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1995 STATISTICS Front of Card: VLBA Image of the Radio Jet at the Center of the Galaxy 3C120

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1995 STATISTICS Front of Card: VLBA Image of the Radio Jet at the Center of the Galaxy 3C120 NATIONAL RADIO S U M M A R Y ASTRONOMY OBSERVATORY 1995 STATISTICS Front of card: VLBA image of the radio jet at the center of the galaxy 3C120. Even at the great distance of this galaxy, 400 million light years from the Earth, the great resolving power of the VLBA makes it possible to image details as small as 1.5 light-years in size within the jet structure which is itself 100 light-years long. "Time-lapse" images made over several years show the jet to be moving outward at an apparent speed faster than the speed of light. Observers: R.C. Walker and J.M. Benson NATIONAL RADIO ASTRONOMY OBSERVATORY Observing Summary 1995 Statistics February 1996 SOME HIGHLIGHTS OF THE 1995 RESEARCH PROGRAM A second Superlmninal Source in the Milky Way has been tracked by the VLBA/VLA combination. The X-ray nova GRO J1655-40, tracked as a target of opportunity during a radio outburst, has been found to exhibit superlmninal motion in radio jets. The two highly collimated relativistic jets, one on each side of the source, expand and decay over a few days. The jet ejection, at 92 percent the speed of light, appears episodic and asymmetric; the alternate brightening and fading of the jets cannot be explained by relativistic beaming. The jets exhibit "wiggles" with a period of about three days. Optical observers have discovered that the system is an eclipsing spectroscopic binary, with an orbital period of 2.62 days for the secondary, within the uncertainty of the 3-day period seen in the radio-jet *<wiggles." These results establish the geometry of the system and the mass of the invisible primary component as between 4 and 5.2 solar masses-above the stable limit of a neutron star. These results thus confirm the suspicion that the primary is a black hole. The fortuitous wealth of known parameters also makes this system, at a distance of only 3.2 kpc, the best known system for studying relativistic jets from accretion disks. A northem-sky survey on the 140 Foot Telescope for millisecond pulsars has resulted in the detection of a pulsar in a binary system (Jl 518+4904). The pulsar has a period of 41 ms and is in an eccentric 8.6 day orbit around its companion, probably a second neutron star. Because neutron stars can be treated as point masses, tight double-neutron-star binaries such as this provide unique laboratories for measuring phenomena of relativistic gravity. Measures of the relativistic precession of the J1518+4904 orbit (0.011 deg/yr) indicate that the summed mass of the pulsar and its companion is 2.62 ± 0.7 solar masses. While the distribution of mass between the pulsar and its companion cannot be determined unambiguously, dynamical considerations place a firm upper limit of 1.8 solar masses on the pulsar mass and constrain the companion mass to be between 0.9 and 2.7 solar masses. Both neutron stars appear to have masses near the Chandrasekhar mass of 1.35 solar masses. This system is only the fifth known relativistic binary system. VLBA images of HjO and SiO masers in the envelopes of evolved M-type giant stars have been made over four epochs covering a three-year period. The SiO masers lie in well-defined thin rings at 2-4 stellar radii from the stars and persist over the three years but vary in size and shape. In TX Cam the rings vary from 28 mas to 42 mas in diameter, with some evidence for two minima, suggesting that the environment close to this pulsating star changes dramatically at different points through its 557-day cycle. The most recent epoch was observed in full polarization mode and shows the presence of highly ordered linearly polarized structure in the maser rings. H20 masers observed around five M-type giant and supergiant stars he systemmatically outside the SiO masers, in accordance with astrochemical predictions. The H2O masers move away from the central star in the expected manner, and the transverse velocities are about as expected from the outflow velocity measured with single dishes, allowing distances to be determined by the cluster-parallax method to within ten percent accuracy. MgCN, a new metal-bearing free radical, has been identified in the circumstellar envelope of the evolved carbon star IRC 10216. The NRAO 12 Meter Telescope and the ERAM 30-m telescope were used. Metals such as magnesium, iron, and sodium must form in supemovae and massive stars, and are known to be abundant in elemental form but not in molecular form in the interstellar gas. The mechanism by which metals leave stars, enter the interstellar medium, and eventually reappear in new stars is poorly understood. It is likely that most of the metals are condensed onto dust grains. The emission from MgCN appears to arise from the outer envelope of IRC 10216, suggesting that MgCN (or Mg) probably enters the interstellar medium in the gas phase. MgCN is the metastable isomer of the earlier-detected species MgNC. An abundance ratio [MgNC]/[MgCN] of 22 is determined. The relatively high abundance of MgCN implies that it is formed by non-equilibrium chemistry. A sequence of VLBI images has revealed expansion of the supernova SN1993J in M81. Using the VLA, the VLBA, and European telescopes, an international VLBI team has made a high-resolution sequence of images that shows the expansion over a one-year period. While the expansion is symmetrical, the radio emission is clearly stronger on one side of the shell. So far, no protrusions such as those seen resulting from instabilities in older supernova events have developed. The images also show that the debris shell has not begun decelerating due to interaction with circumstellar material. The angular expansion rate measured with the VLBI images, combined with expansion speed measured by optical spectroscopy, will provide a refined value for the distance to M81, about 11 million light-years away. Observation with the 140 Foot Telescope of OH emission and absorption spectra toward a sample of compact extragalactic continuum sources show that there are some OH emission features which have no counterpart in either OH absorption or in the spectra of other molecules like CO, HCO+, and HCN. The OH emission lines, however, match the deepest HI absorption components. This implies that interstellar OH must be widespread and must exist in clouds that are mainly atomic and have little H2 or other molecular species. Observations have revealed a collapsing envelope around a protostar. The VLA and the DSN 70-meter telescope were used to make 22 GHz spectral line maps of the molecule CCS toward the core of B335, a young protostellar region. The resulting high-resolution channel maps image the collapsing envelope around the protostar. The velocity structure supports the evidence for inside-out collapse and the high-velocity features are consistent with accretion onto a rotating central disk. The CCS emission is asymmetric and clumpy, implying that the physical conditions of the region are not spherically symmetric and that the infall of gas onto the circumstellar disk may be episodic. OBSERVING HOURS est. est. est. 30 o 20 O 1983 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 Calendar Year 140 Foot Y7A 12 Meter ^ VIA I I VISA Fig. 1. This figure shows the hours scheduled for observing on each telescope during the last decade. Includes astronomical observing, testings, and calibration DISTRIBUTION OF SCHEDULED OBSERVING TIME 12 Meter 140 Foot VLA VLBA o 1—I 3 o mm m illw 1986 87 88 89 90 91 92 93 94 95 1986 87 88 89 90 91 92 93 94 95 1986 87 88 89 90 91 92 93 94 95 1993 94 95 Calendar Year Calendar Year Calendar Year Calendar Year NRAO Staff E55I Visitors Testing and Calibration Fig. 2. These graphs show the number of hours scheduled for testing and calibration, and for observing by the NRAO staff and by visitors on each telescope system. 12 METER RADIO TELESCOPE SUMMARY 100 - A - ^ * j i > \ •\ 80 A r N ■v r S rf 1 A 4 A \ i / 1 t f / r - / g 60 \ A 1 \ A \ A / \ A \ h / 1 h / A ; \ A \ A / \ A / - [ & 40 V - V V \ V \ 20 i V \ V \ s. V \ u L V V V 0 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 Calendar Year Observing Installation, Maintenance and Calibration — — Equipment Failure, Weather and Interference Fig. 3. This summary for each quarter of the calendar year shows the percentage of time the telescope was scheduled for observing; for routine calibration, maintenance, and installation of new experiments; and the percentage of time lost due to equipment failure, bad weather, and radio interference. The telescope is removed from service for a period of four to six weeks each summer during the wet season. This period is used for maintenance and upgrading of the instrument 140 FOOT RADIO TELESCOPE SUMMARY 100 - -> /* ■> V 'V ^ ^ ^ ^ ^ 80 w __i :\...^ * ^V- !^ ^ ^5 S* ^ f - C - V - 3 60 - - £ 40 A - A - -o ^v >- A. •s 20 y \ ^^ X r \ V V * ^ a £t B ► < ■ ■»■ ^ •^ .- i ,-«« -^ > ^.— 3<; .^ ^^ ■ ■■ .-, .-••*" Calendar Year Observing Installation, Maintenance and Calibration — — Equipment Failure, Weather and Interference Fig. 4. This summary for each quarter of the calendar year shows the percentage of time the telescope was scheduled for observing; for routine calibration, maintenance, and installation of new experiments; and the percentage of time lost due to equipment failure, bad weather, and radio interference. Major improvements to the telescope system include holographic surface tests and panel readjustments in 1987.
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