ABSTRACT
We present an analysis of multi-epoch spectroscopic and photometric observations of the WZ Sge-type dwarf nova BW Scl, a period-bouncer candidate. We detected multiple irradiation-induced ...emission lines from the donor star allowing the radial velocity variations to be measured with high accuracy. Also, using the absorption lines Mg ii 4481 Å and Ca ii K originated in the photosphere of the accreting white dwarf (WD), we measured the radial velocity semi-amplitude of the WD and its gravitational redshift. We find that the WD has a mass of 0.85 ± 0.04 M⊙, while the donor is a low-mass object with a mass of 0.051 ± 0.006 M⊙, well below the hydrogen-burning limit. Using NIR data, we put an upper limit on the effective temperature of the donor to be ≲1600 K, corresponding to a brown dwarf of T spectral type. The optically thin accretion disc in BW Scl has a very low luminosity ≲4 × 1030 erg s−1 which corresponds to a very low-mass accretion rate of ≲7 × 10−13 M⊙ yr−1. The outer parts of the disc have a low density allowing the stream to flow down to the inner disc regions. The brightest part of the hotspot is located close to the circularization radius of the disc. The hotspot is optically thick and has a complex elongated structure. Based on the measured system parameters, we discuss the evolutionary status of the system.
The black hole (BH) candidate SWIFT J1753.5−0127 has remained active since the onset of its 2005 outburst. Emission lines in the optical spectrum were observed at the very beginning of the outburst, ...but since then the spectrum has been featureless making a precise BH mass estimation impossible. Here we present results from our optical and ultraviolet (UV) observations of SWIFT J1753.5−0127 taken in 2012–2013. Our new observations show extremely broad, double-peaked emission lines in the optical and UV spectra. The optical data also show narrow absorption and emission features with nearly synchronous and significant Doppler motions. A radial velocity study of these lines which we associate with the secondary star, yields a semi-amplitude of K
2 = 382 km s−1. A time series analysis of the spectral and photometric data revealed a possible orbital periodicity of 2.85 h, significantly shorter than the reported 3.2 h periodic signal by Zurita et al. The observed variability properties argue against a low orbital inclination angle and we present several observational arguments in favour of the BH interpretation. However, the measured radial velocity semi-amplitude of the donor star and the short orbital period imply that SWIFT J1753.5−0127 has one of the lowest measured mass function for a BH in a low-mass X-ray binary. We show that the compact object mass in excess of 5 M⊙ is highly improbable. Thus, SWIFT J1753.5−0127 is a BH binary that has one of the shortest orbital period and hosts probably one of the smallest stellar mass BH found to date.
Abstract
We report on our photometric observations of the 2016 superoutburst of ASASSN-16eg. This object showed a WZ Sge-type superoutburst with prominent early superhumps with a period of ...0.075478(8) d and a post-superoutburst rebrightening. During the superoutburst plateau, it showed ordinary superhumps with a period of 0.077880(3) d and a period derivative of 10.6(1.1) × 10−5 in stage B. The orbital period (Porb), which is almost identical with the period of the early superhumps, is exceptionally long for a WZ Sge-type dwarf nova. The mass ratio (q = M2/M1) estimated from the period of developing (stage A) superhumps is 0.166(2), which is also too large for a WZ Sge-type dwarf nova. This suggests that the 2 : 1 resonance can be reached in such high-q systems, contrary to our expectation. Such conditions are considered to be achieved if the mass-transfer rate is much lower than those in typical SU UMa-type dwarf novae that have comparable orbital periods to ASASSN-16eg, and a resultant accumulation of a large amount of matter on the disk is realized at the onset of an outburst. We examined other candidates for long-period WZ Sge-type dwarf novae for their supercycles, which are considered to reflect the mass-transfer rate, and found that V1251 Cyg and RZ Leo have longer supercycles than those of other WZ Sge-type dwarf novae. This result indicates that these long-period objects including ASASSN-16eg have a low mass-transfer rate in comparison to other WZ Sge-type dwarf novae.
Cataclysmic variables (CVs) are close binary systems in which the primary, the more massive star, is a white dwarf. CVs usually exhibit a number of periodicities, most of which are now understood. ...However, recently a new phenomenon was discovered that does not fit the standard picture. Two objects have been found to show periods that are much longer than orbital and have no relation to it, either in light curves or in radial velocity variations measured from spectroscopy. Here we show that the precession of a fast-rotating magnetically accreting white dwarf can successfully explain these phenomena. The theory of compact objects predicts certain relations between the spin and precession periods, and our finding provides a good test for the theory and establishes a qualitative model to be explored both theoretically and observationally. Detection of precession can become a powerful tool in searching for the internal properties of compact stars, which would be otherwise inaccessible to us.
Abstract
We report on a superoutburst of a WZ Sge-type dwarf nova (DN), ASASSN-15po. The light curve showed the main superoutburst and multiple rebrightenings. In this outburst, we observed early ...superhumps and growing (stage A) superhumps with periods of 0.050454(2) and 0.051809(13) d, respectively. We estimated that the mass ratio of secondary to primary (q) is 0.0699(8) by using P
orb and a superhump period P
SH of stage A. ASASSN-15po P
orb ∼ 72.6 min is the first DN with an orbital period between 67–76 min. Although the theoretical predicted period minimum P
min of hydrogen-rich cataclysmic variables (CVs) is about 65–70 min, the observational cut-off of the orbital period distribution at 80 min implies that the period minimum is about 82 min, and the value is widely accepted. We suggest the following four possibilities: the object is (1) a theoretical period minimum object, (2) a binary with a evolved secondary, (3) a binary with a metal-poor (Popullation II) seconday, or (4) a binary which was born with a brown-dwarf donor below the period minimum.
Continuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for 102 SU UMa-type dwarf novae, observed mainly during the 2014–2015 season, and ...characterized these objects. Our project has greatly improved the statistics of the distribution of orbital periods, which is a good approximation of the distribution of cataclysmic variables at the terminal evolutionary stage, and has confirmed the presence of a period minimum at a period of 0.053 d and a period spike just above this period. The number density monotonically decreased toward the longer period and there was no strong indication of a period gap. We detected possible negative superhumps in Z Cha. It is possible that normal outbursts are also suppressed by the presence of a disk tilt in this system. There was no indication of enhanced orbital humps just preceding the superoutburst, and this result favors the thermal–tidal disk instability as the origin of superoutbursts. We detected superhumps in three AM CVn-type dwarf novae. Our observations and recent other detections suggest that 8% of objects showing dwarf nova-type outbursts are AM CVn-type objects. AM CVn-type objects and EI Psc-type objects may be more abundant than previously recognized. OT J213806, a WZ Sge-type object, exhibited remarkably different features between the 2010 and 2014 superoutbursts. Although the 2014 superoutburst was much fainter, the plateau phase was shorter than the 2010 one, and the course of the rebrightening phase was similar. This object indicates that the O − C diagrams of superhumps can indeed be variable, at least in WZ Sge-type objects. Four deeply eclipsing SU UMa-type dwarf novae (ASASSN-13cx, ASASSN-14ag, ASASSN-15bu, and NSV 4618) were identified. We studied long-term trends in supercycles in MM Hya and CY UMa and found systematic variations of supercycles of ∼20%.
Abstract Continuing the project undertaken by Kato et al. (2009), we collected times of superhump maxima for 56 SU UMa-type dwarf novae mainly observed during the 2013–2014 season and characterized ...these objects. We detected negative superhumps in VW Hyi and indicated that the low number of normal outbursts in some supercycles can be interpreted as a result of disk tilt. This finding, combined with the Kepler observation of V1504 Cyg and V344 Lyr, suggests that disk tilt is responsible for modulating the outburst pattern in SU UMa-type dwarf novae. We also studied the deeply eclipsing WZ Sge-type dwarf nova MASTER OT J005740.99+443101.5 and found evidence of a sharp eclipse during the phase of early superhumps. The profile can be reproduced by a combination of the eclipse of the axisymmetric disk and the uneclipsed light source of early superhumps. This finding shows the lack of evidence for a greatly enhanced hot spot during the early stage of WZ Sge-type outburst. We detected growing (stage A) superhumps in MN Dra and give a suggestion that some of SU UMa-type dwarf novae situated near the critical condition of tidal instability may show long-lasting stage A superhumps. The large negative period derivatives reported in such systems can be understood as a result of the combination of stage A and B superhumps. Two WZ Sge-type dwarf novae, AL Com and ASASSN-13ck, showed a long-lasting (plateau-type) rebrightening. In the early phase of their rebrightenings, both objects showed a precursor-like outburst, suggesting that the long-lasting rebrightening is triggered by a precursor outburst.
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.
We present an analysis of multi-epoch spectroscopic and photometric observations of the WZ Sge-type dwarf nova BW Scl, a period-bouncer candidate. We detected multiple irradiation-induced emission ...lines from the donor star allowing the radial velocity variations to be measured with high accuracy. Also, using the absorption lines Mgii 4481 and Caii K originated in the photosphere of the accreting white dwarf (WD), we measured the radial velocity semi-amplitude of the WD and its gravitational redshift. We find that the WD has a mass of 0.85\(\pm\)0.04 M\(_\odot\), while the donor is a low-mass object with a mass of 0.051\(\pm\)0.006 M\(_\odot\), well below the hydrogen-burning limit. Using NIR data, we put an upper limit on the effective temperature of the donor to be \(\lesssim\)1600 K, corresponding to a brown dwarf of T spectral type. The optically thin accretion disc in BW Scl has a very low luminosity \(\lesssim\)4 \(\times 10^{30}\) erg s\(^{-1}\) which corresponds to a very low mass accretion rate of \(\lesssim\)7 \(\times 10^{-13}\) M\(_\odot\) year\(^{-1}\). The outer parts of the disc have a low density allowing the stream to flow down to the inner disc regions. The brightest part of the hotspot is located close to the circularization radius of the disc. The hotspot is optically thick and has a complex, elongated structure. Based on the measured system parameters, we discuss the evolutionary status of the system.
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