TITLE: GCN CIRCULAR NUMBER: 32643 SUBJECT: Swift J1913.1+1946 : Pre-discovery limits from ZTF DATE: 22/10/10 04:06:07 GMT FROM: Viraj Karambelkar at Indian Inst of Tech,Bombay Viraj Karambelkar (Caltech), Igor Andreoni (Univ. of Maryland), Eric Bellm (University of Washington), Daniel Perley (LJMU), Harsh Kumar (IIT Bombay) on behalf of the ZTF collaboration We performed forced photometry on ~1400 Zwicky Transient Facility (ZTF) images dating back to 2018-03-27 at the location of the bright hard X-ray and optical transient Swift J1913.1+1946 (Dichiara et al., GCN 32632; Lipunov et al., GCN 32634; Perley et al. GCN 32638; Broens et al. GCN 32640). We used the ZTF point-spread-function (PSF) forced photometry service (Masci et al., 2018) on images processed through the ZTF reduction and image subtraction pipelines at IPAC (Masci et al., 2019) No source is detected in the archival ZTF images, which reached a median 5-sigma depth of ~21 mag (AB) in the g, r and i filters. ZTF last observed the field of this transient on JD 2459861.68 in the g-band, and on JD 2459860.59 in the r-band -- 0.4 and 1.5 days prior to the Swift BAT discovery respectively. No source was detected in these images, and the following 5-sigma upper limits were measured - +----------------------+------------+------------------------+--------+----------+ | Date [UT] | JD | days_before_discovery | Filter | mag (AB) | | ---------------------+------------+------------------------+--------+----------+ | 2022-10-09 04:19:12 | 2459861.68 | 0.4 | ZTF-g | >20.1 | | 2022-10-08 02:09:36 | 2459860.59 | 1.5 | ZTF-r | >22.0 | +----------------------+------------+------------------------+--------+----------+ The reported limits are not corrected for Galactic extinction. ZTF is a project led by PI S. R. Kulkarni at Caltech (see ATEL #11266), and includes IPAC; WIS, Israel; OKC, Sweden; JSI/UMd, USA; UW, USA; DESY, Germany; NSTC, Taiwan; UW Milwaukee, USA and LANL USA. ZTF acknowledges the generous support of the NSF under AST MSIP Grant No 1440341. Alert distribution service provided by DIRAC@UW. The ZTF forced-photometry service was funded under the Heising-Simons Foundation grant #12540303 (PI: Graham)