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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,747 records · Page 97
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

IC 3639—a new bona fide compton-thick AGN unveiled by NuSTAR

We analyze high-quality NuSTAR observations of the local (z = 0.011) Seyfert 2 active galactic nucleus (AGN) IC 3639, in conjunction with archival Suzaku and Chandra data. This provides the first broadband X-ray spectral analysis of the source, spanning nearly two decades in energy (0.5–30 keV). Previous X-ray observations of the source below 10 keV indicated strong reflection/obscuration on the basis of a pronounced iron fluorescence line at 6.4 keV. The hard X-ray energy coverage of NuSTAR, together with self-consistent toroidal reprocessing models, enables direct broadband constraints on the obscuring column density of the source. We find the source to be heavily Compton-thick (CTK) with an obscuring column in excess of 3.6×10{sup 24} cm{sup −2}, unconstrained at the upper end. We further find an intrinsic 2–10 keV luminosity of log{sub 10}(L{sub 2--10keV} [erg s{sup −1}])=43.4{sub −1.1}{sup +0.6} to 90% confidence, almost 400 times the observed flux, and consistent with various multiwavelength diagnostics. Such a high ratio of intrinsic to observed flux, in addition to an Fe-Kα fluorescence line equivalent width exceeding 2 keV, is extreme among known bona fide CTK AGNs, which we suggest are both due to the high level of obscuration present around IC 3639. Our study demonstrates that broadband spectroscopic modeling with NuSTAR enables large corrections for obscuration to be carried out robustly and emphasizes the need for improved modeling of AGN tori showing intense iron fluorescence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

A New Compton-thick AGN in Our Cosmic Backyard: Unveiling the Buried Nucleus in NGC 1448 with NuSTAR

NGC 1448 is one of the nearest luminous galaxies (L {sub 8–1000μm} > 10{sup 9} L {sub ⊙}) to ours (z = 0.00390), and yet the active galactic nucleus (AGN) it hosts was only recently discovered, in 2009. In this paper, we present an analysis of the nuclear source across three wavebands: mid-infrared (MIR) continuum, optical, and X-rays. We observed the source with the Nuclear Spectroscopic Telescope Array (NuSTAR), and combined these data with archival Chandra data to perform broadband X-ray spectral fitting (≈0.5–40 keV) of the AGN for the first time. Our X-ray spectral analysis reveals that the AGN is buried under a Compton-thick (CT) column of obscuring gas along our line of sight, with a column density of N {sub H}(los) ≳ 2.5 × 10{sup 24} cm{sup −2}. The best-fitting torus models measured an intrinsic 2–10 keV luminosity of L {sub 2−10,int} = (3.5–7.6) × 10{sup 40} erg s{sup −1}, making NGC 1448 one of the lowest luminosity CTAGNs known. In addition to the NuSTAR observation, we also performed optical spectroscopy for the nucleus in this edge-on galaxy using the European Southern Observatory New Technology Telescope. We re-classify the optical nuclear spectrum as a Seyfert on the basis of the Baldwin–Philips–Terlevich diagnostic diagrams, thus identifying the AGN at optical wavelengths for the first time. We also present high spatial resolution MIR observations of NGC 1448 with Gemini/T-ReCS, in which a compact nucleus is clearly detected. The absorption-corrected 2–10 keV luminosity measured from our X-ray spectral analysis agrees with that predicted from the optical [O iii]λ5007 Å emission line and the MIR 12 μm continuum, further supporting the CT nature of the AGN.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

X-Ray Bolometric Corrections for Compton-thick Active Galactic Nuclei

We present X-ray bolometric correction factors, κ{sub Bol} (≡L{sub Bol}/L{sub X}), for Compton-thick (CT) active galactic nuclei (AGNs) with the aim of testing AGN torus models, probing orientation effects, and estimating the bolometric output of the most obscured AGNs. We adopt bolometric luminosities, L{sub Bol}, from literature infrared (IR) torus modeling and compile published intrinsic 2–10 keV X-ray luminosities, L{sub X}, from X-ray torus modeling of NuSTAR data. Our sample consists of 10 local CT AGNs, where both of these estimates are available. We test for systematic differences in κ{sub Bol} values produced when using two widely used IR torus models and two widely used X-ray torus models, finding consistency within the uncertainties. We find that the mean κ{sub Bol} of our sample in the range of L{sub Bol}≈10{sup 42}--10{sup 45} erg s{sup −1} is log{sub 10} κ{sub Bol} = 1.44 ± 0.12 with an intrinsic scatter of ∼0.2 dex, and that our derived κ{sub Bol} values are consistent with previously established relationships between κ{sub Bol} and L{sub Bol} and κ{sub Bol} and Eddington ratio (λ{sub Edd}). We investigate if κ{sub Bol} is dependent on N{sub H} by comparing our results on CT AGNs to published results on less-obscured AGNs, finding no significant dependence. Since many of our sample are megamaser AGNs, known to be viewed edge-on, and furthermore under the assumptions of AGN unification whereby unobscured AGNs are viewed face-on, our result implies that the X-ray emitting corona is not strongly anisotropic. Finally, we present κ{sub Bol} values for CT AGNs identified in X-ray surveys as a function of their observed L{sub X}, where an estimate of their intrinsic L{sub X} is not available, and redshift, useful for estimating the bolometric output of the most obscured AGNs across cosmic time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Geometrically Thick Obscuration by Radiation-driven Outflow from Magnetized Tori of Active Galactic Nuclei

Near-Eddington radiation from active galactic nuclei (AGNs) has significant dynamical influence on the surrounding dusty gas, plausibly furnishing AGNs with geometrically thick obscuration. We investigate this paradigm with radiative magnetohydrodynamics simulations. The simulations solve the magnetohydrodynamics equations simultaneously with the infrared (IR) and ultraviolet (UV) radiative transfer (RT) equations; no approximate closure is used for RT. We find that our torus, when given a suitable sub-Keplerian angular momentum profile, spontaneously evolves toward a state in which its opening angle, density distribution, and flow pattern change only slowly. This “steady” state lasts for as long as there is gas resupply toward the inner edge. The torus is best described as a midplane inflow and a high-latitude outflow. The outflow is launched from the torus inner edge by UV radiation and expands in solid angle as it ascends; IR radiation continues to drive the wide-angle outflow outside the central hole. The dusty outflow obscures the central source in soft X-rays, the IR, and the UV over three-quarters of solid angle, and each decade in column density covers roughly equal solid angle around the central source; these obscuration properties are similar to what observations imply.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Anchoring Polar Magnetic Field in a Stationary Thick Accretion Disk

We investigate the properties of a hot accretion flow bathed in a poloidal magnetic field. We consider an axisymmetric viscous-resistive flow in the steady-state configuration. We assume that the dominant mechanism of energy dissipation is due to turbulence viscosity and magnetic diffusivity. A certain fraction of that energy can be advected toward the central compact object. We employ the self-similar method in the radial direction to find a system of ODEs with just one varible, θ in the spherical coordinates. For the existence and maintenance of a purely poloidal magnetic field in a rotating thick disk, we find that the necessary condition is a constant value of angular velocity along a magnetic field line. We obtain an analytical solution for the poloidal magnetic flux. We explore possible changes in the vertical structure of the disk under the influences of symmetric and asymmetric magnetic fields. Our results reveal that a polar magnetic field with even symmetry about the equatorial plane makes the disk vertically thin. Moreover, the accretion rate decreases when we consider a strong magnetic field. Finally, we notice that hot magnetized accretion flows can be fully advected even in a slim shape.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Discovery of a Kiloparsec Extended Hard X-Ray Continuum and Fe–Kα from the Compton Thick AGN ESO 428-G014

We report the discovery of kiloparsec-scale diffuse emission in both the hard continuum (3–6 keV) and in the Fe–Kα line in the Compton thick (CT) Seyfert galaxy ESO 428-G014. This extended hard component contains at least ∼24% of the observed 3–8 keV emission, and follows the direction of the extended optical line emission (ionization cone) and radio jet. The extended hard component has ∼0.5% of the intrinsic 2–10 keV luminosity within the bi-cones. A uniform scattering medium of density 1 cm{sup −3} would produce this luminosity in a 1 kpc path length in the bi-cones. Alternatively, higher column density molecular clouds in the disk of ESO 428-G014 may be responsible for these components. The continuum may also be enhanced by the acceleration of charged particles in the radio jet. The steeper spectrum (Γ ∼ 1.7 ± 0.4) of the hard continuum outside of the central 1.″5 radius nuclear region suggests a contribution of scattered/fluorescent intrinsic Seyfert emission. Ultrafast nuclear outflows cannot explain the extended Fe–Kα emission. This discovery suggests that we may need to revise the picture at the base of our interpretation of CT AGN spectra.

79 ASTRONOMY AND ASTROPHYSICS↗