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1.
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   

2.
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   

3.
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4.
Steep soft X-ray (0.1–2 keV) quasars share several unusual properties: narrow Balmer lines, strong Fe  II emission, large and fast X-ray variability, and a rather steep 2–10 keV spectrum. These intriguing objects have been suggested to be the analogues of Galactic black hole candidates in the high, soft state. We present here results from ASCA observations for two of these quasars: NAB 0205 + 024 and PG 1244 + 026.   Both objects show similar variations (factor of ∼ 2 in 10 ks), despite a factor of ∼ 10 difference in the 0.5–10 keV luminosity (7.3 × 1043 erg s−1 for PG 1244 + 026 and 6.4 × 1044 erg s−1 for NAB 0205 + 024, assuming isotropic emission, H 0 = 50.0 and q 0 = 0.0).   The X-ray continuum of the two quasars flattens by 0.5–1 going from the 0.1–2 keV band towards higher energies, strengthening recent results on another half-dozen steep soft X-ray active galactic nuclei.   PG 1244 + 026 shows a significant feature in the '1-keV' region, which can be described either as a broad emission line centred at 0.95 keV (quasar frame) or as edge or line absorption at 1.17 (1.22) keV. The line emission could be a result of reflection from a highly ionized accretion disc, in line with the view that steep soft X-ray quasars are emitting close to the Eddington luminosity. Photoelectric edge absorption or resonant line absorption could be produced by gas outflowing at a large velocity (0.3–0.6 c ).  相似文献   

5.
We present X-ray/ γ -ray spectra of Cyg X-1 observed during the transition from the hard to the soft state and in the soft state by ASCA , RXTE and CGRO /OSSE in 1996 May and June. The spectra consist of a dominant soft component below ∼2 keV and a power-law-like continuum extending to at least ∼800 keV. We interpret them as emission from an optically thick, cold accretion disc and from an optically thin, non-thermal corona above the disc. A fraction f ≳0.5 of total available power is dissipated in the corona.
We model the soft component by multicolour blackbody disc emission taking into account the torque-free inner-boundary condition. If the disc extends down to the minimum stable orbit, the ASCA RXTE data yield the most probable black hole mass of M X≈10 M⊙ and an accretion rate,     , locating Cyg X-1 in the soft state in the upper part of the stable, gas-pressure-dominated, accretion-disc solution branch.
The spectrum of the corona is well modelled by repeated Compton scattering of seed photons from the disc off electrons with a hybrid, thermal/non-thermal distribution. The electron distribution can be characterized by a Maxwellian with an equilibrium temperature of kT e∼30–50 keV, a Thomson optical depth of τ ∼0.3 and a quasi-power-law tail. The compactness of the corona is 2≲ℓh≲7, and a presence of a significant population of electron–positron pairs is ruled out.
We find strong signatures of Compton reflection from a cold and ionized medium, presumably an accretion disc, with an apparent reflector solid angle, Ω/2π∼0.5–0.7. The reflected continuum is accompanied by a broad iron K α line.  相似文献   

6.
We present the Chandra ACIS-S3 data of the old classical nova RR Pic (1925). The source has a count rate of 0.067 ± 0.002 count s−1 in the 0.3–5.0 keV energy range. We detect the orbital period of the underlying binary system in the X-ray wavelengths. We also find that the neutral hydrogen column density differs for orbital minimum and orbital maximum spectra with values  0.25+0.23−0.18× 1022  and  0.64+0.13−0.14× 1022 cm−2  at 3σ confidence level. The X-ray spectrum of RR Pic can be represented by a composite model of bremsstrahlung with a photoelectric absorption, two absorption lines centered around 1.1–1.4 keV and five Gaussian lines centered at emission lines around 0.3–1.1 keV corresponding to various transitions of S, N, O, C, Ne and Fe. The bremsstrahlung temperature derived from the fits ranges from 0.99 to 1.60 keV and the unabsorbed X-ray flux is found to be  2.5+0.4−1.2× 10−13 erg  cm−2 s−1  in the 0.3–5.0 keV range with a luminosity of 1.1 ± 0.2  1031 erg s−1  at 600 pc. We also detect excess emission in the spectrum possibly originating from the reverse shock in the ejecta. A fit with a cooling flow plasma emission model shows enhanced abundances of He, C, N, O and Ne in the X-ray emitting region indicating existence of diffusive mixing.  相似文献   

7.
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   

8.
We study spectral variability of 11 ultraluminous X-ray sources (ULX) using archived XMM–Newton and Chandra observations. We use three models to describe the observed spectra: a power law, a multicolour disc (MCD) and a combination of these two models. We find that seven ULXs show a correlation between the luminosity L X and the photon index Γ. Furthermore, four out of these seven ULXs also show spectral pivoting in the observed energy band. We also find that two ULXs show an   L X–Γ  anticorrelation. The spectra of four ULXs in the sample can be adequately fitted with a MCD model. We compare these sources to known black hole binaries (BHB) and find that they follow similar paths in their luminosity–temperature diagrams. Finally, we show that the 'soft excess' reported for many of these ULXs at ∼0.2 keV seems to roughly follow a trend   L soft∝ T −3.5  when modelled with a power law plus a 'cool' MCD model. This is contrary to the   L ∝ T 4  relation that is expected from theory and what is seen for many accreting BHBs. The observed trend could instead arise from disc emission beamed by an outflowing wind around a  ∼10 M⊙  black hole.  相似文献   

9.
We present BeppoSAX observations of Nova Velorum 1999 (V382 Vel), carried out in a broad X-ray band covering 0.1–300 keV only 15 d after the discovery and again after 6 months. The nova was detected at day 15 with the BeppoSAX instruments which measured a flux F x≃1.8×10−11 erg cm−2 s−1 in the 0.1–10 keV range and a 2 σ upper limit F x<6.7×10−12 erg cm−2 s−1 in the 15–60 keV range. We attribute the emission to shocked nebular ejecta at a plasma temperature kT ≃6 keV . At six months no bright component emerged in the 15–60 keV range, but a bright central supersoft X-ray source appeared. The hot nebular component previously detected had cooled to a plasma temperature kT <1 keV . There was strong intrinsic absorption of the ejecta in the first observation and not in the second, because the column density of neutral hydrogen decreased from N (H)≃1.7×1023 to N (H)≃1021 cm−2 (close to the interstellar value). The unabsorbed X-ray flux also decreased from F x=4.3×10−11 to F x≃10−12 erg cm−2 s−1 .  相似文献   

10.
We present ROSAT High Resolution Imager (HRI) and ASCA observations of the well-known ultraluminous infrared galaxy (ULIRG) IRAS 19254−7245 (the 'Superantennae' ). The object is not detected by ROSAT , implying a 3 σ upper limit of X-ray luminosity L X∼8×1041 erg s−1 in the 0.1–2 keV band. However, we obtain a clear detection by ASCA , yielding a luminosity in the 2–10 keV band of 2×1042 erg s−1. The X-ray spectrum of IRAS 19254−7245 is very hard, equivalent to a photon index of Γ=1.0±0.35. We therefore attempt to model the X-ray data using a 'scatterer' model, in which the intrinsic X-ray emission along our line of sight is obscured by an absorbing screen while some fraction, f , is scattered into our line of sight by an ionized medium; this is the standard model for the X-ray emission in obscured (but non Compton-thick) Seyfert galaxies. We obtain an absorbing column density of N H=2×1023 cm−2 for a power-law photon index of Γ=1.9, an order of magnitude above the column estimated on the basis of optical observations; the percentage of the scattered emission is high (∼20 per cent). Alternatively, a model where most of the X-ray emission comes from reflection on a Compton-thick torus ( N H>1024 cm−2) cannot be ruled out. We do not detect an Fe line at 6.4 keV; however, the upper limit (90 per cent) to the equivalent width of the 6.4 keV line is high (∼3 keV). Overall , the results suggest that most of the X-ray emission originates in a highly obscured Seyfert 2 nucleus.  相似文献   

11.
We report on the discovery of a narrow-emission-line object at z  = 0.672 detected in a deep ASCA survey. The object, AXJ 0341.4–4453, has a flux in the 2–10 keV band of 1.1 ± 0.27 × 10−13 erg s−1 cm−2, corresponding to a luminosity of 1.8 × 1044 erg s−1 ( q 0 = 0.5, H 0 = 50 km s−1 Mpc−1). It is also marginally detected in the ROSAT 0.5–2 keV band with a flux 5.8 × 10−15 erg s−1 cm−2. Both the ASCA data alone and the combined ROSAT/ASCA data show a very hard X-ray spectrum, consistent with either a flat power law (α < 0.1) or photoelectric absorption with a column of n H > 4 × 1022 cm−2 (α = 1). The optical spectrum shows the high-ionization, narrow emission lines typical of a Seyfert 2 galaxy. We suggest that this object may be typical of the hard sources required to explain the remainder of the X-ray background at hard energies.  相似文献   

12.
The X-ray pulsar GX 1+4 was observed with the RXTE satellite for a total of 51 ks between 1996 July 19 and 21. During this period the flux decreased smoothly from an initial mean level of ≈6×1036 erg s−1 to a minimum of ≈4×1035 erg s−1 (2–60 keV, assuming a source distance of 10 kpc) before partially recovering towards the initial level at the end of the observation.
BATSE pulse timing measurements indicate that a torque reversal took place approximately 10 d after this observation. Both the mean pulse profile and the photon spectrum varied significantly. The observed variation in the source may provide important clues as to the mechanism of torque reversals.
The single best-fitting spectral model was based on a component originating from thermal photons with kT 0≈1 keV Comptonized by a plasma of temperature kT ≈7 keV. Both the flux modulation with phase during the brightest interval and the evolution of the mean spectra over the course of the observation are consistent with variations in this model component; with, in addition, a doubling of the column density n H contributing to the mean spectral change.
A strong flare of duration ≲50 s was observed during the interval of minimum flux, with the peak flux ≈20 times the mean level. Although beaming effects are likely to mask the true variation in M ˙ thought to give rise to the flare, the timing of a modest increase in flux prior to the flare is consistent with dual episodes of accretion resulting from successive orbits of a locally dense patch of matter in the accretion disc.  相似文献   

13.
We report BeppoSAX and optical observations of the black hole candidate GX 339–4 during its X-ray 'off' state in 1999. The broad-band (0.8–50 keV) X-ray emission can be fitted by a single power law with spectral index, α ∼1.6. The observed luminosity is 6.6×1033 erg s−1 in the 0.5–10 keV band, which is at the higher end of the flux distribution of black hole soft X-ray transients in quiescence, comparable to that seen in GS 2023+338 and 4U 1630–47. An optical observation just before the BeppoSAX observation shows the source to be very faint at these wavelengths as well ( B =20.1, V =19.2). By comparing with previously reported 'off' and low states (LS), we conclude that the 'off' state is actually an extension of the LS, i.e. an LS at lower intensities. We propose that accretion models such as the advection-dominated accretion flows are able to explain the observed properties in such a state.  相似文献   

14.
LMC X-1 and LMC X-3 are the only known persistent stellar-mass black-hole candidates that have almost always shown spectra that are dominated by a soft, thermal component. We present here results from 170-ks-long Rossi X-ray Timing Explorer ( RXTE ) observations of these objects, taken in 1996 December, where their spectra can be described by a disc blackbody plus an additional soft     high-energy power law (detected up to energies of 50 keV in LMC X-3). These observations, as well as archival Advanced Satellite for Cosmology and Astrophysics ( ASCA ) observations, constrain any narrow Fe line present in the spectra to have an equivalent width ≲90 eV. Stronger, broad lines (≈150 eV EW,     are permitted. We also study the variability of LMC X-1. Its X-ray power spectral density (PSD) is approximately proportional to     between 10−3 and 0.3 Hz with a root-mean-square (rms) variability of ≈7 per cent. At energies >5 keV, the PSD shows evidence of a break at     possibly indicating an outer disc radius of ≲1000  GM c 2 in this likely wind-fed system. Furthermore, the coherence function     a measure of the degree of linear correlation between variability in the >5 keV band and variability in the lower energy bands, is extremely low (≲50 per cent). We discuss the implications of these observations for the mechanisms that might be producing the soft and hard X-rays in these systems.  相似文献   

15.
We use the grid of hydrodynamic accretion disc calculations of Stehle to construct orbital phase‐dependent emission‐line profiles of thin discs carrying spiral density waves. The observational signatures of spiral waves are explored to establish the feasibility of detecting spiral waves in cataclysmic variable discs using prominent emission lines in the visible range of the spectrum. For high Mach number accretion discs ( M v φ c s≃ 15 – 30), we find that the spiral shock arms are so tightly wound that they leave few obvious fingerprints in the emission lines. Only a minor variation of the double peak separation in the line profile at a level of ∼8 per cent is produced. For accretion discs in outburst ( M ≃ 5 – 20) however, the lines are dominated by the emission from an m =2 spiral pattern in the disc. We show that reliable Doppler tomograms of spiral shock patterns can be reconstructed provided that a signal‐to‐noise ratio of at least 15, a wavelength resolution of ∼80 km s−1 and a time resolution of ∼50 spectra per binary orbit are achieved. We confirm that the observed spiral pattern in the disc of IP Pegasi can be reproduced by tidal density waves in the accretion disc and demands the presence of a large, hot disc, at least in the early outburst stages.  相似文献   

16.
X-ray reflection spectra from photoionized accretion discs in active galaxies are presented for a wide range of illumination conditions. The energy, equivalent width (EW) and flux of the Fe K α line are shown to depend strongly on the ratio of illuminating flux to disc flux,   F x/ F disc  , the photon index of the irradiating power law, Γ, and the incidence angle of the radiation, i . When   F x/ F disc≤2  a neutral Fe K α line is prominent for all but the largest values of Γ. At higher illuminating fluxes an He-like Fe K α line at 6.7 keV dominates the line complex. With a high-energy cut-off of 100 keV, the thermal ionization instability seems to suppress the ionized Fe K α line when  Γ≤1.6  . The Fe K α line flux correlates with   F x/ F disc  , but the dependence weakens as iron becomes fully ionized. The EW is roughly constant when   F x/ F disc  is low and a neutral line dominates, but then declines as the line progresses through higher ionization stages. There is a strong positive correlation between the Fe K α EW and Γ when the line energy is at 6.7 keV, and a slight negative one when it is at 6.4 keV. This is a potential observational diagnostic of the ionization state of the disc. Observations of the broad Fe K α line, which take into account any narrow component, would be able to test these predictions. Ionized Fe K α lines at 6.7 keV are predicted to be common in a simple magnetic flare geometry. A model that includes multiple ionization gradients on the disc is postulated to reconcile the results with observations.  相似文献   

17.
We present the results of an analysis of ultraviolet observations of T Tauri stars (TTs). By analysing emission measures taken from the literature, we derive rates of ionizing photons from the chromospheres of five classical TTs in the range  ∼1041–1044  photon s−1, although these values are subject to large uncertainties. We propose that the He  ii /C  iv line ratio can be used as a reddening-independent indicator of the hardness of the ultraviolet spectrum emitted by TTs. By studying this line ratio in a much larger sample of objects, we find evidence for an ionizing flux which does not decrease, and may even increase, as TTs evolve. This implies that a significant fraction of the ionizing flux from TTs is not powered by the accretion of disc material on to the central object, and we discuss the significance of this result and its implications for models of disc evolution. The presence of a significant ionizing flux in the later stages of circumstellar disc evolution provides an important new constraint on disc photoevaporation models.  相似文献   

18.
We present new X-ray and H  I 21-cm data on the poor cluster of galaxies Abell 3581. The ASCA spectrum requires a low temperature, has a strong requirement for excess absorption and shows evidence for multi-temperature components. The ROSAT HRI image shows the strongly peaked emission indicative of a cooling flow. Despite the low temperature (∼ 1.5–2.0 keV) and low luminosity (∼ 2 × 1042 erg s−1 in the 2–10 keV band), Abell 3581 has a mass deposition rate ∼ 80 M⊙ yr−1 which is larger than found for other nearby low-luminosity objects. VLA observations in the 21-cm band set velocity width and spin temperature dependent limits on the column density of atomic hydrogen.  相似文献   

19.
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   

20.
I use ASCA data to investigate the 2–10 keV X-ray emission of active galactic nuclei (AGN) taken from the ROSAT International X-ray Optical Survey (RIXOS). I find that the integrated spectrum of these faint, soft X-ray-selected AGN in the 2–10 keV band is harder (best-fitting α = 0.8 ± 0.1) than the slope measured with ROSAT between 0.1 and 2 keV, but softer than the 2–10 keV X-ray background, and consistent with the average 2–10 keV spectrum of bright, nearby Seyfert galaxies. With this spectral slope and using measurements of the AGN contribution to the 1–2 keV X-ray background, I estimate that the AGN percentage contribution to the 2–10 keV background is 0.60 +0.19−0.14 times the AGN percentage contribution to the 1–2 keV background. Hence AGN produce between 12 and 32 per cent of the 2–10 keV X-ray background. This is only the contribution from the types of AGN which are found in soft X-ray surveys; a population of absorbed AGN could represent an additional component of the 2–10 keV X-ray background.  相似文献   

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