XMM–Newton observations of UW CrB: detection of X-ray bursts and evidence for accretion disc evolution

UW CrB (MS 1603+2600) is a peculiar short-period X-ray binary that exhibits extraordinary optical behaviour. The shape of the optical light curve of the system changes drastically from night to night, without any changes in overall brightness. Here we report X-ray observations of UW CrB obtained with XMM–Newton . We find evidence for several X-ray bursts, confirming a neutron star primary. This considerably strengthens the case that UW CrB is an accretion disc corona system located at a distance of at least 5–7 kpc (3–5 kpc above the Galactic plane). The X-ray and Optical Monitor (ultraviolet–optical) light curves show remarkable shape variation from one observing run to another, which we suggest are due to large-scale variations in the accretion disc shape resulting from a warp that periodically obscures the optical and soft X-ray emission. This is also supported by the changes in phase-resolved X-ray spectra.     

The nature of the hard state of Cygnus X-3

The X-ray binary Cygnus X-3 (Cyg X-3) is a highly variable X-ray source that displays a wide range of observed spectral states. One of the main states is significantly harder than the others, peaking at ∼20 keV, with only a weak low-energy component. Due to the enigmatic nature of this object, hidden inside the strong stellar wind of its Wolf–Rayet companion, it has remained unclear whether this state represents an intrinsic hard state, with truncation of the inner disc, or whether it is just a result of increased local absorption. We study the X-ray light curves from RXTE /ASM and CGRO /BATSE in terms of distributions and correlations of flux and hardness and find several signs of a bimodal behaviour of the accretion flow that are not likely to be the result of increased absorption in a surrounding medium. Using INTEGRAL observations, we model the broad-band spectrum of Cyg X-3 in its apparent hard state. We find that it can be well described by a model of a hard state with a truncated disc, despite the low cut-off energy, provided the accreted power is supplied to the electrons in the inner flow in the form of acceleration rather than thermal heating, resulting in a hybrid electron distribution and a spectrum with a significant contribution from non-thermal Comptonization, usually observed only in soft states. The high luminosity of this non-thermal hard state implies that either the transition takes place at significantly higher   L / L _E  than in the usual advection models, or the mass of the compact object is  ≳20 M_⊙  , possibly making it the most-massive black hole observed in an X-ray binary in our Galaxy so far. We find that an absorption model as well as a model of almost pure Compton reflection also fit the data well, but both have difficulties explaining other results, in particular the radio/X-ray correlation.     

Exploring the spreading layer of GX 9+9 using RXTE and INTEGRAL

We have fitted ∼200 RXTE and INTEGRAL spectra of the neutron star (NS) low-mass X-ray binary (LMXB) GX 9+9 from 2002 to 2007 with a model consisting of a disc blackbody and another blackbody representing the spreading layer (SL), i.e. an extended accretion zone on the NS surface as opposed to the more traditional disc-like boundary layer. Contrary to theory, the SL temperature was seen to increase towards low SL luminosities, while the approximate angular extent had a nearly linear luminosity dependency. Comptonization was not required to adequately fit these spectra. Together with the ∼ 70° upper bound of inclination implied by the lack of eclipses, the best-fitting normalization of the accretion disc blackbody component implies a distance of ∼10 kpc, instead of the usually quoted 5 kpc.     

Neutrinos in neutron stars

It is shown that in neutron stars some of the neutrinos cannot escape from the star. For incompressible fluid sphere models, this part, in the optimum case, is 25%, and for the modelV (B) integrated byTsuruta andCameron (1966b) 1%. As a result a neutrino sea is formed and some radioactive isotopes may become stable.     

Problems of low-mass binary evolution

Some problems connected with low-mass binary evolution (from contact binaries to cataclysmic variables and origin of bursters) are considered. Most attention is given to contact W UMa-stars and to (still unclear) scenarios where the angular momentum loss by magnetic braking may, at least partly, control the contact binary evolution.     

Multisite observations of surface structures on AB Doradus in 1994 November

We present time-resolved optical spectroscopy and broad-band photometry of the rapidly rotating southern K0 dwarf star AB Doradus, obtained during 1994 November. The data were obtained as part of a collaboration dedicated to MUlti-SIte COntinuous Spectroscopy (MUSICOS), and entailed coordinated observations on three continents to obtain the fullest phase coverage possible subject to limitations of local weather conditions. The Doppler images from the three consecutive nights of the run show excellent mutual agreement, with a dark polar cap and numerous intermediate- and low-latitude features. Simultaneous optical photometry showed numerous short-duration U -band flares, and two longer duration optical flares with durations of the order of hours. The latter produced broad-band continuum enhancements throughout the optical spectrum. Where simultaneous spectroscopy was available, both types of flare were seen to have counterparts in H and the Ca  ii H line. Simultaneous time-resolved ultraviolet spectroscopy from the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope , reported elsewhere, shows that at least one of the short-duration U -band flares was also observed in C  iv with the GHRS. Time-series H spectra showed significant evolution of the circumstellar prominence system over five consecutive stellar rotations. One prominence underwent a dramatic increase in distance from the stellar rotation axis. We speculate that this event may have been associated with one of the long-duration flares.     

Scientific rationale for the D-CIXS X-ray spectrometer on board ESA's SMART-1 mission to the Moon

The D-CIXS X-ray spectrometer on ESA's SMART-1 mission will provide the first global coverage of the lunar surface in X-rays, providing absolute measurements of elemental abundances. The instrument will be able to detect elemental Fe, Mg, Al and Si under normal solar conditions and several other elements during solar flare events. These data will allow for advances in several areas of lunar science, including an improved estimate of the bulk composition of the Moon, detailed observations of the lateral and vertical nature of the crust, chemical observations of the maria, investigations into the lunar regolith, and mapping of potential lunar resources. In combination with information to be obtained by the other instruments on SMART-1 and the data already provided by the Clementine and Lunar Prospector missions, this information will allow for a more detailed look at some of the fundamental questions that remain regarding the origin and evolution of the Moon.     

On the X-ray source SCO XR-1

If the assumption of a catastrophic explosion during the formation of a neutron star is correct, the parent systems for klovsky's model of SCO XR-1 seem to have been very short-period white-dwarf binaries. A white dwarf originally in contact with its Roche limit is forced to lose mass. During the ejection of matter the primary may pass the white-dwarf mass limit and become a neutron star. The mass transfer time-scale can change from pulsational to thermal, and a mass flow of 10^–9 M per year needed for SCO XR-1 can be understood, while at the same time the orbital period will increase.     

Magnetic braking in cataclysmic and low-mass X-ray binaries

The chromosphreric-coronal emission of lower Main-Sequence single and binary stars can be correlated with an activity parameter of typeR=g(B-V)P ^–1, whereP is the rotation or orbital period andg(B-V) a function of the color resembling the convective turnover time. Observations indicate that the active region area coverage filling factor grows asR ², and the whole stellar surface becomes filled with closed loop structures atR3. A braking formula is proposed (Equation 4) to include all periods (0.^d1R30^d) and spectral types F-M. On the basis of this equation, the mass transfer rates in compact binaries (driven by the gradual loss of orbital angular momentum) are discussed. It is concluded that the magnetic braking has good chances of being that mechanism which drives the mass transfer in cataclysmic variables and galactic bulge X-ray sources.Paper presented at the Lembang-Bamberg IAU Colloquium No. 80 on Double Stars: Physical Properties and Generic Relations, held at Indonesia, 3–7 June, 1983.On leave from Observatory and Astrophysics Laboratory, University of Helsinki, Helsinki, Finland.