National Aeronautics and Space Administration

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

Last Update: 2012 May 7

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 about 1 million detected X-ray sources.

As of 2000, there were about 220,000 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 100,000 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 2010, for example, the Version 2XMMi-DR3 of the XMM-Newton Serendipitous Source Catalog contained ~263,000 unique X-ray sources detected in about 9.5 years of XMM-Newton's observations. Chandra had detected a similar order of magnitude of sources, e.g., there are 300,000 X-ray sources in Andy Ptak's XAssist Chandra table, in its 10 years of observations. Adding these more recent detections to the earlier 2000 figure of 220,000 detected X-ray sources, means that, by 2010, the total number of known X-ray sources has increased since 2000 by a factor of 3.5, to ~ 780,000.

If the 2000's rate of growth of detections of X-ray sources is as expected in the coming half-decade, i.e., assuming Chandra and XMM-Newton keep operating, there should be about 1.5 million X-ray sources known by the year 2015 (these will be primarily derived from past and upcoming Chandra Observatory and XMM-Newton observations)!

      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        220,000         ROSAT source catalogs
      2010        780,000         above + XMM-Newton & Chandra detected sources
      2012      1,000,000         above + XMM-Newton & Chandra detected sources
circa 2015      1,500,000         predicted number 

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 1,900 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 a factor of 15 or more 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 Fermi LAT 1-Year Catlg
     2011        1,873            Number of sources in Fermi LAT 2-Year Catlg
     2015        6,000            Predicted no. of Fermi LAT sources in 6 Yrs 

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

The current number of detected X-ray and gamma-ray sources is tiny (<1%) compared to the number of known optical and/or infrared objects, and an order of magnitude less than the number of known radio sources, as the comparison below with other energy bands shows:

    Band          Number of             Based on
               Known Sources          

Gamma-ray               1,873          Fermi LAT 2-Yr Catalog
X-ray                  1,000,000          Above estimate
EUV                          3,100          HEASARC EUV Source Catalog
UV                          40,000          ANS, TD1 & Faust Catalogs
Optical       1,000,000,000+        Guide Star Catalog II/USNO B1 Catalog (present)
                 20,000,000,000         Large Synoptic Survey Telescope (after 2020)
IR                   564,000,000         WISE All-Sky Data Release Catalog
Radio               4,000,000+         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