arXiv subjects

Sergey Karpov

Publications and source records attributed to Sergey Karpov.

At least 37 records · Page 2Linked to original sources

arXiv2025

New ultracool dwarf candidates from multi-epoch WISE data

Thirty years after the discovery of brown dwarfs, the search for these objects continues, particularly in the vicinity of the Sun. Objects near the Sun are characterized by large proper motions, making them seen as fast-moving objects. While the Gaia DR3 catalogue is a comprehensive source of proper motions, it lacks the depth needed for discovering fainter objects. Modern multi-epoch surveys, with their greater depth, offer a new opportunity for systematic search for ultracool dwarfs. The study aims to systematically search for high proper motion objects using the newly released catalogue of epochal WISE data in order to identify new brown dwarf candidates in the solar neighborhood, estimate their spectral types, distances and spatial velocities. We used recently released unTimely catalogue of epochal detections in unWISE coadds to search for objects with high proper motions using simple motion detection algorithm, combined with machine learning based artefact rejection routine. This method was used to identify objects with proper motions exceeding approximately 0.3 arcseconds per year. The identified objects were then cross-referenced with data from other large-scale sky surveys to further analyze their characteristics. The search yielded 21,885 moving objects with significant proper motions, 258 of which had not been previously published. All except 6 of them are compatible with being ultracool dwarfs. Among these, at least 33 were identified as most promising new T dwarf candidates, with estimated distances closer than about 40 parsecs, and effective temperatures less than 1300 K.

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arXiv2024

M dwarfs quasi-periodic pulsations at a time resolution of 1 s

Quasi-periodic pulsations (QPPs) of Sun and stars are challenging for stellar flare models. The white light stellar QPPs in the periodicity region of tens of second are unexplored yet. On the basis of observations with the 6-m telescope BTA in U-band of flaring dM-stars EV Lac, Wolf 359, Wolf 424, V577 Mon and UV Ceti we found 13 new QPPs. This composes 30% occurrence among 44 worked flares. These QPPs were found to have periods ranging from 6 to 107 seconds and were detected using both Fourier transform and empirical mode decomposition methods. The observed QPPs were categorized by the evolution of their oscillation envelope and fractional flux amplitudes. There are shown the statistically significant correlations of the QPP period with the duration, the equivalent duration and the amplitude of a flare, and the correlation between the QPP amplitude and flare amplitude.

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arXiv2024

STDWeb: Simple Transient Detection pipeline for the Web

We present a simple web-based tool, STDWeb, for a quick-look photometry and transient detection in astronomical images. It tries to implement a self-consistent and mostly automatic data analysis workflow that would work on any image uploaded to it, allowing to perform basic interactive masking, do object detection, astrometrically calibrate the image, and build the photometric solution based on a selection of catalogues and supported filters, optionally including the color term and positionally varying zero point. It also allows you to do image subtraction using either user-provided or automatically downloaded template images, and do a forced photometry for a specified target in either original or difference images, as well as transient detection with basic rejection of artefacts. The tool may be easily deployed allowing its integration into the infrastructure of robotic telescopes or data archives for effortless analysis of their images.

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arXiv2024

Characterisation of the Atmosphere in Very High Energy Gamma-Astronomy for Imaging Atmospheric Cherenkov Telescopes

Ground-based observations of Very High Energy (VHE) gamma rays from extreme astrophysical sources are significantly influenced by atmospheric conditions. This is due to the atmosphere being an integral part of the detector when utilizing Imaging Atmospheric Cherenkov Telescopes (IACTs). Clouds and dust particles diminish atmospheric transmission of Cherenkov light, thereby impacting the reconstruction of the air showers and consequently the reconstructed gamma-ray spectra. Precise measurements of atmospheric transmission above Cherenkov observatories play a pivotal role in the accuracy of the analysed data, among which the corrections of the reconstructed energies and fluxes of incoming gamma rays, and in establishing observation strategies for different types of gamma-ray emitting sources. The Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes and the Cherenkov Telescope Array Observatory (CTAO), both located on the Observatorio del Roque de los Muchachos (ORM), La Palma, Canary Islands, use different sets of auxiliary instruments for real-time characterisation of the atmosphere. In this paper, historical data taken by MAGIC LIDAR (LIght Detection And Ranging) and CTAO FRAM (F/Photometric Robotic Telescope) are presented. From the atmospheric aerosol transmission profiles measured by the MAGIC LIDAR and CTAO FRAM aerosol optical depth maps, we obtain the characterisation of the clouds above the ORM at La Palma needed for data correction and optimal observation scheduling.

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arXiv2024

Early-Time Observations of SN 2023wrk: A Luminous Type Ia Supernova with Significant Unburned Carbon in the Outer Ejecta

We present extensive photometric and spectroscopic observations of the nearby Type Ia supernova (SN) 2023wrk at a distance of about 40 Mpc. The earliest detection of this SN can be traced back to a few hours after the explosion. Within the first few days the light curve shows a bump feature, while the B - V color is blue and remains nearly constant. The overall spectral evolution is similar to that of an SN 1991T/SN 1999aa-like SN Ia, while the C II $\lambda6580$ absorption line appears to be unusually strong in the first spectrum taken at $t \approx -$15.4 days after the maximum light. This carbon feature disappears quickly in subsequent evolution but it reappears at around the time of peak brightness. The complex evolution of the carbon line and the possible detection of Ni III absorption around 4700 Å and 5300 Å in the earliest spectra indicate macroscopic mixing of fuel and ash. The strong carbon lines is likely related to collision of SN ejecta with unbound carbon, consistent with the predictions of pulsational delayed-detonation or carbon-rich circumstellar-matter interaction models. Among those carbon-rich SNe Ia with strong C II $\lambda6580$ absorption at very early times, the line-strength ratio of C II to Si II and the B-V color evolution are found to exhibit large diversity, which may be attributed to different properties of unbound carbon and outward-mixing $^{56}$Ni.

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arXiv2023

LAMOST J040901.83+329355.6 -- a new Galactic star with Wolf--Rayet characteristics on a post-AGB to CSPN transitional stage

The similarity in physical conditions in winds of low-mass post-asymptotic giant branch stars and evolved massive stars leads to the appearance of an interesting phenomenon of spectral mimicry. Due to that the discovery of every new star with Wolf--Rayet spectrum requires special study of its evolutionary status before it may be included in the list of Galactic Wolf--Rayet (WR) stars. A couple of years ago LAMOST J040901.83+323955.6 (hereafter J0409+3239) was selected as a WR star in LAMOST spectroscopic database by machine learning methods. In this work we investigated its evolutionary status. Analyzing the spatial location of J0409+3239, in the Galaxy and its position in the color-magnitude diagram we concluded what J0409+3239, is instead a low mass object with WR phenomenon. Its luminosity is $L*=1000 L_\odot$ and effective temperature $T_{\rm eff}$=40,000 K. Using new and archival photometric data we detected irregular variability on time scales from hours to tens of days with amplitude up to $\approx0.2$ mag. Comparison of the spectrum obtained in 2022 with the one from 2014 also shows an evidence of spectral variability. The absence of clearly detected circumstellar nebula does not allow to classify J0409+3239, as [WR], i.e. a central star of planetary nebula (CSPN). However, its position in Hertzsprung--Russell diagram suggests that J0409+3239, is a low mass star caught in rare transitional phase to CSPN. Estimation of J0409+3239, mass based on evolutionary tracks shows that it is less than $0.9~M_\odot$, and thus the age of the Galaxy is barely enough for the star to evolve to its current stage.

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arXiv2023

SAINT (Small Aperture Imaging Network Telescope) -- a wide-field telescope complex for detecting and studying optical transients at times from milliseconds to years

(Abridged) In this paper, we present a project of multi-channel wide-field optical sky monitoring system with high temporal resolution -- Small Aperture Imaging Network Telescope (SAINT) -- mostly built from off-the-shelf components and aimed towards searching and studying optical transient phenomena on the shortest time scales. The instrument consists of 12 channels each containing 30cm (F/1.5) objectives mounted on separate mounts with pointing speeds up to 50deg/s. Each channel is equipped with a 4128x4104 pixel, and a set of photometric $griz$ filters and linear polarizers. At the heart of every channel is a custom built reducer-collimator module allowing rapid switching of an effective focal length of the telescope -- due to it the system is capable to operate in either wide-field survey or narrow-field follow-up modes. In the first case, the field of view of the instrument is 470 square degrees and the detection limits (5$σ$ level at 5500$Å$) are 12.5-21 mag for exposure times of 20 ms - 20 min correspondingly. In the second, follow-up regime, all telescopes are oriented towards the single target, and SAINT becomes an equivalent to a 1m telescope, with the field of view reduced to 11$'$ x 11$'$, and the exposure times decreased down to 0.6 ms. Different channels may then have different filters installed, thus allowing a detailed study -- acquiring both color and polarization information -- of a target object with highest possible temporal resolution. The operation of SAINT will allow acquiring an unprecedented amount of data on various classes of astrophysical phenomena, from near-Earth to extragalactic ones, while its multi-channel design and the use of commercially available components allows easy expansion of its scale, and thus performance and detection capabilities.

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arXiv2023

The rate of satellite glints in ZTF and LSST sky surveys

We assess the impact of satellite glints -- rapid flashes produced by reflections of a sunlight from flat surfaces of rotating satellites -- on current and future deep sky surveys such as the ones conducted by the Zwicky Transient Facility (ZTF) and the Vera C. Rubin Observatory upcoming Legacy Survey of Space and Time (LSST). In addition to producing a large number of streaks polluting the images, artificial satellites and space debris also generate great amount of false point-source alerts hindering the search for new rapid astrophysical transients. To investigate the extent of this problem, we perform an analysis of isolated single frame events detected by ZTF in more than three years of its operation, and, using three different methods, assess the fraction of them related to artificial satellites to be at least 20\%. The satellites causing them occupy all kinds of orbits around the Earth, and the duration of flashes produced by their rotation is from a fraction of a second down to milliseconds, with mean all-sky rate of up to 80,000 per hour.

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arXiv2023

Large-Scale Ejecta of Z CMa -- Proper Motion Study and New Features Discovered

Z Canis Majoris is a fascinating early-type binary with a Herbig Be primary and a FU Orionis-type secondary. Both of the stars exhibit sub-arcsecond jet-like ejecta. In addition, the primary is associated with the extended jet as well as with the large-scale outflow. In this study, we investigate further the nature of the large-scale outflow, which has not been studied since its discovery almost three and a half decades ago. We present proper motion measurements of individual features of the large-scale outflow and determine their kinematical ages. Furthermore, with our newly acquired deep images, we have discovered additional faint arc-shaped features that can be associated with the central binary.

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arXiv2023

Revealing the Binarity of HD 36030 -- One of the Hottest Flare Stars

The Kepler and TESS space missions significantly expanded our knowledge of what types of stars display flaring activity by recording a vast amount of super-flares from solar-like stars, as well as detecting flares from hotter stars of A-F spectral types. Currently, we know that flaring occurs in the stars as hot as B-type ones. However, the structures of atmospheres of hot B-A stars crucially differ from the ones of late types, and thus the occurrence of flaring in B-A type stars requires some extension of our theoretical views of flare formation and therefore a detailed study of individual objects. Here we present the results of our spectral and photometric study of HD 36030, which is a B9 V star with flares detected by the TESS satellite. The spectra we acquired suggest that the star is in a binary system with a low-mass secondary component, but the light curve lacks any signs of periodic variability related to orbital motion or surface magnetic fields. Because of that, we argue that the flares originate due to magnetic interaction between the components of the system.

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arXiv2022

Prompt emission and early optical afterglow of VHE detected GRB 201015A and GRB 201216C: onset of the external forward shock

We present a detailed prompt emission and early optical afterglow analysis of the two very high energy (VHE) detected bursts GRB 201015A and GRB 201216C, and their comparison with a subset of similar bursts. Time-resolved spectral analysis of multi-structured GRB 201216C using the Bayesian binning algorithm revealed that during the entire duration of the burst, the low energy spectral index ($α_{\rm pt}$) remained below the limit of the synchrotron line of death. However, statistically some of the bins supported the additional thermal component. Additionally, the evolution of spectral parameters showed that both peak energy (Ep) and $α_{\rm pt}$ tracked the flux. These results were further strengthened using the values of the physical parameters obtained by synchrotron modeling of the data. Our earliest optical observations of both bursts using FRAM-ORM and BOOTES robotic telescopes displayed a smooth bump in their early optical light curves, consistent with the onset of the afterglow due to synchrotron emission from an external forward shock. Using the observed optical peak, we constrained the initial bulk Lorentz factors of GRB 201015A and GRB 201216C to $Γ_0$ = 204 and $Γ_0$ = 310, respectively. The present early optical observations are the earliest known observations constraining outflow parameters and our analysis indicate that VHE-detected bursts could have a diverse range of observed luminosity within the detectable redshift range of present VHE facilities.

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arXiv2022

How long can LBVs sleep? A long-term photometric vaiability and spectral study of the Galactic candidate luminous blue variable MN112

Luminous Blue Variables (LBVs) are massive stars that show strong spectral and photometric variability. The question of what evolutionary stages they represent and what exactly drives their instability is still open, and thus is it important to understand whether LBVs without significant ongoing activity exist, and for how long such dormant LBVs may ``sleep''. In this article we investigate the long-term variability properties of the LBV candidate MN112, by combining its optical and infrared spectral data covering 12 years with photometric data covering nearly a century acquired by both modern time-domain sky surveys and historical photographic plates. We analyze the spectra, derive physical properties of the star by modelling its atmosphere and use a new distance estimate from Gaia data release 3 (DR3) to determine the position of MN112 both inside the Galaxy and in the Hertzsprung--Russell diagram. Distance estimation increased in almost two times in comparison with Gaia DR2. Due to that MN112 moved in upper part of the diagram and according to our modeling it lies on evolutionary track for star with initial mass $M_*=70 M_\odot$ near Humphreys--Davidson limit. Given the absence of any significant variability we conclude that the star is a dormant LBV that has been inactive for at least a century now.

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arXiv2022

GRB 190919B: Rapid optical rise explained as a flaring activity

Following the detection of a long GRB 190919B by INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory), we obtained an optical photometric sequence of its optical counterpart. The light curve of the optical emission exhibits an unusually steep rise ~100 s after the initial trigger. This behaviour is not expected from a 'canonical' GRB optical afterglow. As an explanation, we propose a scenario consisting of two superimposed flares: an optical flare originating from the inner engine activity followed by the hydrodynamic peak of an external shock. The inner-engine nature of the first pulse is supported by a marginal detection of flux in hard X-rays. The second pulse eventually concludes in a slow constant decay, which, as we show, follows the closure relations for a slow cooling plasma expanding into the constant interstellar medium and can be seen as an optical afterglow sensu stricto.

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arXiv2022

Impact of satellite glints on the transient science on ZTF scale

Thousands of active artificial objects are orbiting around Earth along with much more non-operational ones -- derelict satellites or rocket bodies, collision debris, or spacecraft payloads, significant part of them being uncatalogued. They all impact observations of the sky by ground-based telescopes by producing a large number of streaks polluting the images, as well as generating false alerts hindering the search for new astrophysical transients. While the former threat for astronomy is widely discussed nowadays in regard of rapidly growing satellite mega-constellations, the latter one -- false transients -- still lacks attention on the similar level. In this work we assess the impact of satellite glints -- rapid flashes produced by reflections of a sunlight from flat surfaces of rotating satellites -- on current and future deep sky surveys such as the ones conducted by the Zwicky Transient Facility (ZTF) and the Vera Rubin Observatory Legacy Survey of Space and Time (LSST). For that, we propose a simple routine that detects, in a single exposure, a series of repeated flashes along the trajectories of otherwise invisible satellites, and describe its implementation in FINK alert broker. Application of the routine to ZTF alert stream revealed about 73,000 individual events polluting 3.6\% of all ZTF science images between November 2019 and December 2021 and linked to more than 300 different glinting satellites on all kinds of orbits, from low-Earth up to geostationary ones. The timescales of individual flashes are as short as $0.1$--$10^{-3}$ seconds, with instant brightness of 4--14 magnitudes, peak amplitudes of at least 2--4 magnitudes, and generally complex temporal patterns of flashing activity. We expect LSST to see much more such satellite glints of even larger amplitudes due to its better sensitivity.

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arXiv2021

Exceptionally bright optical emission from a rare and distant $γ-$ray burst

Long $\rm γ$-ray bursts (GRBs) are produced by the dissipation of ultra-relativistic jets launched by newly-born black holes after the collapse of massive stars. Right after the luminous and highly variable $γ$-ray emission, the multi-wavelength afterglow is released by the external dissipation of the jet in circumburst medium. We report the discovery of a very bright ($\rm \sim 10$ mag) optical emission $\rm \sim 28$ s after the explosion of the extremely luminous and energetic GRB 210619B located at redshift 1.937. Early multi-filter observations allowed us to witness the end of the shock wave propagation into the GRB ejecta. We observed the spectral transition from a bright reverse to the forward shock emission, demonstrating that the early and late GRB multi-wavelength emission is originated from a very narrow jet propagating into an unusually rarefied interstellar medium. We also find evidence of an additional component of radiation, coming from the jet wings which is able explain the uncorrelated optical/X-ray emission.

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arXiv2021

Search for Ultraviolet Luminous Objects in GALEX data

Selection of extreme objects in the data from large-scale sky surveys is a powerful tool for the detection of new classes of astrophysical objects or rare stages of their evolution. The cross-matching of catalogues and analysis of the color indices of their objects is a usual approach for this problem which has already provided a lot of interesting results. However, the analysis of objects that are found in only one of the surveys, and absent in all others, should also attract close attention, as it may lead to the discovery of both transients and objects with extreme color values. Here we report on the initial study aimed at the detection of objects with a significant UV excess in their spectra by cross-matching of the GALEX all-sky catalogue with several other surveys in different wavelength ranges and analyzing the ones visible in GALEX only, or having extreme UV to optical colors (ultraviolet luminous objects). We describe the methodology for such investigation, explain the selection of surveys for this study, and show the initial results based on the search in a small fraction of the sky. We uncovered several prominent UV-only objects lacking the counterparts in the catalogues of longer wavelengths, and discuss their possible nature. We also detected a single source showing an extreme UV to optical color and corresponding to UV flare on a cool sdM subdwarf star. Finally, we discuss the possible populations of objects that may be revealed in a future larger-scale analysis of this kind.

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arXiv2021

A New Method for Aerosol Measurement using Wide-field Photometry

We present a new method to measure the vertical aerosol optical depth (VAOD) during clear nights using a wide-field imager - a CCD camera with a photographic lens on an equatorial mount. A series of 30-second exposures taken at different altitudes above the horizon can be used to measure the VAOD with a precision better than 0.008 optical depths within a few minutes. Such a measurement does not produce any light and is thus suitable for use at sites where other astronomical instruments are located. The precision of the VAOD measurement depends on laboratory calibration of spectral properties of the system and of the response of the camera electronics to varying illumination levels, as well as careful considerations of details of stellar photometry and modelling of the dependence of measured stellar fluxes star color and position within the field of view. The results obtained with robotic setups at the future sites of the Cherenkov Telescope Array show good internal consistency and agreement with the simultaneous measurements from a Sun/Moon Photometer located at the same site.

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arXiv2021

Evaluation of scientific CMOS sensors for sky survey applications

Scientific CMOS image sensors are a modern alternative for a typical CCD detectors, as they offer both low read-out noise, large sensitive area, and high frame rates. All these makes them promising devices for a modern wide-field sky surveys. However, the peculiarities of CMOS technology have to be properly taken into account when analyzing the data. In order to characterize these, we performed an extensive laboratory testing of two Andor cameras based on sCMOS chips -- Andor Neo and Andor Marana. Here we report its results, especially on the temporal stability, linearity and image persistence. We also present the results of an on-sky testing of these sensors connected to a wide-field lenses, and discuss its applications for an astronomical sky surveys.

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