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1.
We present the most complete multiwavelength coverage of any dwarf nova outburst: simultaneous optical, Extreme Ultraviolet Explorer and Rossi X-ray Timing Explorer observations of SS Cygni throughout a narrow asymmetric outburst. Our data show that the high-energy outburst begins in the X-ray waveband 0.9–1.4 d after the beginning of the optical rise and 0.6 d before the extreme-ultraviolet rise. The X-ray flux drops suddenly, immediately before the extreme-ultraviolet flux rise, supporting the view that both components arise in the boundary layer between the accretion disc and white dwarf surface. The early rise of the X-ray flux shows that the propagation time of the outburst heating wave may have been previously overestimated.
The transitions between X-ray and extreme-ultraviolet dominated emission are accompanied by intense variability in the X-ray flux, with time-scales of minutes. As detailed by Mauche & Robinson, dwarf nova oscillations are detected throughout the extreme-ultraviolet outburst, but we find they are absent from the X-ray light curve.
X-ray and extreme-ultraviolet luminosities imply accretion rates of  3 × 1015 g s−1  in quiescence,  1 × 1016 g s−1  when the boundary layer becomes optically thick, and  ∼1018 g s−1  at the peak of the outburst. The quiescent accretion rate is two and a half orders of magnitude higher than predicted by the standard disc instability model, and we suggest this may be because the inner accretion disc in SS Cyg is in a permanent outburst state.  相似文献   

2.
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   

3.
We report on a comprehensive and consistent investigation into the X-ray emission from GX 339−4. All public observations in the 11 year RXTE archive were analysed. Three different types of model – single power law, broken power law and a disc + power law – were fitted to investigate the evolution of the disc, along with a fixed Gaussian component at 6.4 keV to investigate any iron line in the spectrum. We show that the relative variation in flux and X-ray colour between the two best sampled outbursts are very similar. The decay of the disc temperature during the outburst is clearly seen in the soft state. The expected decay is   S Disc∝ T 4  ; we measure   T 4.75±0.23  . This implies that the inner disc radius is approximately constant in the soft state. We also show a significant anticorrelation between the iron line equivalent width (EW) and the X-ray flux in the soft state while in the hard state the EW is independent of the flux. This results in hysteresis in the relation between X-ray flux and both line flux and EW. To compare the X-ray binary outburst to the behaviour seen in active galactic nuclei (AGN), we construct a disc fraction luminosity diagram for GX 339−4, the first for an X-ray binary. The shape qualitatively matches that produced for AGN. Linking this with the radio emission from GX 339−4 the change in radio spectrum between the disc and power-law-dominated states is clearly visible.  相似文献   

4.
We study protoplanetary disc evolution assuming that angular momentum transport is driven by gravitational instability at large radii, and magnetohydrodynamic (MHD) turbulence in the hot inner regions. At radii of the order of 1 au such discs develop a magnetically layered structure, with accretion occurring in an ionized surface layer overlying quiescent gas that is too cool to sustain MHD turbulence. We show that layered discs are subject to a limit cycle instability, in which accretion on to the protostar occurs in ∼104-yr bursts with ̇ ∼10−5 M⊙ yr−1, separated by quiescent intervals lasting ∼105 yr where ̇ ≈10−8 M⊙ yr−1. Such bursts could lead to repeated episodes of strong mass outflow in young stellar objects. The transition to this episodic mode of accretion occurs at an early epoch ( t ≪1 Myr), and the model therefore predicts that many young pre-main-sequence stars should have low rates of accretion through the inner disc. At ages of a few Myr, the discs are up to an order of magnitude more massive than the minimum-mass solar nebula, with most of the mass locked up in the quiescent layer of the disc at r ∼1 au. The predicted rate of low-mass planetary migration is reduced at the outer edge of the layered disc, which could lead to an enhanced probability of giant planet formation at radii of 1–3 au.  相似文献   

5.
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6.
The high-mass X-ray binary RX J0146.9+6121, with optical counterpart LS I+61°235 (V831 Cas), is an intriguing system on the outskirts of the open cluster NGC 663. It contains the slowest Be type X-ray pulsar known with a pulse period of around 1400 s and, primarily from the study of variation in the emission line profile of Hα, it is known to have a Be decretion disc with a one-armed density wave period of approximately 1240 d. Here we present the results of an extensive photometric campaign, supplemented with optical spectroscopy, aimed at measuring short time-scale periodicities. We find three significant periodicities in the photometric data at, in order of statistical significance, 0.34, 0.67 and 0.10 d. We give arguments to support the interpretation that the 0.34 and 0.10 d periods could be due to stellar oscillations of the B-type primary star and that the 0.67 d period is the spin period of the Be star with a spin axis inclination of  23+10−8  degrees. We measured a systemic velocity of  −37.0 ± 4.3 km s−1  confirming that LS I+61°235 has a high probability of membership in the young cluster NGC 663 from which the system's age can be estimated as 20–25 Myr. From archival RXTE All Sky Monitor (ASM) data we further find 'super' X-ray outbursts roughly every 450 d. If these super outbursts are caused by the alignment of the compact star with the one-armed decretion disc enhancement, then the orbital period is approximately 330 d.  相似文献   

7.
I solve analytically the viscous evolution of an irradiated accretion disc, as seen during outbursts of soft X-ray transients. The solutions predict steep power-law X-ray decays L X ∼ (1 + t/tvisc)−4, changing to L X ∼ (1 − t/t'visc)4 at late times, where t visc, t 'visc are viscous time-scales. These forms closely resemble the approximate exponential and linear decays inferred by King and Ritter in these two regimes. The decays are much steeper than for unirradiated discs because the viscosity is a function of the central accretion rate rather than of local conditions in the disc.  相似文献   

8.
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9.
In an attempt to model the accretion on to a neutron star in low-mass X-ray binaries, we present 2D hydrodynamical models of the gas flow in close vicinity of the stellar surface. First, we consider a gas pressure-dominated case, assuming that the star is non-rotating. For the stellar mass we take   M star= 1.4 × 10−2 M⊙  and for the gas temperature   T = 5 × 106 K  . Our results are qualitatively different in the case of a realistic neutron star mass and a realistic gas temperature of T ≃ 108 K, when the radiation pressure dominates. We show that to get the stationary solution in a latter case, the star most probably has to rotate with the considerable velocity.  相似文献   

10.
We consider the effect of a supernova (SN) explosion in a very massive binary that is expected to form in a portion of Population III stars with the mass higher than  100 M⊙  . In a Population III binary system, a more massive star can result in the formation of a black hole (BH) and a surrounding accretion disc. Such BH accretion could be a significant source of the cosmic reionization in the early Universe. However, a less massive companion star evolves belatedly and eventually undergoes a SN explosion, so that the accretion disc around a BH might be blown off in a lifetime of companion star. In this paper, we explore the dynamical impact of a SN explosion on an accretion disc around a massive BH, and elucidate whether the BH accretion disc is totally demolished or not. For the purpose, we perform three-dimensional hydrodynamic simulations of a very massive binary system, where we assume a BH of  103 M⊙  that results from a direct collapse of a very massive star and a companion star of  100 M⊙  that undergoes a SN explosion. We calculate the remaining mass of a BH accretion disc as a function of time. As a result, it is found that a significant portion of gas disc can survive through three-dimensional geometrical effects even after the SN explosion of a companion star. Even if the SN explosion energy is higher by two orders of magnitude than the binding energy of gas disc, about a half of disc can be left over. The results imply that the Population III BH accretion disc can be a long-lived luminous source, and therefore could be an important ionizing source in the early Universe.  相似文献   

11.
We present results of a Chandra survey of the ultraluminous X-ray sources (ULX) in 13 normal galaxies, in which we combine source detection with X-ray flux measurement. 22 ULX were detected, i.e. with   L x > 1 × 1039 erg s−1 ( L 10)  and 39 other sources were detected with   L x > 5 × 1038 erg s−1 ( L 5)  . We also use radial intensity profiles to remove extended sources from the sample. The majority of sources are not extended, which for a typical distance constrains the emission region size to less than 50 pc. X-ray colour–colour diagrams and spectral fitting results were examined for indicators of the ULX nature. In the case of the brighter sources, spectral fitting generally requires two-component models. In only a few cases do colour–colour diagrams or spectral fitting provide evidence of a black hole nature. We find no evidence of a correlation with stellar mass, however, there is a strong correlation with star formation as indicated by the 60-μm flux as found in previous studies.  相似文献   

12.
In this short paper, we suggest that the missing boundary layer luminosity of dwarf novae in quiescence is released mainly in the ultraviolet (UV) as the second component commonly identified in the far-UV as the 'accretion belt'. We present the well-studied SU UMa-type system VW Hyi in detail as a prototype for such a scenario. We consider detailed multiwavelength observations and in particular the recent Far Ultraviolet Spectroscopic Explorer ( FUSE ) observations of VW Hyi in quiescence, which confirm the presence of a second component (the 'accretion belt') in the far-UV spectrum. The temperature  (≈ 50 000 K)  and rotational velocity  (≈ 3000 km s −1)  of this second component are entirely consistent with the optically thick region  (τ≈ 1)  located just at the outer edge of optically thin boundary layer in the simulations of Popham. This second component contributes about 20 per cent of the far-UV flux. Using geometrical assumptions and taking into account the X-ray luminosity, we find that the total boundary layer luminosity sums up to   L BL= (0.53 ± 0.25) L disc  , while the theory (Kluźniak) predicts, for the rotation rate of VW Hyi's white dwarf,   L BL≈ (0.76 ± 0.03) L disc  . About one-fifth of the boundary layer energy is emitted in the X-ray and the remaining is emitted in the UV. This scenario is consistent with the recent simultaneous X-ray and UV observations of VW Hyi by Pandel, Córdova & Howell, from which we deduce here that the viscosity in the boundary layer region must be of the order of  ν≈ 1013–1014 cm2 s −1  , depending on the white dwarf mass and the size of the boundary layer.  相似文献   

13.
Following the discovery of X-ray absorption in a high-velocity outflow from the bright quasar PG 1211 + 143 we have searched for similar features in XMM–Newton archival data of a second (high accretion rate) quasar PG 0844+349. Evidence is found for several faint absorption lines in both the EPIC and RGS spectra, whose most likely identification with resonance transitions in H-like Fe, S and Ne implies an origin in highly ionized matter with an outflow velocity of order ∼0.2c. The line equivalent widths require a line-of-sight column density of   N H∼ 4 × 1023 cm−2  , at an ionization parameter of log  ξ∼ 3.7  . Assuming a radial outflow being driven by radiation pressure from the inner accretion disc, as suggested previously for PG 1211 + 143, the flow in PG 0844+349 is also likely to be optically thick, in this case within ∼25 Schwarzschild radii. Our analysis suggests that a high-velocity, highly ionized outflow is likely to be a significant component in the mass and energy budgets of active galactic nuclei accreting at or above the Eddington rate.  相似文献   

14.
We report the discovery of a new hysteresis effect in black hole X-ray binary state transitions, that of the near-infrared (NIR) flux (which most likely originates in the jets) versus X-ray flux. We find, looking at existing data sets, that the IR emission of black hole X-ray transients appears to be weaker in the low/hard state rise of an outburst than the low/hard state decline of an outburst at a given X-ray luminosity. We discuss how this effect may be caused by a shift in the radiative efficiency of the inflowing or outflowing matter, or variations in the disc viscosity or the spectrum/power of the jet. In addition we show that there is a correlation (in slope but not in normalization) between IR and X-ray luminosities on the rise and decline, for all three low-mass black hole X-ray binaries with well-sampled IR and X-ray coverage:   L NIR∝ L 0.5–0.7X  . In the high/soft state this slope is much shallower;   L NIR∝ L 0.1–0.2X  , and we find that the NIR emission in this state is most likely dominated by the viscously heated (as opposed to X-ray heated) accretion disc in all three sources.  相似文献   

15.
We compute the continuous part of the ideal-magnetohydrodynamic (ideal-MHD) frequency spectrum of a polar mountain produced by magnetic burial on an accreting neutron star. Applying the formalism developed by Hellsten & Spies, extended to include gravity, we solve the singular eigenvalue problem subject to line-tying boundary conditions. This spectrum divides into an Alfvén part and a cusp part. The eigenfunctions are chirped and anharmonic with an exponential envelope, and the eigenfrequencies cover the whole spectrum above a minimum ωlow. For equilibria with accreted mass  1.2 × 10−6≲ M a/M⊙≲ 1.7 × 10−4  and surface magnetic fields  1011≲ B */G ≲ 1013, ωlow  is approximately independent of   B *  , and increases with M a. The results are consistent with the Alfvén spectrum excited in numerical simulations with the zeus-mp solver. The spectrum is modified substantially by the Coriolis force in neutron stars spinning faster than ∼100 Hz. The implications for gravitational-wave searches for low-mass X-ray binaries are considered briefly.  相似文献   

16.
The X-ray-bright Seyfert 1 galaxy III Zw 2 was observed with XMM–Newton in 2000 July. Its X-ray spectrum can be described by a power law of photon index Γ= 1.7 and an extremely broad (FWHM∼ 140 000 km  s−1  ) Fe Kα line at 6.44 keV. The iron line has an equivalent width of ∼800 eV. To study the long-term X-ray behaviour of the source we have analysed 25 yr of data, from 1975 to 2000. There is no evidence of significant intrinsic absorption within the source or of a soft X-ray excess in the XMM or archival data. We do not detect rapid X-ray variability (a few  × 103 s  ) during any of the individual observations; however, on longer time-scales (a few years) the X-ray light curve shows 10-fold flux variations. We infer a black hole mass of  ∼109 M⊙  (from Hβ FWHM) for III Zw 2 which is much higher than some previous estimates.
A comparison of X-ray variability with light curves at other wavelengths over a 25-yr period reveals correlated flux variations from radio to X-ray wavelengths. We interpret the variable radio to optical emission as synchrotron radiation, self-absorbed in the radio/millimetre region, and the X-rays mainly as a result of Compton up-scattering of low-energy photons by the population of high-energy electrons that give rise to the synchrotron radiation.  相似文献   

17.
We report the appearance and evolution during 1998 of strong neutral helium 3S–3Po absorption at ∼10 830 Å in Sakurai's Object (V4334 Sgr), which is believed to be a planetary nebula nucleus (PNN) undergoing a final helium shell-flash. First detected on 1998 March 18, the profile of the He  i feature is P Cygni-like. The absorption depth has increased in three subsequent spectra in 1998. If this is owing to a wind, the profile indicates a wind velocity of ∼670±50 km s−1. The strong C  i 10 690-Å line seen prior to the appearance of the helium feature has disappeared; however Sr  ii and CN absorption features remain present. We tentatively identify several new features as Si  i . Taken together with other observations we suggest that the data are consistent with Sakurai's Object entering a phase in which it seems to have become a member of the R Coronae Borealis-type class of stars.  相似文献   

18.
We apply our technique for indirect imaging of the accretion stream to the polar HU Aquarii, using eclipse profiles observed when the system was in a high accretion state. The accretion stream is relatively luminous, contributing as much as the accretion region on the white dwarf, or more, to the overall system brightness. We model the eclipse profiles using a model stream consisting of a ballistic trajectory from the L1 point followed by a magnetically channelled trajectory that follows a dipole field line out of the orbital plane. We perform model fits using two geometries: a stream that accretes on to both footpoints of the field line, and a stream that accretes only on to the footpoint of the field line above the orbital plane. The stream images indicate that the distribution of emission along the stream is not a simple function of the radial distance from the white dwarf. The stream is redirected by the magnetic field of the white dwarf at a distance 1.0–1.3×1010 cm from the white dwarf; this implies a mass transfer rate in the range 8–76×1016 g s−1. The absorption dips in the light curve indicate that the magnetically entrained part of the stream moves from 42° to 48° from the line of centres over the three nights of observation. This is in close agreement with the results of the one-footpoint models, suggesting that this is the more appropriate geometry for these data. The stream images show that, in almost all sections of the stream, the flux peaks in B and is successively fainter in U , V and R .  相似文献   

19.
Transonic discs with accretion rates relevant to intrinsically bright Galactic X-ray sources ( L ≈1038–1039 erg s−1) exhibit a time-dependent cyclic behaviour due to the onset of a thermal instability driven by radiation pressure. In this paper we calculate radiation spectra emitted from thermally unstable discs to provide detailed theoretical predictions for observationally relevant quantities. The emergent spectrum has been obtained by solving self-consistently the vertical structure and radiative transfer in the disc atmosphere. We focus on four particular stages of the disc evolution, the maximal evacuation stage and three intermediate stages during the replenishment phase. The disc is found to undergo rather dramatic spectral changes during the evolution, emitting mainly in the 1–10 keV band during outburst and in the 0.1–1 keV band off-outburst. Local spectra, although different in shape from a blackbody at the disc effective temperature, may be characterized in terms of a hardening factor f . We have found that f is more or less constant, both in radius and in time, with a typical value ∼ 1.65.  相似文献   

20.
We present the first of two papers describing an in-depth study of multiwaveband phase-resolved spectroscopy of the unusual dwarf nova WZ Sge. In this paper we present an extensive set of Doppler maps of WZ Sge covering optical and infrared emission lines, and describe a new technique for studying the accretion discs of cataclysmic variables using ratioed Doppler maps. Applying the ratioed Doppler map technique to our WZ Sge data shows that the radial temperature profile of the disc is unlike that predicted for a steady state α disc. Time-averaged spectra of the accretion disc line flux (with the bright spot contribution removed) show evidence in the shapes of the line profiles for the presence of shear broadening in a quiescent non-turbulent accretion disc. From the positions of the bright spots in the Doppler maps of different lines, we conclude that the bright spot region is elongated along the ballistic stream, and that the density of the outer disc is low. The velocity of the outer edge of the accretion disc measured from the H α line is found to be 723±23 km s−1. Assuming that the accretion disc reaches to the 3:1 tidal resonance radius, we derive a value for the primary star mass of 0.82 M⊙. We discuss the implications of our results on the present theories of WZ Sge type dwarf nova outbursts.  相似文献   

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