We studied eight new doubly eclipsing stellar systems. We found that they are all rare examples of quadruple systems of 2 + 2 architecture, where both inner pairs are eclipsing binaries. Until now, ...such a configuration had only been proven for dozens of systems on the whole sky. We enlarged this rare group of systems with four stars in the Small Magellanic Cloud (SMC) galaxy and four brighter stars on the northern sky. These analysed systems are the following: OGLE SMC-ECL-2339 (both eclipsing periods of 0.72884 days and 3.39576 days; mutual orbital period of 5.95 years); OGLE SMC-ECL-3075 (1.35890 d, 2.41587 d, 9.75 yr); OGLE SMC-ECL-4756 (0.91773 d, 2.06047 d, 4.34 yr); OGLE SMC-ECL-6093 (0.90193 d, 2.03033 d, 31.2 yr); GSC 01949-01700 (0.24058 d, 0.75834 d, 21.7 yr); ZTF J171602.61+273606.5 (0.36001 d, 4.51545 d, 19.5 yr); WISE J210935.8+390501 (0.33228 d, 3.51575 d, 1.9 yr); and V597 And (0.46770 d, 0.35250, 20.4 yr). These systems constitute a rare selection of W UMa stars among the doubly eclipsing quadruples. For all of the systems, new dedicated observations were obtained as well. V597 And is definitely the most interesting system for several reasons: (1) the system is the brightest in our sample; (2) it is a rare quintuple (2 + 2) + 1 system; and (3) it is also closest to the Sun. It yielded the predicted angular separation of the two components of 57 mas, which is probably within the detection limits for modern, high-angular-resolution techniques.
ABSTRACT
Brightening of Hen 3-860, previously classified as an Hα emitter, was detected by the All-Sky Automated Survey for Supernovae (ASAS-SN) survey at the end of the year 2016. We have obtained ...the first spectroscopic observations of the transient and supplemented them with photometric data from the Digital Access to a Sky Century at Harvard (DASCH) archive of astronomical plates, All-Sky Automated Survey (ASAS), and ASAS-SN surveys. Based on the results of our analysis, we can classify the object as a classical symbiotic star of the infrared type S, consisting of an M2-3 giant with a temperature of Tg ∼ 3550 K, a radius of Rg ∼ 60–75 R⊙, and a luminosity of Lg ∼ 540–760 L⊙, and a hot and luminous component (Th ∼ 1 × 105–2 × 105 K and $L_{\rm h}\, \sim 10^3\, L_\odot$). The system experienced at least four outbursts in the last 120 years. In addition to the outbursts, its light curves revealed the presence of eclipses of the hot component and its surrounding (relatively cool) shell, which developed during the outburst and redistributed a fraction of the radiation of the hot component into the optical, by the giant, classifying the object as a representative of a group of eclipsing symbiotic stars. The eclipses allowed us to reveal the orbital period of the system to be 602 d.
ABSTRACT
New Online Database of Symbiotic Variables includes several poorly characterized objects and candidate symbiotic stars, not only in the Milky Way but also in other galaxies. The goal of the ...research presented in this paper was to reveal the nature of 2MASS J07363415+6538548, the object discovered as an X-ray source in the field of NGC 2403, and tentatively classified as a symbiotic candidate or a cataclysmic variable. By analysis of available photometric data from ground-based surveys, together with a high-precision photometry from TESS, remarkable astrometric measurements of the Gaia satellite and observations of other surveys spanning from X-rays to infrared, we have found that the object neither not a symbiotic star nor a cataclysmic variable, but rather an active K-type dwarf. The star is located in the distance of 415 pc, has an effective temperature of 4275 K, luminosity of 0.14 L⊙, mass of 0.7 M⊙, and radius of 0.7 R⊙. It has a rotational period ∼ 3 d and is a strong X-ray source with the X-ray luminosity of ∼ $10^{30}\rm \, erg\, s^{-1}$. Gyrochronology and isochrone fitting confirmed that the star is young.
ABSTRACT
Symbiotic stars belong to a group of interacting binaries that display a wide variety of phenomena, including prominent outbursts connected with mass transfer, as well as stellar winds, ...jets, eclipses, or intrinsic variability of the components. Dozens of new symbiotic stars and candidates have been discovered in recent years. However, there are many objects that are still poorly studied. Some symbiotic candidates suspected in the literature have never been studied spectroscopically. In this contribution, we present the first results of the ongoing campaign focused on symbiotic candidates. In the first paper in the series, we study the nature of 10 candidate classical symbiotic stars suspected based on their photometric behaviour, colours or abundance pattern. To confirm or reject the symbiotic nature of the studied candidates, we obtained new spectra and analysed them in detail together with available multifrequency photometric and spectroscopic observations of the objects. Hen 3−860 and V2204 Oph are genuine symbiotic systems showing typical spectral features of burning symbiotic stars and outbursts in the last 100 yr. The first object belongs to the uncommon group of eclipsing symbiotic stars. V1988 Sgr cannot be classified as a genuine burning symbiotic star, but the scenario of an accreting-only symbiotic system cannot be ruled out. Hen 4−204 might be a bonafide symbiotic star due to its similarity with the known symbiotic binary BD Cam. Six other symbiotic candidates (V562 Lyr, IRAS 19050+0001, EC 19249−7343, V1017 Cyg, PN K1−6, V379 Peg) are either single dwarf or giant stars or non-symbiotic binaries.
ABSTRACT
V618 Sgr was previously classified as an R CrB-type variable and later as a possible symbiotic star. Our study aims to analyse the nature of this target, which is currently undergoing ...significant brightening in properties similar to those of known symbiotic novae. We analyse literature information, photometric observations, and 35 new optical spectra. Our findings strongly suggest that V618 Sgr is an eclipsing symbiotic nova currently in outburst. Additionally, since the star has demonstrated at least two similar brightenings in the past, we propose that V618 Sgr could be the first known galactic symbiotic nova observed in repeated outbursts of this type and may host a relatively massive white dwarf.
This is the first in a collection of three papers introducing the science with an ultra-violet (UV) space telescope on an approximately 130 kg small satellite with a moderately fast re-pointing ...capability and a real-time alert communication system approved for a Czech national space mission. The mission, called
Quick Ultra-Violet Kilonova surveyor—QUVIK
, will provide key follow-up capabilities to increase the discovery potential of gravitational wave observatories and future wide-field multi-wavelength surveys. The primary objective of the mission is the measurement of the UV brightness evolution of kilonovae, resulting from mergers of neutron stars, to distinguish between different explosion scenarios. The mission, which is designed to be complementary to the
Ultraviolet Transient Astronomy Satellite—ULTRASAT
, will also provide unique follow-up capabilities for other transients both in the near- and far-UV bands. Between the observations of transients, the satellite will target other objects described in this collection of papers, which demonstrates that a small and relatively affordable dedicated UV-space telescope can be transformative for many fields of astrophysics.
Context.
Besides the astrometric mission of the
Gaia
satellite, its repeated and high-precision measurements also serve as an all-sky photometric transient survey. The sudden brightenings of the ...sources are published as
Gaia
Photometric Science Alerts and are made publicly available, allowing the community to photometrically and spectroscopically follow up on the object.
Aims.
The goal of this paper is to analyze the nature and derive the basic parameters of Gaia18aen, a transient detected at the beginning of 2018. This object coincides with the position of the emission-line star WRAY 15-136. The brightening was classified as a “nova?” on the basis of a subsequent spectroscopic observation.
Methods.
We analyzed two spectra of Gaia18aen and collected the available photometry of the object covering the brightenings in 2018 and also the preceding and following periods of quiescence. Based on this observational data, we derived the parameters of Gaia18aen and discussed the nature of the object.
Results.
Gaia18aen is the first symbiotic star discovered by
Gaia
satellite. The system is an S-type symbiotic star and consists of an M giant of a slightly super-solar metallicity, where
T
eff
∼ 3500 K, a radius of ∼230
R
⊙
, and a high luminosity
L
∼ 7400
L
⊙
. The hot component is a hot white dwarf. We tentatively determined the orbital period of the system ∼487 d. The main outburst of Gaia18aen in 2018 was accompanied by a decrease in the temperature of the hot component. The first phase of the outburst was characterized by the high luminosity
L
∼ 27 000
L
⊙
, which remained constant for about three weeks after the optical maximum, later followed by the gradual decline of luminosity and increase of temperature. Several re-brightenings have been detected on the timescales of hundreds of days.
In this review, the third one in the series focused on a small two-band UV-photometry mission, we assess possibilities for a small UV two-band photometry mission in studying accreting supermassive ...black holes (SMBHs; mass range
∼
10
6
–
10
10
M
⊙
). We focus on the following observational concepts: (i) dedicated monitoring of selected type-I Active Galactic Nuclei (AGN) in order to measure the time delay between the far-UV, the near-UV, and other wavebands (X-ray and optical), (ii) nuclear transients including (partial) tidal disruption events and repetitive nuclear transients, and (iii) the study of peculiar sources, such as changing-look AGN, hollows and gaps in accretion disks, low-luminosity AGN, and candidates for Intermediate-Mass Black Holes (IMBHs; mass range
∼
10
2
–
10
5
M
⊙
) in galactic nuclei. The importance of a small UV mission for the observing program (i) is to provide intense, high-cadence monitoring of selected sources, which will be beneficial for, e.g. reverberation-mapping of accretion disks and subsequently confronting accretion-disk models with observations. For program (ii), a relatively small UV space telescope is versatile enough to start monitoring a transient event within ≲ 20 minutes after receiving the trigger; such a moderately fast repointing capability will be highly beneficial. Peculiar sources within the program (iii) will be of interest to a wider community and will create an environment for competitive observing proposals. For tidal disruption events (TDEs), high-cadence UV monitoring is crucial for distinguishing among different scenarios for the origin of the UV emission. The small two-band UV space telescope will also provide information about the near- and far-UV continuum variability for rare transients, such as repetitive partial TDEs and jetted TDEs. We also discuss the possibilities to study and analyze sources with non-standard accretion flows, such as AGN with gappy disks, low-luminosity active galactic nuclei with intermittent accretion, and SMBH binaries potentially involving intermediate-mass black holes.
We present an extensive study of 162 early-type binary systems located in the LMC galaxy that show apsidal motion and have never been studied before. For the ample systems, we performed light curve ...and apsidal motion modelling for the first time. These systems have a median orbital period of 2.2 days and typical periods of the apsidal motion were derived to be of the order of decades. We identified two record-breaking systems. The first, OGLE LMC-ECL-22613, shows the shortest known apsidal motion period among systems with main sequence components (6.6 years); it contains a third component with an orbital period of 23 years. The second, OGLE LMC-ECL-17226, is an eccentric system with the shortest known orbital period (0.9879 days) and with quite fast apsidal motion period (11 years). Among the studied systems, 36 new triple-star candidates were identified based on the additional period variations. This represents more than 20% of all studied systems, which is in agreement with the statistics of multiples in our Galaxy. However, the fraction should only be considered as a lower limit of these early-type stars in the LMC because of our method of detection, data coverage, and limited precision of individual times of eclipses.
Variations of the emission lines in the spectrum of the yellow symbiotic star AG Draconis (AG Dra) have been studied for over 14 yr (1997-2011), using more than 500 spectra obtained with the 1.5-m ...telescope at Tartu Observatory, Estonia. The time interval covered includes the major (cool) outburst of AG Dra that started in 2006. The main findings can be summarized as follows. (i) The cool and hot outbursts of AG Dra can be distinguished from the variations of optical emission lines. (ii) The Raman scattered emission line of O VI at lambda 6825 almost disappeared during the cool outburst. (iii) The lower excitation emission lines did not change significantly during the cool outburst, but they do vary in hot outbursts and also follow orbital motion. (iv) The similarity of variations in AG Dra to those in the prototypical symbiotic star Z Andromedae allows us to suggest that a combination nova model proposed for the latter object might also be responsible for the outburst behaviour of AG Dra.
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