X-ray and optical observations of 1RXS J154814.5-452845: a new intermediate polar with soft X-ray emission

We report the identification of the ROSAT all-sky survey source 1RXS J154814.5-452845as new intermediate polar and present the results from follow-up optical and X-ray observations. The source shows pulsations with a period of 693 s both in the optical and X-ray light curves and the detection of a synodic frequency strongly suggests that this is the rotation period of the white dwarf. Although the one day aliasing and the sparse optical data coverage does not allow to unambiguously identify the orbital period, the most likely values of 9.37 h and 6.72 h add 1RXS J154814.5-452845 to the intermediate polars with the longest orbital periods known. The optical spectrum displays features from the late type secondary and shows the presence of broad absorption lines at \Hbet and higher order Balmer lines which may be a signature of the white dwarf atmosphere, very similar to V 709 Cas. The average X-ray spectra as obtained by the EPIC instruments on board XMM-Newton show hard emission typical for this class of objects but also the presence of soft blackbody-like emission similar to that seen from soft intermediate polars and thought to arise from the white dwarf surface heated by the hard X-rays. The best fit model comprises thermal emission from multi-temperature plasma in collisional ionization equilibrium with a continuous temperature distribution up to a maximum of $\sim$60 keV, an Fe fluorescence line at 6.4 keV and with equivalent width of 260 eV and a blackbody component with kT of 86 eV. The hard X-ray emission is absorbed by matter covering 47% of the X-ray source with an equivalent hydrogen density of $\sim$\ohcm{23}. The remaining hard emission is absorbed by a much reduced column density of 1.5\hcm{21} as is the soft blackbody emission. (truncated)Comment: 14 pages, Latex, with 19 figures, accepted for publication in Astronomy and Astrophysic

XMM-Newton observations of MR Vel/RX J0925.7-4758

We report on XMM-Newton observations of the galactic supersoft X-ray source RX J0925.7-4758. The RGS spectrum exhibits a wealth of spectral features from iron and oxygen. XMM-Newton data confirm the finding of previous Chandra HETGS/MEG observations that NLTE models of hot white dwarf atmospheres fail to represent the complex spectrum. There are clear evidences for P Cygni profiles with wind velocities of up to 2000 km/s. Small flux variations with time scales larger than 1000s are present. The strongest power is at ~ 0.21d, a period close to that seen in V band optical light curves. A detailed analysis of the associated changes in the RGS and EPIC pn spectra hint at a mostly grey mechanism suggesting a variation of the visibility of the white dwarf due to occulting material in the accretion disk. Finally, we detect radial velocity changes of 173 +/- 47 km/s between two RGS observations obtained half an orbital cycle apart. The amplitude of the RGS velocity shift is consistent with that of the optical He II 4686 and thus supports the idea that most of the He II optical line emission arises from the accretion disk.Comment: Accepted for publication in A&A (8 pages and 9 figures

Multiwavelength Observations of GX 339-4 in 1996. II. Rapid X-ray Variability

As part of our multiwavelength campaign of GX 339-4 observations in 1996 we present the rapid X-ray variability observed July 26 using the RXTE when the source was in a hard state (= soft X-ray low state). We found that the source was extremely variable, with many bright flares. The flares have relatively symmetric time profiles. There are a few time intervals where the flux rises steadily and then drops suddenly, sometimes to a level lower than the average before the increase. Hardness ratios showed that the source was slightly softer when the flux was brighter. The power density spectra (PDS) were also complicated and we found that broken power laws do not provide adequate fits to any of them. Instead a pair of zero-centered Lorentzians gives a good general description of the shape of the PDS. We found several quasi-periodic oscillations (QPO), including some that are harmonically spaced with the most stable frequency at 0.35 Hz. While the overall rms variability of the source was close to being constant throughout the observation (29% integrating between 0.01 and 50 Hz), there is a small but significant change in the PDS shape with time. More importantly, we show that the soft 2-5 keV band is more variable than the harder 5-10 and 10-40 keV bands, which is unusual for this source and for other black hole candidates. Cross correlation functions (CCF) between these bands show that the light curve for the 10-40 keV band lags that of the 2-5 keV band by 5 msec.Comment: Submitted to Astrophysical Journal. 20 pages. 8 figure

Multiwavelength Observations of GX 339-4 in 1996. III. Keck Spectroscopy

As part of our multiwavelength campaign of observations of GX 339-4 in 1996 we present our Keck spectroscopy performed on May 12 UT. At this time, neither the ASM on the RXTE nor BATSE on the CGRO detected the source. The optical emission was still dominated by the accretion disk with V approximately 17 mag. The dominant emission line is H alpha, and for the first time we are able to resolve a double peaked profile. The peak separation Delta v = 370 +/- 40 km/s. Double peaked H alpha emission lines have been seen in the quiescent optical counterparts of many black hole X-ray novae. However, we find that the peak separation is significantly smaller in GX 339-4, implying that the optical emission comes from a larger radius than in the novae. The H alpha emission line may be more akin to the one in Cygnus X-1, where it is very difficult to determine if the line is intrinsically double peaked because absorption and emission lines from the companion star dominate.Comment: Submitted to Astrophysical Journal. 10 pages. 2 figure

Is there a propeller neutron star in γ\gamma Cas?

$\gamma$ Cas is the prototype of a small population of B0-B1.5 III-V classical Be (cBe) stars that emit anomalous and hard X-rays with a unique array of properties. $\gamma$ Cas is known to host, like other cBe stars, a decretion disk and also a low mass companion. Recently Postnov et al. have posited that this companion is a magnetized rapidly spinning neutron star that deflects direct gravitational accretion from a stellar/disk wind via the "propeller mechanism." These authors state that the key X-ray observations are "remarkably well produced" in this scenario. We reexamine this mechanism in detail and conclude that there are a number of fatal objections in its application to the $\gamma$ Cas case. Among other considerations these issues include the prediction under the propeller scenario of a much smaller population of $\gamma$ Cas stars than is observed and the lack of allowance for observed correlations of X-ray and UV and/or optical properties over a variety of timescales.Comment: 9 page