RAiSE III : 3C radio AGN energetics and composition

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2017 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.Kinetic jet power estimates based exclusively on observed monochromatic radio luminosities are highly uncertain due to confounding variables and a lack of knowledge about some aspects of the physics of active galactic nuclei (AGNs). We propose a new methodology to calculate the jet powers of the largest, most powerful radio sources based on combinations of their size, lobe luminosity, and shape of their radio spectrum; this approach avoids the uncertainties encountered by previous relationships. The outputs of our model are calibrated using hydrodynamical simulations and tested against independent X-ray inverse-Compton measurements. The jet powers and lobe magnetic field strengths of radio sources are found to be recovered using solely the lobe luminosity and spectral curvature, enabling the intrinsic properties of unresolved high-redshift sources to be inferred. By contrast, the radio source ages cannot be estimated without knowledge of the lobe volumes. The monochromatic lobe luminosity alone is incapable of accurately estimating the jet power or source age without knowledge of the lobe magnetic field strength and size, respectively. We find that, on average, the lobes of the Third Cambridge Catalogue of Radio Sources (3C) have magnetic field strengths approximately a factor three lower than the equipartition value, inconsistent with equal energy in the particles and the fields at the 5σ level. The particle content of 3C radio lobes is discussed in the context of complementary observations; we do not find evidence favouring an energetically dominant proton population.Peer reviewe

X-ray Reflection from Inhomogeneous Accretion Disks: II. Emission Line Variability and Implications for Reverberation Mapping

One of the principal scientific objectives of the upcoming Constellation-X mission is to attempt to map the inner regions of accretion disks around black holes in Seyfert galaxies by reverberation mapping of the Fe K fluorescence line. This area of the disk is likely radiation pressure dominated and subject to various dynamical instabilities. Here, we show that density inhomogeneities in the disk atmosphere resulting from the photon bubble instability (PBI) can cause rapid changes in the X-ray reflection features, even when the illuminating flux is constant. Using a simulation of the development of the PBI, we find that, for the disk parameters chosen, the Fe K and O VIII Ly\alpha lines vary on timescales as short as a few hundredths of an orbital time. In response to the changes in accretion disk structure, the Fe K equivalent width (EW) shows variations as large as ~100 eV. The magnitude and direction (positive or negative) of the changes depends on the ionization state of the atmosphere. The largest changes are found when the disk is moderately ionized. The O VIII EW varies by tens of eV, as well as exhibiting plenty of rapid, low-amplitude changes. This effect provides a natural explanation for some observed instances of short timescale Fe K variability which was uncorrelated with the continuum (e.g., Mrk 841). New predictions for Fe K reverberation mapping should be made which include the effects of this accretion disk driven line variability and a variable ionization state. Reflection spectra averaged over the evolution of the instability are well fit by constant density models in the 2-10 keV region.Comment: 20 pages, 3 figures. Accepted by Ap

On the X-ray spectra of luminous, inhomogeneous accretion flows

We discuss the expected X-ray spectral and variability properties of black hole accretion discs at high luminosity, under the hypothesis that radiation pressure dominated discs are subject to violent clumping instabilities and, as a result, have a highly inhomogeneous two-phase structure. After deriving the full accretion disc solutions explicitly in terms of the parameters of the model, we study their radiative properties both with a simple two-zones model, treatable analytically, and with radiative transfer simulations which account simultaneously for energy balance and Comptonisation in the hot phase, together with reflection, reprocessing, ionization and thermal balance in the cold phase. We show that, if not only the density, but also the heating rate within these flows is inhomogeneous, then complex reflection-dominated spectra can be obtained for a high enough covering fraction of the cold phase. In general, large reflection components in the observed X-ray spectra should be associated with strong soft excesses, resulting from the combined emission of ionized atomic emission lines. The variability properties of such systems are such that, even when contributing to a large fraction of the hard X-ray spectrum, the reflection component is less variable than the power-law like emission originating from the hot Comptonising phase, in agreement with what is observed in many Narrow Line Seyfert 1 galaxies and bright Seyfert 1. Our model falls within the family of those trying to explain the complex X-ray spectra of bright AGN with ionized reflection, but presents an alternative, specific, physically motivated, geometrical setup for the complex multi-phase structure of the inner regions of near-Eddington accretion flows.Comment: 15 pages, 9 figures. MNRAS, in pres

A systematic look at the Very High and Low/Hard state of GX 339-4: Constraining the black hole spin with a new reflection model

We present a systematic study of GX 339-4 in both its very high and low hard states from simultaneous observations made with XMM-Newton and RXTE in 2002 and 2004. The X-ray spectra of both these extreme states exhibit strong reflection signatures, with a broad, skewed Fe-Kalpha line clearly visible above the continuum. Using a newly developed, self-consistent reflection model which implicitly includes the blackbody radiation of the disc as well as the effect of Comptonisation, blurred with a relativistic line function, we were able to infer the spin parameter of GX 339-4 to be 0.935 +/- 0.01 (statistical) +/- 0.01 (systematic) at 90 per cent confidence. We find that both states are consistent with an ionised thin accretion disc extending to the innermost stable circular orbit around the rapidly spinning black hole.Comment: 10 pages, 10 figures, accepted for publication in MNRAS 17/04/0

Soft X-ray spectral variations of the narrow line Seyfert l galaxy Markarian 766

The X-ray variability of the narrow-line Seyfert 1 galaxy Markarian 766 is studied using nine ROSAT PSPC data sets. The spectrum is well described by a power law combined with a blackbody (kT ∼ 70 eV) soft excess. Examination of flux ratio changes and variability amplitude in three X-ray bands shows that the power-law component varies continuously on time-scales of ∼ 5000 s and is steeper when it is brighter. In contrast, variability of the soft excess is not detected. Spectral modelling of 31 spectra from different observations and at a range of count rates is also consistent with a picture in which the power law is steeper when it is brighter, and in which the soft-excess component does not vary. The power-law variability can be explained if the power law is produced by variable thermal or non-thermal Comptonization of soft photons. This behaviour is similar to that of Galactic black hole candidates in the low state. The X-ray and multiwavelength properties of Markarian 766 are shown to be very similar to those of other narrow-line Seyfert 1s. This may mean that the rapid X-ray variability seen in other narrow-line Seyfert 1s may also not originate in their strong soft-excess components

A softer look at MCG--6-30-15 with XMM-Newton

We present analysis and results from the Reflection Grating Spectrometer during the 320 ks XMM observation of the Seyfert 1 galaxy MCG-6-30-15. The spectrum is marked by a sharp drop in flux at 0.7 keV which has been interpreted by Branduardi-Raymont et al. as the blue wing of a relativistic OVIII emission line and by Lee at al. as a dusty warm absorber. We find that the drop is well explained by the FeI L2,3 absorption edges and obtain reasonable fits over the 0.32-1.7 keV band using a multizone, dusty warm absorber model. Some residuals remain which could be due to emission from a relativistic disc, but at a much weaker level than from any model relying on relativistic emission lines alone. A model based on such emission lines can be made to fit if sufficient (warm) absorption is added, although the line strengths exceed those expected. The EPIC pn difference spectrum between the highest and lowest flux states of the source indicates that this is a power-law in the 3-10 keV band which, if extrapolated to lower energies, reveals the absorption function acting on the intrinsic spectrum, provided that any emission lines do not scale exactly with the continuum. We find that this function matches our dusty warm absorber model well. The soft X-ray spectrum is therefore dominated by absorption structures, with the equivalent width of any individual emission lines in the residuals being below about 30 eV. (abridged)Comment: 8 pages, 10 figures, submitted to MNRA

Duty-cycle and energetics of remnant radio-loud AGN

Deriving the energetics of remnant and restarted active galactic nuclei (AGNs) is much more challenging than for active sources due to the complexity in accurately determining the time since the nucleus switched-off. I resolve this problem using a new approach that combines spectral ageing and dynamical models to tightly constrain the energetics and duty-cycles of dying sources. Fitting the shape of the integrated radio spectrum yields the fraction of the source age the nucleus is active; this, in addition to the flux density, source size, axis ratio, and properties of the host environment, provides a constraint on dynamical models describing the remnant radio source. This technique is used to derive the intrinsic properties of the well-studied remnant radio source B2 0924+30. This object is found to spend $50^{+14}_{-12}$ Myr in the active phase and a further $28^{+6}_{-5}$ Myr in the quiescent phase, have a jet kinetic power of $3.6^{+3.0}_{-1.7}\times 10^{37}$ W, and a lobe magnetic field strength below equipartition at the $8\sigma$ level. The integrated spectra of restarted and intermittent radio sources is found to yield a 'steep-shallow' shape when the previous outburst occurred within 100 Myr. The duty-cycle of B2 0924+30 is hence constrained to be $\delta < 0.15$ by fitting the shortest time to the previous comparable outburst that does not appreciably modify the remnant spectrum. The time-averaged feedback energy imparted by AGNs into their host galaxy environments can in this manner be quantified.Comment: 9 pages, 5 figures, 1 table; accepted in MNRA

‘Vox Twitterati’: Investigating the effects of social media exemplars in online news articles

There is a growing trend among online news outlets to include Twitter posts as an equivalent to the traditional “vox pop” or “man-on-the-street” interview. Media effects research has documented the ability of vox pops to influence consumer perceptions of news issues within the traditional media environment, but there is limited research on the possible effects that including social media exemplars as vox pops within editorially curated articles might have on issue perceptions. Drawing on the exemplification effects literature to inform the experimental design, we conduct two studies on two topics of either low or high national salience and find strong evidence that vox pop tweets can influence perceptions of public opinion and, indirectly, readers’ own opinions on an issue. Results are discussed in light of implications for journalistic practice, media effects research and the wider democratic process