TXS 0128+554: A Young Gamma-Ray Emitting AGN With Episodic Jet Activity
  We have carried out a Chandra X-ray and multi-frequency radio VLBA study of
the AGN TXS 0128+554, which is associated with the Fermi gamma-ray source 4FGL
J0131.2+5547. The AGN is unresolved in a target 19.3 ks Chandra image, and its
spectrum is well fit by a simple absorbed power law model, with no
distinguishable spectral features. Its relatively soft X-ray spectrum compared
to other CSOs may be indicative of a thermal emission component, for which we
were able to obtain an upper temperature limit of kT = 0.08 keV. The compact
radio morphology and measured advance speed of 0.32c +- 0.07c indicate a
kinematic age of only 82 y +- 17 y, placing TXS 0128+554 among the youngest
members of the compact symmetric object (CSO) class. The lack of compact,
inverted spectrum hotspots and an emission gap between the bright inner jet and
outer radio lobe structure indicate that the jets have undergone episodic
activity, and were re-launched a decade ago. The predicted gamma-ray emission
from the lobes, based on an inverse Compton-emitting cocoon model, is three
orders of magnitude below the observed Fermi LAT flux. A comparison to other
Fermi-detected and non-Fermi detected CSOs with redshift z<0.1 indicates that
the gamma-ray emission likely originates in the inner jet/core region, and that
nearby, recently launched AGN jets are primary candidates for detection by the
Fermi LAT instrument.
