arXiv subjects

J. P. Leahy

Publications and source records attributed to J. P. Leahy.

69 records · Page 4Linked to original sources

arXiv2003

Chandra Discovery of an X-ray Jet and Lobes in 3C 15

We report the Chandra detection of an X-ray jet in 3C 15. The peak of the X-ray emission in the jet is 4.1'' (a projected distance of 5.1 kpc) from the nucleus, and coincident with a component previously identified in the radio and optical jets. We examine four models for the X-ray jet emission: (I) weak synchrotron cooling in equip., (II) moderate synchrotron cooling in equip., (III) weak synchrotron plus SSC cooling, and (IV) moderate synchrotron plus SSC cooling. We argue that case (II) can most reasonably explain the overall emission from knot C. Case (III) is also possible, but requires a large departure from equipartition and for the jet power to be comparable to that of the brightest quasars. Diffuse X-ray emission has also been detected, distributed widely over the full extent (63kpc x 25kpc) of the radio lobes. We compare the total energy contained in the lobes with the jet power estimated from knot C, and discuss the energetic link between the jet and the lobes. We argue that the fueling time (t_fuel) and the source age (t_src) are comparable for case (II), whereas t_fuel << t_src is likely for case (III). The latter may imply that the jet has a very small filling factor, ~10^{-3}. We consider the pressure balance between the thermal galaxy halo and non-thermal relativistic electrons in the radio lobes. Finally, we show that the X-ray emission from the nucleus is not adequately fitted by a simple absorbed power-law model, but needs an additional power-law with heavy absorption intrinsic to the source. Such a high column density is consistent with the presence of a dense, dusty torus which obscures the quasar nucleus.

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arXiv2003

Synchrotron Polarization at High Galactic Latitude

We present preliminary results from mapping the high-latitude Galactic polarization with the Effelsberg Telescope at $λ$21 cm. Structures on the resulting maps are mostly on the scale of several degrees. The results show detection of polarization over most of the field, at the level of tens of percent of the synchrotron emission. The evidence of more structure in Stokes Q and U rather than in $\sqrt{Q^2+U^2}$ suggests the existence of Faraday rotation.

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arXiv2003

A multiband study of Hercules A. II. Multifrequency VLA imaging

We have mapped the powerful radio galaxy Hercules A at six frequencies spanning 1295 to 8440 MHz using the VLA in all four configurations. Here we discuss the structure revealed in total intensity, spectral index, polarization, and projected magnetic field. Our observations clearly reveal the relation between the bright jets, radio source. The jets and rings form a coherent structure with a dramatically flatter spectrum than the surrounding lobes and bridge, strongly suggesting that they represent a recently renewed outburst from the active nucleus. The spectrum of the lobes is also steeper than in typical radio sources, and steepens further towards the centre. The compact core is optically thin and also has a remarkably steep spectrum (alpha \~ -1.2). There is some evidence that the old lobe material has been swept up and compressed ahead of the new outburst. We interpret the dramatic asymmetry in the bright structure, and more subtle differences between diffuse lobe structures, in terms of relativistic beaming combined with front-to-back light-travel delays which mean that we view the two lobes at different stages of the outburst. After correcting for Faraday rotation the projected magnetic field closely follows the edge of the lobes, the jets, and the rings; the field pattern in the two lobes is broadly similar. We confirm a strong asymmetry in depolarization and Faraday rotation, with the jet side the less depolarized and the flatter spectrum, consistent with general correlations between these asymmetries. The spectral index asymmetry is clearly present in the `old' lobe material and so, at least in this case, is not due to beaming; but it can be understood in terms of the light-travel delay.

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arXiv2003

The CMB power spectrum out to l=1400 measured by the VSA

We have observed the cosmic microwave background (CMB) in three regions of sky using the Very Small Array (VSA) in an extended configuration with antennas of beamwidth 2 degrees at 34 GHz. Combined with data from previous VSA observations using a more compact array with larger beamwidth, we measure the power spectrum of the primordial CMB anisotropies between angular multipoles l = 160 - 1400. Such measurements at high l are vital for breaking degeneracies in parameter estimation from the CMB power spectrum and other cosmological data. The power spectrum clearly resolves the first three acoustic peaks, shows the expected fall off in power at high l and starts to constrain the position and height of a fourth peak.

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arXiv2003

Cosmological parameter estimation and Bayesian model comparison using VSA data

We constrain the basic comological parameters using the first observations by the Very Small Array (VSA) in its extended configuration, together with existing cosmic microwave background data and other cosmological observations. We estimate cosmological parameters for four different models of increasing complexity. In each case, careful consideration is given to implied priors and the Bayesian evidence is calculated in order to perform model selection. We find that the data are most convincingly explained by a simple flat Lambda-CDM cosmology without tensor modes. In this case, combining just the VSA and COBE data sets yields the 68 per cent confidence intervals Omega_b h^2=0.034 (+0.007, -0.007), Omega_dm h^2 = 0.18 (+0.06, -0.04), h=0.72 (+0.15,-0.13), n_s=1.07 (+0.06,-0.06) and sigma_8=1.17 (+0.25, -0.20). The most general model considered includes spatial curvature, tensor modes, massive neutrinos and a parameterised equation of state for the dark energy. In this case, by combining all recent cosmological data, we find, in particular, 95 percent limit on the tensor-to-scalar ratio R < 0.63 and on the fraction of massive neutrinos f_nu < 0.11; we also obtain the 68 per cent confidence interval w=-1.06 (+0.20, -0.25) on the equation of state of dark energy.

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arXiv2002

First results from the Very Small Array -- II. Observations of the CMB

We have observed the cosmic microwave background temperature fluctuations in eight fields covering three separated areas of sky with the Very Small Array at 34 GHz. A total area of 101 square degrees has been imaged, with sensitivity on angular scales 3.6 - 0.4 degrees (equivalent to angular multipoles l=150-900). We describe the field selection and observing strategy for these observations. In the full-resolution images (with synthesised beam of FWHM ~ 17 arcmin) the thermal noise is typically 45 microK and the CMB signal typically 55 microK. The noise levels in each field agree well with the expected thermal noise level of the telescope, and there is no evidence of any residual systematic features. The same CMB features are detected in separate, overlapping observations. Discrete radio sources have been detected using a separate 15 GHz survey and their effects removed using pointed follow-up observations at 34 GHz. We estimate that the residual confusion noise due to unsubtracted radio sources is less than 14 mJy/beam (15 microK in the full-resolution images), which added in quadrature to the thermal noise increases the noise level by 6 %. We estimate that the rms contribution to the images from diffuse Galactic emission is less than 6 microK. We also present images which are convolved to maximise the signal-to-noise of the CMB features and are co-added in overlapping areas, in which the signal-to-noise of some individual CMB features exceeds 8.

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arXiv2002

Observations of the neutral gas and dust in the radio galaxy 3C305

We present MERLIN and Hubble Space Telescope (HST) observations of the central region of the nearby radio galaxy 3C305 and use them to study the gas and dust in this object. The MERLIN observations are of neutral hydrogen (HI) absorption against the strong non-thermal 20cm continuum seen towards the central 4kpc of 3C305. Our 0.2-arcsec (160 pc) resolution observations show that the HI absorption is highly localised against the south-western radio-emission with column densities 1.9x10^21/cm2. The absorption is broad and red-shifted by 130 km/s relative to the systemic velocity. The HST images in 430nm, 702nm, [OIII] 500.7nm, [FeII] 1.64um and K-band polarization are presented. Evidence is seen for coincidence of the [FeII] emission with the knot at the end of the radio jet, suggesting the presence of shocks. We compare the optical and radio images in order to investigate the relationship between the dust and neutral gas. An unresolved (0.07-arcsec) nucleus is detected in H and K and its properties are consistent with a quasar reddened by A(V)>4. We propose that the absorption arises in a region of neutral gas and dust whose structure is complex but is broadly consistent with an inclined disc encircling, but not covering, the active galactic nucleus. A comparison of the neutral gas observations and previous emission-line observations suggests that both the neutral and ionised gas are undergoing galactic rotation towards the observer in the north-east and away from the observer in the south-west. We propose that the outflow giving rise to the radio emission has a component towards the observer in the north-east and away from the observer in the south-west.

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arXiv2002

Systematics of Microwave Polarimetry with the Planck LFI

The Planck Low Frequency Instrument will recover polarization by differencing the outputs from radiometers sensitive to orthogonal polarizations. We contrast the systematic errors that afflict such a system with those that affect correlation polarimeters; the Planck design has some important advantages when measuring very weak signals such as the CMB. We also review systematic effects arising from the choice of scan strategy for all-sky mapping missions like Planck.[For the Planck-LFI consortium].

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arXiv2001

Probing the pc- and kpc-scale environment of the powerful radio galaxy Hercules A

We present the kpc-scale behaviour of the powerful extragalactic radio source Hercules A and the behaviour of the intracluster gas in which the radio source is situated. We have found that Hercules A exhibits a strong Laing-Garrington effect. The X-ray observations have revealed an extended X-ray emission elongated along the radio galaxy axis. The estimated temperature of the cluster is kT = 2.45 keV and the central electron density is no~7.8 x 10^(-3) cm^(-3) which reveals a hot, dense environment in which Hercules A is situated. From the combined study of the radio and X-ray data we have estimated a central value of 3<Bo (muG)<9. We also present the most recent results from the analysis of the radio data on the pc-scale structure of the radio galaxy, observed at 18 cm by the EVN-MERLIN array. A faint but compact radio source, coincident with the optical centre of Hercules A was detected by the EVN at 18 mas resolution. The total flux density of the EVN core is 14.6 mJy. Its angular size is 18 x 7 mas with a position angle of ~139 degrees. There is also evidence for extended emission in the NW-SE direction, most probably from the eastern pc-scale jet. If this is true then there is a misalignment between the direction of the pc-eastern and the aligned kpc-scale jets of ~35 degrees.

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arXiv2001

Spectral Structure in FRII Radio Galaxies and Jets

Using spectral tomography to separate overlapping spectral features in a sample of FRII radio galaxies, we find a variety of spatial/spectral features that are not easily described in the context of current models. In particular, we find mixtures of flat and steep spectrum features in the hot spot regions, and non-monotonic spectral index gradients along jets. Additional findings include spectral gradients in compact hot spots and possible transitions in jet properties at the downstream ends of the diffuse lobes. The complexity of behaviors uncovered here points to the need for a thorough investigation of numerical models for radio galaxies, as well as detailed observational studies of larger, unbiased samples. We also perform the first quantitative assessment of errors in the use of spectral tomography for spectral index measurements.

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arXiv1999

Chandra X-Ray Detection of the Radio Hotspots of 3C295

An observation of the radio galaxy 3C295 during the calibration phase of the Chandra X-ray Observatory reveals X-ray emission from the core of the galaxy, from each of the two prominent radio hotspots, and from the previously known cluster gas. We discuss the possible emission processes for the hotspots and argue that a synchrotron self-Compton model is preferred for most or all of the observed X-ray emission. SSC models with near equipartition fields thus explain the X-ray emission from the hotspots in the two highest surface brightness FRII radio galaxies, Cygnus A and 3C295. This lends weight to the assumption of equipartition and suggests that relativistic protons do not dominate the particle energy density.

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arXiv1999

Testing minimum energy with powerful radio sources in clusters of galaxies

We analyze ROSAT data for cluster gas surrounding powerful radio galaxies, which is well fitted by a ``beta-model'' gas distribution, after allowing for a compact central source. The cluster thermal pressure at the distance of the radio lobes is typically an order of magnitude larger than the lobe minimum pressure. Since radio lobes are sharply-bounded, the missing pressure is not simply entrained intra-cluster gas. Thus the minimum energy in the lobes is a severe underestimate of the actual energy content. We argue that the extra energy is mostly in the form of particles, so that the magnetic field is below equipartition and thus not a major factor in the lobe dynamics. The large departure from minimum energy has far-reaching implications for the nature of AGN central engines and the supply of mechanical energy to the cluster gas.

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arXiv1998

X-Ray Emission from a Radio Hotspot in 3C 390.3: Evidence for the Deflection of a Radio Jet by a Neighboring Galaxy

By summing a large number of ROSAT High Resolution Imager (HRI) observations of the variable radio galaxy 3C 390.3, we demonstrate that the X-ray emission associated with the northern radio lobe (reported by Prieto 1997) can be identified with hotspot B. None of the other hotspots have been detected. We present evidence that the anomalous X-ray emission is the consequence of a strong shock produced where the northern radio jet impinges onto an external galaxy.

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arXiv1997

Asymmetry of jets, lobe length and spectral index in quasars

The less depolarized lobe of a radio source is generally the lobe containing the jet (Laing-Garrington correlation) but the less depolarized lobe is also generally that with the flatter radio spectrum (Liu-Pooley correlation). Both effects are strong; taken together they would imply a correlation between jet side and lobe spectral index, i.e. between an orientation-dependent feature and one which is intrinsic. We test this prediction using detailed spectral imaging of a sample of quasars with well-defined jets and investigate whether the result can be reconciled with the standard interpretation of one-sided jets in terms of relativistic aberration. Our central finding is that the spectrum of high surface brightness regions is indeed flatter on the jet side, but that the spectrum of low surface brightness regions is flatter on the side with the longer lobe. We discuss possible causes for these correlations and favour explanations in terms of relativistic bulk motion in the high surface brightness regions and differential synchrotron ageing in the extended lobe material.

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arXiv1997

Comment on the Measurement of Cosmological Birefringence

Nodland and Ralston (astro-ph/9704196) claim evidence for a wavelength-independent cosmic birefringence. Although it has been shown (astro-ph/9704247, astro-ph/9704263) that the data analysed did not support this conclusion, the possibility of such an effect remains interesting. Here I present a much more accurate test using high-resolution radio images of distant radio galaxies and quasars. A preliminary application sets an upper limit about thirty times lower than the original claim.

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