Distinguishing between symbiotic stars and planetary nebulae

Number of known symbiotic stars (SySt) is still significantly lower than their predicted population. One of the main problems in finding complete population of SySt is the fact that their spectrum can be confused with other objects, such as planetary nebulae (PNe) or dense H II regions. The problem is reinforced by a fact that in significant fraction of established SySt the emission lines used to distinguish them from other objects are not present. We aim at finding new diagnostic diagrams that could help separate SySt from PNe. Additionally, we examine known sample of extragalactic PNe for candidate SySt. We employed emission line fluxes of known SySt and PNe from the literature. We found that among the forbidden lines in the optical region of spectrum, only the [O III] and [N II] lines can be used as a tool for distinguishing between SySt and PNe, which is consistent with the fact that they have the highest critical densities. The most useful diagnostic that we propose is based on He I lines which are more common and stronger in SySt than forbidden lines. All these useful diagnostic diagrams are electron density indicators that better distinguishes PNe and ionized symbiotic nebulae. Moreover, we found six new candidate SySt in the Large Magellanic Cloud and one in M81. If confirmed, the candidate in M81 would be the furthest known SySt thus far.Comment: 7 pages, 7 figures, submitted to A&

Variability of the symbiotic X-ray binary GX 1+4: Enhanced activity near periastron passage

Context. GX 1+4 belongs to a rare class of X-ray binaries with red giant donors, symbiotic X-ray binaries. The system has a history of complicated variability on multiple timescales in the optical light and X-rays. The nature of this variability remains poorly understood. Aims. We study variability of GX 1+4 on long time-scale in X-ray and optical bands. Methods. The presented X-ray observations are from INTEGRAL Soft Gamma-Ray Imager and RXTE All Sky Monitor. The optical observations are from INTEGRAL Optical Monitoring Camera. Results. The variability of GX 1+4 both in optical light and hard X-ray emission (>17 keV) is dominated by ~50-70d quasi-periodic changes. The amplitude of this variability is highest during the periastron passage, while during the potential neutron star eclipse the system is always at minimum, which confirms the 1161d orbital period that has had been proposed for the system based on radial velocity curve. Neither the quasi-periodic variability or the orbital period are detected in soft X-ray emission (1.3-12.2 keV), where the binary shows no apparent periodicity.Comment: 8 pages, 7 figures, accepted for publication in A&

Symbiotic star T CrB as an extreme SU UMa type dwarf nova

T CrB is a symbiotic recurrent nova that exhibits quiescent and active phases between its classical nova eruptions. The statistical properties of these active phases have been poorly studied thus far. Because of that their nature remained unknown. Here we study statistical properties of the active phases and show that they are consistent with outburst and superoutbursts observed in SU UMa type dwarf novae. The recurrence time of these outbursts is consistent with theoretical predictions for similar systems. Moreover, the visual and X-ray evolution of the last active phase is consistent with a superoutburst. This suggests that T CrB is a dwarf nova with an extremely long orbital period, closely related to SU UMa dwarf novae. The similarities between the last superoutburst and the reported activity preceding the 1946 nova eruption may suggest that next classical nova eruption in T CrB could be indeed soon expected.Comment: 7 pages, 4 figures, accepted to ApJ

LMC S154: the first Magellanic symbiotic recurrent nova

Classical nova outburst has been suggested for a number of extragalactic symbiotic stars, but in none of the systems has it been proven. In this work we study the nature of one of these systems, LMC S154. We gathered archival photometric observations in order to determine the timescales and nature of variability in this system. Additionally we carried out photometric and spectroscopic monitoring of the system and fitted synthetic spectra to the observations. Carbon abundance in the photosphere of the red giant is significantly higher than that derived for the nebula, which confirms pollution of the circumbinary material by the ejecta from nova outburst. The photometric and spectroscopic data show that the system reached quiescence in 2009, which means that for the first time all of the phases of a nova outburst were observed in an extragalactic symbiotic star. The data indicate that most probably there were three outbursts observed in LMC S154, which would make this system a member of a rare class of symbiotic recurrent novae. The recurrent nature of the system is supported by the discovery of coronal lines in the spectra, which are observed only in symbiotic stars with massive white dwarfs and with short-recurrence-time outbursts. Gathered evidence is sufficient to classify LMC S154 as the first bona fide extragalactic symbiotic nova, which is likely a recurrent nova. It is also the first nova with a carbon-rich donor.Comment: 16 pages, 7 figures, accepted for publication in A&