A ‘superoutburst’ in XTEJ1118+480

I propose that the properties of the two outbursts observed in the X‐ray transient XTEJ1118+480 in 2000 are akin to superoutbursts of SU UMa stars. In these systems a ‘normal’ outburst immediately precedes a 5–10 times longer (‘super’) outburst. The optical light curve of the outbursts of XTEJ1118+480 is remarkably similar to that seen in some SU UMa stars, such as UVPer and TLeo, where the precursor outburst is distinct from the superoutburst, but the time scales are a factor of ∼15 different. The first outburst of XTEJ1118+480 was relatively short (∼1 month) while the second outburst was ∼5 times longer. During the second outburst superhumps were seen, a feature characteristic for superoutbursts. The gap of about a month between the two outbursts is longer in X‐rays with respect to the optical, a feature not previously recognized for X‐ray transients. Also in SU UMa stars the precursor outburst becomes more distinct at shorter wavelengths. Finally, I show that the time of appearance of the superhumps in XTEJ1118+480 is consistent with the expected superhump growth time, if the superhump mechanism was triggered during the first outburst. I conclude that the similarity in outburst behaviour in the two types of systems provides further support that a common mechanism is at work to start the long (‘super’) outbursts.     

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.     

Galactic 1.275-MeV emission from ONe novae and its detectability by INTEGRAL/SPI

Models of Galactic 1.275‐MeV emission produced by the decay of the radionuclide ²²Na have been computed. Several frequency–spatial distributions of novae have been investigated using recent results of nova rates and spatial distributions of novae in our Galaxy. These models allow us to estimate the lower limit of the ²²Na mass ejected per ONe nova detectable with the future spectrometer (SPI) of the INTEGRAL observatory as a function of the frequency–spatial distribution of ONe novae in the Galaxy. Calculations using recent estimations of the expected ²²Na mass ejected per ONe nova show that the detection of the Galactic emission of 1.275‐MeV photons will be difficult with the future spectrometer of the INTEGRAL observatory, whereas the cumulative emission around the Galactic Centre has some chance of being detected during the deep survey of the central radian of the Galaxy.     

The nature of TW Pictoris

The results of X-ray and optical observations of the candidate intermediate polar TW Pic are presented in an attempt to understand its nature. We find no sign of the previously proposed ∼2 h white-dwarf spin period and ∼6 h orbital period of TW Pic in its X-ray light curve. There is therefore no convincing evidence in support of its previous classification. The lack of X-ray pulsation could be the result of a low inclination angle, but in that case there would be no reason why an optical pulsation should have been seen previously. Negative results from polarimetry also preclude TW Pic from being a polar. One possibility may be that the shorter of the two periods is in fact the orbital period, whilst the longer one is a harmonic of a disc precession period. Alternatively, both the high accretion rate and period structure of TW Pic indicate that it may be a system that displays persistent negative superhumps. In this case the true orbital period of the binary may be around 6.36 h and the shorter of the two previously identified periods, 1.996 h, represents the (shifted) second harmonic of a negative superhump period of 6.06 h. Under this interpretation, it would be the longest period system to display such a phenomenon. Finally there is also evidence that TW Pic may be a VY Scl star, in which case it would be the longest period member of that subclass too.     

A spectroscopic study of V603 Aquilae: stellar parameters and continuumline variations

A time-resolved spectroscopic study of V603 Aql (Nova Aquilae 1918) is presented. An orbital period of P _orb=01385±00002, consistent with previous results, and a radial velocity semi-amplitude of K =20±3 km s¹ are obtained from the radial velocity variations of the H emission line. Similar K values are also found in H , H , and He  i emission lines. Using the measured FWHM of the H line and assuming that the derived semi-amplitude is that of the white dwarf, we deduce a most likely mass ratio of q =0.24±0.05 and stellar masses of M ₂=0.29±0.04 M and M ₁=1.2±0.2 M for the secondary and primary (the white dwarf) star, respectively. The dynamical solution also indicates a very low orbital inclination, i =13°±2°. We find that the continuum and line variations are modulated with both the positive and the negative superhump periods, indicating that they arise from similar regions of the accretion disc. Moreover, we find, for the first time from spectroscopy, evidence of negative superhumps in addition to the positive superhumps. Positive superhumps are explained within the disc instability model as caused by an eccentric disc surrounding the white dwarf, which is precessing (apsidal advance) because of tidal instabilities, causing the observed positive superhumps. A nodal precession in the accretion disc is currently believed to be the cause of the observed negative superhumps. The low value of q is consistent with the expected value for systems that show superhumps, in accordance with the eccentric disc model. We find no evidence of periodicity associated with the spin period.     

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.     

激变变星中的快速光变

文中综述了激变变星中快速光变的研究进展，讨论了闪变，相干振荡和准周期振荡的观测特征和可能的产生机制。     

Resumption of mass accretion in RS Oph

The latest outburst of the recurrent nova RS Oph occurred in 2006 February. Photometric data presented here show evidence of the resumption of optical flickering, indicating re-establishment of accretion by day 241 of the outburst. Magnitude variations of up to 0.32 mag in V band and 0.14 mag in B band on time-scales of 600–7000 s are detected. Over the two-week observational period, we also detect a 0.5 mag decline in the mean brightness, from   V ≈ 11.4  to 11.9, and record   B ≈ 12.9 mag  . Limits on the mass accretion rate of     are calculated, which span the range of accretion rates modelled for direct wind accretion and Roche lobe overflow mechanisms. The current accretion rates make it difficult for thermonuclear runaway models to explain the observed recurrence interval, and this implies average accretion rates are typically higher than seen immediately post-outburst.