Goals of the ARISE Space VLBI Mission

Supermassive black holes, with masses of 10^6 to more than 10^9 solar masses, are among the most spectacular objects in the Universe, and are laboratories for physics in extreme conditions. The primary goal of ARISE (Advanced Radio Interferometry between Space and Earth) is to use the technique of Space VLBI to increase our understanding of black holes and their environments, by imaging the havoc produced in the near vicinity of the black holes by their enormous gravitational fields. The mission will be based on a 25-meter space-borne radio telescope operating at frequencies between 8 and 86 GHz, roughly equivalent to an orbiting element of the Very Long Baseline Array. In an elliptical orbit with an apogee height of 40,000-100,000 km, ARISE will provide resolution of 15 microarcseconds or better, 5-10 times better than that achievable on the ground. At frequencies of 43 and 86 GHz, the resolution of light weeks to light months in distant quasars will complement the gamma-ray and X-ray observations of high-energy photons, which come from the same regions near the massive black holes. At 22 GHz, ARISE will image the water maser disks in active galaxies more than 15 Mpc from Earth, probing accretion physics and giving accurate measurements of black-hole masses. ARISE also will study gravitational lenses at resolutions of tens of microarcseconds, yielding important information on the dark-matter distribution and on the possible existence of compact objects with masses of 10^3 to 10^6 solar masses.Comment: 6 pages, New Astronomy Reviews, Proceedings of 4th EVN/JIVE Symposium, LaTeX, document class elsart.cls, bibliography style natbib.st

International mission planning for space Very Long Baseline Interferometry

Two spacecraft dedicated to Very Long Baseline Interferometry (VLBI) will be launched in 1996 and 1997 to make observations using baselines between the space telescopes and many of the world's ground radio telescopes. The Japanese Institute of Space and Astronautical Science (ISAS) will launch VSOP (VLBI Space Observatory Program) in September 1996, while the Russian Astro Space Center (ASC) is scheduled to launch RadioAstron in 1997. Both spacecraft will observe radio sources at frequencies near 1.7, 4.8, and 22 GHz; RadioAstron will also observe at 0.33 GHz. The baselines between space and ground telescopes will provide 3-10 times the resolution available for ground VLBI at the same observing frequencies. Ground tracking stations on four continents will supply the required precise frequency reference to each spacecraft measure the two-way residual phase and Doppler on the ground-space link, and record 128 Megabit/s of VLBI data downlinked from the spacecraft. The spacecraft data are meaningless without cross-correlation against the data from Earth-bound telescopes, which must take place at special-purpose VLBI correlation facilities. Therefore, participation by most of the world's radio observatories is needed to achieve substantial science return from VSOP and RadioAstron. The collaboration of several major space agencies and the ground observatories, which generally follow very different models for allocation of observing time and for routine operations, leads to great complexity in mission planning and in day-to-day operations. This paper describes some of those complications and the strategies being developed to assure productive scientific missions

Radio Emission from the Intermediate-mass Black Hole in the Globular Cluster G1

We have used the Very Large Array (VLA) to search for radio emission from the globular cluster G1 (Mayall-II) in M31. G1 has been reported by Gebhardt et al. to contain an intermediate-mass black hole (IMBH) with a mass of ~2 x 10^4 solar masses. Radio emission was detected within an arcsecond of the cluster center with an 8.4 GHz power of 2 x 10^{15} W/Hz. The radio/X-ray ratio of G1 is a few hundred times higher than that expected for a high-mass X-ray binary in the cluster center, but is consistent with the expected value for accretion onto an IMBH with the reported mass. A pulsar wind nebula is also a possible candidate for the radio and X-ray emission from G1; future high-sensitivity VLBI observations might distinguish between this possibility and an IMBH. If the radio source is an IMBH, and similar accretion and outflow processes occur for hypothesized ~ 1000-solar-mass black holes in Milky Way globular clusters, they are within reach of the current VLA and should be detectable easily by the Expanded VLA when it comes on line in 2010.Comment: ApJ Letters, accepted, 11 pages, 1 figur

The Origin of Radio Emission in Low-Luminosity Active Galactic Nuclei: Jets, Accretion Flows, or Both?

The low-luminosity active galactic nuclei in NGC 3147, NGC 4203, and NGC 4579 have been imaged at four frequencies with the Very Long Baseline Array. The galaxies are unresolved at all frequencies, with size upper limits of $10^3-10^4$ times the Schwarzschild radii of their central massive black holes. The spectral indices between 1.7 and 5.0 GHz range from 0.2 to 0.4; one and possibly two of the galaxies show spectral turnovers between 5.0 and 8.4 GHz. The high brightness temperatures ($> 10^9$ K) and relatively straight spectra imply that free-free emission and/or absorption cannot account for the slightly inverted spectra. Although the radio properties of the cores superficially resemble predictions for advection-dominated accretion flows, the radio luminosities are too high compared to the X-ray luminosities. We suggest that the bulk of the radio emission is generated by a compact radio jet, which may coexist with a low radiative efficiency accretion flow.Comment: To appear in ApJ (Letters). 4 page

Blazar Counterparts for 3EG Sources at -40 < decl. < 0: Pushing South Through the Bulge

Supplementing existing survey data with VLA observations, we have extended $\gamma-$ray counterpart identifications down to decl. = -40$^\circ$ using our Figure of Merit approach. We find blazar counterparts for $\sim$ 70% of EGRET sources above decl. = -40$^\circ$ away from the Galaxy. Spectroscopic confirmation is in progress, and spectra for $\sim$ two dozen sources are presented here. We find evidence that increased exposure in the bulge region allowed EGRET to detect relatively faint blazars; a clear excess of non-blazar objects in this region however argues for an additional (new) source class.Comment: ApJ accepte

A Search for Active Galactic Nuclei in Sc Galaxies with H II Spectra

(Abridged) We have searched for nuclear radio emission from a statistically complete sample of 40 Sc galaxies within 30 Mpc that are optically classified as star-forming objects, in order to determine whether weak AGNs might be present. Only three nuclear radio sources were detected, in NGC 864, NGC 4123, and NGC 4535. These galaxies have peak 6-cm radio powers of 10^{20} W/Hz at arcsecond resolution, while upper limits of the non-detected galaxies typically range from 10^{18.4} to 10^{20} W/Hz. The three nuclear radio sources all are resolved and appear to have diffuse morphologies, with linear sizes of ~300 pc. This strongly indicates that circumnuclear star formation has been detected in these three H II galaxies. Comparison with previous 20-cm VLA results for the detected galaxies shows that the extended nuclear radio emission has a flat spectrum in two objects, and almost certainly is generated by thermal emission from gas ionized by young stars in the centers of those galaxies. The 6-cm radio powers are comparable to predictions for thermal emission that are based on the nuclear H-alpha luminosities, and imply nuclear star formation rates of 0.08-0.8 solar masses/yr, while the low-resolution NRAO VLA Sky Survey implies galaxy-wide star formation rates of 0.3-1.0 solar masses/yr in stars above 5 solar masses. Although the presence of active nuclei powered by massive black holes cannot be definitively ruled out, the present results suggest that they are likely to be rare in these late-type galaxies with H II spectra.Comment: To appear in ApJ. 7 page