Ryoko Ishioka

Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae. II: The Second Year (2009-2010)

As an extension of the project in Kato et al. (2009, arXiv:0905.1757), we collected times of superhump maxima for 61 SU UMa-type dwarf novae mainly observed during the 2009-2010 season. The newly obtained data confirmed the basic findings reported in Kato et al. (2009): the presence of stages A-C, as well as the predominance of positive period derivatives during stage B in systems with superhump periods shorter than 0.07 d. There was a systematic difference in period derivatives for systems with superhump periods longer than 0.075 d between this study and Kato et al. (2009). We suggest that this difference is possibly caused by the relative lack of frequently outbursting SU UMa-type dwarf novae in this period regime in the present study. We recorded a strong beat phenomenon during the 2009 superoutburst of IY UMa. The close correlation between the beat period and superhump period suggests that the changing angular velocity of the apsidal motion of the elliptical disk is responsible for the variation of superhump periods. We also described three new WZ Sge-type objects with established early superhumps and one with likely early superhumps. We also suggest that two systems, VX For and EL UMa, are WZ Sge-type dwarf novae with multiple rebrightenings. The O-C variation in OT J213806.6+261957 suggests that the frequent absence of rebrightenings in very short-Porb objects can be a result of sustained superoutburst plateau at the epoch when usual SU UMa-type dwarf novae return to quiescence preceding a rebrightening. We also present a formulation for a variety of Bayesian extension to traditional period analyses.

Repetitive patterns in rapid optical variations in the nearby black-hole binary V404 Cygni.

How black holes accrete surrounding matter is a fundamental yet unsolved question in astrophysics. It is generally believed that matter is absorbed into black holes via accretion disks, the state of which depends primarily on the mass-accretion rate. When this rate approaches the critical rate (the Eddington limit), thermal instability is supposed to occur in the inner disk, causing repetitive patterns of large-amplitude X-ray variability (oscillations) on timescales of minutes to hours. In fact, such oscillations have been observed only in sources with a high mass-accretion rate, such as GRS 1915+105 (refs 2, 3). These large-amplitude, relatively slow timescale, phenomena are thought to have physical origins distinct from those of X-ray or optical variations with small amplitudes and fast timescales (less than about 10 seconds) often observed in other black-hole binaries—for example, XTE J1118+480 (ref. 4) and GX 339−4 (ref. 5). Here we report an extensive multi-colour optical photometric data set of V404 Cygni, an X-ray transient source containing a black hole of nine solar masses (and a companion star) at a distance of 2.4 kiloparsecs (ref. 8). Our data show that optical oscillations on timescales of 100 seconds to 2.5 hours can occur at mass-accretion rates more than ten times lower than previously thought. This suggests that the accretion rate is not the critical parameter for inducing inner-disk instabilities. Instead, we propose that a long orbital period is a key condition for these large-amplitude oscillations, because the outer part of the large disk in binaries with long orbital periods will have surface densities too low to maintain sustained mass accretion to the inner part of the disk. The lack of sustained accretion—not the actual rate—would then be the critical factor causing large-amplitude oscillations in long-period systems.

The 2003/2004 Superoutburst of SDSS J013701.06-091234.9

We report on time-resolved photometry of the superoutburst of an SU UMa-type dwarf nova, SDSS J013701.06−091234.9, in 2003 December–2004 January. The obtained light curves definitely show superhumps with a period of 0.056686(12)d, which is one of the shortest superhump periods among those of SU UMa-type dwarf novae ever observed. Considering quiescent photometric studies, we estimated the fractional superhump excess to be 0.024. Spectroscopic observations by Szkody et al. (2003, AJ, 126, 1499) provided evidence for TiO bands despite the short orbital period, implying that the system has a luminous secondary star. We have drawn a color–color diagram of SU UMa-type dwarf novae in quiescence using 2MASS archives, revealing that the location of this star in the color-color diagram deviates from the general trend. The distance to the system was roughly estimated to be 300 ±80pc, using the empirical period–absolute magnitude relation and based on the proper motion.

Long-term monitoring of the short period SU UMa-type dwarf nova, V844 Herculis

We report on the time-resolved CCD photometry of four outbursts of a short-period SU UMa-type dwarf nova, V844 Herculis. We successfully determined the mean superhump periods to be 0.05584(64) d, and 0.055883(3) d for a 2002 May superoutburst, and a 2006 April–May superoutburst, respectively. During the 2002 October observations, we confirmed that the outburst was a normal outburst, which was the first recorded normal outburst in V844 Her. We also examined superhump period changes during the 2002 May and 2006 April–May superoutbursts, both of which showed an increasing superhump period over the course of the plateau stage. In order to examine the long-term behavior of V844 Her, we analyzed archival data over the past ten years since the discovery of this binary. Although photometry is not satisfactory for some superoutbursts, we found that V844 Her showed no precursors or rebrightenings. Based on the long-term light curve, we further confirmed that V844 Her has shown almost no normal outbursts despite the fact that the supercycle of the system is estimated to be about 300 d. In order to explain the long-term light curves of V844 Her, evaporation in the accretion disk may play a role in the avoidance of several normal outbursts, which does not contradict with the relatively a large X-ray luminosity of V844 Her.

The 2006 November outburst of EG Aquarii: the SU UMa nature revealed

AbstractWe report time-resolved CCD photometry of the cataclysmic variable EG Aquarii during the 2006November outburst During the outburst, superhumps were unambiguously detected with a mean period of0.078828(6) days, firstly classifying the object as an SU UMa-type dwarf nova. It also turned out that theoutburst contained a precursor. At the end of the precursor, immature profiles of humps were observed. Bya phase analysisofthese humps, we interpreted the featuresas superhumps. This is the secondexample thatthe superhumps were shown during a precursor. Near the maximum stage of the outburst, we discoveredan abrupt shift of the superhump period by ∼ 0.002 days. After the supermaximum, the superhump perioddecreased at the rate of P/P˙ =−8.2×10 −5 , which is typical for SU UMa-type dwarf novae. Although theoutburst light curve was characteristic of SU UMa-type dwarf novae, long-term monitoring of the variableshows no outbursts over the past decade. We note on the basic properties of long period and inactive SUUMa-type dwarf novae.Key words: accretion, accretion disks — stars: dwarf novae — stars: individual (EG Aquarii) —stars: novae, cataclysmic variables — stars: oscillations

Photometric Studies of New Southern SU UMa-Type Dwarf Novae, FL Trianguli Australis and CTCV J0549–4921

We report on the time-resolved optical CCD photometry of newly discovered SU UMa-type dwarf novae, FL TrA and CTCV J0549� 4921. During a 2006 August outburst, we detected superhumps with a period of 0.59897(11) days for FL TrA, clarifying the SU UMa nature of the system. On the first night of our observations of FL TrA, the object showed no superhumps. This implies that it takes a few days for the full development of superhumps. The superhump period variation diagram of FL TrA was simila rt o thatobserved in some WZ Sge stars and short-period SU UMa-type stars. This indicates that the system is closely related to WZ Sge stars and SU UMa stars having short orbital periods. For CTCV J0549� 4921, the candidates of the mean superhump period are 0.083249(10) days and 0.084257(8) days, respectively. Due to a lack of observations, we cannot determine the true superhump period, but the latter period is favorable. Using the ASAS-3 archive ,i t turned out that the system showed only four outbursts ove rt he past 6 years. The outburst amplitude of CTCV J0549� 4921 was relatively small, with about 4.5 mag. One possibility is that mass evaporation may play a role during quiescence.

RZ Leonis Minoris bridging between ER Ursae Majoris-type dwarf nova and nova-like system

We observed RZ LMi, which is renowned for the extremely (~19d) short supercycle and is a member of a small, unusual class of cataclysmic variables called ER UMa-type dwarf novae, in 2013 and 2016. In 2016, the supercycles of this object substantially lengthened in comparison to the previous measurements to 35, 32, 60d for three consecutive superoutbursts. We consider that the object virtually experienced a transition to the novalike state (permanent superhumper). This observed behavior extremely well reproduced the prediction of the thermal-tidal instability model. We detected a precursor in the 2016 superoutburst and detected growing (stage A) superhumps with a mean period of 0.0602(1)d in 2016 and in 2013. Combined with the period of superhumps immediately after the superoutburst, the mass ratio is not as small as in WZ Sge-type dwarf novae, having orbital periods similar to RZ LMi. By using least absolute shrinkage and selection operator (Lasso) two-dimensional power spectra, we detected possible negative superhumps with a period of 0.05710(1)d. We estimated the orbital period of 0.05792d, which suggests a mass ratio of 0.105(5). This relatively large mass ratio is even above ordinary SU UMa-type dwarf novae, and it is also possible that the exceptionally high mass-transfer rate in RZ LMi may be a result of a stripped core evolved secondary which are evolving toward an AM CVn-type object.

On the nature of long-period dwarf novae with rare and low-amplitude outbursts

There are several peculiar long-period dwarf-nova like objects, which show rare, low-amplitude outbursts with highly ionized emission lines. 1SWASP J162117

The bridge between ER Ursae Majoris-type dwarf nova and nova-like system

+

Discovery of Repetitive Optical Variation Patterns from the Accretion Disk During the 2015 Outbursts of the Black Hole X-Ray Binary V404 Cyg

441254, BD Pav, and V364 Lib belong to this kind of objects. Some researchers even doubt whether 1SWASP J1621 and V364 Lib have the same nature as normal dwarf novae. We studied the peculiar outbursts in these three objects via our optical photometry and spectroscopy, and performed numerical modeling of their orbital variations to investigate their properties. We found that their outbursts lasted for a long interval (a few tens of days), and that slow rises in brightness were commonly observed during the early stage of their outbursts. Our analyses and numerical modeling suggest that 1SWASP J1621 has a very high inclination, close to 90 deg, plus a faint hot spot. Although BD Pav seems to have a slightly lower inclination (