BAT AGN Spectroscopic Survey - V. X-ray properties of the Swift/BAT 70-month AGN catalog

Abstract: Hard X-ray ($\geq 10$ keV) observations of Active Galactic Nuclei (AGN) can shed light on some of the most obscured episodes of accretion onto supermassive black holes. The 70-month Swift/BAT all-sky survey, which probes the 14-195 keV energy range, has currently detected 838 AGN. We report here on the broad-band X-ray (0.3-150 keV) characteristics of these AGN, obtained by combining XMM-Newton, Swift/XRT, ASCA, Chandra, and Suzaku observations in the soft X-ray band ($\leq 10$ keV) with 70-month averaged Swift/BAT data. The non-blazar AGN of our sample are almost equally divided into unobscured ($N_{\rm H}< 10^{22}\rm cm^{-2}$) and obscured ($N_{\rm H}\geq 10^{22}\rm cm^{-2}$) AGN, and their Swift/BAT continuum is systematically steeper than the 0.3-10 keV emission, which suggests that the presence of a high-energy cutoff is almost ubiquitous. We discuss the main X-ray spectral parameters obtained, such as the photon index, the reflection parameter, the energy of the cutoff, neutral and ionized absorbers, and the soft excess for both obscured and unobscured AGN.

• The paper conducts a comprehensive analysis of 838 AGN, using multi-observatory data to differentiate between unobscured and obscured X-ray spectra.
• It employs advanced spectral modeling to derive crucial parameters such as photon index, high-energy cutoff near 100 keV, and reflection strength.
• The study’s results enhance our understanding of AGN accretion and obscuration, setting the stage for more refined future X-ray surveys.

Overview of BAT AGN Spectroscopic Survey: X-ray Properties of the Swift/BAT 70-month AGN Catalog

The paper "BAT AGN Spectroscopic Survey -- V" offers a comprehensive analysis of X-ray properties for the AGN cataloged over a 70-month period by the Swift Burst Alert Telescope (BAT), which operates in the hard X-ray band (14–195 keV). This research examines 838 AGN, leveraging a combination of data from several prominent X-ray observatories, including Swift/XRT, XMM-Newton, Chandra, ASCA, and Suzaku, to analyze the broad-band X-ray characteristics spanning 0.3–150 keV.

Sample and Methodology

The research meticulously combines time-averaged hard X-ray spectra from the Swift/BAT catalog with data across the 0.3–10 keV range from other observatories. Important distinctions are made between unobscured AGN ($N_{\rm H}< 10^{22}\rm\,cm^{-2}$) and obscured AGN ($N_{\rm H}\geq 10^{22}\rm\,cm^{-2}$), revealing that approximately half of the sample falls into each category. The non-blazar AGN group is directly investigated, while blazar AGN are accounted for differently due to their emission properties.

X-ray Continuum and Spectral Features

The study provides key insights into the X-ray spectral parameters like the photon index, reflection parameter, and high-energy cutoff. It employs various models to fit the data, addressing factors such as neutral and ionized absorption, and explores the complexities of spectral curvature introduced by these parameters. The exploration covers the iron Kα line and Compton humps at ~30 keV, known features indicative of X-ray reprocessing in AGN.

Results

A significant conclusion of this study is the pervasive presence of a high-energy cutoff among non-blazar AGN, with a majority only providing lower limits on its energy, often linked to $E_{\rm C}\sim100 \rm\,keV$. Additionally, the reflection parameter tends to be more pronounced in unobscured AGN than in their obscured counterparts, implying differences in the X-ray reflection environments.

Obscured AGN demonstrate a notable diversity in spectral characteristics. In particular, Compton-thick AGN (CT AGN) that are well constrained stand out with higher reflection parameters stemming from pronounced reprocessing in obscured environments.

Implications and Future Directions

This extensive work on the Swift/BAT 70-month AGN catalog delivers multifaceted insights into the nature of AGN X-ray emission, contributing to the broader understanding of SMBH accretion dynamics and the role of obscuration in cosmic evolution models.

Future advancements in X-ray astronomy could refine the detection and analysis of high-energy features currently challenging to constrain, further elucidating AGN physics. In particular, improvements in resolving the soft excess and characterizing ionized absorbers across broader samples will provide more clarity.

The synthesis of multi-wavelength data, including infrared and optical observations, will continue to be crucial for providing a holistic view of AGN. This paper lays a solid foundation for ongoing surveys and theoretical models addressing the intrinsic luminosities, column densities, and spectra of AGN through the power of broad-band observations.