encouraged.


A second, catalogued ("rank 4") source is also detected in our
observations. Details of both sources are below.

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 5:
  =============
    RA: 	 138.3059 ( = 09h 13m 13.42s) J2000
    Dec:	 +61.0919 ( = +61d 05' 30.8") J2000
    Error:	 +6.3 (arcsec, radius, 90% confidence).
    Peak Rate:	 7.2e-03 +/- 2.9e-03 ct/sec (0.3-10 keV)
    Peak Flux:	 3.1e-13 +/- 1.2e-13 erg cm^-2 s^-1 (0.3-10 keV)
    RASS UL:	 1.8e-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.
    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.


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 4:
  =============
    RA: 	 138.0838 ( = 09h 12m 20.11s) J2000
    Dec:	 +60.9299 ( = +60d 55' 47.6") J2000
    Error:	 +5.1 (arcsec, radius, 90% confidence).
    Peak Rate:	 1.6e-02 +/- 4.4e-03 ct/sec (0.3-10 keV)
    Peak Flux:	 6.7e-13 +/- 1.9e-13 erg cm^-2 s^-1 (0.3-10 keV)
    Cat Source:  1RXS J091222.2+605603 in the ROSAT/RASSFSC catalogue
    Separation:  22.1" from the XRT source
    Cat Rate:	 2.5e-02 +/- 9.4e-03 ct/sec 
    Cat Flux:	 7.1e-13 +/- 2.6e-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.
    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 J09122029+6055503' is 3.1" away.
    There is 1 2MASS object within the source's 3-sigma error radius.

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