GCN Circular 26605

G. P. Smith (Birmingham), M. Nicholl (Birmingham), K. Sharon (Michigan), 
M. Bianconi (Birmingham), W. P. Varricatt (UKIRT), S. Benigni (UKIRT), 
and E. J. Ridley (Birmingham) report on behalf of the Gravitationally 
Lensed Gravitational Wave Hunters:

As part of our search for strongly-lensed gravitational waves and their 
electromagnetic counterparts, we observed a contiguous area of 0.75 
square degrees within the sky localization of the gravitational wave 
trigger S191216ap (GCN #26454) with the WFCAM instrument on UKIRT 
through the z-band.  The observations were centered on the position of 
the HAWC sub-threshold detection of gamma-ray flux reported by 
Martinez-Castellanos (GCN #26472), and encompassed the full extent of 
their 68% confidence region.  The observations comprise two epochs:

Epoch  Date(UT)    Start(UT)  End(UT)   Airmass  FWHM(arcsec)
1      2019-12-20  04:45:21   05:45:57  1.3-1.7  1.2-1.6
2      2019-12-21  04:31:26   05:43:34  1.3-1.8  1.6-1.8

We estimate that Epoch 1 reaches a 5 sigma point source sensitivity of 
AB~21.5.  Epoch 2 is less sensitive than Epoch 1, with noticeable 
variations in Epoch 2 between the four observations required to achieve 
contiguous coverage with WFCAM.

A preliminary comparison of Epoch 1 with templates derived from 
Pan-STARRS1 data (nominal sensitivity of AB=22.3) identified two 
candidate transient sources.  Neither source is known to the Transient 
Name Server, and Minor Planet Center.  The celestial coordinates and 
estimated z-band apparent AB magnitudes of these two candidates are:

Name      RA(J2000)    Dec(J2000)   AB(Epoch1)  AB(Epoch2)
GLGWc19a  21:32:45.50  +05:19:58.0  20.8+/-0.1  ~22
GLGWc19b  21:35:20.11  +04:55:19.8  20.6+/-0.1  >20(tentative)

Neither source is associated with an obvious candidate gravitational 
lens (i.e. massive galaxy, group or cluster) to the depth of the PS1 
data.  However, GLGWc19a is 7 arcsec West, 1 arcsec North of the center 
of an edge-on disk galaxy (possible host?) that is located at 
21:32:45.97 +5:19:57.0.

These sources are observable at Airmass<2 for ~30 minutes following the 
end of evening twilight in the Northern Hemisphere in the next few 
nights, before becoming unobservable through the winter months.  We 
therefore encourage urgent follow-up observations of GLGWc19a, GLGWc19b 
and the candidate host galaxy adjacent to GLGWc19a.

We thank the Director of UKIRT, Klaus Hodapp, for quickly approving our 
urgent DDT proposal.  We are also grateful to Mike Irwin for kindly 
expediting the processing of our data at Cambridge Astronomy Survey Unit 
(CASU).

This circular is citable.