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Last Update: March 26th, 2018

What are the (apparently) brightest X-ray sources in the sky as seen from the Earth?

In the soft X-ray band (0.2 - 5 keV) the Sun is by far the brightest persistent X-ray source (by a factor of about a million) due to its proximity to the Earth. The flaring Sun is also arguably the brightest 'transient' or variable X-ray source, but a giant gamma-ray flare of the magnetar SGR 1806-20 on 27 Dec 2004 had a peak flux (in a hard band from 45 keV to 10 MeV) which rivalled a very large solar flare in its observed flux at the Earth (and covered a much broader energy range, to boot). A few other giant flares have been observed from the handful of other known magnetars.

Next after these sources, come the class of so-called soft X-ray transients (SXTs), sometimes also called X-ray novae, which are actually outbursts of X-ray binary systems containing accreting neutron stars or black holes; SXTs can stay bright for durations of days to months, and in some cases the ratio of maximum to minimum X-ray emission can be huge (many orders of magnitude).

Only one source (excluding the Sun) in the `top ten' brightest sources is a persistently bright X-ray source, namely the low-mass X-ray binary system Scorpius X-1. For completeness, after these 'top ten' sources, the brightest members of some of the other various classes of X-ray sources are listed, first for Galactic objects, then for extragalactic classes of objects.

A name in bold-face indicates that it is a steady or slowly varying X-ray source. A name in italics indicates that it is an X-ray transient source, e.g., an X-ray nova or a gamma-ray burst). An `@" indicates that this is the level seen during a flare (e.g., Algol and HR 1099) or an outburst (e.g., SS Cyg).

The energy range quoted for the X-ray fluxes and luminosities of the non-solar sources is 2-10 keV. The fluxes are given both in terms of cgs units and in multiples of the Crab flux, where 1 Crab = 2.4 x 10-8 erg/s/cm2.

Object
 Distance (parsecs)
 X-ray flux (erg/s/cm2) and in Crabs
 X-ray Luminosity (erg/s)
 Other Name or Comment
Very Large Solar Flare
 5 x 10-6
 7
 2 x 1028
 Peres et al. (2000, ApJ, 528, 537)
Largest Giant Gamma-ray Flare from SGR 1806-20
 15,000
 5
 1 x 1047 (if energy were emitted isotropically)
 Magnetar (Palmer et al. 2005, Nature, 434, 1107)
Sun at Solar Maximum
 5 x 10-6
 2
 5 x 1027
 Peres et al. (2000, ApJ, 528, 537)
Sun at Solar Minimum
 5 x 10-6
 0.1
 3 x 1026
 Peres et al. (2000, ApJ, 528, 537)
GRB 100621A
 1.6 x 109
 3 x 10-6 = 125 Crab
 3 x 1051 (if energy were emitted isotropically)
 Gamma-Ray Burst
A 0620-00
 870.
 1 x 10-6 = 42 Crab
 1 x 1038
 SXT: X-Ray Nova Mon 1975
Cen X-4
 1200.
 1 x 10-6 = 42 Crab
 3 x 1038
 SXT: X-Ray Nova Cen 1969
V404 Cyg
 3500.
 6 x 10-7 = 25 Crab
 1 x 1039
 SXT: X-Ray Nova Cyg 1989
Sgr A* (Milky Way Nucleus) See note below
 8500.
 4 x 10-7 = 17 Crab
 3 x 1039
 Inferred flux of super-massive black hole before ~1900 CE (Terrier et al. 2010, ApJ, 719, 143)
GS 1354-64
 10000.
 3 x 10-7 = 12 Crab
 4 x 1039
 SXT: X-Ray Nova Cen 1967
4U 1543-47
 4000.
 2 x 10-7 = 8 Crab
 7 x 1038
 Low-Mass X-Ray Binary in outburst
Sco X-1
 2800.
 2 x 10-7 = 8 Crab
 2 x 1038
 Low-Mass X-Ray Binary
QZ Vul
 2000.
 2 x 10-7 = 8 Crab
 4 x 1038
 SXT: X-Ray Nova Vul 1988
GRS 1716-249
 2400.
 2 x 10-7 = 8 Crab
 2 x 1038
 SXT: X-Ray Nova Oph 1993
GU Mus
 		5500.
 2 x 10-7 = 8 Crab
 7 x 1038
 SXT: Nova Mus 1991
Other Galactic Sources
 Distance (parsecs)
 X-ray flux (erg/s/cm2)
 X-ray Luminosity (erg/s)
 Other Name or Comment
EV Lac@
 5.1
 5 x 10-8 = 2 Crab
 1.6 x 1032
 dMe Flare Star (Osten et . 2010, ApJ, 721, 785)
Crab SNR
 2000.
 2.4 x 10-8 = 1 Crab (duh!)
 1.2 x 1037
 Supernova Remnant, Pulsar Wind Nebula and Pulsar
II Peg@
 42.
 1 x 10-8 = 417 milliCrab
 2 x 1033
 RS CVn Binary (Osten et . 2007, ApJ, 654, 1052)
HR 1099@
 29.
 4 x 10-9 = 167 milliCrab
 4 x 1032
 RS CVn Binary
AB Dor@
 10.
 4 x 10-9 = 167 milliCrab
 1 x 1032
 Active Young Star
Algol@
 28.
 3 x 10-9 = 125 milliCrab
 3 x 1032
 Active Binary
TWA-7@
 55.
 2 x 10-9 = 83 milliCrab
 1 x 1033
 Pre-Main Sequence Star (Morii et al. 2010, ATel. 2836)
Eta Car (Just before periastron)
 2300.
 3 x 10-10 = 12 milliCrab
 2 x 1035
 Interacting Wind Binary
Capella
 13.
 1 x 10-10 = 4 milliCrab
 2 x 1030
 Wide Pair of Normal Stars
SS Cyg@
 75.
 5 x 10-11 = 2 milliCrab
 3 x 1031
 Cataclysmic Binary
Other Extragalactic Sources
 Distance (parsecs)
 X-ray flux (erg/s/cm2)
 X-ray Luminosity (erg/s)
 Other Name or Comment
Perseus Cluster
 110 x 106
 1 x 10-9 = 42 milliCrab
 1 x 1045
 Cluster of Galaxies
Messier 87
 22 x 106
 5 x 10-10 = 21 MilliCrab
 3 x 1043
 Active Galaxy
3C 273
 750 x 106
 2 x 10-10 = 8 milliCrab
 1 x 1046
 Radio-Loud Quasar
Messier 31
 0.7 x 106
 7 x 10-11 = 3 milliCrab
 4 x 1039
 Normal Galaxy

Note: The Galactic Center source Sgr A* currently has a weak X-ray flux only a millionth of the quoted level, but, based on the observed fading X-ray fluorescence and hard X-ray emission from giant molecular clouds in the Galactic Center, Terrier, Sunyaev and others have argued that, a hundred or more years ago, Sgr A* must have been emitting at this extremely high level.

How do the sensitivity and angular resolution of current X-ray telescopes compare with those of earlier instruments?

The currently operating Chandra and XMM-Newton X-ray observatories have detected X-ray sources that are one ten-billionth (1 x 10-10) of the brightness of the first cosmic X-ray source ever detected, Scorpius X-1, which is also the brightest persistent (non-solar) X-ray source in the sky : i.e., sources as faint as 3 x 10-17 erg cm-2 s-1 are now known e.g., in the Chandra Deep Field South 7-Megasecond source catalog (Luo et al. 2017, ApJS, 228, 2), compared to the flux of 3 x 10-7 erg cm-2 s-1 for Sco X-1 (both fluxes quoted are in the 2-7 keV X-ray band). This improvement in sensitivity, achieved in the 5+ decades from 1962 to the present, of X-ray telescopes is very similar to the improvement achieved over the four centuries of telescopic observations of the sky in the visible (optical) band since Galileo Galilei's first observations. In fact, the faintest objects in the Hubble Extreme Deep Field (cf. Illingworth et al. 2013, ApJS, 209, 6) are of 31st magnitude and have a brightness, which corresponds to 1 x 10-10 (one ten-billionth) of that of the faintest naked-eye stars (which are about 6th magnitude).

The earliest X-ray detectors were wide-angle instruments that had either no or limited (degrees scale) angular resolution. The first fully imaging X-ray telescope put into space was the Einstein (HEAO-2) Observatory, launched in 1978, which had instruments with angular resolution as good as 2 arcseconds (the High Resolution Imager) up to 60 arcseconds (the Imaging Proportional Counter): the ROSAT Observatory which operated during the 1990s had instruments with similar capabilities. The currently operating Chandra X-ray Observatory (launched in 1999) has a mirror which is figured to such an exquisite accuracy that its resolution is about 0.5 arcseconds (3600 arcseconds = 1 degree). Thus, in the 40 years of cosmic X-ray astronomy, the angular resolution has improved by a factor of more than 10,000 in linear scale, equivalent to an improvement of 108 in area on the sky. The latter measure is crucially important when looking for counterparts to X-ray sources in other wavelength regions, since the smaller the sky area to be searched, the smaller the chance of a misidentification with an unrelated object.

Web page author and maintainer: Stephen A. Drake