[vsnet-grb-info 23287] LIGO/Virgo S190814bv: Pan-STARRS imaging indicates DG19sevhc (AT2019npy) is a proper motion star

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25386
SUBJECT: LIGO/Virgo S190814bv: Pan-STARRS imaging indicates DG19sevhc (AT2019npy) is a proper motion star
DATE:    19/08/17 22:45:05 GMT
FROM:    Stephen Smartt at Queen's U/Belfast  <s.smartt at qub.ac.uk>

S. Smartt (QUB), D. Malesani (DARK), K. W. Smith (QUB), M. Huber (IfA,
Univ. of Hawaii), K. Chambers, A.  Schulz (IfA), D. R. Young,
O. McBrien, J. Gillanders. S.  Srivastav, D. O'Neil, P. Clark, S. Sim
(QUB), T. de Boer, J. Bulger, J. Fairlamb, M. Huber, C.-C. Lin,
T. Lowe, E. Magnier, R. J.  Wainscoat, M. Willman (IfA, Univ. of
Hawaii), T.-W, Chen (MPE), A.  Rest (STScI), C. Stubbs (Harvard)

In the search of the skymap of the NSBH event S190814bv (The LIGO
Scientific Collaboration and the Virgo Collaboration, GCN 25333,
25324) the DECam-GROWTH team identified DG19sevhc (AT2019npy; Andreoni
et al. GCN 25362) as a candidate. It was followed up by Rossi et al
(GCN 25383) and Dichiara et al. (GCN 25374).  The latter reported an
unusual and rapid z-band rise, which drew attention.

We found a previous source detection close to this position on
difference images taken during the Pan-STARRS Survey For Transients
(Huber et al. 2015, ATel 7153) at i=20.9, on multiple images from
MJD=58335 (2018-08-05).  However closer inspection revealed that the
source was not a transient.

There is a faint red star which is coincident with background extended
flux (probably a faint, red galaxy) and the star appears to have slow
proper motion. This produced a dipole source in the Pan-STARRS
difference images, and triggered a new source detection.  The motion
between the Pan-STARRS reference and the image from MJD=58335 is
visually clear (about 1.4 arcsec).  

We further inspected the separate Pan-STARRS 3Pi epochs
(Chambers et al. 2016, arXiv:1612.05560C). Using 3 images with
reasonable S/N between 2010 September and 2014 August, plus the public
DECam image from 2019 August, the position of the star traces a vector
towards the S-E. We estimate a proper motion of 0.07 and -0.12
arcsec/yr in RA and Dec, respectively.

Hence we conclude that AT2019npy is not a transient, but resulted
from the proper motion of this star leaving a positive residual
in the DECam images of 2019 August, and the DECam references. 
The unusual spatial coincidence of the moving star and background galaxy 
made the original DECam identification as a transient quite understandable and 
reasonable.