Test for Echo: X-ray Reflection Variability in the Seyfert-2 AGN NGC 4388
arxiv(2024)
摘要
We report on a study of the narrow Fe Kα line and reflection spectrum
in the well-known Seyfert-2 AGN, NGC 4388. X-ray spectra summed from two
extensive NICER monitoring campaigns, separated by years, show strong evidence
of variation in the direct continuum and reflected emission, but only small
variations in the obscuring gas. Fits to the spectra from individual NICER
observations find a strong, positive correlation between the power-law photon
index, Γ, and direct flux that is commonly observed in unobscured AGN. A
search for a reverberation lag between the direct and reflected spectra
-dominated by the narrow Fe Kα emission line - measures a time scale of
t = 16.37^+0.46_-0.38 days, or a characteristic radius of
r=1.374_-0.032^+0.039×10^-2 pc
=3.4_-0.1^+0.1×10^4 GM/c^2. Only one cycle of this tentative lag is
observed, but it is driven by a particularly sharp drop in the direct continuum
that leads to the subsequent disappearance of the otherwise prominent Fe
Kα line. Physically motivated fits to high-resolution Chandra spectra of
NGC 4388 measure a line production radius of r =2.9^+1.2_-0.7 ×
10^4 GM/c^2, formally consistent with the tentative lag. The line profile
also prefers a Compton-thick reflector, indicating an origin in the disk and/or
thick clumps within a wind. We discuss the strengths and weaknesses of our
analysis and methods for testing our results in future observations, and we
note the potential for X-ray reverberation lags to constrain black hole masses
in obscured Seyferts wherein the optical broad line region is not visible.
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