arXiv subjects

D. E. Harris

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

At least 55 records · Page 3Linked to original sources

arXiv2006

On the Anomalous Temperature Distribution of the Intergalactic Medium in the NGC 3411 Group of Galaxies

We present XMM, Chandra and VLA observations of the USGC S152 group and its central elliptical NGC 3411. Imaging of the group X-ray halo suggests it is relaxed with little apparent structure. We investigate the temperature and metal abundance structure of the group halo, and find that while the abundance distribution is fairly typical, the temperature profile is highly unusual, showing a hot inner core surrounded by a cool shell of gas with a radius of \~20-40 kpc, at the center of the larger group halo. Spectral mapping confirms an irregular ring of gas ~0.15 keV cooler than its surroundings. We estimate the total mass, entropy and cooling time profiles within ~200 kpc, and find that the cool shell contains ~9x10^9 Msun of gas. VLA observations at 1.4, 5 and 8 GHz reveal a relatively weak nuclear radio source, with a core radio luminosity L_R=2.7x10^38 erg/s, and a diffuse component extended on scales of a few arcseconds (or more). A lack of evidence for activity at optical or X-ray wavelengths supports the conclusion that the central black hole is currently in a quiescent state. We discuss possible mechanisms for the formation of temperature features observed in the halo, including a previous period of AGN activity, and settling of material stripped from the halo of one of the other group member galaxies.

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arXiv2006

The 300 kpc Long X-ray Jet in PKS 1127-145, z=1.18 Quasar: Constraining X-ray Emission Models

We present a ~100 ksec Chandra X-ray observation and new VLA radio data of the large scale, 300 kpc long X-ray jet in PKS1127-145, a radio loud quasar at redshift z=1.18. With this deep X-ray observation we now clearly discern the complex X-ray jet morphology and see substructure within the knots. The X-ray and radio jet intensity profiles are seen to be strikingly different with the radio emission peaking strongly at the two outer knots while the X-ray emission is strongest in the inner jet region. The jet X-ray surface brightness gradually decreases by an order of magnitude going out from the core. The new X-ray data contain sufficient counts to do spectral analysis of the key jet features. The X-ray energy index of the inner jet is relatively flat with alpha_X = 0.66 +/-0.15 and steep in the outer jet with alpha_X = 1.0 +/-0.2. We discuss the constraints implied by the new data on the X-ray emission models and conclude that ``one-zone'' models fail and at least a two component model is needed to explain the jet's broad-band emission. We propose that the X-ray emission originates in the jet proper while the bulk of the radio emission comes from a surrounding jet sheath. We also consider intermittent jet activity as a possible cause of the observed jet morphology.

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arXiv2006

Detailed Structure of the X-ray Jet in 4C 19.44 (=PKS1354+195)

We investigate the variations of the magnetic field, Doppler factor, and relativistic particle density along the jet of a quasar at z=0.72. We chose 4C 19.44 for this study because of its length and straight morphology. The 18 arcsec length of the jet provides many independent resolution elements in the Chandra X-ray image. The straightness suggests that geometry factors, although uncertain, are almost constant along the jet. We assume the X-ray emission is from inverse Compton scattering of the cosmic microwave background. With the aid of assumptions about jet alignment, equipartition between magnetic-field and relativistic-particle energy, and filling factors, we find that the jet is in bulk relativistic motion with a Doppler factor about 6 at an angle no more than 10 degrees to the line of sight over deprojected distances about 150--600 kpc from the quasar, and with a magnetic field approximately 10 micro Gauss.

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arXiv2006

X-ray Emission from Extragalactic Jets

This review focuses on the X-ray emission processes of extra-galactic jets on scales resolvable by the sub arcsec resolution of the Chandra X-ray Observatory. It is divided into 4 parts. The introductory chapter reviews the classical problems for jets, as well as those associated directly with the X-ray emission. Throughout this section, we deal with the dualisms of low powered radio sources versus high powered radio galaxies and quasars; synchrotron models versus inverse Compton models; and the distinction between the relativistic plasma responsible for the received radiation and the medium responsible for the transport of energy down the jet. The second part collects the observational and inferred parameters for the currently detected X-ray jets and attempts to put their relative sizes and luminosities in perspective. In part 3, we first give the relevant radio and optical jet characteristics, and then examine the details of the X-ray data and how they can be related to various jet attributes. The last section is devoted to a critique of the two non-thermal emission processes and to prospects for progress in our understanding of jets.

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arXiv2006

New Chandra observations of the jet in 3C273. I. Softer X-ray than radio spectra and the X-ray emission mechanism

The jet in 3C273 is a high-power quasar jet with radio, optical and X-ray emission whose size and brightness allow a detailed study of the emission processes acting in it. We present deep Chandra observations of this jet and analyse the spectral properties of the jet emission from radio through X-rays. We find that the X-ray spectra are significantly softer than the radio spectra in all regions of the bright part of the jet except for the first bright "knot A", ruling out a model in which the X-ray emission from the entire jet arises from beamed inverse-Compton scattering of cosmic microwave background photons in a single-zone jet flow. Within two-zone jet models, we find that a synchrotron origin for the jet's X-rays requires fewer additional assumptions than an inverse-Compton model, especially if velocity shear leads to efficient particle acceleration in jet flows.

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arXiv2006

Constraints on the Nature of Jets from kpc Scale X-ray Data

Motivated by the large number of jets detected by the Chandra X-ray Observatory, and by the inverse Compton X-ray emission model (IC/CMB) for relativistic jets, we revisit two basic questions: ``If the medium that carries the jet's energy consists of hot electrons, can we use the physical length of the jet to constrain the maximum electron energy?'' and ``Why do jets have knots?'' Based on the two non-thermal emission processes for X-rays from jets, we consider constraints on the jet medium and other properties from these two simple questions. We argue that hot pairs cannot be the dominant constituent of the medium responsible for the jet's momentum flux and that some mechanisms for producing fluctuating brightness along jets (rather than a monotonically decreasing intensity) are precluded by observed jet morphologies.

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arXiv2005

The Outburst of HST-1 in the M87 Jet

The X-ray intensity of knot HST-1, 0.85" from the nucleus of the radio galaxy M87, has increased by more than a factor of 50 during the last 5 years. The optical increase is similar and our more limited radio data indicate a commensurate activity. We give the primary results of our Chandra X-ray Observatory monitoring program and consider some of the implications of this extreme variability in a relativistic jet. We find that the data support a 'modest beaming synchrotron' model as indicated in our earlier papers. Based on this model, the decay of the X-ray lightcurve appears to be dominated by light travel time across the emitting region of HST-1, rather than synchrotron loss timescales.

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arXiv2005

X-ray Emission from the 3C 273 Jet

We present results from four recent Chandra monitoring observations of the jet in 3C 273 using the ACIS detector, obtained between November 2003 and July 2004. We find that the X-ray emission comes in two components: unresolved knots that are smaller than the corresponding optically emitting knots and a broad channel that is about the same width as the optical interknot region. We compute the jet speed under the assumption that the X-ray emission is due to inverse Compton scattering of the cosmic microwave background, finding that the dimming of the jet X-ray emission to the jet termination relative to the radio emission may be due to bulk deceleration.

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arXiv2005

An X-ray study of magnetic field strengths and particle content in FRII radio sources

We present a Chandra and XMM-Newton study of X-ray emission from the lobes of 33 classical double radio galaxies and quasars. We report new detections of lobe-related X-ray emission in 11 sources. Together with previous detections we find that X-ray emission is detected from at least one radio lobe in ~75 percent of the sample. For all of the lobe detections, we find that the measured X-ray flux can be attributed to inverse-Compton scattering of the cosmic microwave background radiation, with magnetic field strengths in the lobes between (0.3 - 1.3) B_eq, where the value B_eq corresponds to equipartition between the electrons and magnetic field assuming a filling factor of unity. There is a strong peak in the magnetic field strength distribution at B ~ 0.7 B_eq. We find that > 70 percent of the radio lobes are either at equipartition or electron dominated by a small factor. The distribution of measured magnetic field strengths differs for narrow-line and broad-line objects, in the sense that broad-line radio galaxies and quasars appear to be further from equipartition; however, this is likely to be due to a combination of projection effects and worse systematic uncertainty in the X-ray analysis for those objects. Our results suggest that the lobes of classical double radio sources do not contain an energetically dominant proton population, because this would require the magnetic field energy density to be similar to the electron energy density rather than the overall energy density in relativistic particles.

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arXiv2004

On the X-ray emission of z~2 radio galaxies: IC scattering of the CMB & no evidence for fully-formed potential wells

We present the results of 20 ksec Chandra observations for each of 5 radio galaxies in the redshift range 2.0 < z < 2.6. For 4 of the 5 targets we detect unresolved X-ray components coincident with the radio nuclei. From spectral analysis of one of the cores and comparison to the empirical radio to X-ray luminosity ratio correlation, we find that obscuring material (n(HI)~10^22 cm^-2) may be surrounding the nuclei. We detect X-ray emission coincident with the radio hotspots or lobes in 4 of the 5 targets, which can be explained by Inverse-Compton (IC) scattering of CMB photons. The magnetic field strengths of ~100-200 muG that we derive agree with the equipartition magnetic field strengths. The relative ease with which the lobe X-ray emission is detected is a consequence of the (1+z)^4 increase in the energy density of the CMB. An HST image of one of the sources shows that the X-ray emission could also be produced by a reservoir of hot, shocked gas, as evidenced by a bright, optical bow-shock. By stacking our data we created a deep, 100 ksec exposure to search for diffuse X-ray emission from intra-cluster gas. We detect no diffuse emission and derive upper limits of ~1e+44 erg/s, thereby ruling out a virialized structure of cluster-size scale at z~2. The average number of soft X-ray sources in our fields is consistent with the number density of AGN in the Chandra Deep Fields. Their angular distribution shows no evidence for large-scale structure associated with the radio galaxies.

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arXiv2004

From Clark Lake to Chandra: Closing in on the Low End of the Relativistic Electron Spectra in Extragalactic Sources

The limited angular resolutions and sensitivities historically available below 300 MHz have made it difficult to define the low end of the electron energy distribution, N(gamma). We extrapolate down from the well observed segments of radio spectra with almost complete ignorance of what N(gamma) is actually doing. We do not know if there is a low energy cutoff or if there are other deviations from extrapolated power laws. The result is that we really do not have a good estimate of the total energy density and pressure of the relativistic plasmas we study. The situation is even worse for Inverse Compton (IC) X-ray emission, several flavors of which rely on electrons of Lorentz factors, gamma, of 1000, 300, or in some cases of order 50. If our assumed extrapolations are wrong, some IC emission models may have to be abandoned. We present several examples and demonstrate that the Long Wave Array (LWA) should have sufficient sensitivity and resolution to obtain meaningful constraints on N(gamma) at low energies.

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arXiv2004

The X-ray Jet of 3C 120: Evidence for a Non-standard Synchrotron Spectrum

We report on archival data from the Chandra X-ray Observatory for the radio jet of 3C120. We consider the emission process responsible for the X-rays from 4 knots using spectra constructed from radio, optical, and X-ray intensities. While a simple synchrotron model is adequate for three of the knots, the fourth ('k25'), which was previously detected by ROSAT and is now well resolved with Chandra, still represents a problem for the conventional emission processes. If, as we argue, the flat X-ray spectra from two parts of k25 are synchrotron emission, then it appears that either the emission comes from an electron distribution spectrally distinct from that responsible for the radio emission, or at the highest electron energies, there is a significant deviation from the power law describing the electron distribution at lower energies.

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arXiv2004

The origins of X-ray emission from the hotspots of FRII radio sources

We use new and archival Chandra data to investigate the X-ray emission from a large sample of compact hotspots of FRII radio galaxies and quasars from the 3C catalogue. We find that only the most luminous hotspots tend to be in good agreement with the predictions of a synchrotron self-Compton model with equipartition magnetic fields. At low hotspot luminosities inverse-Compton predictions are routinely exceeded by several orders of magnitude, but this is never seen in more luminous hotspots. We argue that an additional synchrotron component of the X-ray emission is present in low-luminosity hotspots, and that the hotspot luminosity controls the ability of a given hotspot to produce synchrotron X-rays, probably by determining the high-energy cutoff of the electron energy spectrum. It remains plausible that all hotspots are close to the equipartition condition.

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arXiv2003

Month-Timescale Optical Variability in the M87 Jet

A previously inconspicuous knot in the M87 jet has undergone a dramatic outburst and now exceeds the nucleus in optical and X-ray luminosity. Monitoring of M87 with the Hubble Space Telescope and Chandra X-ray Observatory during 2002-2003, has found month-timescale optical variability in both the nucleus and HST-1, a knot in the jet 0.82'' from the nucleus. We discuss the behavior of the variability timescales as well as spectral energy distribution of both components. In the nucleus, we see nearly energy-independent variability behavior. Knot HST-1, however, displays weak energy dependence in both X-ray and optical bands, but with nearly comparable rise/decay timescales at 220 nm and 0.5 keV. The flaring region of HST-1 appears stationary over eight months of monitoring. We consider various emission models to explain the variability of both components. The flares we see are similar to those seen in blazars, albeit on longer timescales, and so could, if viewed at smaller angles, explain the extreme variability properties of those objects.

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arXiv2003

Thermal and Non-thermal Plasmas in the Galaxy Cluster 3C 129

We describe new Chandra spectroscopy data of the cluster which harbors the prototypical "head tail" radio galaxy 3C 129 and the weaker radio galaxy 3C 129.1. We combined the Chandra data with Very Large Array (VLA) radio data taken at 0.33, 5, and 8 GHz (archival data) and 1.4 GHz (new data). We also obtained new HI observations at the Dominion Radio Astrophysical Observatory (DRAO) to measure the neutral Hydrogen column density in the direction of the cluster with arcminute angular resolution. The Chandra observation reveals extended X-ray emission from the radio galaxy 3C 129.1 with a total luminosity of 1.5E+41 erg/s. The X-ray excess is resolved into an extended central source of ~2 arcsec (1 kpc) diameter and several point sources with an individual luminosity up to 2.1E+40 erg/s. In the case of the radio galaxy 3C 129, the Chandra observation shows, in addition to core and jet X-ray emission reported in an earlier paper, some evidence for extended, diffuse X-ray emission from a region east of the radio core. The 12 arcsec x 36 arcsec (6 kpc x 17 kpc) region lies "in front" of the radio core, in the same direction into which the radio galaxy is moving. We use the radio and X-ray data to study in detail the pressure balance between the non-thermal radio plasma and the thermal Intra Cluster Medium (ICM) along the tail of 3C 129 which extends over 15 arcmin (427 kpc). Depending on the assumed lower energy cutoff of the electron energy spectrum, the minimum pressure of the radio plasma lies a factor of between 10 and 40 below the ICM pressure for a large part of the tail. We discuss several possibilities to explain the apparent pressure mismatch.

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arXiv2003

X-ray Variability and Emission Process of the Radio Jet in M87

We monitored the M87 jet with the ACIS-S detector on Chandra with 5 observations between 2002 Jan and 2002 Jul. Our goal was to determine the presence and degree of variability in morphology, intensity, and spectral parameters. We find strong variability of the core and HST-1, the knot lying 0.8" from the core. These observations were designed to constrain the X-ray emission process: whereas synchrotron emission would necessitate the presence of extremely high energy electrons with a halflife of a few years or less, inverse Compton emission from a relativistic jet would arise from low energy electrons with very long halflives. Currently, all indications point to a synchrotron process for the X-ray emission from the M87 jet. We give key parameters for a ``modest beaming'' synchrotron model.

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arXiv2003

What can we learn about extragalactic radio jets from X-ray data?

We review the current status of resolved X-ray emission associated with extragalactic radio jets and hotspots. The primary question for any particular jet is to decide if the X-rays come from the synchrotron process or from inverse Compton scattering. There is considerable evidence supporting synchrotron emission for knots in the jets of FRI galaxies. For FRII terminal hotspots detected in the X-ray band, synchrotron self-Compton emission continues to provide viable models with one possible exception (so far). Inverse Compton scattering on photons of the cosmic microwave background is indicated for a few powerful jets, and is expected to be an important contributor if not the dominating mechanism for higher redshift objects. The application of a model generally yields physical parameters and in many cases, these include the Doppler boosting factor.

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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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