arXiv subjects

D. Marchev

Publications and source records attributed to D. Marchev.

17 recordsLinked to original sources

arXiv2026

BV photometry of the cataclysmic variable LY UMa

We performed B- and V-band photometry of the probable dwarf nova LY UMa. We find that it has apparent magnitude V=15.26+/-0.03 on 2024-04-14, V=15.43+/-0.02 on 2025-04-28, and V = 15.57+/-0.02 on 2025-06-02. Using the GAIA distance d=310pc, we find that the lower values correspond to absolute V band magnitude M_V=7.94-8.10 and indicate that the mass donor of LY UMa is a K7-K8V star with M_V=8.0-8.5. Our B-band measurements show slightly bluer B-V index of LY UMa than that of its donor, consistent with its nature as a cataclysmic variable. Close to LY UMa is the Active Galaxy Nucleus Mrk 153, for which we estimate V=14.81+/-0.03.

astro-ph.SR↗

arXiv2026

Is Mira B a low mass white dwarf?

We report photometric observations in Johnson B and V bands of the short term variability (flickering) of Mira (omicron Ceti). The observations were performed during 7 nights in the period August - October 2025, in the course of the last minimum of the Mira pulsations. The observed peak-to-peak amplitude of the flickering is 0.11-0.28 mag in B band. For the flickering source we find luminosity in the range 0.10-0.46 L_sun. Using the amplitude-flux relation, we estimate an average luminosity of the accretion disc L_d = 0.91 +/- 0.28 L_sun. Assuming that the white dwarf accretes material through Wind Roche Lobe Overflow (WRLOF), we find that Mira B is a low mass white dwarf with M_wd = 0.24 +/- 0.04 M_sun accreting at a rate 6.8x10^{-9} M_sun/yr. This value of the mass is in the range of the extremely low mass white dwarfs. The data are available on Zenodo: zenodo.org/records/18756532

astro-ph.SR↗

arXiv2025

Photometry with the new 1.5-meter telescope of the Rozhen Observatory

The new 1.5~m telescope (AZ1500) is operating at the National Astronomical Observatory Rozhen, Bulgaria. This paper gives an overview of the telescope and presents a snapshot of the current performance. Science observations are under way, and we give brief highlights from a number of programs that have been enabled.

astro-ph.IM↗

arXiv2025

Observational signs of limited flare area variation and peak flare temperatures estimations in main-sequence flaring stars

In the study of stellar flares, traditional method of calculating total energy emitted in the continuum assumes the emission originating from a narrow chromospheric condensation region with a constant temperature of 10000 K and variable flare area. However, based on multicolor data from 7 new flares observed in Bia{\l}k\'ow and Shumen observatory and 8 flares from (Howard et al., 2020) observed on 10 main-sequence stars (spectral types M5.5V to K5V) we show that flare areas had a relative change in the range of 10% - 61% (for more than half of the flares this value did not exceed 30%) throughout the events except for the impulsive phase, and had values starting from 50 +/- 30 ppm to 300 +/- 150 ppm for our new flares and from 380 +/- 200 ppm to 7600 +/- 3000 ppm from (Howard et al., 2020) data, while their temperature increased on average by the factor 2.5. The peak flare temperatures for our seven observed flares ranged from 5700 +/- 450 K to 17500 +/- 10050 K. Five of these flares had their temperatures estimated using the Johnson-Kron-Cousins B filter alongside TESS data, one flare was analyzed using the SLOAN g' and r' bandpasses, and another was evaluated using both the SLOAN g' and r' bandpasses and TESS data. Using flare temperature and area from our data and data from Howard et al., 2020, along with the physical parameters of stars where the flares occurred, we developed a semi-empirical grid that correlates a star's effective temperature and flare amplitude in TESS data with the flare's peak temperature. This allows interpolation of a flare's peak temperature based on the star's effective temperature (ranging from 2700 K to 4600 K) and flare amplitude from TESS observations. Applying this grid to 42257 flares from TESS survey, we estimated peak flare temperatures between 5700 K and 38300 K, with most flares showing peak black-body temperatures around 11100 +/- 2400 K.

astro-ph.SR↗

arXiv2025

Red giant component of the recurrent nova T Coronae Borealis

We performed simultaneous V band photometry and spectroscopic observations of the recurrent nova T CrB and estimate the V band magnitude of the red giant. We find for the red giant of T CrB apparent and absolute V-band magnitudes m_V = 10.17 +/- 0.06 and M_V = +0.14 +/- 0.08, respectively. At the maximum of the ellipsoidal variation when these values are obtained, its absolute V-band magnitude is similar but fainter than the typical M4/5III giants. The data are available on : zenodo.org/records/15174720

astro-ph.SR↗

arXiv2024

Size of the accretion disc in the recurrent nova T CrB

We present high resolution (0.06 A/px) spectroscopic observations of the recurrent nova T Coronae Borealis obtained during the last 1.5 years (September 2022 -- January 2024), with the 2.0m RCC telecope of the Rozhen National Astronomical Observatory, Bulgaria. Double-peaked emission is visible in the H-alpha line after the end of the superactive state. We subtract the red giant contribution and measure the distance between the peaks ($\Delta v_a$) of the line. For the period July 2023 -- January 2024, we find that $\Delta v_a$ is in range $90 < \Delta v_a < 120$ km/s. Assuming that the emission is from the accretion disc around the white dwarf, we find average radius of the accretion disc $R_{disc} = 89 \pm 19$ R$_\odot$, which is approximately equal to the Roche lobe size of the white dwarf. Our results indicate that tidal torque plays an important role but that the disc can extend up to the Roche lobe of the accreting star.

astro-ph.SR↗

arXiv2023

Accretion in the recurrent nova T CrB: Linking the superactive state to the predicted outburst

Context: T CrB (NOVA CrB 1946) is a famous recurrent nova with a recurrence timescale of 80 years. Aims: We aim to estimate the colours, luminosity, and mass-accretion rate for T CrB (NOVA CrB 1946) during and after the superactive state. Methods and Results: We performed and analysed $UBV$ photometry of the recurrent nova T~CrB. For the hot component of T~CrB, we find average dereddened colours of $(U-B)_0 = -0.70 \pm 0.08$ and $(B-V)_0 = 0.23 \pm 0.06$, which correspond to an effective temperature of $9400 \pm 500$~K and an optical luminosity of $40-110~L_\odot$ during the superactive state (2016-2022). After the end of the superactive state, the hot component became significantly redder, $(U-B)_0 \approx -0.3$ and $(B-V)_0 \approx 0.6$ in August 2023, and its luminosity decreased markedly to $20-25$~$L_\odot$ in April-May 2023, and to $8-9~L_\odot$ in August 2023. The total mass accreted during the superactive state from 2014 to 2023 is $\sim 2 \times 10^{-7}$~M$_\odot$. This is a significant fraction of the mass required to cause a thermonuclear runaway (TNR). Overall our results support a model in which a large accretion disc acts as a reservoir with increased accretion rate onto the central white dwarf during disc high states, ultimately leading to a TNR explosion, which now seems to be imminent.

astro-ph.SR↗

arXiv2023

Mass outflow from the symbiotic binary RS Oph during its 2021 outburst

RS Oph is a symbiotic recurrent nova containing a massive white dwarf with heavy mass loss during activity. In August 2021, it underwent its seventh optical eruption since the end of the 19th century. The goal of this work is to analyse the structure of the outflows from the outbursting object. Based on broad-band $U$, $B$, $V$, $R_{\rm C}$, and $I_{\rm C}$ photometry and high-resolution H$\alpha$ spectroscopy obtained at days 11--15 of the outburst, we derived some parameters of the system's components and outflows and their changes during our observation. The effective temperature of a warm shell (pseudophotosphere) produced by the ejected material and occulting the hot component of the system was $T_{eff}=15000\pm1000$ K and the electron temperature of the nebula was $T_{e}=17000\pm3000$ K throughout the observations. The effective radius of the pseudophotosphere was $R_{ eff}=13.3\pm2.0$ R$_{\odot}$ and the emission measure of the nebula $EM=(9.50\pm0.59) $10$^{61}$ cm$^{-3}$ for day 11 and $R_{eff}=10.3\pm1.6$ R$_{\odot}$ and $EM=(5.60\pm0.35)$10$^{61}$ cm$^{-3}$ for day 15. To provide this emission measure, the bolometric luminosity of the outbursting object must exceed its Eddington limit. The mass-loss rate of the outbursting object through its wind is much greater than through its streams. The total rate (from wind + streams) was less than $(4-5)$ 10$^{-5}$ (d/1.6kpc)$^{3/2}$ M$_{\odot}$yr$^{-1}$. The streams are not highly collimated. Their mean outflowing velocities are $\upsilon_{b}=-3680\pm60$ km s$^{-1}$ for the approaching stream and $\upsilon_{r}=3520\pm50$ km s$^{-1}$ for the receding one if the orbit inclination is 50$^\circ$.

astro-ph.SR↗

arXiv2022

The hidden symbiotic star SU Lyn -- detection of flickering in U band

We report photometric observations of the hidden symbiotic star SU Lyn in the optical bands. In five nights we detect a weak flickering in U band with amplitude of about 0.05 magnitudes. No intranight variations are found in B, V, g' and r' bands. This is one more indication that the secondary component is a white dwarf accreting at low accretion rate. We also searched for intranight variability of a dozen related object (RR Boo, RT Boo, AM Cyg, AG Peg, BF Cyg, NQ Gem, StHa190, V627 Cas, XX Oph, FS Cet and Y Gem) in which no variability above the observational errors is detected.

astro-ph.SR↗

arXiv2022

Optical spectroscopy of the Be/black hole binary MWC656 -- interaction of a black hole with a circumstellar disc

We present new spectroscopic observations of the Be/black hole binary MWC 656 obtained during the period 2015 - 2021. We measure the equivalent width of H$\alpha$ (EW$_\alpha$), H$\beta$ ($EW_\beta$), and the distance between the peaks of H$\alpha$ ($\Delta V_\alpha$), H$\beta$ ($\Delta V_\beta$), and FeII ($\Delta V_{FeII}$) lines. Combining new and old data, we find that: - the density of the circumstellar disc of MWC 656 ($\Delta V_\alpha$ versus $EW_\alpha$ diagram) is similar to the Be stars. For the Be stars we find the relation $\Delta V_\beta = 0.999 \Delta V_\alpha + 62.4$ km s$^{-1}$, and the position of MWC~656 corresponds to the average behaviour of the Be stars. This means that the presence of the black hole does not change the overall structure of the circumstellar disc. - the periodogram analysis indicates modulation of $EW_\alpha$ with a period $60.4 \pm 0.4$ days, which is identical to the binary orbital period. The maxima of $EW_\alpha$ and $EW_\beta$ are around periastron (phase zero). - around orbital phase zero, $\Delta V_\beta$ and $\Delta V_{FeII}$ decrease by about $30$ km s$^{-1}$. This suggests that we observe an increase of the circumstellar disc size induced by the periastron passage of the black hole and that the entire circumstellar disc pulsates with the orbital period with relative amplitude of 10-20%. The observations also indicate, that the reason for the black hole in MWC 656 to be in deep quiescence is a very low efficiency of accretion ($\sim 2 \times 10^{-6}$).

astro-ph.SR↗

arXiv2021

H-alpha spectroscopy of the recurrent nova RS Oph during the 2021 outburst

We report spectroscopic observations of the $H\alpha$ emission line of the recurrent nova RS~Oph obtained between 12 and 23 August 2021 during the recent outburst. On the basis of the sharp P~Cyg profile superimposed onto the strong H-alpha emission, we estimate that the outflowing velocity of the material surrounding RS Oph is in the range 32 km/s < V_{out} < 68 km/s. The new GAIA distance indicates that the red giant should be probably classified in between II and III luminosity class. The spectra are available upon request from the authors and on Zenodo.

astro-ph.SR↗

arXiv2021

Physical parameters of close binary systems: VIII

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, RWCom, 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 ultra-short 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 confgurations even after several Gyr of evolution.

astro-ph.SR↗

arXiv2020

International observational campaign of the 2014 eclipse of EE Cep

Context. EE Cep is one of few eclipsing binary systems with a dark, dusty disk around an invisible object similar to {\epsilon} Aur. The system is characterized by grey and asymmetric eclipses every 5.6 yr, with a significant variation in their photometric depth, ranging from ~ 0 m .5 to ~ 2 m .0. Aims. The main aim of the observational campaign of the EE Cep eclipse in 2014 was to test the model of disk precession (Galan et al. 2012). We expected that this eclipse would be one of the deepest with a depth of ~ 2 m .0. Methods. We collected multicolor observations from almost 30 instruments located in Europe and North America. This photometric data covers 243 nights during and around the eclipse. We also analyse the low- and high-resolution spectra from several instruments. Results. The eclipse was shallow with a depth of 0 m .71 in V-band. The multicolor photometry illustrates small color changes during the eclipse with a total amplitude of order ~ +0 m . 15 in B-I color index. The linear ephemeris for this system is updated by including new times of minima, measured from the three most recent eclipses at epochs E = 9, 10 and 11. New spectroscopic observations were acquired, covering orbital phases around the eclipse, which were not observed in the past and increased the data sample, filling some gaps and giving a better insight into the evolution of the H {\alpha} and NaI spectral line profiles during the primary eclipse. Conclusions. The eclipse of EE Cep in 2014 was shallower than expected 0 m .71 instead of ~ 2 m . 0. This means that our model of disk precession needs revision.

astro-ph.SR↗

arXiv2019

An eccentric wave in the circumstellar disc of the Be/X-ray binary X Persei

We present spectroscopic observations of the Be/X-ray binary X Per obtained during the period December 2017 - January 2020 (MJD~58095 - MJD~58865). In December 2017 the $H\alpha$, $H\beta$, and HeI 6678 emission lines were symmetric with violet-to-red peak ratio $V/R \approx 1$. During the first part of the period (December 2017 - August 2018) the V/R-ratio decreased to 0.5 and the asymmetry developed simultaneously in all three lines. In September 2018, a third component with velocity $\approx 250$~km~s$^{-1}$ appeared on the red side of the HeI line profile. Later this component emerged in $H\beta$, accompanied by the appearance of a red shoulder in $H\alpha$. Assuming that it is due to an eccentric wave in the circumstellar disc, we find that the eccentric wave appeared first in the innermost part of the disc, it spreads out with outflowing velocity $v_{wave} \approx 1.1 \pm 0.2 $~km~s$^{-1}$, and the eccentricity of the eccentric wave is $e_{wave} \approx 0.29 \pm 0.07$. A detailed understanding of the origin of such eccentricities would have applications to a wide range of systems from planetary rings to AGNs.

astro-ph.SR↗

arXiv2018

Spectral observations of X Persei: Connection between H-alpha and X-ray emission

We present spectroscopic observations of the Be/X-ray binary X Per obtained during the period 1999 - 2018. Using new and published data, we found that during "disc-rise" the expansion velocity of the circumstellar disc is 0.4 - 0.7 km/s. Our results suggest that the disc radius in recent decades show evidence of resonant truncation of the disc by resonances 10:1, 3:1, and 2:1, while the maximum disc size is larger than the Roche lobe of the primary and smaller than the closest approach of the neutron star. We find correlation between equivalent width of H-alpha emission line ($W\alpha$) and the X-ray flux, which is visible when $15 \: \AA \: < W\alpha \le 40 \: \AA$. The correlation is probably due to wind Roche lobe overflow.

astro-ph.SR↗

arXiv2009

Variability and stability in blazar jets on time scales of years: Optical polarization monitoring of OJ287 in 2005-2009

(Abridged) OJ287 is a BL Lac object that has shown double-peaked bursts at regular intervals of ~12 yr during the last ~40 yr. We analyse optical photopolarimetric monitoring data from 2005-2009, during which the latest double-peaked outburst occurred. The aim of this study is twofold: firstly, we aim to analyse variability patterns and statistical properties of the optical polarization light-curve. We find a strong preferred position angle in optical polarization. The preferred position angle can be explained by separating the jet emission into two components: an optical polarization core and chaotic jet emission. The optical polarization core is stable on time scales of years and can be explained as emission from an underlying quiescent jet component. The chaotic jet emission sometimes exhibits a circular movement in the Stokes plane. We interpret these events as a shock front moving forwards and backwards in the jet, swiping through a helical magnetic field. Secondly, we use our data to assess different binary black hole models proposed to explain the regularly appearing double-peaked bursts in OJ287. We compose a list of requirements a model has to fulfil. The list includes not only characteristics of the light-curve but also other properties of OJ287, such as the black hole mass and restrictions on accretion flow properties. We rate all existing models using this list and conclude that none of the models is able to explain all observations. We discuss possible new explanations and propose a new approach to understanding OJ287. We suggest that both the double-peaked bursts and the evolution of the optical polarization position angle could be explained as a sign of resonant accretion of magnetic field lines, a 'magnetic breathing' of the disc.

astro-ph.CO↗

arXiv2008

A massive binary black-hole system in OJ287 and a test of general relativity

Tests of Einstein's general theory of relativity have mostly been carried out in weak gravitational fields where the space-time curvature effects are first-order deviations from Newton's theory. Binary pulsars provide a means of probing the strong gravitational field around a neutron star, but strong-field effects may be best tested in systems containing black holes. Here we report such a test in a close binary system of two candidate black holes in the quasar OJ287. This quasar shows quasi-periodic optical outbursts at 12 yr intervals, with two outburst peaks per interval. The latest outburst occurred in September 2007, within a day of the time predicted by the binary black-hole model and general relativity. The observations confirm the binary nature of the system and also provide evidence for the loss of orbital energy in agreement (within 10 per cent) with the emission of gravitational waves from the system. In the absence of gravitational wave emission the outburst would have happened twenty days later.

astro-ph↗