AGN STORM 2. II. Ultraviolet Observations of Mrk 817 with the Cosmic Origins Spectrograph on the Hubble Space Telescope*

Abstract

We present reverberation mapping measurements for the prominent ultraviolet broad emission lines of the active galactic nucleus Mrk 817 using 165 spectra obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. Our ultraviolet observations are accompanied by X-ray, optical, and near-infrared observations as part of the AGN Space Telescope and Optical Reverberation Mapping Program 2 (AGN STORM 2). Using the cross-correlation lag analysis method, we find significant correlated variations in the continuum and emission-line light curves. We measure rest-frame delayed responses between the far-ultraviolet continuum at 1180 Å and Lyα λ1215 Å ( 10.4 − 1.4 + 1.6 days), N v λ1240 Å ( 15.5 − 4.8 + 1.0 days), Si iv + ]O iv λ1397 Å ( 8.2 − 1.4 + 1.4 days), C iv λ1549 Å ( 11.8 − 2.8 + 3.0 days), and He ii λ1640 Å ( 9.0 − 1.9 + 4.5 days) using segments of the emission-line profile that are unaffected by absorption and blending, which results in sampling different velocity ranges for each line. However, we find that the emission-line responses to continuum variations are more complex than a simple smoothed, shifted, and scaled version of the continuum light curve. We also measure velocity-resolved lags for the Lyα and C iv emission lines. The lag profile in the blue wing of Lyα is consistent with virial motion, with longer lags dominating at lower velocities, and shorter lags at higher velocities. The C iv lag profile shows the signature of a thick rotating disk, with the shortest lags in the wings, local peaks at ±1500 km s−1, and a local minimum at the line center. The other emission lines are dominated by broad absorption lines and blending with adjacent emission lines. These require detailed models, and will be presented in future work.

AGN STORM 2. IV. Swift X-Ray and Ultraviolet/Optical Monitoring of Mrk 817

“…Despite this, both UV/optical continuum reverberation and emission-line reverberation were still observed. Homayouni et al (2023a; hereafter Paper II) presents the HST observations and UV emission-line reverberation from the full campaign. Paper III (Partington et al 2023) presents a detailed analysis of the X-ray spectral variability using NICER.…”

AGN STORM 2. VI. Mapping Temperature Fluctuations in the Accretion Disk of Mrk 817

“…Based on these tests, the errors for the Swift, ground-based, and HST 1709 Å band lightcurves are left unchanged. The errors on the other, bluer HST lightcurves, which were calibrated by Homayouni et al (2023a) to within 2%, are inflated by a factor of 3.3-3.5. While this seems large, the HST errors were inflated by similar values in Cackett et al (2023) when they used PyROA (Donnan et al 2021) to measure the reverberation lags.…”

“…However, if we model the residuals of the unmodified U lightcurve, we find they are consistent with a lag of 0 days for both Mrk 817 and NGC 5548, which is not consistent with BLR continuum contamination, where we should obtain a lag time of order the BLR light travel time. A possible explanation for these residuals is that the emission is reprocessed emission from the UV/X-ray obscurer discussed in Kara et al (2021), Homayouni et al (2023a), and Partington et al (2023. This obscurer is thought to be situated in the inner BLR or further inward.…”

“…The AGN STORM 2 project is a large-scale spectroscopic and photometric reverberation mapping campaign using X-ray through near-infrared observations from space-and ground-based observatories. Previous papers include an overview of the first 100 days of observations (Kara et al 2021), an analysis of the UV spectra obtained with the Hubble Space Telescope (HST; Homayouni et al 2023a), an analysis of the X-ray properties using the Neil Gehrels Swift Observatory (Swift) and the Neutron Star Interior Composition ExploreR (Partington et al 2023), an overview of the UV flux variability and disk reverberation signal using Swift (Cackett et al 2023), an analysis of the anomalous behavior of the broad C IV emission line lightcurve (Homayouni et al 2023b), and an overview of the ground-based optical observations (J. Montano et al 2023, in preparation).…”

Fierce Feedback in an Obscured, Sub-Eddington State of the Seyfert 1.2 Markarian 817

“…, has recently been refined through continued optical BLR reverberation mapping (Lu et al 2021). Recent works by Kara et al (2021) and Homayouni et al (2023) use a smaller black hole mass of M ∼ 3.85 × 10 7 M e based on some similarities to NGC 5548. The fact that optical reverberation mapping sometimes detects a clear lag in Mrk 817 demands that reprocessing of the centrally emitted continuum by the BLR happen during some intervals or states.…”