[vsnet-grb-info 23105] LIGO/Virgo S190728q: Pan-STARRS detection of the transient AT2019lvs/ZTF19abjethn 18hrs before S190728q

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25204
SUBJECT: LIGO/Virgo S190728q:  Pan-STARRS detection of the transient AT2019lvs/ZTF19abjethn 18hrs before S190728q
DATE:    19/07/28 18:13:31 GMT
FROM:    Stephen Smartt at Queen's U/Belfast  <s.smartt at qub.ac.uk>

S. J. Smartt, K. W. Smith (QUB), M. Huber, K. Chambers (IfA), D. R.
Young, S. Srivastav, O. McBrien, J. Gillanders, 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), A. Rest (STScI), C. Stubbs (Harvard)

We imaged part of the localisation map of the gravitational wave
S190728q (GCN 25187) with Pan-STARRS1 (Chambers et al. 2016,
arXiv:1612.05560C) on the night preceding the detection (GW detection
at 58692.28137188518) during routine survey observations.

This coverage included the location of the ZTF reported optical
transient AT2019lvs (ZTF19abjethn; Kasliwal et al. GCN 25199).
Pan-STARRS observed in the NEO sequence of 45s x 4 in the wide
gri-composite filter w-band. Images were processed with the IPP
(Magnier et al. 2016, arXiv:1612.05240) and the w-band reference sky
frame was subtracted during normal processing.

We detected a transient source (PS19dwa) in all four difference
images. In 2 images it is formally below 5-sigma but visually
detected. The 2 other images have 5-sigma detections at AB
magnitudes :

MJD          w     w_err
58691.544552 22.27 0.22 
58691.531772 22.01 0.20

At position: 
RA  =  326.39543 (21:45:34.90)  
DEC =  +20.69064 (+20:41:26.3) 

This is coincident with the ZTF coordinates to 0.2 arcsec, and hence
is almost certainly the same object. Therefore this optical transient
is detected 18 hours before S190728q.

There are no other detections of transient flux at this position in 
about 30 epochs of Pan-STARRS grizy imaging over the last 2000 days
in our detection database. 

AT2019lvs is coincident with a blue source (SDSS J214534.91+204126.2)
in SDSS and Pan-STARRS and DECaLs (Dey et al. 2019, DR7 as reported in
Zhou et al. GCN 25201). The object is detected only in the g-band in
SDSS and PS1 surveys (SDSS g = 22.57 +/- 0.14). It is uncertain if it
is stellar or a faint blue galaxy in all three surveys. It is more
likely to be associated with this faint source than the brighter host
at photoz - 0.23 reported by Zhou et al. (GCN 25201) and Kasliwal et
al. (GCN 25199).

A foreground variable or CV is the most likely explanation, but a more
exotic explanation and association with S190728q can't be ruled out
and a classification or redshift is essential.