Showing papers by "John Reynolds published in 1998"

The Deep X-Ray Radio Blazar Survey. I. Methods and First Results*

Abstract: We have undertaken a survey, the Deep X-Ray Radio Blazar Survey (DXRBS), of archived, pointed ROSAT Position Sensitive Proportional Counter data for blazars by correlating the ROSAT WGACAT database with several publicly available radio catalogs, restricting our candidate list to serendipitous flat radio spectrum sources (αr ≤ 0.70, where Sν ∝ ν). We discuss our survey methods, identification procedure, and first results. Our survey is found to be ~95% efficient at finding flat-spectrum radio-loud quasars (FSRQs; 59 of our first 85 identifications) and BL Lacertae objects (22 of our first 85 identifications), a figure that is comparable to or greater than that achieved by other radio and X-ray survey techniques. The identifications presented here show that all previous samples of blazars (even when taken together) did not representatively survey the blazar population, missing critical regions of (LX, LR) parameter space within which large fractions of the blazar population lie. Particularly important is the identification of a large population of FSRQs (25% of DXRBS FSRQs) with ratios of X-ray to radio luminosity 10-6 (αrx 0.78). In addition, as a result of our greater sensitivity, the DXRBS has already more than doubled the number of FSRQs in complete samples with 5 GHz (radio) luminosities between 1031.5 and 1033.5 ergs s-1 Hz-1, and fills in the region of parameter space between X-ray–selected and radio-selected samples of BL Lac objects. The DXRBS is the very first sample to contain statistically significant numbers of blazars at low luminosities, approaching what should be the lower end of the FSRQ luminosity function.

The Deep X-ray Radio Blazar Survey (DXRBS). I. Methods and First Results

Abstract: We have undertaken a survey of archived, pointed ROSAT PSPC data for blazars by correlating the ROSAT WGACAT database with several publicly available radio catalogs, restricting our candidate list to serendipitous flat radio spectrum sources (alpha_r <= 0.70). Here we discuss our survey methods, identification procedure and first results. Our survey is found to be ~ 95% efficient at finding flat-spectrum radio-loud quasars (FSRQs, 59 of our first 85 IDs) and BL Lacertae objects (22 of our first 85 IDs), a figure which is comparable to or greater than that achieved by other radio and X-ray survey techniques. The identifications presented here show that all previous samples of blazars (even when taken together) did not representatively survey the blazar population, missing critical regions of (L_X,L_R) parameter space within which large fractions of the blazar population lie. Particularly important is the identification of a large population of FSRQs (>~ 25% of DXRBS FSRQs) with ratios of X-ray to radio luminosity >~ 10^-6 (alpha_rx <~ 0.78). In addition, due to our greater sensitivity, DXRBS has already more than doubled the number of FSRQs in complete samples with 5 GHz (radio) luminosities between 10^31.5 and 10^33.5 erg/s/Hz and fills in the region of parameter space between X-ray selected and radio-selected samples of BL Lacs. DXRBS is the very first sample to contain statistically significant numbers of blazars at low luminosities, approaching what should be the lower end of the FSRQ luminosity function.

Overview and initial results of the very long baseline interferometry space observatory programme

Abstract: High angular resolution images of extragalactic radio sources are being made with the Highly Advanced Laboratory for Communications and Astronomy (HALCA) satellite and ground-based radio telescopes as part of the Very Long Baseline Interferometry (VLBI) Space Observatory Programme (VSOP). VSOP observations at 1.6 and 5 gigahertz of the milli-arc-second-scale structure of radio quasars enable the quasar core size and the corresponding brightness temperature to be determined, and they enable the motions of jet components that are close to the core to be studied. Here, VSOP images of the gamma-ray source 1156+295, the quasar 1548+056, the ultraluminous quasar 0014+813, and the superluminal quasar 0212+735 are presented and discussed.

The Subparsec-Scale Structure and Evolution of Centaurus A: The Nearest Active Radio Galaxy

Abstract: ?????The subparsec-scale structure of Cen A is complex, consisting of a bright jet and a fainter counterjet. The bright jet contains components that have subluminal speeds of approximately 0.1c and undergo irregular episodes of rapid internal evolution. The rapid evolution sometimes observed could be interpreted as evidence for an underlying jet flow much faster (>0.45c) than observed from the proper motion of components (~0.1c). Considering the large-scale morphology of the source, the motions and temporal variations in the jet, and the detection of a counterjet, we conclude that the axis of the Cen A jet lies between ~50? and ~80? to our line of sight. We find that the estimated times of component ejection from the compact core are reasonably coincident with enhancements in hard X-ray intensity and 22 GHz flux density. In the context of the radio galaxy population, Cen A is a low-luminosity FR I?type source and in general has the properties observed in other FR I radio galaxies. Overall, the observations of Cen A presented here, and from other investigations, are consistent with the idea that sources with an FR I appearance are not aligned with our line of sight and have relativistic flow on the subparsec scale. The apparently subluminal subparsec-scale jet components are interpreted as being slow patterns on the relativistic flow.

Seyfert galaxies and the radio—far-infrared correlation

Abstract: We have observed a sample of 149 Seyfert galaxies and radio-quiet quasars at 13 cm with both a 275-km radio interferometer and the 6-km compact array of the Australia Telescope The high-resolution observations searched for the presence of compact, high-brightness-temperature radio emission from the active nucleus The low-resolution observations measured the total radio emission from the galaxy disc and Seyfert core and lobes From these we draw the following conclusions (i) Seyfert galaxies that lack compact radio cores display a correlation between radio and far-infrared (FIR) emission similar to the correlation displayed by normal spirals, albeit with greater scatter The correlation is found to be intrinsic and is not an artefact of the richness effect (ii) A very different radio–FIR correlation is displayed by those Seyferts that harbour compact radio cores These tend to be more radio-loud than either normal spirals or the Seyferts that lack compact cores The compact core emission thus seems to be responsible for the generally poor radio–FIR correlation displayed by Seyfert galaxies (iii) The radio–FIR correlation is not significantly improved by subtracting off the 01-arcsec (20- to 200-pc) compact radio emission from the total radio emission This suggests that the emission from the active galactic nucleus has significant structure on scales larger than 01 arcsec Perhaps these structures are the ‘linear’ radio features that have been seen previously in Seyfert nuclei

VLBI Observations of Gamma-Ray-quiet Active Galactic Nuclei: Comparing Radio-Core Brightness Temperatures

Abstract: We present VLBI and Australia Telescope Compact Array images and derive source-frame radio-core brightness temperatures for three prominent, flat-spectrum extragalactic radio sources, notable because they have not been detected as gamma-ray sources with the EGRET instrument. Radio-core brightness temperatures are discussed as an indicator of relativistic beaming. A comparison of source-frame radio-core brightness temperatures for EGRET-identified and gamma-ray-quiet, radio-loud AGNs shows that a wide range in beaming properties is likely to exist, independent of the level of gamma-ray emission. This comparison suggests that, although there is a strong relationship between radio emission and gamma-ray emission in EGRET-identified AGNs, and many EGRET-identified AGNs show evidence for relativistic beaming, there is little evidence for a one-to-one correspondence between the strength of gamma-ray emission and the degree of beaming of the radio emission.

Compact radio emission in 60‐μm peaker galaxies

Abstract: Accepted 1998 July 9. Received 1998 July 2; in original form 1998 March 10 ABSTRA C T We present radio interferometric observations of a well-defined sample of IRAS galaxies with warm far-infrared colours ‐ 60-mm peakers (60PKs). We find the surprising result that the core radio power of Seyfert 60PKs is between those of ‘normal’ Seyfert 2 galaxies and radio ellipticals, and follows the same relationship with respect to total and extended radio emission as low- and high-power radio galaxies. This is consistent with the suggestion that 60PKs represent nascent radio elliptical galaxies of low to intermediate power. The compact radio cores (assumed to be associated with the AGN core) in 60PKs contain typically ,37 per cent of the total radio flux of the galaxy (generally attributed to synchrotron radiation from cosmic ray electrons accelerated by distributed star formation in the galaxy). If this result is true more generally, then it implies that the compact radio core in an active galaxy ‘knows’ about the rate of star formation in the galaxy. Alternatively, the presence of a ‘radio excess’ for Seyfert 60PKs suggests that the extended radio flux may be attributed to radio jets rather than star formation, which explains its relationship with the radio core emission. Taking into consideration the sensitivity limit for the Parkes‐Tidbinbilla Interferometer, we find that none of the 60PKs classified as starburst by optical spectroscopy is detected, while over 90 per cent of the Seyfert 60PKs contain compact radio cores. This leads us to conclude that the starburst 60PKs are not just heavily dust-obscured AGN. Although we have smallnumber statistics, these results are consistent with previous surveys that indicate that the optical spectroscopic classification of emission-line galaxies discriminates between galaxies that contain high brightness temperature radio cores and those that do not. The data presented in this paper are insufficient to determine if there is an evolutionary relationship between the starburst and Seyfert classes of 60PKs.

The Counterjet in the Nucleus of Centaurus A

Abstract: Centaurus A (NGC 5128) is the nearest giant radio galaxy It is a Fanaroff-Riley type 1 (low luminosity) radio source, but the compact radio source in the nucleus is strong enough that VLBI imaging has been possible with both the SHEVE array and the VLBA at several frequencies These observations have detected a sub-parsec scale counterjet This shows that jet formation in at least some FR I sources is intrinsically two-sided over very small distances and the radio jets in Centaurus A are probably only moderately relativistic We also find evidence that the center of activity in Centaurus A is partially obscured by a disk or torus of dense plasma

The Sub-Parsec-Scale Structure and Evolution of Centaurus A

Abstract: We present high-resolution VLBI radio images of the closest classical radio galaxy, Centaurus A, including the highest resolution image yet for this source.

PKS 0405–385 as Seen by the ISM Telescope

Abstract: 1. Observations The ATCA IDV Survey (Kedziora-Chudczer et al., these Proceedings, p. 271) of 125 compact, flat spectrum southern radio sources at four frequencies (1.4, 2.4, 4.8, and 8.6 GHz) led to the discovery of extremely rapid and large amplitude variations in the z=1.285 quasar PKS 0405—385 (Kedziora-Chudczer et al. 1996). The IDV of PKS 0405-385 was first observed in Nov 1993 as a part of the IDV Survey, when strong and rapid changes (17% peak-to-peak within 24 hours) were seen. Continuous monitoring in June 1996 with the ATCA revealed large amplitude, rapid, quasiperiodic variations. These variations were immediately confirmed by data taken with the 26 m telescope of the Hartebeesthoek Radio Astronomy Observatory (HARTRAO) at 5.0 and 8.4 GHz. Figure 1 shows the data from June 1996 where the excellent agreement in the ATCA and HARTRAO flux density measurements is immediately apparent. Remarkably, these variations persisted only for several weeks. They were present at full strength on July 2 and entirely absent on July 21. The flux Figure 1. Combined ATCA and HARTRAO light-curve measured on the 1996 June 8 at four frequencies: 8.6, 4.8, 2.4, and 1.4 GHz, from top to bottom, respectively. ATCA data are plotted as circles, HARTRAO data as triangles.

Evidence of a Low-Mass Companion to AB Doradus

Abstract: A b s t r a c t . From the combination of VLBI phase-referenced observations and Hipparcos satellite data, we have found evidence of a low-mass object orbiting the late-type star AB Doradus. The mass of the new object is near the hydrogen burning limit and will constitute a precise point for calibrating the low end of the main sequence. This represents the first detection of a low-mass stellar companion using the VLBI technique, which could become an important tool in future searches for planets and brown dwarfs orbiting other stars.

Multi-Frequency VLBI Observations of the GPS Source 1934–638

Abstract: PKS 1934–638 is an archetypal GPS source, peaking at 1.4 GHz and exhibits almost no flux density variability. VLBI images at frequencies of .843, 2.3, 4.8, & 8.4 were made with the southern hemisphere VLBI array and they reveal that the source is a 42 mas compact double. There is no detectable change in separation over the last 20 years, yielding an upper limit of ~ 0.03c ± 0.2c on any expansion velocity. The spectral shapes of the two components are remarkably similar, despite indications of finer structure on longer baselines. Magnetic field calculations indicate fields of a few mGauss and the results are consistent with equipartition.

VLBI of Southern EGRET Identifications

Abstract: We have undertaken VLBI observations of 8 Southern Hemisphere EGRET radio sources. Using our data as well as data obtained from the literature we have examined the difference in radio properties between gamma-ray loud and gamma-ray quiet radio sources. In particular, we find no evidence that gamma-ray loud radio sources lie preferentially in sources with straight radio jets as has been suggested.