SUPER SOFTX-RAY SOURCES

Presentation Scope What we know – theoretical model.

What we learn – high resolution X-ray spectroscopy

What we see – observations.

What we still don’t know.

What we know:Theoretical model

General PropertiesSuper soft X-ray sources (SSS) are objects that are:

Soft: all photons with

Have extremely soft spectra: equivalent blackbody temperatures of ().

Luminous: bolometric luminosities of

Their numbers in the disks of ordinary spiral galaxies like our own and M31 are extimated to be of the order of .

Their observed characteristics are consistent with those of white dwarfs, which are steadily or cyclically burning hydrogen-rich matter accreted onto the surface at a rate of order .

The required high accretion rates can be supplied by mass transfer on a thermal time scale () from close companion stars that are more massive than the white dwarf accretor, typically .

Discovery Four luminous SSS (CAL 83, CAL 87 in LMCand 1E0035.4-7230 and 1E0056.8-7154 in SMC)were discovered in 1980 with the Einsteinobservatory. It was recognized that the sources haveunusually soft spectra, but since the Einsteinsatellite had a different energy range and a lowerspectral resolution than ROSAT, they were not recognized as a separate class.

With help of ROSAT (Röntgensatellit, launched on 1 June 1990) SSS could be clearly distinguished from the classical strong point X-ray sources (accreting neutron stars and black holes in binaries) and from the X-ray emission of coronae of nearby solar type stars, on the basis of their observed ROSAT X-ray spectra.

Distinguishing from other X-ray sources

Black Holes:

Since black hole sources often also have quite soft spectra, the first idea was that CAL 83 and CAL 87 are black holes in binary systems. What is called soft in connection with the black hole sources is, however, still a fairly hard type of X-ray (i.e. with a large flux in the 0.1- 1 keV range). In addition, black hole sources always have an important hard X-Ray component, which is completely lacking in the SSS.

Classical strong point X-ray sources (accreting neutron stars and black holes in binaries):

The classical strong point X-ray sources typically have blackbody temperatures on the order of several times K, and therefore their main emission is observed in the hardest ROSAT energy band, with almost no emission in the softest ROSAT band.

X-ray emission of coronae of nearby solar type stars:

Stellar coronae (solar-type stars) typically have temperatures of about 1-3 K, which causes them to produce, in addition to a strong signal in the softest energy bands of ROSAT, a significant signal also in the harder ROSAT band.

ROSAT PSPC count spectra of three objects in the Large Magellanic Cloud (LMC) field: the SSS CAL 83, the dK7e foreground star CAL 69, and the black hole candidate LMC X-1

(P. Kahabka and E. P. J. van den Heuvel, 1997)

Binary Nature of CAL 83 and CAL 87 The optical studies of CAL 83

(by Smale et al (1988)) revealed regular photometric variations with a period of 1.04 days.

The light curve is approximately sinusoidal with an amplitude of about 0.1 magnitude (mag).

Similarly, CAL 87 was found to show regular photometric variations with a period of 10.6 h (Cowley et al 1990).

The light curve resembles that of an eclipsing binary with a deep primary minimum (about 1.3 mag) and a small and variable secondary minimum (Schmidtke et al 1993).

Light Curves Explanation Model

Model with an optically bright accretion disk and a companion that is strongly heated on one side:

The main light sources in the system are the very bright accretion disk and the X-ray heated side of the donor star. In CAL 87 the accretion disk is regularly eclipsed; CAL 83 is seen at low inclination, such that only the heating effect is observed.

Nuclear Burning on the Surface of White Dwarf

Knowing luminosity and temperature, a radius of SSS can be calculated through:

Inserting typical parameters: , , we get that emitting object has a radius of about 9000km, i.e. similar to that of a White Dwarf.

In order to generate , a solar mass White Dwarf with a radius of 9000 km should accrete:

Nuclear Burning on the Surface of White Dwarf

However, with such an accretion rate, the inflowing matter would have an optical thickness large enough to block the soft X-rays from coming out.

As table on the right shows, there is a difference between the energy generation by accretion onto neutron stars/black holes and WDs:

In neutron star/black hole X-ray binaries, the accretion energy is ~15 times greater than that released by nuclear fusion.

But in white dwarf systems, the situation is reversed, with fusion energy exceeding that of accretion by a factor ~30.

If the accreted matter on the surface of a WD begins to burn steadily, a necessary accretion rate is ~30times lower () than the rate required for energy generation purely by accretion.

In this case the soft X-rays are able to escape.

Nuclear Burning on the Surface of White Dwarf

A steady state can be achieved in case the accretion rate is similar to the nuclear burning rate.

For

The accretion rate onto the WD determines the strength of the outburst.

Higher accretion rates lead to less violent outbursts.

For , a fraction of the accreted matter is ejected in a nova outburst.

For , all accreted matter will be ejected during a nova outburst.

For , a red giant envelope will form, part of which is blown off by a strong wind.

Eddington limit

Red Giant configuration

Steady H burning

Non-mass ejecting outbursts (recurrent sources)

Novae

What we learn:High resolution X-ray spectroscopy:What can we learn from the X-ray

spectrum

Learning about processes by spectrum analysis

X-ray spectrum is often a very complicated curve. By analyzing its parts we can learn about processes in the objects that emit such spectrum.

Spectra parts are usually represent either a line or a continuum features.

Continuum spectrum: black body

Black body curve that appears in a spectrum allow us to calculate the object’s temperature by Wien’s law:

𝑇=0.29 [𝑐𝑚 ∙𝐾 ]

𝜆𝑚𝑎𝑥 Main source of the black body spectrum is usually from Compton / Thomson scattering.

Continuum spectrum: Bremsstrahlung

When an electron passes close to a positive ion, the strong electric force causes its trajectory to change. The acceleration of the electron in such a collision causes it to radiate electromagnetic energy, which is called bremsstrahlung.

Black body spectrum is optically thick limit of bremsstrahlung.

Similar to black body, bremsstrahlung spectrum gives us indication about object’s temperature.

Black body

Bremsstrahlung

Continuum spectrum: Synchrotron radiation

A fast electron traversing a region containing a magnetic field will change direction because the field exerts a force perpendicular to the direction of motion. Because the velocity vector changes, the electron is accelerated and consequently emits electromagnetic energy, which is called synchrotron radiation. The spectrum of the resulting synchrotron radiation is a power law.

As a result, if spectrum is a power law over a reasonably large energy range, it is usually taken as a strong indication that the source is emitting synchrotron radiation, and therefore has strong magnetic fields.

Line spectrum: Resonant scattering

Photons emitted into polar directions can be absorbed by ions in excited states that re-emit absorbed photons in all directions. It is called resonant line scattering. Depending on the geometry of the scattering medium, photons can be scattered into the line of sight or out of the line of sight. Since these processes balance out in a spherically symmetric scattering medium, it needs to be asymmetric to allow for observable effects of resonant line scattering.

The contribution of resonant scattering to the emission lines dominates over recombination when the column density through the absorber/re-emitter is low, but decreases as the column increases. Therefore, if the lines are the result of a mixture of processes, then it is difficult to explore them for quantitative diagnostics. Recombination

Resonant line scattering

Line spectrum: Photoelectric absorption

In a photoelectric absorption interaction, an incoming photon transfers virtually all of its energy to an atomic electron, usually the most tightly bound (K-shell) electron of an atom. This electron is ejected from the atom with an energy equal to that of the initial photon minus the binding energy for the atomic electron.

Line spectrum: Red / Blue shifts Sometimes, beside the main emission line there are some

additional smaller lines on the sides. These can indicate on a bipolar outflow from the object.

On the right is an optical spectrum of the SSS RXJ0513.9-6951, which is dominated by strong emission from hydrogen (Hβ) and ionized helium at λ4686. Note the features marked S+ and S− on either side of the two strongest lines, which are believed to be red- and blue-shifted emission from HeII and Hβ that is part of a bipolar outflow from the system.

Line spectrum: He-like triplet Lines from He-like ions are triplets. The first

excited state of the He-like atom has 3 levels all at slightly different energies, dependent on the angular momentum of the excited electron. These 3 states are populated by photon absorption, and they decay by emitting photons at three slightly different wavelengths (around for ). These transitions are, in order of increasing wavelength, called resonance, intercombination and forbidden.

Triplets are very useful in density diagnostics. The forbidden line comes from a long-lived excited state. If the electron density is high enough, collisions will cause a transition from the forbidden state to the intercombination state, which then decays by emitting a photon. As collisions become more frequent, the forbidden line becomes weaker and the intercombination line stronger. Thus the ratio of the intensity of the two lower energy lines is good indication of density.

Line spectrum: He-like triplet

What we see:Observations

X-ray spectrumSSS, CN and RN

As we already saw, SSS emission originates in binary systems containing a WD which burns a material that is accreted from a secondary star. In persistent SSS, the burning rate is roughly the same as the accretion rate.

In CN (classical novae) the burning rate is higher. Because WD is driven by degenerate gas which almost doesn’t depend on temperature, a “stability control” (negative heat capacity) doesn’t work here, leading to explosion – outer shell, that consists of accreted hydrogen is ejected away.

In RN (recurrent novae), process of accretion-explosion repeats multiple times.

The ejecta in CN and RN consists of hydrogen (90%) and some C,N and O material that comes from the inner shells of WD during hydrogen burning on the surface. The ejecta initially form an expanding shell surrounding the white dwarf, which prevents high-energy radiation from escaping. The continuing shrinkage of the photosphere leads to a shift in the peak of the spectral energy distribution into the X-ray regime, producing an SSS spectrum.

X-ray spectrumSub types: SSa and SSe

X-ray spectra of SSS is usually represented by one of the following types:

SSa class: X-ray spectra is dominated by absorption lines.

SSe class: X-ray spectra is dominated by emission lines.

Intermediate: both absorption and emission lines present.

High-resolution X-ray spectra in arbitrary flux units of different SSS (Super soft Sources), CN (Classical Novae) and RN (Recurrent Novae). Left column: SSe. Right column: SSa.Blue thin lines are absorbed blackbody curves, indicating the presence of photospheric emission in all cases.

Comparison plot of grating spectra: Examples of the SSa class: Comparison of high-resolution X-ray spectra of the two CNe KT Eri and V4743 Sgr. X axis: entire wavelength range. Y axis: X-ray spectra flux. Prominent bound-bound transitions are marked with vertical lines at their rest wavelengths, and a label on top of each line gives the corresponding element/ion descriptor.

Examples of the SSe class: Comparison of high-resolution X-ray spectra of Usco (RN) and of the prototype SSS Cal 87, scaled by a factor 10 in brightness.

Examples of intermediate cases: Comparison of high-resolution X-ray spectra of the CNe V5116 Sgr, HVCet, and V1494 Aql. The wavelengths of important transitions are marked and labelled. Lines found inother nova spectra in absorption but without having been identified are included with their wavelength values.

X-ray spectrumOrigin of absorption and emission lines

Blackbody-like continuum emission:The continuum component in all SSSspectra is produced by the photosphereof the WD. We observe some of SSSbinary systems from the angle in whichWD is eclipsed by a companion duringsome periods of time. But even duringeclipse the we still detect continuum emission from the WD. The only explanation of continuum emission detection is Thomson scattering that preserves the spectral shape. Thomson scattering can likely also explain the presence of some if not all continuum emission in SSS.

Thomson scattering:

X-ray spectrumOrigin of absorption and emission lines

Absorption lines:

Main cause of absorption lines is a photoexcitationin the stellar medium of the WD. For example, since wavelength region of SSS photons is typically ,

following atomic bound-bound transitions will result in an absorption:

And these absorption lines are exactly what we saw in X-Ray absorption spectrum.

X-ray spectrumOrigin of absorption and emission lines

Absorption lines:

Some spectra contain large absorption areas as a result of photoelectric absorption.

22.5 [ Å ] → 𝐸=551 [𝑒𝑉 ] → 𝑁𝑖𝑡𝑟𝑜𝑔𝑒𝑛 𝐿− h𝑠 𝑒𝑙𝑙

X-ray spectrumOrigin of absorption and emission lines

Emission lines from blackbody-like continuum:

The strongest emission lines in SSe arise at wavelengths where the continuum is strongest. This is not typical for collisional equilibrium spectra, but can be explained by a resonant line scattering spectrum.

What we still don’t know

X-ray spectrumOrigin of absorption and emission lines

While continuum component in all SSS spectra is well explained, the origin of the emission lines on top of a blackbody-like continuum in SSe spectra is less clear. As we saw, it is suggested that emission lines are a mixture of resonant line scattering and recombination processes, but even if there is no other cause, proof of this model is very difficult.

Regarding absorption lines, beside stellar absorption there is also an interstellar absorption due to stellar winds from the WD and gas clouds between the WD and the abserver, which make it difficult to know which absorption processes happen inside the photosphere and which outside of the WD.

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Kahabka, P., and van den Heuvel, P.J., “Luminous super soft X-ray sources”, Annu. Rev. Astron. Astrophys., 35, 69–100, (1997).

Kahabka, P., “Super Soft Sources”, in Lewin, W.H.G., and van der Klis, M., eds., Compact Stellar X-Ray Sources, Cambridge Astrophysics Series, (Cambridge University Press, Cambridge, U.K., 2006).

Mitra-Kraev, U., Ness, J.U., “Densities of stellar flares from spectral lines” – MSSL, 2006.

Ness, J.U., Osborne, J.P., Henze, M., Dobrotka, A., Drake, J.J., Ribeiro, V., Starrfield, A., Kuulkers, E., Behar, E., Hernanz, M., Schwarz, G., Page, K.L., Beardmore, A., Bode, M.F., “Obscuration effects in Super Soft Source X-ray spectra”, 2013.

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