William A. Dent

I am studying the variability of radio emission from quasars and active galactic nuclei at millimeter and centimeter wavelengths. This research program has two components: observational and interpretational.

Flux density measurements of over 100 known variable sources are made at regular intervals using the 120 ft. Haystack antenna, the 12-meter NRAO antenna on Kitt Peak and the 14-meter FCRAO antenna. Because the work needs a precision of 1%, careful attention must be paid to the pointing accuracy and gain calibration of the antenna. From these measurements which range in frequency from 7.8 GHz to 90 GHz, I can observe the time evolution of the synchrotron radiation spectrum. In many objects a series of outbursts with self-absorbed spectra and duration of a few years are seen along with more slowly varying components.

With an accumulated database of over 20 years, my collaborators and I are now attempting to understand this phenomena and answer the following questions about quasi-stellar radio sources: do the radio outbursts occur periodically; if so, why? How are the radio outbursts correlated with outbursts seen in the optical and X-ray bands? How are the outbursts related to structural changes observed with milliarc second resolution radio interferometer? What does the outburst profile tell us about particle acceleration and energy loss mechanisms occuring in the source? The answers to these questions may help us to understand the origin of the enormous amounts of energy being released in the nuclei of active galaxies.

Recent Publications

  • "The 20 Year Evolution of the Radio Nucleus of NGC 1275," O'Dea, C.P., Dent, W.A., Balonek, T.J., Astrophys. J., 278, 89 (1984).

  • "Cassiopeia A at 86 GHz: Spectral and Rotation Measure Differences," Kenney, J.D. and Dent, W.A., Astrophys. J., 298, 644 (1985).

  • "Broadband Spectral Evolution of Outbursts in Extragalactic Radio Sources," O'Dea, C.P., Dent, W.A., Balonek, T.J., Proceedings of the Manchester Symposium on Active Galactic Nuclei, Reidel Press (1985).

  • "Multifrequency Observations of the Superluminal Quasar 3C345," Bregman, J.N. et al., Astrophys. J., 301, 708-726 (1986).

  • "Multifrequency Radio Observations of the Variable Quasars 0133+476, 0235+164, 1749+096 and 2131-021," O'Dea, C.P., Dent, W.A., Kinzel, W.M. and Balonek, T.J., Astrophys. J., 92 , 1262-1272 (1986).

  • "Multifrequency Observations of the Optically Violent Variable Quasar 3C446," Bregman, J.N. et al., Astrophys. J., 331, 746-763 (1988).

  • "Multifrequency Observations of BL Lacertae," Bregman, J.N. et al., Astrophys. J., 352, 574 (1990).

    dent@donald.phast.umass.edu

    Last Update: 5/23/1993

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