How Many Known X-Ray (and Other) Sources Are There?
Last Update: 2018 March 26
How many individual X-ray sources are known currently? How has the number grown in the 56 years since the field of X-ray astronomy began?
As of the present date, there are around 1 to 1.5 million detected unique X-ray sources. For example, the HEASARC XRAY Master Catalog of X-ray sources detected by the many X-ray observatories in the last 5 decades contains 2.48 million sources. Since there are a significant number of sources which have multiple detections, this number is an upper limit on the actual number of unique X-ray sources.
As of 2000, there were about 340,000 known X-ray sources, with the vast majority discovered by the ROSAT X-ray satellite observatory: about 120,000 sources were found in the ROSAT All-Sky Survey, and about 220,000 more were discovered in pointed ROSAT observations. This compares with 1 known source (excluding the Sun) as of 1962 (Sco X-1), with 59 known sources in 1970 (all from rocket and balloon observations), with about 700 known sources in 1980 (based on Uhuru, Ariel-V and HEAO-1 satellite observations), and with about 8000 known sources by 1990 (derived mostly from observations by the Einstein (HEAO-2) and EXOSAT satellites).
As of 2018, for example, the Version 3XMM DR7 of the XMM-Newton Serendipitous Source Catalog contained ~499,000 unique X-ray sources that have detected in about 17 years of XMM-Newton's observations. Chandra has detected a similar order of magnitude of sources, e.g., there are 370,000 unique X-ray sources in the Chandra Source Catalog, Release 2.0 that were detected in its first 15 years of observations, and the Swift XRT 1SXPS Catalog contains ~152,000 X-ray sources that were detected in Swift's first 8 years of observations. Adding these more recent detections to the earlier 2000 figure of 340,000 detected X-ray sources, means that, by 2018, the total number of X-ray sources listed in the HEASARC master XRAY Catalog reached 2.5 million. There will be many duplicate sources in this compilation of course, so that the final number of unique X-ray sources is likely something like half of this, viz., ~ 1.25 million. Given that there are 41,252.961 square degrees in the whole sky, this implies an average surface density of X-ray sources of 30.3 sources per square degree, or 0.0084 per square arcminute. Thus, within 1 arcminute of a random position in the sky, there ia a 2.6% chance of finding an X-ray source, assuming that they are randomly distributed across the sky.
Year No. Unique X-ray Based on
Sources known
1960 0 (excluding the Sun)
1962 1 Rocket experiments
1965 10 Rocket experiments
1970 60 Rocket & balloon experiments
1974 160 3rd Uhuru Catalog
1980 680 Amnuel et al. (1982) Catalog
1984 840 HEAO A-1 Catalog
1990 8,000 Einstein & EXOSAT source catalogs
2000 340,000 ROSAT source catalogs
2010 780,000 above + XMM-Newton & Chandra detected sources
2017 1,250,000 above + XMM-Newton, Swift & Chandra detected sources
How many individual gamma-ray sources are known currently? How has the number grown in the 3+ decades since this field began?
As of the present date, there are about 5,500 known gamma-ray sources in the 100 MeV to 100 GeV energy range, mostly detected by the Fermi Large Area Telescope (LAT) instrument. At higher energies, there are a much smaller number of known sources, e.g., there are about 210 gamma-ray sources detected in the 50 GeV to 100 TeV energy range, often called the very high energy (VHE) gamma-ray band, that are listed in the TeVCat catalog of VHE sources.
By comparison, as of 2002, there were only 420 known discrete gamma-ray sources, with the majority having been discovered by the EGRET instrument on the Compton Gamma-Ray Observatory. These numbers can be compared to the ~1-2 possible gamma-ray sources known in 1970, and the ~25 confirmed sources by 1990 (most discovered by the COS-B mission):
Year No.of Gamma-ray Based on
Sources known
1967 0
1970 1-2 Rocket and balloon experiments
1973 6 SAS-2
1977 13 COS-B
1981 25 COS-B
1994 50 1st EGRET Catalog
1995 128 2nd EGRET Catalog
1996 156 2Rth EGRET Catalog
1999 270 3rd EGRET Catalog
1999 309 Macomb & Gehrels' Catalog
2002 420 Updated Macomb & Gehrels' Catalog
2010 1,451 Number of sources in 1FGL (Fermi LAT 1-Year Catlg)
2011 1,873 Number of sources in 2FGL (Fermi LAT 2-Year Catlg)
2015 3,034 Number of sources in 3FGL (Fermi LAT 4-Year Catlg)
2018 5,524 Number of sources in Preliminary Fermi LAT 8-Year Point Source List)
How do the number of known (detected) X-ray and gamma-ray sources compare with those in other energy bands?
Band Number of Surface Density Based on
Known Sources Per Square Degree
Gamma-ray 5,524 0.134 Prelim. Fermi LAT 8-Yr Catalog
X-ray 1.25 million 30.3 Above estimate (including correction for duplicates)
EUV ~1,000 0.024 HEASARC EUV Source Catalog (accounting for duplicate sources)
UV 86 million 2,085 GALEX Catalogs
Optical 1 billion+ 24,241+ Guide Star Catalog II/USNO B1 Catalog (present)
Optical 20 billion 484,814 Large Synoptic Survey Telescope (after 2020)
IR 564 million 13,672 WISE All-Sky Data Release Catalog
Radio 5.2 million 126 NVSS + FIRST + GMRT All-Sky + Other Radio Catalogs
These numbers can be compared with the estimates of 300 billion stars in our Milky Way Galaxy and >100 billion galaxies in the observable Universe.
Web page author and maintainer: Stephen A. Drake