Tracking X-ray Variability in Next Generation EHT LLAGN Targets

TL;DR

This study presents a high-cadence X-ray monitoring campaign of two LLAGNs, revealing their spectral properties, variability timescales, and potential emission mechanisms, emphasizing the importance of multi-wavelength studies for understanding low-luminosity AGN physics.

Contribution

It introduces a joint source/background modeling approach for faint X-ray sources and provides detailed variability analysis of LLAGNs, highlighting their inner accretion flow characteristics.

Findings

NGC 4594 shows significant aperiodic variability within 5-7 days.

Both sources exhibit a photon index anti-correlation with flux, consistent with radiatively inefficient flows.

A tentative spectral break at 6 keV suggests synchrotron/inverse Compton emission processes.

Abstract

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 $\Gamma \sim 1.5 - 2$, with NGC 4594 being slightly harder than IC 1459. The thermal contribution in both sources is fainter, but constant, with $kT \sim 0.5$ keV ($\sim 5 \times 10^6$ K). The power law flux and $\Gamma$ are strongly anti-correlated 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 $< 100 R_{g}$, at the scale of the inner accretion flow. A spectral break found at $\sim 6$ keV, while tentative, could arise from synchrotron/inverse compton emission. This high-cadence LLAGN X-ray monitoring campaign underlines the importance of multi-wavelength variability studies for a sample of LLAGNs to truly understand their accretion and outflow physics.