BeppoSAX observations of AM Herculis in intermediate and high states

Temporal and spectral analyses from BeppoSAX observations of AM Her performed during both an intermediate and a high stare are presented and discussed. Optical observations taken a few days after the X-ray ones are also presented. During the intermediate state observation, the source was in its "normal", one-pole accretion mode. In the high state it switched to an hitherto unobserved atypical "two-pole" accretion mode, with significant soft and hard X-ray emission from both poles. The emission from the second pole is much softer than that from the primary pole, while the soft X-ray excess of the primary pole is fairly modest in this accretion mode. These facts suggest that accretion onto the secondary is mainly due to blobs penetrating deeply in the photosphere, while that on the primary pole is mostly via a more homogeneous column, giving rise to the classical standing shock. A strong X-ray flaring activity is also observed in the soft X-ray band, but not in the hard X-ray and optical emissions indicating that flares are due to inhomogeneous blobby-accretion events

High-energy pulse profile of the transient X-ray pulsar SAX J2103.5+4545

Two recent INTEGRAL papers report a timing and spectral analysis of the transient Be/X-ray pulsar SAX J2103.5+4545 at high energies (5-200 keV). In this work we present for the first time a study of the pulse profile at energies above 20 keV using INTEGRAL data. The spin-pulse profile shows a prominent (with a duty cycle of 14%) and broad (with a FWHM of similar to 51 s) peak and a secondary peak which becomes more evident above 20 keV. The pulsed fraction increases with energy from similar to 45% at 5-40 keV to similar to 80% at 40-80 keV. The morphology of the pulse profile also changes as a function of energy, consistent with variations in the spectral components that are visible in the pulse phase resolved spectra. A study of the double peaked profile shows that the difference in the two peaks can be modeled by a different scattering fraction between the radiation from the two magnetic poles

A deep X-ray low state of AM Herculis

We present a BeppoSAX observation of AM Her during a prolonged low state. The source was observed for similar to 4 hrs at a flux level comparable to previous low states, followed by a rapid (similar to 40 min) drop by a factor of similar to 7 to the deepest X-ray low state ever detected. While the active phase X-ray flux is likely to be accretion induced, coronal emission from the secondary may contribute significantly during the inactive phase. The timescale of this dramatic change in the accretion rate is of the order of the dynamical timescale of the secondary star; no available model can satisfactorily explain the evolution of the X-ray flux detected in these BeppoSAX data

The intriguing nature of the high-energy gamma ray source XSS J12270-4859

Context. The nature of the hard X-ray source XSS J12270-4859 is still unclear. It was claimed to be a possible magnetic cataclysmic variable of the Intermediate Polar type from its optical spectrum and a possible 860 s X-ray periodicity in RXTE data. However, recent observations do not support the latter variability, leaving this X-ray source still unclassified. Aims. To investigate its nature we present a broad-band X-ray and gamma ray study of this source based on a recent XMM-Newton observation and archival INTEGRAL and RXTE data. Using the Fermi/LAT 1-year point source catalogue, we tentatively associate XSS J12270-4859 with 1FGLJ1227.9-4852, a source of high-energy gamma rays with emission up to 10GeV. We further complement the study with UV photometry from XMM-Newton and ground-based optical and near-IR photometry. Methods. We have analysed both timing and spectral properties in the gamma rays, X-rays, UV and optical/near-IR bands of XSS J12270-4859. Results. The X-ray emission is highly variable, showing flares and intensity dips. The flares consist of flare-dip pairs. Flares are detected in both X-rays and the UV range, while the subsequent dips are present only in the X-ray band. Further aperiodic dipping behaviour is observed during X-ray quiescence, but not in the UV. The broad-band 0.2-100 keV X-ray/soft gamma ray spectrum is featureless and well described by a power law model with Gamma = 1.7. The high-energy spectrum from 100 MeV to 10 GeV is represented by a power law index of 2.45. The luminosity ratio between 0.1-100 GeV and 0.2-100 keV is similar to 0.8, indicating that the GeV emission is a significant component of the total energy output. Furthermore, the X-ray spectrum does not greatly change during flares, quiescence and the dips seen in quiescence. The X-ray spectrum however hardens during the post-flare dips, where a partial covering absorber is also required to fit the spectrum. Optical photometry acquired at different epochs reveals a period of 4.32 hr that could be ascribed to the binary orbital period. Near-IR, possibly ellipsoidal, variations are detected. Large amplitude variability on shorter (tens mins) timescales is found to be non-periodic. Conclusions. The observed variability at all wavelengths together with the spectral characteristics strongly favour a low-mass atypical low-luminosity X-ray binary and are against a magnetic cataclysmic variable nature. The association with a Fermi/LAT high-energy gamma ray source further strengths this interpretation

The X-ray emission of the polar BL Hydri

We report on the analysis of the ASCA and BeppoSAX X-ray observations of the polar system BL Hyi, per formed in October 94 and September 96, respectively. Emission from both poles is apparent from the folded light curves of both observations; the emission from the second pole varies from cycle to cycle, indicating non-stationary accretion there. The temperature of the post-shock region is estimated to be about 10 keV. Inclusion of both complex absorption and Compton reflection significantly improves the quality of the fit. No soft X-ray component is observed; the BeppoSAX/LECS upper limit to the soft component is in agreement with theoretical expectations for this low magnetic field system

BeppoSAX observations of AM Her type stars

First BeppoSAX-NFI observations obtained in the framework of a program aiming to detect simultaneously the soft and hard X-ray emissions and the long term behaviour of AM Her type stars are presented. BL Hyi, the prototipe AM Her and the peculiar V1309 Ori are systems characterized by a strong variability and are elucidative examples of different accretion patterns and modes which can be present in these systems

Laboratory analogue of a supersonic accretion column in a binary star system.

Astrophysical flows exhibit rich behaviour resulting from the interplay of different forms of energy-gravitational, thermal, magnetic and radiative. For magnetic cataclysmic variable stars, material from a late, main sequence star is pulled onto a highly magnetized (B>10 MG) white dwarf. The magnetic field is sufficiently large to direct the flow as an accretion column onto the poles of the white dwarf, a star subclass known as AM Herculis. A stationary radiative shock is expected to form 100-1,000 km above the surface of the white dwarf, far too small to be resolved with current telescopes. Here we report the results of a laboratory experiment showing the evolution of a reverse shock when both ionization and radiative losses are important. We find that the stand-off position of the shock agrees with radiation hydrodynamic simulations and is consistent, when scaled to AM Herculis star systems, with theoretical predictions