3C 138 is known to shown linearpolarization at high degrees although the source of it remains a mystery. When correcting theFaraday rotation for this object, theelectric field position angle is ~170°, indicating themagnetic field direction as ~ 80°.[2] The source of 3C 138 is shown to emitgamma rays with a powerful flare detected in 2012.[6]
The radio structure of 3C 138 is unresolved at both 0.151 and 0.408GHz. However, when detected through higherfrequencies, it is revealed to be largely linear.[7] The structure is then further divided into separate components consisting of a bright radio core and aradio jet[8] which is polarized and showing a slight increase from 12percent at 5 GHz to 14 percent at 15 GHz.[9] There are several extended knots present within the main jet emission region extending 400milliarcseconds in a 65° position angle. A low surface-brightness counter-jet is located in an opposite direction from the region.[10] Furthermore, 3C 138 has a compactradio lobe located east and a much fainter, diffused radio lobe located west.[11]
3C 138 is one of the four primary calibrators used by theVery Large Array. The other three are3C 48,3C 147 and3C 286. For all visibilities of other sources, they are calibrated via observed visibilities of one of the four calibrators.[12]
