VSOP Observations of the Gamma-Ray Sources J1733−1304, J1625−2527, and J1015+4926

Abstract

Abstract We present images from VSOP (VLBI Space Observatory Programme) and VLBA (Very Long Baseline Array) observations of the active galactic nuclei J1733$-$1304, J1625$-$2527, and J1015$+$4926. These three sources have been identified as possible counterparts to gamma-ray sources in the third EGRET catalog. In general, the counterparts to extragalactic EGRET sources have very compact structure, with brightness temperatures ($T_{\mathrm{b}}$) greater than $10^{11} \hbox{--} 10^{12} \,\mathrm{K}$. Evidence for compactness in gamma-ray emitting AGNs is also given by the variability seen from gamma-ray to radio frequencies. In this report, we compare the images of the three sources with previously published images and also examine jet motions and spectral indices. The measured $T_{\mathrm{b}}$ of J1733$-$1304 are an order of magnitude lower than previous $T_{\mathrm{b}}$ measurements, suggesting that the VSOP observations were made while the source was in a relatively quiescent state after the radio outburst of 1995–1997. The relatively low $T_{\mathrm{b}}$ of J1625$-$2527 is also consistent with the highly variable nature of this source. A comparison of the VSOP observation of J1015$+$4926 with a VLBA image made 2.2 yr earlier suggests the existence of stationary jet components in this source.