Artikel zu "The supersoft X-ray sources — SSS" MIV: Multiwavelength Pulsars
Catalog of Supersoft X-ray Sources by Jochen Greiner M101: X-Ray Sources Point New Black Hole Population From Einstein to ROSAT: M31 in X-rays K1 Catalog of the supersoft X-ray sources K2 Accreting white dwarf model for ultrasoft X-ray sources K2.1 CAL 87 K3 Supersoft X-ray Sources in M31 K3.1 Ten Facts of Life for Distant Supersoft Sources K4 Galactic Glow Gleaned	— : XMM-Newton — : 18 SSS candidates M31 in X-rays K5 An ultraluminous SSS in NGC 4631 K6 V723 Cassiopeia Literatur

SSS — SuperSoft X-ray Sources

K1 Catalog of the supersoft X-ray sources

Introduction

After the discovery of supersoft X-ray sources with Einstein Observatory observations, the ROSAT satellite with its PSPC detector has discovered about four dozen new supersoft sources and has thus established luminous supersoft X-ray sources (SSS) as a new class of objects. Though many different classes of objects emit supersoft X-ray radiation (defined here as emission dominantly below 0.5 keV which corresponds to effective temperatures of the emitting objects of <50 eV), we consider here sources with bolometric luminosities in the range 10³⁶-10³⁸ erg/s.
Optical observations have revealed the binary nature of several of these objects. A white dwarf (WD) model, the so-called close-binary supersoft source (CBSS) model, is perhaps the most promising (van den Heuvel et al. 1992).
It invokes steady-nuclear burning on the surface of an accreting WD as the generator of these systems' prodigious flux. Indeed, SSS temperatures and luminosities as derived from the X-ray data suggest an effective radius comparable to that of WDs.
Eight SSSs have orbital periods between approximately 4 hrs and 3.5 days. These are the candidates for the CBSS model. Mass transfer rates derived from the CBSS model are in the right range for steady nuclear burning of the accreted matter.

Einstein, ROSAT and beyond...

The two most famous supersoft X-ray sources, CAL 83 and CAL 87 (Long et al. 1981), have been discovered with Einstein satellite observations. ROSAT observations established these sources as a distinct class in the early nineties.
During the years 1995-1999 the high-resolution imager (HRI) on ROSAT has been used to improve the coordinates of the newly detected sources down to typically 10'' and to monitor the long-term X-ray intensity. At these soft energies, the HRI count rates are typically a factor of 7.5-8 smaller than those of the PSPC (David et al. 1994, Greiner et al. 1996a). Since 1997, some of the brightest supersoft X-ray sources have been also observed with the low-energy concentrator spectrometer (LECS) onboard BeppoSAX.
With the Chandra and XMM missions starting regular observations, a wealth of new information on the X-ray properties of these supersoft sources can be expected, as well as new discoveries. In particular, the better energy resolution, through-put and location accuracy will improve our understanding.

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	Authors: A. Ibragimov, V. Suleimanov, A. Vikhlinin, N. Sakhibullin
	Journal-ref: Astronomy Report 47 (2003) 186 [astro-ph/0603429 ]
	Title: Supersoft X-ray Sources. Parameters of Stellar Atmospheres
Abstract: ROSAT spectra of 11 supersoft X-ray sources (RX J0439.8-6809, RX J0513.9-6951, RX J0527.8-6954, CAL 87, CAL 83, 1E 0035.4-7230, RX J0048.4-7332, 1E 0056.8-7154, RX J0019.8 +2156, RX J0925.7-4758, AG Draconis) are approximated with theoretical spectra obtained in LTE models for the atmospheres of hot white dwarfs with line blanketing. The confidence intervals of parameters derived from these approximations T_eff, log(g), N_H, and (R/d)² are determined. The results are compared with predictions for a model with stable/recurrent thermonuclear burning on the white-dwarf surface.

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	Authors: V. Suleimanov, A. Ibragimov
	Journal-ref: Astronomy Report 47 (2003) 197 [astro-ph/0603430 ]
	Title: Supersoft X-ray Sources. Basic Parameters
Abstract: The parameters of ten supersoft X-ray sources (RX J0439.8-6809, RX J0513.9-6951, RX J0527.8-6954, CAL 87, CAL 83, 1E 0035.4-7230, RX J0048.4-7332, 1E 0056.8-7154, RX J0019.8 +2156, RX J0925.7-4758) observed by ROSAT obtained using blanketing LTE model atmospheres are analyzed. The consistency of the resulting parameters with a model with stable/recurrent burning on the surface of the white dwarf is studied. The luminosity and sizes of seven of the sources are in good agreement with this model. The masses of the white dwarfs in these sources are estimated. A formula that can be used to estimate the masses of white dwarfs in classical supersoft sources based on their effective temperatures is presented.

K2 Accreting white dwarf model for ultrasoft X-ray sources

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	Authors: van den Heuvel E.P.J., Bhattacharya D., Nomoto K., Rappaport S.A.
	Journal-ref: A&A; 262 (1992) 97 []
	Title: Accreting white dwarf models for CAL 83, CAL 87 and other ultrasoft X-ray sources in the LMC
Abstract: The study demonstrates that the ultrasoft X-ray emission observed in the three strong LMC X-ray sources CAL 83, CAL 87, and RXJ 0527.8-6954 can be explained by steady nuclear burning of hydrogen accreted onto white dwarfs with masses in the range of 0.7 to 1.2 solar mass. The observed optical and X-ray characteristics of the binary systems CAL 83 and CAL 87 are shown to be consistent with such a model. In both systems the companions are main-sequence stars with masses in the range of 1.5 to 2 solar masses. They are transferring mass unstably on a thermal time scale by Roche-lobe overflow, at rates between 1.0 and 4.0 x 10 exp -7 solar mass/yr. It is argued that the stellar wind emanating from the heated star interacts with that from the disk to generate the He II 4686 emission line with a radial velocity amplitude much lower than the actual radial velocity amplitude of the white dwarf, thus yielding an apparently excessively large mass estimate of this compact star. It is suggested that CAL 83 and CAL 87 are the white dwarf analogs of X-ray binaries like Her X-1.

credit: credit: Dan Corbett, Kate Stafford, and Patrick Wright for ThinkQuest.

Luminous Supersoft X-Ray Sources

An average star shines because nuclear fusion in its core generates energy in the form of x-rays and gamma rays, which become visible light by the time they reach the star's surface. However, scientists have recently found that some stars have nuclear fusion occurring just below their surfaces. These stars are white dwarves, or dense stars that have used up their nuclear fuel, in orbit around other ordinary stars.
In this type of arrangement, the dwarf siphons off gas and other materials from the surface of its companion. This material congregates on the dwarf's surface and initiates the unusual type of stellar fusion described above. As a result, the dwarves emit large quantities of x-rays with "soft" wavelengths. When the dwarves accumulate too much gas and thus gain too much mass, they become unstable and either collapse into a very dense neutron star or explode into a Type Ia supernova.

Soft and Hard X-Rays

Hard x-rays are much more commonly researched than their soft counterparts. With typical energies of 1-20 kiloelectron volts (keV), they are much easier to detect, much higher in energy, and can be generated by a number of cosmological processes. They usually imply temperatures of 10-100 million kelvins and often represent neutron stars or black holes "digesting" large amounts of matter, often from a companion star. Soft x-rays, on the other hand, basically represent the boundary line between hard x-rays and ultraviolet light. Their wavelengths are typically 50 to 1000 times smaller than those of visible light, putting their energy in the 0.09-2.5 keV range. They are generally emitted by objects with temperatures in the hundred thousands of kelvins.

Supersoft vs. Traditional Sources

Scientists studying supersoft sources first thought they were neutron stars or black holes in orbit around ordinary stars. Both of these systems would involve the dense star remnant in question siphoning off matter from its companion and using it for nuclear fusion. However, in neutron stars, a large amount of energy released in the form of energetic hard x-rays balances out a lower fusion rate. The x-rays produced by black holes are generally softer than neutron stars because 46% of the matter accreted by a black hole disappears into it. For this reason, supersoft x-ray sources were originally thought to be black holes. However, it was quickly shown that the sources were much softer than any known black-hole system. It was concluded that the sources in question were white dwarves accreting matter onto their surfaces and releasing much lower energies due to their lower gravity and greater efficiency in fusion.

The Life of a Supersoft Source

Supersoft sources begin as binary systems whose stars have significantly different lifespans. Eventually one will use up all the fuel in its core and cease nuclear fusion, becoming a red giant. The orbit then begins to tighten and the giant releases its outermost layers, becoming a white dwarf with no internal nuclear fusion. At this point, the type of the initial system determines its future: if the companion star is an ordinary star or a large red giant, it essentially surrenders its outer layers of gas to the white dwarf. If it is a smaller red giant or is in a wide orbit, its solar winds can drive the supersoft source. When the white dwarf has accumulated enough mass to become unstable, it either collapses still further into a neutron star (thus becoming a hard x-ray source) or explodes as a Type Ia supernova. Supersoft x-ray sources, depending on the rate of accretion of material from the companion star, may also provide explanations for certain types of novae.

The Death of a Supersoft Source

The death of supersoft sources can occur in three ways:

the white dwarf can collapse into a neutron star and thus become a classic source of hard x-rays.
the companion star can begin to "feel" the effects of the loss of its outer layers and thus cease the donation of its matter.
the white dwarf can explode into a Type Ia supernova. This occurs when the white dwarf reaches the Chandrasekhar limit, or the maximum mass it can stably support, and either possesses carbon or was initially smaller than 1.1 solar masses.

Some white dwarves that reach the supernova stage simply amass too much helium and explode completely. In most, however, the helium layer reaches its own critical mass and explodes independently. The explosion then ignites the carbon core of the star, which is converted almost within seconds to nickel and elements between silicon and iron. The nickel then falls into space and decays to cobalt, then iron, in the space of a few hundred days. This is the supernova Type Ia, in which no spectroscopic evidence of hydrogen or helium participation is evident (this distinguishes it from Type Ib, Ic, and II). Type Ia supernovae are thought to be major sources of iron and similar elements in the universe. Type Ia supernovae are also often used by astronomers as "standard candles" to determine distances to other, less well-understood objects.

K2.1 CAL 87

CAL 87 — P_orb = 10.6 h — P/P' = +7.2 ± 1.3 Myr

Authors: A.S. Oliveira, J.E. Steiner

Journal-ref: A&A; (2007) L [0707.3441

]

Title: CAL 87 - an evolved wind-driven supersoft X-ray binary

Abstract: Compact binary supersoft X-ray sources (CBSS) are explained as being associated with hydrostatic nuclear burning on the surface of a white dwarf with high accretion rate. This high mass transfer rate has been suggested to be caused by dynamical instability, expected when the donor star is more massive than the accreting object.
When the orbital period is smaller than ~6 hours, this mechanism does not work and the CBSS with such periods are believed to be fed by a distinct mechanism: the wind-driven accretion.
Such a mechanism has been proposed to explain the properties of objects like SMC 13, T Pyx and V617 Sgr.
One observational property that offers a critical test for discriminating between the above two possibilities is the orbital period change. As systems with wind-driven accretion evolve with increasing periods, some of them may reach quite long orbital periods.
The above critical test may, therefore, also be applied to orbital periods longer than 6 hours.
CAL 87 is an eclipsing system in the LMC with an orbital period of 10.6 hours that could provide the opportunity for testing the hypothesis of the system being powered by wind-driven accretion. We obtained eclipse timings for this system and show that its orbital period increases with a rate of P/P' = +7.2 ± 1.3 Myr.
Contrary to the common belief, we conclude that CAL 87 is the first confirmed case of a wind-driven CBSS with an orbital period longer than 6 hours. The system is probably an evolved object that had an initial secondary mass of M_{2 i}=0.63 M but is currently reduced to about M₂=0.34 M.
We discuss evidence that other CBSS, like CAL 83 and V Sge stars, like WX Cen, are probably also wind-driven systems. This may in fact be the rule, and systems with inverted mass ratio, the exception.

Table 2. Physical parameters and orbital elements of CAL 87

1. Introduction

Compact binary supersoft X-ray sources (CBSS) are a group of objects, first discovered in the Magellanic Clouds, with unusual properties. Their large (~ Eddington) luminosity is mostly radiated in the supersoft X-ray spectral range of 20 to 80 eV. Van den Heuvel et al. (1992) have shown convincingly that this emission is originated from hydrostatic nuclear burning on the surface of a white dwarf. In order for this to happen, a high accretion rate (M'_acc ~ 10^-7 M yr^-1) is required, such as may be realized in systems with inverted mass ratios, in the sense that the mass donor is the more massive of the binary components. Mass transfer is then dynamically unstable and occurs on the Kelvin-Helmholtz timescale, with the orbital period decreasing with time. For mass donor stars with masses in the range 1.3 to 2.5 M, this configuration will provide the required accretion rate of ~ 10^-7 M yr^-1. This framework has been adopted in the literature since it was proposed (van den Heuvel et al. 1992).

3. Assisted stellar suicide – a terminal case?

The first conclusion is that, contrary to the canonical view that considers CBSS as having massive (1–2 M) secondary stars, CAL 87 clearly has a mass ratio which is typical for cataclysmic variables, i.e., M₂ < M₁. This has been anticipated by Cowley et al. (1990, 1998) who found, given the small amplitude of the radial velocity of emission lines, that M₂ = 0.4M and M₁ > 4 M.
For this reason those authors explored the idea that such a massive primary could be a black hole. We now know that the mass accreting star is probably a white dwarf: its mass, derived from fitting the luminosity-temperature diagram with theoretical calculations, is M₁ = 1.35M (Starrfield et al. 2004). The observed supersoft X-ray luminosity, L_X = 4 × 10³⁶ erg s^-1 (Starrfield et al. 2004), implies an accretion rate of M'_acc = 10^-8 M yr^-1.
The initial orbital period was P_i = 5.7 hr and the initial mass of the secondary was about M_{2 i} ~ 0.63M. In other words, 0.29M have already been evaporated or transferred to the primary star. Considering M1 = 1.35M and the accretion rate of about 10^-8 M yr^-1, CAL 87 may explode as a type Ia supernova in 10 Myr. The parameters of CAL 87 are presented in Table 2.

References
Starrfield, S. et al. 2004, ApJ, 612, L53 
  Surface Hydrogen-burning Modeling of Supersoft X-Ray Binaries: Are They Type Ia Supernova Progenitors?
van den Heuvel, E.P.J., Bhattacharya, D., Nomoto, K. & Rappaport, S.A., 1992, A&A; 262, 97

K3 Supersoft X-ray Sources in M31

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Authors: R. Di Stefano, A.K.H. Kong, J. Greiner, F.A. Primini, M.R. Garcia, P. Barmby, P. Massey, P.W. Hodge, B.F. Williams, S.S. Murrary, S. Curry, T.A. Russo

Journal-ref: Astrophys.J. 610 (2004) 247-260 [astro-ph/0306440

]

Title: Supersoft X-ray Sources in M31: I. A Chandra Survey and an Extension to Quasisoft Sources

CHANDRA / R. DI STEFANO

FIG. 1.— Detected VSSs (red dots) overlaid on an optical Digitized Sky Survey image of M31. The fields of view of the three Chandra ACIS-S observations in the disk (black boxes) and the central region (green box) are also shown. Also shown in the figures are the optical position of SNRs (yellow plus signs). The ellipse shows the D25 isophote.

Abstract: We report on very soft X-ray sources (VSSs) in M31. In a survey which was most sensitive to soft sources in four 8'x8' regions covered by Chandra's ACIS-S S3 CCD, we find 33 VSSs that appear to belong to M31. Fifteen VSSs have spectral characteristics mirroring the supersoft X-ray sources studied in the Magellanic Cloud and Milky Way
(k T_eff < 100 eV); we therefore call these ``classical'' supersoft sources, or simply supersoft sources (SSSs).
Eighteen VSSs may either have small (< 10%) hard components, or slightly higher effective temperatures (but still < 350 eV). We refer to these VSSs as quasisoft sources (QSSs). While hot white dwarf models may apply to SSSs, the effective temperatures of QSSs are too high, unless, e.g., the radiation emanates from only a small portion of surface.
Two of the SSSs were first detected and identified as such through ROSAT observations. One SSS and one QSS may be identified with symbiotics, and 2 SSSs with supernova remnants. Both SSSs and QSSs in the disk are found near star-forming regions, possibly indicating that they are young.
VSSs in the outer disk and halo are likely to be old systems; in these regions, there are more QSSs than SSSs, which is opposite to what is found in fields closer to the galaxy center.
The largest density of bright VSSs is in the bulge; some of the bulge sources are close enough to the nucleus to be remnants of the tidal disruption of a giant by the massive central black hole.
By using Chandra data in combination with ROSAT and XMM observations, we find most VSSs to be highly variable, fading from or brightening toward detectability on time scales of months. There is evidence for VSSs with low luminosities
(L_X ~ 10³⁶ erg s^-1).

K3.1 Ten Facts of Life for Distant Supersoft Sources

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	Authors: Rosanne Di Stefano, Albert Kong, Francis A. Primini
	Journal-ref: (2006) [astro-ph/0606364 ]
	Title: Ten Facts of Life for Distant Supersoft Sources
Abstract: First discovered in the Magellanic Clouds and in the Milky Way, the largest pools of luminous supersoft X-ray sources (SSSs) now known lie in M31 and in more distant galaxies. Hundreds of newly-discovered SSSs are helping us to test models for Type Ia supernovae and to identify SSSs that may represent a wider range of physical systems, including accreting intermediate-mass black holes. In this short report we list ten intriguing facts about distant SSSs. INTRODUCTION Luminous supersoft X-ray sources (SSSs) were discovered and defined in terms of properties observed in a small number of sources (~ 18) in the Galaxy and Magellanic Clouds (MCs). Specifically, SSSs are defined in terms of their estimated luminosities ( L_X > 10³⁶ erg s^-1 ) and their broad band spectra, with little or no emission above 1 keV. The physical nature of SSSs is not yet determined. SSS effective radii are comparable to those of WDs. Indeed, roughly half of the SSSs in the MCs and Milky Way with optical IDs have counterparts that are consistent with systems known to contain hot WDs: planetary nebulae, recent novae, and symbiotic binaries. The remaining SSSs in the Galaxy and MCs have measured periods ranging from a few hours to a few days. The first and best-known model for these binary was developed by Ed van den Heuvel and collaborators (1992). The close-binary supersoft (CBSS) model, postulates that the prodigious luminosities are generated through the nuclear burning of material accreted by a WD. In order for nuclear burning to occur, the accretion rates must be high (close to or larger than 10^-7 M yr^-1). These high rates can be sustained only if the donor star is somewhat more massive than the WD and/or slightly evolved. An interesting characteristic of these models is that they allow the WD to increase its mass. Some SSS binaries may therefore be progenitors of Type Ia supernovae

K4 Galactic Glow Gleaned

Ursprung des galaktischen Röntgenlichts geklärt

[21. Februar 2006] Neue Himmelskarte zeigt Millionen bisher unbekannter Objekte.

(Großbild)

Wissenschafter vom Max-Planck-Institut für Astrophysik haben entdeckt, dass das Röntgenleuchten der Milchstraße von rund einer Million Doppelsternen stammt, in denen jeweils ein Weißer Zwergstern von seinem Begleitstern Gas absaugt, sowie von Hunderten von Millionen gewöhnlicher Sterne mit aktiven Gashüllen. Die Entdeckung wurde durch die genaueste Röntgenkarte unserer Galaxie ermöglicht, die auf zehnjährigen Messungen mit dem Rossi XTE-Satelliten beruht. Die Forscher sind überzeugt, dass das Röntgenleuchten der Milchstrasse nicht - wie lange vermutet - diffus ist, sondern von Hunderten von Millionen einzelner Quellen stammt.

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	Authors: S. Sazonov, M. Revnivtsev, M. Gilfanov, E. Churazov, R. Sunyaev
	Journal-ref: A&A; (2006) [astro-ph/0510049 ]
	Title: X-ray luminosity function of faint point sources in the Milky Way
Abstract: We assess the contribution to the X-ray (above 2 keV) luminosity of the Milky Way from different classes of low-mass binary systems and single stars. We begin by using the RXTE Slew Survey of the sky at \|b\|>10 to construct an X-ray luminosity function (XLF) of nearby X-ray sources in the range 10³⁰ erg s^-1 < Lx < 10³⁴ erg s^-1 (where Lx is the luminosity over 2-10 keV), occupied by coronally active binaries (ABs) and cataclysmic variables (CVs). We then extend this XLF down to Lx~10^27.5 erg s^-1 using the Rosat All-Sky Survey in soft X-rays and available information on the 0.1-10 keV spectra of typical sources. We find that the local cumulative X-ray (2-10 keV) emissivities (per unit stellar mass) of ABs and CVs are (2.0 ± 0.8)x10^27 and (1.1 ± 0.3)x10²⁷ erg s^-1/M, respectively. In addition to ABs and CVs, representing old stellar populations, young stars emit locally (1.5 ± 0.4)x10²⁷ erg s^-1/M. We finally attach to the XLF of ABs and CVs a high luminosity branch (up to ~10³⁹ erg s^-1) composed of neutron-star and black-hole low-mass X-ray binaries (LMXBs), derived in previous work. The combined XLF covers ~12 orders of magnitude in luminosity. The estimated combined contribution of ABs and CVs to the 2-10 keV luminosity of the Milky Way is ~2x10³⁸ erg s^-1, or ~3% of the integral luminosity of LMXBs (averaged over nearby galaxies). The XLF obtained in this work is used elsewhere (Revnivtsev et al.) to assess the contribution of point sources to the Galactic ridge X-ray emission.
	Authors: Mikhail Revnivtsev, Sergey Sazonov, M.Gilfanov, E.Churazov, R.Sunyaev
	Journal-ref: A&A; (2006) [astro-ph/0510050 ]
	Title: Origin of the Galactic ridge X-ray emission
Abstract: We analyze a map of the Galactic ridge X-ray emission (GRXE) constructed in the 3-20 keV energy band from RXTE/PCA scan and slew observations. We show that the GRXE intensity closely follows the Galactic near-infrared surface brightness and thus traces the Galactic stellar mass distribution. The GRXE consists of two spatial components which can be identified with the bulge/bar and the disk of the Galaxy. The parameters of these components determined from X-ray data are compatible with those derived from near-infrared data. The inferred ratio of X-ray to near-infrared surface brightness I(3-20 keV) (1e-11 erg/s/cm2/deg2)/I_(3.5µ)(MJy/sr)=0.26 ± 0.05, and the ratio of X-ray to near-infrared luminosity L_(3-20 keV)/L_(3-4 µ)=(4.1 ± 0.3)e-5. The corresponding ratio of the 3-20 keV luminosity to the stellar mass is L_x/M= (3.5 ± 0.5) 10²⁷ erg s^-1, which agrees within the uncertainties with the cumulative emissivity per unit stellar mass of point X-ray sources in the Solar neighborhood, determined in an accompanying paper (Sazonov et al.). This suggests that the bulk of the GRXE is composed of weak X-ray sources, mostly cataclysmic variables and coronally active binaries. The fractional contributions of these classes of sources to the total X-ray emissivity determined from the Solar neighborhood data can also explain the GRXE energy spectrum. Based on the luminosity function of local X-ray sources we predict that in order to resolve 90% of the GRXE into discrete sources a sensitivity limit of ~10^{-16} erg/s/cm2 (2--10 keV) will need to be reached in future observations.

Nahezu 400 Jahre nachdem Galileo Galilei das schwache Leuchten der Milchstraße auf die Beiträge unzähliger Sterne zurückführte, haben Wissenschaftler mit Hilfe des Rossi X-ray Timing Explorer des NASA nun die verteilte Röntgenstrahlung in der Milchstraße auf dieselbe Weise erklärt.

Courtesy NASA / RXTE / M. Revnivtsev et al.

Abb. 1: Die perfekte Übereinstimmung des von Rossi XTE gemessenen Röntgenbildes (Konturlinien) und des vom COBE-Satelliten aufgenommenen Bildes der Milchstraße im nahen Infrarot (Farbe) bedeutet, dass die Röntgenstrahlung die Verteilung der Sterne abbildet und dass der galaktische Röntgenhintergrund von einer sehr großen Zahl von einzelnen schwachen Quellen stammt.

Der Ursprung dieses sogenannten galaktischen Röntgenhintergrunds blieb lange rätselhaft. Die Forscher sind nun überzeugt, dass das Röntgenleuchten nicht wie lange vermutet diffus ist, sondern von Hunderten von Millionen einzelner Quellen stammt, zumeist Weißen Zwergen, also toten Sternen von der Größe der Erde, aber der Masse unserer Sonne.
Falls dieses Ergebnis durch weitere Messungen bestätigt wird, hat das wichtige Konsequenzen für unser Verständnis der Entwicklung der Milchstraße, ihrer Sternentstehungs- und Supernovarate sowie ihrer Sternentwicklung. Es löst damit wichtige theoretische Probleme, weist aber gleichzeitig auf eine erstaunliche Unterschätzung der Zahl von stellaren Objekten hin.
"Vom Flugzeug aus kann man das diffuse Leuchten einer nächtlichen Stadt sehen", sagt Dr. Mikhail Revnivtsev vom MPA, Erstautor einer der beiden Publikationen. "Aber zu sagen, eine Stadt erzeugt Licht, ist nicht genug. Nur beim Näherkommen kann man die einzelnen Quellen erkennen, von denen die Strahlung stammt, die Lichter der Häuser, Straßenlampen, Autos. In diesem Sinne haben wir jetzt die einzelnen Objekte identifiziert, die zum Röntgenleuchten der Milchstraße beitragen. Unser Ergebnis wird viele Wissenschaftler überraschen."
Röntgenstrahlen sind eine Art Licht hoher Energie, unsichtbar für das Auge und weit hochenergetischer als sichtbares Licht oder ultraviolette Strahlung. Unsere Augen sehen einzelne Sterne verteilt auf einem größtenteils dunklen Himmel. Im Röntgenlicht ist der Himmel nirgends dunkel; er zeigt ein überall vorhandenes und gleichbleibendes Glühen.
Frühere Beobachtungen konnten nicht genügend Röntgenquellen finden, um das Röntgenleuchten der Milchstraße zu erklären. Dies zog theoretische Probleme nach sich. Wenn es Röntgenstrahlung von heißem, diffusem Gas war, dann würde dieses Gas irgendwann "aufsteigen" und aus dem Schwerkraftkäfig der Milchstraße entweichen. Außerdem müsste all dieses heiße Gas von Millionen von vergangenen Sternexplosionen, so genannte Supernovae, stammen, was bedeuten würde, dass unsere Schätzungen für die Zahl entstehender und sterbender Sterne völlig falsch wären.

Bild: NASA/Max-Planck-Institut für Astrophysik

Abb. 2: Breitband-Verteilung - über rund 13 Größenordnungen in der Leuchtkraft - der differentiellen Luminosität von galaktischen Röntgenquellen pro Einheit der Sternmasse bei Energien zwischen 2 und 10 keV. Verschiedene hauptsächlich beitragende Quellen sind markiert: aktive Doppelsterne (ABs), kataklysmische Variable (CVs) und Röntgendoppelsterne mit einem Neutronenstern oder Schwarzen Loch (LMXBs).

"Die Strahlung von einzelnen Quellen, die mit Röntgenteleskopen ausgemacht werden konnten, schien nicht mehr als 30 Prozent des Röntgenleuchtens zu erklären", sagt Dr. Jean Swank, Projektwissenschaftler für den Rossi Explorer am NASA Goddard Space Flight Center in Greenbelt, Maryland. "Viele Forscher glaubten, dass der Löwenanteil der Strahlung wirklich diffuser Natur ist, etwa von heißem Gas zwischen den Sternen stammt. Nun scheint es, als könnte man die gesamte Strahlung durch zwei Arten von Sternen erklären."
Die neue Studie basiert auf fast zehnjährigen Messungen mit dem Rossi Explorer, die die genaueste Karte unserer Milchstraße im Röntgenlicht hervorgebracht haben. Das Team von Wissenschaftlern kam zu dem Schluss, dass es in unserer Milchstraße tatsächlich von Sternen mit Röntgenemission nur so wimmelt, die größtenteils aber nicht sehr hell sind und deren Zahl daher früher um das Hundertfache unterschätzt worden war.
Überraschenderweise sind die üblichen Verdächtigen für die Herkunft von Röntgenstrahlung, nämlich Schwarze Löcher und Neutronensterne, hier nicht beteiligt. Bei höheren Energien der Röntgenstrahlung wird nahezu die gesamte Strahlung von so genannten "kataklysmischen Variablen" erzeugt.
Kataklysmische Variable sind Doppelsternsysteme, die aus einem recht gewöhnlichen Stern und einem Weißen Zwerg bestehen, der als Überrest eines Sterns wie der Sonne zurückbleibt, wenn der nukleare Brennstoff im Innern aufgebraucht ist. Für sich allein leuchtet ein Weißer Zwerg nur schwach. In einem Doppelsternsystem jedoch kann er Gas von seinem Begleitstern abziehen und sich dabei in einem Vorgang stark aufheizen, den man "Akkretion" nennt. Das akkretierte Gas wird dann so heiß, dass es intensive Röntgenstrahlung abgibt.
Bei etwas geringeren Röntgenenergien stammt das galaktische Glühen zu einem Drittel von kataklysmischen Variablen und zwei Dritteln von aktiven Vorgängen in der heißen Gashülle, der Korona, von Sternen. In den meisten Fällen findet die Koronaktivität bei Sternen in Doppelsystemen statt, in denen ein naher Begleitstern die äußeren Schalen des Sterns stark "aufrührt". Dies führt zu Ausbrüchen ähnlich den solaren Flares, bei denen Röntgenstrahlung frei wird. Die Wissenschaftlergruppe schätzt, dass es rund eine Million kataklysmischen Variable und nahezu eine Milliarde aktive Sterne in unserer Milchstraße gibt. Beide Zahlen bedeuten, dass frühere Schätzungen deutlich zu niedrig lagen.
"Wie eine medizinische Röntgenaufnahme enthüllt die Karte des galaktischen Röntgenhintergrunds Feinheiten der Struktur unserer Heimatgalaxie", sagt Revnivtsev. "Wir können durch unsere gesamte Milchstraße hindurch sehen und einzelne Röntgenquellen zählen. Dies ist von großer Bedeutung für die Astronomen, die die
Entwicklung von Sternen mit Computern berechnen."
Der Rossi Explorer Satellit wurde im Dezember 1995 gestartet.

Thanks to NASA's Rossi X-ray Timing Explorer (RXTE) satellite, astronomers appear to have solved the long-standing mystery of what produces the diffuse glow of X-ray emission that permeates our galaxy. To the surprise of many, the background glow originates from huge numbers of white dwarfs — the dead cores of roughly solar-mass stars — and the hot outer atmospheres (coronae) of ordinary stars.
The Milky Way's X-ray background was discovered in balloon-borne and satellite experiments in the 1970s. The X-ray emission is smoothly distributed in the galactic plane, with a brightness peak toward the galactic center. Because major X-ray observatories such as Chandra and XMM-Newton were unable to resolve the background into individual objects, many astronomers assumed that the glow came from hot, diffuse interstellar gas.
But in two papers that will appear in the European journal Astronomy & Astrophysics, Mikhail Revnivtsev, Sergey Sazonov (both at Max Planck Institute for Astrophysics, Germany, and the Space Research Institute, Russia), and three colleagues show that interstellar gas can account for only a very small portion of the background. The team found a tight correlation between the X-ray background as mapped in 10 years of RXTE data and our galaxy’s near-infrared glow as mapped by NASA's Cosmic Background Explorer (COBE) satellite in the early 1990s. Since the near-infrared glow emanates from huge numbers of individual stars, the close match between the RXTE and COBE data strongly suggests that the X-ray emission also comes from stars, not diffuse gas.
"From the local population of X-ray sources (less than 300 light-years away), we see that most of the sources are accreting white dwarfs or ordinary stars in binary systems," says Revnivtsev. He and his colleagues propose that approximately a million white dwarfs and a billion normal stars can account for the X-ray background, and that previous Chandra observations had insufficient sensitivity to see them as individual sources.
The white dwarfs slowly accrete gas from close companion stars, a process that produces relatively high-energy X-rays. These binary systems are known as cataclysmic variables (CVs), since occasionally the white dwarf will collect enough matter to flare up in a tremendous explosion. Most of the lower-energy sources are probably ordinary stars in binary systems. The tidal influence of the companion maintains the star's fast rotation, which keeps its corona magnetically active so it can produce detectable levels of low-energy X-rays.
The results suggest that astronomers have underestimated the number of contributing X-ray sources by a factor of a hundred. Ironically, says Revnivtsev, astronomers in the 1970s produced estimates of the number of weak X-ray sources that were much closer to the mark than estimates that have appeared in recent papers. And the fact that the background is not due to hot gas clears up the mystery of why such hot (and therefore fast-moving) material does not escape the gravitational clutches of our home galaxy.
"I personally think the arguments of Revnivtsev and his colleagues are convincing in the overall picture," says Koji Mukai (NASA/Goddard Space Flight Center). "It is now up to the proponents of the diffuse gas origin to re-argue their case, that somehow, somewhere, Revnivtsev's group went wrong. I think that is going to be a tough job indeed." Mukai estimates that our galaxy may host up to 10 million CVs.

K5 An ultraluminous SSS in NGC 4631

NGC 4631 — d = 7.5 Mpc
	Authors: S. Carpano, A. M. T. Pollock, A. R. King, J. Wilms, M. Ehle
	Journal-ref: A&A; 471 (2007) L55 [0707.0933 ]
	Title: An ultraluminous supersoft source with a 4 hour modulation in NGC 4631
Abstract: • Context. Supersoft X-ray sources (SSSs) are characterised by very low temperatures (<100 eV). Classical SSSs have bolometric luminosities in the range of 10^36-38 erg s^-1 and are modelled with steady nuclear burning of hydrogen on the surfaces of white dwarfs. However, several SSSs have been discovered with much higher luminosities. Their nature is still unclear. • Aims. We report the discovery of a 4 h modulation for an ultraluminous SSS in the nearby edge-on spiral galaxy NGC 4631, observed with XMM-Newton in 2002 June. Temporal and spectral analysis of the source is performed. • Methods. We use a Lomb-Scargle periodogram analysis for the period search and evaluate the confidence level using Monte-Carlo simulations. We measure the source temperature, flux and luminosity through spectral fitting. • Results. A modulation of 4.2 ± 0.4 h was found for the SSS with a confidence level >99%. Besides dips observed in the light curve, the flux decreased by a factor of 3 within ~ 10 h. The spectrum can be described with an absorbed blackbody model with kT ~ 67 eV. The absorbed luminosity in the 0.2–2 keV energy band was L_X(0.2-2 keV) = 2.7 × 10³⁸ erg s^-1 while the bolometric luminosity was a hundred times higher L_bol = 3.2 × 10⁴⁰ erg s^-1, making the source one of the most luminous of its class, assuming the best fit model is correct. • Conclusions. This source is another very luminous SSS for which the standard white dwarf interpretation cannot be applied, unless a strong beaming factor is considered. A stellar-mass black hole accreting at a super Eddington rate is a more likely interpretation, where the excess of accreted matter is ejected through a strong optically-thick outflow. The 4 h modulation could either be an eclipse from the companion star or the consequence of a warped accretion disk. 1. Introduction The X-ray binaries known as supersoft sources (SSSs) are characterised by very soft emission with temperatures <100 eV and bolometric luminosities exceeding 10³⁶ erg s^-1. The standard model for such soft and high luminosities was proposed by van den Heuvel et al. (1992) as nuclear burning of hydrogen on the surface of a white dwarf of mass in the range 0.7–1.2 M. On the other hand, if luminosities greatly exceed the Eddington limit for solar mass compact objects, it is not obvious that this type of model could apply unless beaming is considered. Several such very luminous SSSs have been reported in nearby galaxies: two in M101 (Di Stefano & Kong 2003; Kong et al. 2004; Mukai et al. 2005), one in M51 (Di Stefano & Kong 2003), one in M81 (Swartz et al. 2002), one in the Antennae (Fabbiano et al. 2003) and two in NGC 300 (Kong & Di Stefano 2003; Carpano et al. 2006). In one of these SSSs, a short period of 5.4 h has been reported: the source located in the face-on spiral galaxy NGC 300, which was discussed by Kong & Di Stefano (2003) and Carpano et al. (2006). The modulationwas only present during a single XMM-Newton observation but was not visible 6 days earlier, when the source was a few times brighter. In this Letter, we report the discovery of an even more luminous SSS with a slightly shorter modulation, located in the nearby edge-on galaxy NGC 4631. NGC 4631 is a SBc/d type galaxy located at an assumed distance of 7.5 Mpc, with low galactic foreground absorption (N_H = 1.2 × 10²⁰ cm^-2; Dickey & Lockman 1990). A study of the giant diffuse X-ray emitting corona around the galaxy was performed by Wang et al. (2001) with Chandra and by Tüllmann et al. (2006) with XMM-Newton. The point-source population viewed by ROSAT was studied by Vogler & Pietsch (1996) and Read et al. (1997). They reported the detection of 7 sources, including the supersoft source described in this Letter. Spectral fitting of this SSS using a bremsstrahlung model (Read et al. 1997) gave a temperature of 0.1 keV with high (8 × 10²¹ cm^-2) intrinsic absorption; the corresponding absorbed luminosity in the 0.1–2.0 keV energy band was of 1.9 × 10³⁸ erg s^-1. References Di Stefano, R. & Kong, A. K. H. 2003, ApJ, 592, 884 Luminous Supersoft X-Ray Sources in External Galaxies Fabbiano, G., King, A. R., Zezas, A., Ponman, T. J., Rots, A. & Schweizer, F. 2003, ApJ, 591, 843 A Variable Ultraluminous Supersoft X-Ray Source in ``The Antennae'': Stellar-Mass Black Hole or White Dwarf? Kong, A. K. H. & Di Stefano, R. 2003, ApJ, 590, L13 A Luminous Recurrent Supersoft X-Ray Source in NGC 300 Kong, A. K. H., Di Stefano, R. & Yuan, F. 2004, ApJ, 617, L49 Evidence of an Intermediate-Mass Black Hole: Source in M101 during Its 2004 Outburst Mukai, K., Still, M., Corbet, R. H. D., Kuntz, K. D. & Barnard, R. 2005, ApJ, 634, 1085 The X-Ray Properties of M101 ULX-1 = CXOKM101 J140332.74+542102 Read, A. M., Ponman, T. J. & Strickland, D. K. 1997, MNRAS, 286, 626 ROSAT PSPC observations of nearby spiral galaxies - I. The data Swartz, D. A., Ghosh, K. K., Suleimanov, V., Tennant, A. F. & Wu, K. 2002, ApJ, 574, 382 Chandra Discovery of Luminous Supersoft X-Ray Sources in M81 van den Heuvel, E. P. J., Bhattacharya, D., Nomoto, K. & Rappaport, S. A. 1992, A&A;, 262, 97 Accreting white dwarf models for CAL 83, CAL 87 and other ultrasoft X-ray sources in the LMC Vogler, A. & Pietsch, W. 1996, A&A;, 311, 35

K6 V723 Cassiopeia

—
	Authors: J.U-. Ness, G. Schwarz, S. Starrfield, J.P. Osborne, K.L. Page, A.P. Beardmore, R.M. Wagner, C.E. Woodward
	Journal-ref: AJ (2008) [0801.3288 ]
	Title: V723 Cassiopeia still on in X-rays: A bright Super Soft Source 12 years after outburst
Abstract: We find that the classical nova V723 Cas (1995) is still an active X-ray source more than 12 years after outburst and analyze seven X-ray observations carried out with Swift between 2006 January 31 and 2007 December 3. The average count rate is 0.022 cts s^{-1} but the source is variable within a factor of two of the mean and does not show any signs of turning off. We present supporting optical observations which show that between 2001 and 2006 an underlying hot source was present with steadily increasing temperature. In order to confirm that the X-ray emission is from V723 Cas, we extract a ROSAT observation taken in 1990 and find that there was no X-ray source at the position of the nova. The Swift XRT spectra resemble those of the Super Soft X-ray binary Sources (SSS) which is confirmed by RXTE survey data which show no X-ray emission above 2 keV between 1996 and 2007. Using blackbody fits we constrain the effective temperature to between T_eff = (2.6-4.6)x10⁵ K and a bolometric luminosity >5x10³⁵ erg s^-1. In order to confirm that our blackbody fitting technique works, we applied it to a Chandra observation of CAL 83, known to be a Super Soft Source, and obtain results that are consistent with a Non-LTE stellar atmosphere analysis of the same data. We discuss a number of possible explanations for the continuing X-ray activity, including the intriguing possibility of steady hydrogen burning due to renewed accretion. 1. Introduction Classical Novae (CNe) are the observable events caused by thermonuclear runaways on the surfaces of white dwarfs (WDs), fueled by material accreted from a companion star. Material dredged up from below the WD surface is mixed with the accreted material and violently ejected. The outburst continues until either nuclear burning has converted all the remaining hydrogen on theWD surface to helium or it has been ejected from off the WD. The bolometric luminosity at the peak of the explosion is near or exceeds the Eddington luminosity, L_Edd = 1.3 × 10³⁸(M/M) erg s^-1. Initially, the radiative energy output of the nova peaks in the optical, since the binary is surrounded by expanding ejecta that are not transparent to the high-energy radiation produced on the surface of the WD by nuclear burning. As the ejecta expand and thin (and the remaining material on the WD burns hydrostatically in a shell), the photosphere recedes to inner and hotter layers. The peak of the spectral energy distribution then shifts to higher energies (Gallagher & Starrfield 1978). The X-ray bright phase of the evolution has only been observed sporadically, with the first systematic observations obtained with ROSAT (e.g., Krautter 2002). The X-ray spectrum during this phase was found to sometimes resemble that of the class of Super Soft X-ray Binary Sources (SSS; van den Heuvel et al. 1992) such as Cal 83 in the LMC (e.g., Lanz et al. 2005). This phase is now called the SSS phase. V1974 Cyg was the first nova to be followed in X-rays from before the SSS phase until decline. Krautter et al. (1996) reported a slow rise and fast decline of the X-ray brightness after the peak had been reached. The first spectral modeling of V1974 Cyg was carried out by Krautter et al. (1996) using blackbody fits, but Balman et al. (1998) reanalyzed the spectra using LTE atmosphere models, because blackbodies did not fit the spectra. References Gallagher, J.S., & Starrfield, S. 1978, ARAA 16, 171 Krautter, J. 2002, in AIPC 637, 345 Krautter, J., Ögelman, H., Starrfield, S., Wichmann, R., & Pfeffermann, E. 1996, ApJ 456, 788 Krautter, J., & Williams, R.E. 1989, ApJ 341, 968 Lanz, T., Telis, G.A., Audard, M., Paerels, F., Rasmussen, A.P., & Hubeny, I. 2005, ApJ 619, 517 van den Heuvel, E.P.J., Bhattacharya, D., Nomoto, K., & Rappaport, S.A. 1992, A&A; 262, 97

Literatur zu ""
Long K.S., Helfand D.J., Grabelsky D.A.	1981	ApJ 248, 925	"A soft X-ray study of the Large Magellanic Cloud"
Cowley A.P., Schmidtke P.C., Crampton D., Hutchings J.B.	1990	ApJ 350, 288	"CAL 87 - An eclipsing black hole binary?"
van den Heuvel E.P.J., Bhattacharya D., Nomoto K., Rappaport S.A.	1992	A&A; 262, 97	"Accreting white dwarf models for CAL 83, CAL 87 and other ultrasoft X-ray sources in the LMC"
Carpano, S., Pollock, A.M.T., et al.	2007	A&A; 471, L55	" An ultraluminous supersoft source with a 4 hour modulation in NGC 4631"

H. Heintzmann

( Eintrag vom 19.1.2008)

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