# PKS 1954-388: RadioAstron Detection on 80,000 km Baselines and Multiwavelength Observations

## TL;DR

This study reports the first RadioAstron detection of PKS 1954-388 on 80,000 km baselines, revealing extremely high brightness temperatures and multiwavelength variability, supporting Doppler boosting and a one-zone SSC plus EC emission model.

## Contribution

It presents the first RadioAstron interferometric detection of PKS 1954-388 on unprecedented baselines and combines multiwavelength data to model the source with a comprehensive emission scenario.

## Key findings

- Detection of fringes on 80,000 km baselines indicating high brightness temperature
- Confirmation of superluminal jet motion from VLBI imaging
- Radio flare observed with a delay relative to gamma-ray high state

## 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.