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How Many Known X-Ray (and Other) Sources Are There?
Last Update: 2007 September 13
How many individual X-ray sources are known currently? How has the
number grown in the nearly 5 decades since the field of X-ray astronomy
began?
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), and 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). If the rate of growth of detections of X-ray
sources is as expected in the coming half-decade, there should be about a
million X-ray sources known by the year 2010 (these will be primarily derived
from past and upcoming Chandra Observatory and XMM-Newton observations)!
As of the Fall 2007, for example, the 2nd Version of the XMM-Newton
Serendipitous Source Catalog contained ~192,000 unique X-ray sources detected
in about 7 years of XMM-Newton's observations, and Chandra has likely detected
a similar order of magnitude of sources in its 7.5 years of observations:
adding the more recent observations made by these satellites to the earlier
detected X-ray sources, and allowing for some overlapping sources in common,
means that, by 2007, the total number of known X-ray sources has likely
increased since 2000 by a factor of 2.5, to ~ 550,000.
Year No. X-ray Based on
Sources known
1960 0 (or 1 if the Sun is counted)
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
2007 550,000 above + XMM-Newton & Chandra serendip sources
circa2010 1,000,000 predicted " + " " " "
How many individual gamma-ray sources are known currently? How has the
number grown in the 3+ decades since this field began?
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 30
after the GLAST All-Sky Survey is completed in the late 2000's to >10,000.
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. 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
circa2010 10,900 Predicted Number of GLAST detections
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 420 Macomb & Gehrels (1999,2001) Catalog
X-ray 550,000 Above estimate
EUV 1,400 HEASARC EUV Source Catalog
UV 35,400 TD1 & Faust Catalogs
Optical 1,000,000,000+ Guide Star Catalog II/USNO B1 Catalog
IR 470,000,000 2-micron All-Sky Survey (2MASS) Catalog
Radio 3,500,000+ NVSS + FIRST + Other Radio Catalogs
Web page author and maintainer: Stephen A.
Drake
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Last modified: Thursday, 13-Sep-2007 15:05:07 EDT
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