National Aeronautics and Space Administration

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?

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 4 less than the number of known radio sources, as the comparison below 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          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