17721( 10) - 09-Dec-2025 18:52:32 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17721 Version: 10 Check-in Time: 09-Dec-2025 18:52:32 STScI Edit Number: 5 Title Building an ANT-hill: STIS UV Follow-up of Ambiguous Nuclear Transients ------------------------------------------------------------------------------------ Type Cycle GO 32 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Jason Hinkle University of Illinois at Urbana - Y Champaign CoI: Prof. Benjamin John Shappee University of Hawaii N CoI: Dr. Chris S. Kochanek The Ohio State University N CoI: Dr. Katie Auchettl University of Melbourne N CoI: Dr. Krzysztof Z. Stanek The Ohio State University N CoI: Dr. Anna Payne Space Telescope Science Institute N ------------------------------------------------------------------------------------ Abstract Due in large part to the rapid growth of all-sky transient surveys, a rich diversity of nuclear transients has been discovered. The two most common such transients are tidal disruption events (TDEs) and active galactic nucleus (AGN) flares. However, it has become increasingly clear that there is a growing class of nuclear flares with characteristics distinct from typical TDEs and AGNs, currently known as ambiguous nuclear transients (ANTs). These ANTs likely represent a window into a hitherto unseen or poorly understood mode of accretion onto supermassive black holes, possibly a TDE occurring within an AGN disk. Here we propose to obtain 2 epochs of STIS UV spectroscopy each for 2 newly-discovered UV-bright ANTs in Cycles 32, 33, and 34 for a total of 12 spectra of 6 ANTs. For each ANT, emission lines in the UV spectra will identify if the source is more TDE-like or AGN-like in nature, providing physical insight into the complex interplay between the existing AGN disk and the disrupted tidal debris. The UV spectra will also allow for crucial insights into outflows launched by ANTs through measurements of broad absorption lines, photoionization physics from measurements of low- and high-ionization lines, and properties of circumnuclear gas through multi-ion curve of growth analysis. Obtaining two spectra per ANT separated by roughly 100 days will probe the temporal evolution of the emitting region, which will be compared with predictions from theoretical models of TDEs in AGN disks. ------------------------------------------------------------------------------------ Observations Description ------------------------ The goal is to obtain a sequence of 2 STIS FUV-MAMA/G140L and NUV-MAMA/G230L UV spectra of two ambiguous nuclear transients (ANTs) per cycle, with a roughly 100-day spacing between the two epochs. This requires 2 orbits per spectroscopic epoch. After identifying a UV-bright ANT (a Swift UVM2 flux above 1e-15 erg/s/cm^2/A), we will trigger one of our 12 ToOs (4 triggers per cycle). Although these are non-disruptive ToOs, we request that the first epoch occur as close to peak as possible. At the same time, we will trigger a second epoch roughly 100 days later as scheduling allows. The exact timing of this second epoch can be 17721( 10) - 09-Dec-2025 18:52:32 - [ 2] flexible, it is most important to have a long baseline between the two epochs. We have set the acquisition image for a point source with the clear/50CCD filter and a 5-second exposure time. This is appropriate for bright ANTs, which should have V~16-17 mag and yield a S/N of ~150 according to the STIS acquisition ETC. This exposure time will be fine for targets with 18.5 > V > 14.5 mag, outside of which we will tweak the exposure time to avoid saturation and ensure a sufficient S/N. We select the 50CCD filter as the contrast between the target (with a blue SED) and host galaxy will be larger than with the standard F28X50LP filter. The source should be well above the sky, so the 52x0.2 slit should be fine for both the FUV and NUV observations. ------------------------------------------------------------------------------------ 17721( 10) - 09-Dec-2025 18:52:32 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2024ZKR EXT-STAR, ACCRETION RA=01H54M22.9560S, J2000 Z = V = 18.6 +/- 0.1 1.2e-15 erg/s/cm^2/A DISK, NUCLEUS DEC=-05D59'50.42" 0.078 Reference Frame: ICRS Comments: Flux in the Swift UVW2 filter. ------------------------------------------------------------------------------------------------------------------------------------ 4 AT2021ZFA EXT-STAR, ACCRETION RA=04H52M18.4007S, J2000 Z = V = 16.25 +/- 0.07 2.0e-15 erg/s/cm^2/A DISK, NUCLEUS DEC=-25D37'5.36" 0.0245 Reference Frame: ICRS Comments: The V magnitude is the host+transient measurement from Swift UVOT. The other flux provided is the transient-only flux in the Swift UVW2 filter. The archival GALEX NUV magnitude of the full host galaxy is ~18.9 mag, at least one magnitude fainter than the transient. ------------------------------------------------------------------------------------------------------------------------------------ 5 EP251020A EXT-STAR, ACCRETION RA=04H57M6.3768S, J2000 Z = V = 16.5 +/- 0.06 2.3e-16 erg/s/cm^2/A DISK, NUCLEUS DEC=-11D43'40.19" 0.026145 Reference Frame: ICRS Extended: NO Comments: The V magnitude is the host+transient measurement from Swift UVOT. The other flux provided is the transient-only flux in the Swift UVW1 filter. ------------------------------------------------------------------------------------------------------------------------------------ 17721( 10) - 09-Dec-2025 18:52:32 - [ 4] TARGET LIST Generic Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Selection Criteria | Flux data No | Name | Description | | ------------------------------------------------------------------------------------------------------------------------------------ 1 NEW-ANT-1 SWIFT UV FLUX AT 2250 A ABOVE V = 16 10^-15 ERG/S/CM^2/A Comments: The V mag is approximate, and will be updated when a suitable target is discovered. Target would also have to meet STIS/MAMA safety criteria, based on Swift follow-up to measure the source UV mag. ------------------------------------------------------------------------------------------------------------------------------------ 2 NEW-ANT-2 SWIFT UV FLUX AT 2250 A ABOVE V = 16 10^-15 ERG/S/CM^2/A Comments: The V mag is approximate, and will be updated when a suitable target is discovered. Target would also have to meet STIS/MAMA safety criteria, based on Swift follow-up to measure the source UV mag. ------------------------------------------------------------------------------------------------------------------------------------ 17721( 10) - 09-Dec-2025 18:52:32 - [ 5] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type STIS-ALONG-SLIT Pattern_Purpose DITHER Number_Of_Points 2 Point_Spacing 0.3829 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 90.0 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type STIS-ALONG-SLIT Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 0.3829 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 90.0 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 17721( 10) - 09-Dec-2025 18:52:32 - [ 6] Visit: 01 (Epoch 1 (AT2021zfa)) Visit Requirements: SCHED 100%; ON HOLD ; TOO RESPONSE TIME 21.0D On Hold Comments: Requires discovery of a sufficiently bright ambiguous nuclear transient. Additional Comments: First UV spectroscopic epoch of AT2021zfa. 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 AT2021ZFA STIS/CCD ACQ 50CCD MIRROR 1 5 S (5 S) Comments: We select the clear/50CCD filter as our target will be blue and we want to maximize contrast from the redder host galaxy. The nominal exposure time of 5 seconds with the clear 50CCD filter will provide sufficient S/N for sources with V magnitudes between 14.5 and 18.5. Outside of this range we will adjust the acquistion exposure time slightly as needed. ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2021ZFA STIS/NUV TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=490 1 5 S -MAMA G see below (980 S) Comments: The buffer time is calculated under the assumption that the source will be faint enough that the count rate is less than 4e6/exptime, which should generally be true for our sources. If the source is significantly brighter in the UV, we will lower the buffer time as recommended in Chapter 11.1 of the STIS instrument handbook. ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2021ZFA STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=250 1 5 S -MAMA G see below (500 S) Comments: The buffer time is calculated under the assumption that the source will be faint enough that the count rate is less than 4e6/exptime, which should nearly always be true for our ANTs in the FUV. Nevertheless, we will confirm the appropriate buffer time based on Swift observations of the UV SED of the source. ------------------------------------------------------------------------------------------------------------------------------------ 17721( 10) - 09-Dec-2025 18:52:32 - [ 7] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 4 AT2021ZFA STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1250 1 5 S -MAMA G see below (2505 S) Comments: The buffer time is calculated under the assumption that the source will be faint enough that the count rate is less than 4e6/exptime, which should nearly always be true for our ANTs in the FUV. Nevertheless, we will confirm the appropriate buffer time based on Swift observations of the UV SED of the source. ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2021ZFA 1 STIS/CCD ACQ 50CCD MIRROR none none none none 1 N/A AT2021ZFA 2 STIS/NUV TIME-TA 52X0.2 G230L 2376 none none none none 1 980.0 S -MAMA G AT2021ZFA 3 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 1 500 S -MAMA G AT2021ZFA 4 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 2 2505.0 S -MAMA G 17721( 10) - 09-Dec-2025 18:52:32 - [ 8] Visit: 02 (Epoch 2 (AT2021zfa)) Visit Requirements: SCHED 100%; AFTER 01 BY 80 D TO 110 D; ON HOLD On Hold Comments: Requires discovery of a sufficiently bright ambiguous nuclear transient. Additional Comments: Second UV spectroscopic epoch of AT2021zfa. 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 AT2021ZFA STIS/CCD ACQ 50CCD MIRROR 1 5 S (5 S) Comments: We select the clear/50CCD filter as our target will be blue and we want to maximize contrast from the redder host galaxy. The nominal exposure time of 5 seconds with the clear 50CCD filter will provide sufficient S/N for sources with V magnitudes between 14.5 and 18.5. Outside of this range we will adjust the acquistion exposure time slightly as needed. ------------------------------------------------------------------------------------------------------------------------------------ 2 AT2021ZFA STIS/NUV TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=500 1 5 S -MAMA G see below (980 S) Comments: The buffer time is calculated under the assumption that the source will be faint enough that the count rate is less than 4e6/exptime, which should generally be true for our sources. If the source is significantly brighter in the UV, we will lower the buffer time as recommended in Chapter 11.1 of the STIS instrument handbook. ------------------------------------------------------------------------------------------------------------------------------------ 3 AT2021ZFA STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=250 1 5 S -MAMA G see below (500 S) Comments: The buffer time is calculated under the assumption that the source will be faint enough that the count rate is less than 4e6/exptime, which should nearly always be true for our ANTs in the FUV. Nevertheless, we will confirm the appropriate buffer time based on Swift observations of the UV SED of the source. We use the 52x0.2 aperture as our default, although if the source fades more quickly than expected over the 100 day timespan between epochs, it may be necessary to use the 52X0.2D1 pseudo-aperture to avoid the FUV-MAMA glow region. ------------------------------------------------------------------------------------------------------------------------------------ 17721( 10) - 09-Dec-2025 18:52:32 - [ 9] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 4 AT2021ZFA STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1250 1 5 S -MAMA G see below (2505 S) Comments: The buffer time is calculated under the assumption that the source will be faint enough that the count rate is less than 4e6/exptime, which should nearly always be true for our ANTs in the FUV. Nevertheless, we will confirm the appropriate buffer time based on Swift observations of the UV SED of the source. We use the 52x0.2 aperture as our default, although if the source fades more quickly than expected over the 100 day timespan between epochs, it may be necessary to use the 52X0.2D1 pseudo-aperture to avoid the FUV-MAMA glow region. ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ AT2021ZFA 1 STIS/CCD ACQ 50CCD MIRROR none none none none 1 N/A AT2021ZFA 2 STIS/NUV TIME-TA 52X0.2 G230L 2376 none none none none 1 980 S -MAMA G AT2021ZFA 3 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 1 500 S -MAMA G AT2021ZFA 4 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 2 2505.0 S -MAMA G 17721( 10) - 09-Dec-2025 18:52:32 - [ 10] Visit: 03 (Epoch 1 (EP251020A)) Visit Requirements: SCHED 100%; TOO RESPONSE TIME 21.0D On Hold Comments: Additional Comments: First UV spectroscopic epoch of EP251020A. 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 EP251020A STIS/CCD ACQ 50CCD MIRROR 1 5 S (5 S) Comments: We select the clear/50CCD filter as our target will be blue and we want to maximize contrast from the redder host galaxy. The nominal exposure time of 5 seconds with the clear 50CCD filter will provide sufficient S/N for sources with V magnitudes between 14.5 and 18.5. Outside of this range we will adjust the acquistion exposure time slightly as needed. ------------------------------------------------------------------------------------------------------------------------------------ 2 EP251020A STIS/NUV TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=500 1 5 S -MAMA G see below (967 S) Comments: The buffer time is calculated under the assumption that the source will be faint enough that the count rate is less than 4e6/exptime, which should generally be true for our sources. If the source is significantly brighter in the UV, we will lower the buffer time as recommended in Chapter 11.1 of the STIS instrument handbook. ------------------------------------------------------------------------------------------------------------------------------------ 3 EP251020A STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=250 1 5 S -MAMA G see below (467 S) Comments: The buffer time is calculated under the assumption that the source will be faint enough that the count rate is less than 4e6/exptime, which should nearly always be true for our ANTs in the FUV. Nevertheless, we will confirm the appropriate buffer time based on Swift observations of the UV SED of the source. ------------------------------------------------------------------------------------------------------------------------------------ 17721( 10) - 09-Dec-2025 18:52:32 - [ 11] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 4 EP251020A STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1250 1 5 S -MAMA G see below (2459 S) Comments: The buffer time is calculated under the assumption that the source will be faint enough that the count rate is less than 4e6/exptime, which should nearly always be true for our ANTs in the FUV. Nevertheless, we will confirm the appropriate buffer time based on Swift observations of the UV SED of the source. ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ EP251020A 1 STIS/CCD ACQ 50CCD MIRROR none none none none 1 N/A EP251020A 2 STIS/NUV TIME-TA 52X0.2 G230L 2376 none none none none 1 967.0 S -MAMA G EP251020A 3 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 1 467.0 S -MAMA G EP251020A 4 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 2 2459.0 S -MAMA G 17721( 10) - 09-Dec-2025 18:52:32 - [ 12] Visit: 04 (Epoch 2 (EP251020A)) Visit Requirements: SCHED 100%; ON HOLD On Hold Comments: Requires discovery of a new ambiguous nuclear transient. Additional Comments: Second UV spectroscopic epoch of EP251020A. 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 EP251020A STIS/CCD ACQ 50CCD MIRROR 1 5 S (5 S) Comments: We select the clear/50CCD filter as our target will be blue and we want to maximize contrast from the redder host galaxy. The nominal exposure time of 5 seconds with the clear 50CCD filter will provide sufficient S/N for sources with V magnitudes between 14.5 and 18.5. Outside of this range we will adjust the acquistion exposure time slightly as needed. ------------------------------------------------------------------------------------------------------------------------------------ 2 EP251020A STIS/NUV TIME-TA 52X0.2 G230L 2376 BUFFER-TIME=500 1 5 S -MAMA G see below (1000 S) Comments: The buffer time is calculated under the assumption that the source will be faint enough that the count rate is less than 4e6/exptime, which should generally be true for our sources. If the source is significantly brighter in the UV, we will lower the buffer time as recommended in Chapter 11.1 of the STIS instrument handbook. ------------------------------------------------------------------------------------------------------------------------------------ 3 EP251020A STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=250 1 5 S -MAMA G see below (500 S) Comments: The buffer time is calculated under the assumption that the source will be faint enough that the count rate is less than 4e6/exptime, which should nearly always be true for our ANTs in the FUV. Nevertheless, we will confirm the appropriate buffer time based on Swift observations of the UV SED of the source. We use the 52x0.2 aperture as our default, although if the source fades more quickly than expected over the 100 day timespan between epochs, it may be necessary to use the 52X0.2D1 pseudo-aperture to avoid the FUV-MAMA glow region. ------------------------------------------------------------------------------------------------------------------------------------ 17721( 10) - 09-Dec-2025 18:52:32 - [ 13] Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 4 EP251020A STIS/FUV TIME-TA 52X0.2 G140L 1425 BUFFER-TIME=1250 1 5 S -MAMA G see below (2500 S) Comments: The buffer time is calculated under the assumption that the source will be faint enough that the count rate is less than 4e6/exptime, which should nearly always be true for our ANTs in the FUV. Nevertheless, we will confirm the appropriate buffer time based on Swift observations of the UV SED of the source. We use the 52x0.2 aperture as our default, although if the source fades more quickly than expected over the 100 day timespan between epochs, it may be necessary to use the 52X0.2D1 pseudo-aperture to avoid the FUV-MAMA glow region. ------------------------------------------------------------------------------------------------------------------------------------ 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 | ------------------------------------------------------------------------------------------------------------------------------------ EP251020A 1 STIS/CCD ACQ 50CCD MIRROR none none none none 1 N/A EP251020A 2 STIS/NUV TIME-TA 52X0.2 G230L 2376 none none none none 1 1000.0 S -MAMA G EP251020A 3 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 1 500 S -MAMA G EP251020A 4 STIS/FUV TIME-TA 52X0.2 G140L 1425 none none none none 2 2500.0 S -MAMA G 17721( 10) - 09-Dec-2025 18:52:32 - [ 14] Summary Form for Proposal 17721 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures 50CCD, 52X0.2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations STIS/CCD, STIS/FUV-MAMA, STIS/NUV-MAMA ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACQ, TIME-TAG ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters BUFFER-TIME=1250, BUFFER-TIME=250, BUFFER-TIME=490, BUFFER-TIME=500 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01 BY 80 D TO 110 D, ON HOLD , SCHED 100%, TOO RESPONSE TIME 21.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements G140L, G230L, MIRROR ------------------------------------------------------------------------------------------------------------------------------------ Target Names AT2021ZFA, EP251020A ------------------------------------------------------------------------------------------------------------------------------------