We report on a 1-day ASCA observation of the eclipsing dwarf nova HT Cas. We confirm the presence of an X-ray eclipse, which is narrow and deep. The data are consistent with the X-rays originating entirely from the immediate neighborhood of the white dwarf. We draw some preliminary conclusions on the boundary layer and other relevant issues.

In non-magnetic cataclysmic variables the accreted matter forms an accretion disk around the white dwarf. In the boundary layer between the white dwarf and the accretion disk the accreted matter decelerates from Keplerian velocities to the rotation velocity of the white dwarf. If the accretion rate is high the boundary layer would be optically thick and cool (T ~ 105K), and if the accretion rate is low the boundary layer would be optically thin and hot (T ~ 108K) (Pringle & Savonije 1979).

There are several observational problems with this simple picture: a soft X-ray component could only be detected so far in 5 dwarf novae in outburst and not in any nova-like variable. Also in high-accretion-rate systems there is a hot optically thin X-ray source, which has, however, an X-ray luminosity which is much less than the UV luminosity of the system (van Teeseling & Verbunt 1994). Finally, there is evidence for orbital modulation in the X-rays from some systems (e.g. van Teeseling et al. 1995).

The COMPTEL experiment on the Compton Gamma-Ray Observatory measures γ–radiation in the energy range from 0.75 to 30 MeV [10]. X-ray binaries form one category of potential γ-ray sources. Up to now only two X-ray binaries, Cyg X-l and Nova Per, have been seen by COMPTEL.

Cyg X-l. Preliminary results were reported by [6] and indicate that (i) there was no evidence for any hardening of the spectrum near lMeV, (ii) the plasma temperature suggested by a Wien spectral model was much higher than that implied by hard X-ray observations. This latter conclusion seems to require a revision in the standard spectral model for Cyg X-l.

The α–ray flux from Cyg X-l is constant up to several MeV. The time history of the flux in the 0.75… 2.0 MeV energy range shows that there are no significant variations near 1 MeV. The average photon spectrum derived from all observations up to 1994 July shows evidence for significant emission extending well above 2 MeV, with a data point in the 2…5 MeV range at 4.3 σ significance. However, the observed flux near 1 MeV is significantly below that reported by HEAO-3 and several other balloon observations [6]. The COMPTEL spectral data show a broad line-like feature around ~ 4 MeV with the best fit of a single power law plus Gaussian. If we assume that the Gaussian represents a single broadened nuclear line of 12C*, then the width of 1.38 MeV demands a temperature of ~ 2 1012 K [7].

Multi-wavelength observations have shown that, after optical decline, the stellar remnants of classical nova outbursts evolve at constant, near-Eddington, bolometric luminosity to high effective temperature (> 2 105 K), before turning off. Here we briefly review the observations of classical novae in this phase of evolution, and discuss what the soft X-ray observations tell us about the mass of the underlying white dwarf and the rate of mass change due to stellar winds and accretion from the stellar companion.

We observed a number of classical and recurrent novae in the Galaxy and the LMC with the ROSAT X-ray telescope and searched the archival data for other serendipitous observations. Preliminary results show that only 9 out of 37 observed objects were bright enough in X-rays to be detected with ROSAT, either in outburst or at quiescence.

Three basic mechanisms can cause X-ray emission from classical or re-current novae. The first is hot hydrogen burning in a thin shell of the remnant envelope left on the white dwarf after the nova explosion. Hydrogen burning post-novae should be blackbody like emitters at nearly Eddington luminosity (as per the ‘supersoft’ X-ray sources). In our sample, only GQ Mus (Nova Mus 1983, see Ögelman et al. 1993; Shanley et al. 1995) and V1974 Cyg 1992 (Krautter et al. 1996) had these characteristics. Remarkably, among 10 LMC novae that had an outburst in the last 47 yr none was detected as a ‘supersoft’ X-ray source. The 3 σ upper limits for the black-body temperatures of the post-nova white dwarfs are mainly in the range 20… 30 eV. A post-nova can also emit X-rays because of shocks occurring in the ejected shell (e.g. O’Brien et al. 1994). Three out of four classical novae that were observed in outburst displayed a hard X-ray component in the ROSAT energy band, which might be due to a shocked shell. Finally, X-ray emission is expected from quiescent nearby novae because of accretion. Only four nearby accreting sources were detected; the ROSAT upper limits for the non-detected quiescent novae are Lx < 1031… 1032 ergs−1, assuming a thermal plasma at kT = a few keV.

We report preliminary results from a new program of observations of novae in nearby galaxies. The principal goal of the program is to explore the population of nova progenitors through observations of galaxies having a wide range of Hubble types. Here we present preliminary results from observations of the spiral systems M51 and M101, and the giant elliptical, M87. Based on available data, we find no evidence that the luminosity specific nova rate varies across the Hubble sequence.

Novae in outburst can be used as distance indicators. The distance measurement process is considerably simplified if novae could be regarded as standard candles. The term standard candles in the context of this paper means that the absolute magnitude at maximum for different novae is, to a good approximation, constant. The t3 – Mb diagram (Schmidet 1957) implies that this is the case for M31 and LMC novae (Della Valle & Livio 1995). However, this conclusion does not necessarily apply to non Local Group novae (Livio 1992; Della Valle & Livio 1995); for example the Virgo novae are not similar to Local Group novae (Della Valle & Livio 1995). Here we focus on a question of principle, namely: Does the fact that novae have similar spectra imply that they have similar luminosities?

Spherically symmetric models for classical novae as thermonuclear run-aways (TNR) of hydrogen-rich, accreted, envelopes on top of cold white dwarfs (WD) are in very good agreement with observations (Starrfield, Sparks & Truran 1974, 1985; Prialnik, Shara & Shaviv 1978; Prialnik 1986; MacDonald 1980). Multi-dimensional investigations are needed since the burning rate is very sensitive to the temperature and small fluctuations in temperature can lead to large differences between the flash times of different points on the burning shell; these differences can be much larger than the rise time of the nova light curve. Convection is assumed to be the mechanism that couples the burning process at different points in the shell so that the overall phenomena is spherically symmetric.

The theory of nova outbursts invokes accretion of hydrogen-rich material by a white dwarf (WD) in a close binary system, due to Roche lobe overflow of a red dwarf companion, leading to periodic outbursts — powered by thermonuclear runaways — that result in mass ejection. Observed characteristics of novae are reproduced by varying the values of three basic and independent parameters: the accreting white dwarf’s mass, its core temperature, and the mass transfer rate. In a recent study (Prialnik & Kovetz 1995) we have carried out a systematic investigation of nova outbursts on WD progenitors composed of C and O in equal mass fractions, concluding that the entire range of observed nova characteristics could be accounted for. A subset of 34 out of 64 evolutionary sequences (obtained with different parameter combinations) reproduced classical novae, and here we focus on the break-up of the heavy element abundances in the ejecta of these models, in comparison with observations, based on a recent analysis of observed nova abundances by Livio & Truran (1994).

We report on preliminary hydrodynamic simulations of thermonuclear runaways in the hydrogen-rich envelopes of accreting white dwarfs, leading to nova outbursts. A one-dimensional, Lagrangian, implicit, hydrodynamic code (Kutter & Sparks 1972) has been used to reproduce the gross features of a classical nova, from the onset of accretion, through the progress of the outburst up to the expansion stage, where significant mass loss takes place. Special interest is focused on the synthesis of 22Na and 26Al, for their interest in γ-ray line astrophysics. The γ-ray spectrum is built by means of a Monte Carlo simulation.

Consequences of a detailed photoionization model study of the shell ejected by Nova Mus 1983 (GQ Mus) are presented for the period from early 1983 to 1990 (Morisset & Péquignot 1996).

The drastic time variation of the emission line spectrum, including the transition from a nebular to a coronal stage in 1986, can be quantitatively understood in terms of a smooth evolution of both the expanding shell and the hot thermal source. This transition is due to a decrease of the shell density with time, and not to an increase of the source effective temperature.

The model shell comprises two components so that not all directions become optically thin to ionizing radiation simultaneously. The range of density and geometrical thickness of the emitting material is of the order of a factor 2.

Observations of spatially resolved nova shells and reconstructions from emission line profiles show that a ring/blob asymmetry in the ejecta of novae is very common (Martin 1989). The polar blobs expand more rapidly than the equatorial rings, giving the ejecta an overall prolate geometry.

Observations of the formation of carbonaceous and silicate dusts in the same nova provide unambiguous evidence that abundance gradients exist in the ejecta. Carbonaceous dusts form where carbon is more abundant than oxygen and vice versa for silicates (Evans 1990; Gehrz et al. 1992; Smith, Aitken & Roche 1994; Snijders et al. 1987).

A deep, predominantly Ha, imaging survey of 37 classical nova remnants has just been completed using the William Herschel and Anglo-Australian Telescopes. This has resulted in the detection of new nebular remnants and the resolution of detailed structure in previously known remnants. These data indicate a correlation between nova speed class and the ellipticity of the resulting remnants – those of faster novae tend to comprise randomly distributed clumps of ejecta superposed on spherically symmetric diffuse material, whilst slower novae produce more structured ellipsoidal remnants with at least one, and sometimes several, rings of enhanced emission.

Several years after outburst, extended optical emission can be seen around many classical novae. Common features include shells, polar ‘blobs’, equatorial rings and, at least in the case of DQ Her (1934), tropical rings above and below the ‘equator’. We present hydrodynamic calculations of the dynamical effects of the underlying binary system on the material ejected during outburst, and show that many of the features observed in the optical remnants of novae can be reproduced. Polar blobs, banded shells and spherical shells can all be obtained in the model, depending on the speed class of the nova.

We used the Wisconsin Ultraviolet Photo-Polarimeter Experiment during the Astro-2 mission aboard the Space Shuttle Endeavour, to obtain ultraviolet spectropolarimetry of three classical novae that had recently gone into outburst. All three novae appear to have intrinsic polarization, with polarization changes across emission lines. This result indicates that, geometrically, the ejecta were quite aspherical.

A model for nova shells which is a combination of wind model and spherically symmetric, Hubble flow shell model, fits the light curve, image, and nebular line shape data for Nova V1974 Cyg 1992. The results show that after outburst the shell is ejected by a wind-like outflow with a linear velocity gradient, and that after 180 d this outflow stops. The shape of the elliptical inner boundary at this time determines the subsequent emission measure structure seen in images, light curves, and nebular line profiles.

V1974 Cyg erupted on 1992 February 19, and was the brightest nova since V1500 Cyg. It soon became an object of massive international multiwavelength observational study.

Some major photometric features of the nova were investigated by DeYoung & Schmidt (1994), and Semeniuk et al. (1994,1995). They found two periodicities in the nova light curve. While the shorter 0.081263 d period was present during the 1993 and 1994 observations, the longer 0.0850 d period was only observed from 1994 July. Semeniuk et al. (1995) interpreted the shorter period as the orbital period of the binary system, and the longer one as caused by the rotation of the white dwarf due to a magnetic field.

Nova Cas 1993 (=V705 Cas) was discovered ten days before it reached its maximum brightness in the V colour (Vmax = 5.53 mag and Bmax = 6.14 mag) on 1993 December 17.29 (Munari et al. 1994b). The pre-nova varied around 18… 18.5 mag and 17… 17.5 mag in B and V, respectively. No more prominent light fluctuations were found (Munari et al. 1994a). The nova started to fade rapidly, entering the deep minimum typical for the transition period of the DQ Her-subclass of novae in 1994 mid-February. The time development of the UV luminosity of the nova was discussed by Hauschildt et al. (1994). In this paper, we analyze our original UBV photometric data and all the available data obtained by the other authors. We derive the basic parameters of the nova and present the results of our search for short-term periodicities.

Nova V705 Cas was discovered on 1993 December 7, several days before maximum light. The light curve indicates that t2 ≃ 40 d (see e.g. [4]), characteristic of dust-forming novae. The light curve went into decline on day 58, indicating the formation of an optically thick dust shell. IR spectra have been obtained regularly since outburst using the cooled grating spectrometer CGS4 on the United Kingdom InfraRed Telescope (UKIRT). We present here the results of ongoing IR spectroscopic monitoring of this nova.

We present infrared (IR) photometry and optical spectroscopy of the eclipsing old nova WY Sge. According to hibernation theorists the disc in a system a few centuries after outburst should be significantly fainter than in other, more recent, old novae but similar to the discs of dwarf novae in quiescence. Despite the apparent faintness of the late type star we have sufficient information to infer that the face closest to the white dwarf (WD) is irradiated.