The V~17.6 mag optical counterpart of the bright, soft, high-galactic latitude X-ray source RX J0512.2–3241 detected during the ROSAT All-Sky Survey, has been identified as a new, asynchronously rotating, magnetic cataclysmic variable (intermediate polar). The X-ray spectrum of RXJ0512–32 is similar to that of polars, it shows a soft component with no intrinsic absorption and a blackbody temperature kTbb~38 eV. From our optical follow-up B and V CCD photometry (cf. Fig. 1) we derive most probable spin and orbital periods of (863.5 ± 0.7) s and (3.45 ± 0.03) h respectively. A lower limit for the distance to the system is d > 740 pc. From this evidence we suggest that RXJ0512-32 is a further member of the ROSAT discovered class of soft X-ray intermediate polars (for details see Burwitz et al., 1996, A&A 310, L25). This still small class of systems (see Haberl and Motch 1995, A&A 297, L37) has X-ray characteristics similar to those of low magnetic field polars and may be their long sought evolutionary progenitors.

Observed cyclotron spectra of AM Herculis binaries imply a large range of mass flow rates in the accretion spot on the white dwarf. We derive this ṁ–distribution for the case of UZ For. For AM Her stars with different field strength B, such a distribution is shifted in ṁ proportional to B. This fact can account for the observed dominance of bremsstrahlung at low B and of quasi-blackbody emission at high B values.

Observational results of compact objects are best understood using advective accretion flows (Chakrabarti, 1996, 1997). We present here the results of numerical simulations of all possible types of such flows.

Two parameter (specific energy ε and specific angular momentum λ) space of solutions of inviscid advective flow is classified into ‘SA’ (shocks in accretion), ‘NSA’ (no shock in accretion), ‘I’ (inner sonic point only), ‘O’ (outer sonic point only) etc. (Fig. 1 of Chakrabarti, 1997 and references therein). Fig. 1a shows examples of solutions (Molteni, Ryu & Chakrabarti, 1996; Eggum, in preparation) from ‘SA’, ‘I’ and ‘O’ regions where we superpose analytical (solid) and numerical simulations (short dashed curve is with SPH code and medium dashed curve is with TVD code; very long dashed curve is with explicit/implicit code). The agreement is excellent. In presence of cooling effects, shocks from ‘SA’ oscillate (Fig. 1b) when the cooling timescale roughly agrees with postshock infall time scale (Molteni, Sponholz & Chakrabarti, 1996). The solid, long dashed and short dashed curves are drawn for T1/2 (bremsstrahlung), T0.4 and T0.75 cooling laws respectively. In the absence of steady shock solutions, shocks for parameters from ‘NSA’ oscillate (Fig. 2) even in the absence of viscosity (Ryu et al. 1997). The oscillation frequency and amplitude roughly agree with those of quasi-periodic oscillation of black hole candidates. When the flow starts from a cool Keplerian disk, it simply becomes sub-Keplerian before it enters through the horizon. Fig. 3a shows this behaviour where the ratio of λ/λKeplerian is plotted. When the flow deviates from a hot Keplerian disk, it may develop a standing shock as well (Fig. 3b) (Molteni et al. 1996).

Centrifugally driven winds from the surfaces of magnetized accretion disks are a leading candidate for the origin of bipolar outflows and have also been recognized as an attractive mechanism for removing the angular momentum of the accreted matter. The origin of the open magnetic field lines that thread the disk in this scenario is, however, still uncertain. One possibility is that the field lines are transported through the disk, but previous studies have shown that this process is inefficient in disks with turbulent viscosity and diffusivity. Here we examine whether the efficiency can be increased if angular momentum is transported from the disk surfaces by large-scale magnetic fields instead of radially by viscous stresses. In this picture, the removal of angular momentum is associated with the establishment of a global poloidal electric current driven by the radial EMF in the disc, and it does not necessarily need to involve super-Alfvénic outflows. We address this problem in the context of protostellar systems and present representative solutions of the time evolution of a resistive disk that is initially threaded by a uniform vertical field anchored at a large distance from its surfaces. We assume that the angular momentum transport in the disk is controlled by the large-scale magnetic field and take into account the influence of the field on the disk structure.

Spectropolarimetric observations of 3 broad line radio galaxies exhibiting double peaked broad Hα lines, show that two of the sources (Arpl02B and 3C390.3) have single peaked line profiles in polarized flux with a position angle aligned with the jet axis. The third source (3C382) exhibits no polarized line emission at all. The consequences of these observations for the relativistic accretion disk and other BLR models are discussed.

Relativistic outflows from AGN can be parameterized by θ, the angle subtended by the direction of the outflow and the line of sight to the observer, and γ, the bulk Lorentz factor of the outflow. The Doppler factor, δ, and the apparent speed in the plane of the sky, βapp, are combinations of θ and γ. The Doppler factor can be estimated using either the equipartition Doppler factor, δeq (Readhead 1994), or the inverse Compton Doppler factor, δIC. These Doppler factor estimates are combined with observed βapp to solve for θ and γ for different categories of AGN.

Ghisellini et al. (1993) compute δIC for 105 compact radio sources, and Güijosa & Daly (1996) compute δeq for the same sample. Daly, Guerra, & Güijosa (1996) estimate θ and γ for the 43 sources that have βapp listed by Vermeulen & Cohen (1994) and δeq computed by Güijosa & Daly (1996).

Solutions and errors for θ and γ are presented in Figures 1 and 2 using δeq and δIC respectively. Guerra & Daly (1996) discuss these estimates and errors in greater detail. These AGN fall into the following categories: BL Lacertae objects (BL Lacs), core-dominated high-polarization quasars (CDHPQ), core-dominated low-polarization quasars (CDLPQ), core-dominated quasars with no polarization information (CDQ(NPI)), lobe-dominated quasars (LDQ), and radio galaxies (RG).

High resolution optical spectroscopy of V795 Herculis shows complex time- and phase-dependent behaviour of the disk emission. Separate low- and high-velocity fluctuations phased on the 2.6 hr orbital period are observed in the wings of the Balmer lines, prompting a gas stream overflow model.

GX339-4, a black hole candidate, has been monitored since more than 30 years by X and gamma-ray spectrometers on board a dozen of satellites. One of the main characteristics of this source is its bimodal behaviour (high and low states). These states, apparently driven by accretion of matter from a companion onto a compact object, appear to be periodic, with a 14.5 month repetitivity, and an approximative 4 month duration. We discuss, in this paper, on periodic instabilities in the accretion disk surrounding GX339–4 to explain this bimodal behaviour.

Hot optically thin accretion flow solutions are often used to model X-ray binaries and active galactic nuclei. Recently two new classes of advection-dominated models have been proposed: a two-temperature solution (Narayan & Yi 1995, ApJ, 452, 710; Abramowicz et al. 1995, ApJ, 438, L37), and a one-temperature solution (Esin et al. 1996, ApJ, 465, 312). Though detailed numerical spectra of advection-dominated flows include global radiative transfer effects (e.g. Narayan 1996, ApJ, 462, 136), analytical calculations of the physical properties of these accretion solutions generally ignore the effects of non-local radiative transfer (e.g. Narayan & Yi 1995; Esin et al. 1996). However, the optical depth for electron scattering in such flows is generally much lower than unity, and therefore, radiation emitted at one radius can potentially heat or cool the gas at other radii through Compton scattering. In this paper we investigate the importance of this effect. We discuss three situations:

1. 1. Radiation from the inner regions of an advection-dominated flow Compton cooling gas at intermediate radii and Compton heating gas at large radii.

2. 2. Soft radiation from an outer thin accretion disk Compton cooling a hot one- or two-temperature flow on the inside.

3. 3. Soft radiation from an inner thin accretion disk Compton cooling hot gas in a surrounding one-temperature flow.

We describe how previous results are modified by these non-local interactions. We find that Compton heating or cooling of the gas by the radiation emitted in the inner regions of a hot flow is not important. Likewise, Compton cooling by the soft photons from an outer thin disk is negligible when the transition from a cold to a hot flow occurs at a radius greater than some minimum Rtr,min ~ 102.8(Ṁ / ṀEdd)3.5α−7Rschw. However, if the hot flow terminates at R < Rtr,mim, non-local cooling of the hot gas near the transition radius is so strong that the flow is cooled to a thin disk configuration. As a result, the transition radius decreases and the hot gas at the new boundary experiences even stronger external cooling. The resulting runaway process continues until the entire hot flow assumes the thin disk configuration. In the case of a thin disk surrounded by a hot one-temperature flow, we find that Compton cooling by soft radiation dominates over local cooling in the hot gas for Ṁ ≳ 10−3αṀEdd. As a result, the maximum accretion rate for which an advection-dominated one-temperature solution exists, decreases by a factor of ~ 10, compared to the value computed under an assumption of local energy balance.

We present multiwavelength observations of the superluminal jet source GRS 1915+105 in 1996 April–May, over which period a variety of phenomena, including radio QPO, strong infrared emission lines and rapid X-ray flickering and outbursts were observed.

Observations of the inter-outburst behaviour of dwarf novae can put constraints on theoretical models for the structure of the accretion disk and the outburst mechanism. The white-dwarf properties, such as temperature, postoutburst cooling timescale, and rotational velocity play a major rôle in the understanding of the processes in the boundary layer. Alas, the white dwarf could be detected directly only in a small number of dwarf novae. We have re-analysed the available IUE observations of the southern dwarf nova EK TrA, testing the suggestion by Verbunt (1987, A&AS 71, 339) that in this system the white dwarf is a significant source of UV radiation during quiescence.

We have decomposed the spectrum obtained closest to quiescence (Fig. 1) into accretion disc and white dwarf contribution following the method described by Gansicke & Beuermann (1996, A&A 309, L47): we use the observed spectrum close to outburst maximum to describe the disc contribution and a grid (log g = 8.0 and 11 000K ≤ Twd ≤ 30 000K, solar abundances) of calculated model spectra kindly provided by I. Hubeny for the white dwarf. Following our experience with VW Hyi (Gänsicke & Beuermann, 1996, A&A 309, L47), we allow for Gaussian emission lines of N V λ 1240, Si IV λ 1400 and C IV λ 1549. The observed spectrum was then fitted with the sum of these components using an evolutionary strategy algorithm. The best fit is achieved for a white dwarf of Twd = 16 000 K and the accretion disc contributing 21% and 77% of the integrated SWP flux, respectively (Fig. 1). Assuming an 0.6 M⊙ white dwarf, the scaling factor of the white dwarf model implies a distance of ~ 140 pc, roughly twice that of VW Hyi. However, EK TrA was never observed in a truly quiescent state; at the time of the IUE observations the residual accretion disc or accretion belt still dominates the UV light and the derived white-dwarf parameters remain necessarily uncertain. Comparison with VW Hyi teaches us that the decline of the spectrum towards Lyα should not, at face value, be taken as a direct measure of the white dwarf contribution to the overall flux.

In the outer parts of disks in active galactic nuclei and around young stellar objects (YSOs) the internal sources of heat may be too weak to prevent gravitational instability. We are thus motivated to examine the nonlinear outcome of gravitational instability in an optically thick disk. We use height-integrated, two-dimensional numerical simulations that include pressure (the gas has a polytropic equation of state), self-gravity, and cooling. The cooling function is calculated using a one-zone model and is characterized by a single dimensionless parameter τcool, which is the ratio of the cooling time to the orbital period. The simulations are done in the context of a local model of the disk (the “shearing sheet”). We also include the possibility of an effective “viscosity” characterized by a dimensionless parameter α.

We find that the gravitational instability in thin, Keplerian disks leads to sustained angular momentum transport that is essentially local in character. Instability drives Q toward 1.7, and provides an angular momentum flux αeff ≃ 0.4/τcool. The disk breaks up when the cooling time is shorter than the dynamical time, which is equivalent to saying that αeff < 1. We conclude that gravitational instability can raise the angular momentum transport rate in the outer parts of AGN and YSO disks (although YSO disks may be too thick for our local approach to be strictly applicable). This can occur over a factor of a few in radius. The gravitationally dominated region is bounded on the inside by that radius where other heating processes (e.g. MHD turbulence driven by the Balbus-Hawley instability) make Q > 1. It is bounded on the outside by that radius where the cooling time is shorter than the dynamical time, or the disk is optically thin so that the equation of state is soft. At larger radius the disk will be clumpy.

We have obtained optical spectra and IR photometry in an attempt to determine the orbital parameters of the Aql X-1 system. Spectra from the MMT in October 1995 appear to detect the secondary, but are not sufficient to measure the mass function. IR photometry in and out of mini-outburst find K ~ 15.4, and may indicate that the disk contributionat K is modest during the mini-outburst.

RX J1940.1-1025 is a newly discovered magnetic Cataclysmic Variable and one of only three known polars with a slightly non-synchronous (< 3 %) rotation of the white dwarf primary. The accretion geometry between the infalling accretion stream and the magnetic field of the WD thus changes periodically with the beat period of 50.0 d between the orbital and the spin period of the WD. The accretion thus occurs along different field lines, depending on the beat phase.

For the first time we have observed this effect as a periodic shift in the timings of the absorption troughs in the optical and X-ray pulse profiles of RX J1940.1-1025 with phase of the beat period. There is evidence for a secular decrease of the spin period of RX J1940.1-1025 with a synchronization time scale of the order of 102 y.

After the discovery of GRS 1915+105 (Castro-Tirado et al. 1992) we obtained pointed ROSAT observations every six months (12 until now). The flux in the ROSAT (0.1–2.4 keV) band is strikingly different from the simultaneous BATSE (25–50 keV) flux which was obtained by integrating the best fit power law (Fig. 1). Motivated by the different intensity evolution in the soft and hard X-ray band we have selected BATSE monitoring data collected simultaneously to ROSATdata and performed joint spectral fitting with XSPEC. As a result, we never got an acceptable fit (see Fig. 2): The BATSE power law (upper dotted line) is too steep to match the ROSAT band, and even allowing for an increased absorbing column (lower dotted line) does not solve the problem. Alternatively, neither a thermal bremsstrahlung fit (solid line) nor a power law fit (lower dash-dot line) to the ROSAT data match the BATSE flux. The upper dash-dot line is a -2.5 powerlaw which would match the BATSE data while giving too much 1–2 keV emission. A similar, but less stringent result is obtained when folding the best fit BATSE power law models with the HRI detector response to compare the expected count rate with the observed one. We therefore conclude that the spectrum during all simultaneous ROSAT/BATSE observations seemingly consists of two different spectral components.

The detection of a Hell absorption trough in the spectra of three high-redshift quasars provides new constraints on the spectral shape of the UV extragalactic background. The relative strengths of the observed flux decrements at the rest-frame wavelengths of 1216 and 304 Å require a relatively soft radiation field at 4 ryd compared to 1 ryd. If the ionizing metagalactic flux is dominated by the integrated light from QSOs, and the Hell reionization of the universe was completed well before z ~ 3, then the UV/soft X-ray spectrum of individual quasars at these epochs must, once the cosmological “filtering” through material along the line of sight is taken into account, satisfy similar constraints on the average. We model the propagation of AGN-like ionizing radiation through the intergalactic medium using CUBA, a numerical code developed in our prior work. We show that, in order to explain the reported Hell absorption, any thermal component responsible for the “soft X-ray excess” observed in the spectra of AGNs at z ≲ 0.3 must, at z ~ 3, have a typical temperature ≳ 80 eV, and a luminosity not exceeding ≈ 20% of that of the “UV bump”.

We have measured optical spectra of several BeXRBs. Their Be disks apparently do not differ significantly from those in single Be’s.

We observed the new intermediate polar RX J0558+53 withthe 4.2m WHT in March 1995. We detect 2–3%% pulsations of the He ɪɪ emission line on the 545s spin period. The spin-resolved Hell spectrogram shows a complex structure with two roughly anti-phased components moving from red to blue (double pulse). This is most likely produced by non-axisymmetric gas flow in the rotating magnetosphere of the white dwarf.

Here we present a model of hard X-ray flares and hot plasma outflows (optical jets) observed in protostars. Assuming that the dipole magnetic field of a protostar threads the protostellar disk, we carried out 2.5-dimensional magnetohydrodynamic (MHD) simulations of the diskstar interaction. The closed magnetic loops connecting the central star and the disk are twisted by the rotation of the disk. In the presence of resistivity, magnetic reconnection takes place in the current sheet formed inside the expanding loops. Hot, outgoing plasmoid and post flare loops are formed as a result of the reconnection. Numerical results are consistent with the observed plasma temperature (107 – 108K), the length of the flaring loop (1011 – 1012cm), and the speed of optical jets (200 – 400 km s−1 ).

We present preliminary results of a one year monitoring campaign of a sample of active galactic nuclei. We discuss the implications of the observed variations and colour changes in terms of accretion disk models.