S2 Pulsar Baseband Processing System
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ultrafast signal flucturations pulse timing. S2 da↗
Engineering topics
Bartel, N.
Publications and source records attributed to Bartel, N..
Position and morphology of the compact non-thermal radio source at the Galactic Center
We have determined with VLBI the position of the compact nonthermal radio source at the Galactic Center, commonly referred to as SgrA*, in the J2000.0 reference frame of extragalactic radio sources. We have also determined the size of SgrA* at 1.3, 3.6, and 13 cm wavelengths and found that the apparent size of the source increases proportionally to the observing wavelength squared, as expected from source size broadening by interstellar scattering and as reported previously by other authors. We have also established an upper limit of about 8 mJy at 3.6 cm wavelength for any ultracompact component. The actual size of the source is less than 15 AU. Fourier analysis of our very sensitive 3.6 cm observations of this source shows no significant variations of correlated flux density on time scales from 12 to 700 s.
Marcaide, J. M.↗
A high-resolution radio image of a young supernova
A VLBI radio images of the bright supernova 1986J, which occurred in the galaxy NGC891 at a distance of about 12 Mpc, is presented. No detailed image of any supernova or remnant has been obtained before so soon after the explosion. The image shows a shell of emission with jetlike protrusions. Analysis of the images should advance understanding of the dynamics of the expanding debris, the dissipation of energy into the surrounding circumstellar medium, and the evolution of the supernova into the remnant.
Bartel, N.↗
Supernova 1987A - A radiosphere resolved with VLBI five days after the neutrino burst
The results of VLBI observations of SN1987A are reported. No emission from the supernova above a level of about 20 percent of the supernova's total flux density was detected, although signals were detected from two calibrator sources with amplitudes roughly equal to those determined in earlier VLBI observations. It is inferred that the supernova's radiosphere was resolved, and a lower bound on the radiosphere's radius of 2.2 mas is estimated from an epoch 5.2 days after the neutrino burst. Given the photometric data from the supernova, a distance to the LMC of 50 + or - 5 kpc, and an apparent expansion velocity that varied systematically with time from 18,000-16,000 km/s, as estimated from the blue-shifted H-alpha absorption lines on the days preceding the observations, it is concluded that 5.2 days after the neutrino burst the supernova's radiosphere was at least 2.5 times larger than the inferred blackbody photosphere, and at least as large as the H-alpha line-forming region.
Jauncey, D. L.↗
Second epoch of simultaneous lambda 3.6 and lambda 13 cm observations of the pair of quasars 1038+528 A,B
It is found that the quasar 1038+528 B is a good reference for studying the internal motions in quasar 1038+528 A. The apparent superluminal motion of a feature in quasar A takes place with respect to a neary stationary core; its proper motion is bounded from above by about 40 micro-as/yr. The wavelength dependence of the location of the core brightness peak of A is confirmed.
Marcaide, J. M.↗
VLBI observations of Supernova 1987 A
VLBI observations at 2.3 GHz of SN1987 A on February 28, 1987 yielded no fringes, implying, for an optically thin shell, a lower bound on the (outer) diameter of 1.9 mas. From the comparison of the VLBI and optical results, it is inferred that the radiosphere of SN1987 A was either about equal to, or larger than, the photosphere of the supernova five days after the explosion.
Shapiro, I.↗
VLBI observations of the 0957 + 561 gravitational lens system
A series of VLBI observations of the gravitational lens system 0957 + 561 at a wavelength of 13 cm has yielded the positions of the A and B images, the relative magnification of their largest discernible radio structures, and the time variability of their smallest discernible radio structures. These observations have also allowed upper limits to be placed on the flux density of an expected third image. The positions and relative magnification of the A and B images provide new information with which to constrain models of the lens that forms the images. The detection of variations in the flux densities of the cores of A and B suggests that observations at shorter wavelengths may reveal superluminal motion, which may in turn provide a means to measure the relative time delay.
Gorenstein, M. V.↗
Radio observations of SN 1987A
The study of radio emission from SN 1987A after the neutrino burst is reviewed. The observed maximum flux density of the burst at radio wavelengths was about 140 mJy and occurred at 1.4 GHz. The radio light curves for SN 1987A at four frequencies are compared with data for SN 1979C and SN 1980K. VLBI observations of SN 1987A at 2.3 GHz and other radio monitoring programs are examined.
Bartel, N.↗
VLBI limits on the proper motion of the 'core' of the superluminal quasar 3C345
VLBI (very-long-baseline interferometry) observations between 1971 and 1983 have been used to determine the positions of the 'core' of the quasar 3C345 relative to the more distant compact quasar NRAO512 with a fractional uncertainty as small as two parts in a hundred million. The core of 3C345 appears stationary in right ascension to within 20 arc microsec/yr, a subluminal bound corresponding to 0.7c. The apparent velocities of the jets are superluminal, up to 14c in magnitude.
Bartel, N.↗
VLBI observations of the compact components in M82
VLBI observations at 2.3 and 8.4 GHz of 15 compact components in M82, obtained on May 19, 1983, are examined. Angular sizes and linear sizes of the compact sources in M82 are estimated; it is detected that most of the compact sources studied have linear sizes in excess of 0.3 pc. The brightness distribution of the strongest of the 15 compact sources, 41.9 + 58, is investigated; two dominant, elongated components oriented at an angle of about 20 deg and having maximum dimensions of 10-30 mas are observed.
Bartel, N.↗
Quasar 4C39.25 is not contracting
A map has been made of the quasar 4C39.25, which has been described as consisting of two components whose angular separation remains constant at about 2 marc-sec, using VLBI observations made at 3.6 cm during 1983. From the map it is concluded that Shaffer's (1984) suggestion that this source may have been contracting superluminally during 1979-82 is not correct. Three distinct components are found in the quasar structure, two separated by 2.0 marc-sec and a third, presumably new and not previously reported, situated between the other two. It is possible either that the third component is stationary and that its flux density has rapidly increased to render it visible, or that it has recently been ejected from the westernmost component.
Marcaide, J. M.↗
Simultaneous dual-wavelength VLBI observations of the compact radio source near the Galactic center
Preliminary results from simultaneous VLBI observations at 3.6 and 13 cm confirm that, in this wavelength range, the size of the compact radio source near the Galactic center varies as lambda-squared, where lambda is the wavelength of observation. These results also establish a flux density upper limit at 3.6 cm of 5 mJy for any source in Sgr A West smaller than 8 mas (80 AU).
Marcaide, J. M.↗
The nucleus of M81 - Simultaneous 2.3 and 8.3 GHz Mark III VLBI observations
Mark III very long baseline interferometry (VLBI) observations have been made of the nucleus in the normal galaxy M81 (NGC 3031) simultaneously at 2.3 and 8.3 GHz, and it was found that nearly 100% of the flux density of the nuclear region originates in an elongated radio core with linear dimensions 1000-4000 AU, dependent on frequency. This galactic nucleus is the most compact observed in any extragalactic source. The position coincides with that of the nucleus at optical and X-ray wavelengths within the larger uncertainties of the latter two. The position angle of the major axis of the M81 nucleus is within 3 deg of 75 deg at 2.3 GHz and within 6 deg of 50 deg at 8.3 GHz and is frequency dependent at the 4 sigma level. These values straddle the 62 deg position angle of the projection on the sky of the rotation axis of the galaxy. The spectrum of the core is slightly inverted, and the length of its major axis decreases with frequency. These results are consistent with the emission's being incoherent electron synchrotron radiation from an inhomogeneous region with an optical depth varying along the rotation axis of the galaxy.
Bartel, N.↗