[Paper Review] Broad band energy distribution of ROSAT detected quasars II: Radio-quiet objects

Wei Yuan, W. Brinkmann+2 more|ArXiv.org|May 4, 1998

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study presents a broad-band energy distribution analysis of 846 radio-quiet quasars from ROSAT All-Sky and pointed observations, confirming steeper soft X-ray spectra in these objects compared to radio-loud quasars, with spectral flattening up to z ≈ 2 and no significant X-ray spectral evolution at z > 2.5, while also finding a tight correlation between X-ray and UV luminosities with minimal excess absorption at high redshifts.

ABSTRACT

A database of radio-quiet quasars detected with ROSAT is presented containing 846 quasars seen in the All-Sky Survey and/or in pointed PSPC observations. About 70% of the objects have been detected in X-rays for the first time. We present the soft X-ray fluxes and spectra, if available. Using an optically selected subsample compiled from this database, we study the broad band properties of radio-quiet quasars with high statistical significance. We confirm that radio-quiet quasars have in general steeper soft X-ray spectra than radio-loud. The spectral differences persist to high redshifts. A spectral flattening with increasing redshift is confirmed up to redshift around 2, beyond which the spectral slopes seem to be independent of redshift. The spectral slopes of the ROSAT radio-quiet quasars at z>2.5 are consistent with those found for nearby quasars in the medium energy band (2-10keV), implying that the X-ray spectral evolution is not important. We show that there is, in a statistical sense, little or no excess absorption for most of the radio-quiet objects at z>2. The existence of a correlation between the X-ray luminosity and the luminosity at 2500A is confirmed. Individual objects show a large scatter from this correlation. The X-ray loudness alpha_ox appears to be independent of z, but the analyses indicate a slight increase of alpha_ox with optical luminosity. However, this behavior is likely not caused by physical properties inherent to the quasars but is the result of the intrinsic dispersion. Finally, we find a small fraction of objects which appear to be relatively `X-ray quiet' compared to the bulk of the other quasars.

Motivation & Objective

• To characterize the broad-band energy distribution of radio-quiet quasars using ROSAT All-Sky and pointed observations.
• To investigate the soft X-ray spectral properties of radio-quiet quasars across a wide redshift range.
• To examine the evolution of X-ray spectral slopes and luminosity correlations with redshift and optical luminosity.
• To assess the presence of excess absorption in high-redshift radio-quiet quasars.
• To determine the dependence of X-ray loudness (α_ox) on redshift and optical luminosity.

Proposed method

• Compilation of a database of 846 radio-quiet quasars detected in ROSAT All-Sky Survey and pointed PSPC observations, with ~70% detected in X-rays for the first time.
• Selection of an optically selected subsample for high-significance statistical analysis of broad-band properties.
• Spectral fitting of soft X-ray data (0.1–2.4 keV) to derive photon indices and assess spectral hardness.
• Cross-correlation of X-ray fluxes with UV luminosities at 2500 Å to study the X-ray loudness parameter α_ox.
• Redshift-dependent analysis of spectral slopes and luminosity correlations using statistical modeling.
• Assessment of excess absorption via spectral fitting and comparison with expected intrinsic absorption models.

Experimental results

Research questions

• RQ1 Do radio-quiet quasars exhibit steeper soft X-ray spectra than radio-loud quasars across cosmic time?
• RQ2 How does the soft X-ray spectral slope evolve with redshift, particularly beyond z ≈ 2?
• RQ3 Is there significant excess absorption in high-redshift radio-quiet quasars (z > 2)?
• RQ4 Is the correlation between X-ray luminosity and 2500 Å UV luminosity consistent across redshifts?
• RQ5 Does the X-ray loudness parameter α_ox depend on redshift or optical luminosity?

Key findings

• Radio-quiet quasars exhibit significantly steeper soft X-ray spectra than radio-loud quasars, a difference that persists to high redshifts.
• Spectral slopes flatten with increasing redshift up to z ≈ 2, after which they stabilize and show no further evolution.
• At z > 2.5, the X-ray spectral slopes are consistent with those of nearby quasars in the 2–10 keV band, indicating no significant X-ray spectral evolution.
• Statistical analysis shows little or no excess absorption in radio-quiet quasars at z > 2, contrary to some earlier suggestions.
• A tight correlation between X-ray luminosity and 2500 Å UV luminosity is confirmed, though individual objects show large scatter.
• The X-ray loudness parameter α_ox shows no significant dependence on redshift, but a slight increase with optical luminosity, likely due to intrinsic dispersion rather than physical causality.