Publications by Katherine Blundell

X-ray emission around the z=4.1 radio galaxy TNJ1338-1942 and the potential role of far-infrared photons in AGN Feedback

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I Smail, K Blundell

We report the discovery in an 80-ks observation of spatially-extended X-ray emission around the high-redshift radio galaxy TNJ1388-1942 (z=4.11) with the Chandra X-ray Observatory. The X-ray emission extends over a ~30-kpc diameter region and although it is less extended than the GHz-radio lobes, it is roughly aligned with them. We suggest that the X-ray emission arises from Inverse Compton (IC) scattering of photons by relativistic electrons around the radio galaxy. At z=4.11 this is the highest redshift detection of IC emission around a radio galaxy. We investigate the hypothesis that in this compact source, the Cosmic Microwave Background (CMB), which is ~700x more intense than at z~0 is nonetheless not the relevant seed photon field for the bulk of the IC emission. Instead, we find a tentative correlation between the IC emission and far-infrared luminosities of compact, far-infrared luminous high-redshift radio galaxies (those with lobe lengths of <100kpc). Based on these results we suggest that in the earliest phases of the evolution of radio-loud AGN at very high redshift, the far-infrared photons from the co-eval dusty starbursts occuring within these systems may make a significant contribution to their IC X-ray emission and so contribute to the feedback in these massive high-redshift galaxies.

The inverse-Compton ghost HDF 130 and the giant radio galaxy 6C 0905+3955: matching an analytic model for double radio source evolution

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P Mocz, AC Fabian, KM Blundell, PT Goodall, SC Chapman, DJ Saikia

We present new GMRT observations of HDF 130, an inverse-Compton (IC) ghost of a giant radio source that is no longer being powered by jets. We compare the properties of HDF 130 with the new and important constraint of the upper limit of the radio flux density at 240 MHz to an analytic model. We learn what values of physical parameters in the model for the dynamics and evolution of the radio luminosity and X-ray luminosity (due to IC scattering of the cosmic microwave background (CMB)) of a Fanaroff-Riley II (FR II) source are able to describe a source with features (lobe length, axial ratio, X-ray luminosity, photon index and upper limit of radio luminosity) similar to the observations. HDF 130 is found to agree with the interpretation that it is an IC ghost of a powerful double-lobed radio source, and we are observing it at least a few Myr after jet activity (which lasted 5--100 Myr) has ceased. The minimum Lorentz factor of injected particles into the lobes from the hotspot is preferred to be $\gamma\sim10^3$ for the model to describe the observed quantities well, assuming that the magnetic energy density, electron energy density, and lobe pressure at time of injection into the lobe are linked by constant factors according to a minimum energy argument, so that the minimum Lorentz factor is constrained by the lobe pressure. We also apply the model to match the features of 6C 0905+3955, a classical double FR II galaxy thought to have a low-energy cutoff of $\gamma\sim10^4$ in the hotspot due to a lack of hotspot inverse-Compton X-ray emission. The models suggest that the low-energy cutoff in the hotspots of 6C 0905+3955 is $\gamma\gtrsim 10^3$, just slightly above the particles required for X-ray emission.

Inverse Compton X-rays from Giant Radio Galaxies at z~1

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T Laskar, AC Fabian, KM Blundell, MC Erlund

We report XMM-Newton observations of three FR II radio galaxies at redshifts between 0.85 and 1.34, which show extended diffuse X-ray emission within the radio lobes, likely due to inverse-Compton up-scattering of the cosmic microwave background. Under this assumption, through spectrum-fitting together with archival VLA radio observations, we derive an independent estimate of the magnetic field in the radio lobes of 3C 469.1 and compare it with the equipartition value. We find concordance between these two estimates as long as the turnover in the energy distribution of the particles occurs at a Lorentz factor in excess of ~ 250. We determine the total energy in relativistic particles in the radio emitting lobes of all three sources to range between 3e59 and 8e59 erg. The nuclei of these X-ray sources are heavily-absorbed powerful AGN.

Multiwavelength study of Cygnus A III. Evidence for relic lobe plasma

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KC Steenbrugge, I Heywood, KM Blundell

We study the particle energy distribution in the cocoon surrounding Cygnus A, using radio images between 151 MHz and 15 GHz and a 200 ks Chandra ACIS-I image. We show that the excess low frequency emission in the the lobe further from Earth cannot be explained by absorption or excess adiabatic expansion of the lobe or a combination of both. We show that this excess emission is consistent with emission from a relic counterlobe and a relic counterjet that are being re-energized by compression from the current lobe. We detect hints of a relic hotspot at the end of the relic X-ray jet in the more distant lobe. We do not detect relic emission in the lobe nearer to Earth as expected from light travel-time effects assuming intrinsic symmetry. We determine that the duration of the previous jet activity phase was slightly less than that of the current jet-active phase. Further, we explain some features observed at 5 and 15 GHz as due to the presence of a relic jet.

The extended X-ray emission around HDF130 at z=1.99: an inverse Compton ghost of a giant radio source in the Chandra Deep Field North

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AC Fabian, S Chapman, CM Casey, F Bauer, KM Blundell

One of the six extended X-ray sources found in the Chandra DeepField North is centred on HDF130, which has recently been shown to be a massive galaxy at z=1.99 with a compact radio nucleus. The X-ray source has a roughly double-lobed structure with each lobe about 41 arcsec long, or 345 kpc at the redshift of HDF130. We have analyzed the 2 Ms X-ray image and spectrum of the source and find that it is well fit by a power-law continuum of photon index 2.65 and has a 2--10 keV luminosity of 5.4x10^{43}ergps (if at z=1.99). Any further extended emission within a radius of 60 arcsec has a luminosity less than half this value, which is contrary to what is expected from a cluster of galaxies. The source is best explained as an inverse Compton ghost of a giant radio source, which is no longer being powered, and for which Compton losses have downgraded the energetic electrons, \gamma> 10^4, required for high-frequency radio emission. The lower energy electrons, \gamma~1000, produce X-rays by inverse Compton scattering on the Cosmic Microwave Background. Depending on the magnetic field strength, some low frequency radio emission may remain. Further inverse Compton ghosts may exist in the Chandra deep fields.

On the origin of radio core emission in radio-quiet quasars

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K Blundell, Z Kuncic

We present a model for the radio emission from radio-quiet quasar nuclei. We show that a thermal origin for the high brightness temperature, flat spectrum point sources (known as radio ``cores'') is possible provided the emitting region is hot and optically-thin. We hence demonstrate that optically-thin bremsstrahlung from a slow, dense disk wind can make a significant contribution to the observed levels of radio core emission. This is a much more satisfactory explanation, particularly for sources where there is no evidence of a jet, than a sequence of self-absorbed synchrotron components which collectively conspire to give a flat spectrum. Furthermore, such core phenomena are already observed directly via milli-arcsecond radio imaging of the Galactic microquasar SS433 and the active galaxy NGC1068. We contend that radio-emitting disk winds must be operating at some level in radio-loud quasars and radio galaxies as well (although in these cases, observations of the radio cores are frequently contaminated/dominated by synchrotron emission from jet knots). This interpretation of radio core emission mandates mass accretion rates that are substantially higher than Eddington. Moreover, acknowledgment of this mass-loss mechanism as an AGN feedback process has important implications for the input of energy and hot gas into the inter-galactic medium (IGM) since it is considerably less directional than that from jets.

The Compton-thick quasar at the heart of the high-redshift giant radio galaxy 6C 0905+39

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MC Erlund, AC Fabian, KM Blundell, CS Crawford

Our XMM-Newton spectrum of the giant, high-redshift (z=1.88) radio galaxy 6C 0905+39 shows that it contains one of the most powerful, high-redshift, Compton-thick quasars known. Its spectrum is very hard above 2 keV. The steep XMM spectrum below that energy is shown to be due to extended emission from the radio bridge using Chandra data. The nucleus of 6C 0905+39 has a column density of 3.5 (+1.4,-0.4) X 10^24 cm^-2 and absorption-corrected X-ray luminosity of 1.7 (+0.9,-0.1) X 10^45 erg/s in the 2-10 keV band. A lower redshift active galaxy in the same field, SDSS J090808.36+394313.6, may also be Compton-thick.

Detection of a relic X-ray jet in Cygnus A

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KC Steenbrugge, KM Blundell, P Duffy

We present a 200 ks Chandra ACIS-I image of Cygnus A, and discuss a long linear feature seen in its counterlobe. This feature has a non-thermal spectrum and lies on the line connecting the brighter hotspot on the approaching side and the nucleus. We therefore conclude that this feature is (or was) a jet. However, the outer part of this X-ray jet does not trace the current counterjet observed in radio. No X-ray counterpart is observed on the jet side. Using light-travel time effects we conclude that this X-ray 50 kpc linear feature is a relic jet that contains enough low-energy plasma (gamma ~ 10^3) to inverse-Compton scatter cosmic microwave background photons, producing emission in the X-rays.

The 6C** sample of steep-spectrum radio sources: II - Redshift distribution and the space density of high-redshift radio galaxies

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MJ Cruz, MJ Jarvis, S Rawlings, KM Blundell

We use the 6C** sample to investigate the co-moving space density of powerful, steep-spectrum radio sources. This sample, consisting of 68 objects, has virtually complete K-band photometry and spectroscopic redshifts for 32 per cent of the sources. In order to find its complete redshift distribution, we develop a method of redshift estimation based on the K-z diagram of the 3CRR, 6CE, 6C* and 7CRS radio galaxies. Based on this method, we derive redshift probability density functions for all the optically identified sources in the 6C** sample. Using a combination of spectroscopic and estimated redshifts, we select the most radio luminous sources in the sample. Their redshift distribution is then compared with the predictions of the radio luminosity function of Jarvis et al. We find that, within the uncertainties associated with the estimation method, the data are consistent with a constant co-moving space density of steep-spectrum radio sources beyond z > 2.5, and rule out a steep decline.

The 6C** sample of steep-spectrum radio sources: I - Radio data, near-infrared imaging and optical spectroscopy

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MJ Cruz, MJ Jarvis, KM Blundell, S Rawlings, S Croft, H-R Kloeckner, RJ McLure, C Simpson, TA Targett, CJ Willott

We present basic observational data on the 6C** sample. This is a new sample of radio sources drawn from the 151 MHz 6C survey, which was filtered with radio criteria chosen to optimize the chances of finding radio galaxies at z > 4. The filtering criteria are a steep-spectral index and a small angular size. The final sample consists of 68 sources from a region of sky covering 0.421 sr. We present VLA radio maps, and the results of K-band imaging and optical spectroscopy. Near-infrared counterparts are identified for 66 of the 68 sources, down to a 3-sigma limiting magnitude of K ~ 22 mag in a 3-arcsec aperture. Eight of these identifications are spatially compact, implying an unresolved nuclear source. The K-magnitude distribution peaks at a median K=18.7 mag, and is found to be statistically indistinguishable from that of the similarly selected 6C* sample, implying that the redshift distribution could extend to z > 4. Redshifts determined from spectroscopy are available for 22 (32 per cent) of the sources, over the range of 0.2 < z < 3.3 . We measure 15 of these, whereas the other 7 were previously known. Six sources are at z > 2.5. Four sources show broad emission lines in their spectra and are classified as quasars. Three of these show also an unresolved K-band identification. Eleven sources fail to show any distinctive emission and/or absorption features in their spectra. We suggest that these could be (i) in the so-called `redshift desert' region of 1.2 < z < 1.8, or (ii) at a greater redshift, but feature weak emission line spectra.

Clues from microquasars to the origin of radio-loudness of quasars

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C Nipoti, KM Blundell, J Binney

We analysed the long-term variability of four microquasars (GRS 1915+105, Cyg X-1, Cyg X-3, and Sco X-1) in radio and X rays. The results of our analysis indicate the existence of two distinct modes of energy output, which we refer to as the `coupled' mode and the `flaring' mode. The coupled mode is responsible for mildly fluctuating, flat-spectrum radio emission, coupled with the X-ray emission; the flaring mode produces powerful, steep-spectrum radio flares, with no significant counterpart in X rays. We find that the fraction of time spent by a typical microquasar in the flaring mode is similar to the fraction of quasars that are radio-loud. This is consistent with the hypothesis that radio-loudness of quasars is a function of the epoch at which the source is observed.

Time-sequenced Multi-Radio-Frequency Observations of Cygnus X-3 in Flare

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JCA Miller-Jones, KM Blundell, MP Rupen, AJ Mioduszewski, P Duffy, AJ Beasley

Multifrequency observations from the VLA, VLBA and OVRO Millimeter Array of a major radio outburst of Cygnus X-3 in 2001 September are presented, measuring the evolution of the spectrum of the source over three decades in frequency, over a period of six days. Following the peak of the flare, as the intensity declines the high-frequency spectrum at frequency nu steepens from nu^{-0.4} to nu^{-0.6}, after which the spectral index remains at this latter terminal value; a trend previously observed but hitherto not satisfactorily explained. VLBA observations, for the first time, track over several days the expansion of a sequence of knots whose initial diameters are approximately 8 milliarcseconds. The light-crossing time within these plasmons is of the same order as the time-scale over which the spectrum is observed to evolve. We contend that properly accounting for light-travel time effects in and between plasmons which are initially optically thick, but which after expansion become optically thin, explains the key features of the spectral evolution, for example the observed timescale. Using the VLBA images, we have directly measured for the first time the proper motions of individual knots, analysis of which shows a two-sided jet whose axis is precessing. The best-fit jet speed is roughly beta = 0.63 and the precession period is about 5 days, significantly lower than fitted for a previous flare. Extrapolation of the positions of the knots measured by the VLBA back to zero-separation shows this to occur approximately 2.5 days after the detection of the rise in flux density of Cygnus X-3.

Lowering Inferred Cluster Magnetic Field Strengths - the radio galaxy contributions

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L Rudnick, K Blundell

We present a detailed examination of the relationship between the magnetic field structures and the variations in Faraday Rotation across PKS1246-410, a radio source in the Centaurus cluster of galaxies, using data from Taylor, Fabian and Allen. We find a significant relationship between the intrinsic position angle of the polarization and the local amount of Faraday Rotation. The most plausible explanation is that most or all of the rotation is local to the source. We suggest that the rotations local to cluster radio galaxies may result from either thermal material mixed with the radio plasma, or from thin skins of warm, ionized gas in pressure balance with the observed galaxy or hot cluster atmospheres. We find that the contribution of any unrelated cluster Rotation Measure variations on scales of 2 - 10 arcsec are less than 25 rad/m^2; the standard, although model dependent, derivation of cluster fields would then lead to an upper limit of approximately 0.4 microGauss on these scales. Inspection of the distributions of Rotation Measure, polarisation angle and total intensity in 3C75, 3C465 and Cygnus A also shows source-related Faraday effects in some locations. Many effects can mask the signatures of locally-dominated RMs, so the detection of even isolated correlations can be important, although difficult to quantify statistically. In order to use radio sources such as shown here to derive {\it cluster-wide} magnetic fields, as is commonly done, one must first remove the local contributions; this is not possible at present.

Optical spectroscopy of radio galaxies in the 7C Redshift Survey

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CJ Willott, S Rawlings, KM Blundell, M Lacy, GJ Hill, SE Scott

We present optical spectroscopy of all 49 radio galaxies in the 7C-I and 7C-II regions of the 7C Redshift Survey (7CRS). The low-frequency (151 MHz) selected 7CRS sample contains all sources with flux-densities S_151 > 0.5 Jy in three regions of the sky; 7C-I and 7C-II were chosen to overlap with the 5C6 and 5C7 surveys respectively, and cover a total sky area of 0.013 sr. The sample has been completely identified and spectroscopy of the quasars and broad-lined radio galaxies has been presented in Willott et al. (1998). Only seven of the radio galaxies do not have redshift determinations from the spectroscopy, giving a redshift completeness for the sample of >90%. The median redshift of the 7CRS is 1.1. We present a composite 0.2<z<0.8 7CRS radio galaxy spectrum and investigate the strengths of the 4000 Angstrom breaks in these radio galaxies. We find an anti-correlation between the 4000 Angstrom break strength and emission line luminosity, indicating that departures from old elliptical galaxy continuum shapes are most likely due to non-stellar emission associated with the active nucleus.

The properties of the X-ray holes in the intracluster medium of the Perseus cluster

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AC Fabian, A Celotti, KM Blundell, NE Kassim, RA Perley

High resolution X-ray and low frequency radio imaging now allow us to examine in detail the interaction and physical properties of the radio source 3C84 and the surrounding thermal gas. The radiative and dynamical properties of the inner X-ray holes, which coincide with the radio lobes, indicate that the ratio of the energy factor k and filling factor f is in the range 180<k/f<500. We define k to be the ratio of the total particle energy to that of the electrons radiating above a fiducial frequency of 10MHz. The relativistic plasma and magnetic field are not in equipartition, since the field must be a factor of 4 or more lower than required for pressure balance. Unexpected steep-spectrum spurs in the low-frequency radio maps point to outer X-ray holes, which are plausibly buoyant old radio lobes. The evidence that the inner lobes are currently expanding subsonically, yet have not detached due to buoyancy, and the requirement that the synchrotron cooling time must exceed the age of the hole, enable us to constrain the jet power of the nucleus to between 10^44 and 10^45 ergps, depending on the filling factor of the relativistic plasma.

On the redshift cut-off for steep-spectrum radio sources

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MJ Jarvis, S Rawlings, CJ Willott, KM Blundell, S Eales, M Lacy

We use three samples (3CRR, 6CE and 6C*) selected at low radio frequency to constrain the cosmic evolution in the radio luminosity function (RLF) for the `most luminous' steep-spectrum radio sources. Although intrinsically rare, such sources give the largest possible baseline in redshift for the complete flux-density-limited samples currently available. Using parametric models to describe the RLF which incorporate distributions in radio spectral shape and linear size as well as the usual luminosity and redshift, we find that the data are consistent with a constant comoving space density between z~2.5 and z~4.5. We find this model is favoured over a model with similar evolutionary behaviour to that of optically-selected quasars (i.e. a roughly Gaussian distribution in redshift) with a probability ratio of ~25:1 and ~100:1 for spatially-flat cosmologies with Omega_Lambda = 0 and Omega_Lambda = 0.7 respectively. Within the uncertainties, this evolutionary behaviour may be reconciled with the shallow decline preferred for the comoving space density of flat-spectrum sources by Dunlop & Peacock (1990) and Jarvis & Rawlings (2000), in line with the expectations of Unified Schemes.

The radio galaxy K-z relation to z ~ 4.5

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MJ Jarvis, S Rawlings, S Eales, KM Blundell, CJ Willott

Using a new radio sample, 6C* designed to find radio galaxies at z > 4 along with the complete 3CRR and 6CE sample we extend the radio galaxy K-z relation to z~4.5. The 6C* K-z data significantly improve delineation of the K-z relation for radio galaxies at high redshift (z > 2). Accounting for non-stellar contamination, and for correlations between radio luminosity and estimates of stellar mass, we find little support for previous claims that the underlying scatter in the stellar luminosity of radio galaxies increases significantly at z > 2. This indicates that we are not probing into the formation epoch until at least z > 3.

The Nature and Evolution of Classical Double Radio Sources from Complete Samples

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KM Blundell, S Rawlings, CJ Willott

We present a study of the trends in luminosity, linear size, spectral index, and redshift of classical double radio sources from three complete samples selected at successively fainter low radio-frequency flux-limits. We have been able to decouple the effects of the tight correlation between redshift and luminosity (inherent in any single flux-limited sample) which have hitherto hindered interpretation of the relationships between these four source properties. The major trends found are that (i) spectral indices increase with linear size, (ii) rest-frame spectral indices have a stronger dependence on luminosity than on redshift except at high (GHz) frequencies, and that (iii) the linear sizes are smaller at higher redshifts. We reproduce the observed dependences in a model for radio sources (born throughout cosmic time according to a radio-source birth function) whose lobes are fed with a synchrotron-emitting population (whose energy distribution is governed by compact hotspots), and which suffer inverse Compton, synchrotron and adiabatic expansion losses. In simulating the basic observed dependences, we find that there is no need to invoke any systematic change in the environments of these objects with redshift if the consequences of imposing a survey flux-limit on our simulated datasets are properly included in the model. We present evidence that for a radio survey there is an unavoidable `youth--redshift degeneracy', even though radio sources are short-lived relative to the age of the Universe; it is imperative to take this into account in studies which seemingly reveal correlations of source properties with redshift such as the `alignment effect'.

A sample of 6C radio sources designed to find objects at redshift > 4: the radio data

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KM Blundell, S Rawlings, SA Eales, GB Taylor, AD Bradley

We describe the selection of a sample of 34 radio sources from the 6C survey (Hales, Baldwin & Warner 1993) from a region of sky covering 0.133 sr. The selection criteria for this sample, hereafter called 6C*, were chosen to optimise the chances of finding radio galaxies at redshift z > 4. Optical follow-up observations have already led to the discovery of the most distant known radio galaxy at z = 4.41 (Rawlings et al. 1996). We present VLA radio maps and derive radio spectra for all the 6C* objects.

A complete sample of quasars from the 7C redshift survey

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CJ Willott, S Rawlings, KM Blundell, M Lacy

We present details of a new sample of radio-loud quasars drawn from 0.013 sr of the 7C Redshift Survey. This sample is small (21 quasars) but complete in that every object with an unresolved nucleus and/or broad emission lines with S(151MHz) > 0.5 Jy has been discovered. The dependence of the quasar fraction with redshift and radio luminosity is investigated, providing new evidence supporting the unification of radio-loud quasars and powerful radio galaxies. This 7C sample is compared with optically-selected quasars, in order to determine whether there are systematic biases in the different selection techniques. There are no lightly reddened (Av approx. 1) quasars in our sample amongst the 14 with z < 2. The discovery of a reddened quasar at z = 2.034 and its implications are discussed. A tight correlation between radio luminosity and optical/near infrared continuum luminosity for a subset of the sample is also found.