Institute for Particle, Nuclear and Astronomical Sciences (IPNAS)
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Browsing Institute for Particle, Nuclear and Astronomical Sciences (IPNAS) by Author "Dennison, B."
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- The Contribution Of Galactic Free-Free Emission to Anisotropies in the Cosmic Microwave Background Found by the Saskatoon ExperimentSimonetti, John H.; Dennison, B.; Topasna, G. A. (IOP PUBLISHING LTD, 1996-02)We made a sensitive, wide-field Ha image of the north celestial polar region. Using this image, we constrain the contribution of irregularities in interstellar free-free emission to the degree-scale anisotropies in the cosmic microwave background detected in recent observations at Saskatoon by the Princeton group. The analysis of the Her image mimics the Saskatoon data analysis: the resulting signal is the strength of irregularities sampled with the Saskatoon beam (i.e., degree-scale) along the 85 degrees declination circle. We found no such irregularities that could be attributed to Her emission. The implied upper bound on the rms variation in free-free brightness temperature is less than 4.6 mu K at 27.5 GHz. The observed cosmic microwave background anisotropies are much larger. Therefore, the contribution of irregularities in interstellar free-free emission to the observed anisotropies is negligible.
- Detection in Hα of a Supershell Associated with W4Dennison, B.; Topasna, G. A.; Simonetti, John H. (IOP PUBLISHING LTD, 1997-01)From H I observations, Normandeau, Taylor, & Dewdney have identified a possible Galactic chimney emanating from W4. We observed a 10 degrees diameter field centered on this region in the Her line using a CCD camera sensitive to faint extended emission. Our image shows an apparent shell of H II, which we interpret as the ionized inner wall of a superbubble produced by stellar winds from the very young star cluster OCl 352. An analysis of the ionization balance indicates that much of the Lyman continuum radiation from the star cluster is absorbed and does not escape from the disk The shell appears to close 6 degrees (or about 230 pc) above the star cluster, and at a Galactic latitude of 7 degrees. The shell is quite elongated, with its major axis approximately perpendicular to the Galactic plane, as predicted for a superbubble formed in a stratified Galactic disk. The large size of the shell leads to an estimated age between 6.4 and 9.6 Myr, which exceeds that of OCl 352 (less than or similar to 2.5 Myr). The reason for this discrepancy is unclear, although it is possible that an earlier epoch of stellar outflow has contributed to the growth of the W4 superbubble.
- The Extreme Scattering Event toward PKS 1741-038: VLBI ImagesLazio, T. J. W.; Fey, A. L.; Dennison, B.; Mantovani, F.; Simonetti, John H.; Alberdi, A.; Foley, A. R.; Fiedler, R.; Garrett, M. A.; Hirabayashi, H.; Jauncey, D. L.; Johnston, K. J.; Marcaide, J.; Migenes, V.; Nicolson, G. D.; Venturi, T. (IOP PUBLISHING LTD, 2000-05)We report multiepoch VLBI observations of the source PKS 1741-038 (OT 068) as it underwent an extreme scattering event (ESE). Observations at four epochs were obtained, and images were produced at three of these. One of these three epochs was when the source was near the minimum flux density of the ESE, the other two were as the flux density of the source was returning to its nominal value. The fourth epoch was at the maximum flux density during the egress from the ESE, but the VLBI observations had too few stations to produce an image. During the event the source consisted of a dominant, compact component, essentially identical to the structure seen outside the event. However, the source's diameter increased slightly at 13 cm, from near 0.6 mas outside the ESE to near 1 mas during the ESE. An increase in the source's diameter is inconsistent with a simple refractive model in which a smooth refractive lens drifted across the line of sight to PKS 1741-038. We also see no evidence for ESE-induced substructure within the source or the formation of multiple images, as would occur in a strongly refractive lens. However, a model in which the decrease in flux density during the ESE occurs solely because of stochastic broadening within the lens requires a larger broadening diameter during the event than is observed. Thus, the ESE toward PKS 1741-038 involved both stochastic broadening and refractive defocusing within the lens. If the structure responsible for the ESE has a size of order I AU, the level of scattering within an ESE lens may be a factor of 10(7) larger than that in the ambient medium. A filamentary structure could reduce the difference between the strength of scattering in the lens and ambient medium, but there is no evidence for a refractively induced elongation of the source. We conclude that, if ESEs arise from filamentary structures, they occur when the filamentary structures are seen lengthwise. We are able to predict the amount of pulse broadening that would result from a comparable lens passing in front of a pulsar. The pulse broadening would be no more than 1.1 mu s, consistent with the lack of pulse broadening detected during ESEs toward the pulsars PSR B1937+21 and PSR J1643-1224. The line of sight toward PKS 1741-038 is consistent with a turbulent origin for the structures responsible for ESEs. The source PKS 1741-038 lies near the radio Loop I and is seen through a local minimum in 100 mu m emission.
- Galactic Foregrounds in Owens Valley Radio Observatory and UCSB South Pole 1994 Cosmic Microwave Background Anisotropy DataMukherjee, P.; Dennison, B.; Ratra, B.; Simonetti, John H.; Ganga, K.; Hamilton, J. C. (IOP PUBLISHING LTD, 2002-11)We study Galactic emission foreground contamination of the Owens Valley Radio Observatory and UCSB South Pole 1994 cosmic microwave background anisotropy data by cross-correlating with templates of infrared dust emission and new high-resolution Virginia Tech Spectral Line Survey (VTSS) and Southern Halpha Sky Survey Atlas (SHASSA) Halpha data. Halpha data provide rough upper limits on the level of free-free emission in the data sets. The cross-correlation analysis does not contradict a two-component foreground emission hypothesis, with the two dust-correlated components being free-free emission and spinning-dust emission.