[vsnet-grb-info 23100] LIGO/Virgo S190728q: Optical Candidate from the Zwicky Transient Facility ZTF19abjethn

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25199
SUBJECT: LIGO/Virgo S190728q: Optical Candidate from the Zwicky Transient Facility ZTF19abjethn
DATE:    19/07/28 15:06:42 GMT
FROM:    Mansi M. Kasliwal at Caltech/Carnegie  <mansikasliwal at gmail.com>

Mansi M. Kasliwal (Caltech), Leo P. Singer (NASA GSFC), Eric C. Bellm (UW),
Michael W. Coughlin (Caltech), Daniel A. Perley (LJMU), Jesper Sollerman
(OKC), Igor Andreoni (Caltech), Erik Kool (OKC), Danny Goldstein (Caltech),
Rahul Biswas (OKC), Richard Walters (Caltech), Varun Bhalerao (IITB), G. C.
Anupama (IIA), Shreya Anand (Caltech), Tomas Ahmuda (UMD), Brad Cenko (NASA
GSFC), Sara Webb (Swinburne), Jeff Cooke (Swinburne), Kirsty Taggart
(LJMU), Sudhanshu Barway (IIAP), Alessandra Corsi (TTU), Mattia Bulla
(OKC), Albert Kong (Taiwan), Chris Copperwheat (LJMU)

On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of
Observatories Watching Transients Happen (GROWTH) collaborations

We observed the localization region of the gravitational wave trigger
S190728q (GCN 25187) with the Palomar 48-inch telescope equipped with the
47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). A new
tiling was automatically optimally determined and triggered using the
GROWTH Target of Opportunity marshal (Coughlin et al. 2019a, Kasliwal et
al. 2019b). We enclosed 64% of the probability and obtained a sequence of
300s exposures in g-band, r-band and g-band before sunrise.

The images were processed through the ZTF reduction and image subtraction
pipelines at IPAC to search for potential counterparts (Masci et al. 2019).
We applied standard filtering criterion including rejecting stellar sources
(Tachibana and Miller 2018) and moving objects and applying machine
learning algorithms (Mahabal et al. 2019, Duev et al. 2019).

A promising candidate with the first detection after the GW merger time and
a fast intra-night rise in both filters is:
-------------------------------------------------------------------------------------------------------------------
 ZTF Name     | RA (deg)    | DEC (deg)   | Filter | Mag   |  Magerr |
Filter | Mag   | Magerr
--------------------------------------------------------------------------------------------------------------------
ZTF19abjethn  | 326.395431 | 20.690590    | r      | 19.65 |  0.11   | g
  | 19.82 |  0.16
--------------------------------------------------------------------------------------------------------------------

We reported this candidate to TNS and it was given the name AT2019lvs. The
candidate is offset from a galaxy with SDSS phot-z estimate of 0.228 ±
0.0286. We serendipituously observed this field the previous night with ZTF
and did not detect the candidate.

Additional analysis and continued follow-up is in progress and encouraged.

ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC,
USA, WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA; DESY,
Germany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan;
IIT-B, India; IIA, India; LJMU, UK; TTU, USA; and USyd, Australia. ZTF
acknowledges the generous support of the NSF under AST MSIP Grant No
1440341. GROWTH acknowledges generous support of the NSF under PIRE Grant
No 1545949. Alert distribution service provided by DIRAC at UW (Patterson et
al. 2019). Alert filtering and follow-up co-ordination is being undertaken
by the GROWTH marshal system (Kasliwal et al. 2019).