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

A1 Refereed original research article in a scientific journal

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

Authors: Ford, Nicole M.; Nowak, Michael; Ramakrishnan, Venkatessh; Haggard, Daryl; Dage, Kristen; Nair, Dhanya G.; Chan, Chi-kwan

Publisher: Institute of Physics Publishing

Publishing place: BRISTOL

Publication year: 2025

Journal: Astrophysical Journal

Journal name in source: The Astrophysical Journal

Journal acronym: ASTROPHYS J

Article number: 126

Volume: 981

Issue: 2

Number of pages: 14

ISSN: 0004-637X

eISSN: 1538-4357

DOI: https://doi.org/10.3847/1538-4357/adae0f

Web address : https://doi.org/10.3847/1538-4357/adae0f

Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/491413459

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 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.

Downloadable publication

This is an electronic reprint of the original article.
This reprint may differ from the original in pagination and typographic detail. Please cite the original version.

Ford_2025_ApJ_981_126.pdf

Funding information in the publication:
This research has made use of the NASA/IPAC Extragalactic Database (NED), which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology.N.M.F. and D.H. acknowledge funding from the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant program and the Canada Research Chairs (CRC) program. N.M.F. acknowledges funding from the Fondes de Recherche Nature et Technologies (FRQNT) Doctoral research scholarship. N.M.F. and D.H. acknowledge support from the Canadian New Frontiers in Research Fund (NFRF)-Explorations program and the Trottier Space Institute at McGill. The authors acknowledge support from the Centre de recherche en astrophysique du Quebec, un regroupement strategique du FRQNT. V.R. acknowledges the financial support from the visitor and mobility program of the Finnish Centre for Astronomy with ESO (FINCA), funded by the Academy of Finland grant number 306531. D.G.N. acknowledges funding from ANID through Fondecyt Postdoctorado (project code 3220195) and Nucleo Milenio TITANs (project NCN2022002).