Monitoring X-ray Variability in Nearby Low Luminosity AGNs with NICER and NuSTAR Telescopes

Nicole Ford ( Université McGill )

The Event Horizon Telescope Collaboration (EHTC) has achieved the first horizon-scale images of supermassive black holes (SMBHs) buried in the centers of galaxies. The successful radio and multi-wavelength (MWL) imaging of the SMBHs in Messier 87 and Sagittarius A* (our own Galactic center) can now be expanded to a sample of nearby active galactic nuclei (AGN). In anticipation of upcoming EHT observations and in coordination with the MWL Science WG, we performed NICER monitoring in 2022 of three new EHT targets identified as having SMBH mass ≳ 10⁹ solar masses and ring size > 1 μas: NGC 4594 (Sombrero galaxy/M104; scheduled for 2024 EHT+ALMA observations), NGC 3998 (observed with GMVA in 2023), and IC 1459 (observed with GMVA+ALMA in 2023) . We measure the soft X-ray flux for these low luminosity AGN using an absorbed power law and thermal model, and demonstrate the utility of NICER's new SCORPEON background model for constraining emission in faint, background-dominated targets. Using the estimated X-ray flux, we track the sources' soft X-ray variability and compare with concurrent hard X-ray emission detected with NuSTAR. This allows us to study low-luminosity AGN in fine detail and set a baseline for longer term variability and broadband SED studies with the combined power of EHT radio and MWL observations.