Max-Planck-Institut für extraterrestrische Physik

The Physics of Galactic Compact Objects

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X-Ray Astronomy
Research Activities
Compact Objects

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White Dwarf Binaries
X-Ray Binaries
High mass
Low Mass
Isolated Neutron stars
Rotation Powered

White Dwarf Binaries
Nova Velorum 1999 (=V382 Vel)
The evolution of Nova Velorum 1999 was monitored with Chandra ACIS-S and LETGS (see Fig. 1). A great wealth of emission lines were observed in the LETGS spectrum (see Fig. 2). Its broadened OVII and NVI lines are explained by the expansion of the shell in the novae. This spectrum allows us for the first time to detect a lack of Fe and Na in the Novae ejecta material.
Fig. 1: X-ray flux evolution of Nova Velorum 1999 (=V382 Vel) from 4 Chandra observations.
Fig. 2: The Chandra LETGS spectrum of Nova Velorum 1999 (=V382 Vel) observed in Feb. 2000.

Magnetic Cataclysmic Variables
Magnetic cataclysmic variables (mCVs) are short period binary systems that consist of roche-lobe filling red dwarf star that accretes matter onto a strongly magnetized white dwarf (B~1-200 MegaGauss). In these systems matter flows from the L2 point toward the white dwarf. In the tidally locked systems (Pspin = Porbital), the "polars" the matter couples directly onto the magnetic field at the point where the ram pressure equates the magnetic field pressure. From there it is channeld along the magnetic field lines toward the accretion pole where eventually releases most of it kinetic energy as hard X-rays in shock just above the white dwarf surface. These hard X-rays are to a great extent reprocessed in the surface of the white dwarf creating a large soft X-ray component. In the "intermediate polars" with lower magnetic fields and Pspin >> Porbital the matter passes through an accretion disc and then is threaded onto the magnetic field lines.
Artists view of a "polar"
Artists view of the accretion funnel just above the surface of the white dwarf.
Artists view of an "Intermediate Polar"
A Chandra LETGS observation of prototype "polar" AM Herculis
Results from the analysis of high resolution X-ray spectra obtainedwith the low energy transmission grating spectrometer (LETGS) of the magnetic cataclysmic variables (mCVs) AM Her are presented here. Also a Simultaneous These spectra with lambda (FWHM) =0.06 Angstrom between 5-170 Angstrom were obtained with the low energy transmission grating spectrometer (LETGS) on-board the X-ray observatory Chandra. The LETGS spectra clearly show the hard X-ray component from the standoff shock above the white dwarf surface and its soft blackbody-like component reprocessed in the atmosphere of the white dwarf. For the first time the hard X-ray component can be studied in detail (figure 2). It shows emission lines characteristic of a thin heated plasma. The state of the plasma can be studied using the spectrally resolved density sensitive OVII He-like triplett. In addition details of the geometry of the accretion funnel can be determined from the phase dependant Doppler-shifts in these emissionlines.
Figure 2: Phase folded orbital X-ray lightcurve of AM Her (right). Left the corresponding spectrum of the He-like triplett of OVII in 5 phase bins.
Fig. 1 Simultaneous Chandra LETGS hard and soft X-ray lightcurves (bottom black and gold). Optical UBVRI color photometry (top ) obtained with the MCCP at the Skinakas 1.3-m telescope, Crete.
The ROSAT discovered polar RX J0953.5+1458
For the ROSAT-discovered magnetic cataclysmic variable RXJ0953.5+1458, strong variability in the optical (photometry + spectroscopy) and X-rays (Fig. 1) indicates that it is a self-eclipsing system in which the accretion pole and column periodically dis-appear behind the limb of the white dwarf. This is a short period system with an orbital period of 103.75 minutes.
Fig.1 : Lightcurve of the magnetic cataclysmic variable RX J0953.5+1458, showing strong modulation due to varying viewing angle of the accretion column.The self-eclipse of the accretion pole by the white dwarf occurs at phases 0.75-1.15.

Isolated Neutron Stars
RX J1856.5-3754
For the neutron star RX J1856.5-3754, XMM-Newton and Chandra observations yield a nearly-perfect black-body spectrum with a temperature of 7.4 105K (Figure 1). All other classical neutron star atmosphere models can be ruled out. With all these data an upper limit of 1.3% could be found for periodic variations.
Fig. 1 Count rate spectra of a 505 ksec Chandra LETGS and a 57 ksec XMM-Newton (EPIC-pn, -MOS and RGS) observation of the isolated neutron star RX J1856.5-3754. The best-fit spectrum is given by a single blackbody of temperature 7.4 105K.
Fig. 2 (a) two component black-body model to fit the overall optical to X-ray spectrum of RXJ1856. (b) model with a continuous temperature distribution.
RX J0720.4-3125 and RBS122
XMM-Newton observations revealed broad absorption lines in the X-ray spectra of at least three radio-quiet isolated neutron stars. If interpreted as proton cyclotron resonance absorption the line center energies indicate magnetic field strengths in the range of 1013-14 G.

Presently seven thermally emitting isolated neutron stars are known. Their X-ray spectra are characterized by soft blackbody-like emission (kT ~ 45 – 120 eV) without indication for harder, non-thermal components. These stars apparently show no radio emission and no association with supernova remnants. Four of them exhibit pulsations in their X-ray flux with periods in the range of 3.45 s to 11.37 s. XMM-Newton observations revealed broad absorption lines in the X-ray spectra of at least three of the stars. From the two pulsars RX J0720.4-3125 and RBS1223 variations of the depth of the line with pulse phase are observed.

The XMM-Newton spectra of RBS1223, RX J0720.4-3125 and RX J1605.3+3249 show deviations from a Planckian energy distribution which can be modeled by a broad Gaussian-shaped absorption line. The first two neutron stars are pulsars with 10.31 s and 8.39 s spin period and show spectral variations with pulse phase (Figure 3).

Pulse-phase spectroscopy for RX J0720.4-3125 shows that a large fraction of the spectral variations can be attributed to changes in the equivalent width of the absorption line (Figure 4). The observed dependence of temperature and equivalent width on pulse phase may be due to the change in the viewing geometry of the inclined magnetic rotator.

Fig. 3 Folded light curves in soft and hard energy bands together with hardness ratio (the ratio of count rates in the hard and soft band) of the pulsars RBS1223 (top) and RX J0720.4-3125 (bottom).

Burwitz, V., Haberl, F., Neuhäuser, R., Predehl, P., Trümper, J., Zavlin, V. E. 2003, A&A 399, 1109

Haberl, F., Schwope, A.D., Hambaryan, V., Hasinger, G., Motch, C. 2003, A&A 403, L19

Haberl, F., Zavlin, V.E., Trümper, J., Burwitz, V. 2004, A&A 419, 1077

Haberl, F., Motch, C., Zavlin, V.E., et al. 2004, A&A 424, 635

Fig. 4: Fig. 2 Phase-resolved EPIC-pn spectra of RX J0720.4–3125 from phases of high and low hardness ratios. The upper pair of spectra shows the combined data from 3 thin filter observations, the lower pair corresponds to the medium filter observation. The upper and lower spectra in each pair are extracted from the phases of low and high hardness ratio. The spectra demonstrate that spectral variations mainly originate below 0.5 keV where they can be described by changes in the depth of the absorption line (by a factor of ~2). In contrast temperature variations, determined by the spectrum above 0.5 keV are small (2 – 3 eV).


© X-Ray Group at MPE (group)
last update:10-11-2004, editor of this page:Vadim Burwitz

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