Title: PKS 1954-388: RadioAstron Detection on 80,000 km Baselines and Multiwavelength Observations,
Abstract: We present results from a multiwavelength study of the blazar PKS 1954-388 at
radio, UV, X-ray, and gamma-ray energies. A RadioAstron observation at 1.66 GHz
in June 2012 resulted in the detection of interferometric fringes on baselines
of 6.2 Earth-diameters. This suggests a source frame brightness temperature of
greater than 2x10^12 K, well in excess of both equipartition and inverse
Compton limits and implying the existence of Doppler boosting in the core. An
8.4 GHz TANAMI VLBI image, made less than a month after the RadioAstron
observations, is consistent with a previously reported superluminal motion for
a jet component. Flux density monitoring with the Australia Telescope Compact
Array confirms previous evidence for long-term variability that increases with
observing frequency. A search for more rapid variability revealed no evidence
for significant day-scale flux density variation. The ATCA light-curve reveals
a strong radio flare beginning in late 2013 which peaks higher, and earlier, at
higher frequencies. Comparison with the Fermi gamma-ray light-curve indicates
this followed ~9 months after the start of a prolonged gamma-ray high-state --
a radio lag comparable to that seen in other blazars. The multiwavelength data
are combined to derive a Spectral Energy Distribution, which is fitted by a
one-zone synchrotron-self-Compton (SSC) model with the addition of external
Compton (EC) emission.