0 Assigned ID: 6506 ----- Attempting Submission 1 (Thu Apr 10 16:09:29 GMT 2025) ----- HST Phase I Proposal 6506 successfully submitted. Receipt: # 6506-1 ----- Attempting Submission 2 (Thu Apr 10 22:07:22 GMT 2025) ----- HST Phase I Proposal 6506 successfully submitted. Receipt: # 6506-2 ----- Attempting Submission 3 (Thu Apr 10 22:34:15 GMT 2025) ----- Updated submission reflecting changes to this Cycle's programme (dropping STIS and increasing the COS integration time) that were not reflected in the previous submission. Specifically, I increased the Exposure Time on both visits from 1000s to 1500s. (I did *not* increase the buffer time since there will be uncertainty about the target brightness, and I figured keeping it as it was was the more conservative choice.) The Orbit Planner is still not working on my PC, so parts of the program that rely on Orbit Planner calculations should be checked carefully. -562632599 ----- Attempting Submission 1 (Mon Aug 04 21:09:46 GMT 2025) ----- ----- Attempting Submission 2 (Sun Oct 12 14:28:44 GMT 2025) ----- The Orbit Planner is still not working on my system - I get an error message saying that it could not connect to the server and that APT needs to be restarted, but restarting does not fix the problem. (I have got this error on several occasions, so it appears to be a bug or a system problem, not an issue with the server.) However, this is a Generic target, so the exposure times are only estimates anyway. Recent surveys have identified a population of rapidly evolving, very luminous extragalactic transients which radiate primarily in the ultraviolet (even out to late times) and are accompanied by powerful X-ray and radio emission. These transients lack any discernable supernova features in optical spectra and ground-based observations have been unable to definitively pin down their origins, although a combination of extreme energetics and low ejecta masses indicate they are probably linked to accretion events involving very massive (but not supermassive) black holes. We propose to obtain a far-ultraviolet spectrum of a newly-identified transient of this nature in Cycle 33 or 34, which would be only the second such spectrum obtained to date. The abundant strong line features in the FUV make it a uniquely powerful spectral regime for testing models for the structure, composition, and velocity of the outflow and allow for a crucial test of competing models: whether these events represent the collapses of massive stars into black holes (and if so, which types of stars), or the disruption and accretion of stars onto supermassive black holes. Our objective is to carry out low-resolution FUV and NUV spectroscopy of a fast-luminous UV transient with COS (G140L) and STIS (G230L). Fast turnaround is helpful for many reasons - both to optimize the S/N (the target will fade rapidly) and to decrease the risk of acquisition failure if the target's UV flux evolution is different than expected. A turnaround of less than 1 week is ideal, although for a sufficiently nearby event we may accept a longer delay. Using fewer guide stars may be acceptable to achieve faster turnaround. It is desirable in principle to obtain the COS and STIS observations close in time to each other but observing sooner in one band will almost always be preferred if the option is available. Rapid two-way communication with STScI will be necessary to correctly configure the observations and perform all safety checks to ensure a successful acquisition. The PI's mobile number is +44 745 6339330 and can be contacted 24 hours a day. Our group has extensive experience with ultraviolet spectroscopy and with fast optical transients. PI D. Perley, an associate professor at LJMU, led an international collaboration to follow and has published a comprehensive paper on the early-through-late UVOIR observations of AT2018cow. He also discovered AT2020xnd (another Cow-like event), and is PI of the C31/C32 HST-COS/STIS programme to observe AT2024wpp, a recent LFBOT. Anna Ho, an assistant professor at Cornell, led the long-wavelength follow-up and published papers on the X-ray, submillimeter and radio properties of these events. She developed the tools that are used to scan for new fast transients in the ZTF data stream and which were used to discover nearly all known LFBOTs to date. Shri Kulkarni is a professor at Caltech and is an expert in transient astronomy and former PI of the Zwicky Transient Facility. Lin Yan and Ragnhild Lunnan are experts in superluminous supernovae (another class of hot, UV transient) and have published several papers on UV spectroscopy, and led previous HST ToO proposals on those events. Our team has proprietary access to ZTF, a fast-cadence wide-area optical survey that is well-suited to finding future examples of this class. We also have access to a variety of robotic follow-up resources to help rapidly confirm the fast-luminous nature of any candidate transient, including the Palomar 60-inch telescope and the Liverpool Telescope; plus target-of-opportunity programs on Palomar, Gemini, Keck, and VLT. We are frequent users of Swift and have programs to study fast transients at other wavelengths, including at the VLA, NOEMA, and ALMA. Stellar Physics and Stellar Types Supermassive Black Holes and Active Galaxies 6 false true false false false false SUPPORTED STAR SUPERNOVA The target will be identified from ZTF or potentially another high-cadence optical survey. The trigger will be submitted at or after peak and there will be a clear upper limit on the flux of the target based on this. The source will probably be close to V=18.5, UV=17.5 mag at peak (AB). The source is expected to fade rapidly after peak, but this is difficult to project. The target is a point source and we expect it will be brighter than its host galaxy (in UV) but will confirm at the time of triggering. Fast rise (<3 days) to high luminosity (M ~ -20) without cooling COS/FUV Spectroscopic No No G140L 1280 Exposure times will need to be adjusted depending on the visibility of the actual target at time of trigger. false -1 -1 Target of opportunity awaiting a suitable trigger from ZTF or another survey. The anticipated rate of fading is 0.25 mag/day, so sooner observations is always better. A longer response may be acceptable depending on the initial brightness of the event. There will be uncertainty in the target magnitude, although most of the uncertainty will be in the downward direction (how fast will it fade) once the time of observation is determined. We may elect to use MIRRORA if the target is faint. Exposure times will need to be adjusted depending on the visibility of the actual target at time of trigger. This visit is currently identical to the other COS visit. The observation is broken into two separate visits to increase scheduling flexibility. Our general expectation is that the two visits will be executed back to back, but if the TOO target is sufficiently bright we may request a time constraint on the second visit to delay the observation for a few days to study time evolution. false -1 -1 Target of opportunity awaiting a suitable trigger from ZTF or another survey. The anticipated rate of fading is 0.25 mag/day, so sooner observations is always better. A longer response may be acceptable depending on the initial brightness of the event. There will be uncertainty in the target magnitude, although most of the uncertainty will be in the downward direction (how fast will it fade) once the time of observation is determined. We may elect to use MIRRORA if the target is faint.