ALBERT

Description: We present an analysis of the 3–79 keV NuSTAR spectrum of the low-luminosity active galactic nucleus NGC 7213. In agreement with past observations, we find a lower limit to the high-energy cut-off of E c  〉 140 keV, no evidence for a Compton-reflected continuum and the presence of an iron K α complex, possibly produced in the broad-line region. From the application of the mytorus model, we find that the line-emitting material is consistent with the absence of a significant Compton reflection if arising from a Compton-thin torus of gas with a column density of $5.0^{+2.0}_{-1.6}\times 10^{23}$ cm –2 . We report variability of the equivalent width of the iron lines on the time-scale of years using archival observations from XMM–Newton , Chandra and Suzaku . This analysis suggests a possible contribution from dusty gas. A fit with a Comptonization model indicates the presence of a hot corona with a temperature kT e  〉 40 keV and an optical depth   1, assuming a spherical geometry.

Description: Superbursts are hours-long X-ray flares attributed to the thermonuclear runaway burning of carbon-rich material in the envelope of accreting neutron stars. By studying the details of the X-ray light curve, properties of carbon combustion can be determined. In particular, we show that the shape of the rise of the light curve is set by the slope of the temperature profile left behind by the carbon flame. We analyse Rossi X-ray Timing Explorer /Proportional Counter Array observations of 4U 1636–536 and separate the direct neutron star emission from evolving photoionized reflection and persistent spectral components. This procedure results in the highest quality light curve ever produced for the superburst rise and peak, and interesting behaviour is found in the tail. The rising light curve between 100 and 1000 s is inconsistent with the idea that the fuel burned locally and instantaneously everywhere, as assumed in some previous models. By fitting improved cooling models, we measure for the first time the radial temperature profile of the superbursting layer. We find d ln T /d ln P 1/4. Furthermore, 20 per cent of the fuel may be left unburned. This gives a new constraint on models of carbon burning and propagation in superbursts.

Description: NuSTAR observed the bright Compton-thin, narrow-line Seyfert 1 galaxy, NGC 5506, for about 56 ks. In agreement with past observations, the spectrum is well fitted by a power law with  ~ 1.9, a distant reflection component and narrow ionized iron lines. A relativistically blurred reflection component is not required by the data. When an exponential high-energy cutoff is added to the power law, a value of 720 $^{+130}_{-190}$  keV (90 per cent confidence level) is found. Even allowing for systematic uncertainties, we find a 3 lower limit to the high-energy cutoff of 350 keV, the highest lower limit to the cutoff energy found so far in an AGN by NuSTAR .

Description: Obscured active galactic nuclei (AGNs) provide an opportunity to study the material surrounding the central engine. Geometric and physical constraints on the absorber can be deduced from the reprocessed AGN emission. In particular, the obscuring gas may reprocess the nuclear X-ray emission producing a narrow Fe Kα line and a Compton reflection hump. In recent years, models of the X-ray reflection from an obscuring torus have been computed; however, although the reflecting gas may be dusty, the models do not yet take into account the effects of dust on the predicted spectrum. We study this problem by analysing two sets of models, with and without the presence of dust, using the one-dimensional photoionization code cloudy . The calculations are performed for a range of column densities (22 〈 log[ N H (cm – 2 )] 〈 24.5) and hydrogen densities (6 〈 log[ n H (cm – 3 )] 〈 8). The calculations show the presence of dust can enhance the Fe Kα equivalent width (EW) in the reflected spectrum by factors up to 8 for Compton thick (CT) gas and a typical interstellar medium grain size distribution. The enhancement in EW with respect to the reflection continuum is due to the reduction in the reflected continuum intensity caused by the anisotropic scattering behaviour of dust grains. This effect will be most relevant for reflection from distant, predominately neutral gas, and is a possible explanation for AGNs which show a strong Fe Kα EW and a relatively weak reflection continuum. Our results show it is an important to take into account dust while modelling the X-ray reflection spectrum, and that inferring a CT column density from an observed Fe Kα EW may not always be valid. Multidimensional models are needed to fully explore the magnitude of the effect.

Description: We present a broad-band spectral analysis of the joint XMM–Newton and Nuclear Spectroscopic Telescope Array observational campaign of the narrow-line Seyfert 1 SWIFT J2127.4+5654, consisting of 300 ks performed during three XMM–Newton orbits. We detect a relativistic broadened iron Kα line originating from the innermost regions of the accretion disc surrounding the central black hole, from which we infer an intermediate spin of $a = 0.58^{+0.11}_{-0.17}$ . The intrinsic spectrum is steep ( = 2.08 ± 0.01) as commonly found in narrow-line Seyfert 1 galaxies, while the cutoff energy ( $E_{\rm c}=108^{+11}_{-10}$ keV) falls within the range observed in broad-line Seyfert 1 galaxies. We measure a low-frequency lag that increases steadily with energy, while at high frequencies, there is a clear lag following the shape of the broad Fe K emission line. Interestingly, the observed Fe K lag in SWIFT J2127.4+5654 is not as broad as in other sources that have maximally spinning black holes. The lag amplitude suggests a continuum-to-reprocessor distance of about 10–20 r g . These timing results independently support an intermediate black hole spin and a compact corona.

Description: The broad-band X-ray spectra of active galactic nuclei (AGNs) contains information about the nuclear environment from Schwarzschild radii scales (where the primary power law is generated in a corona) to distances of ~1 pc (where the distant reflector may be located). In addition, the average shape of the X-ray spectrum is an important input into X-ray background synthesis models. Here, local ( z 0) AGN luminosity functions (LFs) in five energy bands are used as a low-resolution, luminosity-dependent X-ray spectrometer in order to constrain the average AGN X-ray spectrum between 0.5 and 200 keV. The 15–55 keV LF measured by Swift -BAT is assumed to be the best determination of the local LF, and then a spectral model is varied to determine the best fit to the 0.5–2 keV, 2–10 keV, 3–20 keV and 14–195 keV LFs. The spectral model consists of a Gaussian distribution of power laws with a mean photon-index 〈〉 and cutoff energy E cut , as well as contributions from distant and disc reflection. The reflection strength is parametrized by varying the Fe abundance relative to solar, A Fe , and requiring a specific Fe Kα equivalent width (EW). In this way, the presence of the X-ray Baldwin effect can be tested. The spectral model that best fits the four LFs has 〈〉 = 1.85 ± 0.15, $E_{\mathrm{cut}}=270^{+170}_{-80}$ keV, $A_{\mathrm{Fe}}=0.3^{+0.3}_{-0.15}$ . The sub-solar A Fe is unlikely to be a true measure of the gas-phase metallicity, but indicates the presence of strong reflection given the assumed Fe Kα EW. Indeed, parametrizing the reflection strength with the R parameter gives $R=1.7^{+1.7}_{-0.85}$ . There is moderate evidence for no X-ray Baldwin effect. Accretion disc reflection is included in the best-fitting model, but it is relatively weak (broad iron Kα EW 〈 100 eV) and does not significantly affect any of the conclusions. A critical result of our procedure is that the shape of the local 2–10 keV LF measured by HEAO-1 and MAXI is incompatible with the LFs measured in the hard X-rays by Swift -BAT and RXTE . We therefore present a new determination of the local 2–10 keV LF that is consistent with all other energy bands, as well as the de-evolved 2–10 keV LF estimated from the XMM–Newton Hard Bright Survey. This new LF should be used to revise current measurements of the evolving AGN LF in the 2–10 keV band. Finally, the suggested absence of the X-ray Baldwin effect points to a possible origin for the distant reflector in dusty gas not associated with the AGN obscuring medium. This may be the same material that produces the compact 12 μm source in local AGNs.

Description: Deep X-ray surveys have provided a comprehensive and largely unbiased view of active galactic nuclei (AGN) evolution stretching back to z  ~ 5. However, it has been challenging to use the survey results to connect this evolution to the cosmological environment that AGN inhabit. Exploring this connection will be crucial to understanding the triggering mechanisms of AGN and how these processes manifest in observations at all wavelengths. In anticipation of upcoming wide-field X-ray surveys that will allow quantitative analysis of AGN environments, this paper presents a method to observationally constrain the conditional luminosity function (CLF) of AGN at a specific z . Once measured, the CLF allows the calculation of the AGN bias, mean dark matter halo mass, AGN lifetime, halo occupation number, and AGN correlation function– all as a function of luminosity. The CLF can be constrained using a measurement of the X-ray luminosity function and the correlation length at different luminosities. The method is illustrated at z 0 and 0.9 using the limited data that are currently available, and a clear luminosity dependence in the AGN bias and mean halo mass is predicted at both z , supporting the idea that there are at least two different modes of AGN triggering. In addition, the CLF predicts that z 0.9 quasars may be commonly hosted by haloes with M h  ~ 10 14 M . These ‘young cluster’ environments may provide the necessary interactions between gas-rich galaxies to fuel luminous accretion. The results derived from this method will be useful to populate AGN of different luminosities in cosmological simulations.

Description: The orientation-based unification model of active galactic nuclei (AGNs) posits that the principle difference between obscured (Type 2) and unobscured (Type 1) AGNs is the line of sight into the central engine. If this model is correct then there should be no difference in many of the properties of AGN host galaxies (e.g. the mass of the surrounding dark matter haloes). However, recent clustering analyses of Type 1 and Type 2 AGNs have provided some evidence for a difference in the halo mass, in conflict with the orientation-based unified model. In this work, a method to compute the conditional luminosity function (CLF) of Type 2 and Type 1 AGNs is presented. The CLF allows many fundamental halo properties to be computed as a function of AGN luminosity, which we apply to the question of the host halo masses of Type 1 and 2 AGNs. By making use of the total AGN CLF, the Type 1 X-ray luminosity function, and the luminosity-dependent Type 2 AGN fraction, the CLFs of Type 1 and 2 AGNs are calculated at z 0 and 0.9. At both z , there is no statistically significant difference in the mean halo mass of Type 2 and 1 AGNs at any luminosity. There is marginal evidence that Type 1 AGNs may have larger halo masses than Type 2s, which would be consistent with an evolutionary picture where quasars are initially obscured and then subsequently reveal themselves as Type 1s. As the Type 1 lifetime is longer, the host halo will increase somewhat in mass during the Type 1 phase. The CLF technique will be a powerful way to study the properties of many AGNs subsets (e.g. radio-loud, Compton-thick) as future wide-area X-ray and optical surveys substantially increase our ability to place AGNs in their cosmological context.