TITLE:   GCN CIRCULAR
NUMBER:  26501
SUBJECT: LIGO/Virgo S191213g: Swift UVOT/XRT observations of Pan-STARRS1 candidate: PS19hgw
DATE:    19/12/19 11:13:16 GMT
FROM:    Alice Breeveld at MSSL-UCL  <a.breeveld@ucl.ac.uk>

S. R. Oates (U. of Birmingham), P. A. Evans (U. Leicester), A. A. Breeveld (MSSL-UCL), N. P. M. Kuin 
(UCL-MSSL), F. E. Marshall (NASA/GSFC), P. Brown (TAMU), M. De Pasquale (Istanbul U.), C. Gronwall 
(PSU), M. J. Page (UCL-MSSL),  M. H. Siegel (PSU), A. D'Ai (INAF-IASFPA), P. D'Avanzo (INAF-OAB), S. 
D. Barthelmy (NASA/GSFC), A. P. Beardmore (U. Leicester), D. N. Burrows (PSU), S. Campana 
(INAF-OAB), S. B. Cenko (NASA/GSFC), G. Cusumano (INAF-IASF PA), V. D’Elia (ASDC), P. Giommi (ASI), 
D. Hartmann (Clemson U.), K. L. Page (U.Leicester), J. A. Kennea (PSU), N. J. Klingler (PSU), H. A. 
Krimm (NSF), A. Y. Lien (GSFC/UMBC), D. B. Malesani (DTU Space), A. Melandri (INAF-OAB), J. A. 
Nousek (PSU), P. T. O'Brien (U. Leicester), J. P. Osborne (U. Leicester), C. Pagani (U. Leicester), 
D.M. Palmer (LANL), M. Perri (ASDC), J. L. Racusin (NASA/GSFC),  T. Sakamoto (AGU), B. Sbarufatti 
(INAF-OAB/PSU), G. Tagliaferri (INAF-OAB), A. Tohuvavohu (U Toronto), and E. Troja (NASA/GSFC/UMCP) 
report on behalf of the Swift team:

We report Swift/UVOT and XRT follow-up observations of PS19hgw (AT2019wxt; McBrien et al., GCN Circ. 
26485) found during the search for the EM counterpart of the LVC event S191213g (LIGO/VIRGO 
Collaboration GCN Circ. 26402).

This object has also been observed by MASTER (Lipunov et al., GCN Circ. 26497), TNG (D'Avanzo et 
al., GCN Circ. 26499) and SEDM (Fremling et al., GCN Circ. 26500) and spectroscopically by ePESSTO 
(Müller Bravo et al., GCN Circ. 26494), HCT (Dutta et al., GCN Circ. 26490), NOT (Izzo et al., GCN 
Circ. 26491) and LT (Srivastav et al., GCN Circ. 26493), indicating this transient is at z = 0.037.

Swift/UVOT and XRT observations began 5.7 days after the LIGO/VIRGO trigger (LIGO/VIRGO 
Collaboration GCN Circ. 26402). Photometry is complicated by the host galaxy, however, in the Swift 
archive a pre-explosion image is available for the u filter. Comparing the u band images we measure 
a ~0.8 mag enhancement in flux. For all filters, we have attempted to subtract the host by measuring 
the host flux at a position offset from PS19hgw, but of a similar brightness as  the location of 
PS19hgw in the pre-explosion image.

In the following we give preliminary magnitudes using the UVOT photometric system (Breeveld et al. 
2011, AIP Conf. Proc. 1358, 373):

Filter   T_start(s)   T_stop(s) Exp(s)     Mag

v          489252       494253   157     18.53 +\- 0.34
b          488842	493844   157	 19.30 +\- 0.32
u          488758	493759   157	 18.45 +\- 0.24
w1         488594	493675   315	 18.37 +\- 0.23
m2         489336	494468   1163	 19.31 +\- 0.32
w2         488928	494169   629	 18.82 +\- 0.27

We have checked the reliability of our host subtraction by determining the host subtracted u band 
magnitude using the host flux measured at the location of PS19hgw in the pre-explosion image. This 
method results in a u filter magnitude for PS19hgw of 18.49 +\- 0.21, consistent with the u 
magnitude in the table above.

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of 
E(B-V) = 0.06 in the direction of the burst (Schlegel et al. 1998).

In XRT, no source was found at the location of AT2019wxt in 2.6 ks of exposure, starting 5.65 d 
after the GW trigger. The 3-sigma upper limit is 4.74e-3 ct s^-1. This corresponds to a 0.3-10 keV 
flux upper limit of 2.04e-13 erg cm^2 s^-1, assuming a power-law spectrum with NH=3e20 cm^-2 and 
Gamma=1.7.

This circular is an official product of the Swift team.