0 Assigned ID: 541 ----- Attempting Submission 1 (Fri Mar 06 16:34:35 GMT 2020) ----- HST Phase I Proposal 541 successfully submitted. Receipt: # 541-1 PI added BETWEEN constraints based on the Target Viewing Time Report. The schedulability has been changed to 100%. -427709755 ----- Attempting Submission 1 (Tue Jun 23 11:51:49 GMT 2020) ----- HST Phase II Proposal 16317 successfully submitted. Receipt: # 16317-1 ----- Attempting Submission 2 (Tue Jun 23 18:16:37 GMT 2020) ----- The advent of GALEX and COS have revolutionized our view of HeII reionization, the final major phase transition of the intergalactic medium. Efficient COS surveys have provided a statistical sample of strongly fluctuating HeII Lyman alpha absorption at 2.7<z<3, evoking a picture of overlapping HeIII zones around quasars at the end of HeII reionization. Unexpectedly, the seven rare quasars probing z>3.5 have revealed a set of HeII transmission 'spikes' and larger regions with nonzero transmission that suggest HeII reionization was well underway by z=4. This is in striking conflict with radiative transfer simulations of a HeII reionization driven by bright quasars. Explaining these measurements may require either faint quasars or more exotic sources of hard photons at z>4, with concomitant implications for HI reionization. A recent survey for high-redshift quasars in the southern hemisphere has uncovered the highest-redshift far-UV-bright quasar known (z=3.95, FUV=21.7). We propose here for COS G140L follow-up spectroscopy to (i) confirm its utility for HeII absorption studies by identifying possible interloping low-z HI absorbers, (ii) provide accurate measurements of the HeII Lyman alpha effective optical depth that are limited by systematics for much fainter already observed quasars, and (iii) constrain the lifetime of the background quasar from its ionized proximity zone. Moreover, the proposed observations would provide only the third z>3.5 sightline that would allow for high-resolution G130M spectroscopy before the end of HST's mission. We will obtain a COS FUV G140L spectrum of the recently discovered highest-redshift UV-bright quasar at z=3.95 to characterize intergalactic HeII Lyman alpha absorption along its sightline. Our target has a sufficiently accurate flux in the quasar continuum just redward of the HeII break (GALEX FUV=21.7+/-0.4), and is safe to observe with COS. Its precision Gaia astrometry enables efficient acquisition with ACQ/IMAGE. The target is not detected in GALEX NUV, and its spectral energy distribution suggests NUV~23.9 (Fig. 5 in the proposal). With the COS ETC we estimate a NUV imaging acquisition exposure time of 400 seconds to reach the recommended S/N=20. The G140L grating provides the wavelength coverage to define the quasar continuum and the necessary resolution to measure the HeII absorption on scales of ~40 comoving Mpc. We require a S/N~3 per resolution element at 1500A, which results in a total request of 4 orbits. Continuous wavelength coverage at good sensitivity (lambda>1100A) is required to maximize the coverage of the HeII Lyman series absorption, only provided by the G140L 800A setup. In accord with our Phase I proposal and considering the significant acquisition overhead, the target will be observed in a single 4-orbit visit, using all FP-POS offsets in consecutive orbits. Spectra will be recorded in TIME-TAG mode with concurrent wavelength calibration (TAGFLASH). Time intervals spent in the Earth's shadow will be used to correct for geocoronal OI and NI emission. Given our faint source, airglow will dominate the COS count rates. However, even at the high airglow conditions conservatively assumed in our ETC calculations, the buffer time is longer than the exposure time. All exposure times have been adjusted to use the full visibility period in each orbit. In order to limit detector gain sag and contamination of the HeII absorption region by scattered geocoronal Lyman alpha emission, the observations should not be scheduled in visibility periods with large daytime portions. We have selected suitable time windows from the target viewing time report using BETWEEN constraints and setting the schedulability to 100%. Gain sag limits the accuracy of the COS dark current subtraction, which is critical to reach our science goals. We have developed a custom pipeline to determine the dark current in the COS PSA by post-processing COS dark monitoring exposures (Worseck et al. 2016, ApJ, 825, 144). For this calibration we require COS dark monitoring exposures taken at the same detector voltage as the science observations. Fortunately, up to now G140L science exposures are being taken at the same detector voltage as the dark monitoring exposures, such that special calibrations are unlikely to be required (see our program GO-15356). Reduced gyro operations will not affect our program, because we do not have orientation constraints (other than the BETWEEN constraints above). APT indicates a sufficient number of scheduling windows for our 4-orbit visit. Our team combines the observational, theoretical and technical expertise to perform the proposed research project. The PI, Dr. Gabor Worseck, is an expert in observational studies to constrain the reionization epochs of hydrogen and helium with quasar spectroscopy. He is a renowned expert in the reduction and analysis of HST/COS data in the low-count Poisson regime with custom software that correctly estimates dark current and scattered light in the COS aperture (e.g. Worseck et al. 2016, ApJ, 825, 144; Worseck et al. 2019, ApJ, 875, 111). He has led five related HST/COS programs in Cycles 17-25 that have resulted in 8 published papers so far. Dr. Frederick B. Davies is an expert in semi-analytic modeling of the fluctuating HeII-ionizing background (Davies & Furlanetto 2014, MNRAS, 437, 1141; Davies et al. 2017, MNRAS, 465, 2886), and the proximity effect (Davies et al. 2020, MNRAS, 493, 1330). Dr. Ilya Khrykin is supplying radiative transfer simulations of HeII QSO proximity zones for comparison with the HST/COS data to infer the quasar lifetime (Khrykin et al. 2016, ApJ, 824, 133; Khrykin et al. 2019, MNRAS, 484, 3897). Professors Joseph F. Hennawi and J. Xavier Prochaska provide senior advice on data analysis, modeling, and interpretation of the HST/COS results. The other team members are conducting the survey for bright southern quasars that has led to the discovery of the remarkable target proposed here for HST/COS follow-up (Calderone et al. 2019, ApJ, 887, 268). They are experts in surveys for high-redshift quasars and galaxies (e.g. Grazian et al. 2017, A&A, 602, A18; Grazian et al. 2018, A&A, 613, A44; Boutsia et al. 2018, ApJ, 869, 20), and on the intergalactic and circumgalactic medium (e.g. D'Odorico et al. 2016, MNRAS, 463, 2690; D'Odorico et al. 2018, ApJ, 863, L29). They provided the discovery spectrum for analysis of the intervening Lyman limit system to explain the GALEX photometry (Fig. 5 in the proposal). In particular, Dr. Konstantina Boutsia took the discovery spectrum at Las Campanas Observatory. Professor Stefano Cristiani checked the acquisition image to rule out possible foreground objects that could be responsible for the GALEX detection due to the broad 5 arcsec GALEX PSF. Intergalactic Medium and the Circumgalactic Medium 6 false false false false false false false false SUPPORTED EXT-MEDIUM ABSORPTION LINE SYSTEM IGM NO J2000 J2000 GALEX FUV=21.73+/-0.40 NUV~23.9 COS/FUV Spectroscopic No No G140L 800 1592933383663 The scheduling data was generated by CASM 28.2. CASM was run in Phase II mode. 1609814606000 40.0 (Tic-Visit :proposal-id 16317 :visit-id "01" :proposal-version -1 :trans-version "20200518" :date "23-Jun-2020 19:29:43.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 4 :saa-model "31" :si-bright-earth-limb-angle 20.0 :data-volume 269728 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 16317 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16317 :visit-id "01" :orbit-number 1 :pcs-time 393 :si-vis-utilization 2536 :al-overhead-at-end 0 :internal-dur-at-end 76 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2929 :pcs-gap 0 ) (Tic-Orbit :proposal-id 16317 :visit-id "01" :orbit-number 2 :pcs-time 392 :si-vis-utilization 2537 :al-overhead-at-end 0 :internal-dur-at-end 105 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2929 :pcs-gap 0 ) (Tic-Orbit :proposal-id 16317 :visit-id "01" :orbit-number 3 :pcs-time 392 :si-vis-utilization 2537 :al-overhead-at-end 0 :internal-dur-at-end 105 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2929 :pcs-gap 0 ) (Tic-Orbit :proposal-id 16317 :visit-id "01" :orbit-number 4 :pcs-time 392 :si-vis-utilization 2537 :al-overhead-at-end 0 :internal-dur-at-end 398 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2929 :pcs-gap 0 ) false 4 0