AstroNote 2024-261

The ZTF Flexible Weekends experiment (Rehemtulla et al. 2024-216), conducted August 10 - 12 2024 UTC, was focused on discovering infant SNe (~days after explosion) with a special emphasis on finding a Type II SN suitable for follow-up through HST Flexible Thursday program 17507 (PI Jacobson-Galán). The observing strategy involved taking two 60 second exposures per field (1 each in g and r band) per night over ZTF extragalactic fields which were also visible to HST (field id list). During the three nights of the experiment, an average of 238k alerts were sent per night, with 101 total new transients reported to TNS. This is an increased number of new transients discovered by ZTF compared to prior 3 day observing increments e.g., 89 transients (Aug. 7 - 9 UTC), 74 transients (Aug. 4 - 6 UTC) and 55 transients (Aug. 1 - 3 UTC).

Following completion of the ZTF experiment, we triggered HST UV/optical spectroscopy on the Type II SN 2024ryv, first reported to TNS by ATLAS (Tonry et al. 2024) on MJD 60534.29. ZTF forced photometry revealed a likely first r-band detection of the SN on MJD 60533.36 (m = 21.5 mag; M = -12.6 mag for D=65.7 Mpc) from the previous night of observations, indicating that the transient was extremely young and rising rapidly. This deep forced photometry detection was made possible by the experiment’s long exposures. We obtained a classification spectrum with the Liverpool Telescope on 2024-08-13, which revealed a blue, featureless continuum. The transient was later confirmed to be a Type II SN (Angus et al. 2024). HST UV/optical spectroscopy was obtained at MJD 60537.54. Using the time of the first ZTF forced photometry detection, the UV spectrum of SN 2024ryv occurred at a phase of ~4.2 days after first light. We will conduct similar observations preceding upcoming HST Flexible Thursday triggers during Cycles 31 and 32 through ZTF partnership time.

ZTF is supported by the National Science Foundation under Grants No. AST-1440341 and AST-2034437 and a collaboration including current partners Caltech, IPAC, the Oskar Klein Center at Stockholm University, the University of Maryland, University of California, Berkeley, the University of Wisconsin at Milwaukee, University of Warwick, Ruhr University, Cornell University, Northwestern University and Drexel University.

Zwicky Transient Facility access for N.R., and S.S. was supported by Northwestern University and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).