When Changing-Look Quasars Wake-Up, Do They Look the Same as Before They Went to Sleep?

0 Assigned ID: 6282 ----- Attempting Submission 1 (Wed Apr 09 20:52:05 GMT 2025) ----- 12/21/2025 -- updated STIS exposure time on visit 4 to accurately reflect required S/N, updated STIS exposures to TIMETAG mode instead of ACCUM. Update 10/18/2025 -- updated ETC runs to use consistent GALEX NUV magnitudes for exposure scaling. Observing mode was initially accidentally set to ACCUM for COS exposures -- here it is updated to TIMETAG mode, and the subsequent exposure times have increased. -1657680531 ----- Attempting Submission 1 (Thu Aug 14 15:04:20 GMT 2025) ----- ----- Attempting Submission 2 (Thu Sep 11 12:33:45 GMT 2025) ----- HST Phase II Proposal 18122 successfully submitted. Receipt: # 18122-2 ----- Attempting Submission 3 (Sat Oct 18 18:59:17 GMT 2025) ----- ----- Attempting Submission 5 (Sun Dec 21 18:57:20 GMT 2025) ----- When Changing-Look Quasars Wake-Up, Do They Look the Same as Before They Went to Sleep? Changing-look quasars (CLQs) show dramatic variability in their broad emission lines and non-stellar continuum strengths on time scales of months to years, challenging models for quasar accretion flows. The currently favored theory for the observed behavior is that the transition from a high, quasar-dominated state to a low, galaxy-dominated state is caused by a changing accretion rate leading to a transition in the accretion flow structure. The typical, geometrically thin, optically thick disk changes to an advection dominated accretion flow (ADAF), as is seen in X-ray binaries. Direct observations of such changes in the accretion flow structure, however, have not yet been made. Thus, we propose to observe two CLQs that have recently returned to a high state to thoroughly test the changing-accretion flow hypothesis. Testing this hypothesis is important for understanding the stability of quasar accretion flows and has implications for feedback from the AGN onto the host galaxy. Both targets were well-characterized in their low states with HST UV spectroscopy, and both have recently turned back on according to optical observations. With new UV spectra, we will probe the structure of the accretion flow by comparing the high state UV-optical spectral energy distributions (SEDs) with low-state UV-optical SEDs and with theoretical models of ADAFs and thin disk SEDs. We will also probe the structure of the broad line region (BLR) by characterizing UV broad emission lines and comparing measurements to predictions from various BLR models. This will serve as a conclusive test of the hypothesis that changes in the accretion flow structure are responsible for the changing look. Target Selection We propose to observe two targets at z~0.24: J1011+5442 and J2336+0017. Recent ZTF g-band magnitudes are ~19 and ~20, respectively. The combination of COS+G140L and STIS+G230L+NUV-MAMA spectra will probe the Lya, N V, Si IV, C IV, and C III] broad emission lines at high significance for both objects. Our proposed targets are the only two known turn-off CLQs that have (a) gone back and forth between high and low states, (b) been observed in both the rest-frame UV and optical in their low states, and (c) since returned to a high state. Archival HST observations of these two targets in their low states also show hallmarks of ADAFs and allow us to design an economical observing program. Instrument Choice and Exposure Times For our first test, we will observe the continuum shape of the two targets in the rest-frame bandpass 1000--2500 Angstrom. This wavelength range covers the portion of the UV SED that differs the most between thin disk quasar SEDs and ADAF SEDs. To cover the longer-wavelength range of the above bandpass, we request STIS+G230L+NUV-MAMA. Although the COS+G230L configuration is more sensitive than the STIS+G230L+NUV-MAMA, the latter is considerably more efficient because it can obtain a spectrum over the entire wavelength range of interest (~1800-2500 Angstrom, rest-frame) with one grating setting. The spectral resolution of STIS+G230L (~200 km/s) is considerably lower than that of the COS+G230L, but that is fine for our purposes since we are interested in wide continuum windows and emission lines of FWHM of a few thousand km/s. For our second test, we wish to observe strong UV emission lines. In the low-state UV spectra, only a modest Lya line was detectable. Thus, we choose COS+G140L for the short-wavelength portion of the UV spectrum, because this instrument is much more sensitive than STIS+G140L. To supplement the UV spectra, we will also obtain contemporaneous optical spectra, within 2--3 weeks of the HST observations, using facilities to which we have institutional access to cover the long-wavelength portion of the SED over rest-frame wavelengths ~4000-8000 Angstrom. We carried out simulations with the ETCs, assuming the SDSS composite quasar spectrum, available in the COS and STIS ETCs, as high-state CLQs resemble normal quasars spectroscopically. We estimated exposure times to attain high significance detections of both the continuum and any broad emission lines. We find the following. For the COS+G140L, after binning the spectrum to 150 km/s bins, one 2400 s exposure per target will yield S/N~15 in the lines and ~10 in the continuum, which will allow us to make the requisite measurements. Hence, considering orbital visibility (52--58 minutes) and acquisition, as well as other overheads, one orbit per target will be sufficient. We note that prior COS+G140L observations of both targets in their low states were able to obtain significant detections of both Lya and the continuum in one orbit. Both targets have brightened significantly in the optical in the time since those observations, thus, even if the UV spectra have not changed in the same way as the optical, we can obtain the required exposure time in one orbit per target. For the STIS G230L, the emission lines of interest are CIV and CIII] Because the spectral bins of the STIS+G230L are much wider than the COS+G140L, we only need to bin the spectra to two pixels per 400 km/s bin. Thus, in order to achieve S/N of ~15 in the lines and ~10 in the continuum, we require 5000 seconds of exposure per target. Considering orbital visibility and including overhead, this amounts to two orbits per object. Our total time request is six orbits; three for J1011+5442, and three for J2336+0017 (one orbit for each with COS+G140L and two orbits for each with STIS+G230L). L. Duffy is an expert on the UV observations of changing-look quasars, and has experience analyzing Hubble UV observations of CLQs, as well as CLQs that undergo multiple state changes. Her optical monitoring campaigns have also led to the identification of the targets of this proposal and the results of her recent work have motivated this proposal. M. Eracleous has experience on a wide variety of topics related to CLQs. These include accretion flows across a broad range of scales and AGN types and associated models. He has experience both modeling and analyzing broad emission-line spectra of quasars. He also has expertise in the spectral energy distributions of low luminosity AGN and CLQs. J. Runnoe & J. Ruan have a great deal of experience in case studies of changing-look quasars. Both have characterized the SEDs of changing-look quasars across a variety of states. J. Runnoe is an expert on quasar SEDs in general. J. Ruan is an expert on the X-ray observations of CLQs and the SEDs of X-ray binaries as analogs for CLQs. Q. Yang is an expert on large spectroscopic studies of populations of CLQs. She also has expertise on the identification and analysis of 'turn-on' CLQs, and has completed X-ray tests of CLQ accretion flows with Chandra data. Supermassive Black Holes and Active Galaxies 6 false true false false false false SUPPORTED GALAXY BLR NUCLEUS QSO QUASAR NO This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. SDSS-J101152.97+544206.4 J2000 J2000 GALEX_FUV =19.067, GALEX_NUV = 18.790 GALAXY BLR NUCLEUS QSO QUASAR NO This object was generated by the targetselector and retrieved from the NED database. SDSS-J233602.98+001728.7 J2000 J2000 GALEX_FUV = 21.486, GALEX_NUV = 21.459 COS/FUV Spectroscopic No No G140L 1280 STIS/NUV-MAMA Spectroscopic No No G230L 2376 COS/FUV Spectroscopic No No G140L 1280 STIS/NUV-MAMA Spectroscopic No No G230L 2376 1766343089945 /private/var/folders/5x/d53sfc292yd033l5v_mgmmlr0000gn/T/OPVisit7510237896817342888.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18122 :visit-id "01" :proposal-version -1 :trans-version "20250624" :date "21-Dec-2025 13:51:29.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 52416 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18122 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18122 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2645 :al-overhead-at-end 0 :internal-dur-at-end 348 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3026 :pcs-gap 0 ) true 1 0 1766343090325 The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. /private/var/folders/5x/d53sfc292yd033l5v_mgmmlr0000gn/T/OPVisit16575905205537301010.xml (Tic-Visit :proposal-id 18122 :visit-id "02" :proposal-version -1 :trans-version "20250624" :date "21-Dec-2025 13:51:30.0" :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 452672 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18122 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18122 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2646 :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 3026 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18122 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2646 :al-overhead-at-end 0 :internal-dur-at-end 601 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3026 :pcs-gap 0 ) true 2 0 1766343090642 /private/var/folders/5x/d53sfc292yd033l5v_mgmmlr0000gn/T/OPVisit5553993144909310735.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18122 :visit-id "03" :proposal-version -1 :trans-version "20250624" :date "21-Dec-2025 13:51:30.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 52416 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18122 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18122 :visit-id "03" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2597 :al-overhead-at-end 0 :internal-dur-at-end 348 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 1 0 1766343091209 The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. /private/var/folders/5x/d53sfc292yd033l5v_mgmmlr0000gn/T/OPVisit11086235650924019832.xml (Tic-Visit :proposal-id 18122 :visit-id "04" :proposal-version -1 :trans-version "20250624" :date "21-Dec-2025 13:51:31.0" :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 313504 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18122 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18122 :visit-id "04" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2599 :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 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18122 :visit-id "04" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2599 :al-overhead-at-end 0 :internal-dur-at-end 601 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0