ABSTRACT The detection of periodicity in the broadband non-thermal emission of blazars has so far been proven to be elusive. However, there are a number of scenarios that could lead to quasi-periodic ...variations in blazar light curves. For example, an orbital or thermal/viscous period of accreting matter around central supermassive black holes could, in principle, be imprinted in the multi-wavelength emission of small-scale blazar jets, carrying such crucial information about plasma conditions within the jet launching regions. In this paper, we present the results of our time series analysis of the ∼9.2 yr long, and exceptionally well-sampled, optical light curve of the BL Lac object OJ 287. The study primarily used the data from our own observations performed at the Mt. Suhora and Kraków Observatories in Poland, and at the Athens Observatory in Greece. Additionally, SMARTS observations were used to fill some of the gaps in the data. The Lomb-Scargle periodogram and the weighted wavelet Z-transform methods were employed to search for possible quasi-periodic oscillations in the resulting optical light curve of the source. Both methods consistently yielded a possible quasi-periodic signal around the periods of ∼400 and ∼800 days, the former with a significance (over the underlying colored noise) of . A number of likely explanations for this are discussed, with preference given to a modulation of the jet production efficiency by highly magnetized accretion disks. This supports previous findings and the interpretation reported recently in the literature for OJ 287 and other blazar sources.
Various scenarios of contact binary evolution have been proposed in the past, giving hints of (sometimes contradictory) evolutionary sequences connecting A- and W-type systems. As the components of ...close detached binaries approach each other and contact binaries are formed, following evolutionary paths transforms them into systems of two categories: A-type and W-type. The systems evolve in a similar way but under slightly different circumstances. The mass/energy transfer rate is different, leading to quite different evolutionary results. An alternative scenario of evolution in contact is presented and discussed, based on the observational data of over one hundred low-temperature contact binaries. It results from the observed correlations among contact binary physical and orbital parameters. Theoretical tracks are computed assuming angular momentum loss from a system via stellar wind, accompanied by mass transfer from an advanced evolutionary secondary to the main-sequence primary. A good agreement is seen between the tracks and the observed graphs. Independently of details of the evolution in contact and a relation between A- and W-type systems, the ultimate fate of contact binaries involves the coalescence of both components into a single fast rotating star.
We present a study on low-mass contact binaries (LMCB) with orbital periods shorter than 0.3 days and total mass lower than about 1.4 M sub(sun). We show that such systems have a long pre-contact ...phase, which lasts for 8-9 Gyrs, while the contact phase takes only about 0.8 Gyr, which is rather a short fraction of the total life. With low mass transfer rate during contact, moderate mass ratios prevail in LMCBs since they do not have enough time to reach extreme mass ratios often observed in higher mass binaries. During the whole evolution both components of LMCBs remain within the MS band. The evolution of cool contact binaries toward merging is controlled by the interplay between the evolutionary expansion of the less massive component resulting in the mass transfer to the more massive component and the angular momentum loss from the system by the magnetized wind. In LMCB the angular momentum loss prevails. As a result, the orbital period systematically decreases until the binary overflows the outer critical Roche surface and the components merge into a single fast rotating star of a solar type surrounded by a remnant disk carrying excess angular momentum. The disk can be a place of planet formation with the age substantially lower than the age of a host star. The calculated theoretical tracks show good agreement with the physical properties of LMCB from the available observations. Estimates of the frequency of occurrence of LMCB and the merger formation rate indicate that about 40 LMCBs and about 100 low mass merger products is expected to exist within 100 pc from the Sun.
ABSTRACT
Binary and multiple stellar systems are numerous in our solar neighbourhood with 80 per cent of the solar-type stars being members of systems with high order multiplicity. The ...Contact Binaries Towards Merging (CoBiToM) Project is a programme that focuses on contact binaries and multiple stellar systems, as a key for understanding stellar nature. The goal is to investigate stellar coalescence and merging processes, as the final state of stellar evolution of low-mass contact binary systems. Obtaining observational data of approximately 100 eclipsing binaries and multiple systems and more than 400 archival systems, the programme aspires to give insights for their physical and orbital parameters and their temporal variations, e.g. the orbital period modulation, spot activity etc. Gravitational phenomena in multiple-star environments will be linked with stellar evolution. A comprehensive analysis will be conducted, in order to investigate the possibility of contact binaries to host planets, as well as the link between inflated hot Jupiters and stellar mergers. The innovation of CoBiToM Project is based on a multimethod approach and a detailed investigation, that will shed light for the first time on the origin of stellar mergers and rapidly rotating stars. In this work, we describe the scientific rationale, the observing facilities to be used and the methods that will be followed to achieve the goals of CoBiToM Project and we present the first results as an example of the current research on evolution of contact binary systems.
ABSTRACT OJ 287 is a quasi-periodic quasar with roughly 12 year optical cycles. It displays prominent outbursts that are predictable in a binary black hole model. The model predicted a major optical ...outburst in 2015 December. We found that the outburst did occur within the expected time range, peaking on 2015 December 5 at magnitude 12.9 in the optical R-band. Based on Swift/XRT satellite measurements and optical polarization data, we find that it included a major thermal component. Its timing provides an accurate estimate for the spin of the primary black hole, . The present outburst also confirms the established general relativistic properties of the system such as the loss of orbital energy to gravitational radiation at the 2% accuracy level, and it opens up the possibility of testing the black hole no-hair theorem with 10% accuracy during the present decade.
The eclipsing binary system DV Psc has been known to be magnetically active for almost two decades. However, there has been no evidence of a magnetic cycle on this system until recently. This study ...focuses on the long-term photometric monitoring of DV Psc between 2005-2017. A total of 50 individual light curves in BVRI optical bands were collected, in order to investigate its magnetic activity and cycle, as well as determine its orbital and physical properties. The combined photometric and spectroscopic observations of this study resulted in a unified model for the system, which describes accurately the light curves throughout the years, as a result of the variable spot activity. A total of 105 new times of minimum light are calculated through the entire time span of observations and they are combined with the 203 bibliographic ones since 1997, increasing significantly the existing sample. This resulted in an accurate ephemeris and an updated O-C diagram for a total span of 20 yr (1997-2017). It is found that the system exhibits intense magnetic activity, which is shown through the strong asymmetries on the light curves (O'Connell effect) and the periodic variation of the O-C diagram. The existence of a third body, orbiting the eclipsing binary with a period of Porb=9.79±0.60 yr in an eccentric orbit with eccentricity e=0.83±0.24, as well as a magnetic cycle of 14.74±0.84 yr are most likely connected with this variability. The absolute physical parameters of the system are calculated for a new and unified model, which explains the light curves through the entire observing season. The evolution state of DV Psc is studied through the mass-radius and temperature-luminosity (HR) diagrams, as well as the location of the two components with respect to the ZAMS and TAMS region.
ABSTRACT
Ultra-short orbital period contact binaries (Porb < 0.26 d) host some of the smallest and least massive stars. These systems are faint and rare, and it is believed that they have reached a ...contact configuration after several Gyrs of evolution via angular momentum loss, mass transfer, and mass loss through stellar wind processes. This study is conducted in the frame of the Contact Binaries Towards Merging (CoBiToM) Project and presents the results from light curve and orbital analysis of 30 ultra-short orbital period contact binaries, with the aim to investigate the possibility of them being red nova progenitors, eventually producing merger events. Approximately half of the systems exhibit orbital period modulations, as a result of mass transfer or mass loss processes. Although they are in contact, their fill-out factor is low (less than 30 per cent), while their mass ratio is larger than the one in longer period contact binaries. This study investigates the orbital stability of these systems and examines their physical and orbital parameters in comparison to those of the entire sample of known and well-studied contact binaries, based on combined spectroscopic and photometric analysis. It is found that ultra-short orbital period contact binaries have very stable orbits, while very often additional components are gravitationally bound in wide orbits around the central binary system. We confirmed that the evolution of such systems is very slow, which explains why the components of ultra-short orbital period systems are still Main Sequence stars after several Gyrs of evolution.
ABSTRACT
This paper presents the results of a combined spectroscopic and photometric study of 20 contact binary systems: HV Aqr, OO Aql, FI Boo, TX Cnc, OT Cnc, EE Cet, RW Com, KR Com, V401 Cyg, ...V345 Gem, AK Her, V502 Oph, V566 Oph, V2612 Oph, V1363 Ori, V351 Peg, V357 Peg, Y Sex, V1123 Tau, and W UMa, which was conducted in the frame of the W UMa Project. Together with 51 already covered by the project and an additional 67 in the existing literature, these systems bring the total number of contact binaries with known combined spectroscopic and photometric solutions to 138. It was found that mass, radius, and luminosity of the components follow certain relations along the MS and new empirical power relations are extracted. We found that 30 per cent of the systems in the current sample show extreme values in their parameters, expressed in their mass ratio or fill-out factor. This study shows that, among the contact binary systems studied, some have an extremely low mass ratio (q < 0.1) or an ultrashort orbital period (Porb < 0.25 d), which are expected to show evidence of mass transfer progress. The evolutionary status of these components is discussed with the aid of correlation diagrams and their physical and orbital parameters compared to those in the entire sample of known contact binaries. The existence of very short orbital periods confirms the very slow nature of the merging process, which seems to explain why their components still exist as MS stars in contact configurations even after several Gyr of evolution.
ABSTRACT
We present a dynamical study of the intermediate polar and dwarf nova cataclysmic variable GK Persei (Nova Persei 1901) based on a multisite optical spectroscopy and R-band photometry ...campaign. The radial velocity curve of the evolved donor star has a semi-amplitude $K_2=126.4 \pm 0.9 \, \mathrm{km}\, \mathrm{s}^{-1}$ and an orbital period $P=1.996872 \pm 0.000009 \, \mathrm{d}$. We refine the projected rotational velocity of the donor star to $v_\mathrm{rot} \sin i = 52 \pm 2 \, \mathrm{km}\, \mathrm{s}^{-1}$ that, together with K2, provides a donor star to white dwarf mass ratio q = M2/M1 = 0.38 ± 0.03. We also determine the orbital inclination of the system by modelling the phase-folded ellipsoidal light curve and obtain i = 67° ± 5°. The resulting dynamical masses are $M_{1}=1.03^{+0.16}_{-0.11} \, \mathrm{M}_{\odot }$ and $M_2 = 0.39^{+0.07}_{-0.06} \, \mathrm{M}_{\odot }$ at 68 per cent confidence level. The white dwarf dynamical mass is compared with estimates obtained by modelling the decline light curve of the 1901 nova event and X-ray spectroscopy. The best matching mass estimates come from the nova light curve models and an X-ray data analysis that uses the ratio between the Alfvén radius in quiescence and during dwarf nova outburst.
ABSTRACT
We present a dynamical study of the eclipsing intermediate polar XY Ari based on time-resolved near-infrared spectroscopy obtained with the EMIR spectrograph on the 10.4-m Gran Telescopio ...Canarias. Using main sequence template spectra taken with the same instrument setup as the target spectra, we measure a radial velocity amplitude of the late K- type donor star K2 = 256 ± 2 km s−1. We also obtain the rotational broadening of its photospheric lines vrot sin i = 141 ± 3 km s−1. From these and the eclipse geometry, we derive a donor-to-white dwarf mass ratio q = M2/M1 = 0.62 ± 0.02, an orbital inclination i = 80.8° ± 0.5° and dynamical masses $M_{1} = 1.21 \pm 0.04 \, \mathrm{M}_{\odot }$ and $M_2 = 0.75 \pm 0.04 \, \mathrm{M}_{\odot }$ (1σ). This result places the white dwarf in XY Ari as one of the three most massive known in a cataclysmic variable. Comparison with white dwarf mass estimates from X-ray spectral studies could indicate the necessity of an improvement of the X-ray models and/or analysis techniques, as a number of X-ray white dwarf masses are in disagreement with the dynamical mass value.
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