LIGO/Virgo S190814bv: Pan-STARRS imaging indicates DG19sevhc (AT2019npy) is a proper motion star
Abstract
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.
- Publication:
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GRB Coordinates Network
- Pub Date:
- August 2019
- Bibcode:
- 2019GCN.25386....1S