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.
Abstract
We report on multicolor photometry of the short-period dwarf nova CSS130418:174033+414756 during the 2013 superoutburst. The system showed an unusually short superhump period of 0.046346(67) ...d during stage A, which is one of the shortest periods among dwarf novae below the period minimum. We found that the bluest peaks in g΄ − Ic color variations tended to coincide with the brightness minima of the superhump modulations. We also studied nightly-averaged superhump amplitudes in the g΄, Rc, and Ic bands and found that they have less dependence on wavelength. These properties are likely to be in common with dwarf novae exhibiting superhumps. We successfully obtained g΄ − Rc and Rc − Ic colors during the temporal dip. The color indices were significantly bluer compared with other dips of WZ Sge-type dwarf novae. By using the period of the growing superhumps, we estimated the mass ratio to be q = 0.077(5), which is much larger than the previous study.
We report on our time-resolved CCD photometry during the 2005 June superoutburst of a WZ Sge-type dwarf nova candidate, ASAS 160048
$-$
4846.2. The ordinary superhumps of the object underwent a ...complex evolution during the superoutburst. The superhump amplitude experienced a regrowth and had two peaks. The superhump period decreased when the superhump amplitude reached the first maximum, subsequently increased gradually until the second maximum of the amplitude, and finally decreased again. Investigating other SU UMa-type dwarf novae, which show an increase in superhump period, we found the same trend of the superhump evolution in superoutbursts of them. We speculate that the superhump regrowth in the amplitude has a direct relation to the increase in superhump period, and all SU UMa-type dwarf novae with a superhump regrowth follow the same evolution of the ordinary superhumps as that of ASAS 160048
$-$
4846.2.
Abstract
We present our spectroscopic observations of V455 Andromedae during the 2007 superoutburst. Our observations cover this superoutburst from around the optical peak of the outburst to the ...post-superoutburst stage. During the early superhump phase, the emission lines of the Balmer series, He i, He ii, Bowen blend, and C iv/N iv blend were detected. The He ii 4686 line exhibited a double-peaked emission profile, where Balmer emission lines were single-peaked, which is unexpected from its high inclination. In the ordinary superhump phase, the Balmer series transitioned to double-peaked emission profiles, and high-ionization lines were significantly weakened. These transitions of the line profiles should be related to the structural transformation of the accretion disk, as expected between the early and ordinary superhump transition in the thermal–tidal instability model. The Doppler map of Hα during the early superhump phase exhibits a compact blob centered at the primary white dwarf. In analogy to SW Sex-type cataclysmic variables, this feature could emerge from the disk wind and/or the mass accretion column on to the magnetized white dwarf. The Doppler map of He ii 4686 Å is dominated by the ring-like structure and imposed two flaring regions with the velocity of ∼300 km s−1, which is too slow for a Keplerian accretion disk. The phase of the flaring regions was coincident with the inner spiral arm structure identified during the early superhump phase. Our disk wind model with the enhanced emission from the wind component launched from the inner arm structure successfully reproduced the observed properties of He ii 4686 Å. Therefore, V455 And is the first case in dwarf nova outbursts where the presence of the disk wind is inferred from an optical spectrum.
Abstract
Continuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for 127 SU UMa-type dwarf novae observed mainly during the 2016–2017 season and ...characterized these objects. We provide updated statistics of the relation between the orbital period and the variation of superhumps, the relation between period variations and the rebrightening type in WZ Sge-type objects. We obtained the period minimum of 0.05290(2) d and confirmed the presence of the period gap above the orbital period ∼0.09 d. We note that four objects (NY Her, 1RXS J161659.5+620014, CRTS J033349.8−282244, and SDSS J153015.04+094946.3) have supercycles shorter than 100 d but show infrequent normal outbursts. We consider that these objects are similar to V503 Cyg, whose normal outbursts are likely suppressed by a disk tilt. These four objects are excellent candidates to search for negative superhumps. DDE 48 appears to be a member of ER UMa-type dwarf novae. We identified a new eclipsing SU UMa-type object, MASTER OT J220559.40−341434.9. We observed 21 WZ Sge-type dwarf novae during this interval and report 18 of them in this paper. Among them, ASASSN-16js is a good candidate for a period bouncer. ASASSN-16ia showed a precursor outburst for the first time in a WZ Sge-type superoutburst. ASASSN-16kg, CRTS J000130.5+050624, and SDSS J113551.09+532246.2 are located in the period gap. We have newly obtained 15 orbital periods, including periods from early superhumps.
Continuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for 128 SU UMa-type dwarf novae observed mainly during the 2015–2016 season and ...characterized these objects. The data have improved the distribution of orbital periods, the relation between the orbital period and the variation of superhumps, and the relation between period variations and the rebrightening type in WZ Sge-type objects. Coupled with new measurements of mass ratios using growing stages of superhumps, we now have a clearer and statistically greatly improved evolutionary path near the terminal stage of evolution of cataclysmic variables. Three objects (V452 Cas, KK Tel, and ASASSN-15cl) appear to have slowly growing superhumps, which is proposed to reflect the slow growth of the 3 : 1 resonance near the stability border. ASASSN-15sl, ASASSN-15ux, SDSS J074859.55+312512.6, and CRTS J200331.3−284941 are newly identified eclipsing SU UMa-type (or WZ Sge-type) dwarf novae. ASASSN-15cy has a short (∼0.050 d) superhump period and appears to belong to EI Psc-type objects with compact secondaries having an evolved core. ASASSN-15gn, ASASSN-15hn, ASASSN-15kh, and ASASSN-16bu are candidate period bouncers with superhump periods longer than 0.06 d. We have newly obtained superhump periods for 79 objects and 13 orbital periods, including periods from early superhumps. In order that future observations will be more astrophysically beneficial and rewarding to observers, we propose guidelines on how to organize observations of various superoutbursts.
Abstract
ASASSN-14dx showed an extraordinary outburst whose features are a small outburst amplitude (∼2.3 mag) and long duration (>4 yr). Because we found a long observational gap of 123 d before the ...outburst detection, we propose that the main outburst plateau was missed and that this outburst is just a “fading tail” often seen after the WZ Sge-type super-outbursts. In order to distinguish between WZ Sge and SU UMa-type dwarf novae (DNe), we investigated Gaia Data Release 2 (DR2) statistically. We applied a logistic regression model and succeeded in classifying by using absolute Gaia magnitudes MG and Gaia colors GBP–GRP. Our new classifier also suggests that ASASSN-14dx is the best candidate of a WZ Sge-type DN. We estimated distances from the Earth of known WZ Sge stars by using Gaia DR2 parallaxes. The result indicates that ASASSN-14dx is the third-nearest WZ Sge star (next to WZ Sge and V455 And), and hence the object can show the third-brightest WZ Sge-type super-outburst whose maximum is V = 8–9 mag.
Continued from Kato et al. (2009, PASJ, 61, S395), we collected the times of superhump maxima for 68 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. (ibid.): the presence of stages A–C and 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 the period derivatives for the systems with superhump periods longer than 0.075 d between this study and Kato et al. (ibid.). We suggest that this difference was possibly caused by a 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. A close correlation between the beat period and the superhump period suggests that the changing angular velocity of the apsidal motion of the elliptical disk is responsible for the variation of the superhump periods. We also described three new WZ Sge-type objects with established early superhumps and one with likely early superhumps. We 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-
$P_{\rm orb}$
objects can be the result of a 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 extensions to traditional period analyses.
Abstract We observed RZ LMi, which is renowned for its extremely short (∼19 d) 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, and 60 d for three consecutive superoutbursts. We consider that the object virtually experienced a transition to the nova-like state (permanent superhumper). This observed behavior reproduced the prediction of the thermal-tidal instability model extremely well. 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 an orbital period of 0.05792 d, which suggests a mass ratio of 0.105(5). This relatively large mass ratio is even above that of 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 secondary with an evolved core in a system evolving toward an AM CVn-type object.
Abstract
We report on time-resolved photometry of the 2015 February–March superoutburst of QZ Virginis. The superoutburst consisted of a separated precursor, main superoutburst, and rebrightening. We ...detected superhumps with a period of 0.061181(42) d between the precursor and main superoutburst. Based on analyses of period changes and amplitudes of superhumps, the observed superhumps were identified as growing superhumps (stage A superhumps). The duration of the stage A superhumps was about 5 d, unusually long for SU UMa-type dwarf novae. Using the obtained stage A superhump period, we estimated the mass ratio of QZ Vir to be 0.108(3). This value suggests that QZ Vir is an SU UMa-type dwarf nova evolving toward the period minimum. Based on the present and previous observations regarding long-lasting stage A superhumps, the time scale for stage A superhumps is likely to be determined by the mass ratio of the system and the temperature of the accretion disk.
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