arXiv subjects

Yanko Nikolov

Publications and source records attributed to Yanko Nikolov.

7 recordsLinked to original sources

arXiv2025

Polarimetric and spectropolarimetric observations with FoReRo2: Instrument overview and standard star monitoring

In this paper, we present a description and characterisation of the 2-Channel Focal Reducer Rozhen (FoReRo2), with an emphasis on its polarimetric and spectropolarimetric observation modes. FoReRo2 is a multimode instrument mounted at the Ritchey-Chretien (RC) focus of the 2-meter Ritchey-Chretien-Coude (RCC) telescope at the Bulgarian National Astronomical Observatory (BNAO) - Rozhen. The primary targets of this instrument include novae, Be/X-ray binaries, symbiotic stars, asteroids, and comets. Standard stars with a high degree of polarisation are an essential part of the polarimetric observations. In our sample, HD 204827 shows short-term variability in the position angle (PA), making it unsuitable for calibration. HD 183143 displays intrinsic polarisation variability, but remains stable in terms of the PA. For the first time, we present a statistical study of the K and lambda_max parameters of Serkowski's law, including the mean value, standard deviation, and distribution. To demonstrate FoReRo2's polarimetric capabilities, we present several examples. The recurrent nova RS Oph shows the variability of Serkowski's K parameter, which is due to dust formation within the first few days after the 2021 outburst. These examples also include the imaging polarimetry of comet C/2019 Y4 (ATLAS) and spectropolarimetric observations of the symbiotic star Z And.

astro-ph.SR↗

arXiv2023

Intra-night optical flux and polarization variability of BL~Lacertae during its 2020 $-$ 2021 high state

In this work, we report the presence of rapid intra-night optical variations in both -- flux and polarization of the blazar BL Lacertae during its unprecedented 2020--2021 high state of brightness. The object showed significant flux variability and some color changes, but no firmly detectable time delays between the optical bands. The linear polarization was also highly variable in both -- polarization degree and angle (EVPA). The object was observed from several observatories throughout the world, covering in a total of almost 300 hours during 66 nights. Based on our results, we suggest, that the changing Doppler factor of an ensemble of independent emitting regions, travelling along a curved jet that at some point happens to be closely aligned with the line of sight can successfully reproduce our observations during this outburst. This is one of the most extensive variability studies of the optical polarization of a blazar on intra-night timescales.

astro-ph.HE↗

arXiv2023

Polarimetric Reverberation Mapping in Medium-Band Filters

Earlier, we suggested the "reload" concept of the polarimetric reverberation mapping of active galactic nuclei (AGN), proposed for the first time more than 10 years ago. We have successfully tested this approach of reverberation mapping of the broad emission line on the galaxy Mrk 6. It was shown that such an idea allows one to look at the AGN central parsec structure literally in a new light. However, the method originally assumed the use of spectropolarimetric observations, expensive in terms of telescope time, and implemented on rare large telescopes. Currently, we propose an adaptation of the polarimetric reverberation mapping of broad lines in medium-band filters following the idea of the photometric reverberation mapping, when filters are selected so that their bandwidth is oriented to the broad line and the surrounding continuum near. In this paper, we present the progress status of such monitoring conducted jointly at the Special astrophysical observatory and Asiago Cima Ekar observatory (OAPd/INAF) with support from Rozhen National Astronomical Observatory (NAO), some first results for the most frequently observed AGNs Mrk 335, Mrk 509, and Mrk 817, and the discussion of the future perspectives of the campaign.

astro-ph.GA↗

arXiv2022

Recent humps and superhumps observations and an estimation of outburst parameters of the AM CVn star CR Boo

We present our observational results of AM CVn star CR Boo in the UBVR bands. Our observational campaign includes data obtained over 5 nights with the National Astronomical Observatory Rozhen, Belogradchik and the AS Vidojevica telescopes. During the whole time of our observations the brightness of the system varied between $13.95 - 17.23$ in B band. We report the appearance of humps during the period of quiescence and superhumps during the active state of the object, (where the latter are detected in two nights). We obtain the superhumps periodicity for two nights, $P_{sh}\approx 24.76 - 24.92$ min. The color during maximum brightness is estimated as $ - 0.107 < (B-V)_{0} < 0.257$ and the corresponding temperature is in the range as $7700 [K] < T(B-V)_{0} < 11700 [K]$. We found that CR Boo varies from bluer to redder in the nights with outbursts activity. The star becomes bluer during the times of superhumps.

astro-ph.SR↗

arXiv2022

Interstellar polarization and extinction toward the Recurrent Nova T CrB

Spectropolarimetry is a powerful tool for diagnostic of interstellar matter and gives information on the geometry of the ejected material after the novae outbursts. In this paper are presented spectropolarimetric observations of the recurrent nova T CrB at quiescence obtained with FoReRo2 attached to the Cassegrain focus of the 2.0m RCC telescope of the Bulgarian Rozhen National Astronomical Observatory. The interstellar polarization toward T CrB was estimated using the field stars method. The spectropolarimetric observations were obtained from February 2018 to August 2021. In the wavelength range from 4800~Å~ to 8200~Å~the maximum of the degree of linear polarization is $P_{max}(obs)(\%) = 0.46 \pm 0.01$ at $λ\approx 5200$ Å. The position angle is $P.A._{obs}=100^{\circ}.8 \pm 0^{\circ}.9$. During the observations, there is no intrinsic polarization in T CrB, and the derived values represent interstellar polarization. The polarization toward T CrB is due to the foreground interstellar dust located at the distance up to $\approx$ 400 pc. Based on the degree of polarization the interstellar extinction toward the T CrB is $E_{B-V} \approx 0.07$.

astro-ph.SR↗

arXiv2021

Mid-Cycle Observations of CR Boo and Estimation of the System's Parameters

We present observations (with NAO Rozhen and AS Vidojevica telescopes) of the AM Canum Venaticorum (AM CVn) type binary star CR Bootis (CR Boo) in the UBV bands. The data were obtained in two nights in July 2019, when the V band brightness was in the range of 16.1-17.0 mag. In both nights, a variability for a period of $25 (\pm 1)$ min and amplitude of about 0.2 magnitudes was visible. These brightness variations are most likely indications of "humps". During our observational time, they appear for a period similar to the CR Boo orbital period. A possible reason of their origin is the phase rotation of the bright spot, placed in the contact point of the infalling matter and the outer disc edge. We estimated some of the parameters of the binary system, on the base of the observational data.

astro-ph.SR↗

arXiv2007

Observations of Comet C/LINEAR (2004B1) between2 and 3 AU heliocentric distance

We present R-band observations of comet C2004B1 obtained in the period June, 21 - August 20, 2006. The data have been reduced to surface brightness maps, light curves, and mean radial profiles of the coma. In two cases a decrease of the brightness was recorded, which lasted for several days. The brightness decrease was accompanied by morphological changes in the coma.

astro-ph↗