TITLE: GCN CIRCULAR NUMBER: 25235 SUBJECT: LIGO/Virgo S190728q: ZTF Forced Photometry for PS1 and DECam-GROWTH optical candidates, and a note on DECam Photometry from GCN 25215 DATE: 19/07/31 18:43:12 GMT FROM: Igor Andreoni at Caltech Igor Andreoni (Caltech), Erik Kool (OKC), Danny Goldstein (Caltech), Frank Masci (IPAC), Mansi Kasliwal (Caltech), Matthew Graham (Caltech) On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations We performed forced photometry on images acquired with the Zwicky Transient Facility (ZTF; Bellm et al. 2019, Graham et al. 2019) for the the optical transients reported so far by the Pan-STARRS (PS1; Huber et al., GCN #25224) and DECam-GROWTH (Goldstein et al., GCN #25215) teams. Forced photometry was performed on images processed through the ZTF reduction and image subtraction pipelines at IPAC (Masci et al. 2019). The following table includes candidates with no detections before the trigger time of S190728q (2019-07-28 06:45:10.529 UT, GCN #25187). For these sources, we report the latest relevant upper limits or confirmation detections. An association between these candidates and S190728q cannot be excluded with our ZTF observations. +----------------------------------------------------+ | Name | TNS name | Date (UT) | ZTF phot | +----------------------------------------------------+ |PS19dxc | AT2019lzb | 2019-07-06 09:03 | r > 21.8 | | | | 2019-07-28 10:51 | r > 21.8 | +----------------------------------------------------+ |PS19dxb | AT2019lzc | 2019-07-21 07:03 | i > 20.3 | +----------------------------------------------------+ |PS19dxn | AT2019lzt | 2019-07-26 09:20 | r > 21.5 | | | | 2019-07-28 10:31 | r = 21.0 | | | | 2019-07-30 08:41 | r > 21.7 | +----------------------------------------------------+ |PS19dxf | AT2019lzg | 2019-07-26 09:20 | r > 21.5 | | | | 2019-07-30 08:42 | r > 21.6 | +----------------------------------------------------+ |PS19dxg | AT2019lzf | 2019-01-10 02:24 | r > 18.8 | | | | 2019-07-28 10:46 | r = 22.1 | | | | 2019-07-29 04:08 | r > 20.5 | +----------------------------------------------------+ |PS19dwz | AT2019lyz | 2019-07-05 11:26 | g > 21.3 | +----------------------------------------------------+ |PS19dxm | AT2019lzu | 2019-07-14 11:05 | r = 21.5 | +----------------------------------------------------+ |PS19dxd | AT2019lze | 2019-07-24 09:33 | r > 21.3 | | | | 2019-07-28 08:04 | r = 21.6 | | | | 2019-07-31 08:07 | r = 21.5 | +----------------------------------------------------+ |DG19arsob | AT2019lvw | 2019-01-07 02:20 | r > 18.6 | | | | 2019-07-28 10:42 | r = 22.1 | | | | 2019-07-29 04:34 | r > 20.0 | +----------------------------------------------------+ |DG19tfmlb | AT2019lvv | 2019-07-26 09:20 | r > 21.5 | | | | 2019-07-30 08:42 | r > 21.6 | +----------------------------------------------------+ |DG19zrjmb | AT2019lvx | 2019-07-06 07:31 | r = 21.5 | +----------------------------------------------------+ We note that PS19dxn / AT2019lzt may be evolving rapidly. The candidate is detected by both PS1 and ZTF hours after S190728q (ZTF detection on 2019-07-28 10:31 UT, PS1 detection on 2019-07-28 12:43 UT) and it is not detected by ZTF on the following days. The following table includes candidates with ZTF pre-detections. For these candidates, an association with S190728q can be ruled out: +----------------------------------------------------+ | Name | TNS name | Date (UT) | ZTF phot | +----------------------------------------------------+ |PS19dxk | AT2019lzs | 2019-07-06 07:31 | r = 20.9 | |PS19dxl | AT2019lzr | 2019-07-06 07:32 | r = 21.2 | |PS19dwv | AT2019lyv | 2019-07-08 08:51 | r = 21.7 | |PS19dxo | AT2019lzv | 2019-07-08 08:32 | r = 20.9 | |PS19dxe | AT2019lzd | 2019-07-13 07:06 | r = 20.4 | |DG19jufnb | AT2019lvu | 2019-07-06 09:38 | i = 20.7 | +----------------------------------------------------+ Other candidates were reported during the follow-up of S190827q, but their association with the gravitational wave event has already been ruled out by photometric pre-detection or by spectroscopic classification. In particular, those include DG19rxclb (GCN #25215, GCN #25224), DG19blknb (GCN #25215 GCN #25221), and DG19bfhpb (GCN #25215). We caution that we plan to shortly revise the DECam photometry in GCN #25215 as it was impacted by the poor quality of some of the available reference imaging from the DECam Legacy Survey DR7. We thank Stephen Smartt for useful discussions. The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant #12540303 (PI: Graham). 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 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.