A universal physical relation for quasars

Since the ‘80s there is a general consensus that every single massive galaxy hosts a central black hole. For example, in the centre of our own Galaxy sits a black hole of about 4 million times the mass of the Sun. Yet, such black hole is relatively silent, making the entire Milky Way a fairly quiet place. Nonetheless, about 10% of the galaxies in the Universe host very active black holes, whose growth, up to a mass of billions of Suns, is the power source of the so-called active galactic nuclei (AGN in short), which are amongst the most remarkable objects in the cosmos. The most luminous active galaxies are also known as quasars or QSOs, which stand for quasi-stellar (radio) objects after their discovery by Maarten Schmidt in 1963. QSOs outshine typical galaxies like our own Milky Way by several orders of magnitude in luminosity, and they show peculiar properties in specific regions of their electromagnetic spectrum.

Harvey Tananbaum and his collaborators have first observed a relation between the soft X-ray and the optical/ultraviolet emission in AGN in 1979. Such relation is nonlinear, and it is usually parameterized as dependence between the logarithm of the monochromatic luminosities at 2500 Å (L_2500Å) and 2 keV (L_2keV). Since then, the distribution of X-ray and optical-UV properties in AGN and their possible dependencies upon redshift have been the subject of active investigations for more than 30 years.

The L_2500Å–L_2keVrelation is a first step towards the understanding of the structure of the AGN accretion disc and of its interplay with the mysterious X-ray corona (i.e., a hot plasma of relativistic electrons that Compton up-scatter the photons coming from the accretion disc). Yet, the dispersion along the L_2500Å–L_2keVrelationship is found to be rather high, typically ~0.35–0.4 in log units. Such a large dispersion could suggest that, even if the correlation exists, it may not be considered a good diagnostic to shed new light on the nature of the energy generation mechanism in AGN.

In this framework, Elisabeta Lusso (INAF-Arcetri Observatory) and Guido Risaliti (UniFI, INAF-Arcetri Observatory) have recently analyzed the L_2500Å–L_2keVrelationship on a sample of 2685 optically selected quasars from the Sloan Digital Sky Survey (SDSS) Data Release 7, with available X-ray data from the 3XMM-DR5 source catalog. The aim of this work is to understand the origin of the observed dispersion, and to evaluate its intrinsic dispersion.

This analysis shows that, even though the initial dispersion observed in the main quasar sample is ~0.45 dex, most of this scatter is caused by the combination of multiple effects, including variability/not-simultaneous observations, poor optical-UV and X-ray data quality, and the contamination from QSOs with intrinsically red continua and/or host galaxy contamination. All these factors add substantial noise to the correlation, with the effect of disguising the intrinsic dispersion of the L_2500Å–L_2keVrelationship.

Once optically selected quasars with homogeneous spectral energy distributions and reliable X-ray detection are selected, the measured dispersion drops to significantly lower values (i.e., 0.24 dex). Figure 1 presents the results for the sub-sample of quasars where dust-reddened and/or gas-obscured objects are not included. This work also shows that the residual dispersion is due, to some extent, to variability, and to remaining measurement uncertainties. The real physical dispersion is found to be less than 0.21 dex.

The observed L_2500Å–L_2keVrelationship in quasars indicates that there is a good empirical coupling between the disc, emitting the primary optical/UV radiation, and the hot-electron corona, emitting in the X-rays. Such a tight relation, presented in the article “The tight relation between X-ray and ultraviolet luminosity of quasars” to be published in the Astrophysical Journal, is valid over 4 decades in luminosity, hence it must be the manifestation of an intrinsic (and universal) physical relation between the disk and the corona. Yet, at present, the details of the physics governing this crucial interplay are still not well understood.

Figure 1: Rest-frame monochromatic luminosities log L 2 _keV against log L ₂₅₀₀ for the X-ray detected quasar samples. The results from the statistical analysis are shown with the dashed and thin solid lines. The regression results for the dispersion, slope, and intercept (along with their uncertainties) are also reported. The lower panel shows the residuals of the log L _{2 keV}- log L ₂₅₀₀ relation with respect to the best- fit line (figure adapted from Lusso & Risaliti 2016, ApJ in press).

 Edited by Elisabeta Lusso & Laura Magrini