skip to main content

Title: The spectral variability of the GHZ-Peaked spectrum radio source PKS 1718-649 and a comparison of absorption models

Using the new wideband capabilities of the ATCA, we obtain spectra for PKS 1718-649, a well-known gigahertz-peaked spectrum radio source. The observations, between approximately 1 and 10 GHz over 3 epochs spanning approximately 21 months, reveal variability both above the spectral peak at ∼3 GHz and below the peak. The combination of the low- and high-frequency variability cannot be easily explained using a single absorption mechanism, such as free–free absorption or synchrotron self-absorption. We find that the PKS 1718-649 spectrum and its variability are best explained by variations in the free–free optical depth on our line of sight to the radio source at low frequencies (below the spectral peak) and the adiabatic expansion of the radio source itself at high frequencies (above the spectral peak). The optical depth variations are found to be plausible when X-ray continuum absorption variability seen in samples of active galactic nuclei is considered. We find that the cause of the peaked spectrum in PKS 1718-649 is most likely due to free–free absorption. In agreement with previous studies, we find that the spectrum at each epoch of observation is best fit by a free–free absorption model characterized by a power-law distribution of free–free absorbing clouds. Thismore » agreement is extended to frequencies below the 1 GHz lower limit of the ATCA by considering new observations with Parkes at 725 MHz and 199 MHz observations with the newly operational Murchison Widefield Array. These lower frequency observations argue against families of absorption models (both free–free and synchrotron self-absorption) that are based on simple homogenous structures.« less
Authors:
; ; ; ;  [1] ; ;  [2] ; ; ;  [3] ; ;  [4] ; ;  [5] ;  [6] ;  [7] ; ;  [8] ;  [9] ;  [10] more »; « less
  1. International Centre for Radio Astronomy Research, Curtin University, Bentley, WA 6102 (Australia)
  2. University of Western Sydney, Locked Bag 1797, Penrith, NSW 2751 (Australia)
  3. CSIRO Astronomy and Space Science (CASS), PO Box 76, Epping, NSW 1710 (Australia)
  4. Sydney Institute for Astronomy, School of Physics (SiFA), The University of Sydney, NSW 2006 (Australia)
  5. Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611 (Australia)
  6. Square Kilometre Array South Africa (SKA SA), Cape Town 7405 (South Africa)
  7. School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287 (United States)
  8. MIT Haystack Observatory, Westford, MA 01886 (United States)
  9. Raman Research Institute, Bangalore 560080 (India)
  10. Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139 (United States)
Publication Date:
OSTI Identifier:
22342106
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astronomical Journal (New York, N.Y. Online); Journal Volume: 149; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; APPROXIMATIONS; COMPARATIVE EVALUATIONS; DISTRIBUTION; EXPANSION; GALAXIES; GALAXY NUCLEI; GHZ RANGE; LINEAR ABSORPTION MODELS; SELF-ABSORPTION; SPECTRA; SYNCHROTRONS; X RADIATION