Space Telescope and Optical Reverberation Mapping Project. X. Understanding the Absorption-line Holiday in NGC 5548

TY - JOUR

T1 - Space Telescope and Optical Reverberation Mapping Project. X. Understanding the Absorption-line Holiday in NGC 5548

AU - Dehghanian, M.

AU - Ferland, G. J.

AU - Kriss, G. A.

AU - Peterson, B. M.

AU - Mathur, S.

AU - Mehdipour, M.

AU - Guzmán, F.

AU - Chatzikos, M.

AU - Van Hoof, P. A.M.

AU - Williams, R. J.R.

AU - Arav, N.

AU - Barth, A. J.

AU - Bentz, M. C.

AU - Bisogni, S.

AU - Brandt, W. N.

AU - Crenshaw, D. M.

AU - Bonta, E. Dalla

AU - De Rosa, G.

AU - Fausnaugh, M. M.

AU - Gelbord, J. M.

AU - Goad, M. R.

AU - Gupta, A.

AU - Horne, Keith

AU - Kaastra, J.

AU - Knigge, C.

AU - Korista, K. T.

AU - McHardy, I. M.

AU - Pogge, R. W.

AU - Starkey, D. A.

AU - Vestergaard, M.

PY - 2019/6/1

Y1 - 2019/6/1

N2 - The Space Telescope and Optical Reverberation Mapping Project (AGN STORM) on NGC 5548 in 2014 is one of the most intensive multiwavelength AGN monitoring campaigns ever. For most of the campaign, the emission-line variations followed changes in the continuum with a time lag, as expected. However, the lines varied independently of the observed UV-optical continuum during a 60-70 day "holiday," suggesting that unobserved changes to the ionizing continuum were present. To understand this remarkable phenomenon and to obtain an independent assessment of the ionizing continuum variations, we study the intrinsic absorption lines present in NGC 5548. We identify a novel cycle that reproduces the absorption line variability and thus identify the physics that allows the holiday to occur. In this cycle, variations in this obscurer's line-of-sight covering factor modify the soft X-ray continuum, changing the ionization of helium. Ionizing radiation produced by recombining helium then affects the level of ionization of some ions seen by the Hubble Space Telescope. In particular, high-ionization species are affected by changes in the obscurer covering factor, which does not affect the optical or UV continuum, and thus appear as uncorrelated changes, a "holiday." It is likely that any other model that selectively changes the soft X-ray part of the continuum during the holiday can also explain the anomalous emission-line behavior observed.

AB - The Space Telescope and Optical Reverberation Mapping Project (AGN STORM) on NGC 5548 in 2014 is one of the most intensive multiwavelength AGN monitoring campaigns ever. For most of the campaign, the emission-line variations followed changes in the continuum with a time lag, as expected. However, the lines varied independently of the observed UV-optical continuum during a 60-70 day "holiday," suggesting that unobserved changes to the ionizing continuum were present. To understand this remarkable phenomenon and to obtain an independent assessment of the ionizing continuum variations, we study the intrinsic absorption lines present in NGC 5548. We identify a novel cycle that reproduces the absorption line variability and thus identify the physics that allows the holiday to occur. In this cycle, variations in this obscurer's line-of-sight covering factor modify the soft X-ray continuum, changing the ionization of helium. Ionizing radiation produced by recombining helium then affects the level of ionization of some ions seen by the Hubble Space Telescope. In particular, high-ionization species are affected by changes in the obscurer covering factor, which does not affect the optical or UV continuum, and thus appear as uncorrelated changes, a "holiday." It is likely that any other model that selectively changes the soft X-ray part of the continuum during the holiday can also explain the anomalous emission-line behavior observed.

KW - galaxies: active

KW - galaxies: individual (NGC 5548)

KW - galaxies: nuclei

KW - galaxies: Seyfert

KW - line: formation

UR - http://www.scopus.com/inward/record.url?scp=85068958720&partnerID=8YFLogxK

U2 - 10.3847/1538-4357/ab1b48

DO - 10.3847/1538-4357/ab1b48

M3 - Journal article

AN - SCOPUS:85068958720

SN - 0004-637X

VL - 877

JO - Astrophysical Journal

JF - Astrophysical Journal

IS - 2

M1 - 119

ER -