{
  "bibcode": "2022GCN.32643....1K",
  "body": "Viraj Karambelkar (Caltech), Igor Andreoni (Univ. of Maryland), Eric Bellm\n(University of Washington), Daniel Perley (LJMU), Harsh Kumar (IIT Bombay)\non behalf of the ZTF collaboration\n\nWe performed forced photometry on ~1400 Zwicky Transient Facility (ZTF)\nimages dating back to 2018-03-27 at the location of the bright hard X-ray\nand optical transient Swift J1913.1+1946 (Dichiara et al., GCN 32632;\nLipunov et al., GCN 32634; Perley et al. GCN 32638; Broens et al. GCN\n32640). We used the ZTF point-spread-function (PSF) forced photometry\nservice (Masci et al., 2018) on images processed through the ZTF reduction\nand image subtraction pipelines at IPAC (Masci et al., 2019)\n\nNo source is detected in the archival ZTF images, which reached a median\n5-sigma depth of ~21 mag (AB) in the g, r and i filters.\n\nZTF last observed the field of this transient on JD 2459861.68 in the\ng-band, and on JD 2459860.59 in the r-band -- 0.4 and 1.5 days prior to the\nSwift BAT discovery respectively. No source was detected in these images,\nand the following 5-sigma upper limits were measured -\n\n+----------------------+------------+------------------------+--------+----------+\n\n| Date [UT]            |  JD        |  days_before_discovery | Filter | mag\n(AB) |\n\n|\n---------------------+------------+------------------------+--------+----------+\n\n| 2022-10-09 04:19:12  | 2459861.68 |         0.4            | ZTF-g  |\n>20.1    |\n\n| 2022-10-08 02:09:36  | 2459860.59 |         1.5            | ZTF-r  |\n>22.0    |\n\n+----------------------+------------+------------------------+--------+----------+\n\nThe reported limits are not corrected for Galactic extinction.\n\n\n\nZTF is a project led by PI S. R. Kulkarni at Caltech (see ATEL #11266), and\nincludes IPAC; WIS, Israel; OKC, Sweden; JSI/UMd, USA; UW, USA; DESY,\nGermany; NSTC, Taiwan; UW Milwaukee, USA and LANL USA. ZTF acknowledges the\ngenerous support of the NSF under AST MSIP Grant No 1440341. Alert\ndistribution service provided by DIRAC@UW. The ZTF forced-photometry\nservice was funded under the Heising-Simons Foundation grant #12540303 (PI:\nGraham)",
  "circularId": 32643,
  "createdOn": 1665374767000,
  "email": "karambelkarvraj21197@gmail.com",
  "subject": "Swift J1913.1+1946 : Pre-discovery limits from ZTF",
  "submitter": "Viraj Karambelkar at Indian Inst of Tech,Bombay  <karambelkarvraj21197@gmail.com>"
}