National Aeronautics and Space Administration

Last Update: November 30th, 2017

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 L_x 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 L_x can reach 5 x 10²⁷ erg s^-1, while during a very large solar flare, values of L_x >~ 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 "extrinsic" due to either the reflection of solar X-rays from their surfaces, charge exchange of their atmospheres with the highly ionized solar wind and/or solar-wind initiated auroral emission) include the Moon, the planets Jupiter, Saturn, Pluto, 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
Mars        4.0E13 - 1.6E14   4 - 16 MW
Comets      1.0E14 - 5.0E16   10 - 5000 MW   Within 2 Au of the Sun
Earth       3.0E14            30 MW          My guess
Venus       5.5E14            55 MW
Saturn      8.7E14            87 MW
Io Plasma   1.0E15            100 MW
Torus
Pluto       2.0E15            200 MW         Surprisingly large!
Uranus     <6.0E15           <600 MW
Neptune    <1.2E16          <1200 MW
Jupiter     2.2E16           2200 MW

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/M_sun) erg s^-1, E.g., the XRB Sco X-1 has L_x ~ 2 x 10³⁸ erg s^-1. Some X-ray binaries can have outbursts that for a limited time exceed the Eddington Luminosity, e.g., the binary GS 1354-64 in 1967 reached 4 x 10³⁹ 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 (the upper limit to the X-ray luminosity of Vega, an A0V star, is 3 x 10²⁵ erg s^-1), cool white dwarf stars (the upper limit to the X-ray luminosity of GD 356, a DAH white dwarf, is 6 x 10²⁵ erg s^-1), and single red (late K- and M-type) giant and supergiant stars (the upper limit to the X-ray luminosity of Aldebaran, a K5 III star, is 7 x 10²⁵ erg s^-1). [The small fraction of A-type and red giant and supergiant stars that are detected as X-ray sources are likely binary systems in which the emission is produced by the companion directly and/or interaction of the stellar wind with the companion].

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 Nebula. 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 (4 x 10⁶ solar masses) black hole Sgr A* at the center of our Galaxy is currently in a low-luminosity, very sub-Eddington state (L_x <~ 10³⁴ erg s^-1), but it is widely believed that about a century ago its was much more luminous (with L_x ~ 3 x 10³⁹ erg s^-1, which is about the 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 (L_x ~ 5 x 10⁴⁴ erg s^-1), its luminosity would exceed the bolometric luminosity of the entire Milky Way galaxy (~ 5 x 10⁴³ erg s^-1) by a factor of 10! As seen from the Earth, the observed X-ray flux of Sgr A* in this state would be 5.8 x 10-2 erg/s/cm² and would far outshine in X-rays every other object in the Sky as seen from the Earth, with the sole exception of the Sun!

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