P. C. Gregory

The separation of synchrotron and bremsstrahlung radio emission in spiral galaxies

Spectra consisting of both bremsstrahlung and synchrotron components are fit to the observed radio-continuum spectra of 41 spiral galaxies. The detection of the bremsstrahlung component in 11 galaxies is reported. Very low upper limits are placed on another 20. For the remaining 11, model spectra cannot be fit with statistical significance. It is found that the thermal fraction (ratio of bremsstrahlung to total radio emission) spans at least two orders of magnitude among the 31 galaxies with good fits. The spectral index of the synchrotron component is found to vary over a larger range than previously suggested. A possible trend in the thermal fraction with the van den Bergh luminosity class is reported. The dependence is discussed in the context of cosmic-ray production, star formation, and theories of spiral structure, and it is concluded that, if the correlation is real, a three-way relationship may exist. 31 references.

Radio observations and analysis of Nova V1500 Cygni

We present the results and analysis of radio observations of Nova V1500 Cygni (1975). The data are consistent with thermal bremmstrahlung from an expanding cloud of ionized gas. Our data, combined with curves published by Hjellming (1977), are fit to thick shell models with density laws of the form n/sub e/=A/r/sup n/. The models require a large velocity dispersion with the outer shell boundary having V=3600 D km s/sup -1/, where D is the distance to the nova in kpc. The radio data therefore indicate the presence of a high velocity component not readily detectable in the optical emission line data, although it is detectable in the prenebular absorption line data. By combining the radio data with the infrared data of Ennis et al. (1977), we show that the ejected shell must have been ionization bounded during the first several weeks after the outburst, and density bounded thereafter. The mass and kinetic energy in the shell for a distance of 1.4 kpc are approx.1.4 x 10/sup -4/ M/sub sun/ and 1.1 x 10/sup 46/ erg, respectively.

Radio and optical observations of SS 433

We present the results of radio and limited optical spectroscopic observations of the peculiar emission variable star SS 433, which appears to be associated with the X-ray source A 1909 + 04 and the supernova remnant W50. The radio source is nonthermal and shows clear evidence for variability on a time scale as short as a few hours. Neither linear nor circular polarization was detected. The optical spectra indicate intense Balmer and other lines superimposed on a red continuum, with evidence for variability in the H..beta.. profile. The significance of these observations is briefly discussed.

VLA structure of variable radio sources in the Galactic plane

VLA observations of 17 new variable and three nonvariable radio sources from the Galactic Patrol Survey of Gregory and Taylor (1981) are presented. The variable-source morphologies range from unresolved point sources to doubles and triples with jet-lobe structures. No optical counterparts were found for the variable sources to the plate limits of the Palomar Sky Survey, but a 15th-magnitude stellar-like object was found for the nonvariable source GT 0007 + 635. The new observations are combined with previous data to study the collective properties of 28 variable sources. Their spectra, morphologies, and variability characteristics suggest that they fall into two broad categories. The first category consists of 15 unresolved point sources which are characterized by flat inverted spectra between 6 and 20 cm and strong variability. The second category consists of a heterogeneous mixture of resolved sources having varying degrees of structure. The bimodal distribution of variability properties of the patrol sources, reported by Gregory and Taylor, can, in part, be explained by these results. The presence or absence of nonvariable extended emission, resolved in the VLA observations but not in the Patrol observations, appears to regulate the variabilities of these sources. 13 references.

2-EPOCH VLA OBSERVATIONS OF VARIABLE GALACTIC PLANE RADIO-SOURCES

Two-epoch VLA observations of seven variable radio sources from the galactic plane survey of Gregory and Taylor (1981) are presented. Three of the sources are found to be highly variable. They exhibit inverted spectra between 6 and 20 cm and are unresolved to 0.07-0.1 arcsec. Four other sources have morphologies reminiscent of jets and lobes and exhibit no structural changes between the two epochs. All sources contain at least one compact component, and in the case of the double and triple sources comparisons with survey data suggest that these undergo transient and dramatic changes in flux. The object GT 2203 + 559 presents the exciting possibility of another Sco X-1 candidate. Two sources have close optical counterparts. 12 references.