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Last Update: May 13th, 2008
What are the (apparently) brightest X-ray sources in the sky as seen
from the Earth?
In the soft X-ray band (0.12 - 5 keV) the Sun is by far the brightest X-ray
source (by a factor of about a million) due to its proximity to the Earth.
Next after the Sun 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 a soft X-ray
transient source (also known as an X-ray nova). An `@" indicates that this
is the level seen during a flare (e.g., Algol and HR 1099) or an outburst
(e.g., SS Cyg).
Object
|
Distance (parsecs)
|
X-ray flux (erg/s/cm2)
|
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)
|
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)
|
A 0620-00
|
870.
|
1 x 10-6
|
1 x 1038
|
SXT: X-Ray Nova Mon 1975
|
Cen X-4
|
1200.
|
1 x 10-6
|
3 x 1038
|
SXT: X-Ray Nova Cen 1969
|
V404 Cyg
|
3500.
|
6 x 10-7
|
1 x 1039
|
SXT: X-Ray Nova Cyg 1989
|
GS 1354-64
|
10000.
|
3 x 10-7
|
4 x 1039
|
SXT: X-Ray Nova Cen 1967
|
Sgr A* (Milky Way Nucleus) See note below
|
8500.
|
3 x 10-7
|
2 x 1039
|
Inferred flux of super-massive black hole circa 1600 CE
|
4U 1543-47
|
4000.
|
2 x 10-7
|
7 x 1038
|
Low-Mass X-Ray Binary in outburst
|
Sco X-1
|
2800.
|
2 x 10-7
|
2 x 1038
|
Low-Mass X-Ray Binary |
QZ Vul
|
2000.
|
2 x 10-7
|
4 x 1038
|
SXT: X-Ray Nova Vul 1988
|
GRS 1716-249
|
2400.
|
2 x 10-7
|
2 x 1038
|
SXT: X-Ray Nova Oph 1993 |
GU Mus
|
5500.
|
2 x 10-7
|
7 x 1038
|
SXT: Nova Mus 1991
|
Other Galactic Sources
|
|
|
|
|
Crab SNR
|
2000.
|
6 x 10-8
|
3 x 1037
|
Supernova Remnant
|
EV Lac@
|
5.
|
3 x 10-8
|
9 x 1031
|
dMe Flare Star: flare on April 25th 2008 caught by Swift
|
II Peg@
|
42.
|
6 x 10-9
|
1 x 1033
|
RS CVn Binary
|
HR 1099@
|
29.
|
4 x 10-9
|
4 x 1032
|
RS CVn Binary
|
AB Dor@
|
10.
|
4 x 10-9
|
1 x 1032
|
Active Young Star
|
Algol@
|
28.
|
3 x 10-9
|
3 x 1032
|
Active Binary
|
Eta Car (Just before periastron)
|
2300.
|
3 x 10-10
|
2 x 1035
|
Interacting Wind Binary
|
Capella
|
13.
|
1 x 10-10
|
2 x 1030
|
Wide Pair of Normal Stars
|
SS Cyg@
|
75.
|
5 x 10-11
|
3 x 1031
|
Cataclysmic Binary
|
Other Extragalactic Sources
|
|
|
|
|
Perseus Cluster
|
110 x 106
|
1 x 10-9
|
1 x 1045
|
Cluster of Galaxies
|
Messier 87
|
22 x 106
|
5 x 10-10
|
3 x 1043
|
Active Galaxy
|
Messier 31
|
0.7 x 106
|
7 x 10-11
|
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 X-ray fluorescence from giant molecular clouds in the Galactic Center,
Sunyaev and others have argued that, several hundred years ago,
Sgr A* must have been emitting at an 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 two ten-billionths (2 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 6 x 10-17 erg cm-2
s-1 are now known, compared to 3 x 10-7 erg
cm-2 s-1 for Sco X-1 (both
in the 2 - 10 keV X-ray band). This improvement in sensitivity (achieved in
about 4 decades from 1962 to the mid-2000s) of
X-ray telescopes is slightly more than that achieved over the many centuries of
observations of the sky in the visible or optical band, e.g., the faintest
objects in the Hubble Ultra-Deep Field are of 29th magnitude brightness, which
corresponds to six ten-billionths of the brightness of the faintest naked eye
stars (about 6th magnitude).
The currently operating Chandra X-ray Observatory has a mirror which is
figured to such an exquisite accuracy that its resolution is better than
1 arcsecond (3600 arcseconds = 1 degree). The earliest X-ray detectors
were wide-angle instruments that had either no or poor (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), as did the ROSAT Observatory
which operated during the 1990s. 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, or 108 in area on the sky. The latter 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
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