arXiv subjects

Dev R Sadaula

Publications and source records attributed to Dev R Sadaula.

3 recordsLinked to original sources

arXiv2025

The response of warm absorbers to the variations in the ionizing continuum in the active galaxy NGC 4051

We present a time-resolved X-ray spectral analysis of the warm absorbers in the Seyfert galaxy NGC 4051, which has an active galactic nucleus (AGN), using observations from the Neutron Star Interior Composition Explorer (NICER). Despite NICER's moderate spectral resolution, its high-cadence monitoring allows us to probe the response of the ionized outflows, also known as warm absorbers, on timescales of approximately 5500 seconds. We detect two distinct components of ionized absorbers in this source. The ionization parameter of the low-ionization warm absorber component tracks changes in the ionizing flux with no measurable time lag. This rapid response implies photoionization equilibrium and places a lower limit on the electron density of about 9 x 10^6 cm^-3, based on the most abundant ionic species, O VII. The absorber is located within approximately 0.02 parsecs of the central source, consistent with an origin in the inner regions of the active nucleus. In contrast, the high-ionization absorber remains consistently under-ionized relative to equilibrium predictions. This suggests that it may be collisional plasma, as also indicated in previous studies. These results demonstrate that time-resolved spectroscopy, even with moderate-resolution instruments, can provide valuable constraints on the density and location of warm absorbers in AGN. As a potential contributor to AGN feedback, the study of these ionized outflows is crucial to understanding AGN--host galaxy interactions.

astro-ph.GA↗

arXiv2023

Time Dependent Photoionization Modeling of Warm Absorbers: High-Resolution Spectra and Response to Flaring Light Curves

Time dependent photoionization modeling of warm absorber outflows in active galactic nuclei can play an important role in understanding the interaction between warm absorbers and the central black hole. The warm absorber may be out of the equilibrium state because of the variable nature of the central continuum. In this paper, with the help of time dependent photoionization modeling, we study how the warm absorber gas changes with time and how it reacts to changing radiation fields. Incorporating a flaring incident light curve, we investigate the behavior of warm absorbers using a photoionization code that simultaneously and consistently solves the time dependent equations of level population, heating and cooling, and radiative transfer. We simulate the physical processes in the gas clouds, such as ionization, recombination, heating, cooling, and the transfer of ionizing radiation through the cloud. We show that time dependent radiative transfer is important and that calculations which omit this effect quantitatively and systematically underestimate the absorption. Such models provide crucial insights into the characteristics of warm absorbers and can constrain their density and spatial distribution.

astro-ph.GA↗

arXiv2022

Time Dependent Photoionization Modeling of Warm Absorbers in Active Galactic Nuclei

Warm absorber spectra contain bound-bound and bound-free absorption features seen in the X-ray and UV spectra from many active galactic nuclei (AGN). The widths and centroid energies of these features indicate they occur in outflowing gas, and the outflow can affect the gas within the host galaxy. Thus the warm absorber mass and energy budgets are of great interest. Estimates for these properties depend on models which connect the observed strengths of the absorption features with the density, composition, and ionization state of the absorbing gas. Such models assume that the ionization and heating of the gas come primarily from the strong continuum near the central black hole. They also assume that the various heating, cooling, ionization, and recombination processes are in a time-steady balance. This assumption may not be valid, owing to the intrinsic time-variability of the illuminating continuum, or other factors which change the cloud environment. This paper presents models for warm absorbers which follow the time dependence of the ionization, temperature, and radiation field in warm absorber gas clouds in response to a changing continuum illumination. We show that the effects of time variability are important over a range of parameter values, that time dependent models differ from equilibrium models in important ways, and that these effects should be included in models which derive properties of warm absorber outflows.

astro-ph.HE↗