# 3C 48

3C48 is a quasar discovered in 1960; it was the second source conclusively identified as such.
3C48 was the first source in the Third Cambridge Catalogue of Radio Sources for which an optical identification was found by Allan Sandage and Thomas A. Matthews in 1960 through interferometry. In 1963 Jesse L. Greenstein and Thomas Matthews found that it had a redshift of 0.367, making it one of the highest redshift sources then known. It was not until 1982 that the surrounding faint galactic "nebulosity" was confirmed to have the same redshift as 3C48, cementing its identification as an object in a distant galaxy. This was also the first solid identification of a quasar with a surrounding galaxy at the same redshift.
3C 48 is one of four primary calibrators used by the Very Large Array (along with 3C 138 and 3C 147, and 3C 286). Visibilities of all other sources are calibrated using observed visibilities of one of these four calibrators.

## Nomenclature

The name of the object “3C 48” consists of two significant parts. The first part, “3C,” means that the object belongs to the Third Cambridge Catalog of Radio Sources. The second part - “48” - is the serial number in the catalog ordered by right ascension.

## History

3C 48 was the first source in the Third Cambridge Catalog of Radio Sources to be optically identified by Allan Sandage and Thomas Matthews in 1960 using interferometry.
Jesse Greenstein and Thomas Matthews found that it had a redshift of 0.367, one of the highest redshifts of any source known at the time. It was not until 1982 that a surrounding faint galactic "nebula" was measured to have the same redshift as 3C 48, confirming its identification as an object in a distant galaxy. This was also the first reliable identification of a quasar with a surrounding galaxy of the same redshift.
   On September 26, 1960, Matthews and Sandage photographed an area of the sky containing one such source, 3C48, with the 200-inch telescope. To their surprise, there were no objects within the error rectangle of the coordinates in this area except for a star of 16.2$^m$. True, there were traces of a faint small nebula around it, but the object certainly looked like a star. Doubts about the possibility of identifying the radio source with a star disappeared on October 22, 1960, when Sandage obtained the spectrum of the object. It contained an unusual combination of broad emission lines that defied identification. The color indices of 3C48 were also unusual: Sandage found that they were consistent with hot white dwarfs and former Nova - very hot objects with an ultraviolet excess.
   In December 1960, Sandage reported the results of the first optical observations of 3C48 at the 107th meeting of the American Astronomical Society. Testing the hypothesis that 3C48 is a stellar remnant of a nova or even a supernova ("the first true radio star"), H. Smith and D. Hoffleit looked through the Harvard Sky Survey's collection of plates for 1897-1958 and did not find any noticeable fluctuations in the object's brightness exceeding 0.3m, which are common in former novae. The most likely possibility seemed to be that it was a neutron star - a remnant of a supernova.