Tracking X-Ray Variability in Next-generation EHT Low-luminosity Active Galactic Nucleus Targets

Abstract

<p>We present a 5 month NICER X-ray monitoring campaign for two low-luminosity active galactic nuclei (LLAGNs)-NGC 4594 and IC 1459-with complementary Swift and NuSTAR observations. Utilizing an absorbed power-law and thermal source model combined with NICER's SCORPEON background model, we demonstrate the effectiveness of joint source-background modeling for constraining emission from faint, background-dominated targets. Both sources are dominated by nuclear power-law emission with photon indices Gamma similar to 1.5-2, with NGC 4594 being slightly harder than IC 1459. The thermal contribution in both sources is fainter, but constant, with kT similar to 0.5 keV (similar to 5 x 10⁶ K). The power-law flux and Gamma are strongly anticorrelated in both sources, as has been seen for other LLAGNs with radiatively inefficient accretion flows. NGC 4594 is the brighter source and exhibits significant aperiodic variability. Its variability timescale with an upper limit of 5-7 days indicates emission originating from less than or similar to 100 r_g, at the scale of the inner accretion flow. A spectral break found at similar to 6 keV, while tentative, could arise from synchrotron/inverse Compton emission. This high-cadence LLAGN X-ray monitoring campaign underlines the importance of multiwavelength variability studies for a sample of LLAGNs to truly understand their accretion and outflow physics.<br></p>