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Investigating Properties of Active Galactic Nuclei Through Reverberation Mapping

  • Author(s): Pei, Liuyi
  • Advisor(s): Barth, Aaron J
  • et al.
Abstract

Reverberation mapping is a time-domain technique used to resolve the supermassive black hole’s sphere of influence in active galactic nuclei. We carried out a nine-month reverberation mapping campaign to measure the broad-line region size and estimate the mass of the black hole in KA1858+4850, a narrow-line Seyfert 1 galaxy at redshift 0.078 and among the brightest active galaxies monitored by the Kepler mission. We obtained spectroscopic data using the Kast Spectrograph at the Lick 3-m telescope and complementary V-band images from five other ground-based telescopes. We measured the H light curve lag with respect to the V -band continuum light curve, and combined this lag with the H root-mean-square line profile width to obtain a virial estimate of MBH = 8.06 (+1.59, −1.72) × 10^6 M⊙ for the mass of the central black hole and an Eddington ratio of L/LEdd ≈ 0.2.

I also used reverberation mapping to study in detail the broad line region in NGC 5548, a Seyfert 1 galaxy at redshift 0.017. Optical spectroscopic data targeting NGC 5548 were taken in 2014 as part of a larger multi-wavelength reverberation mapping campaign. The ground-based spectra spanned six months and achieved almost daily cadence with observations from five telescopes. We computed the H and He II 4686 lags relative to both the optical continuum and the UV continuum measured by the Hubble Space Telescope, and found the H–UV lag to be ∼50% longer than the H–optical lag. This suggests that the true broad-line region size is 50% larger than the size that would be inferred from optical data alone. We also measured velocity-resolved lags for H and found a complex velocity-lag structure with shorter lags in the line wings. The responsivity of both the H and He II lines decreased halfway through the campaign, an anomalous phenomenon also observed for the UV emission lines during the same monitoring period. Finally, we showed that, given the optical luminosity of NGC 5548 during our campaign, the measured H lag is a factor of five shorter than the expected value based on the past behavior of NGC 5548.

To efficiently process large amounts of reverberation mapping photometry data, I developed an IDL pipeline that is able to automatically extract the aperture photometry magnitude of the AGN, calibrate the individual exposures for nightly variations using reference stars, and construct the relative optical continuum light curve combining data from multiple telescopes. This pipeline has been used in several collaborations, both to monitor AGN variability in real time and to construct photometry light curves from archival data, and its applications can be extended to time-domain studies of any variable object.

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