TITLE:   GCN CIRCULAR
NUMBER:  24096
SUBJECT: LIGO/Virgo S190408an: Pan-STARRS observations and transients in the skymap 
DATE:    19/04/11 00:30:40 GMT
FROM:    Ken Smith at Queen's University Belfast  <k.w.smith@qub.ac.uk>

K. W. Smith, D. R. Young, M. Huber, K. C. Chambers, S. J. Smartt,
O. McBrien, J. Gillanders. S. Srivastav, D. O'Neil,  P. Clark, 
S. Sim (QUB), E. Magnier, A. Schultz, (IfA, Univ. Hawaii), 
L. Denneau, H. Flewelling, A. Heinze, J. Tonry, H. Weiland,  
A. Rest (STScI),  B. Stalder (LSST), C. Stubbs (Harvard)

We report observations of the BAYESTAR skymap of the BBH event S190804an 
(The LIGO Scientific Collaboration and the Virgo Collaboration, GCN 
24069) with the Pan-STARRS1 telescope (Chambers et al. 2016, 
arXiv:1612.05560C). Images were taken in the PS1 w,z and y-bands 
(Tonry et al. 2012, ApJ 750, 99) in the standard NEO search sequence. 
At each pointing position a sequence of quads (4 x 45 sec) was taken.
This observing sequence ensures exactly the same pointing position 
for each of the quads. We estimate that the active 
pixel area covered 72 square degrees and a summed probability of 
11% of the BAYESTAR skymap. We began taking data at 2019-04-09
14:30 (UTC) 20hrs after the GW trigger.

The images were processed with the IPP (Magnier et al. 2016, 
arXiv:1612.05240) and difference images were produced using 
the Pan-STARRS1 Science Consortium 3Pi images as reference 
frames. Transient candidates were run through our standard 
filtering procedures, combined with a machine learning algorithm 
(Wright et al. 2015, MNRAS, 449, 451) were applied and all candidates were
spatially cross-matched with known minor planets, and major 
star, galaxy, AGN and multi-wavelength catalogues (as described in 
Smartt et al. 2016, MNRAS, 462 4094). 

We find 3 new transients. None of which appear to possess any 
peculiar feature that indicates any link to S190804an. 

bPC denotes the percentage probability contour within which 
the transient is found according to the bayestar.fits map. 

Object RA         DEC        Disc Mag  filt MJD         z      bPC  Notes
-------------------------------------------------------------------------
PS19pn 339.15090  +33.16465  20.40 0.21  w  58582.6099  -      60%  (1)
PS19pm 337.33525  +25.26419  19.87 0.14  w  58582.6061  ~0.195 80%  (2)
PS19pl 335.17707  +31.17752  19.44 0.12  w  58582.6086  ~0.083 90%  (3)  

(1) Orphan, no host object 
(2) Nuclear Transient coincident with SDSS J222920.46+251551.0; an r=18.31 
mag galaxy found in the SDSS DR12 PhotoObjAll Table catalogue. It's located 
0.2" from the galaxy core. A host photoZ=0.195 (±0.031) implies absolute
mag of M = -20. There is some variability history at similar flux levels in 
Pan-STARRS data.
(3) Probably associated with SDSS J222042.60+311038.3; an r=17.26 mag
galaxy found in the SDSS DR12 PhotoObjAll Table catalogue. It's located
0.69" N, 1.35" W from the galaxy centre. A host photoZ=0.083 (±0.020)
implies an absolute mag of M = -18.5