How Many Known X-Ray (and Other) Sources Are There?

Last Update: 2015 September 03

How many individual X-ray sources are known currently? How has the number grown in the 5 decades since the field of X-ray astronomy began?

As of the present date, there are likely around 1 million detected X-ray sources. For example, the HEASARC XRAY Master Catalog contains 1.8 million sources: since there are a significant number of sources which have multiple detections, we have divided this number by a factor of two to take care of this effect.

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 2015, for example, the Version 3XMM DR5 of the XMM-Newton Serendipitous Source Catalog contained ~397,000 unique X-ray sources detected in about 14 years of XMM-Newton's observations. Chandra had detected a similar order of magnitude of sources, e.g., there are 400,000 X-ray sources in the Chandra Source Catalog, Release 2.0, detected in its first 14+ years of observations, and the Swift XRT 1SXPS Catalog contains ~152,000 X-ray sources 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 2015, the total number of known X-ray sources has increased since 2000 by a factor of nearly 4, to ~ 1,250,000. There will be some overlap between these compilations of course, so that the final number of truly unique X-ray sources is likely something like 2/3 of this, viz., 850,000. Ignoring this correction, and 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 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. 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
      2013      1,100,000         above + XMM-Newton & Chandra detected sources
      2015      1,250,000         above + XMM-Newton & 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 3,000 detected gamma-ray sources in the 100 MeV to 100 GeV energy range. At higher energies, there are a much smaller number of known sources, e.g., about 107 detected gamma-ray sources in the 100 GeV to 100 TeV energy range, often called the very high energy (VHE) gamma-ray band, are listed by Robert Wagner on his VHE Gamma-Ray Sky Map and Source Catalog web page.

As of 2002, there were 420 known discrete gamma-ray sources, with the majority having been discovered by the Egret instrument on the Compton Gamma-Ray Observatory, but this number should increase by an order of magnitude after the Fermi (formerly GLAST) Large-Area Telescope (LAT) All-Sky Survey is extended into the mid-2010's to be ~6,000. These numbers can be compared to the ~1-2 possible gamma-ray sources known circa 1970, and the ~25 confirmed sources by 1990 (mostly 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)

How do the number of known (detected) X-ray and gamma-ray sources compare with those in other energy bands?

The current number of detected EUV, X-ray and gamma-ray sources is tiny (<~ 1%) compared to the number of known optical, ultraviolet and/or infrared objects, and a factor of 3-4 times less than the number of known radio sources, as the comparison with the numbers of known sources in other energy bands shows:

    Band             Number of          Surface Density        Based on
                        Known Sources   Per Square Degree

Gamma-ray          3,034                   0.074             Fermi LAT 4-Yr Catalog
X-ray                  1.25 million            30.3               Above estimate
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                  4 million+             97+                NVSS + FIRST + 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

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Last modified: Wednesday, 09-Sep-2015 15:31:30 EDT