| 4th of July 2017 |
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| A radio outburst from PKS 1954-388 |
| by Phil Edwards (CASS) |
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PKS 1954-388 is an active galaxy about 6 billion light years from our
own Galaxy (at a redshift of 0.63). It is regularly observed with the
ATCA as part of part of a multi-frequency program monitoring galaxies
observed with NASA's Fermi gamma-ray satellite. Observations are made
every month or two in the 5.5, 9, 17 and 38 GHz bands in order to
study the variability at radio wavelengths and to search for
correlations with gamma-ray activity. As PKS 1954-388 is a bright
radio source, the ATCA observations can be made in "snap-shot" mode,
with observations of several minutes sufficient to measure the flux
density of the source. A portion of the resulting light-curve is
shown above, providing a text-book example of the evolution of a radio
flare. The radio emission from active galaxies is synchrotron
radiation, produced by relativistic electrons under the influence of a
magnetic field. A radio outburst can be generated by an increase in
the number of high energy electrons, which produce higher frequency
radio emission, and as they lose energy gradually produce more
emission at lower frequencies. The plot above shows the outburst peaks
first at 38 and 17 GHz, then later at 9 GHz, and later again at 5.5
GHz. It is notable that the peaks are also sharper, and stronger, at
the higher frequencies, in line with expectations. The data are presented as part of a multi-wavelength study of this source in the paper "PKS 1954-388: RadioAstron Detection on 80,000 km Baselines and Multiwavelength Observations" by Edwards et al., published in the Publications of the Astronomical Society of Australia. |