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SwiftMonitoring of NGC 4151: Evidence for a Second X-Ray/UV Reprocessing

  • Author(s): Edelson, R
  • Gelbord, J
  • Cackett, E
  • Connolly, S
  • Done, C
  • Fausnaugh, M
  • Gardner, E
  • Gehrels, N
  • Goad, M
  • Horne, K
  • McHardy, I
  • Peterson, BM
  • Vaughan, S
  • Vestergaard, M
  • Breeveld, A
  • Barth, AJ
  • Bentz, M
  • Bottorff, M
  • Brandt, WN
  • Crawford, SM
  • Bontà, E Dalla
  • Emmanoulopoulos, D
  • Evans, P
  • Jaimes, R Figuera
  • Filippenko, AV
  • Ferland, G
  • Grupe, D
  • Joner, M
  • Kennea, J
  • Korista, KT
  • Krimm, HA
  • Kriss, G
  • Leonard, DC
  • Mathur, S
  • Netzer, H
  • Nousek, J
  • Page, K
  • Romero-Colmenero, E
  • Siegel, M
  • Starkey, DA
  • Treu, T
  • Vogler, HA
  • Winkler, H
  • Zheng, W
  • et al.

© 2017. The American Astronomical Society. All rights reserved. Swift monitoring of NGC 4151 with an ∼6 hr sampling over a total of 69 days in early 2016 is used to construct light curves covering five bands in the X-rays (0.3-50 keV) and six in the ultraviolet (UV)/optical (1900-5500 Å). The three hardest X-ray bands (>2.5 keV) are all strongly correlated with no measurable interband lag, while the two softer bands show lower variability and weaker correlations. The UV/optical bands are significantly correlated with the X-rays, lagging ∼3-4 days behind the hard X-rays. The variability within the UV/optical bands is also strongly correlated, with the UV appearing to lead the optical by ∼0.5-1 days. This combination of ≳3 day lags between the X-rays and UV and ≲1 day lags within the UV/optical appears to rule out the "lamp-post" reprocessing model in which a hot, X-ray emitting corona directly illuminates the accretion disk, which then reprocesses the energy in the UV/optical. Instead, these results appear consistent with the Gardner & Done picture in which two separate reprocessings occur: first, emission from the corona illuminates an extreme-UV-emitting toroidal component that shields the disk from the corona; this then heats the extreme-UV component, which illuminates the disk and drives its variability.

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