The impact of accretion disc winds on the X-ray spectrum of AGN – I. xscort

The accretion disc in active galactic nucleus (AGN) is expected to produce strong outflows, in particular an ultraviolet (UV)-line-driven wind. Several observed spectral features, including the soft X-ray excess, have been associated with the accretion disc wind. However, current spectral models of the X-ray spectrum of AGN observed through an accretion disc wind, known to provide a good fit to the observed X-ray data, are ad hoc in their treatment of the outflow velocity and density of the wind material. In order to address these limitations we adopt a numerical computational method that links a series of radiative transfer calculations, incorporating the effect of a global velocity field in a self-consistent manner { xstar Simulation Chain for Outflows with Radiative Transfer ( xscort )}. We present a series of example spectra from the xscort code that allow us to examine the shape of AGN X-ray spectra seen through a smooth wind with terminal velocity of 0.3 c , as appropriate for a UV-line-driven wind. We calculate spectra for a range of different acceleration laws, density distributions, total column densities and ionization parameters, but all these have sharp features that contrast strongly with both the previous 'smeared absorption' models, and with the observed smoothness of the soft X-ray excess. This rules out absorption in a radiatively driven accretion disc wind as the origin of the soft X-ray excess, though a larger terminal velocity, possibly associated with material in a magnetically driven outflow/jet, may allow outflow models to recover a smooth excess.     

Alignment and precession of a black hole with a warped accretion disc

We consider the shape of an accretion disc whose outer regions are misaligned with the spin axis of a central black hole and calculate the steady state form of the warped disc in the case where the viscosity and surface densities are power laws in the distance from the central black hole. We discuss the shape of the resulting disc in both the frame of the black hole and that of the outer disc. We note that some parts of the disc and also any companion star maybe shadowed from the central regions by the warp. We compute the torque on the black hole caused by the Lense–Thirring precession, and hence compute the alignment and precession time-scales. We generalize the case with viscosity and hence surface density independent of radius to more realistic density distributions for which the surface density is a decreasing function of radius. We find that the alignment time-scale does not change greatly but the precession time-scale is more sensitive. We also determine the effect on this time-scale if we truncate the disc. For a given truncation radius, the time-scales are less affected for more sharply falling density distributions.     

Rossi XTE monitoring of 4U 1636–53 – I. Long-term evolution and kHz quasi-periodic oscillations

We have monitored the atoll-type neutron star low-mass X-ray binary 4U 1636−53 with the Rossi X-ray Timing Explorer ( RXTE ) for more than 1.5 yr. Our campaign consisted of short (∼2 ks) pointings separated by 2 d, regularly monitoring the spectral and timing properties of the source. During the campaign we observed a clear long-term oscillation with a period of ∼30–40 d, already seen in the light curves from the RXTE All-Sky Monitor, which corresponded to regular transitions between the hard (island) and soft (banana) states. We detected kilohertz (kHz) quasi-periodic oscillations (QPOs) in about a third of the observations, most of which were in the soft (banana) state. The distribution of the frequencies of the peak identified as the lower kHz QPO is found to be different from that previously observed in an independent data set. This suggests that the kHz QPOs in the system shows no intrinsically preferred frequency.     

Circumnuclear regions in barred spiral galaxies – I. Near-infrared imaging

We present subarcsecond-resolution, ground-based, near-infrared images of the central regions of a sample of 12 barred galaxies with circumnuclear star formation activity, which is organized in ring-like regions typically 1 kpc in diameter. We also present Hubble Space Telescope near-infrared images of 10 of our sample galaxies, and compare them with our ground-based data. Although our sample galaxies were selected for the presence of circumnuclear star formation activity, our broad-band near-infrared images are heterogeneous, showing a substantial amount of small-scale structure in some galaxies, and practically none in others. We argue that, where it exists, this structure is caused by young stars, which also cause the characteristic bumps or changes in slope in the radial profiles of ellipticity, major axis position angle, surface brightness and colour at the radius of the circumnuclear ring in most of our sample galaxies. In seven out of 10 HST images, star formation in the nuclear ring is clearly visible as a large number of small emitting regions, organized into spiral arm fragments, which are accompanied by dust lanes. Near-infrared colour index maps show much more clearly the location of dust lanes and, in certain cases, regions of star formation than single broad-band images. Circumnuclear spiral structure thus outlined appears to be common in barred spiral galaxies with circumnuclear star formation.     

Quasars from the 7C survey – I. Sample selection and radio maps

We describe the selection of candidate radio-loud quasars obtained by cross-matching radio source positions from the low-frequency (151-MHz) 7C survey with optical positions from five pairs of EO POSS-I plates scanned with the Cambridge Automatic Plate-measuring Machine (APM). The sky region studied is centred at RA 10^h 28^m, Dec.+41° and covers ≈0.057 sr. We present VLA observations of the quasar candidates, and tabulate various properties derived from the radio maps. We discuss the selection criteria of the resulting '7CQ' sample of radio-loud quasars. The 70 confirmed quasars, and some fraction of the 36 unconfirmed candidates, constitute a filtered sample with the following selection criteria: 151-MHz flux density S ₁₅₁>100 mJy; POSS-I E -plate magnitude E ≈ R <20; POSS-I colour ( O E )<1.8; the effective area of the survey drops significantly below S ₁₅₁≈200 mJy. We argue that the colour criterion excludes few if any quasars, but note, on the basis of recent work by Willott et al., that the E magnitude limit probably excludes more than 50 per cent of the radio-loud quasars.     

The effect of radiation stress on gas motions in AGN accretion discs

In this paper we look at one of the effects of irradiation on a warped accretion disc in the context of active galactic nuclei (AGN). A warp will catch a substantial amount of the radiation emitted by the central object. We consider the fluid motions that may arise inside a warped disc when the surface is subject to a radiation stress, and also the net mass flows that result. We find that, to first order, we have a balance of the viscous and Coriolis-type forces. The radial radiation stress causes outward motion of the surface layer, but only the azimuthal Poynting–Robertson drag leads to an increase in the net accretion rate. We investigate the distribution of the velocity perturbations and find them to be significant in determining the local structure of the disc.
An unexpected result is that the picture changes significantly when we take into account the periodic illumination of the warped disc. A type of resonance at the local Keplerian rotation frequency causes a flow that penetrates the whole thickness of the disc; these flows are faster than the flows due to unchanging illumination. They totally dominate the induced flows in terms of sheer mass, but significant impact on disc structure still occurs only near the surface, where velocity perturbations typically go up to some kilometres per second.     

High-frequency X-ray variability as a mass estimator of stellar and supermassive black holes

There is increasing evidence that supermassive black holes in active galactic nuclei (AGN) are scaled-up versions of Galactic black holes. We show that the amplitude of high-frequency X-ray variability in the hard spectral state is inversely proportional to the black hole mass over eight orders of magnitude. We have analysed all available hard-state data from RXTE of seven Galactic black holes. Their power density spectra change dramatically from observation to observation, except for the high-frequency (≳10 Hz) tail, which seems to have a universal shape, roughly represented by a power law of index −2. The amplitude of the tail,   C _M   (extrapolated to 1 Hz), remains approximately constant for a given source, regardless of the luminosity, unlike the break or quasi-periodic oscillation frequencies, which are usually strongly correlated with luminosity. Comparison with a moderate-luminosity sample of AGN shows that the amplitude of the tail is a simple function of black hole mass,   C _M = C / M   , where   C ≈ 1.25 M_⊙ Hz⁻¹  . This makes   C _M   a robust estimator of the black hole mass which is easy to apply to low- to moderate-luminosity supermassive black holes. The high-frequency tail with its universal shape is an invariant feature of a black hole and, possibly, an imprint of the last stable orbit.     

The response of the Fe Kα line to changes in the X-ray illumination of accretion discs

X-ray reflection spectra from photoionized accretion discs in active galaxies are presented for a wide range of illumination conditions. The energy, equivalent width (EW) and flux of the Fe K α line are shown to depend strongly on the ratio of illuminating flux to disc flux,   F _x/ F _disc  , the photon index of the irradiating power law, Γ, and the incidence angle of the radiation, i . When   F _x/ F _disc≤2  a neutral Fe K α line is prominent for all but the largest values of Γ. At higher illuminating fluxes an He-like Fe K α line at 6.7 keV dominates the line complex. With a high-energy cut-off of 100 keV, the thermal ionization instability seems to suppress the ionized Fe K α line when  Γ≤1.6  . The Fe K α line flux correlates with   F _x/ F _disc  , but the dependence weakens as iron becomes fully ionized. The EW is roughly constant when   F _x/ F _disc  is low and a neutral line dominates, but then declines as the line progresses through higher ionization stages. There is a strong positive correlation between the Fe K α EW and Γ when the line energy is at 6.7 keV, and a slight negative one when it is at 6.4 keV. This is a potential observational diagnostic of the ionization state of the disc. Observations of the broad Fe K α line, which take into account any narrow component, would be able to test these predictions. Ionized Fe K α lines at 6.7 keV are predicted to be common in a simple magnetic flare geometry. A model that includes multiple ionization gradients on the disc is postulated to reconcile the results with observations.