0 Assigned ID: 7298 ----- Attempting Submission 1 (Thu Apr 16 05:11:06 GMT 2026) ----- HST Phase I Proposal 7298 successfully submitted. Receipt: # 7298-1 ----- Attempting Submission 2 (Thu Apr 16 18:11:14 GMT 2026) ----- Initial phase II submission -141041301 ----- Attempting Submission 1 (Tue Aug 18 18:00:25 GMT 2026) ----- Ultra-rapid HST Target-of-Opportunity observations of a kilonova in LIGO-Virgo-KAGRA IR1 The upcoming LVK run (IR1) will detect new neutron star mergers in gravitational waves, with wide-field optical and infrared survey support for electromagnetic follow up, but interpretation depends on day-timescale kilonova evolution: the early UV and faint optical are where ground-based follow-up is weakest. Only GW170817/AT 2017gfo has been characterized in both gravitational waves and across the electromagnetic spectrum in enough detail to map ejecta composition, geometry, and heating. A comparable event would again require rapid space-based UV spectroscopy and imaging while the photosphere is hot. We request one ultra-disruptive, ultra-rapid HST Target-of-Opportunity trigger in Cycle 34 (overlapping IR1), totaling 10 orbits for coordinated WFC3/UVIS imaging and STIS spectroscopy matched to a ground-based discovery, prioritizing UV throughput and HST's rapid response. Our science goals are (1) to separate weak- and heavy-r-process signatures using early UV/blue photometry with JWST data, (2) to combine electromagnetic ejecta constraints with gravitational-wave chirp mass to inform neutron star radii, equation of state, and maximum mass, and (3) to test models for early UV and optical emission. Theory predicts blue-to-red kilonova diversity, and our HST window extends to ~12 days post-merger while redder optical/NIR monitoring continues from the ground and with JWST. Joint gravitational-wave and electromagnetic analysis will constrain merger dynamics and the remnant. We will coordinate openly, document triggering criteria, and waive proprietary periods so the multimessenger community can use these data immediately. This Phase II program implements one ultra-disruptive Target-of-Opportunity activation (10 orbits) for early UV/optical observations of a kilonova counterpart to a LIGO-Virgo-KAGRA IR1 gravitational-wave event. We will not trigger unless the first visit can begin within 5 days of GW detection, aiming for first-epoch observations within <=3 days of merger for counterparts with r < 22 mag. Following the proposal Special Requirements, APT uses a simple five-epoch, two-orbit sequence (10 orbits total), with all visits On Hold until trigger, Sequential Observations in Time (After Observation links with variable cadence), and Non-Interruptible Sequential Observations within each visit. Exposure times and filter/instrument choices follow Scenario A of Table 1 (GW170817-like, m_F606W~19.5 at t~1 d), and will be optimized toward Scenario A or B at trigger. Epoch 1 (t~1.0 d): WFC3/UVIS G280 spectroscopy with F225W/F275W/F336W imaging and STIS/CCD G430L (52x2 slit). Epoch 2 (t~1.5 d): STIS G430L plus F225W/F275W/F336W. Epoch 3 (t~3.0 d): F275W/F336W/F438W. Epoch 4 (t~5.0 d): F336W/F438W. Epoch 5 (t~8-12 d): F438W. Early cadence is 0.5-2 days; later cadence widens to a few days, ending by ~12 days post-merger. Within each visit, STIS exposures are placed first (STIS/CCD ACQ with MIRROR+F28X50LP, then G430L spectroscopy), followed by WFC3/UVIS imaging/spectroscopy, minimizing SI changes and placing target acquisition immediately before STIS science. Visit 01 uses an Ultra Disruptive response window of 1-2 days; visits 02-05 use Disruptive windows of 1-14 days to accommodate After Observation cadence. PI Charles Kilpatrick: As PI, Kilpatrick leads program science, counterpart vetting, integration with ground and JWST data, and application of analysis frameworks to infer ejecta properties, nucleosynthesis, and equation-of-state constraints from the proposed HST observations. Co-PI David Coulter brings end-to-end expertise in gravitational-wave counterpart searches, rapid-response optical and infrared follow-up, and practical execution of HST Target-of-Opportunity campaigns during LVK runs. As Co-PI he anchors coordination with STScI on triggering, orbit planning, and ultra-disruptive ToO logistics so that IR1 discoveries can be translated into the UV spectroscopy and matched photometry this science requires. Co-PI Armin Rest contributes HST time-domain programs, rapid UV imaging and spectroscopy, supernova and exotic transient science, and technical optimization of short-turnaround observations. James Annis contributes leadership in wide-field optical surveys such as DES, statistical characterization of transient populations, and discovery of faint extragalactic transients at the scale needed for LVK-era follow-up. Iair Arcavi contributes high-cadence optical spectroscopy and photometry of fast transients, kilonovae, and tidal disruption events, and applies classification and physical modeling methods to diverse electromagnetic counterparts. Annalisa Calamida contributes precision HST/WFC3 photometry in crowded fields, differential photometry for variable sources, and vetting of faint transients for space-based follow-up. Jeff Cooke contributes deep imaging and rapid follow-up of fast transients, searches for kilonova counterparts, and characterization of faint extragalactic transients at early phases. Dougal Dobie contributes radio and optical time-domain science, wide-field surveys such as VAST, and joint radio and optical constraints on afterglow and kilonova emission. Maria Drout contributes kilonova phenomenology, stellar progenitors and rates, survey strategy for rare transients, and electromagnetic characterization of neutron star mergers. Wen-fai Fong contributes kilonova and short-GRB afterglow physics, host-galaxy demographics and environments, and multi-epoch optical and infrared campaigns for relativistic and thermal counterparts. Ryan J. Foley contributes kilonova discovery and characterization, high-cadence optical spectroscopy and imaging, and coordination of ground-based campaigns that complement space UV data. Francisco Forster contributes survey operations, real-time brokered classification of transients, and statistical selection of counterparts in wide-field streams. Ori D. Fox contributes HST UV spectroscopy of extragalactic transients, interpretation of slitless and slit spectroscopy, and UV time-domain programs tied to cosmic explosions. Suvi Gezari contributes extragalactic time-domain science across wavelengths, flare and tidal disruption phenomenology, and statistical modeling of variable populations that inform counterpart priors. Sebastian Gomez contributes supernova and kilonova light-curve modeling, comparison to radiative-transfer predictions, and open software for reproducible inference. Daryl Haggard contributes X-ray, optical, and radio follow-up of compact-object mergers, constraints on jets and merger remnants, and multi-wavelength context for kilonova emission. Daichi Hiramatsu contributes spectroscopy and imaging of interacting supernovae and unusual transients, time-series analysis for rapidly evolving sources, and classification under limited prior information. Andrew Howell contributes robotic global follow-up networks, rapid imaging and spectroscopy of transients, and management of high-cadence campaigns for counterpart confirmation. Jacob Jencson contributes infrared imaging and spectroscopy of transients, JWST follow-up strategies, and constraints on red kilonova components and dust. Alexander Ji contributes stellar abundances, r-process signatures in stars, and near-field cosmology that grounds nucleosynthesis tests from kilonova spectra. Saurabh W. Jha contributes nearby supernovae and transients, near-infrared spectroscopy for faint red sources, and precision abundance and velocity measurements for comparison to kilonova models. David O. Jones contributes supernova cosmology from surveys, transient photometric classification at scale, and host-galaxy transient association in deep fields. Daniel Kasen contributes radiative transfer and nucleosynthesis modeling for kilonovae, prediction of UV, optical, and infrared spectra and light curves, and interpretation of ejecta composition and geometry. Curtis McCully contributes high-resolution imaging with robotic facilities, image subtraction and photometry for faint transients on bright hosts, and AGN versus transient discrimination. Gautham Narayan contributes machine learning for transient classification and photometric inference, probabilistic cosmology with supernovae, and scalable pipelines for streaming survey data. Brendan O'Connor contributes gamma-ray bursts and kilonovae, JWST spectroscopy of high-redshift and faint counterparts, and physical modeling of engine and afterglow contributions. Antonella Palmese contributes gravitational-wave cosmology, galaxy catalog cross-matching for counterpart searches, and large-scale structure context for merger localization. Justin D. Pierel contributes supernova and kilonova light-curve fitting, open-source inference tools, and integration of HST photometry with ground-based data and model grids. Anthony L. Piro contributes theoretical models of merger outflows, central-engine activity, and early UV and optical emission from shocks and winds in compact-object mergers. Enrico Ramirez-Ruiz contributes dynamics and nucleosynthesis of tidal and disk ejecta, formation of r-process elements in mergers, and hydrodynamic and radiative modeling tied to observations. Russell Ryan contributes HST calibration, slitless spectroscopy including G280, extraction and astrometry of faint traces, and validation of flux calibration for science-ready spectra. David J. Sand contributes discovery and follow-up of extragalactic transients, host-galaxy mapping and redshifts, and environmental constraints on merger products. Marcelle Soares-Santos contributes gravitational-wave optical counterpart searches such as DES, cosmological inference with standard sirens, and survey statistics for binary neutron star events. Lou Strolger contributes HST rapid-response and ToO policy, scheduling trade-offs for disruptive programs, and supernova and transient science with space-based data. Stefano Valenti contributes spectroscopy and photometry of young extragalactic transients, diversity of stellar explosions, and early-phase characterization relevant to fast kilonova evolution. Yossef Zenati contributes kilonova radiative transfer, r-process nucleosynthesis observables, and multi-messenger constraints on merger physics and ejecta. Other Co-Is on this proposal bring additional expertise in time-domain survey infrastructure and gravitational-wave cosmology, kilonova and merger theory, ZTF pipelines and time-series statistics, HST data reduction and UV spectroscopy, supernova science and host-galaxy environments, student-led and survey follow-up, high-energy and engineering support, observational support and transient classification, and broader institutional and operations support across the collaboration. Stellar Physics and Stellar Types 0 false true false false false false false SUPPORTED EXT-STAR SUPERNOVA Electromagnetic counterpart (kilonova) to a LIGO-Virgo-KAGRA IR1 GW event with at least one neutron star, at a distance consistent with the GW localization, and with r < 22 mag at trigger. WFC3/UVIS Spectroscopic No No G280 WFC3/UVIS Imaging No No F225W WFC3/UVIS Imaging No No F275W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F225W WFC3/UVIS Imaging No No F275W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F275W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F438W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F438W WFC3/UVIS Imaging No No F438W POS-TARG DITHER 0.149 LINE false false 2 14.04 WFC3-UVIS-DITHER-LINE 1 false /Users/ckilpatrick/phaseII/OP-cache/phaseII_JP-v01.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. Scenario A, t~1.0 d (2 orbits): WFC3/UVIS G280 slitless spectroscopy and F225W/F275W/F336W imaging, plus STIS/CCD G430L spectroscopy (52x2 slit), per Table 1. (Tic-Visit :proposal-id 18358 :visit-id "01" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 09:43:24.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 134144 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18358 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18358 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18358 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2596 :al-overhead-at-end 0 :internal-dur-at-end 420 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection. /Users/ckilpatrick/phaseII/OP-cache/phaseII_JP-v02.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. Scenario A, t~1.5 d (2 orbits): STIS/CCD G430L spectroscopy and WFC3/UVIS F225W/F275W/F336W imaging, per Table 1. (Tic-Visit :proposal-id 18358 :visit-id "02" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 09:43:07.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 83104 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18358 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18358 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2594 :al-overhead-at-end 0 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18358 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 47 :internal-dur-at-end 329 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection. /Users/ckilpatrick/phaseII/OP-cache/phaseII_JP-v03.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. Scenario A, t~3.0 d (2 orbits): WFC3/UVIS F275W/F336W/F438W imaging, per Table 1. The Table 1 t~2 d epoch is absorbed into the cadence between epochs 2 and 3. (Tic-Visit :proposal-id 18358 :visit-id "03" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 09:43:08.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18358 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18358 :visit-id "03" :orbit-number 1 :pcs-time 301 :si-vis-utilization 2677 :al-overhead-at-end 126 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection. /Users/ckilpatrick/phaseII/OP-cache/phaseII_JP-v06.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. Scenario A, t~3.0 d (2 orbits): WFC3/UVIS F275W/F336W/F438W imaging, per Table 1. The Table 1 t~2 d epoch is absorbed into the cadence between epochs 2 and 3. (Tic-Visit :proposal-id 18358 :visit-id "06" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 09:43:19.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2309968 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18358 :visit-id "06" :obset-id "06" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18358 :visit-id "06" :orbit-number 1 :pcs-time 301 :si-vis-utilization 2675 :al-overhead-at-end 126 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18358 :visit-id "06" :orbit-number 2 :pcs-time 301 :si-vis-utilization 2677 :al-overhead-at-end 126 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection. /Users/ckilpatrick/phaseII/OP-cache/phaseII_JP-v04.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. Scenario A, t~5.0 d (2 orbits): WFC3/UVIS F336W/F438W imaging. Table 1 allocates 1 orbit here; the second orbit deepens F438W in this simplified 5x2 APT sequence. (Tic-Visit :proposal-id 18358 :visit-id "04" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 09:43:09.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1154976 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18358 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18358 :visit-id "04" :orbit-number 1 :pcs-time 301 :si-vis-utilization 2677 :al-overhead-at-end 126 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection. /Users/ckilpatrick/phaseII/OP-cache/phaseII_JP-v05.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. Scenario A, t~8-12 d (2 orbits): WFC3/UVIS F438W imaging combining the Table 1 t~8 d and t~12 d epochs. (Tic-Visit :proposal-id 18358 :visit-id "05" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 09:43:09.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 577488 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18358 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18358 :visit-id "05" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 123 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection. /Users/ckilpatrick/phaseII/OP-cache/phaseII_JP-v07.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. Scenario A, t~8-12 d (2 orbits): WFC3/UVIS F438W imaging combining the Table 1 t~8 d and t~12 d epochs. (Tic-Visit :proposal-id 18358 :visit-id "07" :proposal-version -1 :trans-version "20260618" :date "18-Aug-2026 09:43:10.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 577488 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18358 :visit-id "07" :obset-id "07" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18358 :visit-id "07" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 123 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 On hold until trigger criteria are met: LVK IR1 GW event with >=1 neutron star, EM kilonova counterpart with r < 22 mag, and first visit schedulable within 5 days of GW detection.