Hisashi Hirabayashi

The VSOP 5 GHz Continuum Survey: The Prelaunch VLBA Observations

We have imaged with milliarcsecond resolution at 5 GHz 374 strong flat-spectrum radio sources north of declination -44° using the VLBA in 1996 June. The source sample was compiled from the source list for the VLBI Space Observatory Programme (VSOP) Survey Program. About 250 of the sources observed with the VLBA had correlated flux densities ≥0.3 Jy on the longest VLBA baseline and these sources are currently being observed with VSOP at 5 GHz. This paper presents the results from the VLBA prelaunch observations: the correlated flux density dependence with u-v distance; the contour display of the images; the model fit parameters for each radio component; and improved positions for 62 sources. Comparisons and extensive cross-referencing with other major VLBI surveys are also given.

15 GHz space VLBI observations using an antenna on a TDRSS satellite

A 4.9 m diameter antenna in earth orbit, part of the Tracking and Data Relay Satellite System, was previously used for space VLBI observations at 2.3 GHz. It has now been used successfully with two ground antennas for VLBI measurements at 15 GHz. Although the sensitivity (gain/system temperature) of the orbiting antenna was a factor of 6 poorer at 15 GHz than at 2.3 GHz, 11 out of 22 extragalactic sources were detected on space-ground baselines, up to a maximum baseline of 1.63 earth diameters. The results suggest that the distribution of source visibilities at 15 and 2.3 GHz are similar for these baseline lengths. Model-dependent brightness temperatures of 1-2.5 x 10 to the 12th K were derived for six sources. This suggests that sources with brightness temperatures in this range are as common at 15 GHz as at 2.3 GHz. The coherence on space-ground baselines (this includes effects due to the phase transfer from the ground to the satellite and the reconstruction of the spacecraft orbit) was 0.76 for 340 s integrations. 11 refs.

The VSOP 5 GHz active galactic nucleus survey. IV. The angular size/brightness temperature distribution

The VLBI Space Observatory Programme (VSOP) mission is a Japanese-led project to study radio sources with submilliarcsecond angular resolution, using an orbiting 8 m telescope on board the satellite HALCA with a global Earth-based array of telescopes. A major program is the 5 GHz VSOP Survey Program, which we supplement here with Very Long Baseline Array observations to produce a complete and flux density - limited sample. Using statistical methods of analysis of the observed visibility amplitude versus projected (u, v) spacing, we have determined the angular size and brightness temperature distribution of bright radio emission from active galactic nuclei. On average, the cores have a diameter ( full width, half-power) of 0.20 mas, which contains about 20% of the total source emission, and 14% +/- 6% of the cores are less than 0.04 mas in size. About 20% +/- 5% of the radio cores have a source frame brightness temperature T-b > 1.0 x 10(13) K, and 3% +/- 2% have T-b > 1.0 x 10(14) K. A model of the high brightness temperature tail suggests that the radio cores have brightness temperatures approximate to1 x 10(12) K and are beamed toward the observer with an average bulk motion of beta = 0.993 +/- 0.004.

VSOP Space VLBI and Geodetic VLBI Investigations of Southern Hemisphere Radio Sources

We present images from VLBI Space Observatory Programme (VSOP) observations of 14 compact extragalactic southern hemisphere radio sources, including a description of the observations, the data reduction techniques, and the parameters of the resulting images and model fits. These images provide the highest resolution information to date for many of these objects. Comparisons are made between VSOP and previous ground-based VLBI results, including images from data extracted from the geodetic VLBI archive at the United States Naval Observatory. From the VSOP data, we find that the two radio galaxies observed have lower peak brightness temperatures than the 12 quasars. Also, these data show (1) no evidence for obvious differences between the brightness temperature distributions of gamma-ray-loud and gamma-ray-quiet radio-loud active galactic nuclei and (2) no evidence for obvious correlations between brightness temperature and spectral index, radio polarization, flux density, or month timescale modulation index. These results are consistent with previous work by Lister, Tingay, & Preston, who found that the only observable significantly correlated with VSOP-derived brightness temperature is intraday variability, which is strongly correlated with many relativistic beaming indicators. For one source, PKS 1127-145, we undertake a detailed investigation of the milliarcsecond-scale component positions as a function of time, taking data from the literature and the current work, to estimate proper motions. As a result, we suggest that two components previously reported as stationary, C1 and C2, have apparent transverse speeds of (9.1 ± 3.8) and (5.3 ± 2.3) h-1c, respectively. We also make the first investigation of the apparent motion in the nearest GHz-peaked spectrum radio galaxy, PKS 1718-649, finding an upper limit on the apparent separation speed of 0.08c. Comparison of geodetic VLBI and VSOP data show no significant detection of component motion in PKS 0208-512, (2.4 ± 3.1) h-1c, and only a tentative detection in PKS 0537-441, (2.8 ± 2.2) h-1c. A significant detection of component motion is found in PKS 1610-771, solely from the geodetic VLBI data, (9.4 ± 3.5) h-1c.

The VSOP 5 GHz active galactic nucleus survey. V. Imaging results for the remaining 140 sources

In 1997 February, the Japanese radio astronomy satellite HALCA was launched to provide the space-bourne element for the VLBI Space Observatory Program (VSOP) mission. Approximately 25% of the mission time was dedicated to the VSOP survey of bright compact active galactic nuclei (AGNs) at 5 GHz. This paper, the fifth in the series, presents images and models for the remaining 140 sources not included in the third paper in the series, which contained 102 sources. For most sources, the plots of the (u, v) coverage, the visibility amplitude versus (u, v) distance, and the high-resolution image are presented. Model fit parameters to the major radio components are determined, and the brightness temperature of the core component for each source is calculated. The brightness temperature distributions for all of the sources in the VSOP AGN survey are discussed.

The VSOP 5 GHz Active Galactic Nucleus Survey. III. Imaging results for the first 102 sources

The VLBI Space Observatory Programme (VSOP) mission is a Japanese-led project to study radio sources with sub-milliarcsec resolution using an orbiting 8 m telescope, HALCA, along with global arrays of Earth-based telescopes. Approximately 25% of the observing time is devoted to a survey of compact active galactic nuclei (AGNs) that are stronger than 1 Jy at 5 GHz-the VSOP AGN Survey. This paper, the third in the series, presents the results from the analysis of the first 102 Survey sources. We present high-resolution images and plots of visibility amplitude versus projected baseline length. In addition, model-fit parameters to the primary radio components are listed, and from these the angular size and brightness temperature for the radio cores are calculated. For those sources for which we were able to determine the source frame core brightness temperature, a significant fraction (53 out of 98) have a source frame core brightness temperature in excess of 10(12) K. The maximum source frame core brightness temperature we observed was 1.2x10(13) K. Explaining a brightness temperature this high requires an extreme amount of relativistic Doppler beaming. Since the maximum brightness temperature one is able to determine using only ground-based arrays is of the order of 10(12) K, our results confirm the necessity of using space VLBI to explore the extremely high brightness temperature regime.

Measuring the brightness temperature distribution of extragalactic radio sources with space VLBI

We have used VLBI Space Observatory Programme (VSOP) observations to measure the brightness temperature distribution of a well-defined subset of the Pearson-Readhead sample of extragalactic radio sources. VLBI, which is restricted to Earth-diameter baselines, is not generally sensitive to emitting regions with brightness temperatures greater than approximately 1012 K, coincidentally close to theoretical estimates of brightness temperature limits, 1011-1012 K. We find that a significant proportion of our sample have brightness temperatures greater than 1012 K; many have unresolved components on the longest baselines, and some remain completely unresolved. These observations begin to bridge the gap between the extended jets seen with ground-based VLBI and the microarcsecond structures inferred from intraday variability, evidenced here by the discovery of a relationship between intraday variability and VSOP-measured brightness temperature, likely due to the effects of relativistic beaming. Also, lower limits on jet Lorentz factors, estimated from space VLBI observations, are starting to challenge numerical simulations that predict low Lorentz factor jets.

[ITAL]Chandra[/ITAL] Discovery of a 100 kiloparsec X-Ray Jet in PKS 0637−752

The quasar PKS 0637-753, the first celestial X-ray target of the Chandra X-ray Observatory, has revealed asymmetric X-ray structure extending from 3 to 12 arcsec west of the quasar, coincident with the inner portion of the jet previously detected in a 4.8 GHz radio image (Tingay et al. 1998). At a redshift of z=0.651, the jet is the largest (~100 kpc) and most luminous (~10^{44.6} ergs/s) of the few so far detected in X-rays. This letter presents a high resolution X-ray image of the jet, from 42 ks of data when PKS 0637-753 was on-axis and ACIS-S was near the optimum focus. For the inner portion of the radio jet, the X-ray morphology closely matches that of new ATCA radio images at 4.8 and 8.6 GHz. Observations of the parsec scale core using the VSOP space VLBI mission show structure aligned with the X-ray jet, placing important constraints on the X-ray source models. HST images show that there are three small knots coincident with the peak radio and X-ray emission. Two of these are resolved, which we use to estimate the sizes of the X-ray and radio knots. The outer portion of the radio jet, and a radio component to the east, show no X-ray emission to a limit of about 100 times lower flux. The X-ray emission is difficult to explain with models that successfully account for extra-nuclear X-ray/radio structures in other active galaxies. We think the most plausible is a synchrotron self-Compton (SSC) model, but this would imply extreme departures from the conventional minimum-energy and/or homogeneity assumptions. We also rule out synchrotron or thermal bremsstrahlung models for the jet X-rays, unless multicomponent or ad hoc geometries are invoked.

The Extreme Scattering Event toward PKS 1741–038: VLBI Images

We report multiepoch VLBI observations of the source PKS 1741-038 (OT 068) as it underwent an extreme scattering event (ESE). Observations at four epochs were obtained, and images were produced at three of these. One of these three epochs was when the source was near the minimum flux density of the ESE, the other two were as the flux density of the source was returning to its nominal value. The fourth epoch was at the maximum flux density during the egress from the ESE, but the VLBI observations had too few stations to produce an image. During the event the source consisted of a dominant, compact component, essentially identical to the structure seen outside the event. However, the sources diameter increased slightly at 13 cm, from near 0.6 mas outside the ESE to near 1 mas during the ESE. An increase in the sources diameter is inconsistent with a simple refractive model in which a smooth refractive lens drifted across the line of sight to PKS 1741-038. We also see no evidence for ESE-induced substructure within the source or the formation of multiple images, as would occur in a strongly refractive lens. However, a model in which the decrease in flux density during the ESE occurs solely because of stochastic broadening within the lens requires a larger broadening diameter during the event than is observed. Thus, the ESE toward PKS 1741-038 involved both stochastic broadening and refractive defocusing within the lens. If the structure responsible for the ESE has a size of order 1 AU, the level of scattering within an ESE lens may be a factor of 107 larger than that in the ambient medium. A filamentary structure could reduce the difference between the strength of scattering in the lens and ambient medium, but there is no evidence for a refractively induced elongation of the source. We conclude that, if ESEs arise from filamentary structures, they occur when the filamentary structures are seen lengthwise. We are able to predict the amount of pulse broadening that would result from a comparable lens passing in front of a pulsar. The pulse broadening would be no more than 1.1 μs, consistent with the lack of pulse broadening detected during ESEs toward the pulsars PSR B1937+21 and PSR J1643-1224. The line of sight toward PKS 1741-038 is consistent with a turbulent origin for the structures responsible for ESEs. The source PKS 1741-038 lies near the radio Loop I and is seen through a local minimum in 100 μm emission.

Milliarcsecond-Scale Spectral Properties and Jet Motions in M 87

We have combined high resolution VLBI Space Observatory Programme (VSOP) data at 1.6 and 4.8GHz with Very Long Baseline Array (VLBA) data at higher frequencies and with similar resolutions to study the spectral properties of the core of M 87 with milliarcsec resolution. The VSOP data allows a more accurate measurement of the turnover frequency, and hence more reliable determination of associated physical parameters of the source. Comparison of the images with previously published images yields no evidence for significant motions of components in the parsec-scale jet. In addition, the brightness temperatures obtained from modelfits to the core are well below the inverse Compton limit, suggesting that the radio emission we are observing is not strongly Doppler boosted.