arXiv subjects

Katherine M. Blundell

Publications and source records attributed to Katherine M. Blundell.

At least 55 records · Page 3Linked to original sources

arXiv2007

The Distance to SS433/W50 and its Interaction with the ISM

[ABRIDGED] The distance to the relativistic jet source SS433 and the related supernova remnant W50 is re-examined using new observations of HI in absorption from the VLA, HI in emission from the GBT, and 12CO emission from the FCRAO. The new measurements show HI in absorption against SS433 to a velocity of 75 km/s but not to the velocity of the tangent point, which bounds the kinematic distance at 5.5 < d_k < 6.5 kpc. This is entirely consistent with a 5.5 +/- 0.2 kpc distance determined from light travel-time arguments (Blundell & Bowler 2004). The HI emission map shows evidence of interaction of the lobes of W50 with the interstellar medium near the adopted systemic velocity of V_LSR = 75 km/s. The western lobe sits in a cavity in the HI emission near the Galactic plane, while the eastern lobe terminates at an expanding HI shell. The expanding shell has a radius of 40 pc, contains 8 +/- 3 x 10^3 M_sun of HI and has a measured kinetic energy of 3 +/- 1.5 x 10^{49} ergs. There may also be a static HI ring or shell around the main part of W50 itself at an LSR velocity of 75 km/s, with a radius of 70 pc and a mass in HI of 3.5 - 10 x 10^4 M_sun. We do not find convincing evidence for the interaction of the system with any molecular cloud or with HI at other velocities. The HI emission data suggest that SS433 lies in an interstellar environment substantially denser than average for its distance from the Galactic plane. This Population I system, now about 200 pc below the Galactic plane, most likely originated as a runaway O-star binary ejected from a young cluster in the plane. New astrometric data on SS433 show that the system now has a peculiar velocity of a few tens of km/s in the direction of the Galactic plane. From this peculiar velocity and the symmetry of the W50 remnant we derive a time since the SN of < 10^5 yr.

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arXiv2007

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

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.

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arXiv2006

Determining the nature of the faint X-ray source population near the Galactic Centre

We present results of a multi-wavelength program to study the faint discrete X-ray source population discovered by Chandra in the Galactic Centre (GC). From IR imaging obtained with the VLT we identify candidate K-band counterparts to 75% of the X-ray sources in our sample. By combining follow-up VLT K-band spectroscopy of a subset of these candidate counterparts with the magnitude limits of our photometric survey, we suggest that only a small percentage of the sources are HMXBs, while the majority are likely to be canonical LMXBs and CVs at the distance of the GC. In addition, we present our discovery of highly structured small-scale (5-15") extinction towards the Galactic Centre. This is the finest-scale extinction study of the Galactic Centre to date. Finally, from these VLT observations we are able to place constraints on the stellar counterpart to the ``bursting pulsar'' GRO J1744-28.

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arXiv2006

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

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.

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arXiv2006

Galactic centre X-ray sources

We report on a campaign to identify the counterparts to the population of X-ray sources discovered at the centre of our Galaxy by Wang et al.(2002) using Chandra. We have used deep, near infrared images obtained on VLT/ISAAC to identify candidate counterparts as astrometric matches to the X-ray positions. Follow up Ks-band spectroscopic observations of the candidate counterparts are used to search for accretions signatures in the spectrum, namely the Brackett-Gamma emission line (Bandyopadhyay et al.1997). From our small initial sample, it appears that only a small percentage, ~2-3% of the ~1000 X-ray sources are high mass X-ray binaries or wind accreting neutron stars, and that the vast majority will be shown to be canonical low mass X-ray binaries and cataclysmic variables.

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arXiv2006

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

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.

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arXiv2006

Extended inverse-Compton emission from distant, powerful radio galaxies

We present Chandra observations of two relatively high redshift FRII radio galaxies, 3C 432 and 3C 191 (z=1.785 and z=1.956 respectively), both of which show extended X-ray emission along the axis of the radio jet or lobe. This X-ray emission is most likely to be due to inverse-Compton scattering of Cosmic Microwave Background (CMB) photons. Under this assumption we estimate the minimum energy contained in the particles responsible. This can be extrapolated to determine a rough estimate of the total energy. We also present new, deep radio observations of 3C 294, which confirm some association between radio and X-ray emission along the NE-SW radio axis and also that radio emission is not detected over the rest of the extent of the diffuse X-ray emission. This, together with the offset between the peaks of the X-ray and radio emissions may indicate that the jet axis in this source is precessing.

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arXiv2006

Complex small-scale structure in the infrared extinction towards the Galactic Centre

A high level of complex structure, or ``granularity'', has been observed in the distribution of infrared-obscuring material towards the Galactic Centre (GC), with a characteristic scale of 5arcsec - 15arcsec, corresponding to 0.2 - 0.6pc at a GC distance of 8.5kpc. This structure has been observed in ISAAC images which have a resolution of 0.6arcsec, significantly higher than that of previous studies of the GC. We have discovered granularity throughout the GC survey region, which covers an area of 1.6deg x 0.8deg in longitude and latitude respectively (300pc x 120pc at 8.5kpc) centred on Sgr A*. This granularity is variable over the whole region, with some areas exhibiting highly structured extinction in one or more wavebands and other areas displaying no structure and a uniform stellar distribution in all wavebands. The granularity does not appear to correspond to longitude, latitude or radial distance from Sgr A*. We find that regions exhibiting high granularity are strongly associated with high stellar reddening.

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arXiv2005

Radio-loud flares from microquasars and radio-loudness of quasars

The low-frequency power spectra of the X-ray and radio emission from four microquasars suggest that two distinct modes of energy output are at work: (i) the `coupled' mode in which the X-ray and radio luminosities are closely coupled and vary only weakly, and (ii) the `flaring' mode, which dramatically boosts the radio luminosity but makes no impact on the X-ray luminosity. The systems are in the flaring mode only a few percent of the time. However, flares completely dominate the power spectrum of radio emission, with the consequence that sources in which the flaring mode occurs, such as GRS 1915+105 and Cyg X-3, have radio power spectra that lie more than an order of magnitude above the corresponding X-ray power spectra. Of the four microquasars for which we have examined data, in only one, Cyg X-1, is the flaring mode seemingly inactive. While Cyg X-1 is a black-hole candidate, one of the three flaring sources, Sco X-1, is a neutron star. Consequently, it is likely that both modes are driven by the accretion disk rather than black-hole spin. Radio imaging strongly suggests that the flaring mode involves relativistic jets. A typical microquasar is in the flaring mode a few percent of the time, which is similar to the fraction of quasars that are radio loud. Thus there may be no essential difference between radio-loud and radio-quiet quasars; radio loudness may simply be a function of the epoch at which the source is observed.

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arXiv2004

Radio Variability of Radio Quiet and Radio Loud Quasars

The majority of quasars are weak in their radio emission, with flux densities comparable to those in the optical, and energies far lower. A small fraction, about 10%, are hundreds to thousands of times stronger in the radio. Conventional wisdom holds that there are two classes of quasars, the radio quiets and radio louds, with a deficit of sources having intermediate power. Are there really two separate populations, and if so, is the physics of the radio emission fundamentally different between them? This paper addresses the second question, through a study of radio variability across the full range of radio power, from quiet to loud. The basic findings are that the root mean square amplitude of variability is independent of radio luminosity or radio-to-optical flux density ratio, and that fractionally large variations can occur on timescales of months or less in both radio quiet and radio loud quasars. Combining this with similarities in other indicators, such as radio spectral index and the presence of VLBI-scale components, leads to the suggestion that the physics of radio emission in the inner regions of all quasars is essentially the same, involving a compact, partially opaque core together with a beamed jet.

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arXiv2004

Jet evolution, flux ratios and light-travel time effects

Studies of the knotty jets in both quasars and microquasars frequently make use of the ratio of the intensities of corresponding knots on opposite sides of the nucleus in order to infer the product of the intrinsic jet speed (beta) and the cosine of the inclination angle of the jet-axis (cos{theta}), via the formalism I_{a}/I_{r} = ((1+beta cos{theta})/(1-beta cos{theta}))^{3+alpha}, where alpha relates the intensity I_{nu} as a function of frequency nu as I_{nu} propto nu^{-alpha}. Where beta cos{theta} is determined independently, the intensity ratio of a given pair of jet to counter-jet knots is over-predicted by the above formalism compared with the intensity ratio actually measured from radio images. As an example in the case of Cygnus X-3 the original formalism predicts an intensity ratio of about 185, whereas the observed intensity ratio at one single epoch is about 3. Mirabel and Rodriguez (1999) have refined the original formalism, and suggested measuring the intensity ratio of knots when they are at equal angular separations from the nucleus. This method is only applicable where there is sufficient time-sampling with sufficient physical resolution to interpolate the intensities of the knots at equal distances from the nucleus, and can therefore be difficult to apply to microquasars and is impossible to apply to quasars. Accounting for both the light-travel time between the knots and the simple evolution of the knots themselves reconciles this over-prediction and renders the original formalism obsolete.

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arXiv2003

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

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.

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arXiv2002

Near-infrared imaging and the K-z relation for radio galaxies in the 7C Redshift Survey

We present K-band imaging 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. We combine the K-band magnitudes of the 7CRS radio galaxies with those from the 3CRR, 6CE and 6C* samples to investigate the nature of the relationship between K-magnitude and redshift and whether there is any dependence upon radio luminosity. We find that radio galaxies appear to belong to a homogeneous population which formed the bulk of their stars at high redshifts (z_f>5) and evolved passively from then until they reach a mean present-day luminosity of 3L*. We find a significant difference between the K-magnitudes of the 7CRS and 3CRR radio galaxies with the 7CRS galaxies being \~0.55 mag fainter at all redshifts. The cause of this weak correlation between stellar and radio luminosities probably lies in mutual correlations of these properties with the central black hole mass. We compare the evolution-corrected host luminosities at a constant radio luminosity and find that the typical host luminosity (mass) increases by approximately 1L* from z~2 to z~0.5 which, although a much smaller factor than predicted by semi-analytic models of galaxy formation, is in line with results on optically-selected quasars. Our study has therefore revealed that the small dispersion in stellar luminosity of radio galaxies around 3L* includes subtle but significant differences between the host galaxies of extreme- and moderate-power radio sources at fixed redshift, and between those of high- and low-redshift radio sources at fixed radio luminosity.

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arXiv2002

Optical spectroscopy of radio galaxies in the 7C Redshift Survey

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.

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arXiv2001

The mass of radio galaxies from low to high redshift

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). In a spatially flat universe with a cosmological constant (Omega_M=0.3 and Omega_Lambda=0.7), the most luminous radio sources appear to be associated with galaxies with a luminosity distribution with a high mean (~5L*), and a low dispersion (~0.5 mag) which formed their stars at epochs corresponding to z >~ 2.5.

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arXiv2001

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

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.

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arXiv2001

A sample of 6C radio sources designed to find objects at redshift > 4: II --- spectrophotometry and emission line properties

(Abridged) This is the second in a series of three papers which present and interpret basic observational data on the 6C* 151-MHz radio sample: a low-frequency selected sample which exploits filtering criteria based on radio properties (steep spectral index and small angular size) to find radio sources at redshift z > 4 within a 0.133sr patch of sky. We present results of a programme of optical spectroscopy which has yielded redshifts in the range 0.5 < z < 4.4 for the 29 sources in the sample, all but six of which are secure. We find that the fil tering criteria used for 6C* are very effective in excluding the low-redshift, low-luminosity radio sources: the median redshift of 6C* is z~1.9 compared to z~1.1 for a complete sample matched in 151-MHz flux density. By combining the emission-line dataset for the 6C* radio sources with those for the 3CRR, 6CE and 7CRS samples we establish that z > 1.75 radio galaxies follow a rough proportionality between Lyalpha- and 151 MHz-luminosity which, like similar correlations seen in samples of lower-redshift radio sources, are indicative of a primary link between the power in the source of the photoionising photons (most likely a hidden quasar nucleus) and the power carried by the radio jets. We argue that radio sources modify their environments and that the range of emission-line properties seen is determined more by the range of source age than by the range in ambient environment. This is in accord with the idea that all high-redshift, high-luminosity radio sources are triggered in similar environments, presumably recently collapsed massive structures.

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arXiv2001

A sample of 6C radio sources designed to find objects at redshift > 4: III --- imaging and the radio galaxy K-z relation

In this paper, the third and final of a series, we present complete K-band imaging and some complementary I-band imaging of the filtered 6C* sample. We find no systematic differences between the K-z relation of 6C* radio galaxies and those from complete samples, so the near-infrared properties of luminous radio galaxies are not obviously biased by the additional 6C* radio selection criteria (steep spectral index and small angular size). 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 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. In a particular spatially-flat universe with a cosmological constant, the most luminous radio sources appear to be associated with galaxies with a luminosity distribution with a high mean (~5 Lstar), and a low dispersion (sigma ~ 0.5 mag) which formed their stars at epochs corresponding to z >~2.5. This result is in line with recent sub-mm studies of high-redshift radio galaxies and the inferred ages of extremely red objects from faint radio samples.

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