X-ray Bright Active Galactic Nuclei Research Articles

X-Ray Bright Active Galactic Nuclei in Local Dwarf Galaxies: Insights from eROSITA

Although supermassive black holes (SMBHs) reside in the heart of virtually every massive galaxy, it remains debated whether dwarf galaxies commonly host SMBHs. Because low-mass galaxies may retain memory of the assembly history of their black holes (BHs), probing the BH occupation fraction of local dwarf galaxies might offer insights into the growth and seeding mechanisms of the first BHs. In this work, we exploit the Western half of the eROSITA all-sky survey (covering 20,000 deg2) and compile a catalog of accreting SMBHs in local (D < 200 Mpc) dwarf galaxies. Cleaning our sample from X-ray background sources, X-ray binaries, and ultraluminous X-ray sources, we identify 74 active galactic nucleus (AGN)–dwarf galaxy pairs. Using this large and uniform sample, we derive the luminosity function of the dwarf galaxy AGN, fitting it with a power-law function and obtaining dN/dLX=(15.9±2.2)×LX−1.63±0.05 . Measuring the offset between the dwarf galaxies' centroids and the X-ray sources, we find that ≈50% of the AGN are likely off-nuclear, in agreement with theoretical predictions. We compare the BH-to-stellar mass relation of our sample with the local and high-redshift relations, finding that our sources better adhere to the former, which suggests that local AGN across different mass scales undergo similar growth histories. Finally, we compare our sources with semianalytical models: while our sample’s shallowness prevents distinguishing between different seeding models, we find that the data favor models that keep SMBHs in dwarf galaxies active at a moderate rate, motivating model improvement by comparison to AGN in the dwarf galaxy regime.

Unified properties of supermassive black hole winds in radio-quiet and radio-loud AGN

ABSTRACT Powerful supermassive black hole (SMBH) winds in the form of ultra-fast outflows (UFOs) are detected in the X-ray spectra of several active galactic nuclei (AGNs) seemingly independently of their radio classification between radio quiet (RQ) and radio loud (RL). In this work, we explore the physical parameters of SMBH winds through a uniform analysis of a sample of X-ray bright RQ and RL AGN. We explored several correlations between different wind parameters and with respect to the AGN bolometric and Eddington luminosities. Our analysis shows that SMBH winds are not only a common trait of both AGN classes but also that they are most likely produced by the same physical mechanism. Consequently, we find that SMBH winds do not follow the radio-loudness dichotomy seen in jets. On average, a comparable amount of material accreted by the SMBH is ejected through such winds. The average wind power corresponds to about 3 per cent of the Eddington luminosity, confirming that they can drive AGN feedback. Moreover, the most energetic outflows are found in the most luminous sources. We find a possible positive correlation of the wind energetics, renormalized to the Eddington limit, with respect to $\lambda _{\mathrm{ Edd}}$, consistent with the correlation found with bolometric luminosity. We also observe a possible positive correlation between the energetics of the outflow and the X-ray radio-loudness parameter. In general, these results suggest an underlying relation between the acceleration mechanisms of accretion disc winds and jets.

On joint analysing &lt;i&gt;XMM-NuSTAR&lt;/i&gt; spectra of active galactic nuclei

A recently released &lt;i&gt;XMM-Newton&lt;/i&gt; note revealed a significant calibration issue between nuclear spectroscopic telescope array (&lt;i&gt;NuSTAR)&lt;/i&gt; and &lt;i&gt;XMM-Newton&lt;/i&gt; European Photon Imaging Camera (EPIC) and provided an empirical correction to the EPIC effective area. To quantify the bias caused by the calibration issue in the joint analysis of &lt;i&gt;XMM-NuSTAR&lt;/i&gt; spectra and verify the effectiveness of the correction, in this work, we perform joint-fitting of the &lt;i&gt;NuSTAR&lt;/i&gt; and EPIC-pn spectra for a large sample of 104 observation pairs of 44 X-ray bright active galactic nuclei (AGN). The spectra were extracted after requiring perfect simultaneity between the &lt;i&gt;XMM-Newton&lt;/i&gt; and &lt;i&gt;NuSTAR&lt;/i&gt; exposures (merging Good Time Intervals, GTIs from two missions) to avoid bias due to the rapid spectral variability of the AGN. Before the correction, the EPIC-pn spectra are systematically harder than the corresponding &lt;i&gt;NuSTAR&lt;/i&gt; spectra by &lt;inline-formula&gt;&lt;tex-math id="M1"&gt;\begin{document}$\Delta \varGamma \sim 0.1 $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="JUSTC-2023-0160_M1.jpg"/&gt;&lt;graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="JUSTC-2023-0160_M1.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, subsequently yielding significantly underestimated cutoff energy &lt;i&gt;E&lt;/i&gt;&lt;sub&gt;cut&lt;/sub&gt; and the strength of reflection component &lt;i&gt;R&lt;/i&gt; when performing joint-fitting. We confirm that the correction is highly effective and can commendably erase the discrepancy in best-fit &lt;i&gt;Γ&lt;/i&gt;, &lt;i&gt;E&lt;/i&gt;&lt;sub&gt;cut&lt;/sub&gt;, and &lt;i&gt;R&lt;/i&gt;. We thus urge the community to apply the correction when joint-fitting &lt;i&gt;XMM&lt;/i&gt;-&lt;i&gt;NuSTAR&lt;/i&gt; spectra, but note that the correction is limited to 3–12 keV and therefore not applicable when the soft X-ray band data are included. Besides, we show that as merging GTIs from two missions would cause severe loss of &lt;i&gt;NuSTAR&lt;/i&gt; net exposure time, in many cases, joint-fitting yields no advantage compared with utilizing &lt;i&gt;NuSTAR&lt;/i&gt; data alone. We finally present a technical note on filtering periods of high background flares for &lt;i&gt;XMM-Newton&lt;/i&gt; EPIC-pn exposures in the Small Window mode.

Where Do Obscured AGN Fit in a Galaxy’s Timeline?

Abstract Many X-ray bright active galactic nuclei (AGNs) are predicted to follow an extended stage of obscured black hole growth. In support of this picture we examine the X-ray undetected AGNs in the COSMOS field and compare their host galaxies with X-ray bright AGNs. We examine galaxies with M * &gt; 109.5 M ⊙ for the presence of AGNs at redshifts z = 0.5–3. We select AGNs in the infrared using Spitzer and Herschel detections and use color selection techniques to select AGNs within strongly star-forming hosts. We stack Chandra X-ray data of galaxies with an infrared (IR) detection but lacking an X-ray detection to obtain soft and hard fluxes, allowing us to measure the energetics of these AGNs. We find a clear correlation between X-ray luminosity and IR AGN luminosity in the stacked galaxies. We also find that X-ray undetected AGNs all lie on the main sequence—the tight correlation between the star formation rate and M * that holds for the majority of galaxies, regardless of mass or redshift. This work demonstrates that there is a higher population of obscured AGNs than previously thought.

Triggering nuclear and galaxy activity in the Bullet cluster

The analysis of a cluster environment is a valuable instrument to investigate the origin of gas fuelling and trigger mechanisms in active galactic nuclei (AGN) and star-forming galaxies. To this purpose, we present a detailed analysis of the point-like X-ray sources in the Bullet cluster field. Based on ∼600 ks Chandra observations, we produced a catalogue of 381 X-ray point sources up to a distance of ∼1.5 virial radius and with flux limits ∼1 × 10−16 and ∼8 × 10−16 erg cm−2 s−1 in the 0.5–2 keV and 2–10 keV bands, respectively. We find a strong (up to a factor 1.5–2) and significant (≥4σ) over-density in the full region studied 0.3R200 &lt; R &lt; 1.5R200. We identified optical (R band) and infrared (Spitzer IRAC) counterparts for ∼84% and ∼48% of the X-ray sources, respectively. We obtained new spectroscopic redshifts for 106 X-ray sources and collected from the literature additional 13 spectroscopic and 8 photometric redshifts of X-ray sources. Twenty-nine X-ray sources turned out to be cluster members. Spectroscopic and photometric redshifts of optical and infrared sources were also collected, and these sources were used as ancillary samples. We used these multi-wavelength data to characterise the nature of the Bullet cluster X-ray point sources. We find that the over-density in the region 0.3R200 &lt; R &lt; R200 is likely due to X-ray AGN (mostly obscured) and star-forming galaxies associated with the cluster, while in the more external region this over-density is likely to be mostly due to background AGN. The fraction of cluster galaxies hosting an X-ray detected AGN is 1.0 ± 0.4%, which is nearly constant with the radius; this fraction is similar to that reported in other clusters of galaxies at similar redshift. The fraction of X-ray bright AGN (L2 − 10 keV &gt; 1043 ergs s−1) in the region 0.3R200 &lt; R &lt; R200 is 0.5−0.2+0.6%, which is higher than that in other clusters at similar redshift and more similar to the AGN fraction in the field. Finally, the spatial distributions of AGN and star-forming galaxies, which we selected for their infrared emission, appear similar, thus suggesting that both are triggered by the same mechanism.

Infrared Contributions of X-Ray Selected Active Galactic Nuclei in Dusty Star-forming Galaxies

Abstract We investigate the infrared (IR) contribution from supermassive black hole activity versus host galaxy emission in the mid- to far-IR spectrum for a large sample of X-ray bright active galactic nuclei (AGN) residing in dusty, star-forming host galaxies. We select 703 AGN with erg s−1 at 0.1 &lt; z &lt; 5 from the Chandra XBoötes X-ray Survey with rich multiband observations in the optical to far-IR. This is the largest sample to date of X-ray AGN with mid- and far-IR detections that uses spectral energy distribution (SED) decomposition to determine intrinsic AGN and host galaxy IR luminosities. We determine weak or nonexistent relationships when averaging star formation activity as a function of AGN activity, but see stronger positive trends when averaging L X in bins of star-forming activity for AGN at low redshifts. We estimate an average dust covering factor (CF) of 33% based on IR SEDs and bolometric AGN luminosity, corresponding to a Type 2 AGN population of roughly a third. We also see a population of AGN that challenge the inclination-based unification model with individual dust CFs that contradict the nuclear obscuration expected from observed X-ray hardness ratios. We see no strong connection between AGN fractions in the IR and corresponding total IR, 24 μm, or X-ray luminosities. The average rest-frame AGN contribution as a function of IR wavelength shows significant (∼80%) contributions in the mid-IR that trail off at λ &gt; 30 μm. Additionally, we provide a relation between observed L X and pure AGN IR output for high-z AGN, allowing future studies to estimate AGN IR contribution using only observed X-ray flux density estimates.

The Halo Occupation Distribution of obscured quasars: revisiting the unification model

We model the projected angular two-point correlation function (2PCF) of obscured and unobscured quasars selected using the Wide-field Infrared Survey Explorer (WISE), at a median redshift of $z \sim 1$ using a five-parameter Halo Occupation Distribution (HOD) parameterization, derived from a cosmological hydrodynamic simulation by Chatterjee et al. The HOD parameterization was previously used to model the 2PCF of optically selected quasars and X-ray bright active galactic nuclei (AGN) at $z \sim 1$. The current work shows that a single HOD parameterization can be used to model the population of different kinds of AGN in dark matter halos suggesting the universality of the relationship between AGN and their host dark matter halos. Our results show that the median halo mass of central quasar hosts increases from optically selected ($4.1^{+0.3}_{-0.4} \times 10^{12} \; h^{-1} \; {M_{sun}}$) and infra-red (IR) bright unobscured populations ($6.3^{+6.2}_{-2.3} \times 10^{12} \; h^{-1} \; {M_{sun}}$) to obscured quasars ($10.0^{+2.6}_{-3.7} \times 10^{12} \; h^{-1} \; {M_{sun}}$), signifying an increase in the degree of clustering. The projected satellite fractions also increase from optically bright to obscured quasars and tend to disfavor a simple `orientation only' theory of active galactic nuclei unification. Our results also show that future measurements of the small-scale clustering of obscured quasars can constrain current theories of galaxy evolution where quasars evolve from an IR- bright obscured phase to the optically bright unobscured phase.

Properties of AGN coronae in theNuSTARera

The focusing optics of NuSTAR have enabled high signal-to-noise ratio spectra to be obtained from many X-ray bright active galactic nuclei (AGN) and galactic black hole binaries (BHB). Spectral modelling then allows robust characterization of the spectral index and upper energy cutoff of the coronal power-law continuum, after accounting for reflection and absorption effects. Spectral-timing studies, such as reverberation and broad iron line fitting, of these sources yield coronal sizes, often showing them to be small and in the range of 3 to 10 gravitational radii in size. Our results indicate that coronae are hot and radiatively compact, lying close to the boundary of the region in the compactness–temperature (Θ − l) diagram which is forbidden due to runaway pair production. The coincidence suggests that pair production and annihilation are essential ingredients in the coronae of AGN and BHB and that they control the shape of the observed spectra.

X-ray bright active galactic nuclei in massive galaxy clusters - III. New insights into the triggering mechanisms of cluster AGN

We present the results of a new analysis of the X-ray selected Active Galactic Nuclei (AGN) population in the vicinity of 135 of the most massive galaxy clusters in the redshift range of 0.2 < z < 0.9 observed with Chandra. With a sample of more than 11,000 X-ray point sources, we are able to measure, for the first time, evidence for evolution in the cluster AGN population beyond the expected evolution of field AGN. Our analysis shows that overall number density of cluster AGN scales with the cluster mass as $\sim M_{500}^{-1.2}$. There is no evidence for the overall number density of cluster member X-ray AGN depending on the cluster redshift in a manner different than field AGN, nor there is any evidence that the spatial distribution of cluster AGN (given in units of the cluster overdensity radius r_500) strongly depends on the cluster mass or redshift. The $M^{-1.2 \pm 0.7}$ scaling relation we measure is consistent with theoretical predictions of the galaxy merger rate in clusters, which is expected to scale with the cluster velocity dispersion, $\sigma$, as $ \sim \sigma^{-3}$ or $\sim M^{-1}$. This consistency suggests that AGN in clusters may be predominantly triggered by galaxy mergers, a result that is further corroborated by visual inspection of Hubble images for 23 spectroscopically confirmed cluster member AGN in our sample. A merger-driven scenario for the triggering of X-ray AGN is not strongly favored by studies of field galaxies, however, suggesting that different mechanisms may be primarily responsible for the triggering of cluster and field X-ray AGN.

The optical emission lines of type 1 X-ray bright Active Galactic Nuclei

A strong X-ray emission is one of the defining signatures of nuclear activity in galaxies. According to the Unified Model for Active Galactic Nuclei (AGN), both the X-ray radiation and the prominent broad emission lines, characterizing the optical and UV spectra of Type 1 AGNs, are originated in the innermost regions of the sources, close to the Super Massive Black Holes (SMBH), which power the central engine. Since the emission is concentrated in a very compact region (with typical size r⩽0.1 pc) and it is not possible to obtain resolved images of the source, spectroscopic studies of this radiation represent the only valuable key to constrain the physical properties of matter and its structure in the center of active galaxies. Based on previous studies on the physics of the Broad Line Region (BLR) and on the X-ray spectra of broad (FWHMHβ⩾2000kms−1) and narrow line (1000 km s−1 ⩽FWHMHβ⩽2000kms−1) emitting objects, it has been observed that the kinematic and ionization properties of matter close to the SMBHs are related together, and, in particular, that ionization is higher in narrow line sources. Here we report on the study of the optical and X-ray spectra of a sample of Type 1 AGNs, selected from the Sloan Digital Sky Survey (SDSS) database, within an upper redshift limit of z=0.35, and detected at X-ray energies. We present analysis of the broad emission line fluxes and profiles, as well as the properties of the X-ray continuum and Fe Kα emission and we use these parameters to assess the consistency of our current AGN understanding.

X-ray bright active galactic nuclei in massive galaxy clusters – II. The fraction of galaxies hosting active nuclei

We present a measurement of the fraction of cluster galaxies hosting X-ray bright active galactic nuclei (AGN) as a function of clustercentric distance scaled in units of r500. Our analysis employs high-quality Chandra X-ray and Subaru optical imaging for 42 massive X-ray-selected galaxy cluster fields spanning the redshift range 0.2 < z < 0.7. In total, our study involves 176 AGN with bright (R < 23) optical counterparts above a 0.5–8.0 keV flux limit of 10-14 erg cm-2 s-1. When excluding central dominant galaxies from the calculation, we measure a cluster galaxy AGN fraction in the central regions of the clusters that is ∼3 times lower than the field value. This fraction increases with clustercentric distance before becoming consistent with the field at ∼2.5r500. Our data exhibit similar radial trends to those observed for star formation and optically selected AGN in cluster member galaxies, both of which are also suppressed near cluster centres to a comparable extent. These results strongly support the idea that X-ray AGN activity and strong star formation are linked through their common dependence on available reservoirs of cold gas.

X-ray bright active galactic nuclei in massive galaxy clusters – I. Number counts and spatial distribution

We present an analysis of the X-ray bright point source population in 43 massive clusters of galaxies observed with the Chandra X-ray Observatory. We have constructed a catalog of 4210 rigorously selected X-ray point sources in these fields, which span a survey area of 4.2 square degrees. This catalog reveals a clear excess of sources when compared to deep blank-field surveys, which amounts to roughly 1 additional source per cluster, likely Active Galactic Nuclei (AGN) associated with the clusters. The excess sources are concentrated within the virial radii of the clusters, with the largest excess observed near the cluster centers. The average radial profile of the excess X-ray sources of the cluster are well described by a power law (N(r) ~ r^\beta) with an index of \beta ~ -0.5. An initial analysis using literature results on the mean profile of member galaxies in massive X-ray selected clusters indicates that the fraction of galaxies hosting X-ray AGN rises with increasing clustercentric radius, being approximately 5 to 10 times higher near the virial radius than in the central regions. This trend is qualitatively similar to that observed for star formation in cluster member galaxies.

Active galaxy 4U 1344-60: did the relativistic line disappear?

X-ray bright active galactic nuclei represent a unique astrophysical laboratory for studying accretion physics around super-massive black holes. 4U 1344-60 is a bright Seyfert galaxy which revealed relativistic reflection features in the archival XMM-Newton observation. We present the spectroscopic results of new data obtained with the Suzaku satellite and compare them with the previous XMM-Newton observation. The X-ray continuum of 4U 1344-60 can be well described by a power-law component with the photon index ~ 1.7 modified by a fully and a partially covering local absorbers. We measured a substantial decrease of the fraction of the partially absorbed radiation from around 45% in the XMM-Newton observation to less than 10% in the Suzaku observation while the power-law slope remains constant within uncertainties. The iron line in the Suzaku spectrum is relatively narrow, $\sigma=(0.08 \pm 0.02)$ keV, without any suggestion for relativistic broadening. Regarding this, we interpret the iron line in the archival XMM-Newton spectrum as a narrow line of the same width plus an additional red-shifted emission around 6.1 keV. No evidence of the relativistic reflection is present in the Suzaku spectra. The detected red-shifted iron line during the XMM-Newton observation could be a temporary feature either due to locally enhanced emission or decreased ionisation in the innermost accretion flow.

EVOLUTION OF THE QUASAR LUMINOSITY FUNCTION OVER 3 &lt;z&lt; 5 IN THE COSMOS SURVEY FIELD

We investigate the high-redshift quasar luminosity function (QLF) down to an apparent magnitude of I(AB) = 25 in the Cosmic Evolution Survey (COSMOS). Careful analysis of the extensive COSMOS photometry and imaging data allows us to identify and remove stellar and low-redshift contaminants, enabling a selection that is nearly complete for type-1 quasars at the redshifts of interest. We find 155 likely quasars at z > 3.1, 39 of which have prior spectroscopic confirmation. We present our sample in detail and use these confirmed and likely quasars to compute the rest-frame UV QLF in the redshift bins 3.1 < z < 3.5 and 3.5 < z < 5. The space density of faint quasars decreases by roughly a factor of four from z \sim 3.2 to z \sim 4, with faint-end slopes of {\beta} \sim -1.7 at both redshifts. The decline in space density of faint optical quasars at z > 3 is similar to what has been found for more luminous optical and X-ray quasars. We compare the rest-frame UV luminosity functions found here with the X-ray luminosity function at z > 3, and find that they evolve similarly between z \sim 3.2 and z \sim 4; however, the different normalizations imply that roughly 75% of X-ray bright active galactic nuclei (AGN) at z \sim 3 - 4 are optically obscured. This fraction is higher than found at lower redshift and may imply that the obscured, type-2 fraction continues to increase with redshift at least to z \sim 4. Finally, the implications of the results derived here for the contribution of quasars to cosmic reionization are discussed.

The ‘harder when brighter’ X-ray behaviour of the low-luminosity active galactic nucleus NGC 7213

We present the first robust evidence of an anti-correlation between the X-ray photon index, \Gamma, and the X-ray luminosity in a single low luminosity active galactic nuclei (LLAGN), NGC 7213. Up to now, such anti-correlation trends have been seen only in large samples of LLAGN that span a wide range of X-ray fluxes, although the opposite behaviour (i.e. a positive correlation between \Gamma and X-ray luminosity) has been extensively studied for individual X-ray bright active galactic nuclei. For NGC 7213, we use the long-term X-ray monitoring data of Rossi X-ray Timing Explorer (RXTE), regularly obtained on average every two days from March 2006 to December 2009. Based on our X-ray data, we derive the \Gamma versus flux and the hardness ratio versus flux relations, indicating clearly that NGC 7213 follows a `harder when brighter' spectral behaviour. Additionally, by analysing radio and optical data, and combining data from the literature, we form the most complete spectral energy distribution (SED) of the source across the electromagnetic spectrum yielding a bolometric luminosity of 1.7*10^43 erg s^-1. Phenomenologically, the SED of NGC 7213 is similar to that of low-ionization nuclear emission-line region. The robust anti-correlation trend that we find between \Gamma and X-ray luminosity together with the low accretion rate of the source, 0.14 per cent that of Eddington limit, make NGC 7213 the first LLAGN exhibiting a similar spectral behaviour with that of black hole X-ray binaries in `hard state'.

AGN with extreme X-ray amplitude variations

We present active galactic nuclei (AGNs) that have shown extreme amplitude variability in X-rays. These AGN appear as bright for long periods, but then suddenly become extremely X-ray weak sources. Most likely this behavior is due to strong absorption along the line of sight or by relativistically blurred reflection. Two extreme examples are the Narrow Line Seyfert 1 (NLS1) galaxies WPVS 007 and Mkn 335. WPVS 007 is a peculiar AGN because it combines the properties of a normal NLS1 with those of a broad-absorption line (BAL) quasar, which typically only appear in high-luminousity, high black hole mass systems. Mkn 335 has appeared an an X-ray bright AGN for most of the past few decades, but was caught by Swift in a deep X-ray flux minimum state in 2007, and has remained in that state for most of the time since. One potential explanation for this low state is absorption. Several Other AGN have been in deep minimum X-ray flux states, including PG 0844+349 and 1H 0707–495, for which the dramatic drops in X-ray flux have been explained by blurred X-ray reflection.

EUV spectroscopy in astrophysics: The role of compact objects

The bulk of radiation from million-degree plasmas is emitted at EUV wavelengths. Such plasmas are ubiquitous in astrophysics, and examples include the atmospheres of white dwarfs, accretion phenomena in cataclysmic variables (CVs) and some active galactic nuclei (AGN), the coronae of active stars, and the interstellar medium (ISM) of our own galaxy as well as of others. Internally, white dwarfs are formally analogous to neutron stars, being stellar configurations where the thermal contribution to support is secondary. Both stellar types have various intrinsic and environmental parameters. Comparison of such analogous systems using scaled parameters can be fruitful. Source class characterization is mature enough that such analogies can be used to compare theoretical ideas across a wide dynamic range in parameters, one example being theories of quasiperiodic oscillations. However, the white dwarf side of this program is limited by the available photometry and spectroscopy at EUV wavelengths, where there exist critical spectral features that contain diagnostic information often not available at other wavelengths. Moreover, interstellar absorption makes EUV observations challenging. Results from an observation of the hot white dwarf G191-B2B are presented to demonstrate the promise of high-resolution EUV spectroscopy. Two types of CVs, exemplified by AM Her and EX Hya, are used to illustrate blending of spectroscopy and timing measurements. Dynamical timescales and envisioned performance parameters of next-generation EUV satellites (effective area >20 cm 2, spectral resolution >10,000) make possible a new level of source modeling. The importance of the EUV cannot be overlooked given that observations are continually being pushed to cosmological distances, where the spectral energy distributions of X-ray bright AGNs, for example, will have their maxima redshifted into the EUV. Sometimes wrongly dismissed for limitations of small bandwidth or local view from optical depth limitations, the EUV is instead a gold mine of information bearing upon key issues in compact objects, but it is information that must be won through the triple combination of high-spectral resolution, large area, and application of advanced theory.

MCG-6-30-15: long time-scale X-ray variability, black hole mass and active galactic nuclei high states

We present a detailed study of the long time-scale X-ray variability of the Seyfert 1 Galaxy MCG-6-30-15, based on eight years of frequent monitoring observations with the Rossi X-ray Timing Explorer. When combined with the published short-time-scale XMM-Newton observations, we derive the power-spectral density (PSD) covering six decades of frequency from ∼10 -8 to ∼10 -2 Hz. As with NGC 4051, another narrow-line Seyfert 1 galaxy (NLS 1), we find that the PSD of MCG-6-30-15 is a close analogue of the PSD of a galactic black hole X-ray binary system (GBH) in a 'high' rather than a 'low' state. As with NGC 4051 and the GBH Cygnus X-1 in its high state, a smoothly bending model is a better fit to the PSD of MCG-6-30-15, giving a derived break frequency of 7.6 +10 -3 x 10 -5 Hz. Assuming linear scaling of the break frequency with black hole mass, we estimate the black hole mass in MCG-6-30-15 to be ∼2.9 +1.8 -1.6 × 10 6 M ○. . Although, in the X-ray band, it is one of the best observed Seyfert galaxies, there has as yet been no accurate determination of the mass of the black hole in MCG-6-30-15. Here we present a mass determination using the velocity dispersion (M BH -σ*) technique and compare it with estimates based on the width of the Ha line. Depending on the calibration relationship assumed for the M BH -σ* relationship, we derive a mass of between 3.6 and 6 x 10 6 M ○. , consistent with the mass derived from the PSD. Using the newly derived mass and break time-scale, and revised reverberation masses for other active galactic nuclei (AGN) from Peterson et al., we update the black hole mass-break-time-scale diagram. The observations are still generally consistent with narrow-line Seyfert 1 galaxies having shorter break time-scales, for a given mass, than broad-line AGN, probably reflecting a higher accretion rate. However, the revised, generally higher, masses (but unchanged break time-scales) are also consistent with perhaps all of the X-ray bright AGN studied so far being high-state objects. This result may simply be a selection effect, based on their selection from high-flux X-ray all-sky catalogues, and their consequent typically high X-ray/radio ratios, which indicate high-state systems.