Astronomer's Toolbox

Where do X-ray sources get their names?

You've probably seen some very strange names for X-ray sources. You may have wondered "how DO they come up with those names?" Well, X-ray sources get their names from the constellations, from famous catalogs, from the satellites that discovered them and their coordinates in Right Ascension and Declination (like longitude and latitude), other coordinate systems and the year they were discovered, just to name a few.

In the early days of X-ray astronomy, new objects were named after the constellation they were in. Objects like Cygnus X-1, LMC X-4, and Cen X-3 have this form. After it became obvious that there were going to be more than 20 or 30 X-ray sources, this naming convention was abandoned. Unfortunately, a single convention has never been agreed upon. Following are some examples of X-ray source names and where they came from.

Sco X-1

The first cosmic X-ray source ever discovered (after the Sun). It's in the constellation Scorpius. Each new X-ray source in a constellation gets an X-#. There is a Cygnus X-1, Cygnus X-2, and a Cygnus X-3. The Large Magellanic Cloud also has several sources with names of this form, they're called LMC X-1, LMC X-2, LMC X-3, and LMC X-4.

U Gem

This is another source that's named after its constellation. Usually, names of this form use a letter of the alphabet to order the stars in a constellation by optical brightness. However, this only applies to stars up through the letter Q. Names of this form that start after Q are *variable stars*. U Gem is a Cataclysmic Variable in the constellation Gemini. Tell me more about this strange naming convention

Many of the X-ray sources have names that come from a combination of a catalog abbreviation and the Right Ascension (RA) and Declination (Dec) of the object. Those funny things that look like backward phone numbers (0748-676) really list the location of the object. The above example source is at an RA of 07 hours, 48 minutes and a Dec of -67.6 degrees. Here are some other examples of this form of naming X-ray sources:

4U 0115+63		4th Uhuru catalog - one of the earliest X-ray satellites
3S 1820-30		SAS-3 discovery - another early X-ray satellite
EXO 0748-676		EXOSAT discovery
PKS 2155-304		Parkes catalog
H 2252-035		HEAO-1 A2 satellite survey
A 1916-05		Ariel catalog
2A 1822-371		2nd Ariel catalog
GS 2000+25		Ginga satellite discovery
G 21.5-0.9		Lowell Proper Motion Surveys of optical stars
MSH 15-52		Mills, Slee & Hill (1958) catalog of radio sources
PSR 1855-09		PSR=Pulsar (normally radio pulsars)
X 1608-52		X-ray source (general)
GX 301-2		This name describes the Galactic coordinates of this X-ray source. In this coordinate system, the center of our Galaxy is defined as 0,0. To find this source, you would go 301 degrees around the plane of the Galaxy (as seen from Earth) and then 2 degrees below the plane. If the source was called GX 4+1, you would go 4 degrees around the plane and 1 degree above the plane. GX 4+1 is very close to the center of our Galaxy (as seen from Earth).

Many objects get their names from a reference number in a catalog. Although these catalogs are often ordered by RA and Dec, one can't tell from the reference number where the object is in the sky. Some objects of this form are:

HD 93162		Henry Draper Catalog (1919-1925)
SS 433		The Stephenson & Sanduleak catalog
M 15		Messier catalog of non-stellar objects
NGC 6624		New General Catalog of Nebulae and Clusters of Stars (published 1888 by Dreyer)
IC 443		Index Catalogue (published 1895 by Dreyer)
Mrk 297		Appears in B. E. Markarian's ultraviolet catalog of galaxies. Sometimes listed as Mkn instead of Mrk.
Abell 2256		George Abell's catalog of clusters of galaxies
3C 273		The 3rd Cambridge catalogue
CTB 109		Cal Tech radio observation reports (catalog B)
AC 211		Auriere and Cordoni catalog of stars in M15
W 44		Westerhout (1958) catalog of radio sources
HZ 43		Humason & Zwicky (1946)
RCW 103		Rodgers, Campbell & Whiteoak catalog of HII regions (1960)
MCG 6-30-15		Morphological Catalogue of Galaxies (a compilation of information for approximately 34,000 galaxies found and examined on the Palomar Observatory Sky Survey (POSS)). The numbers correspond to the zone of the POSS.
I Zw 18		First Zwicky Catalogue of Clusters of Galaxies. The Zwicky clusters were identified by F. Zwicky in 560 POSS fields. They are rich clusters, each having at least 50 members within 3 magnitudes of the brightest member. There are II Zw and III Zw sources, as well.
SN 1987a		This is an object that was in the news in 1987. The SN means that it is a supernova. 1987 is the year it appeared and the letter "a" denotes that that it was the first supernova found in that year. Can you guess which supernova of 1993 was named SN 1993j? Did you know the Chinese have records of the supernova, SN 1006? When there are more supernovae than letters of the alphabet, they add a letter: SN 1995aa, SN 1995ab, etc. The last supernova discovered in 1995 was given the name SN 1995bd!
Tycho		Another supernova (observed by Tycho Brahe in 1572). Can you guess who discovered the Kepler supernova?