A. C. Fabian

Broad line emission from iron K- and L-shell transitions in the active galaxy 1H 0707-495

Since the 1995 discovery of the broad iron K-line emission from the Seyfert galaxy MCG–6-30-15 (ref. 1), broad iron K lines have been found in emission from several other Seyfert galaxies, from accreting stellar-mass black holes and even from accreting neutron stars. The iron K line is prominent in the reflection spectrum created by the hard-X-ray continuum irradiating dense accreting matter. Relativistic distortion of the line makes it sensitive to the strong gravity and spin of the black hole. The accompanying iron L-line emission should be detectable when the iron abundance is high. Here we report the presence of both iron K and iron L emission in the spectrum of the narrow-line Seyfert 1 galaxy 1H 0707-495. The bright iron L emission has enabled us to detect a reverberation lag of about 30 s between the direct X-ray continuum and its reflection from matter falling into the black hole. The observed reverberation timescale is comparable to the light-crossing time of the innermost radii around a supermassive black hole. The combination of spectral and timing data on 1H 0707-495 provides strong evidence that we are witnessing emission from matter within a gravitational radius, or a fraction of a light minute, from the event horizon of a rapidly spinning, massive black hole.

An explanation for the soft X-ray excess in active galactic nuclei

We present a large sample of type 1 active galactic nuclei (AGN) spectra taken with XMM-Newton, and fit them with both the conventional model (a power law and black body) and the relativistically-blurred photoionized disc reflection model of Ross & Fabian (2005). We find the disc reflection model is a better fit. The disc reflection model successfully reproduces the continuum shape, including the soft excess, of all the sources. The model also reproduces many features that would conventionally be interpreted as absorption edges. We are able to use the model to infer the properties of the sources, specifically that the majority of black holes in the sample are strongly rotating, and that there is a deficit in sources with an inclination > 70 ◦ . We conclude that the disc reflection model is an important tool in the study of AGN X-ray spectra.

An explanation for the soft X-ray excess in AGN

We present a large sample of type 1 active galactic nuclei (AGN) spectra taken with XMM-Newton, and fit them with both the conventional model (a power law and black body) and the relativistically-blurred photoionized disc reflection model of Ross & Fabian (2005). We find the disc reflection model is a better fit. The disc reflection model successfully reproduces the continuum shape, including the soft excess, of all the sources. The model also reproduces many features that would conventionally be interpreted as absorption edges. We are able to use the model to infer the properties of the sources, specifically that the majority of black holes in the sample are strongly rotating, and that there is a deficit in sources with an inclination > 70 ◦ . We conclude that the disc reflection model is an important tool in the study of AGN X-ray spectra.

ACCRETION ONTO THE SUPERMASSIVE BLACK HOLE IN M87

Chandra X-ray observations of the giant elliptical galaxy M87 resolve the thermal state of the hot interstellar medium into the accretion (Bondi) radius of its central 3 × 109 M☉ black hole. We measure the X-ray gas temperature and density profiles and calculate the Bondi accretion rate, Bondi ~ 0.1 M☉ yr-1. The X-ray luminosity of the active nucleus of M87 observed with Chandra is LX,0.5-7 keV ~ 7 × 1040 ergs s-1. This value is much less than the predicted nuclear luminosity, LBondi ~ 5 × 1044 ergs s-1, for accretion at the Bondi rate with a canonical accretion radiative efficiency of 10%. If the black hole in M87 accretes at this rate it must do so at a much lower radiative efficiency than the canonical value. The multiwavelength spectrum of the nucleus is consistent with that predicted by an advection-dominated flow. However, as is likely, the X-ray nucleus is dominated by jet emission then the properties of flow must be modified, possibly by outflows. We show that the overall energetics of the system are just consistent with the predicted Bondi nuclear power. This suggests that either most of the accretion energy is released in the relativistic jet or the central engine of M87 undergoes on-off activity cycles. We show that, at present, the energy dumped into the ISM by the jet may reduce the accretion rate onto the black hole by a factor ∝ (vj/cs)-2, where vj is the jet velocity and cs the ISM sound speed, and that this is sufficient to account for the low nuclear luminosity.

The X-ray virial relations for relaxed lensing clusters observed with Chandra

We examine the relations linking mass, X-ray temperature and bolometric luminosity for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra Observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Within radii corresponding to a fixed overdensity with respect to the critical density at the redshifts of the clusters, the observed temperature profiles, scaled in units of T2500 and r2500, exhibit an approximately universal form which rises within and then remains approximately constant out to r2500. We obtain best-fitting slopes for the mass–temperature and temperature–luminosity relations consistent with the predictions from simple scaling arguments i.e. and , respectively. We confirm the presence of a systematic offset of ∼40 per cent between the normalizations of the observed and predicted mass–temperature relations for both SCDM and ΛCDM cosmologies.

The role of black holes in galaxy formation and evolution

Virtually all massive galaxies, including our own, host central black holes ranging in mass from millions to billions of solar masses. The growth of these black holes releases vast amounts of energy that powers quasars and other weaker active galactic nuclei. A tiny fraction of this energy, if absorbed by the host galaxy, could halt star formation by heating and ejecting ambient gas. A central question in galaxy evolution is the degree to which this process has caused the decline of star formation in large elliptical galaxies, which typically have little cold gas and few young stars, unlike spiral galaxies.

Cosmological constraints from the X-ray gas mass fraction in relaxed lensing clusters observed with Chandra

We present precise measurements of the X-ray gas mass fraction for a sample of luminous, relatively relaxed clusters of galaxies observed with the Chandra Observatory, for which independent confirmation of the mass results is available from gravitational lensing studies. Parameterizing the total (luminous plus dark matter) mass profiles using the model of Navarro, Frenk & White (1997), we show that the X-ray gas mass fractions in the clusters asymptote towards an approximately constant value at a radius r_2500, where the mean interior density is 2500 times the critical density of the Universe at the redshifts of the clusters. Combining the Chandra results on the X-ray gas mass fraction and its apparent redshift dependence with recent measurements of the mean baryonic matter density in the Universe and the Hubble Constant determined from the Hubble Key Project, we obtain a tight constraint on the mean total matter density of the Universe, Omega_m = 0.30^{+0.04}_{-0.03}, and measure a positive cosmological constant, Omega_Lambda = 0.95^{+0.48}_{-0.72}. Our results are in good agreement with recent, independent findings based on analyses of anisotropies in the cosmic microwave background radiation, the properties of distant supernovae, and the large-scale distribution of galaxies.

X-ray reflection spectra from ionized slabs

X-ray reflection spectra are an important component in the X-ray spectra of many active galactic nuclei and Galactic black hole candidates. It is likely that reflection takes place from highly ionized surfaces of the accretion disc in some cases. This can lead to strong Comptonization of the emergent iron, and other absorption and emission features. We present such reflection spectra here, computed in a self-consistent manner with the method described by Ross & Fabian. In particular we emphasize the range where the ionization parameter (the flux to density ratio) ξ is around and above 104. Such spectra may be relevant to the observed spectral features found in black hole candidates such as Cygnus X-1 in the low/hard state.

X-Ray Spectroscopic Evidence for Intermediate-Mass Black Holes: Cool Accretion Disks in Two Ultraluminous X-Ray Sources

We have analyzed an XMM-Newton observation of the nearby spiral galaxy NGC 1313, which contains two ultraluminous X-ray (ULX) sources. We measure isotropic luminosities of LX = 2.0 × 1040 ergs s-1 and LX = 6.6 × 1039 ergs s-1 for NGC 1313 X-1 and X-2 (0.2-10.0 keV, assuming a distance of 3.7 Mpc). The spectra statistically require soft and hard spectral components to describe the continuum emission; some prior studies of ULX sources have claimed cool soft components with lower statistics. The improvement over several single-component models exceeds the 8 σ level of confidence for X-1; the improvement for X-2 is significant at the 3 σ level. The soft components in these ULX spectra are well fitted by multicolor disk blackbody models with color temperatures of kT 150 eV. This temperature differs markedly from those commonly measured in the spectra of stellar mass (10 M☉) black holes in their brightest states (kT 1 keV). It is expected that the temperature of an accretion disk orbiting a black hole should decrease with increasing black hole mass. If the soft components we measure are due to emission from the inner region of an accretion disk, and disks extend close to the innermost stable circular orbit at the accretion rates being probed, the low color temperatures may be interpreted as spectroscopic evidence of black holes with intermediate masses: MBH 103 M☉. Simple Eddington scaling arguments suggest a minimum mass of MBH ~ 102 M☉. NGC 1313 X-1 and X-2 are found in optical nebulae, which may indicate that anisotropic emission geometries are unlikely to account for the fluxes observed.

A Long, Hard Look at the Low/Hard State in Accreting Black Holes

We present the first results of coordinated multiwavelength observations of the Galactic black hole GX 339-4 in a canonical low/hard state, obtained during its 2004 outburst. XMM-Newton observed the source for two revolutions, or approximately 280 ks; RXTE monitored the source throughout this long stare. The resulting data offer the best view yet obtained of the inner accretion flow geometry in the low/hard state, which is thought to be analogous to the geometry in low-luminosity active galactic nuclei. The XMM-Newton spectra clearly reveal the presence of a cool accretion disk component and a relativistic Fe K emission line. The results of fits made to both components strongly suggest that a standard thin disk remains at or near to the innermost stable circular orbit, at least in bright phases of the low/hard state. These findings indicate that potential links between the inner disk radius and the onset of a steady compact jet, and the paradigm of a radially recessed disk in the low/hard state, do not hold universally. The results of our observations can best be explained if a standard thin accretion disk fuels a corona that is closely related to, or consistent with, the base of a compact jet. In a brief examination of archival data, we show that Cygnus X-1 supports this picture of the low/hard state. We discuss our results within the context of disk-jet connections and prevailing models for accretion onto black holes.