18077( 3) - 13-Oct-2025 14:47:40 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18077 Version: 3 Check-in Time: 13-Oct-2025 14:47:40 STScI Edit Number: 0 Title Ultraviolet Spectroscopy of a Luminous Fast Blue Optical Transient ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Daniel Perley Liverpool John Moores University Y CoI: Anna Yen Qin Ho Cornell University N CoI: Dr. Christoffer Fremling California Institute of Technology N CoI: Dr. Lin Yan California Institute of Technology N CoI: Prof. Shrinivas R. Kulkarni California Institute of Technology N CoI: Dr. Ragnhild Lunnan Stockholm University N ------------------------------------------------------------------------------------ Abstract 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. ------------------------------------------------------------------------------------ Observations Description ------------------------ 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. 18077( 3) - 13-Oct-2025 14:47:40 - [ 2] ------------------------------------------------------------------------------------ 18077( 3) - 13-Oct-2025 14:47:40 - [ 3] TARGET LIST Generic Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Selection Criteria | Flux data No | Name | Description | | ------------------------------------------------------------------------------------------------------------------------------------ 1 LFBOT STAR, SUPERNOVA FAST RISE (<3 DAYS) TO HIGH V = 19 LUMINOSITY (M ~ -20) WITHOUT COOLING Comments: 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. ------------------------------------------------------------------------------------------------------------------------------------ 18077( 3) - 13-Oct-2025 14:47:40 - [ 4] Visit: 01 (COS FUV spectrum) Visit Requirements: ON HOLD ; TOO RESPONSE TIME 7.0D On Hold Comments: 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. Additional Comments: Exposure times will need to be adjusted depending on the visibility of the actual target at time of trigger. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 LFBOT COS/NUV ACQ/IMA PSA MIRRORB 1 180 S (180 GE S) Comments: 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. ------------------------------------------------------------------------------------------------------------------------------------ 2 LFBOT COS/FUV TIME-TA PSA G140L 1105 FLASH=YES, BUFFER-TIME=6400, 1 1500 S G FP-POS=ALL (6000 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ LFBOT 1 COS/NUV ACQ/IMA PSA MIRRORB none none none none tbd N/A GE LFBOT 2 COS/FUV TIME-TA PSA G140L 1105 none none none 1 tbd N/A G LFBOT 2 COS/FUV TIME-TA PSA G140L 1105 none none none 2 tbd N/A G LFBOT 2 COS/FUV TIME-TA PSA G140L 1105 none none none 3 tbd N/A G LFBOT 2 COS/FUV TIME-TA PSA G140L 1105 none none none 4 tbd N/A G 18077( 3) - 13-Oct-2025 14:47:40 - [ 5] Visit: 02 (COS FUV spectrum) Visit Requirements: ON HOLD ; TOO RESPONSE TIME 7.0D On Hold Comments: 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. Additional Comments: 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. Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 LFBOT COS/NUV ACQ/IMA PSA MIRRORB 1 180 S (180 GE S) Comments: 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. ------------------------------------------------------------------------------------------------------------------------------------ 2 LFBOT COS/FUV TIME-TA PSA G140L 1105 FLASH=YES, BUFFER-TIME=6400, 1 1500 S G FP-POS=ALL (6000 S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ LFBOT 1 COS/NUV ACQ/IMA PSA MIRRORB none none none none tbd N/A GE LFBOT 2 COS/FUV TIME-TA PSA G140L 1105 none none none 1 tbd N/A G LFBOT 2 COS/FUV TIME-TA PSA G140L 1105 none none none 2 tbd N/A G LFBOT 2 COS/FUV TIME-TA PSA G140L 1105 none none none 3 tbd N/A G LFBOT 2 COS/FUV TIME-TA PSA G140L 1105 none none none 4 tbd N/A G 18077( 3) - 13-Oct-2025 14:47:40 - [ 6] Summary Form for Proposal 18077 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures PSA ------------------------------------------------------------------------------------------------------------------------------------ Configurations COS/FUV, COS/NUV ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ/IMAGE, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=6400, FLASH=YES, FP-POS=ALL ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ON HOLD , TOO RESPONSE TIME 7.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, MIRRORB ------------------------------------------------------------------------------------------------------------------------------------ Target Names LFBOT ------------------------------------------------------------------------------------------------------------------------------------