Sergei Yu. Shugarov

X-RAY/OPTICAL STUDIES OF TWO OUTBURSTS OF THE INTERMEDIATE POLAR YY (DO) DRACONIS

YY Draconis (likely the same variable called DO Draconis) is one of a small number of intermediate polars (IPs) that show outburst behavior. We report results from Target of Opportunity observations with the Rossi X-Ray Timing Explorer, together with ground-based optical photometry during outbursts in 1999 September and 2000 November. Similar behavior was evident in both outbursts. At outburst, the X-ray flux increased by more than a factor of 12, and the spectrum became hotter and more absorbed compared to quiescence. While the spin pulse at 529 s is clearly present in the X-ray data at 2–4 days past outburst peak and during quiescence, it was not detected in the X-ray data closest to outburst (1.5 days). This is contrary to the large increase in spin pulse amplitude that has been seen during outbursts of the IPs GK Per, XY Ari, and EX Hya. The differences in YY Dra are likely due to its unique geometry, with two relatively equal poles located near the white dwarf equator. The equal enhancement of both poles near outburst could account for the low pulse amplitude, while unequal feeding of the poles as the magnetosphere recedes during decline could explain the changes in amplitude and pulse shape. The changing height of the shocks may also have an effect on the visibility of both poles.

Superhumps in Cataclysmic Binaries. XIV. V592 Cassiopeiae

ABSTRACT We report a spectroscopic and photometric study of the novalike variable V592 Cassiopeiae (=LS I 55° 8). The spectrum is that of a typical UX UMa–type star, with weak, narrow Balmer emission superposed on broad absorption. The 0.1–2.2 μm flux distribution also looks fairly normal, suggesting disk accretion at ∼10−8 M⊙ yr−1 and a distance of 330 pc. The emission lines move with P = 0.115063(1) days, which is presumably the underlying orbital period of the binary. Photometry reveals a different period, namely, 0.12228(1) days. The presence of this wave in a short‐period cataclysmic variable, and the value of the period excess at 6.3%, suggests identification as a “permanent superhump.” After subtraction of this large signal, the residual time series appears to contain a weak feature at 0.11193(5) days. The star evidently shows positive and negative superhumps simultaneously. Its binary period puts it among a modest number of nonmagnetic cataclysmic variables occupying the 2–3 hr period “gap.”

Optical studies of the X-ray transient XTE J2123-058 — I. Photometry

We present optical photometry of the X-ray transient XTE J2123-058, obtained in July-October 1998. The light curves are strongly modulated on the 5.95hrs orbital period, and exhibit dramatic changes in amplitude and form during the decline. We used synthetic models which include the effect of partial eclipses and X-ray heating effects, to estimate the system parameters, and we constrain the binary inclination to be i=73+-4 degrees. The model is successful in reproducing the light curves at different stages of the decay by requiring the accretion disc to become smaller and thinner by 30% as the system fades by 1.7 mags in the optical. From Aug 26 the system reaches quiescence with a mean magnitude of R=21.7+-0.1 and our data are consistent with the optical variability being dominated by the companions ellipsoidal modulation.We present optical photometry of the X-ray transient XTE J2123-058, obtained in July-October 1998. The light curves are strongly modulated on the 5.95hrs orbital period, and exhibit dramatic changes in amplitude and form during the decline. We used synthetic models which include the effect of partial eclipses and X-ray heating effects, to estimate the system parameters, and we constrain the binary inclination to be i=73+-4 degrees. The model is successful in reproducing the light curves at different stages of the decay by requiring the accretion disc to become smaller and thinner by 30% as the system fades by 1.7 mags in the optical. From Aug 26 the system reaches quiescence with a mean magnitude of R=21.7+-0.1 and our data are consistent with the optical variability being dominated by the companions ellipsoidal modulation.

SDSS J080434.20+510349.2: Eclipsing WZ Sge-Type Dwarf Nova with Multiple Rebrightenings

We observed the 2006 superoutburst of SDSS J080434.20+510349.2 during its plateau phase, rebrightening phase, and post-superoutburst final decline. We found that this object is a grazing eclipsing system with a period of 0.0590048(2) d. Well-defined eclipses were only observed during the late stage of the superoutburst plateau and the depth decreased during the subsequent stages. We determined the superhump period during the superoutburst plateau to be 0.059539(11) d, giving a fractional superhump excess of 0.90(2)%. During the rebrightening and post-superoutburst phases, persisting superhumps have periods longer than those of superhumps during the plateau phase: 0.059632(6) d during the rebrightening phase and 0.05969(4) d during the final fading. This phenomenon is very well in line with the previously known long-period “late superhumps” in GW Lib, V455 And, and WZ Sge. The amplitudes of orbital humps between different states of rebrightenings suggest that these humps do not arise from the classical hot spot, but are more likely to be a result of projection effect in a high-inclination system. There was no clear evidence for an enhanced hot spot during the rebrightening phase. We also studied previously reported “mini-outbursts” in the quiescent state, and found evidence that superhumps were transiently excited during these mini-outbursts. The presence of grazing eclipses and distinct multiple rebrightenings in SDSS J080434.20+510349.2 would provide a unique opportunity for understanding the mechanism of rebrightenings in WZ Sge-type dwarf novae.

Discovery of a WZ Sge-Type Dwarf Nova, SDSS J102146.44+234926.3: Unprecedented Infrared Activity during a Rebrightening Phase

Several SU UMa-type dwarf novae and WZ Sge-type stars tend to exhibit rebrightenings after superoutbursts. The rebrightening phenomenon is problematic for the disk instability theory of dwarf novae, since it requires a large amount of remnant matter in the disk, even after superoutbursts. Here, we report on our optical and infrared observations during the first-ever outburst of a new dwarf nova, SDSS J102146.44 + 234926.3. During the outburst, we detected superhumps with a period of 0.056281 ˙ 0.000015 d, which is typical for superhump periods in WZ Sge stars. In conjunction with the appearance of a long-lived rebrightening, we concluded that the object is a new member of WZ Sge stars. Our observations, furthermore, revealed infrared behaviors for the first time in the rebrightening phase of WZ Sge stars. We discovered prominent infrared superhumps. We calculated the color temperature of the infrared superhump source to be 4600–6400 K. These temperatures are too low to be explained by a fully ionized disk appearing during dwarf-nova outbursts. We also found a Ks-band excess over the hot disk component. These unprecedented infrared activities provide evidence for the presence of mass reservoir at the outermost part of the accretion disk. We propose that a moderately high mass-accretion rate at this infrared active region leads to the long-lived rebrightening observed in SDSS J102146.44 + 234926.3.

Detection of Change in Supercycles in ER Ursae Majoris

We examined data from observations of ER UMa during a period of � 20 years available in the AAVSO, VSNET, AFOEV, NSVS, and VSOLJ databases together with published light curves. The obtained OC diagram revealed a systematic change of the supercycle (time interval between two successive superoutbursts) ranging from 43.6 to 59.2 d. The time-scale of this cycle variation is from 300 to � 1900 d. The number of normal outbursts within the supercycles also varied between 4 and 6, although no strong correlation between this number and the supercycle length was found. We suggest that the appearance of negative superhumps is responsible for the observed varia- tions in the number of normal outbursts. Our results generally confirm the expectations based on the thermal-tidal instability theory.

X-Ray Observations of VY Scl-Type Nova-Like Binaries in the High and Low State

Four VY Scl-type nova-like systems were observed in X-rays during both the low- and the high-optical states. We examined Chandra, ROSAT, Swift and Suzaku archival observations of BZ Cam, MV Lyr, TT Ari and V794 Aql. The X-ray flux of BZ Cam is higher during the low state, but there is no supersoft X-ray source (SSS) as hypothesized in previous articles. No SSS was detected in the low state of the any of the other systems, with the X-ray flux decreasing by a factor between 2 and 50. The best fit to the Swift X-ray spectra is obtained with a multicomponent model of plasma in collisional ionization equilibrium. The high-state high-resolution spectra of TT Ari taken with Chandra Advanced CCD Imaging Spectrometer (ACIS-S) and the Chandra High Energy Transmission Grating (HETG) shows a rich emission line spectrum, with prominent lines of Mg, Si, Ne and S. The complexity of this spectrum seems to have origin in more than one region, or more than one single physical mechanism. While several emission lines are consistent with a cooling flow in an accretion stream, there is at least an additional component. We discuss the origin of this component, which is probably arising in a wind from the system. We also examine the possibility that the VY Scl systems may be intermediate polars, and that while the boundary layer of the accretion disc emits only in the extreme ultraviolet, part of the X-ray flux may be due to magnetically driven accretion.

The Photometry of V1974 Cyg = N Cyg 1992

The results of photoelectric and CCD U BV RI observations and extensive CCD and TV monitoring of V1974 Cas during the decline phase are shown. Strong features in the ultraviolet light and colour curves are caused by the temporal appearance of forbidden neon emissions. The improved orbital period of 0.08125970(±5) day is determined. The “superhump” period of 0.08522 day is still observed, too.

Discovery of a bright microlensing event with planetary features towards the Taurus region: a super-Earth planet

The transient event labeled as TCP J05074264+2447555 recently discovered towards the Taurus region was quickly recognized to be an ongoing microlensing event on a source located at distance of only

Superoutburst of a New Sub-Period-Minimum Dwarf Nova CSS130418 in Hercules

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