J1955+5131 is a radio source and a luminous active galactic nucleus (AGN). It is at a redshift of 1.223, which means that the radio waves were emitted from it when the Universe was only 5.1 billion years old (or 37% of the present age of the Universe). An AGN is a supermassive black hole accreting material, which produces a very bright point of emission in both visible light and radio waves. J1955+5131 is also a blazar, which means that the accretion rate is dynamically changing. The black hole has a mass of approximately 3 to 5 billion times the mass of the Sun. It's difficult to measure the size of the black hole, but the nearby supermassive black hole in M87 has an inner accretion disk diameter of about 350 AU (approximately 10 times the orbit of Neptune), which is a very small region in extragalactic terms.
The inner radio source (or core) of J1955+5131 is very tiny and bright, so it can be used to measure very accurate positions. Hence, it is used to tie together the positions of objects in radio and visible light. The position of the radio emission from J1955+5131 is known to an overall absolute positional accuracy of ~40 microarcseconds. It is one of 295 defining sources used in the International Celestial Reference Frame (ICRF) to accurately tie the radio and optical sky coordinate systems together. This is where the title of the episode about golf on the Moon comes in. The full Moon subtends angle of 31 arcminutes (about 0.5 degrees) when viewed from the Earth. 40 milliarcseconds is the apparent length of the 6-Iron golf club head used by Alan Shepard to play golf on the lunar surface. (The golf ball diameter is 20 microarcseconds.)
J1955+5131 is so bright because the jets are aligned almost along the line of sight to us. Plasma jet blobs in the jet aimed at us are moving close to the speed of light towards us and, due to relativistic effects, 'beamed' towards us like a searchlight. The receding jet is not seen at all since it is beamed away from us.
Blazars like J1955+5131 show variability in optical and radio brightness from changes in their accretion rates. Studying the radio jets can constrain accretion models. Also, the interactions of the jets with the gas in the galaxy can probe the nature of the interstellar medium in the quasar and in the early Universe. Tom Muxlow has proposals submitted to observe J1955+5131 with e-MERLIN and EVN to investigate whether a secondary knot seen within the northern lobe hotspot is associated with a restarted jet. He will also investigate the motion of another knot in the jet first seen in archival MERLIN data from 1992.