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Register a new userNew outburst of the symbiotic nova AG Peg after 165 years
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AG Peg is known as the slowest symbiotic nova, which experienced its nova-like outburst around 1850. After 165 years, during June of 2015, it erupted again showing characteristics of the Z And-type outburst. The primary objective is to determine basic characteristics, the nature and type of the 2015 outburst of AG Peg. We achieved this aim by modelling the spectral energy distribution using low-resolution spectroscopy (330-750 nm), medium-resolution spectroscopy (420-720 nm; R=11000), and $UBVR_{rm C}I_{rm C}$ photometry covering the 2015 outburst with a high cadence. Optical observations were complemented with the archival HST and FUSE spectra from the preceding quiescence. During the outburst, the luminosity of the hot component was in the range of 2-11$times 10^{37}(d/0.8{rm kpc})^2$ erg/s. To generate the maximum luminosity the white dwarf (WD) had to accrete at $sim 3times 10^{-7}$ M$_{odot}yr^{-1}$, which exceeds the stable-burning limit and thus led to blowing optically thick wind from the WD. We determined its mass-loss rate to a few $times 10^{-6}$ M$_{odot}yr^{-1}$. At the high temperature of the ionising source, $1.5-2.3times 10^5$ K, the wind converted a fraction of the WDs photospheric radiation into the nebular emission that dominated the optical. A one order of magnitude increase of the emission measure, from a few $times 10^{59}(d/0.8 {rm kpc})^2$ cm$^{-3}$ during quiescence, to a few $times 10^{60}(d/0.8,{rm kpc})^2$ cm$^{-3}$ during the outburst, caused a 2 mag brightening in the LC, which is classified as the Z And-type of the outburst. The very high nebular emission and the presence of a disk-like HI region encompassing the WD, as indicated by a significant broadening and high flux of the Raman-scattered OVI 6825 AA line during the outburst, is consistent with the ionisation structure of hot components in symbiotic stars during active phases.
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Symbiotic stars often contain white dwarfs with quasi-steady shell burning on their surfaces. However, in most symbiotics, the origin of this burning is unclear. In symbiotic slow novae, however, it is linked to a past thermonuclear runaway. In June 2015, the symbiotic slow nova AG Peg was seen in only its second optical outburst since 1850. This recent outburst was of much shorter duration and lower amplitude than the earlier eruption, and it contained multiple peaks -- like outbursts in classical symbiotic stars such as Z And. We report Swift X-ray and UV observations of AG Peg made between June 2015 and January 2016. The X-ray flux was markedly variable on a time scale of days, particularly during four days near optical maximum, when the X-rays became bright and soft. This strong X-ray variability continued for another month, after which the X-rays hardened as the optical flux declined. The UV flux was high throughout the outburst, consistent with quasi-steady shell burning on the white dwarf. Given that accretion disks around white dwarfs with shell burning do not generally produce detectable X-rays (due to Compton-cooling of the boundary layer), the X-rays probably originated via shocks in the ejecta. As the X-ray photo-electric absorption did not vary significantly, the X-ray variability may directly link to the properties of the shocked material. AG Pegs transition from a slow symbiotic nova (which drove the 1850 outburst) to a classical symbiotic star suggests that shell burning in at least some symbiotic stars is residual burning from prior novae.
We present an analysis of the XMM-Newton observation of the symbiotic star AG Peg, obtained after the end of its 2015 outburst. The X-ray emission of AG Peg is soft and of thermal origin. AG Peg is an X-ray source of class beta of the X-ray sources amongst the symbiotic stars, whose X-ray spectrum is well matched by a two-temperature optically-thin plasma emission (kT_1 ~ 0.14 keV and kT_2 ~ 0.66 keV). The X-ray emission of the class beta sources is believed to originate from colliding stellar winds (CSW) in binary system. If we adopt the CSW picture, the theoretical CSW spectra match well the observed properties of the XMM-Newton spectra of AG Peg. However, we need a solid evidence that a massive-enough hot-star wind is present in the post-outburst state of AG Peg to proof the validity of the CSW picture for this symbiotic binary. No short-term X-ray variability is detected while the UV emission of AG Peg shows stochastic variability (flickering) on time-scales of minutes and hours.
The nova outburst experienced in 2010 by the symbiotic binary Mira V407 Cyg has been extensively studied at optical and infrared wavelengths with both photometric and spectroscopic observations. This outburst, reminiscent of similar events displayed by RS Oph, can be described as a very fast He/N nova erupting while being deeply embedded in the dense wind of its cool giant companion. The hard radiation from the initial thermonuclear flash ionizes and excites the wind of the Mira over great distances (recombination is observed on a time scale of 4 days). The nova ejecta is found to progressively decelerate with time as it expands into the Mira wind. This is deduced from line widths which change from a FWHM of 2760 km/s on day +2.3 to 200 km/s on day +196. The wind of the Mira is massive and extended enough for an outer neutral and unperturbed region to survive at all outburst phases.
Optical spectroscopy study of the recent AG Peg outburst observed during the second half of 2015 is presented. Considerable variations of the intensity and the shape of the spectral features as well as the changes of the hot component parameters, caused by the outburst, are discussed and certain similarities between the outburst of AG Peg and the outburst of a classical symbiotic stars are shown. It seems that after the end of the symbiotic nova phase, AG Peg became a member of the classical symbiotic stars group.
We present and analyze optical photometry and high resolution SALT spectra of the symbiotic recurrent nova V3890 Sgr at quiescence. The orbital period, P=747.6 days has been derived from both photometric and spectroscopic data. Our double-line spectroscopic orbits indicate that the mass ratio is q=M_g/M_WD=0.78+/-0.05, and that the component masses are M_WD=1.35+/-0.13 Msun, and M_g=1.05+/-0.11 Msun. The orbit inclination is approximately 67-69 degr. The red giant is filling (or nearly filling) its Roche lobe, and the distance set by its Roche lobe radius, d=9 kpc, is consistent with that resulting from the giant pulsation period. The outburst magnitude of V3890 Sgr is then very similar to those of RNe in the Large Magellanic Cloud. V3890 Sgr shows remarkable photometric and spectroscopic activity between the nova eruptions with timescales similar to those observed in the symbiotic recurrent novae T CrB and RS Oph and Z And-type symbiotic systems. The active source has a double-temperature structure which we have associated with the presence of an accretion disc. The activity would be then caused by changes in the accretion rate. We also provide evidence that V3890 Sgr contains a CO WD accreting at a high, a few 1e-8 - 1e-7 Msun/yr, rate. The WD is growing in mass, and should give rise to a Type Ia supernova within about 1,000,000 yrs - the expected lifetime of the red giant.
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