Engineering topics

Mitchell, K. J.

Publications and source records attributed to Mitchell, K. J..

10 records

NASA NTRS1994

Results of a five-year program of multifrequency monitoring of low-frequency variable radio sources

We present the results of a detailed multifrequency monitoring program of low-frequency variable radio sources. This consists of light curves at 318, 430, 606, 880, and 1400 MHz over a 5 year period. The observations were carried out with the Arecibo 305 m radiotelescope and the Green Bank 91 m radio telescope. The spectral characteristics of the variations confirm the general picture that at least two mechanisms are responsible. The first is clearly intrinsic evolution of synchrotron-emitting components as it is manifested by variations that appear first and most strongly at high frequencies, subsequently drifting to lower frequencies with diminished amplitude. The more common type of low-frequency variability, however, dominates at frequencies below approximately 800 MHz, while the variations near 1 GHz are often quite weak. This spectral property is strong evidence that these variations are interstellar refractive scintillation.

Mitchell, K. J.↗

NASA NTRS1992

Blue galaxies identified with submilliJansky radio sources in the 1300 + 3034 field

We have obtained deep B and R CCD frames in order to optically identify sub-mJy radio sources discovered in a deep 1.49 GHz VLA survey field centered at alpha (1950) = 13h00m37s, delta(1950) = + 30 deg 34 arcmin. B and R photometry is presented for 37 optical identifications. Using spectral evolution models, we conclude that galaxies brighter than B about 25 identified with sub-mJy radio sources are much bluer than giant elliptical galaxies, but are similar to starburst galaxies with absolute magnitude M(v) = about -23-20 and redshifts in the range 2 = about 0.1-0.8. This population of blue galaxies is the same as that found by Thuan and Condon (1987) in the 0852 + 1716 field using different techniques (optical and near-infrared photometry), suggesting that this population of starburst galaxies at intermediate redshifts is universal.

Thuan, Trinh X.↗

NASA NTRS1988

The size of the z = 0.437 H I absorption region toward the QSO 3C 196

VLBI spectral line observations made at the frequency of the z = 0.437 H I absorption line toward the extended radio QSO 3C 196 demonstrate that the absorption does not occur against the more compact continuum structure in this object. Apparently the H I absorption line results from absorption of the lower brightness continuum emission by an intervening cloud with an angular diameter which must be greater than 0.63 arcsec. For a standard cosmology, this corresponds to a linear extent greater than 2.25 kpc. For a spherical cloud, the mass of absorbing gas is near 7 x 10 to the 7th solar masses. Thus this particular QSO absorbing cloud is surprisingly large and massive.

Brown, R. L.↗

NASA NTRS1988

Radio variability of the blazar AO 0235 + 164

The high-redshift blazar A0 0235 + 164 exhibits flux-density variations which are primarily of the less common variety in which low-frequency flux-density variations track the high-frequency variations but are delayed and of smaller amplitude. Observational results based on five years of monitoring are presented which are correlated over at least a factor of 50 frequency range in the sense expected for an expanding synchrotron component: outbursts propagating toward lower frequencies with diminishing amplitudes. A simple, semiempirical jet model is developed which accounts reasonably well for the radio properties of the object. The predictions of the model are compared with observations, examining the radio flux-density histories, the radio spectral evolution, the radio structure, and evidence for relativistic bulk motion.

O'Dell, S. L.↗

NASA NTRS1987

The U.S. survey for faint blue objects

A spectrophotometric study of the blue and UV-excess starlike objects in the U.S. survey (Usher, 1981) has been conducted. Observations were obtained with a resolution of about 20 A over the 3500-7000-A wavelength range. Considered within the sample are 42 DA white dwarfs, 4 DB/DO white dwarfs, 13 subdwarf B stars, 12 subdwarf O stars, and 13 horizontal branch stars. The sample is analyzed using numerical convolution photometry.

Mitchell, K. J.↗

NASA NTRS1986

Confining hot spots in 3C 196 - Implications for QSO-companion galaxies

VLBI observations of the extremely compact hot spot in the northern radio lobe of the QSO 3C 196 reveal the angular size of its smallest substructure to be 0.065 arcsec x 0.045 arcsec or about 300 pc at the redshift distance. The morphology of the hot spot and its orientation relative to the more diffuse radio emission suggest that it is formed by an oblique interaction between the nuclear QSO jet and circum-QSO cloud. The inferred density in this cloud, together with its apparent size, imply that the cloud contains a galactic mass, greater than a billion solar masses of gas. The effect of the jet will be to hasten gravitational collapse of the cloud. If many QSOs such as 3C 196 are formed or found in gas-rich environments, the QSO radio phase may commonly stimulate the metamorphosis of circum-QSO gas to QSO-companion galaxies or it may play a significant part in catalyzing star formation in existing companions.

Brown, R. L.↗

NASA NTRS1984

Multifrequency light curves of low-frequency variable radio sources

Light curves for the low-frequency variable sources AO 0235 + 16, NRAO 140, PKS 1117 + 14, DA 406, CTA 102, and 3C 454.3, obtained in monthly observations at 318, 430, and 606 MHz using the 305-m telescope at Arecibo and in bimonthly observations at 880 MHz and 1.4 GHz using the 91-m Green Bank transit telescope during 1980-1983, are presented and analyzed. AO 0235 + 16 is found to have basically canonical variability which is attributed to relativistically moving evolving synchrotron components; but in the other sources, strong simultaneous variations at 318, 430, and 606 MHz are observed to be greatly diminished in amplitude at 880 MHz and 1.4 GHz, confirming the existence of the intermediate-frequency gap at about 1 GHz proposed by Spangler and Cotton (1981). The possibility that a second variability mechanism is active in these sources is explored.

Altschuler, D. R.↗

NASA NTRS1984

A medium-bright quasar sample - New quasar surface densities in the magnitude range from 16.4 to 17.65 for B

A new medium-bright quasar sample (MBQS) is constructed from spectroscopic observations of 140 bright objects selected for varying degrees of blue and ultraviolet excess (B-UVX) in five Palomar 1.2 m Schmidt fields. The MBQS contains 32 quasars with B less than 17.65 mag. The new integral surface densities in the B range from 16.45 to 17.65 mag are approximately 40 percent (or more) higher than expected. The MBQS and its redshift distribution increase the area of the Hubble diagram covered by complete samples of quasars. The general spectroscopic results indicate that the three-color classification process used to catalog the spectroscopic candidates (1) has efficiently separated the intrinsically B-UVX stellar objects from the Population II subdwarfs and (2) has produced samples of B-UVX objects which are more complete than samples selected by (U - B) color alone.

Mitchell, K. J.↗

NASA NTRS1984

The spectral evolution of low-frequency variable radio sources

The dynamic spectra of several low frequency extragalactic radio sources are presented. The observations were made at 318, 430, 606, 880, and 1400 MHz at several different radio observatories around the U.S. Two outbursts were observed in AO 0235 + 16 at 1.4 GHz, followed by a diminished variation at the lower frequencies. The dynamic frequencies of NRAO 140, PKS 1117 + 14, DA 406, CTA 102, and 3C 454.3 do not fit the same pattern. These radio sources displayed the following characteristics: (1) departure from straight or curved spectra at the frequencies of variation; (2) no obvious frequency drifting; and (3) negligible variation at 1.4 GHz. Possible explanations for this behavior are briefly discussed.

Dennison, B.↗

NASA NTRS1983

Spectroscopy of blue stellar objects

Spectra have been obtained for the brightest objects from a list of blue stellar objects found in a Palomar Schmidt field centered on Kapteyn Selected Area 28. Four of the objects presented here comprise a complete sample of objects with UV excess and magnitudes brighter than or equal to B = 16.3 mag. The object with the largest UV excess is a previously undiscovered quasar of redshift 0.25 and cataloged B magnitude of 15.6 mag. The object shows some evidence of variability. Spectroscopy for one bright object in a companion field centered on Selected Area 29 is also presented.

Mitchell, K. J.↗