TITLE:   GCN CIRCULAR
NUMBER:  20415
SUBJECT: LIGO/Virgo G268556: Swift-XRT confirms that ATLAS17aeu is fading in X-rays
DATE:    17/01/10 09:45:26 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), 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 continued to observed ATLAS17aeu (Tonry et al., LVC Circ.
20382), gathering a total of 21.3 ks of data, from T0+222 ks to T0+470
ks (where T0 is the GW trigger time). The X-ray counterpart (Evans et
al., LVC Circ. 20390) is now confirmed to be fading.

Assuming the trigger time from the GW event (LVC Circ. 20364) the light
curve is decaying with a power-law index of 2.2 (+1.1, -1.2). If instead
we take T0 from GRB 170105A (Marcinkowski et al., GCN Circ. 20387), the
decay index is 1.7 (+1.1, -0.9). At the mid-point of the latest 
observation, GW_T0+453 ks (=GRB_T0+381 ks) the 0.3-10 keV XRT count-rate
was 0.0017 (+/- 0.0006) ct/sec.

A spectrum compiled from the data can be fitted with an absorbed
power-law, with NH consistent with the Galactic value of 4.6e20 cm^-2
(Willingale et al. 2013) and photon index Gamma=1.60 (+0.4, -0.3).

Using this spectrum the count-rate above corresponds to an observed
(unabsorbed) 0.3-10 keV flux of (6.7+/-2.4)e-14  ( [7.2+/-2.6]e-14) erg
cm^-2 s^-1.

The spectral and temporal indices are consistent with the values
typically seen for X-ray GRB afterglows (e.g. the Swift-XRT GRB
catalogue: http://www.swift.ac.uk/xrt_live_cat/; Evans et al. 2009).

This circular is an official product of the Swift GW follow up team.