This target is not used in this proposal. This target is not used in this proposal. This target is not used in this proposal. This pattern is not used in this proposal. This pattern is not used in this proposal. ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN Phase II submission for new target EP251020a. 871656375 ----- Attempting Submission 4 (Tue Dec 09 15:55:16 GMT 2025) ----- Some targets and pattenrs are retained from earlier submissions. The orbit overruns are minor and can be addressed when more detailed visibility is known. 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. 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 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. This is a proposal by the ASAS-SN team to observe a new ambiguous nuclear transient (ANT) with UV spectroscopy using STIS. ANTs likely either represent unique TDEs, extreme forms of AGN activity, or a TDE in an AGN. ASAS-SN has revolutionized the TDE field through its discovery and characterization of bright, nearby TDEs. This includes the discoveries of ASASSN-14ae (Holoien et al. 2014, Brown et al. 2016), ASASSN-14li (Holoien et al. 2016, Brown et al. 2017a), ASASSN-15oi (Holoien et al. 2016, 2018), ASASSN-18jd (Neustadt et al. 2020), ASASSN-18pg (Holoien et al. 2020), ASASSN-19bt (Holoien et al. 2019b), ASASSN-19dj (Hinkle et al. 2021), and ASASSN-23bd (Hoogendam et al. 2024). The paper on iPTF16fnl includes our STIS UV spectra from a previous proposal, although the campaign was halted after three observations because the source faded so rapidly. The paper on ASASSN-18jd contains a sequence of STIS spectra we obtained. We have also published UV spectra of the ANT ASASSN-17jz (Holoien et al. 2021). The PI and CoIs have extensive experience with both TDE and AGN science. The analysis and paper(s) resulting from this proposal will be led by the PI. Supermassive Black Holes and Active Galaxies 6 false true false false false false SUPPORTED EXT-STAR ACCRETION DISK NUCLEUS Flux in the Swift UVW2 filter. ZTF18ABUXIWJ J2000 1.2e-15 erg/s/cm^2/A EXT-STAR ACCRETION DISK NUCLEUS 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. ATLAS21BIYB J2000 2.0e-15 erg/s/cm^2/A EXT-STAR ACCRETION DISK NUCLEUS NO 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. J2000 2.3e-16 erg/s/cm^2/A 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. Swift UV flux at 2250 A above 10^-15 erg/s/cm^2/A 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. Swift UV flux at 2250 A above 10^-15 erg/s/cm^2/A STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 POS-TARG DITHER 0.3829 LINE false false 2 90.0 STIS-ALONG-SLIT 1 false POS-TARG DITHER 0.3829 LINE false false 3 90.0 STIS-ALONG-SLIT 2 false 1765295575708 Timing link suitability includes constraints ([02 After 01 by 80.0 to 110.0 days]) The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. First UV spectroscopic epoch of AT2021zfa. 1755604728000 252.18054535789435 (Tic-Visit :proposal-id 17721 :visit-id "01" :proposal-version -1 :trans-version "20250624" :date "09-Dec-2025 09:52:55.0" :trans-errors '(:WARNINGS 2 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 539632 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17721 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17721 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2533 :al-overhead-at-end 0 :internal-dur-at-end 178 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) (Tic-Orbit :proposal-id 17721 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2533 :al-overhead-at-end 0 :internal-dur-at-end 582 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) true 2 2 Requires discovery of a sufficiently bright ambiguous nuclear transient. 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. 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. 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. 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. 1765295576925 Timing link suitability includes constraints ([02 After 01 by 80.0 to 110.0 days]) The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. Second UV spectroscopic epoch of AT2021zfa. (Tic-Visit :proposal-id 17721 :visit-id "02" :proposal-version -1 :trans-version "20250624" :date "09-Dec-2025 09:52:56.0" :trans-errors '(:WARNINGS 2 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 539632 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17721 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17721 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2533 :al-overhead-at-end 0 :internal-dur-at-end 178 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) (Tic-Orbit :proposal-id 17721 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2533 :al-overhead-at-end 0 :internal-dur-at-end 582 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2864 :pcs-gap 0 ) true 2 2 Requires discovery of a sufficiently bright ambiguous nuclear transient. 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. 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. 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. 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. 1765302012616 Timing link suitability includes constraints ([04 After 03 by 50.0 to 300.0 days]) The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. /System/Volumes/Data/solaris/user1/mdocher/hst-work/OP-cache/17721-v03.xml First UV spectroscopic epoch of EP251020A. (Tic-Visit :proposal-id 17721 :visit-id "03" :proposal-version -1 :trans-version "20250929" :date "09-Dec-2025 12:40:12.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 470048 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17721 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17721 :visit-id "03" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2487 :al-overhead-at-end 0 :internal-dur-at-end 178 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2867 :pcs-gap 0 ) (Tic-Orbit :proposal-id 17721 :visit-id "03" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2487 :al-overhead-at-end 0 :internal-dur-at-end 582 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2867 :pcs-gap 0 ) true 2 0 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. 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. 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. 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. 1765295579057 Timing link suitability includes constraints ([04 After 03 by 50.0 to 300.0 days]) The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. Second UV spectroscopic epoch of EP251020A. (Tic-Visit :proposal-id 17721 :visit-id "04" :proposal-version -1 :trans-version "20250624" :date "09-Dec-2025 09:52:58.0" :trans-errors '(:WARNINGS 2 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "24" :si-bright-earth-limb-angle 20.0 :data-volume 470048 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17721 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17721 :visit-id "04" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2553 :al-overhead-at-end 0 :internal-dur-at-end 178 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2867 :pcs-gap 0 ) (Tic-Orbit :proposal-id 17721 :visit-id "04" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2528 :al-overhead-at-end 0 :internal-dur-at-end 582 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2867 :pcs-gap 0 ) true 2 2 Requires discovery of a new ambiguous nuclear transient. 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. 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. 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. 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.