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Register a new userMulticolor photometry of SU UMa and U Gem during quiescence, outburst and superoutburst
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The results of time-resolved observations of SU UMa and U Gem obtained over two-years are presented. Both stars are prototypes of different classes of dwarf novae. We studied brightness variations on different time scales: orbital, QPO and flickering. The multicolor BVRI photometry allows to distinguisch the geometrical and physical sources of these variations.
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Chandra HETG spectra of the prototypical dwarf novae SS Cyg and U Gem in quiescence and outburst are presented and discussed. When SS Cyg goes into outburst, it becomes dimmer in hard X-rays and displays a dramatic shift in its relative line strengths, whereas when U Gem goes into outburst, it becomes brighter in hard X-rays and displays only a minor shift in its relative line strengths. In both systems, the emission lines become significantly broader in outburst, signaling the presence of high velocity gas either in Keplerian orbits around the white dwarf or flowing outward from the system.
We present an in-depth photometric study of the 2013 superoutburst of the recently discovered cataclysmic variable V1838 Aql and subsequent photometry near its quiescent state. A careful examination of the development of the superhumps is presented. Our best determination of the orbital period is Porb = 0.05698(9) days, based on the periodicity of early superhumps. Comparing the superhump periods at stages A and B with the early superhump value we derive a period excess of {epsilon} = 0.024(2) and a mass ratio of q = 0.10(1). We suggest that V1838 Aql is approaching the orbital period minimum and thus has a low-mass star as a donor instead of a sub-stellar object.
SU UMa stars are characterized by superoutbursts which are brighter at maximum light and which last much longer than the more frequent ordinary outbursts of these dwarf novae. Although there are now more than 1180 SU UMa type dwarf novae catalogued, our knowledge on their superoutburst cycle length Cso was hitherto limited to about 6$%$ of the entire sample of known SU UMa stars. Using public data bases we have determined new Cso values for a total of 206 additional SU UMa stars in the range 17 d $<$ Cso $<$ 4590 d (including some ER UMa and WZ Sge type representants) within total time intervals between 2 and 57 years, and with an estimated uncertainty of $pm$11$%$. This way, we are increasing our present knowledge of Cso values by a factor $sim$3.8. Its distribution is characterized by a broad maximum around Cso $approx$ 270 days, and slowly decreasing numbers till Cso $approx$ 800 d. The domain Cso $>$ 450 d was unexplored until now; we add here 106 cases ($sim$51$%$ of our total sample) in this range of long cycles, implying a better statistical basis for future studies of their distribution. Our sample contains 16 known WZ Sge stars, and we propose WZ Sge membership for 5 others hitherto classified as ordinary SU UMa stars. Individual superoutburst timings deviate in average about $pm$7$%$ of the cycle length from their overall linear ephemeris, conrming a pronounced quasi-periodic repeatability of superoutbursts. All relevant parameters are listed with their errors, and a table with individual superoutburst epochs of our targets is given, enabling future researchers to combine our results with other (past or future) observations.
BZ UMa is a cataclysmic variable star whose specific classification has eluded researchers since its discovery in 1968. It has outburst and spectral properties consistent with both U Gem class dwarf novae and intermediate polars. We present new photometric and polarimetric measurements of recent outbursts, including the first detected superoutburst of the system. Statistical analysis of these and archival data from outbursts over the past 40 years present a case for BZ UMa as a non-magnetic, U Gem class, SU-UMa subclass dwarf novae.
We explore the accretion mechanisms in EX Lupi, prototype of EXor variables, during its quiescence and outburst phases. We analyse high-resolution optical spectra taken before, during, and after its 2008 outburst. In quiescence and outburst, the star presents many permitted emission lines, including typical CTTS lines and numerous neutral and ionized metallic lines. During the outburst, the number of emission lines increases to over a thousand, with narrow plus broad component structure (NC+BC). The BC profile is highly variable on short timescales (24-72h). An active chromosphere can explain the metallic lines in quiescence and the outburst NC. The dynamics of the BC line profiles suggest an origin in a hot, dense, non-axisymmetric, and non-uniform accretion column that suffers velocity variations along the line-of-sight on timescales of days. Assuming Keplerian rotation, the emitting region would be located at ~0.1-0.2 AU, consistent with the inner disk rim, but the velocity profiles of the lines reveal a combination of rotation and infall. Line ratios of ions and neutrals can be reproduced with a temperature of T~6500 K for electron densities of a few times 10$^{12}$cm$^{-3}$ in the line-emitting region. The data confirm that the 2008 outburst was an episode of increased accretion, albeit much stronger than previous EX Lupi and typical EXors outbursts. The line profiles are consistent with the infall/rotation of a non-axisymmetric structure that could be produced by clumpy accretion during the outburst phase. A strong inner disk wind appears in the epochs of higher accretion. The rapid recovery of the system after the outburst and the similarity between the pre-outburst and post-outburst states suggest that the accretion channels are similar during the whole period, and only the accretion rate varies, providing a superb environment for studying the accretion processes.
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