arXiv subjects

D. E. Harris

Publications and source records attributed to D. E. Harris.

87 records · Page 5Linked to original sources

arXiv2000

A Comparison of the Extra Nuclear X-ray and Radio Features in M87

ROSAT High Resolution Imager (HRI) data from eight observations have been co-added to obtain an effective exposure of 230 ksec. We have identified a number of features and regions with excess X-ray brightness over that from a circularly symmetric model of the well known hot gas component. A prominent `spur' extends 4' from the vicinity of knot A towards the south-west. The brightness to the south and east of this spur is significantly higher than that to the north and west. Excess brightness is also found to the East of the nucleus, with a local maximum centered on the eastern radio lobe 3' from the core. There are two well known relationships between radio and x-ray emission for radio galaxies in clusters: coincidence of emissions because the X-rays come from inverse Compton or synchrotron processes; and anti-coincidence caused by exclusion of hot gas from radio entities. We present a radio/X-ray comparison to determine if either of these relationships can be isolated in M87. The greatest obstacle we face is the unknown projection which affects both bands.

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arXiv2000

Chandra X-ray Observations of the Hydra A Cluster: An Interaction Between the Radio Source and the X-Ray-Emitting Gas

We present Chandra X-ray Observations of the Hydra A cluster of galaxies, and we report the discovery of structure in the central 80 kpc of the cluster's X-ray-emitting gas. The most remarkable structures are depressions in the X-ray surface brightness, $\sim 25-35$ kpc diameter, that are coincident with Hydra A's radio lobes. The depressions are nearly devoid of X-ray-emitting gas, and there is no evidence for shock-heated gas surrounding the radio lobes. We suggest the gas within the surface brightness depressions was displaced as the radio lobes expanded subsonically, leaving cavities in the hot atmosphere. The gas temperature declines from 4 keV at 70 kpc to 3 keV in the inner 20 kpc of the brightest cluster galaxy (BCG), and the cooling time of the gas is $\sim 600$ Myr in the inner 10 kpc. These properties are consistent with the presence of a $\sim 34 \msunyr$ cooling flow within a 70 kpc radius. Bright X-ray emission is present in the BCG surrounding a recently-accreted disk of nebular emission and young stars. The star formation rate is commensurate with the cooling rate of the hot gas within the volume of the disk, although the sink for the material cooling at larger radii remains elusive.

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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

X-ray Emission from the Radio Jet in 3C 120

We report the discovery of X-ray emission from a radio knot at a projected distance of 25" from the nucleus of the Seyfert galaxy, 3C 120. The data were obtained with the ROSAT High Resolution Imager (HRI). Optical upper limits for the knot preclude a simple power law extension of the radio spectrum and we calculate some of the physical parameters for thermal bremsstrahlung and synchrotron self-Compton models. We conclude that no simple model is consistent with the data but if the knot contains small regions with flat spectra, these could produce the observed X-rays (via synchrotron emission) without being detected at other wavebands.

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arXiv1998

Spatial Corrections of ROSAT HRI Observations

X-ray observations with the ROSAT High Resolution Imager (HRI) often have spatial smearing on the order of 10 arcsec (Morse 1994). This degradation of the intrinsic resolution of the instrument (5 arcsec) can be attributed to errors in the aspect solution associated with the wobble of the space craft or with the reacquisition of the guide stars. We have developed a set of IRAF/PROS and MIDAS/EXSAS routines to minimize these effects. Our procedure attempts to isolate aspect errors that are repeated through each cycle of the wobble. The method assigns a 'wobble phase' to each event based on the 402 second period of the ROSAT wobble. The observation is grouped into a number of phase bins and a centroid is calculated for each sub-image. The corrected HRI event list is reconstructed by adding the sub-images which have been shifted to a common source position. This method has shown approx. 30% reduction of the full width half maximum (FWHM) of an X-ray observation of the radio galaxy 3C 120. Additional examples are presented.

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arXiv1998

Multiwavelength observations of 26W20, a radio galaxy which displays BL Lac characteristics

26W20, a radio galaxy located at a projected distance of 2.2 Mpc from the center of Abell 754 (z=0.054) exhibits core emission properties similar to those of a BL Lac object. New radio observations and VLA archival data show parsec scale features in the core (L_{r} approx.10^{41}) ergs/s with a flat spectrum. From observations with ROSAT, we demonstrate that the X-ray core emission (L_{x}=3X10^{43}) ergs/s is non-thermal and variable. An 18% increase in the X-ray luminosity was observed in two observations separated by five days. The optical morphology is that of a large, normal elliptical galaxy (m_{v}=15.6) and the optical spectrum lacks strong emission lines. If the innermost segment of the radio jet is close to our line of sight, Doppler boosting could explain both the observed X-ray intensity and the absence of emitting line regions.

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arXiv1998

X-Ray Observations of Radio Galaxies

We review some of the ways that X-ray observations provide unique information on radio galaxies. Thermal bremsstrahlung X-ray emission provides detailed data on ambient densities and temperatures. These parameters in turn can be used for pressure balance calculations and can demonstrate how the ambient gas affects radio source structure. Additionally, many signatures of the interaction of radio jets and lobes with the hot gas are found in high resolution X-ray maps. Non-thermal X-ray emission from knots and hotspots of radio jets can give us constraints on the relativistic electron population for energies greater that that normally sampled in the radio (in the case of synchrotron emission) or can give us an independent estimate of the average magnetic field strength (if inverse Compton emission is the origin of the X-rays). From recent ROSAT HRI observations of 3C 390.3 and 3C 120, we show evidence that X-ray emission from knots and hotspots appears to be associated with regions of large gradients in the radio surface brightness; i.e. at the location of powerful shocks.

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arXiv1998

The Large Scale X-ray Emission from M87

We describe asymmetrical features in a long exposure X-ray map of M87 made with the ROSAT High Resolution Imager (HRI). A bright triangular region is marked by a linear `spur' along one edge. The structure of this spur suggests an interpretation of a tangential view of a shock front 18 kpc long. None of the brighter features are spatially coincident with radio or optical structures so we concur with earlier investigators that most of the emission arises from thermal processes.

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arXiv1998

X-ray Variability in M87: 1992 - 1998

Beginning in 1995 June, we have obtained an observation of M87 with the ROSAT High Resolution Imager (HRI) every 6 months. We present the measurements of X-ray intensity for the core and knot A through 1998 January. We find significant changes in both components. For the core, intensities measured in 95 Jun, 96 Dec, and 97 Dec are roughly 30% higher than values obtained at three intervening times. For knot A, a secular decrease of approximately 15% is interrupted only by an intensity jump (3 sigma) in 1997 Dec. Because the background used for subtraction is probably underestimated, we suspect the actual variation is somewhat greater than these values indicate.

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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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arXiv1998

Radio Observations of High Redshift Radio Galaxies

We review some aspects of radio continuum polarimetric imaging of high redshift radio galaxies. The correlation between extreme values of Faraday rotation observed toward radio emitting structures in nearby radio galaxies, and X-ray emitting cluster atmospheres, is presented as a method for targeting objects at high redshift for deep X-ray searches. We present an X-ray detection of the extreme rotation measure radio galaxy PKS 1138-262 at z = 2.156, and we argue that the X-ray emission is from a cluster atmosphere with a luminosity of 1.7x10^{44} h^{-2} ergs sec^{-1}. We also present results on the correlation between size and redshift for a sample of ultra-luminous radio galaxies between 0 < z < 4.3. Source sizes decrease systematically with redshift, suggesting either denser environments, or younger sources, at high redshift. An alternative explanation is significant inverse Compton losses off the microwave background at high redshift.

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arXiv1998

Erratum for: An X-ray Cluster at Redshift 2.156?

A third possible explanation for the X-ray emission from the z=2.156 radio galaxy 1138-262 is thermal emission from a very dense, sub-cluster `halo', perhaps associated with the (forming) cD galaxy on a scale < 100 kpc. For instance, increasing the gas density from 0.01 cm^{-3} to 0.1 cm^{-3} would decrease the required hot gas mass by the same factor, and could possibly alleviate constraints on cosmological structure formation models. The pressure in this gas would be very high (10^{-9} dynes cm^{-2}), comparable to the pressure in the optical line emitting nebulosity and to the minimum pressure in the radio source, and the cooling time would be short (< fewx10^{8} years). Circumstantial evidence for such very dense, hot gas enveloping some high z radio sources has been reviewed by Fabian et al. (1986). Fabian (1991) suggests that in some cases the `inferred pressures are close to the maximum that can be obtained by gas cooling in a potential well of a galaxy' (ie. the cooling time = gravitational free-fall time), and he designates such systems as `maximal cooling flows', with implied cooling flow rates up to 2000 M_solar year^{-1}. High resolution X-ray imaging with AXAF should be able to test whether 1138-262 has a normal cluster atmosphere, a `maximal cooling flow', or an unusually X-ray loud AGN.

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arXiv1997

An X-ray Cluster at Redshift 2.156?

We present X-ray observations of the narrow line radio galaxy 1138-262 at z = 2.156 with the High Resolution Imager (HRI) on ROSAT. Observations at other wave-bands, and in particular extremely high values of Faraday rotation of the polarized radio emission, suggest that the 1138-262 radio source is in a dense environment, perhaps a hot, cluster-type atmosphere. We detect X-ray emission from the vicinity of 1138-262, and we discuss possible origins for this emission. The X-ray, optical, and radio data all favor thermal emission from a hot cluster atmosphere as the mechanism responsible for the X-rays, although we cannot rule out a contribution from the active nucleus. If this interpretation is correct, then 1138-262 becomes the most distant, by far, of known X-ray emitting clusters. The X-ray luminosity for 1138-262 is 6.7+/-1.3 x 10**44 ergs/sec for emitted energies between 2 keV and 10 keV.

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arXiv1996

X-ray variability in M87

We present the evidence for X-ray variability from the core and from knot A in the M87 jet based on data from two observations with the Einstein Observatory High Resolution Imager (HRI) and three observations with the ROSAT HRI. The core intensity showed a 16% increase in 17 months ('79-'80); a 12% increase in the 3 years '92 to '95; and a 17% drop in the last half of 1995. The intensity of knot A appears to have decreased by 16% between 92Jun and 95Dec. Although the core variability is consistent with general expectations for AGN nuclei, the changes in knot A provide constraints on the x-ray emission process and geometry. Thus we predict that the x-ray morphology of knot A will differ significantly from the radio and optical structure.

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arXiv1994

The EINSTEIN On-Line Service

The Einstein On-Line Service (EOLS) is a simple menu-driven system which provides an intuitive method of querying over one hundred database catalogs. In addition, the EOLS contains over 30 CDROMs of images from the Einstein X-ray Observatory which are available for downloading. The EOLS provides all of our databases to the NASA Astrophysics Data System (ADS) and our documents which describe each table are written in the ADS format. In conjunction with the IAU working group on Radioastronomical Databases, the EOLS serves as an experimental platform for on-line access to radio source catalogs. The number of entries in these catalogs exceeds half a million.

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