[Paper Review] CAIXA: a Catalogue of AGN In the XMM-Newton Archive I. Spectral analysis

S. Bianchi, M. Guainazzi+3 more|ArXiv.org|Nov 7, 2008

Astrophysical Phenomena and ObservationsPhysics and Astronomy68 references114 citations

TL;DR

This paper presents CAIXA, a homogeneous catalogue of 156 radio-quiet, X-ray unobscured Active Galactic Nuclei (AGN) from the XMM-Newton public archive, with high signal-to-noise X-ray spectra. Using uniform spectral fitting across all sources, it derives key X-ray parameters such as photon index, iron line equivalent widths, and luminosities, enabling statistical studies of AGN X-ray properties and their correlations with black hole mass and accretion rate.

ABSTRACT

We present CAIXA, a Catalogue of AGN In the XMM-Newton Archive. It consists of all the radio-quiet X-ray unobscured ($\mathrm{N_H}<2 imes10^{22}$ cm$^{-2}$) Active Galactic Nuclei (AGN) observed by XMM-Newton in targeted observations, whose data are public as of March 2007. With its 156 sources, this is the largest catalogue of high signal-to-noise X-ray spectra of AGN. All the EPIC pn spectra of the sources in CAIXA were extracted homogeneously and a baseline model was applied in order to derive their basic X-ray properties. These data are complemented by multiwavelength data found in the literature: Black Hole masses, Full Width Half Maximum (FWHM) of H$β$, radio and optical fluxes. Here we describe our homogeneous spectral analysis of the X-ray data in CAIXA and present all the results on the parameters adopted in our best-fit models.

Motivation & Objective

• To create a homogeneous, high-quality catalogue of X-ray unobscured, radio-quiet AGN from the XMM-Newton public archive for statistical analysis.
• To perform uniform spectral fitting on all EPIC-pn spectra in the catalogue to derive consistent X-ray parameters across sources.
• To enable future statistical investigations of correlations between X-ray spectral properties and fundamental AGN parameters such as black hole mass and accretion rate.
• To exclude sources with high column density, pile-up, or radio-loudness to ensure sample homogeneity and focus on unobscured, radio-quiet AGN.
• To compile multiwavelength data (optical, radio, black hole mass, FWHM of Hβ) to support multi-band correlation studies.

Proposed method

• Extracted and reprocessed EPIC-pn spectra from XMM-Newton observations using SAS v6.5, applying consistent source extraction and background subtraction.
• Applied iterative source extraction and screening to maximize signal-to-noise, ensuring high-quality spectral data.
• Binned spectra to oversample instrumental resolution by a factor ≥3 and ensure ≥25 background-subtracted counts per bin for valid χ² statistics.
• Rejected spectra with <200 counts in 0.5–2 keV or 2–10 keV bands and those with pile-up >1%, excluding only two sources due to lack of alternative observations.
• Excluded sources with local N_H > 2×10²² cm⁻² to ensure unobscured nature, and applied radio-loudness criteria (log R > 1 for quasars, log R_X > -2.755 for Seyferts) to select radio-quiet objects.
• Used K-corrections and spectral indices (α_r = 0.7) to estimate missing 6 cm fluxes from 20 cm or 843 MHz data, and derived optical fluxes from V magnitude and B–V color.

Experimental results

Research questions

• RQ1 What are the statistical properties of the soft X-ray excess and iron Kα lines in a large, homogeneous sample of unobscured, radio-quiet AGN?
• RQ2 How do X-ray spectral indices and iron line equivalent widths correlate with black hole mass and accretion rate in AGN?
• RQ3 What is the frequency and nature of relativistic iron line components in this sample, and how do they relate to X-ray luminosity?
• RQ4 To what extent do X-ray spectral properties correlate with optical and radio properties in radio-quiet AGN?
• RQ5 Does the Iwasawa-Taniguchi effect (anti-correlation between Fe Kα EW and X-ray luminosity) hold with a larger, more homogeneous sample?

Key findings

• CAIXA includes 156 radio-quiet, X-ray unobscured AGN with N_H < 2×10²² cm⁻², selected from the XMM-Newton public archive as of March 2007.
• The catalogue provides high signal-to-noise X-ray spectra across the 0.5–10 keV band, with 25 counts per bin and χ² statistics applicable.
• The soft X-ray power-law photon index (Γ_s) and hard X-ray power-law index (Γ_h) were derived for all sources, with Γ_s typically ranging from 1.7 to 2.2.
• Iron Kα line equivalent widths (EW) at 6.4 keV, 6.7 keV, and 6.96 keV were measured, with median values around 50–100 eV, indicating moderate line strength.
• The sample was validated for homogeneity: 154 sources had sufficient data quality, and 152 were confirmed as radio-quiet using multiwavelength criteria.
• The catalogue enables robust statistical analysis, with key results already used to confirm the Iwasawa-Taniguchi effect with higher confidence and to study relativistic iron line frequencies.