[Paper Review] The XMM deep survey in the CDF-S VIII. X-ray properties of the two brightest sources

K. Iwasawa, C. Vignali+8 more|arXiv (Cornell University)|Dec 17, 2014

Astrophysical Phenomena and Observations41 references8 citations

TL;DR

This study presents high-quality XMM-Newton X-ray spectra of two of the brightest AGN in the Chandra Deep Field South (CDFS), PID 203 (z=0.544) and PID 319 (z=0.742), with 2.5 Ms exposure over 10 years. It reveals moderate X-ray flux variability (~10–20% rms), strong and moderately broad Fe K emission lines (EW ~0.2 keV) that vary in shape and flux, indicating a disk origin rather than a distant torus, and hints at variable ionization states in PID 319 linked to Eddington ratio variations.

ABSTRACT

We present results from the deep XMM-Newton observations of the two brightest X-ray sources in the Chandra Deep Field South (CDFS), PID 203 (z=0.544) and PID 319 (z=0.742). The long exposure of 2.5 Ms over a 10 year period (net 4 yr with a 6 yr gap) makes it possible to obtain high quality X-ray spectra of these two Type I AGN with X-ray luminosity of 10^44 erg/s, typical luminosity for low-redshift PG quasars, track their X-ray variability both in flux and spectral shape. Both sources showed X-ray flux variability of ~10-20 per cent in rms which is similar in the soft (0.5-2 keV) and hard (2-7 keV) bands. PID 203, which has evidence for optical extinction, shows modest amount of absorption (nH~1e21cm^-2) in the X-ray spectrum. Fe K emission is strongly detected in both objects with EW~0.2 keV. The lines in both objects are moderately broad and exhibit marginal evidence for variability in shape and flux, indicating that the bulk of the line emission come from their accretion disks rather than distant tori.

Motivation & Objective

• To study the X-ray spectral and variability properties of the two brightest X-ray sources in the CDFS, which are among the most luminous AGN at z>0.5.
• To investigate the origin of strong Fe K emission lines in these high-redshift AGN, distinguishing between disk and torus contributions.
• To probe long-term X-ray variability in moderate-luminosity Type I AGN beyond the local universe.
• To assess the impact of absorption and optical extinction on the X-ray spectra of these high-redshift AGN.

Proposed method

• Utilized 2.5 Ms of deep XMM-Newton observations (2001–2010) on two bright CDFS sources, grouped into six time intervals (X1–X6) for variability analysis.
• Combined XMM-Newton data from pn, MOS1, and MOS2 cameras with 4 Ms Chandra data to improve signal-to-noise and time resolution.
• Performed spectral fitting using XSPEC to model continuum, absorption, and Fe K line components, including Gaussian and relativistic diskline models.
• Tracked flux and line variability across multiple epochs, comparing XMM and Chandra data despite intercalibration challenges.
• Used the αOX and Eddington ratio to assess ionization state changes, particularly in PID 319.
• Applied rest-frame 2–10 keV luminosity corrections and cosmological parameters (H₀=70 km s⁻¹ Mpc⁻¹, ΩΛ=0.72, ΩM=0.28).

Experimental results

Research questions

• RQ1 What is the origin of the strong Fe K emission lines in high-redshift Type I AGN, and do they originate from the accretion disk or a distant torus?
• RQ2 How do the X-ray flux and spectral shape of these AGN vary over 10 years, and what does this imply about the central engine?
• RQ3 Is the observed Fe K line variability consistent with a disk origin, and what does it reveal about the innermost accretion environment?
• RQ4 How do the X-ray properties of these z=0.5–0.7 AGN compare to local PG quasars and other high-redshift samples?
• RQ5 Can variability in Fe K line energy and width be linked to changes in the Eddington ratio or ionization state?

Key findings

• The two sources, PID 203 (z=0.544) and PID 319 (z=0.742), exhibit X-ray flux variability of ~10–20% rms in both soft (0.5–2 keV) and hard (2–7 keV) bands.
• Both sources show strong Fe K emission lines with equivalent widths of ~0.2 keV, significantly higher than expected from cold distant material (EW ~0.04 keV), indicating a dominant disk origin.
• The Fe K lines are moderately broad (σ ≈ 0.2–0.3 keV), and their shape and flux vary over time, especially in PID 203, where the line became broader over 6 years, supporting a disk origin.
• In PID 319, the Fe K line energy shifted from 6.5–6.9 keV (unresolved) in 2001–2002 to 6.21 keV (broadened) in 2008–2010, indicating variable ionization, possibly linked to a higher Eddington ratio in the earlier epoch.
• The source with higher αOX (PID 319, αOX ≈ −1.4) shows evidence of highly ionized Fe K emission (Fe xxv/xxvi) in 2001–2002, suggesting a transient high-ionization state correlated with UV luminosity.
• Despite moderate broadening, the dynamical timescale for a 10⁹ M⊙ black hole at ~40 rg is ~3 months, consistent with observed short-timescale Fe K line variability, supporting a relativistic disk origin.