TITLE:   GCN CIRCULAR
NUMBER:  24458
SUBJECT: LIGO-Virgo S190426c: Pan-STARRS non-detection of ZTF19aassfws
DATE:    19/05/10 17:55:38 GMT
FROM:    Stephen Smartt at Queen's U/Belfast  <s.smartt@qub.ac.uk>

M. Huber, (IfA), S. J. Smartt (QUB), K. Chamber (IfA) K. W. Smith, D. R. Young, O. McBrien, J. Gillanders. S. Srivastav, S. J. Smartt, D. O'Neil, P. Clark, S. Sim (QUB), T. de Boer, J. Bulger, J. Fairlamb, C.-C. Lin, T. Lowe, E. Magnier A. Schultz, R. J. Wainscoat,
M. Willman (IfA, University of Hawaii)

We imaged ZTF19aassfws (Fremling et al. GCN 24433, Cenko et al. GCN 24430, Perley et al. GCN 24331, Coughlin et al. GCN 24283) with Pan-STARRS1 on UT 2019-05-09. This is the ZTF candidate within the skymap and distance range of s190426c
(the possible NS-BH candidate), which was detected at r=21.41.

A set of 6x300s exposures in i and z band filters were taken and stacked together. A custom made reference image in each filter was constructed (deeper than the public 3Pi stack of Chambers et al. 2016, arXiv:1612.05560)  and subtracted this from the night stacks. The image quality was 0.75 arcseconds in the stacked frames.

We find no clear detection of significant excess flux at the positon of ZTF19aassfws, or at the core of the host galaxy in either band.  There is some residual positive flux coincident with the galaxy core in each filter but this is not PSF in shape and is almost certainly a minor residual from the convolution and subtraction. Forcing photometry at the positon we find i =23.5 +/- 0.5 and z=22.5 +/- 0.3. We do not consider either to be significant detections and conclude there is no transient flux above these estimates.