X-ray and ultraviolet observations of the dwarf nova VW Hyi in quiescence     

Dirk Pandel  France A. Córdova  Steve B. Howell 《Monthly notices of the Royal Astronomical Society》2003,346(4):1231-1241

We present an analysis of X-ray and ultraviolet (UV) data of the dwarf nova VW Hyi that were obtained with XMM–Newton during the quiescent state. The X-ray spectrum indicates the presence of an optically thin plasma in the boundary layer that cools as it settles on to the white dwarf. The plasma has a continuous temperature distribution that is well described by a power law or a cooling flow model with a maximum temperature of 6–8 keV. We estimate from the X-ray spectrum a boundary layer luminosity of  8 × 10³⁰ erg s^-1  , which is only 20 per cent of the disc luminosity. The rate of accretion on to the white dwarf is  5 × 10⁻¹² M_⊙ yr⁻¹  , about half of the rate in the disc. From the high-resolution X-ray spectra, we estimate that the X-ray emitting part of the boundary layer is rotating with a velocity of 540 km s⁻¹, which is close to the rotation velocity of the white dwarf but is significantly smaller than the Keplerian velocity. We detect a 60-s quasi-periodic oscillation of the X-ray flux, which is likely to be due to the rotation of the boundary layer. The X-ray and the UV flux show strong variability on a time-scale of ∼1500 s. We find that the variability in the two bands is correlated and that the X-ray fluctuations are delayed by ∼100 s. The correlation indicates that the variable UV flux is emitted near the transition region between the disc and the boundary layer and that accretion rate fluctuations in this region are propagated to the X-ray emitting part of the boundary layer within ∼100 s. An orbital modulation of the X-ray flux suggests that the inner accretion disc is tilted with respect to the orbital plane. The elemental abundances in the boundary layer are close to their solar values.  相似文献   

Photoevaporation of protoplanetary discs – I. Hydrodynamic models     

R. D. Alexander  C. J. Clarke  J. E. Pringle 《Monthly notices of the Royal Astronomical Society》2006,369(1):216-228

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X-ray spectral evolution of low-mass X-ray binary GX 349+2     

V. K. Agrawal  P. Sreekumar 《Monthly notices of the Royal Astronomical Society》2003,346(3):933-939

We present the results of a systematic investigation of spectral evolution in the Z source GX 349+2, using data obtained during 1998 with the Proportional Counter Array (PCA) on-board the RXTE satellite. The source traced a extended normal branch (NB) and flaring branch (FB) in the colour–colour diagram (CD) and the hardness-intensity diagram (HID) during these observations. The spectra at different positions of the Z-track were best fitted by a model consisting of a disc blackbody and a Comptonized spectrum. A broad (Gaussian) iron line at ∼6.7 keV is also required to improve the fit. The spectral parameters showed a systematic and significant variation with the position along the Z-track. The evolution in spectral parameters is discussed in view of the increasing mass accretion rate scenario, proposed to explain the motion of Z sources in the CD and the HID.  相似文献   

2D/3D Dust Continuum Radiative Transfer Codes to Analyze and Predict VLTI Observations     

I. Pascucci  Th. Henning  J. Steinacker  S. Wolf 《Astrophysics and Space Science》2003,286(1-2):113-118

Radiative Transfer (RT) codes with image capability are a fundamental tool for preparing interferometric observations and for interpreting visibility data. In view of the upcoming VLTI facilities, we present the first comparison of images/visibilities coming from two 3D codes that use completely different techniques to solve the problem of self-consistent continuum RT. In addition, we focus on the astrophysical case of a disk distorted by tidal interaction with by-passing stars or internal planets and investigate for which parameters the distortion can be best detected in the mid-infrared using the mid-infrared interferometric device MIDI. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Dwarf nova oscillations and quasi-periodic oscillations in cataclysmic variables – IV. Observations of frequency doubling and tripling in VW Hyi     

Brian Warner  Patrick A. Woudt 《Monthly notices of the Royal Astronomical Society》2006,367(4):1562-1576

We present new observations of the rapid oscillations in the dwarf nova VW Hyi, made late in outburst. These dwarf nova oscillations (DNOs) increase in period until they reach 33 s, when a transition to a strong 1st harmonic and weak fundamental takes place. After further period increase, the 2nd harmonic appears; often all three components are present simultaneously. This 1:2:3 frequency suite is similar to what has been seen in some neutron star and black hole X-ray binaries, but has not previously been seen in a cataclysmic variable. When studied in detail, the fundamental and 2nd harmonic vary similarly in phase, but the 1st harmonic behaves independently, though keeping close to twice the frequency of the fundamental. The fundamental period of the DNOs, as directly observed or inferred from the harmonics, increases to ∼100 s before the oscillation disappears as the star reaches quiescence. Its maximum period is close to that of the 'longer-period' DNOs observed in VW Hyi. The quasi-periodic oscillations (QPOs), which have fundamental periods 400–1000 s, behave in the same way, showing 1st and 2nd harmonics at approximately the same times as the DNOs. We explore some possible models. One in which the existence of the 1st harmonic is due to the transition from viewing a single accretion region to viewing two regions, and the rate of accretion on to the primary is modulated at the frequency of the 1st harmonic, as in the 'beat frequency model', can generate the suite of DNO frequencies observed. But the behaviour of the QPOs is not yet understood.  相似文献   

Is there a disc in the superluminal quasars?     

E. Rokaki  A. Lawrence  F. Economou  A. Mastichiadis 《Monthly notices of the Royal Astronomical Society》2003,340(4):1298-1308

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A ROSAT WFC observation of SW UMa: the EUV behaviour of dwarf novae in superoutburst explained     

M.R. Burleigh  J.P. Pye  S.W. Poulton  K.B. Sohl  P.J. Wheatley  G.A. Wynn 《Monthly notices of the Royal Astronomical Society》2001,325(4):1458-1462

During re-processing and analysis of the entire ROSAT Wide Field Camera (WFC) pointed observations data base, we discovered a serendipitous, off-axis detection of the cataclysmic variable SW UMa at the onset of its 1997 October superoutburst. Although long outbursts in this SU UMa-type system are known to occur every ∼ 450 d, none had ever been previously observed in the extreme ultra-violet (EUV) by ROSAT . The WFC observations began just ≈13 hr after the optical rise was detected. With a peak count rate of ∼ 4.5 count s⁻¹ in the S1 filter, SW UMa was temporarily the third brightest object in the sky in this waveband. Over the next ≈19 hr the measured EUV flux dropped to < 2 count s⁻¹, while the optical brightness remained essentially static at m _v∼11 . Similar behaviour has also been recently reported in the EUV light curve of the related SU UMa-type binary OY Car during superoutburst, as reported by Mauche & Raymond. In contrast, U Gem-type dwarf novae show no such early EUV dip during normal outbursts. Therefore, this feature may be common in superoutbursts of SU UMa-like systems. We expand on ideas first put forward by Osaki and Mauche & Raymond and offer an explanation for this behaviour by examining the interplay between the thermal and tidal instabilities that affect the accretion discs in these systems.  相似文献   

The Flared Disc Project: RXTE and ASCA observations of X 1822−371     

S. Heinz  M. A. Nowak 《Monthly notices of the Royal Astronomical Society》2001,320(2):249-260

We present archival Rossi X-ray Timing Explorer ( RXTE ) and simultaneous Advanced Satellite for Cosmology and Astrophysics ( ASCA ) data of the eclipsing low mass X-ray binary (LMXB) X 1822−371. Our spectral analysis shows that a variety of simple models can fit the spectra relatively well. Of these models, we explore two in detail through phase-resolved fits. These two models represent the case of a very optically thick and a very optically thin corona. While systematic residuals remain at high energies, the overall spectral shape is well approximated. The same two basic models are fitted to the X-ray light curve, which shows sinusoidal modulations interpreted as absorption by an opaque disc rim of varying height. The geometry we infer from these fits is consistent with previous studies: the disc rim reaches out to the tidal truncation radius, while the radius of the corona (approximated as spherical) is very close to the circularization radius. Timing analysis of the RXTE data shows a time-lag from hard to soft consistent with the coronal size inferred from the fits. Neither the spectra nor the light curve fits allow us to rule out either model, leaving a key ingredient of the X 1822−371 puzzle unsolved. Furthermore, while previous studies were consistent with the central object being a 1.4 M_⊙ neutron star, which has been adopted as the best guess scenario for this system, our light curve fits show that a white dwarf or black hole primary can work just as well. Based on previously published estimates of the orbital evolution of X 1822−371, however, we suggest that this system contains either a neutron star or a low mass (≲2.5 M_⊙) black hole and is in a transitional state of duration shortward of 10⁷ yr.  相似文献   

Accretion stream mapping of HU Aquarii     

Sonja Vrielmann  Axel D. Schwope 《Monthly notices of the Royal Astronomical Society》2001,322(2):269-279

We present a new mapping algorithm, the Accretion Stream Mapping (ASM), which uses the full phase-coverage of a light curve to derive spatially resolved intensity distributions along the accretion stream in magnetic cataclysmic variables of AM Herculis type (polars). The surface of the accretion stream is approximated as a 12-sided (duodecadon-shaped) tube. After successfully testing this method on artificial data we applied it to emission-line light curves of H β , H γ and He  ii λ 4686 of the bright eclipsing polar HU Aqr. We find hydrogen and helium line emission bright in the threading region of the stream where the stream couples on to magnetic field lines. It is particularly interesting that the stream is bright on the irradiated side facing the white dwarf, which highlights the interplay of collisional and radiative excitation/ionization.  相似文献   

An analytical expression for apsidal superhump precession and comparisons with numerical simulations and dwarf nova observations     

M.M. Montgomery 《Monthly notices of the Royal Astronomical Society》2001,325(2):761-766

We compare analytical expressions of precession rates from apsidal (positive) superhumps in close binary systems with numerical disc simulation results and observed values. In the analytical expressions, we include both the dynamical effects on the precession of the disc and effects caused by pressure forces that have been theorized to provide a retrograde effect (i.e. slowing) on the prograde disc precession. We establish new limits on density wave pitch angle to a normalized disc sound speed 60≥Ω_orb  d  tan  i / c >2.214 . Using average values for the density wave pitch angle i and speed of sound c , we find good correlation between numerical simulations and the analytical expression for the apsidal superhump period excess, which includes both the prograde and retrograde effects, for mass ratios of 0.025≤ q ≤0.33 . We also show good correlations with the four known eclipsing systems, OY Car, Z Cha, HT Cas, and WZ Sge. Our analytical expression for apsidal superhump period excess as a function of orbital period is consistent with the trend found in observed systems.  相似文献