H. Kobayashi

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.

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.

Space VLBI Observations of Southern Hemisphere Gamma-Ray and Non-Gamma-Ray AGN: First Results for PKS 0637-752

Abstract We present the first results from our VSOP proposal, “γ-ray loud and quiet AGN with VSOP and SHEVE at 5 GHz”. This project is aimed at determining what, if anything, differentiates γ-ray-identified radio sources from similar strong, flat-spectrum radio sources which have not been identified as sources of greater than 100 MeV γ-rays. Five VSOP observations have been successfully completed as part of this project during VSOP AO1. One observation has been correlated thus far, a 4.970 GHz observation of PKS 0637-752 from 1997 November 21. We present the VSOP image for PKS 0637-752 and make comparisons between the space VLBI image and ground-based VLBI and Australia Telescope Compact Array (ATCA) images, as well as present ATCA flux density monitoring at 4.8 GHz. A brightness temperature for PKS 0637-752 of 0.7 × 1012 K has been determined from the VSOP data and a tentative apparent superluminal speed, from comparison to the ground-based VLBI image, of ∼9c.

Imaging capability of the Mitaka VSOP correlator

Abstract We have made a comparison between data between the Mitaka and Penticton correlators, using VSOP observations of NRAO530 at 1.6 GHz. After calibrating the data with AIPS we find excellent agreement with the closure phase between the data processed at the two correlators. The images made from both data sets also agree well. We have made other VSOP images of OQ208 at 1.6 GHz, and 3C395, J2011-15 at 5 GHz from data correlated by the Mitaka correlator. These images are all consistent with ground-only VLBI results at higher frequencies, and demonstrate the capability of the Mitaka correlator as a VSOP correlator.

HALCA onboard VLBI observing system performance

Abstract The space VLBI satellite HALCA was launched on February 12, 1997. It is the first Radio astronomy satellite. We report the performance of onboard radio astronomy system. Typical system noise temperatures are 70K and 90K at L and C band respectively. For K band, the apparent system noise temperature is 400 K. It is caused by the attenuation between the main antenna and the K band LNA. Bandpass characteristics of each observing band are almost the same as the ground test. The phase stability of local oscillators are around 5 degrees r.m.s. at C band. And the total gain of the receiving system and the bit distribution of high rate samplers are checked. Basically the in-orbit performance of the VLBI observing system matches the pre-launch ground test results.

VSOP monitoring of the quasar 1928+738

Abstract In the paper we describe the first results from a campaign to monitor the relatively low redshift (z=0.3) circumpolar superluminal quasar 1928+738 with VSOP. The three epochs of data that we have analyzed show that there have been substantial structural changes in this source near the core on the time-scale of a few months.

VSOP observations of BL Lacertae at 5 GHz

Abstract We have made VSOP observations of BL Lacertae (2200+420) using HALCA and the VLBA on 8 December 1997 at a resolution of 0.6 × 0.3 milliarcsec. The source structures are identified with the core and superluminal jet-components that are seen in VLBA images at 43 GHz from 1996 (Denn and Mutel 1998). Extended structure out to 20 milliarcsec from the core has been detected.

Halca VLBI amplitude calibration performance

Abstract The amplitude calibration properties of the HALCA VLBI system are summarized. The HALCA System Equivalent Flux Densities (SEFD) are typically ∼ 19500 Jy at L-band and ∼ 15800 Jy at C-band, and are stable between and during orbits.

Measuring the Size of the Vela Pulsar's Radio Emission Region

We describe measurements of the size of the Vela pulsar via scintillation, using both fits to the distribution of intensity and measurements of the modulation index. We briefly discuss systematic effects other than source size that can affect the distribution, including gain variations, self-noise, scintillation shot noise, and correlator saturation. Modulation index, a single number, can be biased by all of these, whereas the distribution of intensity is affected in different ways by different effects, providing means of distinguishing among them. Self-noise and gain variations are likely more important at long observing wavelengths, and correlator saturation and scintillation shot noise at short wavelengths. We find a size of about 500 km at decimeter wavelengths. Interestingly, this agrees with measurements of modulation index by Roberts & Ables at the same wavelength. Their results (and more recently that reported by Macquart et al.) suggest that size decreases with increasing wavelength. Although consistent with the observations, this conclusion is perhaps surprising from the standpoint of the traditional radius-to-frequency mapping. However, these measurements are of size rather than height; and of course systematic effects may play a role.

The first space VLBI image of 3C279

Abstract The first 5 GHz VSOP (VLBI Space Observatory Programme) observation of the quasar 3C279 was conducted on 10 January 1998 with the satellite HALCA and an array of ground radio telescopes. The high dynamic range image presented here reveals that the core and the secondary component ∼3 milli-arcseconds from the core dominate the compact radio emission, but that lower level features at intermediate distances from the core are also evident.