Last Update: May 2nd, 2012

What kinds of cosmic objects have been detected as X-ray emitters?

The short answer to this is: a large number of them! In our own Solar System, for example, the Sun is the strongest emitter of X-rays. The Sun was first detected as an X-ray source as long ago as 1948, and it emits X-rays with a typical luminosity or power output Lx of 1027 erg s-1 or 1011 Giga Watts (GW) in the soft X-ray (0.2 - 5 keV) band. At Solar Maximum, the Sun's persistent Lx can reach 5 x 1027 erg s-1, while during a very large solar flare, values of Lx >~ 2 x 1028 erg s-1 can be attained.

Other objects in our Solar System which have been detected as apparent X-ray `emitters' (actually not intrinsic but due to the reflection of solar X-rays from their surfaces and/or charge exchange of their atmospheres with the highly ionized solar wind) include the Moon, the planets Jupiter, Saturn, Venus, Earth and Mars, two or three of the moons of Jupiter, and the Io Plasma Torus, as well as a number of comets.

Object      Lx (erg/s)        Lx (SI)        Comments

Moon        7.3E11            73 kW
Europa      1.5E13            1.5 MW
Io          2.0E13            2 MW
Comets      1.0E14 - 5.0E16   10 - 5000 MW   Within 2 Au of the Sun
Mars        4.0E13 - 1.6E14   4 - 16 MW
Earth       3.0E14            30 MW          My guess
Venus       5.5E14            55 MW
Saturn      8.7E14            87 MW
Io Plasma   1.0E15            100 MW
Jupiter     2.2E16            2.2 GW

In our Milky Way Galaxy, the brightest individual X-ray emitters, with persistent X-ray luminosities of up to 2 x 1038 erg s-1 which are 100 billion (1011) times greater than that of the `Quiet' Sun, are the X-ray binaries (XRBs). XRBs are close binary systems in which one member is a neutron star or black hole that is accreting matter from the other [normal] companion star, and in the process releasing enormous amounts of energy, much of it in the X-ray band. If well-supplied with matter from a donor, they can radiate persistently at a maximum level called the Eddington Luminosity, which is 1.3 x 1038 (M/Msun) erg s-1, e.g., the XRB Sco X-1 has Lx ~ 2 x 1038 erg s-1.

Almost every type of star from the most massive Wolf-Rayet and OB stars to low-mass M dwarf stars, single white dwarf and neutron stars, and even some sub-stellar mass brown dwarfs, have been detected as X-ray sources, with X-ray luminosities in the range from 1025 erg s-1, e.g., low-activity very-low-mass stars and brown dwarfs such as the M9 V ultra-cool dwarf star DENIS-P J104814.7-395606 (Stelzer et al. 2012, A&A, 537, A94), up to 1035 erg s-1, e.g., the colliding-wind massive binary system Eta Carina (discussed in many papers by the HEASARC scientist Mike Corcoran!). Some types of extended objects, such as planetary nebulae, H II regions, the Local Bubble, etc., have also been detected as X-ray sources. The only types of stars that have not been confirmed as intrinsic X-ray sources are the A-type stars, cool white dwarf stars, and single red (M-type) giant and supergiant stars.

Exploding stars can emit copious X-rays! Supernovae can reach peak X-ray luminosities of 1041 erg s-1, e.g., SN 1998bw, and their remnants can have X-ray luminosities of up to 3 x 1037 erg s-1 for hundreds to thousands of years after their formation, e.g., the Crab SNR. The nova outbursts triggered in cataclysmic binary systems, where material transferred onto a white dwarf companion eventually ignites in a thermonuclear runaway, can have peak X-ray luminosities of up to about 1035 erg s-1.

The integrated current X-ray luminosity of our entire Galaxy is estimated to be about 3 x 1039 erg s-1 = 15 times the luminosity of the persistent XRB Sco X-1. The supermassive black hole Sgr A* at the center of our Galaxy is currently in a low-luminosity, very sub-Eddington state (Lx <~ 1035 erg s-1), but it is widely believed that about a century ago its was much more luminous (Lx ~ 3 x 1039 erg s-1 ~ current X-ray luminosity of the entire Galaxy): see Terrier et al. (2010, ApJ, 719, 143) for more details. If Sgr A* ever were to get a sufficient supply of accreting matter and radiate at the Eddington Limit for such a massive object (Lx ~ 5 x 1044 erg s-1), it would far outshine every other object in the Sky in X-rays as seen from the Earth, with the sole exception of the Sun! Interestingly, a dense gas cloud is now nearing Sgr A* on a very eccentric orbit, with perigalacticon predicted to occur in Summer 2013. This is expected to produce a major outburst of Sgr A*, although probably not to its Eddington luminosity! Stay tuned....

In the entire Universe, the most luminous X-ray sources are the active galactic nuclei (AGN), which can have X-ray luminosities of as high as 1047 erg s-1, and rich clusters of galaxies, which can have X-ray luminosities of up to about 3 x 1045 erg s-1. Typical individual `normal' galaxies, on the other hand, have much lower X-ray luminosities in the range from 1038 erg s-1 to 3 x 1042 erg s-1.

Web page author and maintainer: Stephen A. Drake

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Last modified: Wednesday, 02-May-2012 15:49:05 EDT