Gamma-ray emission from individual classical novae

Classical novae are important producers of radioactive nuclei, such as ⁷Be, ¹³N, ¹⁸F, ²²Na and ²⁶Al. The disintegration of these nuclei produces positrons (except for ⁷Be) that through annihilation with electrons produce photons of energies 511 keV and below. Furthermore, ⁷Be and ²²Na decay producing photons with energies of 478 and 1275 keV, respectively, well in the γ-ray domain. Therefore, novae are potential sources of γ-ray emission. We have developed two codes in order to analyse carefully the γ-ray emission of individual classical novae: a hydrodynamical one, which follows both the accretion and the explosion stages, and a Monte Carlo one, able to treat both production and transfer of γ-ray photons. Both codes have been coupled in order to simulate realistic explosions. The properties of γ-ray spectra and γ-ray light curves (for the continuum and for the lines at 511, 478 and 1275 keV) have been analysed, with a special emphasis on the difference between carbon–oxygen and oxygen–neon novae. Predictions of detectability of individual novae by the future SPI spectrometer on board the INTEGRAL satellite are made. Concerning ²⁶Al, its decay produces photons of 1809 keV but this occurs on a time-scale much longer than the typical time interval between nova outbursts in the Galaxy, making it undetectable in individual novae. The accumulated emission of ²⁶Al from many Galactic novae has not been modelled in this paper.     

A search for radio emission from Galactic supersoft X-ray sources

We have made a deep search for radio emission from all the northern hemisphere supersoft X-ray sources using the Very Large Array (VLA) and multi-element radio-linked interferometer network (MERLIN) telescopes, at 5 and 8.4 GHz. Three previously undetected sources, T Pyx, V1974 Cygni and RX J0019.8+2156, were imaged in quiescence using the VLA in order to search for any persistent emission. No radio emission was detected in any of the VLA fields down to a typical 1 σ rms noise of 20 μJy beam⁻¹, however, 17 new point sources were detected in the fields with 5-GHz fluxes between 100 and 1500 μJy, giving an average 100-μJy source density of ∼200 deg⁻², comparable to what was found in the MERLIN Hubble Deep Field survey. The persistent source AG Draconis was observed by MERLIN to provide a confirmation of previous VLA observations and to investigate the source at a higher resolution. The core is resolved at the milliarcsec scale into two components that have a combined flux of ∼1 mJy. It is possible that we are detecting nebulosity, which is becoming resolved out by the higher MERLIN resolution. We have investigated possible causes of radio emission from a wind environment, both directly from the secondary star, and also consequently, of the high X-ray luminosity from the white dwarf. There is an order of magnitude discrepancy between observed and modelled values that can be explained by the uncertainty in fundamental quantities within these systems.     

X‐ray observations of classical novae: Theoretical implications

Detection of X‐rays from classical novae, both in outburst and post‐outburst, provides unique and crucial information about the explosion mechanism. Soft X‐rays reveal the hot white dwarf photosphere, whenever hydrogen (H) nuclear burning is still on and expanding envelope is transparent enough, whereas harder X‐rays give information about the ejecta and/or the accretion flow in the reborn cataclysmic variable. The duration of the supersoft X‐ray emission phase is related to the turn‐off of the classical nova, i.e., of the H‐burning on top of the white dwarf core. A review of X‐ray observations is presented, with a special emphasis on the implications for the duration of post‐outburst steady H‐burning and its theoretical explanation. The particular case of recurrent novae (both the “standard” objects and the recently discovered ones) will also be reviewed, in terms of theoretical feasibility of short recurrence periods, as well as regarding implications for scenarios of type Ia supernovae (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Time-resolved optical observations of five cataclysmic variables detected by INTEGRAL

The European Space Agency γ-ray telescope, INTEGRAL , is detecting relatively more intrinsically rare cataclysmic variables (CVs) than were found by surveys at lower energies. Specifically, a large fraction of the CVs that are INTEGRAL sources consists of asynchronous polars and intermediate polars (IPs). IP classifications have been proposed for the majority of CVs discovered by INTEGRAL , but, in many cases, there is very little known about these systems. In order to address this, I present time-resolved optical data of five CVs discovered through INTEGRAL observations. The white dwarf spin modulation is detected in high-speed photometry of three of the new CVs (IGR J15094−6649, IGR J16500−3307 and IGR J17195−4100), but two others (XSS J12270−4859 and IGR J16167−4957) show no evidence of magnetism, and should be considered unclassified systems. Spectroscopic orbital period ( P _orb) measurements are also given for IGR J15094−6649, IGR J16167−4957, IGR J16500−3307 and IGR J17195−4100.     

Galactic models and white dwarf populations

We make use of a previous well-tested Galactic model, but describing the observational behaviour of the various stellar components in terms of suitable assumptions on their evolutionary status. In this way we are able to predict the expected distribution of Galactic white dwarfs (WDs), with results which appear in rather good agreement with recent estimates of the local WD luminosity function. The predicted occurrence of WDs in deep observations of selected Galactic fields is presented, and we discuss the role played by WDs in star counts. The effects on the theoretical predictions of different white dwarf evolutionary models, ages, initial mass functions and relations between progenitor mass and WD mass are also discussed.     

On the asphericity of nova remnants caused by rotating white dwarf envelopes

The effect of rotating white dwarf envelopes in determining the structure of nova shells is examined. This is achieved by numerical hydrodynamic simulations of the flows around a binary star system. In previous studies of remnant formation, this rotation has not been included.
It is found that the structures formed in the flow are more consistent with observations of nova shells than the previous theoretical studies. The shells produced by the nova become more prolate with increasing white dwarf envelope rotation. Hence the rotation of white dwarf envelopes must be included in any future discussion of remnant formation.
A possible method of identifying the dominant process by which mixing of accreted and white dwarf matter takes place is suggested.     

Swift observations of CSS081007:030559+054715

CSS081007:030559+054715 was discovered by the Catalina Real‐time Transient Survey. Optical spectroscopy revealed a multi‐peaked Hα emission line profile with radial velocities exceeding 1500 km/s, as well as strong Ne emission, suggestive of a neon nova. We monitored the source extensively with the Swift satellite, obtaining a unique dataset spanning 270 days in the soft X‐ray and UV bands. The data reveal a soft, blackbody‐like spectrum with a temperature around 55 eV (though dependent on the modelling), variable X‐ray and UV light curves with a 1.77 day period in both the X‐ray and UV bands, a longer timescale modulation of ∼ 50 days, followed by a slowly declining trend in the soft X‐ray and UV flux. We highlight the Swift observations and their implications for the SSS nature of this object (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

V4745 Sgr – a nova above the period gap and an intermediate polar candidate

A period analysis of CCD unfiltered photometry of V4745 Sgr (Nova Sgr 2003 #1) performed during 23 nights in the years 2003–2005 is presented. The photometric data are modulated with a period of  0.20 782 ± 0.00 003 d (4.98 768 ± 0.00 072 h)  . Following the shape of the phased light curve and the presence of the periodicity in all data sets with no apparent change in its value, we interpret this periodicity as orbital in nature and this is consistent with a cataclysmic variable above the period gap. We found a probable short-term periodicity of  0.017 238 ± 0.000 037 d (24.82 272 ± 0.05 328 min)  which we interpret as the probable spin period of the white dwarf or the beat period between the orbital and spin period. Therefore, we propose that nova V4745 Sgr should be classified as an intermediate polar candidate, supporting the proposed link between transition-oscillation novae and intermediate polars. The mass–period relation for cataclysmic variables yields a secondary mass of about  0.52 ± 0.05 M_⊙  .     

The theory of steady-state super-Eddington winds and its application to novae

We present a model for steady-state winds of systems with super-Eddington luminosities. These radiatively driven winds are expected to be optically thick and clumpy as they arise from an instability-driven porous atmosphere. The model is then applied to derive the mass loss observed in bright classical novae. The main results are as follows.
(i) A general relation between the mass-loss rate and the total luminosity in super-Eddington systems.
(ii) A natural explanation of the long-duration super-Eddington outflows that are clearly observed in at least two cases (Novae LMC 1988 #1 and FH Serpentis).
(iii) A qualitative agreement between the prediction and observations of the mass loss and temperature evolution.
(iv) An agreement between the predicted average integrated mass loss of novae as a function of white dwarf mass and its observations.
(v) A natural explanation for the 'transition phase' of novae.
(vi) Agreement with η Carinae, which was used to double check the theory: the prediction for the mass shed in the star's great eruption agrees with observations to within the measurement error.     

Cluster AgeS Experiment (CASE): deficiency of observed dwarf novae in globular clusters

We present the results of a search for dwarf novae (DNe) in globular clusters (GCs). It is based on the largest available homogeneous sample of observations, in terms of the time-span, number of observations and number of clusters. It includes 16 Galactic GCs and yielded two new certain DNe: M55-CV1 and M22-CV2. All previously known systems located in our fields were recovered, too. We surveyed M4, M5, M10, M12, M22, M30, M55, NGC 288, NGC 362, NGC 2808, NGC 3201, NGC 4372, NGC 6362, NGC 6752, ω Centauri (NGC 5139) and 47 Tucanae (NGC 104). The discovery of two DNe, namely M55-CV1 and M22-CV2, was already reported by Kaluzny et al. and Pietrukowicz et al., respectively. In the remaining 14 GCs, we found no certain new DNe. Our result raises the total number of known DNe in the Galactic GCs to 12 DNe, distributed among seven clusters. Our survey recovered all three already known erupting cataclysmic variables (CVs) located in our fields, namely M5-V101, M22-CV1, and V4 in the foreground of M30. To assess the efficiency of the survey, we analysed images with inserted artificial stars mimicking outbursts of the prototype DNe SS Cygni and U Geminorum. Depending on the conditions, we recovered between 16–100 per cent of these artificial stars. The efficiency seems to be predominantly affected by duty cycle/time-sampling and much less by distance/magnitude. Except for saturated tiny collapsed cores of M30, NGC 362 and NGC 6752 (and also the dense core of NGC 2808), crowding effects in the V band were avoided by our image subtraction technique augmented with auxiliary unsaturated B -band images. Our results clearly demonstrate that in GCs common types of DNe are very rare indeed. However, great care must be taken before these conclusions can be extended to the CV population in GCs.     

Gravitational radiation from strongly magnetized white dwarfs

The magnetic fields of white dwarfs distort their shape generating an anisotropic moment of inertia. A magnetized white dwarf that rotates obliquely relative to the symmetry axis has a mass quadrupole moment that varies in time, so it will emit gravitational radiation. The Laser Interferometer Space Antenna ( LISA ) mission may be able to detect the gravitational waves from two nearby, rapidly rotating white dwarfs.