Norberts's X-ray Binaries Page

Overview

X-ray binaries are among the brightest X-ray sources in the sky and have dominated the focus of research during the first two decades of X-ray astrononmy. Up to date roughly 200 X-ray binaries containing either a neutron star or maybe even a black hole have been classified. However it is not only the nature of the compact object that determines the X-ray emission properties of an accreting binary system, but also the dynamics and geometry of the accretion flow from the stellar companion. The involved phenomena include magnetic field strengths neutron stars, masses of a possible black holes, mass and spectral type of the companion star, disk or spherical accretions flows, just to name a few. For a detailed description of these properties we refer to the beautifully written essays in X-ray Binaries edited by E.H.G.Lewin, J. van Paradijs and E.P.J. van den Heuvel, Cambridge AS 26, and the latest edition of Accretion Power in Astrophysics by J. Frank, A. King, anD D. Raine, Cambridge AS 21.

In the following we identified several key science projects which address fundamental questions in the accretions dynamics of compact binaries as well as the nature of the involved compact object. They also will help to set standards in plasma diagnostics of highly ionized optical thin plasmas. These project involve

Recombination Line Spectroscopy
X-ray Burst Emission in LMXBs
Fe-K Line Diagnosics BH Binaries
Absorption Edges from the ISM
Accretion Disk Models in X-ray Binaries

Recombination Line Spectroscopy

TITLE: Strong X-ray emission line complexes in the highly compact binary pulsar 4U1626-67.

4U1626-67 is 7.7s X-ray pulsar in a compact (40 min) binary system X-ray continuum consists of 0.6 keV blackbody and a variable power law of 0.8 - 1.2. ASCA discovered a 4.4 c/s (SIS0) strong emission-line complex at 1 keV (Angelini et al. 1995) accompanied by a broad feature at 1.42 keV, probably Ne(H) recombination edge (1.36 keV), Mg(H) at 1.47 keV and Mg(He) at 1.34 keV. The whole complex, however, remained unresolved. Blends with unresolved O-K and Fe-K line complexes seemed highly probable. The data indicated an overabundance of Ne of 6 times, O and Mg 2 times the solar abundance. Several questions arise from these indications: Why is there so much Ne, O, Mg? Would that point towards a Ne or He-white dwarf companion? Could we possibly constrain evolutionary scenarios such as an AIC from a CO or ONeMg white dwarf or a type II supernova of the progenitor star. If it was a type I, where is the Si and S?

TITLE: Low ionization plasma diagnostics in the wide binary GX 301-2.

GX 301-2 is a wide binary with an orbital period of 41.5 d. Its pulsar has a rather slow spin period of 696 s. Because of the wide orbit there is there is probably no significant disc accretion but accretion from a strong wind from its very early type B-star companion. The median X-ray luminosity is log L$_x \sim$ 36.4 at a distance of 1.8 kpc, depending on the separation of the neutron star and the main star. ASCA detected 6 K_alpha lines identified with neutral matter or low (I) ionization states (Mg-K_alpha at 1.254 keV, Si-K_alpha at 1.740 keV, S-K_alpha at 2.307 keV, Ar-K_alpha at 2.956 keV, Ca-K_alpha at 3.688 keV and Fe-K_alpha at 6.391 keV). The SIS0 + SIS1 spectrum also indicated a Fe-K absorption edge at 7.2 keV. From the HETG spectra we hope to test accreting wind models in the neutral matter domain, and probe the circumstellar matter in wide binary systems.

TITLE: Medium ionization plasma diagnostics in the eclipsing binary Vela X-1.

Vela X-1 is an eclipsing X-ray binary pulsar with a spin period of 283s and an orbital period of 8.97 days. Its X-ray luminosity is highly variable with a median log L$_x \sim$ 36.5 and also phase dependent. Like GX 301-2 the binary companion is a very early type B-star (B0.5Ib), but in a much closer orbit. The main accretion process is also via stellar wind. There is evidence that a significant portion of the accreted material is fed into a disc. ASCA detected 15 lines, mostly He-like ions through radiative recombination in the stellar wind (Ne-K_alpha IX,X at 1 keV, Mg-K_alpha XI,XII at 1.34 keV, Si-K_alpha XIII,XIV at 1.84 keV, S-K_alpha XV,XIV at 2.5 keV, Ar-K_alpha XVIII at 3.12 keV, Fe-K_alpha XXV,XXVI). Some cold matter line were also detected at low (I) ionization states (Mg-K, Si-K, Ca-K and Fe-K). We will be able to test accreting wind models in the moderately ionized domain, probe the close circumstellar matter of the binary companion.

X-ray Burst Emission in LMXBs

TITLE: The origin of absoption lines in X-ray burst spectra from 4U1636-536.

Significant dips were observed in the X-ray spectra of several type I X-ray bursts. Most of them were observed in bursts from 4U 1636-536, but several other sources showed them as well. The dips appear to be predominantely around 4.1 keV, although some were seen as high as 5.7 keV. The origin of these dips is highly disputed. The most prominent explanation is, that those absorption dips originate from helium-like ions of a heavy element, which are redshifted by the high gravitational field near the surface of the neutron star. To be realistic, the once observed dips obviously only appear sporadically and, because they are broad, we wouldn't need the high resolution of the HETGS. However, they indicate that there is a screening effect of material orbiting very close to the neutron star with the X-ray burst emission, which can be use to probe the mass-radius function of the neutron star through redshift measurements. With the proposed observation we may get as much as 5 to 6 X-ray burst events that allows us to accumulate sufficiently significant signal-to-noise high resolution spectra of the pre-, peak-, and post-burst emission.

Line Diagnostics in Black Hole Binaries.

TITLE: Line Diagnostics in the Jets of SS 433

SS 433 is quite a unique galactic X-ray binary. The nature of the compact object, neutron star or black hole has not yet been determined. The X-ray emission from emerging jets are of thermal origin. The cooling hydrodynamical outflow in form of a jet has dimensions of the binary separation. A recent attempt to map the X-ray emission with the binary and precessional phase based on an ASCA observation found no significant systematic difference between optical and X-ray Doppler shifts. Several lines were detected in the soft and hard band, which mostly beared unresolved alpha and beta-line componenents from Ni and Fe and Mg, Si, S, Ar respectively. Also a stationary flourescent iron line from the outer accretion disc was found. We aim to study the accelaration mechanisms of a prececessing jet, constrain hydrodynamical out flow models, and determine the nature of the compact object.

TITLE: Measuring black hole mass through variable line profiles in BH X-ray binaries

Broad Fe K$\alpha$ emission lines at 6-7 keV are observed in a number of bright X-ray binaries containing a black hole. Fe K_alpha lines broader than 1 keV and significantly redshifted are seen in several candidates. ASCA observations of Cyg X-1 found only an Fe K_alpha line at 6.4 keV with and an actual width of less than 200 eV. The line position varied depending on orbital phase between 6.34 and 6.49, however within 1$\sigma$ error limits. The line profile itself was only poorly determined, because of insufficient resolving power. Cyg X-1 was in its low state during that observation. As part of a program to generally study Fe-line emission of Low state BH binaries, we will obtain a first cut high reolution spectrum of Cyg X-1. It may allow us to Determine gravitational redshift and limit mass of the object.

Accretion disk models in X-ray binaries

TITLE: Accretion disc coronal line emission in low-mass X-ray binaries

Several moderately bright LMXB, usually burst sources, showed indications of a significant accretion disc corona. ADCs in X-ray binaries have been proposed for several Sco X-1 like sources and even Her X-1, but were never confirmed. The Einstein objective grating spectrometer detected a variety of line features around the Fe-L domain, which could have originated from corona excitation by the central source. However, in order to test appropriate models one hase to separate emission originating from a collisional excited region and emission from a photoionized region excited directly by the central source. We will use the observation of 4U 1636-53 and Cyg X-2 (cal) for O-K, Fe-L line and edge diagnostics, which may separate photoionization from collisional excitation lines and allows us to test illuminated disk models.

TITLE: Reprocessed X-ray reflection off the disk in the low state of black hole candidates

An intriguing property of BHCs is the existence of very distinct spectral states. The spectral and time-dependent behaviour of BHCs contains information about he underlying physics of the accretion process. The properties of the low hard state and the power-law component in the high state imply the existence of optically thin hot matter. A recent model by Chen and Taam (1996) describes the various states in terms of varying mass accretion rate allowing one ar a combination of either an optically thick disk, advection dominated disk, corona-disk and a non-steady state disk. If the low state is dominated by a hot thin optically thin accretion disk corona, we should see corona exitation similar to the LMXB case. In fact in BHCs the corona should extend fairly close to the inner edge of the accretion flow. We intend to use the Cyg X-1 observation (see above) to test advection dominated accretion flows versus corona-disk model as well as non-magnetispheric effects in inner accretion disks of accreting black hole

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