Richard A. Sramek

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Proper motions and radial velocities of luminous infrared stars in the galactic center have provided strong evidence for a dark mass of 2.5 × 10 M⊙ in the central 0.05 pc of the galaxy. The leading hypothesis for this mass is a black hole. High angular resolution measurements at radio wavelengths find a compact radio source, Sagittarius (Sgr) A, that is(More)
We present our extensive observations of the radio emission from supernova (SN) 1993J, in M81 (NGC 3031), made with the Very Large Array, at 90, 20, 6, 3.6, 2, 1.2, and 0.7 cm, as well as numerous measurements from other telescopes and at other wavelengths. The combined data set constitutes probably the most detailed set of measurements ever established for(More)
New observations of SN 1980K made with the VLA at 20 and 6 cm from 1994 April through 1996 October show that the supernova (SN) has undergone a significant change in its radio emission evolution, dropping by a factor of ∼ 2 below the flux density S ∝ t power-law decline with time t observed earlier. However, although S at all observed frequencies has(More)
We present new radio observations of the supernova SN 1979C made with the VLA at 20, 6, 3.6, and 2 cm from 1991 July to 1998 October, which extend our previously published observations (Weiler et al. 1986, 1991), beginning 8 days after optical maximum in 1979 April and continuing through 1990 December. We find that the radio emission from SN 1979C has(More)
We present and discuss the radio observations of 27 Type Ia supernovae (SNe Ia) observed over two decades with the Very Large Array. No SN Ia has been detected so far in the radio, implying a very low density for any possible circumstellar material established by the progenitor, or progenitor system, before explosion. We derive 2σ upper limits to a steady(More)
Observations of the Type II-P (plateau) Supernova (SN) 1999em and Type IIn (narrow emission line) SN 1998S have enabled estimation of the profile of the SN ejecta, the structure of the circumstellar medium (CSM) established by the pre-SN stellar wind, and the nature of the shock interaction. SN 1999em is the first Type II-P detected at both X-ray and radio(More)
The low-luminosity active galactic nucleus of M81 has been monitored at centimeter wavelengths since early 1993 as a by-product of radio programs to study the radio emission from Supernova 1993J. The extensive data sets reveal that the nucleus experienced several radio outbursts during the monitoring period. At 2 and 3.6 cm, the main outburst occurred(More)
We report here on multi-frequency VLBA observations of three extragalactic sources within 1 degree of the Galactic Center. These sources have been used as astrometric reference sources for VLA and VLBA determinations of the proper motion of Sagittarius A*, the compact nonthermal radio source in the Galactic Center. Each source has a main component with a(More)
Long-term monitoring of the radio emission from supernovae with the Very Large Array (VLA) shows that the radio “light curves” evolve in a systematic fashion with a distinct peak flux density (and thus, in combination with a distance, a peak spectral luminosity) at each frequency and a well-defined time from explosion to that peak. Studying these two(More)
We analyze 3. ′′5 resolution, high sensitivity radio continuum images of the nearby spiral galaxy NGC 4258 at 6 and 20 cm derived from multiple observations used to monitor the radio supernova SN 1981K (Van Dyk et al. 1992, [ApJ, 396, 1995]). Seven bright H II region and five supernova remnant candidates are identified. Extinctions to the H II regions are(More)