|
| ||
|
|
||
Accretion Disc (accretion + disc)
Terms modified by Accretion Disc Selected AbstractsThe outburst duration and duty cycle of GRS 1915+105MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009Patrick Deegan ABSTRACT The extraordinarily long outburst of GRS 1915+105 makes it one of the most remarkable low-mass X-ray binaries (LMXBs). It has been in a state of constant outburst since its discovery in 1992, an eruption which has persisted ,100 times longer than those of more typical LXMBs. The long orbital period of GRS 1915+105 implies that it contains large and massive accretion disc which is able to fuel its extreme outburst. In this paper, we address the longevity of the outburst and quiescence phases of GRS 1915+105 using smooth particle hydrodynamics (SPH) simulations of its accretion disc through many outburst cycles. Our model is set in the two-, framework and includes the effects of the thermoviscous instability, tidal torques, irradiation by central X-rays and wind mass loss. We explore the model parameter space and examine the impact of the various ingredients. We predict that the outburst of GRS 1915+105 should last a minimum of 20 yr and possibly up to ,100 yr if X-ray irradiation is very significant. The predicted recurrence times are of the order of 104 yr, making the X-ray duty cycle a few 0.1 per cent. Such a low duty cycle may mean that GRS 1915+105 is not an anomaly among the more standard LMXBs and that many similar, but quiescent, systems could be present in the Galaxy. [source] Surprising evolution of the parsec-scale Faraday Rotation gradients in the jet of the BL Lac object B1803+784MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2009M. Mahmud ABSTRACT Several multifrequency polarization studies have shown the presence of systematic Faraday Rotation gradients across the parsec-scale jets of active galactic nuclei, taken to be due to the systematic variation of the line-of-sight component of a helical magnetic (B) field across the jet. Other studies have confirmed the presence and sense of these gradients in several sources, thus providing evidence that these gradients persist over time and over large distances from the core. However, we find surprising new evidence for a reversal in the direction of the Faraday Rotation gradient across the jet of B1803+784, for which multifrequency polarization observations are available at four epochs. At our three epochs and the epoch of Zavala & Taylor, we observe transverse rotation measure (RM) gradients across the jet, consistent with the presence of a helical magnetic field wrapped around the jet. However, we also observe a ,flip' in the direction of the gradient between 2000 June and 2002 August. Although the origins of this phenomenon are not entirely clear, possibly explanations include (i) the sense of rotation of the central supermassive black hole and accretion disc has remained the same, but the dominant magnetic pole facing the Earth has changed from north to south, (ii) a change in the direction of the azimuthal B field component as a result of torsional oscillations of the jet and (iii) a change in the relative contributions to the observed RMs of the ,inner' and ,outer' helical fields in a magnetic-tower model. Although we cannot entirely rule out the possibility that the observed changes in the RM distribution are associated instead with changes in the thermal-electron distribution in the vicinity of the jet, we argue that this explanation is unlikely. [source] The shape of an accretion disc in a misaligned black hole binaryMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2009Rebecca G. Martin ABSTRACT We model the overall shape of an accretion disc in a semidetached binary system in which mass is transferred on to a spinning black hole the spin axis of which is misaligned with the orbital rotation axis. We assume the disc is in a steady state. Its outer regions are subject to differential precession caused by tidal torques of the companion star. These tend to align the outer parts of the disc with the orbital plane. Its inner regions are subject to differential precession caused by the Lense,Thirring effect. These tend to align the inner parts of the disc with the spin of the black hole. We give full numerical solutions for the shape of the disc for some particular disc parameters. We then show how an analytic approximation to these solutions can be obtained for the case when the disc surface density varies as a power law with radius. These analytic solutions for the shape of the disc are reasonably accurate even for large misalignments and can be simply applied for general disc parameters. They are particularly useful when the numerical solutions would be slow. [source] Optical photometry and spectroscopy of the accretion-powered millisecond pulsar HETE J1900.1 , 2455MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2008P. Elebert ABSTRACT We present phase resolved optical photometry and spectroscopy of the accreting millisecond pulsar HETE J1900.1,2455. Our R -band light curves exhibit a sinusoidal modulation, at close to the orbital period, which we initially attributed to X-ray heating of the irradiated face of the secondary star. However, further analysis reveals that the source of the modulation is more likely due to superhumps caused by a precessing accretion disc. Doppler tomography of a broad H, emission line reveals an emission ring, consistent with that expected from an accretion disc. Using the velocity of the emission ring as an estimate for the projected outer disc velocity, we constrain the maximum projected velocity of the secondary to be 200 km s,1, placing a lower limit of 0.05 M, on the secondary mass. For a 1.4 M, primary, this implies that the orbital inclination is low, ,20°. Utilizing the observed relationship between the secondary mass and the orbital period in short-period cataclysmic variables, we estimate the secondary mass to be ,0.085 M,, which implies an upper limit of ,2.4 M, for the primary mass. [source] X-ray reflection in accreting stellar-mass black hole systemsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007R. R. Ross ABSTRACT The X-ray spectra of accreting stellar-mass black hole systems exhibit spectral features due to reflection, especially broad iron K, emission lines. We investigate the reflection by the accretion disc that can be expected in the high/soft state of such a system. First, we perform a self-consistent calculation of the reflection that results from illumination of a hot, inner portion of the disc with its atmosphere in hydrostatic equilibrium. Then, we present reflection spectra for a range of illumination strengths and disc temperatures under the assumption of a constant-density atmosphere. Reflection by a hot accretion disc differs in important ways from that of a much cooler disc, such as that expected in an active galactic nucleus. [source] Magnetic fields and accretion flows on the classical T Tauri star V2129 Oph,MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007J.-F. Donati ABSTRACT From observations collected with the ESPaDOnS spectropolarimeter, we report the discovery of magnetic fields at the surface of the mildly accreting classical T Tauri star (cTTS) V2129 Oph. Zeeman signatures are detected, both in photospheric lines and in the emission lines formed at the base of the accretion funnels linking the disc to the protostar, and monitored over the whole rotation cycle of V2129 Oph. We observe that rotational modulation dominates the temporal variations of both unpolarized and circularly polarized line profiles. We reconstruct the large-scale magnetic topology at the surface of V2129 Oph from both sets of Zeeman signatures simultaneously. We find it to be rather complex, with a dominant octupolar component and a weak dipole of strengths 1.2 and 0.35 kG, respectively, both slightly tilted with respect to the rotation axis. The large-scale field is anchored in a pair of 2-kG unipolar radial field spots located at high latitudes and coinciding with cool dark polar spots at photospheric level. This large-scale field geometry is unusually complex compared to those of non-accreting cool active subgiants with moderate rotation rates. As an illustration, we provide a first attempt at modelling the magnetospheric topology and accretion funnels of V2129 Oph using field extrapolation. We find that the magnetosphere of V2129 Oph must extend to about 7R, to ensure that the footpoints of accretion funnels coincide with the high-latitude accretion spots on the stellar surface. It suggests that the stellar magnetic field succeeds in coupling to the accretion disc as far out as the corotation radius, and could possibly explain the slow rotation of V2129 Oph. The magnetospheric geometry we derive qualitatively reproduces the modulation of Balmer lines and produces X-ray coronal fluxes typical of those observed in cTTSs. [source] Parallel tracks in infrared versus X-ray emission in black hole X-ray transient outbursts: a hysteresis effect?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007David M. Russell ABSTRACT 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: LNIR,L0.5,0.7X. In the high/soft state this slope is much shallower; LNIR,L0.1,0.2X, 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. [source] Electromagnetic fields in jetsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007B. D. Sherwin ABSTRACT The magnetic fields and energy flows in an astronomical jet described by our earlier model are calculated in detail. Though the field distribution varies with the external pressure function p(z), it depends only weakly on the other boundary conditions. Individual field lines were plotted; the lines become nearly vertical at the bottom and are twisted at the top. An animation of a field line's motion was made, which shows the line being wound up by the accretion disc's differential rotation and rising as a result of this. The distribution of Poynting flux within the jet indicates that much of the energy flows up the jet from the inside of the accretion disc but a substantial fraction flows back down to the outside. [source] A toy model for magnetic connection in black hole accretion discMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007Ding-Xiong Wang ABSTRACT A toy model for magnetic connection in black hole (BH) accretion disc is discussed based on a poloidal magnetic field generated by a single electric current flowing around a Kerr BH in the equatorial plane. We discuss the effects of the coexistence of two kinds of magnetic connection (MC) arising, respectively, from (1) the closed field lines connecting the BH horizon with the disc (henceforth MCHD) and (2) the closed field lines connecting the plunging region with the disc (henceforth MCPD). The magnetic field configuration is constrained by conservation of magnetic flux and a criterion of the screw instability of the magnetic field. Two parameters , and ,m are introduced to describe our model instead of resolving the complicated magnetohydrodynamic equations. Compared with MCHD, energy and angular momentum of the plunging particles are extracted via MCPD more effectively, provided that the BH spin is not very high. It turns out that negative energy can be delivered to the BH by the plunging particles without violating the second law of BH thermodynamics, however it cannot be realized via MCPD in a stable way. [source] Keck infrared observations of GRO J0422+32 in quiescenceMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007Mark T. Reynolds ABSTRACT We present Keck K -band photometry and low-resolution H - and K -band spectroscopy of the X-ray nova GRO J0422+32 obtained while the system was in the quiescent state. No clear ellipsoidal modulation is present in the light curve, which is instead dominated by a strong flickering component. In the K band, we observe strong Br, emission, with an equivalent width of 38 ± 5 Å. From this, we conclude that the accretion disc is the most likely source of the observed photometric contamination, and that previous infrared-based attempts to constrain the mass of the putative black hole in this system are prone to considerable uncertainty. We finally proceed to show how it is possible to place meaningful constraints on some of the binary parameters of this system, even in the presence of a relatively high level of contamination from the disc. [source] VW Hyi: optical spectroscopy and Doppler tomographyMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006Amanda J. Smith ABSTRACT We present high-quality optical spectroscopy of the SU UMa-subtype dwarf nova, VW Hyi taken while the system was in quiescence. An S-wave is executed by the emission cores of the hydrogen Balmer lines and by the emission lines of He i, Ca ii, Fe ii and He ii. Using Doppler tomography, we show it originates in the accretion stream,disc impact region. The He ii emission is strongly phase-dependent, suggesting it originates exclusively within a hot cavity at the initial impact. We map the ionization structure of the stream,disc interaction region. One possible interpretation of this is that the Balmer hotspot lies downstream of the He ii hotspot in the outer accretion disc, with the He i and Ca ii hotspots at intermediate locations between the two. This suggests that Balmer emission is suppressed until material has cooled somewhat downstream of the impact site and is able to recombine. We favour a phase offset of 0.15 ± 0.04 between the photometric ephemeris and inferior conjunction of the mass donor. The white dwarf contributes significantly to the optical continuum, with broad Balmer absorption and narrow Mg ii ,4481 absorption clearly apparent. This latter feature yields the gravitational redshift: vgrav= 38 ± 21 km s ,1, so M1= 0.71+0.18,0.26 M,. This implies M2= 0.11 ± 0.03 M, and hence the donor is not a brown dwarf. A prominent Balmer jump is also observed. We note that the previously accepted system parameters for both VW Hyi and WX Hyi incorporate an algebraic error, and we provide a recalculated M1(q) plane for WX Hyi. [source] Evidence of a change in the long-term spin-down rate of the X-ray pulsar 4U 1907+09MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006A. Baykal ABSTRACT We analysed RXTE archival observations of 4U 1907+09 between 1996 February 17 and 2002 March 6. The pulse timing analysis showed that the source stayed at almost constant period around 1998 August and then started to spin-down at a rate of (,1.887 , 0.042) × 10,14 Hz s,1 which is ,0.60 times lower than the long-term (,15 yr) spin-down rate. Our pulse-frequency measurements for the first time resolved significant spin-down rate variations since the discovery of the source. We also presented orbital phase resolved X-ray spectra during two stable spin-down episodes during 1996 November,1997 December and 2001 March,2002 March. The source has been known to have two orbitally locked flares. We found that X-ray flux and spectral parameters except hydrogen column density agreed with each other during the flares. We interpreted the similar values of X-ray fluxes as an indication of the fact that the source accretes not only via transient retrograde accretion disc but also via the stellar wind of the companion, so that the variation of the accretion rate from the disc does not cause significant variation in the observed X-ray flux. Lack of significant change in spectral parameters except hydrogen column density was interpreted as a sign of the fact that the change in the spin-down rate of the source was not accompanied by a significant variation in the accretion geometry. [source] Magnetic jets from swirling discsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2006D. Lynden-Bell ABSTRACT A broad swathe of astrophysical phenomena, ranging from tubular planetary nebulae through Herbig,Haro objects, radio galaxy and quasar emissions to gamma-ray bursts and perhaps high-energy cosmic rays, may be driven by magnetically dominated jets emanating from accretion discs. We give a self-contained account of the analytic theory of non-relativistic magnetically dominated jets wound up by a swirling disc and making a magnetic cavity in a background medium of any prescribed pressure, p(z). We solve the time-dependent problem for any specified distribution of magnetic flux P(R, 0) emerging from the disc at z= 0, with any specified disc angular velocity ,d(R). The physics required to do this involves only the freezing of the lines of force to the conducting medium and the principle of minimum energy. In a constant pressure environment, the magnetically dominated cavity is highly collimated and advances along the axis at a constant speed closely related to the maximum circular velocity of the accretion disc. Even within the cavity the field is strongly concentrated towards the axis. The twist in the jet field ,B,,/,|Bz|, is close to and the width of the jet decreases upwards. By contrast, when the background pressure falls off with height with powers approaching z,4, the head of the jet accelerates strongly and the twist of the jet is much smaller. The width increases to give an almost conical magnetic cavity with apex at the source. Such a regime may be responsible for some of the longest strongly collimated jets. When the background pressure falls off faster than z,4, there are no quasi-static configurations of well-twisted fields and the pressure confinement is replaced by a dynamic effective pressure or a relativistic expansion. In the regimes with rapid acceleration, the outgoing and incoming fields linking the twist back to the source are almost anti-parallel so there is a possibility that magnetic reconnections may break up the jet into a series of magnetic ,smoke-rings' travelling out along the axis. [source] Interpretation of the 1998 outburst of the unique X-ray transient CI Camelopardalis (XTE J0421+560)MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2006imon ABSTRACT We present an analysis of the 1998 outburst of the peculiar X-ray binary and X-ray transient CI Cam (XTE J0421+560). We discuss the observations in the framework of several possible models and argue that this outburst can be explained by the thermal instability of the accretion disc, analogous to the outbursts of soft X-ray transients. Applying the model by King & Ritter and Shahbaz, Charles & King on the X-ray light curve, we obtain a realistic mass of the disc at the peak of outburst to be Mh(0) , 1.5 × 1023 g (the distance d= 5 kpc) or 3.8 × 1022 g (d= 2.5 kpc). The disc radius at this moment is then Rh(0) , 2.5 × 1010 cm (d= 5 kpc) or 1.6 × 1010 cm (d= 2.5 kpc), provided that the factor f (the ratio of the mass of the hot disc at that moment with respect to its maximum possible mass) is close to unity. Even if we take a quite low f= 0.05, we still obtain Rh(0) by only 2.7 times larger. The reddening in the outburst maximum and brighter peak absolute magnitude of CI Cam with respect to those of soft X-ray transients in outbursts can be explained if the disc in CI Cam heats up an extended envelope and/or a strong jet is formed. We thus bring firm arguments for Robinson, Ivans & Welsh's hypothesis. On the other hand, we bring the arguments against the mass transfer burst from the donor and the periastron passage of the compact object. [source] A ZZ Ceti white dwarf in SDSS J133941.11+484727.5MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2006B. T. Gänsicke ABSTRACT We present time-resolved spectroscopy and photometry of the cataclysmic variable (CV) SDSS J133941.11+484727.5 (SDSS 1339) which has been discovered in the Sloan Digital Sky Survey (SDSS) Data Release 4. The orbital period determined from radial velocity studies is 82.524(24) min, close to the observed period minimum. The optical spectrum of SDSS 1339 is dominated to 90 per cent by emission from the white dwarf (WD). The spectrum can be successfully reproduced by a three-component model (white dwarf, disc, secondary) with TWD=12 500 K for a fixed log g= 8.0, d= 170 pc, and a spectral type of the secondary later than M8. The mass-transfer rate corresponding to the optical luminosity of the accretion disc is very low, , 1.7 × 10,13 M, yr,1. Optical photometry reveals a coherent variability at 641 s with an amplitude of 0.025 mag, which we interpret as non-radial pulsations of the white dwarf. In addition, a long-period photometric variation with a period of either 320 or 344 min and an amplitude of 0.025 mag is detected, which bears no apparent relation with the orbital period of the system. Similar long-period photometric signals have been found in the CVs SDSS J123813.73,033933.0, SDSS J204817.85,061044.8, GW Lib and FS Aur, but so far no working model for this behaviour is available. [source] Non-linear bending waves in Keplerian accretion discsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2006G. I. Ogilvie ABSTRACT The non-linear dynamics of a warped accretion disc is investigated in the important case of a thin Keplerian disc with negligible viscosity and self-gravity. A one-dimensional evolutionary equation is formally derived that describes the primary non-linear and dispersive effects on propagating bending waves other than parametric instabilities. It has the form of a derivative non-linear Schrödinger (DNLS) equation with coefficients that are obtained explicitly for a particular model of a disc. The properties of this equation are analysed in some detail and illustrative numerical solutions are presented. The non-linear and dispersive effects both depend on the compressibility of the gas through its adiabatic index ,. In the physically realistic case , < 3, non-linearity does not lead to the steepening of bending waves but instead enhances their linear dispersion. In the opposite case , > 3, non-linearity leads to wave steepening and solitary waves are supported. The effects of a small effective viscosity, which may suppress parametric instabilities, are also considered. This analysis may provide a useful point of comparison between theory and numerical simulations of warped accretion discs. [source] Iron K, line profiles and the inner boundary condition of accretion flowsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2003A. Merloni ABSTRACT Recent X-ray observations have shown evidence for exceptionally broad and skewed iron K, emission lines from several accreting black hole systems. The lines are assumed to be due to fluorescence of the accretion disc illuminated by a surrounding corona and require a steep emissivity profile increasing into the innermost radius. This appears to question both standard accretion disc theory and the zero-torque assumption for the inner boundary condition, both of which predict a much less extreme profile. Instead it argues that a torque may be present due to magnetic coupling with matter in the plunging region or even to the spinning black hole itself. Discussion so far has centred on the torque acting on the disc. However, the crucial determinant of the iron line profile is the radial variation of the power radiated in the corona. Here we study the effects of different inner boundary conditions on the coronal emissivity and on the profiles of the observable Fe K, lines. We argue that in the extreme case where a prominent highly redshifted component of the iron line is detected, requiring a steep emissivity profile in the innermost part and a flatter one outside, energy from the gas plunging into the black hole is being fed directly to the corona. [source] Modelling the extreme ultraviolet emission during the low state of Hercules X-1MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2003D. A. Leahy ABSTRACT Hercules X-1 was observed for extended periods during its low state by the Extreme Ultraviolet Explorer (EUVE). These observations yield low-state light curves in the extreme ultraviolet (EUV) which are compared with a composite model here. The model includes reflection of soft X-rays off the companion HZ Her, including the shadowing of HZ Her by the accretion disc, and emission from the accretion disc surface. Four different geometries for the accretion disc were adopted, all derived from the RXTE All-Sky Monitor (ASM) 35-day light-curve modelling. Three were thin disc models for different system inclinations, i, and the fourth was a disc with a thick inner ring for i= 85°. With the HZ Her reflection model, with no free parameters except normalization, and a simple model for the disc emission, the models fit the data well. The disc emission accounts for about half of the EUV flux, depending on which accretion disc geometry is used. The disc geometry that best fits the EUV light curves is the disc with a thick inner ring, which is the same model that gives the best fit to the RXTE/ASM light curve. [source] An XMM,Newton observation of Ton S180: constraints on the continuum emission in ultrasoft Seyfert galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002S. Vaughan Abstract We present an XMM,Newton observation of the bright, narrow-line, ultrasoft type 1 Seyfert galaxy Ton S180. The 0.3,10 keV X-ray spectrum is steep and curved, showing a steep slope above 2.5 keV (,, 2.3) and a smooth, featureless excess of emission at lower energies. The spectrum can be adequately parametrized using a simple double power-law model. The source is strongly variable over the course of the observation but shows only weak spectral variability, with the fractional variability amplitude remaining approximately constant over more than a decade in energy. The curved continuum shape and weak spectral variability are discussed in terms of various physical models for the soft X-ray excess emission, including reflection off the surface of an ionized accretion disc, inverse Compton scattering of soft disc photons by thermal electrons, and Comptonization by electrons with a hybrid thermal/non-thermal distribution. We emphasize the possibility that the strong soft excess may be produced by dissipation of accretion energy in the hot, upper atmosphere of the putative accretion disc. [source] The stellar mass ratio of GK PerseiMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2002L. Morales-Rueda We study the absorption lines present in the spectra of the long-period cataclysmic variable GK Per during its quiescent state, which are associated with the secondary star. By comparing quiescent data with outburst spectra we infer that the donor star appears identical during the two states and the inner face of the secondary star is not noticeably irradiated by flux from the accreting regions. We obtain new values for the radial velocity semi-amplitude of the secondary star, , a projected rotational velocity, and consequently a measurement of the stellar mass ratio of GK Per, . The inferred white dwarf radial velocities are greater than those measured traditionally using the wings of Doppler-broadened emission lines suspected to originate in an accretion disc, highlighting the unsuitability of emission lines for mass determinations in cataclysmic variables. We determine mass limits for both components in the binary, and . [source] Relativistic ionized accretion disc models of MCG,6-30-15MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2001D.R. Ballantyne We present results from fitting of ionized accretion disc models to three long ASCA observations of the Seyfert 1 galaxy MCG,6-30-15. All three data sets can be fitted by a model consisting of ionized reflection from the inner region of the accretion disc (with twice solar Fe abundance) and a separate disc-line component from farther out on the disc. The disc-line is required to fit the height of the observed Fe K, line profile. However, we show that a much simpler model of reflection from a very weakly ionized constant-density disc also fits the data. In this case only a single cold Fe K, line at 6.4 keV is required to fit the observed line. The ionized disc models predict that O viii K,, C vi K,, Fe xvii L, and Fe xviii L, lines will appear in the soft X-ray region of the reflection spectrum, but are greatly blurred as a result of Compton scattering. The equivalent width (EW) of O viii K, is estimated to be about 10 eV and seems to be as strong as the blend of the Fe L lines. This result creates difficulty for the claim of a strong relativistic O viii line in the XMM - Newton grating spectrum of MCG,6-30-15, although we cannot strictly rule it out since MCG,6-30-15 was in an anomalously low state during that observation. We find that increasing the O abundance or breaking the continuum below 2 keV will not significantly strengthen the line. The second Fe K, line component in the ionized disc model may arise from neutral reflection from a flared disc, or from a second illumination event. The data cannot distinguish between the two cases, and we conclude that single-zone ionized disc models have difficulty fitting these hard X-ray data of MCG,6-30-15. [source] Testing models of X-ray reflection from irradiated discsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2001C. Done We model the reflected spectrum expected from localized magnetic flares above an ionized accretion disc. We concentrate on the case of very luminous magnetic flares above a standard accretion disc extending down to the last stable orbit, and use a simple parametrization to allow for an X-ray-driven wind. Full disc spectra including relativistic smearing are calculated. When fitted with the constant-density reflection models, these spectra give both a low reflected fraction and a small linewidth as seen in the hard spectra from galactic black hole binaries and active galactic nuclei. We fit our calculated spectra to real data from the low/hard state of Nova Muscae and Cyg X-1 and show that these models give comparable ,2 to those obtained from the constant-density reflection models, which implied a truncated disc. This explicitly demonstrates that the data are consistent either with magnetic flares above an ionized disc extending down to the last stable orbit around a black hole, or with non-ionized, truncated discs. [source] The mass of the white dwarf in the recurrent nova U ScorpiiMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2001T.D. Thoroughgood We present spectroscopy of the eclipsing recurrent nova U Sco. The radial velocity semi-amplitude of the primary star was found to be from the motion of the wings of the He ii,4686-Å emission line. By detecting weak absorption features from the secondary star, we find its radial velocity semi-amplitude to be . From these parameters, we obtain a mass of for the white dwarf primary star and a mass of for the secondary star. The radius of the secondary is calculated to be , confirming that it is evolved. The inclination of the system is calculated to be , consistent with the deep eclipse seen in the light-curves. The helium emission lines are double-peaked, with the blueshifted regions of the disc being eclipsed prior to the redshifted regions, clearly indicating the presence of an accretion disc. The high mass of the white dwarf is consistent with the thermonuclear runaway model of recurrent nova outbursts, and confirms that U Sco is the best Type Ia supernova progenitor currently known. We predict that U Sco is likely to explode within ,700 000 yr. [source] Mirror eclipses in the cataclysmic variable IP PegMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2001S.P. Littlefair We present time resolved K -band infrared spectra of the dwarf nova (DN) IP Peg in early quiescence. The Brackett , and He i (,2.0581) lines in our data show hitherto unseen behaviour, which we term a mirror eclipse, and interpret as an eclipse of the secondary star by an optically thin accretion disc. Mirror eclipses are a direct probe of the structure and physical conditions of accretion discs. For example, on assuming the relevant level populations to be in local thermal equilibrium, we constrain the temperature and density of the optically thin material, causing the mirror eclipse in IP Peg to be and respectively. In order to match our data, we find that at least the outermost 20 per cent of the disc (in radius) must be entirely optically thin. Implications for time-dependent disc models are examined. [source] The response of a turbulent accretion disc to an imposed epicyclic shearing motionMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2000Ulf Torkelsson We excite an epicyclic motion, the amplitude of which depends on the vertical position, z, in a simulation of a turbulent accretion disc. An epicyclic motion of this kind may be caused by a warping of the disc. By studying how the epicyclic motion decays, we can obtain information about the interaction between the warp and the disc turbulence. A high-amplitude epicyclic motion decays first by exciting inertial waves through a parametric instability, but its subsequent exponential damping may be reproduced by a turbulent viscosity. We estimate the effective viscosity parameter, ,v, pertaining to such a vertical shear. We also gain new information on the properties of the disc turbulence in general, and measure the usual viscosity parameter, ,h, pertaining to a horizontal (Keplerian) shear. We find that, as is often assumed in theoretical studies, ,v is approximately equal to ,h and both are much less than unity, for the field strengths achieved in our local box calculations of turbulence. In view of the smallness (,0.01) of ,v and ,h we conclude that for ,pgaspmag,10 the time-scale for diffusion or damping of a warp is much shorter than the usual viscous time-scale. Finally, we review the astrophysical implications. [source] The kinetic power of jets magnetically accelerated from advection-dominated accretion flows in radio galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2010Shuang-Liang Li ABSTRACT There is a significant non-linear correlation between the Eddington ratio (Lbol/LEdd) and the Eddington-scaled kinetic power (Lkin/LEdd) of jets in low luminosity active galactic nuclei (AGNs). It is believed that these low luminosity AGNs contain advection-dominated accretion flows (ADAFs). We adopt the ADAF model developed by Li & Cao, in which the global dynamics of ADAFs with magnetically driven outflows is derived numerically, to explore the relation between the bolometric luminosity and the kinetic power of jets. We find that the observed relation, , can be well reproduced by the model calculations with reasonable parameters for ADAFs with magnetically driven outflows. Our model calculations are always consistent with the slope of the correlation independent of the values of the parameters adopted. Compared with the observations, our results show that over 60 per cent of the accreted gas at the outer radius escapes from the accretion disc in a wind before the gas falls into the black holes. The observed correlation between the Eddington-scaled kinetic power and Bondi power can also be qualitatively reproduced by our model calculations. Our results show that the mechanical efficiency varies from 10,2 to10,3, which is roughly consistent with that required in AGN feedback simulations. [source] Stellar dynamical evidence against a cold disc origin for stars in the Galactic CentreMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008Jorge Cuadra ABSTRACT Observations of massive stars within the central parsec of the Galaxy show that, while most stars orbit within a well-defined disc, a significant fraction have large eccentricities and/or inclinations with respect to the disc plane. Here, we investigate whether this dynamically hot component could have arisen via scattering from an initially cold disc , the expected initial condition if the stars formed from the fragmentation of an accretion disc. Using N -body methods, we evolve a variety of flat, cold, stellar systems, and study the effects of initial disc eccentricity, primordial binaries, very massive stars and intermediate mass black holes. We find, consistent with previous results, that a circular disc does not become eccentric enough unless there is a significant population of undetected 100,1000 M, objects. However, since fragmentation of an eccentric disc can readily yield eccentric stellar orbits, the strongest constraints come from inclinations. We show that none of our initial conditions yields the observed large inclinations, regardless of the initial disc eccentricity or the presence of massive objects. These results imply that the orbits of the young massive stars in the Galactic Centre are largely primordial, and that the stars are unlikely to have formed as a dynamically cold disc. [source] The outburst properties of AM CVn starsASTRONOMISCHE NACHRICHTEN, Issue 2 2010I. Kotko Abstract We briefly summarise the observational properties of ultra-compact binaries called AM CVn stars. We analyse their outbursts originating from the thermal-viscous instability in helium accretion disc. We present our preliminary results in applying the model of Dwarf Novae outbursts to helium discs. We can calculate models of outbursts of reasonable amplitude of 2 mag with a constant , parameter throughout the calculation. Setting the mass transfer rate close to its upper critical value produces model lightcurves that resemble short superoutbursts (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] General Relativity effects and line emissionASTRONOMISCHE NACHRICHTEN, Issue 10 2006G. Matt Abstract General Relativity effects (gravitational redshift, light bending, ,) strongly modify the characteristics of the lines emitted close to the Black Hole in Active Galactic Nuclei and Galactic Black Hole systems. These effects are reviewed and illustrated, with particular emphasis on line emission from the accretion disc. Methods, based on the iron line, to measure the two astrophysically relevant parameters of a Black Hole, the mass and spin, are briefly discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Power spectra from ,spotted' accretion discsASTRONOMISCHE NACHRICHTEN, Issue 10 2006T. Pechá Abstract We are carrying out a project to calculate power spectra of variability, assuming a model of a ,spotted' accretion disc near a black hole. We consider relativistic effects that change photon energy and produce light-bending and time-delays acting on the X-ray signal received by an observer. We assume that the life-time and the intrinsic emissivity of individual.aring events are described in terms of a simple stochastic process. This allows us to give approximate analytical formulae and compare them with numerical computations. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||