Klaus Reinsch

Evolution of the polarization of the optical afterglow of the γ-ray burst GRB030329

The association of a supernova with GRB030329 strongly supports the ‘collapsar’ model of γ-ray bursts, where a relativistic jet forms after the progenitor star collapses. Such jets cannot be spatially resolved because γ-ray bursts lie at cosmological distances; their existence is instead inferred from ‘breaks’ in the light curves of the afterglows, and from the theoretical desire to reduce the estimated total energy of the burst by proposing that most of it comes out in narrow beams. Temporal evolution of the polarization of the afterglows may provide independent evidence for the jet structure of the relativistic outflow. Small-level polarization (∼1–3 per cent) has been reported for a few bursts, but its temporal evolution has yet to be established. Here we report polarimetric observations of the afterglow of GRB030329. We establish the polarization light curve, detect sustained polarization at the per cent level, and find significant variability. The data imply that the afterglow magnetic field has a small coherence length and is mostly random, probably generated by turbulence, in contrast with the picture arising from the high polarization detected in the prompt γ-rays from GRB021206 (ref. 18).

ROSAT discovered soft x-ray intermediate polars: UU Col and RX J0806.3+1527

We present new results from our ROSAT HRI observations of soft high-galactic latitude ROSAT all-sky survey sources. A 42 ksec pointing on UU Col reveals that the two independent coherent periodicities known from our optical photometry are also present in the soft X-ray flux confirming the soft X-ray intermediate polar nature of this system. In a 13.4 ksec pointing on RX J0806.3+1527 a (321.5393±0.0004) s pulsation with a modulation amplitude of ∼100% is detected as the only consistent signal in the 0.0002–0.0052 Hz frequency range. Based on its X-ray and optical properties, we suggest that RX J0806.3+1527 is another member of the new class of soft X-ray intermediate polars. Alternatively, it could be a double-degenerate polar.

Zeeman Tomography of Magnetic White Dwarfs: General Method and Application to EF Eridani

We have developed a new method to derive the magnetic field distribution on the surfaces of rotating magnetic white dwarfs from phase-resolved flux and circular polarization spectra. An optimization code based on an evolutionary strategy is used to fit synthetic Zeeman spectra for a variety of model geometries described in the framework of a truncated multipole expansion. We demonstrate that the code allows the reconstruction of relatively complex fields using noise-added synthetic input spectra. As a first application, we analyze flux and circular polarization spectra of the polar EF Eri in a low state of accretion taken with FORS1 at the ESO VLT.

Accretion onto selected magnetic white dwarfs as seen in X‐rays

Magnetic cataclysmic variables of AM Her type comprise an accreting white dwarf with a strong magnetic field. Under its influence, the accretion stream is channeled along the field lines towards the poles of the white dwarf, preventing the formation of an accretion disk and allowing for direct insight into the accretion regions. Due to the high temperatures developing in the accretion process, a considerable fraction of the total emission is found at X‐ray energies: hard X‐ray emission from the material which is decelerated above the white‐dwarf surface, and soft X‐ray and FUV emission from the heated photosphere, where the hard emission is reprocessed. We perform dedicated XMM‐Newton observations to study the spectral components, their flux contributions, and the physical structure of the accretion region of several AM Her systems selected by their distinct soft X‐ray fluxes. Modeling the spectral signature of these system components involves approaches to the complex and still widely unknown temperature...

The Polarization Evolution of the Optical Afterglow of GRB 030329

We report 31 polarimetric observations of the afterglow of GRB 030329 with high signal‐to‐noise and high sampling frequency. The data imply that the afterglow magnetic field has small coherence length and is mostly random, probably generated by turbulence.

Magnetic Field Topology of Accreting White Dwarfs

We report first results of our systematic investigation of the magnetic field structure of rotating single magnetic white dwarfs and of white dwarfs in magnetic cataclysmic variables. The global magnetic field distributions on the isolated white dwarf HE1045-0908 and the accreting white dwarfs in EF Eri and CP Tuc have been derived from phase-resolved flux and polarization spectra obtained with FORS1 at the ESO VLT using the systematic method of Zeeman tomography.

XMM observations of the long period polar V1309 Ori

We present the first full-orbit X-ray light curve of the remarkable eclipsing magnetic Cataclysmic Variable V1309 Ori obtained with XMM-Newton. The photometric behaviour and the extreme softto-hard flux ratio of the bright phase emission suggest that accretion occurs entirely via dense blobs. 1. Observations and initial results With an orbital period of ∼ 8 hrs V1309 Ori is longest period system known among the synchronised polars. Information about the accretion region is still sparse, since emission from the UV to infra-red wavelengths is dominated by the accretion stream, as is evident from the orbital and eclipse light curves. We report on a 31 ksec XMM-Newton pointing taken in 2001 March, which provides the first X-ray light curve with full-orbital coverage (Figure 1). It is characterised by a relatively short bright phase between φecl = 0.4. − 0.7, where countrates up to 15 cts/sec are reached. The phasing of the bright phase is rather peculiar compared to most polars, although it is possibly masked by the non-stationary nature of the accretion or absorption between phase φecl = 0.7− 1. If it is really only of geometrical origin, this would require the accretion region to be located at an azimuth ψ = 160 and a colatitude β ≥ 160. Only the polar HY Eri (Burwitz et al. 1999) has a primary accretion region at the far side of the white dwarf as seen from the secondary. Closer inspection of the light curve shows that a large fraction can be resolved into individual X-ray flares. These events have rise and decay times of about 10 sec and are likely to be connected to the subsurface accretion of single gas blobs. The energies measured for 4 typical and well-separated flares range between 2 − 10 × 10erg (assuming a distance of 625 pc and a standard white dwarf of 0.6 M⊙), which correspond to blob masses of 2 − 10 × 10 g. An indirect confirmation of this scenario, is given by the spectral properties of the bright phase. The absence of corresponding flares in the hard X-ray light curve and phase-resolved EPIC PN spectra reveals that the bright phase flux is entirely due to reprocessed, soft thermal emission, with photon energies below 1 keV. The upper limit of the hard/soft flux-ratio implied by the photon statistics