arXiv subjects

M. A. Burlak

Publications and source records attributed to M. A. Burlak.

36 records · Page 2Linked to original sources

arXiv2021

The curtain remains open: NGC 2617 continues in a high state

Optical and near-infrared photometry, optical spectroscopy, and soft X-ray and UV monitoring of the changing look active galactic nucleus NGC 2617 show that it continues to have the appearance of a type-1 Seyfert galaxy. An optical light curve for 2010-2016 indicates that the change of type probably occurred between 2010 October and 2012 February and was not related to the brightening in 2013. In 2016 NGC 2617 brightened again to a level of activity close to that in 2013 April. We find variations in all passbands and in both the intensities and profiles of the broad Balmer lines. A new displaced emission peak has appeared in H$β$. X-ray variations are well correlated with UV-optical variability and possibly lead by $\sim$ 2-3 d. The $K$ band lags the $J$ band by about 21.5 $\pm$ 2.5 d. and lags the combined $B+J$ filters by $\sim$ 25 d. $J$ lags $B$ by about 3 d. This could be because $J$-band variability arises from the outer part of the accretion disc, while $K$-band variability comes from thermal re-emission by dust. We propose that spectral-type changes are a result of increasing central luminosity causing sublimation of the innermost dust in the hollow biconical outflow. We briefly discuss various other possible reasons that might explain the dramatic changes in NGC 2617.

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arXiv2021

Multi-wavelenth monitoring of the changing-look AGN NGC 2617 during state changes

Optical and near-infrared photometry, optical spectroscopy, and soft X-ray and UV monitoring of the changing-look active galactic nucleus NGC 2617 show that it continues to have the appearance of a type-1 Seyfert galaxy. An optical light curve for 2010$-$2017 indicates that the change of type probably occurred between 2010 October and 2012 February and was not related to the brightening in 2013. In 2016 and 2017 NGC 2617 brightened again to a level of activity close to that in 2013 April. However, in 2017 from the end of the March to end of July 2017 it was in very low level and starting to change back to a Seyfert 1.8. We find variations in all passbands and in both the intensities and profiles of the broad Balmer lines. A new displaced emission peak has appeared in H$β$. X-ray variations are well correlated with UV$-$optical variability and possibly lead by $\sim$ 2$-$3 d. The $K$ band lags the $J$ band by about 21.5 $\pm$ 2.5 d and lags the combined $B + J$ bands by $\sim$ 25 d. $J$ lags $B$ by $\sim$ 3 d. This could be because $J$-band variability arises predominantly from the outer part of the accretion disc, while $K$-band variability is dominated by thermal re-emission by dust. We propose that spectral-type changes are a result of increasing central luminosity causing sublimation of the innermost dust in the hollow bi-conical outflow. We briefly discuss various other possible reasons that might explain the dramatic changes in NGC 2617.

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arXiv2021

Multi-Wavelength Monitoring and Reverberation Mapping of a Changing Look Event in the Seyfert Galaxy NGC 3516

We present the results of photometric and spectroscopic monitoring campaigns of the changing look AGN NGC 3516 carried out in 2018 to 2020 covering the wavelength range from the X-ray to the optical. The facilities included the telescopes of the CMO SAI MSU, the 2.3-m WIRO telescope, and the XRT and UVOT of Swift. We found that NGC 3516 brightened to a high state and could be classified as Sy1.5 during the late spring of 2020. We have measured time delays in the responses of the Balmer and He II 4686 lines to continuum variations. In the case of the best-characterized broad H-beta line, the delay to continuum variability is about 17 days in the blue wing and is clearly shorter, 9 days, in the red, which is suggestive of inflow. As the broad lines strengthened, the blue side came to dominate the Balmer lines, resulting in very asymmetric profiles with blueshifted peaks during this high state. During the outburst the X-ray flux reached its maximum on 1 April 2020 and it was the highest value ever observed for NGC 3516 by the Swift observatory. The X-ray hard photon index became softer, about 1.8 in the maximum on 21 Apr 2020 compared to the mean about 0.7 during earlier epochs before 2020. We have found that the UV and optical variations correlated well (with a small time delay of 1-2 days) with the X-ray until the beginning of April 2020, but later, until the end of Jun. 2020, these variations were not correlated. We suggest that this fact may be a consequence of partial obscuration by Compton-thick clouds crossing the line of sight.

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arXiv2021

The formation of the Milky Way halo and its dwarf satellites: a NLTE-1D abundance analysis. IV. Segue 1, Triangulum II, and Coma Berenices UFDs

We present atmospheric parameters and abundances for chemical elements from carbon to barium in metal-poor stars in Segue 1 (seven stars), Coma Berenices (three stars), and Triangulum II (one star) ultra-faint dwarf galaxies (UFDs). The effective temperatures rely on new photometric observations in the visible and infra-red bands, obtained with the 2.5 m telescope of the SAI MSU Caucasian observatory. Abundances of up to fourteen chemical elements were derived under the non-local thermodynamic equilibrium (NLTE) line formation, and LTE abundances were obtained for up to five more elements. For the first time we present abundance of oxygen in Seg 1 S1 and S4, silicon in ComaBer S2 and Tri II S40, potassium in Seg 1 S1-S6 and ComaBer S1-S3, and barium in Seg 1 S7. Three stars in Segue 1, two stars in Coma Berenices, and Triangulum II star have very low [Na/Mg] of -1.08 to -1.67 dex, which is usually attributed in the literature to an odd-even effect produced by nucleosynthesis in massive metal-free stars. We interpret this chemical property as a footprint of first stars, which is not blurred due to a small number of nucleosynthesis events that contributed to chemical abundance patterns of the sample stars. Our NLTE abundances of Sr and Ba in Coma Berenices, Segue 1, and Triangulum II report on lower [Sr/Ba] abundance ratio in the UFDs compared to that in classical dwarf spheroidal galaxies and the Milky Way halo. However, in UFDs, just as in massive galaxies, [Sr/Ba] is not constant and it can be higher than the pure r-process ratio. We suggest a hypothesis of Sr production in metal-poor binaries at the earliest epoch of galactic evolution.

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arXiv2020

A flare in the optical spotted in the changing-look Seyfert NGC 3516

We present observations from the short-term intensive optical campaign (from Sep2019 to Jan2020) of the changing-look Seyfert NGC 3516. This active galactic nucleus is known to have strong optical variability and has changed its type in the past. It has been in the low-activity state in the optical since 2013, with some rebrightening from the end of 2015 to the beginning of 2016, after which it remained dormant. We aim to study the photometric and spectral variability of NGC 3516 from the new observations in U- and B-bands and examine the profiles of the optical broad emission lines in order to demonstrate that this object may be entering a new state of activity. NGC 3516 has been monitored intensively for the past 4 months with an automated telescope in U and B filters, enabling accurate photometry of 0.01 precision. Spectral observations were triggered when an increase in brightness was spotted. We support our analysis of past-episodes of violent variability with the UV and X-ray long-term light curves constructed from the archival Swift data. An increase of the photometric magnitude is seen in both U and B filters to a maximum amplitude of 0.25mag and 0.1 mag, respectively. During the flare, we observe stronger forbidden high-ionization iron lines than reported before, as well as the complex broad Ha and Hb lines. This is especially seen in Ha which appears to be double-peaked. It seems that a very broad component of ~10,000 km/s in width in the Balmer lines is appearing. The trends in the optical, UV, and X-ray light curves are similar, with the amplitudes of variability being significantly larger in the case of UV and X-ray bands. The increase of the continuum emission, the variability of the coronal lines, and the very broad component in the Balmer lines may indicate that the AGN of NGC 3516 is finally leaving the low-activity state in which it has been for the last ~3 years.

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arXiv2020

Near-Infrared Photometry of Superthin Edge-on Galaxies

We perform near-infrared photometry of a large sample of 49 superthin edge-on galaxies. These galaxies are selected based on optical photometry because of high radial-to-vertical scale ratio in their stellar disks. The Near Infrared (NIR) H and K observations were conducted with the cryogenic-cooled camera ASTRONIRCAM on the 2.5m telescope at the Caucasus Mountain Observatory of Lomonosov Moscow State University. A majority of galaxies in our sample show comparable or better photometric depth than the Sloan Digital Sky Survey (SDSS) optical images. We estimate the structural parameters of stellar disks in the galaxies and find that the NIR scale height of stellar disks is comparable to that estimated from the optical, SDSS g, r and i, whereas the H and K scale length of the stellar disks is significantly shorter than in the g, r and i. We investigate if a realistic distribution of dust alone can explain the difference in the scale length and find that in the majority of the galaxies the radial variation of the stellar population is actually responsible for the color distribution. The latter suggests a younger age of the disks periphery, and the inside out building up of stellar disks in the superthin galaxies.

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arXiv2020

Surprising Variability of the Planetary Nebula IC 4997 = QV Sge

We present the results of a new epoch of a long-term photometric monitoring of the variable planetary nebula IC~4997. The integral $UBV$ light curves display a continuing brightening of 0.$^{m}15$ in $V$, a slight rise (<0.$^{m}1$) in $B$, and constancy in $U$. The $B-V$ color has got redder from 0.$^{m}4$ in 2000 to 0.$^{m}7$ in 2019, whereas the $U-B$ color has not changed significantly. New $JHKL$ photometry was obtained in 2019. We found the source to be fainter by 0.$^{m}4$ in $L$ and bluer by 0.$^{m}2$ in the $K-L$ color if compared to the data of 1999-2006. The long-term brightness variations are due mostly to the changing input of emission lines to the integral light. Low-dispersion spectroscopic observations carried out in 2010-2019 revealed a continuing decrease in the $\lambda4363/\lambda4340$ ratio: it decreased by a factor of $\sim3$ in 30 years and reached the level of 1960-1970. The absolute intensities of [OIII] nebualr lines increased by a factor of $>2$ from 1990 to 2019, whereas the [OIII] $λ$4363 line had weakened by a factor of 2 comparing to the maximum value observed in 2000. The variation of H$β$ absolute intensity in 1960-2019 was shown to be similar to that of [OIII] nebular lines, but of smaller amplitude. The electron density in the outer part of the nebula was estimated from the [SII] and [ClIII] lines. Basing on the data on absolute intensities we propose a possible scenario describing the change of $N_e,T_e$ in 1970-2019. The spectral variability of IC~4997 could be explained by a variation of electron temperature in the nebula caused by not so much the change in ionizing flux from the central star as the variable stellar wind and related processes. The photometric and spectral changes observed for IC~4997 in 1960-2019 may be interpreted as an observable consequence of a single episode of enhanced mass loss from the variable central star.

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arXiv2019

Spectrum of the Yellow Symbiotic Star LT Delphini before, during, and after the 2017 Outburst

LT Del is a yellow symbiotic system that consists of a bright K3-type giant and a hot subdwarf with a temperature of about 10$^5$ K. We present the results of our spectroscopic observations of LT Del over the period 2010-2018. In 2017 the star experienced a second low-amplitude ($ΔV\sim0.^{m}7$) outburst in the history of its studies. The emission spectrum of the star represented in the optical range by hydrogen, neutral and ionized helium lines underwent significant changes in the outburst. The fluxes in the HI and HeI emission lines increased by a factor of 5-6, the HeII $λ$4686 line grew by a factor of 10. According to our estimates, in the 2017 outburst, the temperature of the exciting star rose to $T_{\text{hot}}\sim$130~000 K, while during the first 1994 outburst the change in temperature was insignificant. This suggests cool and hot outbursts of LT Del by analogy with similar events of another yellow symbiotic star, AG Dra.

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arXiv2018

Absolute parameters and period variation in a semidetached eclipsing variable V2247 Cyg

We aim to determine the absolute parameters of the components of a poorly studied Algol-type eclipsing binary V2247 Cyg. The data analysis is based on our numerous precise UBVRcIc photometric observations and low resolution spectra. The photometric solution reveals a semi-detached configuration with a less-massive component filling its Roche lobe. The mass ratio, inclination, effective temperatures, and fractional radii were determined. Masses and radii of the components were obtained by a non-direct method. The Eclipse Time Variation (ETV) diagram revealed the period changes.

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arXiv2018

Multicolor photometry and spectroscopy of the yellow supergiant with dust envelope HD 179821=V1427 Aquilae

We present the results of multicolor (UBVJHKLM) photometry (2009-2017) and low-resolution spectroscopy (2016-2017) of the semi-regular variable V1427 Aql=HD 179821, a yellow supergiant with gas-dust envelope. The star displays low-amplitude (ΔV<0.2 mag) semi-periodic brightness variation superimposed on a long-term trend. The light curve shape and timescale change from cycle to cycle. There are temperature variations characteristic for pulsations, and brightness oscillations with no significant change of color are also observed. The UBV data for the 2009-2011 interval are well reproduced by a superposition of two periodic components with P=170 d and P=141 d (or P=217 d - the one year alias of P=141 d). The variation became less regular after 2011, the timescale increased and exceeded 250 days. In 2009-2016 the annual average brightness monotonically increased in V, J, K, whereas it decreased in U and B. The annual average U-B, B-V, and J-K colors grew, the star was getting redder. The cooling and expanding of the star photosphere along with the increasing of luminosity may explain the long-term trend in brightness and colors. Based on our photometric data we suppose that the photosphere temperature decreased by ~400 K in the 2008-2016 interval, the radius increased by ~24 %, and the luminosity grew by ~19 %. Low-resolution spectra in the λ4000-9000 Å wavelength range obtained in 2016-2017 indicate significant changes in the spectrum of V1427 Aql as compared with the 1994-2008 interval, i.e. the BaII and near-infrared CaII triplet absorptions have turned stronger while the OI λ7771-4 triplet blend has weakened that points out the decrease of temperature in the region where the absorptions are formed. The evolutionary stage of the star is discussed. We also compare V1427 Aql with post-AGB stars and yellow hypergiants.

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arXiv2018

Spectroscopic and photometric observations of Symbiotic Nova PU Vul during 2009-2016

A new set of low-resolution spectral and UBVJHKL-photometric observations of the symbiotic nova PU Vul is presented. The binary has been still evolving after the symbiotic nova outburst in 1977 and now it's in the nebular stage. It is found that the third orbital cycle (after 1977) was characterized by great changes in light curves. Now PU Vul demonstrates a sine-wave shape of all light curves (with an amplitude in the U band of about 0.7 mag), which is typical for symbiotic stars in quiescent state. Brightness variability due to cool component pulsations is now clearly visible in the VRI light curves. The amplitude of the pulsations increases from 0.5 mag in V band to 0.8 mag in I band. These two types of variability, as well as a very slow change of the hot component physical parameters due to evolution after the outburst of 1979, influence the spectral energy distribution of the system. The emission lines variability is highly complex. Only hydrogen lines fluxes vary with orbital phase. An important feature of the third orbital cycle is the first appearance of the OVI, 6828A Raman scattering line. We determined the hot component temperature by means of Zanstra method applied to the He II, 4686 line. Our estimate is about 150000 K for the spectrum obtained near orbital maximum in 2014. The VO spectral index derived near pulsation minimum corresponds to M6 spectral class for the cool component of PU Vul.

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arXiv2017

Optical and J,K-photometry of black hole X-ray nova A0620-00 in passive and active stages of quiescence

Photometric observations of the low-mass X-ray binary system A0620-00=V616 Mon are performed in the optical (unfiltered light, lambda_eff~6400A) and the near-infrared J and K-bands. The mean system flux, the orbital light curve shape and the flickering amplitude dependences on wavelength are examined for two activity stages of the system remaining in quiescence. In 2015-16 A0620-00 was in passive stage (as by Cantrell et al., 2008) exhibiting the regular orbital light curves and low flickering. In less than 230 days in 2016-17 the system switched into active stage: the brightness increased by ~0.2-0.3 mag, the orbital light curve changed while the flickering amplitude increased more than twice. The object regular orbital light curves were fitted by models with "cold" spots on the optical star surface and without those. These models reproduce the observed orbital light curves both in passive and in active stages. The dependence of the mean square flickering amplitude (in fluxes, extinction corrected) on wavelength is computed in the lambda 6400-22000AA range. In active stage, the observed flickering amplitude decreases over the whole studied range and may be represented as Delta F_fl~lambda^{-2} which corresponds to the free-free emission of optically thin high-temperature plasma. In passive stage, flickering obeys Delta F_fl~lambda^{-4} law in the range 6400-12500AA that corresponds to the thermal radiation of optically thick high-temperature plasma. At longer wavelengths the flickering amplitude dependence is flat which may imply existence of a synchrotron component of the relativistic jets emission. These flickering features let us propose that the flickering mechanism includes at least two components: thermal and, apparently, synchrotron, that agrees with the recent discovery of the variable linear polarization of the IR system emission (Russell et al., 2016).

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arXiv2017

The jet of the young star RW Aur A and related problems

Comparing the images of the jet of the young star RW Aur A, separated by a period of 21.3 years, we found that the outermost jet's knots have emerged $\approx 350$ yr ago. We argue that at that moment the jet itself has appeared and intensive accretion onto the star has began due to the rearrangement of its protoplanetary disk structure caused by the tidal effect of the companion RW Aur B. More precisely, we assume that the increase of accretion is a response to changing conditions in the outer disk regions, which followed after the sound wave, generated by these changes, crossed the disk in a radial direction. The difference in the parameters of the blue and red lobes of the RW Aur A jet, according to our opinion, is a result of the asymmetric distribution of the circumstellar matter above and below the disk, due to a fly-by of the companion. It was found from the analysis of RW Aur historical light curve that deep and long $(Δt>150$ days) dimmings of RW Aur A observed after 2010 yr, had no analogues in the previous 110 years. We also associate the change in the character of the photometric variability of the star with the rearrangement of the structure of inner $(r<1$ a.u.) regions of its protoplanetary disk and discuss why these changes began only 350 years after the beginning of the active accretion phase.

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arXiv2016

Photometric and Spectroscopic Study of the Supergiant with an Infrared Excess V1027 Cygni

We present the results of our $UBV$ and $JHKLM$-photometry for the semiregular pulsating variable V1027~Cyg, a supergiant with an infrared excess, over the period from 1991 to 2015. Our search for a periodicity in the $UBV$ brightness variations has led to several periods from $P=212^{d}$ to $P=320^{d}$ in different time intervals. We have found the period $P=237^{d}$ based on our infrared photometry. The variability amplitude, the light-curve shape, and the magnitude of V1027~Cyg at maximum light change noticeably from cycle to cycle. An ambiguous correlation of the $B-V$ and $U-B$ colors with the brightness has been revealed. The spectral energy distribution for V1027~Cyg from our photometry in the range 0.36 ($U$)-5.0 ($M$) $μ$m corresponds to spectral types from G8I to K3I at different phases of the pulsation cycle. Low-resolution spectra of V1027 Cyg in the range $λ$4400--9200 Å were taken during 16 nights over the period 1995--2015. At the 1995 and 2011 photometric minima the star's spectrum exhibited molecular TiO bands whose intensity corresponded to spectral types M0--M1, while the photometric data point to a considerably earlier spectral type. We hypothesize that the TiO bands are formed in the upper layers of the extended stellar atmosphere. We have measured the equivalent widths of the strongest absorption lines, in particular, the infrared Ca~II triplet in the spectrum of V1027~Cyg. The calcium triplet (Ca T) with $W_λ(\mathrm{Ca~T})=20.3\pm1.8$ Å as a luminosity indicator for supergiants places V1027 Cyg in the region of the brightest G--K supergiants. V1027 Cyg has been identified with the infrared source IRAS~20004+2955 and is currently believed to be a candidate for post-AGB stars. The evolutionary status of the star and its difference from other post-AGB objects are discussed.

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arXiv2015

IRAS 19135+3937: An SRd variable as interacting binary surrounded by a circumbinary disc

Semi-regular (SR) variables are not a homogeneous class and their variability is often explained due to pulsations and/or binarity. This study focuses on IRAS 19135+3937, an SRd variable with an infra-red excess indicative of a dusty disc. A time-series of high-resolution spectra, UBV photometry as well as a very accurate light curve obtained by the Kepler satellite, allowed us to study the object in unprecedented detail. We discovered it to be a binary with a period of 127 days. The primary has a low surface gravity and an atmosphere depleted in refractory elements. This combination of properties unambiguously places IRAS 19135+3937 in the subclass of post-Asymptotic Giant Branch stars with dusty discs. We show that the light variations in this object can not be due to pulsations, but are likely caused by the obscuration of the primary by the circumbinary disc during orbital motion. Furthermore, we argue that the double-peaked Fe emission lines provide evidence for the existence of a gaseous circumbinary Keplerian disc inside the dusty disc. A secondary set of absorption lines has been detected near light minimum, which we attribute to the reflected spectrum of the primary on the disc wall, which segregates due to the different Doppler shift. This corroborates the recent finding that reflection in the optical by this type of discs is very efficient. The system also shows a variable Halpha profile indicating a collimated outflow originating around the companion. IRAS 19135+3937 thus encompasses all the major emergent trends about evolved disc systems, that will eventually help to place these objects in the evolutionary context.

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arXiv2014

Variability and possible rapid evolution of the hot post-AGB stars Hen 3-1347, Hen 3-1428, and LSS 4634

We present the results of spectroscopic and photometric observations for three hot southern-hemisphere post-AGB objects, Hen 3-1347 = IRAS 17074-1845, Hen 3-1428 = IRAS17311-4924, and LSS 4634 = IRAS 18023-3409. In the spectrograms taken with the 1.9-m telescope of the South African Astronomical Observatory (SAAO) in 2012, we have measured the equivalent widths of the most prominent spectral lines. Comparison of the new data with those published previously points to a change in the spectra of Hen 3-1428 and LSS 4634 in the last 20 years. Based on ASAS data, we have detected rapid photometric variability in all three stars with an amplitude up to 0.3 mag, 0.4 mag in the $V$ band. A similarity between the patterns of variability for the sample stars and other hot protoplanetary nebulae is pointed out. We present the results of $UBV$ observations for Hen 3-1347, according to which the star undergoes rapid irregular brightness variations with maximum amplitudes $ΔV$=0.25 mag, $ΔB$=0.25 mag, and $ΔU$=0.30 mag and shows color-magnitude correlations. Based on archival data, we have traced the photometric history of the stars over more than 100 years. Hen 3-1347 and LSS 4634 have exhibited a significant fading on a long time scale. The revealed brightness and spectrum variations in the stars, along with evidence for their enhanced mass, may be indicative of their rapid post-AGB evolution.

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arXiv2011

Variability of Hot Post-AGB Star IRAS 19336-0400 in the Early Phase of its Planetary Nebula Ionization

We present photoelectric and spectral observations of a hot candidate proto-planetary nebula - early B-type supergiant with emission lines in spectrum - IRAS 19336-0400. The light and color curves display fast irregular brightness variations with maximum amplitudes dV=0.30 mag, dB=0.35 mag, dU=0.40 mag and color-brightness correlations. By the variability characteristics IRAS 19336-0400 appears similar to other hot proto-planetary nebulae. Based on low-resolution spectra in the range lambda 4000-7500 A we have derived absolute intensities of the emission lines H_alpha, H_beta, H_gamma, [SII], [NII], physical conditions in gaseous nebula: n_e=10^4 cm^{-3}, T_e=7000 \pm 1000 K. The emission line H_alpha, H_beta equivalent widths are found to be considerably variable and related to light changes. By UBV-photometry and spectroscopy the color excess has been estimated: E(B-V)=0.50-0.54. Joint photometric and spectral data analysis allows us to assume that the star variability is caused by stellar wind variations.

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arXiv2011

Variability of Hot Supergiant IRAS 19336-0400 in the Early Phase of its Planetary Nebula Ionization

We present photoelectric and spectral observations of a hot candidate proto-planetary nebula - early B-type supergiant with emission lines in spectrum - IRAS 19336-0400. The light and color curves display fast irregular brightness variations with maximum amplitudes Delta V=0.30 mag, Delta B=0.35 mag, Delta U=0.40 mag and color-brightness correlations. By the variability characteristics IRAS 19336-0400 appears similar to other hot proto-planetary nebulae. Based on low-resolution spectra in the range lambda 4000-7500 A we have derived absolute intensities of the emission lines H_alpha, H_beta, H_gamma, [SII], [NII], physical conditions in gaseous nebula: n_e=10^4 cm^{-3}, T_e=7000 \pm 1000 K. The emission line H_alpha, H_beta equivalent widths are found to be considerably variable and related to light changes. By UBV-photometry and spectroscopy the color excess has been estimated: E_{B-V}=0.50-0.54. Joint photometric and spectral data analysis allows us to assume that the star variability is caused by stellar wind variations.

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