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Spectral energy distributions (SEDs) from X-ray to far-infrared (FIR) wavelengths are presented for a sample of 1246 X-ray-luminous active galactic nuclei (AGNs;L_{0.5–10 keV}> 10⁴³erg s⁻¹), withz_spec< 1.2, selected from Stripe 82X, COSMOS, and GOODS-N/S. The rest-frame SEDs show a wide spread (∼2.5 dex) in the relative strengths of broad continuum features at X-ray, ultraviolet (UV), mid-infrared (MIR), and FIR wavelengths. A linear correlation (log–log slope of 0.7 ± 0.04) is found betweenL_MIRandL_X. There is significant scatter in the relation between theL_UVandL_Xowing to heavy obscuration; however, the most luminous and unobscured AGNs show a linear correlation (log–log slope of 0.8 ± 0.06) in the relation above this scatter. The relation betweenL_FIRandL_Xis predominantly flat, but with decreasing dispersion atL_X> 10⁴⁴erg s⁻¹. The ratio between the “galaxy-subtracted” bolometric luminosity and the intrinsicL_Xincreases from a factor of ∼10 to 70 from logL_bol/(erg s⁻¹) = 44.5 to 46.5. Characteristic SED shapes have been determined by grouping AGNs based on relative strengths of the UV and MIR emission. The averageL_1μmis constant for the majority of these SED shapes, while AGNs with the strongest UV and MIR emission have elevatedL_1μm, consistent with the AGN emission dominating their SEDs at optical and near-infrared wavelengths. A strong correlation is found between the SED shape and both theL_XandL_bol, such thatL_bol/L_X= 20.4 ± 1.8, independent of the SED shape. This is consistent with an evolutionary scenario of increasingL_bolwith decreasing obscuration as the AGN blows away circumnuclear gas.

 

Abstract We report the discovery of a new “changing-look” active galactic nucleus (CLAGN) event, in the quasar SDSS J162829.17+432948.5 at z = 0.2603, identified through repeat spectroscopy from the fifth Sloan Digital Sky Survey (SDSS-V). Optical photometry taken during 2020–2021 shows a dramatic dimming of Δ g ≈ 1 mag, followed by a rapid recovery on a timescale of several months, with the ≲2 month period of rebrightening captured in new SDSS-V and Las Cumbres Observatory spectroscopy. This is one of the fastest CLAGN transitions observed to date. Archival observations suggest that the object experienced a much more gradual dimming over the period of 2011–2013. Our spectroscopy shows that the photometric changes were accompanied by dramatic variations in the quasar-like continuum and broad-line emission. The excellent agreement between the pre- and postdip photometric and spectroscopic appearances of the source, as well as the fact that the dimmest spectra can be reproduced by applying a single extinction law to the brighter spectral states, favor a variable line-of-sight obscuration as the driver of the observed transitions. Such an interpretation faces several theoretical challenges, and thus an alternative accretion-driven scenario cannot be excluded. The recent events observed in this quasar highlight the importance of spectroscopic monitoring of large active galactic nucleus samples on weeks-to-months timescales, which the SDSS-V is designed to achieve.