Super-soft x-ray source
Encyclopedia
A super soft X-ray source (SSXS, or SSS) is an astronomical source of very low energy X-rays. Soft X-rays have energies in the 0.09 to 2.5 keV
range, whereas hard X-rays are in the 1-20 keV range. SSXSs are in most cases only detected below 0.5 keV, so that within our own galaxy they are usually hidden by interstellar absorption in the galactic disk. They are readily evident in external galaxies, with ~10 found in the Magellanic Clouds and at least 15 seen in M31.
As of early 2005, more than 100 SSSs have been reported in ~20 external galaxies, the Large Magellanic Cloud
(LMC), Small Magellanic Cloud
(SMC), and the Milky Way
(MW). Those with luminosities below ~3 x 1038 erg/s are consistent with steady nuclear burning in accreting white dwarf
s (WD)s or post-novae. There are a few SSS with luminosities ≥1039 erg/s.
Super soft X-rays are believed to be produced by steady nuclear fusion
on a white dwarf
's surface of material pulled from a binary companion
, the so-called close-binary supersoft source (CBSS). This requires a flow of material sufficiently high to sustain the fusion. Contrast this with the nova
, where less flow causes the material to only fuse sporadically. Super soft X-ray sources can evolve into type Ia supernova
, where a sudden fusion of material destroys the white dwarf, and neutron stars, through collapse.
Super soft X-ray sources were first discovered by the Einstein Observatory
. Further discoveries were made by ROSAT
. Many different classes of objects emit supersoft X-radiation (emission dominantly below 0.5 keV).
Apparently, luminous SSSs can have equivalent blackbody temperatures as low as ~15 eV and luminosities ranging from 1036 to 1038 erg/s. The numbers of luminous SSSs in the disks of ordinary spiral galaxies such as the MW and M31 are estimated to be on the order of 103.
(WD) as the generator of the prodigious super soft X-ray flux. As of 1999, eight SSXSs have orbital periods between ~4 hr and 1.35 d: RX J0019.8+2156 (MW), RX J0439.8-6809 (LMC), RX J0513.9-6951 (LMC), RX J0527.8-6954 (LMC), RX J0537.7-7034 (LMC), CAL 83 (LMC), CAL 87 LMC), and 1E 0035.4-7230 (SMC).
binary star
system in which a red giant
has expanded its outer envelope and is shedding mass
quickly, and another hot star (often a white dwarf
) is ionizing the gas. Three symbiotic binaries as of 1999 are SSXSs: AG Dra (BB, MW), RR Tel (WD, MW), and RX J0048.4-7332 (WD, SMC).
sources. The accretion disk may be prone to instability
leading to dwarf nova
outbursts: a portion of the disk material falls onto the white dwarf, the cataclysmic outbursts occur when the density
and temperature
at the bottom of the accumulated hydrogen layer rise high enough to ignite nuclear fusion
reactions, which rapidly burn the hydrogen layer to helium.
Apparently the only SSXS nonmagnetic cataclysmic variable is V Sge: bolometric luminosity of (1 - 10) x 1037, a binary including a blackbody (BB) accretor at T < 80 eV, and an orbital period of 0.514195 d.
The accretion disk can become thermally stable in systems with high mass-transfer rates (Ṁ). Such systems are called nova-like (NL) stars, because they lack outbursts characteristic of dwarf novae.
The high luminosity (6.5 x 1036 erg/s) is particularly hard to understand in the context of VY Scl stars generally, because observations suggest that the binaries become simple red dwarf + white dwarf pairs at quiescence (the disk mostly disappears). "A high luminosity in soft X-rays poses an additional problem of understanding why the spectrum is of only modest excitation." The ratio He II λ4686/Hβ did not exceed ~0.5 in any of the spectra recorded up to 2001, which is typical for accretion-powered CVs and does not approach the ratio of 2 commonly seen in supersoft binaries (CBSS).
Pushing the edge of acceptable X-ray fits toward lower luminosity suggests that the luminosity should not exceed ~2 x 1033 ergs/s, which gives only ~4 x 1031 ergs/s of reprocessed light in the WD about equal to the secondary's expected nuclear luminosity.
With temperatures in the range 11,000 to 15,000 K, all the WDs with the most extreme fields are far too cool to be detectable EUV/X-ray sources, e.g., Grw +70°8247, LB 11146, SBS 1349+5434, PG 1031+234 and GD 229.
Most highly magnetic WDs appear to be isolated objects, although G 23-46 (7.4 MG) and LB 1116 (670 MG) are in unresolved binary systems.
RE J0317-853 is the hottest magnetic WD at 49,250 K, with an exceptionally intense magnetic field of ~340 MG, and implied rotation period of 725.4 s. Between 0.1 and 0.4 keV, RE J0317-853 was detectable by ROSAT, but not in the higher energy band from 0.4 to 2.4 keV. RE J0317-853 is associated with a blue star 16 arcsec from LB 9802 (also a blue WD) but not physically associated. A centered dipole field is not able to reproduce the observations, but an off-center dipole 664 MG at the south pole and 197 MG at the north pole does.
Until recently (1995) only PG 1658+441 possessed an effective temperature > 30,000 K. Its polar field strength is only 3 MG.
The ROSAT
Wide Field Camera (WFC) source RE J0616-649 has an ~20 MG field.
PG 1031+234 has a surface field that spans the range from ~200 MG to nearly 1000 MG and rotates with a period of 3h24m.
The magnetic fields in CVs are confined to a narrow range of strengths, with a maximum of 7080 MG for RX J1938.4-4623.
None of the single magnetic stars has been seen as of 1999 as an X-ray source, although fields are of direct relevance to the maintenance of coronae in main sequence stars.
dominated by carbon and oxygen in their atmospheres.
PG 1159 stars reach luminosities of ~1038 erg/s but form a rather distinct class. RX J0122.9-7521 has been identified as a galactic PG 1159 star.
U Sco, a recurrent nova as of 1999 unobserved by ROSAT
, is a WD (74-76 eV), Lbol ~ (8-60) x 1036 erg/s, with an orbital period of 1.2306 d.
Kev
Kev can refer to:*Kev Hawkins, a fictional character.*Kevin, a given name occasionally shortened to "Kev".*Kiloelectronvolt, a unit of energy who symbol is "KeV".* Krefelder Eislauf-VereinKEV can refer to:...
range, whereas hard X-rays are in the 1-20 keV range. SSXSs are in most cases only detected below 0.5 keV, so that within our own galaxy they are usually hidden by interstellar absorption in the galactic disk. They are readily evident in external galaxies, with ~10 found in the Magellanic Clouds and at least 15 seen in M31.
As of early 2005, more than 100 SSSs have been reported in ~20 external galaxies, the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
(LMC), Small Magellanic Cloud
Small Magellanic Cloud
The Small Magellanic Cloud is a dwarf galaxy. It has a diameter of about 7,000 light-years and contains several hundred million stars. It has a total mass of approximately 7 billion times the mass of our Sun....
(SMC), and the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
(MW). Those with luminosities below ~3 x 1038 erg/s are consistent with steady nuclear burning in accreting white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
s (WD)s or post-novae. There are a few SSS with luminosities ≥1039 erg/s.
Super soft X-rays are believed to be produced by steady nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
on a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
's surface of material pulled from a binary companion
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
, the so-called close-binary supersoft source (CBSS). This requires a flow of material sufficiently high to sustain the fusion. Contrast this with the nova
Nova
A nova is a cataclysmic nuclear explosion in a star caused by the accretion of hydrogen on to the surface of a white dwarf star, which ignites and starts nuclear fusion in a runaway manner...
, where less flow causes the material to only fuse sporadically. Super soft X-ray sources can evolve into type Ia supernova
Type Ia supernova
A Type Ia supernova is a sub-category of supernovae, which in turn are a sub-category of cataclysmic variable stars, that results from the violent explosion of a white dwarf star. A white dwarf is the remnant of a star that has completed its normal life cycle and has ceased nuclear fusion...
, where a sudden fusion of material destroys the white dwarf, and neutron stars, through collapse.
Super soft X-ray sources were first discovered by the Einstein Observatory
Einstein Observatory
Einstein Observatory was the first fully imaging X-ray telescope put into space and the second of NASA's three High Energy Astrophysical Observatories...
. Further discoveries were made by ROSAT
ROSAT
ROSAT was a German Aerospace Center-led satellite X-ray telescope, with instruments built by Germany, the UK and the US...
. Many different classes of objects emit supersoft X-radiation (emission dominantly below 0.5 keV).
Luminous supersoft X-ray sources
Luminous super soft X-ray sources have a characteristic blackbody temperature of a few tens of eV (~20-100 eV) and a bolometric luminosity of ~1038 erg/s (below ~ 3 x 1038 erg/s).Apparently, luminous SSSs can have equivalent blackbody temperatures as low as ~15 eV and luminosities ranging from 1036 to 1038 erg/s. The numbers of luminous SSSs in the disks of ordinary spiral galaxies such as the MW and M31 are estimated to be on the order of 103.
Milky Way SSXSs
SSXSs have now been discovered in our galaxy and in globular cluster M3. MR Velorum (RX J0925.7-4758) is one of the rare MW super soft X-ray binaries. "The source is heavily reddened by interstellar material, making it difficult to observe in the blue and ultraviolet." The period determined for MR Velorum at ~4.03 d is considerably longer than that of other supersoft systems, which is usually less than a day.Close-binary supersoft source (CBSS)
The CBSS model invokes steady nuclear burning on the surface of an accreting white dwarfWhite dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
(WD) as the generator of the prodigious super soft X-ray flux. As of 1999, eight SSXSs have orbital periods between ~4 hr and 1.35 d: RX J0019.8+2156 (MW), RX J0439.8-6809 (LMC), RX J0513.9-6951 (LMC), RX J0527.8-6954 (LMC), RX J0537.7-7034 (LMC), CAL 83 (LMC), CAL 87 LMC), and 1E 0035.4-7230 (SMC).
Symbiotic binary
A symbiotic binary star is a variableVariable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
system in which a red giant
Red giant
A red giant is a luminous giant star of low or intermediate mass in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower...
has expanded its outer envelope and is shedding mass
Stellar wind
A stellar wind is a flow of neutral or charged gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.Different types of stars have...
quickly, and another hot star (often a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
) is ionizing the gas. Three symbiotic binaries as of 1999 are SSXSs: AG Dra (BB, MW), RR Tel (WD, MW), and RX J0048.4-7332 (WD, SMC).
Noninteracting white dwarfs
The youngest, hottest WD is very close to 100,000 K, of type DO and is the first single WD recorded as an X-ray source with ROSAT.Cataclysmic variables
"Cataclysmic variables (CVs) are close binary systems consisting of a white dwarf and a red-dwarf secondary transferring matter via the Roche lobe overflow." Both fusion- and accretion-powered cataclysmic variables have been observed to be X-rayX-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
sources. The accretion disk may be prone to instability
Instability
In numerous fields of study, the component of instability within a system is generally characterized by some of the outputs or internal states growing without bounds...
leading to dwarf nova
Dwarf nova
A U Geminorum-type variable star, or dwarf nova is a type of cataclysmic variable starhttp://www.sai.msu.su/groups/cluster/gcvs/gcvs/iii/vartype.txt consisting of a close binary star system in which one of the components is a white dwarf, which accretes matter from its companion...
outbursts: a portion of the disk material falls onto the white dwarf, the cataclysmic outbursts occur when the density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
and temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
at the bottom of the accumulated hydrogen layer rise high enough to ignite nuclear fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
reactions, which rapidly burn the hydrogen layer to helium.
Apparently the only SSXS nonmagnetic cataclysmic variable is V Sge: bolometric luminosity of (1 - 10) x 1037, a binary including a blackbody (BB) accretor at T < 80 eV, and an orbital period of 0.514195 d.
The accretion disk can become thermally stable in systems with high mass-transfer rates (Ṁ). Such systems are called nova-like (NL) stars, because they lack outbursts characteristic of dwarf novae.
VY Scl cataclysmic variables
Among the NL stars is a small group which shows a temporary reduction or cessation of Ṁ from the secondary. These are the VY Scl-type stars or anti-dwarf novae.V751 Cyg
V751 Cyg (BB, MW) is a VY Scl CV, has a bolometric luminosity of 6.5 x 1036 erg/s, and emits soft X-rays at quiescence. The discovery of a weak soft X-ray source of V751 Cyg at minimum presents a challenge as this is unusual for CVs which commonly display weak hard X-ray emission at quiescence.The high luminosity (6.5 x 1036 erg/s) is particularly hard to understand in the context of VY Scl stars generally, because observations suggest that the binaries become simple red dwarf + white dwarf pairs at quiescence (the disk mostly disappears). "A high luminosity in soft X-rays poses an additional problem of understanding why the spectrum is of only modest excitation." The ratio He II λ4686/Hβ did not exceed ~0.5 in any of the spectra recorded up to 2001, which is typical for accretion-powered CVs and does not approach the ratio of 2 commonly seen in supersoft binaries (CBSS).
Pushing the edge of acceptable X-ray fits toward lower luminosity suggests that the luminosity should not exceed ~2 x 1033 ergs/s, which gives only ~4 x 1031 ergs/s of reprocessed light in the WD about equal to the secondary's expected nuclear luminosity.
Magnetic cataclysmic variables
X-rays from magnetic cataclysmic variables are common because accretion provides a continuous supply of coronal gas. A plot of number of systems vs. orbit period shows a statistically significant minimum for periods between 2 and 3 hr which can probably be understood in terms of the effects of magnetic braking when the companion star becomes completely convective and the usual dynamo (which operates at the base of the convective envelope) can no longer give the companion a magnetic wind to carry off angular momentum. The rotation has been blamed on asymmetric ejection of planetary nubulae and winds and the fields on in situ dynamos. Orbit and rotation periods are synchronized in strongly magnetized WDs. Those with no detectable field never are synchronized.With temperatures in the range 11,000 to 15,000 K, all the WDs with the most extreme fields are far too cool to be detectable EUV/X-ray sources, e.g., Grw +70°8247, LB 11146, SBS 1349+5434, PG 1031+234 and GD 229.
Most highly magnetic WDs appear to be isolated objects, although G 23-46 (7.4 MG) and LB 1116 (670 MG) are in unresolved binary systems.
RE J0317-853 is the hottest magnetic WD at 49,250 K, with an exceptionally intense magnetic field of ~340 MG, and implied rotation period of 725.4 s. Between 0.1 and 0.4 keV, RE J0317-853 was detectable by ROSAT, but not in the higher energy band from 0.4 to 2.4 keV. RE J0317-853 is associated with a blue star 16 arcsec from LB 9802 (also a blue WD) but not physically associated. A centered dipole field is not able to reproduce the observations, but an off-center dipole 664 MG at the south pole and 197 MG at the north pole does.
Until recently (1995) only PG 1658+441 possessed an effective temperature > 30,000 K. Its polar field strength is only 3 MG.
The ROSAT
ROSAT
ROSAT was a German Aerospace Center-led satellite X-ray telescope, with instruments built by Germany, the UK and the US...
Wide Field Camera (WFC) source RE J0616-649 has an ~20 MG field.
PG 1031+234 has a surface field that spans the range from ~200 MG to nearly 1000 MG and rotates with a period of 3h24m.
The magnetic fields in CVs are confined to a narrow range of strengths, with a maximum of 7080 MG for RX J1938.4-4623.
None of the single magnetic stars has been seen as of 1999 as an X-ray source, although fields are of direct relevance to the maintenance of coronae in main sequence stars.
PG 1159 stars
PG 1159 stars are a group of very hot, often pulsating WDs for which the prototype is PG 1159PG 1159-035
PG 1159-035 is the prototypical PG 1159 star after which the class of PG 1159 stars was named. It was discovered in the Palomar-Green survey of ultraviolet-excess stellar objects and, like the other PG 1159 stars, is in transition between being the central star of a planetary nebula and being a...
dominated by carbon and oxygen in their atmospheres.
PG 1159 stars reach luminosities of ~1038 erg/s but form a rather distinct class. RX J0122.9-7521 has been identified as a galactic PG 1159 star.
Nova
There are three SSXSs with bolometric luminosity of ~1038 erg/s that are novae: GQ Mus (BB, MW), V1974 Cyg (WD, MW), and Nova LMC 1995 (WD). Apparently, as of 1999 the orbital period of Nova LMC 1995 if a binary was not known.U Sco, a recurrent nova as of 1999 unobserved by ROSAT
ROSAT
ROSAT was a German Aerospace Center-led satellite X-ray telescope, with instruments built by Germany, the UK and the US...
, is a WD (74-76 eV), Lbol ~ (8-60) x 1036 erg/s, with an orbital period of 1.2306 d.