arXiv subjects

A. S. Wilson

Publications and source records attributed to A. S. Wilson.

At least 37 records · Page 2Linked to original sources

arXiv2004

Ionization Mechanisms in Jet-Dominated Seyferts: A Detailed Case Study

For the past 10 years there has been an active debate over whether fast shocks play an important role in ionizing emission line regions in Seyfert galaxies. To investigate this claim, we have studied the Seyfert 2 galaxy Mkn 78, using HST UV/optical images and spectroscopy. Since Mkn 78 provides the archetypal jet-driven bipolar velocity field, if shocks are important anywhere they should be important in this object. Having mapped the emission line fluxes and velocity field, we first compare the ionization conditions to standard photoionization and shock models. We find coherent variations of ionization consistent with photoionization model sequences which combine optically thick and thin gas, but are inconsistent with either autoionizing shock models or photoionization models of just optically thick gas. Furthermore, we find absolutely no link between the ionization of the gas and its kinematic state, while we do find a simple decline of ionization degree with radius. We feel this object provides the strongest case to date against the importance of shock related ionization in Seyferts.

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arXiv2004

Motion and Properties of Nuclear Radio Components in Seyfert Galaxies Seen with VLBI

We report EVN, MERLIN and VLBA observations at 18 cm, 6 cm and 3.6 cm of the Seyfert galaxies NGC 7674, NGC 5506, NGC 2110 and Mrk 1210 to study their structure and proper motions on pc scales and to add some constraints on the many possible causes of the radio-quietness of Seyferts. The component configurations in NGC 7674 and NGC 2110 are simple, linear structures, whereas the configurations in NGC 5506 and Mrk 1210 have multiple components with no clear axis of symmetry. We suggest that NGC 7674 is a low-luminosity compact symmetric object. Comparing the images at different epochs, we find a proper motion in NGC 7674 of (0.92+-0.07) c between the two central components separated by 282 pc and, in NGC 5506, we find a 3 sigma upper limit of 0.50 c for the components separated by 3.8 pc. Our results confirm and extend earlier work showing that the outward motion of radio components in Seyfert galaxies is non-relativistic on pc scales. We briefly discuss whether this non-relativistic motion is intrinsic to the jet-formation process or results from deceleration of an initially relativistic jet by interaction with the pc or sub-pc scale interstellar medium. We combined our sample with a list compiled from the literature of VLBI observations made of Seyfert galaxies, and found that most Seyfert nuclei have at least one flat-spectrum component on the VLBI scale, which was not seen in the spectral indices measured at arcsec resolution. We found also that the bimodal alignment of pc and kpc radio structures displayed by radio galaxies and quasars is not displayed by this sample of Seyferts, which shows a uniform distribution of misalignment between 0 deg and 90 deg. The frequent misalignment could result from jet precession or from deflection of the jet by interaction with gas in the interstellar medium.

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arXiv2003

Chandra X-ray Observations of NGC 4258: Iron Absorption Lines from the Nucleus

We report sub-arcsecond resolution X-ray imaging spectroscopy of the low luminosity active galactic nucleus of NGC 4258 and its immediate surroundings with the Chandra X-ray Observatory. NGC 4258 was observed four times, with the first two observations separated by one month, followed over a year later by two consecutive observations. The spectrum of the nucleus is well described by a heavily absorbed, hard X-ray power law of variable luminosity, plus a constant, thermal soft X-ray component. We do not detect an iron K alpha emission line with the upper limit to the equivalent width of a narrow, neutral iron line ranging between 94 and 887 eV (90% confidence) for the different observations. During the second observation on 2000-04-17, two narrow absorption features are seen with >99.5% confidence at ~6.4 keV and ~6.9 keV, which we identify as resonant absorption lines of Fe XVIII - Fe XIX K alpha and Fe XXVI K alpha, respectively. In addition, the 6.9 keV absorption line is probably variable on a timescale of ~6000 sec. The absorption lines are analyzed through a curve of growth analysis, which allows the relationship between ionic column and kinematic temperature or velocity dispersion to be obtained for the observed equivalent widths. We discuss the properties of the absorbing gas for both photo and collisionally ionized models. Given that the maser disk is viewed at an inclination i = 82 degrees, the gas responsible for the 6.9 keV absorption line may be in an inner disk, a disk-wind boundary layer or be thermal gas entrained at the base of the jet. The gas which gives rise to the photoelectric absorption may be the same as that which causes the 6.4 keV Fe K alpha absorption provided that the gas has a bulk velocity dispersion of a few thousand km/s. (Abstract abridged).

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arXiv2003

The X-ray nebula around the Seyfert 2 galaxy NGC4388

We report on X-ray emission from the Seyfert 2 galaxy NGC4388 observed with the Chandra X-ray Observatory. A hard X-ray peak is found at the position of the active nucleus suggested by optical and radio observations. Extended soft X-ray emission correlates well with the ionization cone found in optical line emission. A large soft X-ray extension is found up to 16 kpc to the north of the galaxy. Photoionized gas with low ionization parameters (xi<3) appears to be the likely explanation of this emission. The same ionized gas clouds could be responsible for the optical [OIII] emission. Fe K line emission from cold material is found to be extended by a few kpc.

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arXiv2003

Flaring X-ray Emission from HST-1, a Knot in the M87 Jet

We present Chandra X-ray monitoring of the M87 jet in 2002, which shows that the intensity of HST-1, an optical knot 0.8" from the core, increased by a factor of two in 116 days and a factor of four in 2 yrs. There was also a significant flux decrease over two months, with suggestive evidence for a softening of the spectrum. From this variability behavior, we argue that the bulk of the X-ray emission of HST-1 comes from synchrotron emission. None of the other conceivable emission processes can match the range of observed characteristics. By estimating synchrotron model parameters for various bulk relativistic velocities, we demonstrate that a model with a Doppler factor, delta, in the range 2 to 5 fits our preliminary estimates of light travel time and synchrotron loss timescales.

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arXiv2003

A Survey for H2O Megamasers III. Monitoring Water Vapor Masers in Active Galaxies

We present single-dish monitoring of the spectra of 13 extragalactic water megamasers taken over a period of 9 years and a single epoch of sensitive spectra for 7 others. Our data include the first K-band science observations taken with the new 100 m Green Bank Telescope (GBT). In the context of a circumnuclear, molecular disk model, our results suggest that either (a) the maser lines seen are systemic features subject to a much smaller acceleration than present in NGC 4258, presumably because the gas is farther from the nuclear black hole, or (b) we are detecting ``satellite'' lines for which the acceleration is in the plane of the sky. We also report a search for water vapor masers towards the nuclei of 58 highly inclined, nearby galaxies.

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arXiv2002

Discovery of water vapor megamaser emission from Mrk1419 (NGC 2690): An analogue of NGC 4258?

Water vapor emission at 22 GHz is reported from the nucleus of the LINER galaxy Mrk 1419 (NGC 2960). Single-dish spectra of the maser source show properties that are similar to those seen in NGC 4258, namely (1) a cluster of systemic H2O features, (2) two additional H2O clusters, one red- and one blue-shifted by about 475 km/s, (3) a likely acceleration of the systemic features, and (4) no detectable velocity drifts in the red- and blue-shifted features. Interpreting the data in terms of the paradigm established for NGC 4258, i.e. assuming the presence of an edge-on Keplerian circumnuclear annulus with the systemic emission arising from the near side of its inner edge, the following parameters are derived: Rotational velocity: 330-600 km/s; radius: 0.13-0.43 pc; binding mass: about 10 million solar masses. With the galaxy being approximately ten times farther away than NGC 4258, a comparison of linear and angular scales (the latter via Very Long Baseline Interferometry) may provide an accurate geometric distance to Mrk 1419 that could be used to calibrate the cosmic distance scale.

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arXiv2002

Chandra Imaging of the X-ray Core of the Virgo Cluster

We report sub-arcsecond X-ray imaging spectroscopy of M87 and the core of the Virgo cluster with the Chandra X-ray Observatory. The X-ray morphology shows structure on arcsecond (~100 pc) to ten arcminute (~50 kpc) scales, the most prominent feature being an "arc" running from the east, across the central region of M87 and off to the southwest. A ridge in the radio map, ending in an "ear"-shaped structure, follows the arc to the east. Depressions in the X-ray surface brightness correspond to the inner radio lobes and there is no evidence of shock-heated gas surrounding them. There are also at least two approximately circular (centered near the nucleus) "edges" in the X-ray brightness distribution, the radii of which are slightly larger than the nuclear distances of the inner radio lobes and intermediate radio ridges, respectively. We speculate that these discontinuities may be spherical pulses or "fronts" driven by the same jet activity as is responsible for the radio structure; such pulses are found in recent numerical simulations. All these results provide good evidence that the nuclear activity affects the intra-cluster medium. We present a temperature map of the intra-cluster medium, and obtain the temperature, pressure and cooling time as a function of nuclear distance for the arcs and the ambient intra-cluster medium. We show that the gas in the arcs is cooler than, and probably over-pressured with respect to, the ambient intra-cluster medium. The metal abundances of the cooler gas in the arc are somewhat enhanced relative to the ambient intra-cluster medium, favoring a ``buoyant plume'' origin for the X-ray arc, in which ambient gas near the nucleus is entrained by buoyant radio plasma and carried to larger nuclear distances. (Abstract truncated).

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arXiv2002

Radio Sources in Low-Luminosity Active Galactic Nuclei. III. "AGNs" in a Distance-Limited Sample of "LLAGNs"

(abbreviated): This paper presents the results of a high resolution radio imaging survey of all known (96) low-luminosity active galactic nuclei (LLAGNs) at D<19Mpc. We find that almost half of all LINERs and low-luminosity Seyferts have flat-spectrum radio cores when observed at 150mas resolution. Higher (2mas) resolution observations of a flux-limited subsample have provided a 100% (16 of 16) detection rate of pc-scale radio cores, with implied brightness temperatures > 10^8 K. The five LLAGNs with the highest core radio fluxes also have pc-scale `jets.' Compact radio cores are almost exclusively found in massive ellipticals and in type1 nuclei. The core radio power is correlated with the nuclear optical `broad' Halpha luminosity, the nuclear optical `narrow' emission line luminosity and width, and with the galaxy luminosity. In these correlations LLAGNs fall close to the low-luminosity extrapolations of more powerful AGNs. About half of all LLAGNs with multiple epoch data show significant inter-year radio variability. Investigation of a sample of ~150 nearby bright galaxies, most of them LLAGNs, shows that the nuclear (<150mas size) radio power is strongly correlated with both the black hole mass and the galaxy bulge luminosity; linear regression fits to all ~150 galaxies give: log P(2cm) = 1.31 log M_blackhole + 8.77 and log P(2cm) = 1.89 log L_B(bulge) - 0.17. Low accretion rates are implied in both advection- and jet-type models. In brief, all evidence points towards the presence of accreting massive black holes in a large fraction, perhaps all, of LLAGNs.

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arXiv2002

Cygnus A

We report Chandra imaging-spectroscopy and RXTE spectroscopy of the nearby, powerful radio galaxy Cygnus A. Various aspects of the results are discussed, including the X-ray properties of the nucleus, the radio hot spots, the cluster of galaxies, the prolate cavity in the ICM inflated by the radio jets and ``bands'' of thermal gas which encircle the cavity in its equatorial plane. The hard X-ray emission of the nucleus extends to 100 keV and originates from an unresolved source absorbed by a large column density (N$_{\rm H}$ $\simeq$ 2 $\times$ 10$^{23}$ cm$^{-2}$) of gas. The soft ($<$ 2 keV) nuclear emission exhibits a bi-polar structure which extends $\simeq$ 2 kpc from the nucleus and is strongly correlated with both optical continuum and emission-line morphologies. It is suggested that this nebulosity is photoionized by the nucleus and that the extended X-rays are electron-scattered nuclear radiation. All four radio hot spots are detected in X-rays, with the emission resulting from synchrotron self-Compton radiation in an approximately equipartition field. The temperature of the X-ray emitting intracluster gas drops from $\simeq 8$ keV more than 100 kpc from the center to $\simeq 5$ keV some 80 kpc from the center, with the coolest gas immediately adjacent to the radio galaxy. There is a metallicity gradient in the X-ray emitting gas, with the highest metallicities ($\sim$ solar) found close to the center, decreasing to $\sim 0.3$ solar in the outer parts. (Abstract truncated).

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arXiv2002

Chandra X-ray Imaging and Spectroscopy of the M87 Jet and Nucleus

We report X-ray imaging - spectroscopy of the jet of M87 at sub arc second resolution with the Chandra X-ray Observatory. The galaxy nucleus and all the knots seen at radio and optical wavelengths, as far from the nucleus as knot C, are detected in the X-ray observations. There is a strong trend for the ratio of X-ray to radio, or optical, flux to decline with increasing distance from the nucleus. At least three knots are displaced from their radio/optical counterparts, being tens of pc closer to the nucleus at X-ray than at radio or optical wavelengths. The X-ray spectra of the nucleus and knots are well described by power laws absorbed by cold gas, with only the unresolved nucleus exhibiting intrinsic absorption. In view of the similar spectra of the nucleus and jet knots, and the high X-ray flux of the knots closest to the nucleus, we suggest that the X-ray emission coincident with the nucleus may actually originate from the pc - or sub-pc - scale jet rather than the accretion disk. Arguments are given that the X-ray emission process is unlikely to be inverse Compton scattering. Instead, we favor synchrotron radiation. Plotted as $ν$S$_{\rm ν}$, the spectra of the knots generally peak in or just above the optical - near infrared band. However, the overall spectra of at least three knots cannot be described by simple models in which the spectral index monotonically increases with frequency, as would result from synchrotron losses or a high energy cut-off to the injected electron spectrum. Instead, these spectra must turn down just above the optical band and then flatten in the X-ray band. (Abstract truncated)

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arXiv2002

Gigahertz-Peaked Spectrum Radio Sources in Nearby Galaxies

There is now strong evidence that many low-luminosity AGNs (LLAGNs) contain accreting massive black holes and that the nuclear radio emission is dominated by parsec-scale jets launched by these black holes. Here, we present preliminary results on the 1.4 GHz to 667 GHz spectral shape of a well-defined sample of 16 LLAGNs. The LLAGNs have a falling spectrum at high GHz frequencies. Several also show a low-frequency turnover with a peak in the 1-20 GHz range. The results provide further support for jet dominance of the core radio emission. The LLAGNs show intriguing similarities with gigahertz-peaked spectrum (GPS) sources.

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arXiv2001

Chandra X-ray Observation of NGC 4151

We present Chandra X-ray observations of the nearby Seyfert 1.5 galaxy NGC 4151. The images show the extended soft X-ray emission on the several hundreds of pc scale with better sensitivity than previously obtained. The spectrum of the unresolved nuclear source may be described by a heavily absorbed (N_{H} \simeq 3 \times 10^{22} cm^-2), hard power-law (Γ\simeq 0.3) plus soft emission from either a power-law (Γ\simeq 2.6) or a thermal (kT \simeq 0.6 keV) component. The flux of the high energy component has decreased from that observed by ASCA in 1993 and the spectrum is much harder.The large difference between the soft and hard spectral shapes does not favor the partial covering or scattering model of the ``soft excess''. Instead, it is likely that the hard and soft nuclear components represent intrinsically different X-ray sources. Spectra of the extended emission to almost 1 kpc NE and SW of the nucleus have also been obtained. The spectra of these regions may be described by either thermal bremsstrahlung (kT \simeq 0.4-0.7 keV) or power-law (Γ\simeq 2.5-3.2) continua plus 3 emission lines. There is an excellent correlation between the extended X-ray and [O III]λ5007 line emissions. We discuss the nature of the extended X-ray emission. Upper limit to the electron scattering column was obtained. This upper limit is much too low for the soft X-rays to be electron scattered nuclear radiation.

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arXiv2001

Chandra Observations and the Nature of the Anomalous Arms of NGC 4258 (M 106)

This paper presents high resolution X-ray observations with Chandra of NGC 4258 and infers the nature of the so called ``anomalous arms'' in this galaxy. The anomalous arms dominate the X-ray image; diffuse X-ray emission from the ``plateaux'' regions, seen in radio and H$α$ imaging, is also found. X-ray spectra have been obtained at various locations along the anomalous arms and are well described by thermal (mekal) models with kT in the range 0.37 - 0.6 keV. The previously known kpc-scale radio jets are surrounded by cocoons of hot X-ray emitting gas for the first 350 pc of their length. The radio jets, seen in previous VLBA and VLA observations, propagate perpendicular to the compact nuclear gas disk (imaged in water vapor maser emission). The angle between the jets and the rotation axis of the galactic disk is 60$^{\circ}$. The jets shock the normal interstellar gas along the first 350 pc of their length, causing the hot, X-ray emitting cocoons noted above. At a height of z = 175 pc from the disk plane, the jets exit the normal gas disk and then propagate though the low density halo until they reach ``hot spots'' (at 870 pc and 1.7 kpc from the nucleus), which are seen in radio, optical line and X-ray emission. These jets must drive mass motions into the low density halo gas. This high velocity halo gas impacts on the dense galactic gas disk and shock heats it along and around a ``line of damage'', which is the projection of the jets onto the galactic gas disk as viewed down the galaxy disk rotation axis. However, because NGC 4258 is highly inclined ($i$ = 64$^{\circ}$), the ``line of damage'' projects on the sky in a different direction to the jets themselves. We calculate the expected p.a. of the ``line of damage'' on the sky and find that it coincides with the anomalous arms to within 2$^{\circ}$. (Abstract truncated).

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arXiv2001

A Chandra X-ray Study of NGC 1068 - I. Observations of Extended Emission

We report sub arc-second resolution X-ray imaging-spectroscopy of the archetypal type 2 Seyfert galaxy NGC 1068 with the Chandra X-ray Observatory. The observations reveal the detailed structure and spectra of the 13 kpc-extent nebulosity previously imaged at lower resolution with ROSAT. The Chandra image shows a bright, compact source coincident with the brightest radio and optical emission; this source is extended by \simeq 1.5 arcsec (165 pc) in the same direction as the nuclear optical line and radio continuum emission. Bright X-ray emission extends \simeq 5 arcsec (550 pc) to the NE and coincides with the NE radio lobe and gas in the narrow line region. The large-scale emission shows trailing spiral arms and other structures. Numerous point sources associated with NGC 1068 are seen. There is a very strong correlation between the X-ray emission and the high excitation ionized gas seen in HST and ground-based [O III] λ5007 images. The X-rays to the NE of the nucleus are absorbed by only the Galactic column density and thus originate from the near side of the disk of NGC 1068. In contrast the X-rays to the SW are more highly absorbed and must come from gas in the disk or on the far side of it. This geometry is similar to that inferred for the narrow line region and radio lobes. (Abstract truncated).

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arXiv2001

Chandra X-ray Observations of Cygnus A and Pictor A

We describe observations of the two nearest, powerful radio galaxies with the Chandra X-ray Observatory: Cygnus A (z = 0.0562) and Pictor A (z = 0.035). Since the early results from these observations are published elsewhere (Wilson, Young & Shopbell 2000, 2001), we here confine ourselves to a summary of the main conclusions and a few remarks about other aspects of the results on Cygnus A, which will be amplified in a future paper.

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arXiv2000

The Nature of Flat-Spectrum Nuclear Radio Emission in Seyfert Galaxies

Parsec-scale VLBA imaging of five Seyfert galaxies with flat-spectrum radio nuclei was conducted to determine whether the flat spectrum represents thermal emission from the accretion disk/obscuring torus or nonthermal, synchrotron self-absorbed emission. Four of the five show emission consistent with synchrotron self-absorption, with intrinsic sizes ~0.05-0.2 pc (or 10^4 gravitational radii for a 10^8 Msun black hole for the smallest). In contrast, NGC 4388, which was detected with MERLIN but not the VLBA, shows thermal emission with similar properties to that detected in NGC 1068. It is notable that the two Seyfert galaxies with detected thermal nuclear radio emission both have large X-ray absorbing columns, suggesting that columns in excess of \~10^{24} cm^{-2} are needed for such disks to be detectable.

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arXiv2000

Parsec-Scale Jets and Tori in Seyfert Galaxies

What causes the dichotomy between very powerful and very weak radio emission from AGNs? Perhaps the engines are the same but the jets get disrupted by dense ISM in radio-quiet objects, or else the engines are intrinsically different with jet power scaling with, say, black hole spin. To distinguish, one can look for interaction between the jets and the NLR and measure the jet speed close to the core using VLBI, before environmental effects become important. We find that in radio-quiet AGN, the jets appear slower and have a greater tendency to bend, and that one-sidedness and flat-spectrum cores are probably due to obscuration.

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