arXiv subjects

Dmitry Blinov

Publications and source records attributed to Dmitry Blinov.

At least 19 recordsLinked to original sources

arXiv2026

WALOP-South: a four camera one shot imaging polarimeter for the PASIPHAE survey. Paper III -- PSF modelling

The two WALOP instruments, built for the PASIPHAE survey, will measure the linear polarization of large numbers of stars in the Galactic polar regions in the SDSS-$r$ band. They are designed with a wide field-of-view, enabling measurement of the Stokes parameters $I$, $q$, and $u$ for multiple stars simultaneously within a $35^\prime \times 35^\prime$ region of sky for WALOP-South and $30^\prime \times 30^\prime$ region for WALOP-North. In this paper, we present a polar shapelet-based PSF photometry framework for well-sampled stellar point sources applicable to WALOP-type wide-field polarimeters. Polar shapelets are a set of orthogonal basis functions, constructed from Gauss-Hermite or Gauss-Laguerre polynomials, that are well-suited to modelling localized PSF in a compact and efficient way. We developed an efficient PSF modelling method that uses polar shapelets as basis functions to reconstruct the spatial variation of the PSF shape across the CCD using Zemax-simulated images of one of the WALOP instruments, and show that a limited number of shapelet coefficients are sufficient to capture this variation consistently across different CCD locations. To simulate realistic star images, we introduce random sub-pixel shifts in the star centroids in the Zemax-simulated images, and account for this using a two-step iterative method that alternately estimates the PSF model and the sub-pixel centroid shift. Applying PSF photometry to the target faint stars, we demonstrate that the photometric accuracy of approximately 0.15% is achievable, and that the reconstructed PSF model can be incorporated into the photometry across different seeing conditions, meeting the polarimetric science requirements of the PASIPHAE survey.

astro-ph.IM↗

arXiv2026

Spectroscopic redshifts of selected flat-spectrum radio sources I

We present the first results of a spectroscopic campaign carried out as part of the Search for Milli-Lenses (SMILE) program, which aims to constrain the prevalence of gravitational lens systems on milli-arcsecond angular scales (milli-lenses) using high-resolution Very Long Baseline Interferometry (VLBI) imaging. The SMILE parent sample contains ~ 5000 radio-loud active galaxies, selected as a flux-limited, complete subsample of CLASS (The Cosmic Lens All-Sky Survey) sources. We compiled redshift information for the full sample from multiple literature and catalog sources and found that 491 sources have no available redshift estimate, either spectroscopic or photometric. A further 948 sources have only photometric redshifts, many of which show substantial discrepancies between catalogs. Reliable redshifts are essential for VLBI radio-source studies because they convert angular measurements into physical linear scales, enable estimates of intrinsic luminosities and jet kinematics, and allow robust cosmological and population studies. To address this key limitation for lensing and population studies, we initiated a dedicated spectroscopic campaign to secure reliable redshifts for as many targets as possible. This paper focuses on the brightest sources in the SMILE sample. We report newly determined spectroscopic redshifts for 6 targets out of 15 observed with the Skinakas 1.3 m telescope.

astro-ph.CO↗

arXiv2026

DHARA: Data Handling and Automated Reduction pipeline for AIMPOL

We present an automated data-reduction and analysis for optical linear polarimetric data obtained from a dual-beam polarimeter. The pipeline is optimized for observations acquired with the ARIES Imaging POLarimeter mounted at the Cassegrain focus of the 1.04-m Sampurnanand Telescope at ARIES. It is implemented using interactive Python routines that process raw images to derive the Stokes parameters, from which the degree of polarization and polarization angle are computed along with their uncertainties. The pipeline framework is designed to handle the reduction of both single-source and crowded-field observations. The identification of extraordinary (e-ray) and ordinary (o-ray) image pairs is a crucial step and is performed using different strategies for single-source and crowded-field data, while subsequent stages, such as photometric and polarimetric analysis, follow a common procedure. We validate the pipeline using observations of polarized standard stars acquired over multiple epochs between 2017 to 2025, and compare the results with values reported in the literature. To demonstrate the applicability of the pipeline to crowded-field observations, we apply it to polarimetric data of the Alessi~1 open cluster and compare with previously published results derived using traditional reduction methods. In both cases, the polarization parameters derived using the pipeline agree with literature values within $2\sigma$ uncertainties. Although developed for AIMPOL, the pipeline is readily adaptable to any dual-channel imaging polarimeter in which the e-ray and o-ray images are recorded in a FITS image from a single CCD.

astro-ph.IM↗

arXiv2026

Automating the detection of polarization angle rotations in blazars. Re-analysis of RoboPol data reveals 27 new rotations

We present an automated pipeline for the detection of EVPA rotations in blazars, integrating correction of the 180$^\circ$ ambiguity, Bayesian Blocks segmentation, and statistical validation. Applied to RoboPol monitoring data, the method identified 48 rotations across 25 sources, including multiple events in RBPLJ2232+1143, RBPLJ1751+0939, RBPLJ1800+7828, and RBPLJ2253+1608. The rotations span amplitudes from 90.8$^\circ$ to 359.7$^\circ$, durations between 7.0 and 111.3 days, and rotation rates averaging 5.0$^\circ$/day. Comparison with previous catalogs reveals systematic differences: Bayesian Blocks rotations are on average $\sim$10\% larger in amplitude, about twice as long in duration, and roughly two-thirds slower in angular velocity, reflecting systematic biases between adaptive binning and manual segmentation. In addition, we report 27 previously unreported rotations, including 11 from the final 2016--2017 season. A correlation analysis with contemporaneous Fermi--LAT $\gamma$-ray light curves shows that longer rotations tend to coincide with enhanced $\gamma$-ray activity, while rotation amplitude alone is not predictive of $\gamma$-ray brightness. Our pipeline minimizes subjective biases, expands the list of known EVPA rotations, and provides a reproducible framework for future multiwavelength studies of blazar jet dynamics and particle acceleration.

astro-ph.HE↗

arXiv2026

Tracking down the broadband polarimetric properties of PG 1553+113

We report on a nine-month monitoring campaign of the blazar PG 1553+113, relying on three observations carried out in 2025 with the Imaging X-ray Polarimetry Explorer (IXPE) and supported by multi-wavelength facilities. The source displayed pronounced variability across the electromagnetic spectrum, with X-ray flux changes by up to a factor of $\sim5$ and complex evolution of the optical polarization properties, including one of the largest (exceeding $150^{\circ}$) and fastest rotations in the electric vector position angle (EVPA) ever recorded. This swing of the EVPA was also accompanied by a temporary drop of the optical polarization degree to nearly zero. Significant X-ray polarization was observed during the third IXPE pointing, with a polarization degree $\Pi_{\rm X}\,=(\,18.4\,\pm\,5.8)\%$ and $\Psi_{\rm X}\,=\,74^{\circ} \pm 9^{\circ}$ in the 2--8~keV band, while only upper limits were obtained in the first two epochs. The optical data show that the second IXPE observation occurred shortly after a dramatic optical polarization event characterized by a rapid EVPA swing and strong depolarization. Two possible scenarios may explain the broadband polarimetric behavior: (i) the superposition of two emitting regions with nearly orthogonal magnetic field configurations and variable relative contributions, and (ii) the interaction of a single emitting region with a shock that temporarily reorders the magnetic field. In both cases, the data support a picture in which the X-ray and optical emissions arise from closely related but not strictly co-spatial regions within a dynamically evolving, magnetically structured jet.

astro-ph.HE↗

arXiv2025

Intra-night optical polarization monitoring of blazars

Blazars are known for their extreme variability across the electromagnetic spectrum. Variability at very short timescales can push the boundaries between competing models offering us much needed discriminating power. This is particularly true for polarization variability that allows us to probe particle acceleration and high-energy emission models in blazars. Here we present results from the first pilot study of intra-night optical polarization monitoring conducted using RoboPol at the Skinakas Observatory and supplemented by observations from the Calar Alto, Perkins, and Sierra Nevada observatories. Our results show that while variability patterns can widely vary between sources, variability on timescales as short as minutes is prevalent in blazar jets. The amplitude of variations are typically small, a few percent for the polarization degree and less than 20 degrees for the polarization angle, pointing to a significant contribution to the optical emission from a turbulent magnetic field component, while the overall stability of the polarization angle over time points to a preferred magnetic field orientation.

astro-ph.HE↗

arXiv2025

Evidence for cloud-to-cloud variations in the ratio of polarized thermal dust emission to starlight polarization

The correlation between optical starlight polarization and polarized thermal dust emission can be used to infer intrinsic dust properties. This correlation is quantified by the ratio Rp/p, which has been measured to be 5.42 +/- 0.05 MJy sr^-1 at 353 GHz when averaged over large areas of the sky. We investigate this correlation using newly published stellar polarimetric data densely sampling a continuous sky region of ~4 square degrees at intermediate Galactic latitude. We combine RoboPol optical polarization measurements for 1,430 stars with submillimeter data from the Planck satellite at 353 GHz. We perform linear fits between the Planck (Qs, Us) and optical (qv, uv) Stokes parameters, accounting for the differences in resolution between the two datasets as well as the distribution of clouds along the line of sight. We find that in this region of the sky the Rp/p value is 3.67 +/- 0.05 MJy sr^-1, indicating a significantly shallower slope than that found previously using different stellar samples. We also find significant differences in the fitted slopes when fitting the Qs-qv and Us-uv data separately. We explore two explanations using mock data: miscalibration of polarization angle and variations in Rp/p along the line of sight due to multiple clouds. We show that the former can produce differences in the correlations of Qs-qv and Us-uv, but large miscalibration angles would be needed to reproduce the magnitude of the observed differences. Our simulations favor the interpretation that Rp/p differs between the two dominant clouds that overlap on the sky in this region. The difference in Rp/p suggests that the two clouds may have distinct dust polarimetric properties. With knowledge from the tomographic decomposition of the stellar polarization, we find that one cloud appears to dominate the correlation of Us-uv, while both clouds contribute to the correlation of the Qs-qv data.

astro-ph.GA↗

arXiv2025

Unveiling blazar synchrotron emission: a multiwavelength polarimetric study of HSP and LSP populations

Polarimetric properties of blazars allow us to put constraints on the acceleration mechanisms that fuel their powerful jets. By studying the multiwavelength polarimetric behaviour of high synchrotron peaked (HSP) and low synchrotron peaked (LSP) blazars, we aim to explore differences in their emission mechanisms and magnetic field structure in the acceleration region. In this study, we take advantage of several X-ray polarisation observations of HSP by the IXPE, including four new observations of Mrk 501, and optical polarisation observations of LSP from RoboPol and many others. We find that the polarisation degree (PD) distribution of HSP in X-rays is systematically higher than in optical and mm-radio wavelengths, as reported in previous IXPE publications. The distribution of the X-ray electric vector position angles (PA) is centered around the jet axis with most of the observations consistent with zero difference within uncertainties. In fact, the distribution of the offset of the PA from the jet axis is consistent between the LSP and HSP populations (with PA measured in optical for the first, X-ray for the latter), suggesting a common magnetic field structure close to the acceleration region, in strong support of the emerging energy stratified picture of particle acceleration followed by energy loss in blazar jets.

astro-ph.HE↗

arXiv2025

Wide Area Linear Optical Polarimeter Control Software

The WALOPControl software is designed to facilitate comprehensive control and operation of the WALOP (Wide Area Linear Optical Polarimeter) polarimeters, ensuring safe and concurrent management of various instrument components and functionalities. This software encompasses several critical requirements, including control of the filter wheel, calibration half-wave plate, calibration polarizer, guider positioning, focusers, and 4 concurrent CCD cameras. It also manages the host telescope and dome operations while logging operational parameters, user commands, and environmental conditions for troubleshooting and stability. It provides a user-friendly graphical user interface, secure access control, a notification system for errors, and a modular configuration for troubleshooting are integral to the software's architecture. It is accessible over the internet with the backend developed using NodeJS and ExpressJS, featuring a RESTful API that interacts with a MongoDB database, facilitating real-time status updates and data logging. The frontend utilizes the React.JS framework, with Redux for state management and Material UI for the graphical components. The system also allows for automatic observations based on user-defined schedules. A Continuous Integration and Continuous Deployment (CI CD) pipeline ensures the software's reliability through automated testing and streamlined deployment. The WALOPControl software is a key component of the PASIPHAE (Polar-Areas Stellar Imaging in Polarimetry High Accuracy Experiment) project, which aims to study the dust and magnetic field of the Milky Way by observing the polarization of starlight.

astro-ph.IM↗

arXiv2025

Determining the origin of the X-ray emission in blazars through multiwavelength polarization

The origin of the high-energy emission in astrophysical jets from black holes is a highly debated issue. This is particularly true for jets from supermassive black holes that are among the most powerful particle accelerators in the Universe. So far, the addition of new observations and new messengers have only managed to create more questions than answers. However, the newly available X-ray polarization observations promise to finally distinguish between emission models. We use extensive multiwavelength and polarization campaigns as well as state-of-the-art polarized spectral energy distribution models to attack this problem by focusing on two X-ray polarization observations of blazar BL Lacertae in flaring and quiescent $\gamma$-ray states. We find that regardless of the jet composition and underlying emission model, inverse-Compton scattering from relativistic electrons dominates at X-ray energies.

astro-ph.HE↗

arXiv2025

Variability of X-ray polarization of Cyg X-1

We present the results of a three-year X-ray, optical, and radio polarimetric monitoring campaign of the prototypical black hole X-ray binary Cyg X-1, conducted from 2022 to 2024. The X-ray polarization of Cyg X-1 was measured 13 times with the Imaging X-ray Polarimetry Explorer (IXPE), covering both hard and soft spectral states. The X-ray polarization degree (PD) in the hard state was found to be $\approx4.0\%$, roughly twice as high as in the soft state, where it was around $2.2\%$. In both states, a statistically significant increase of PD with the energy was found. Moreover, a linear relation between PD and spectral hardness suggests a gradual and continuous evolution of the polarization properties, rather than an abrupt change of polarization production mechanism between states. The polarization angle (PA) was independent of the spectral state and showed no trend with the photon energy. The X-ray PA is well aligned with the orientation of the radio jet, as well as the optical and radio PAs. We find significant orbital changes of PA in the hard state, which we attribute to scattering of X-ray emission at intrabinary structure. No significant superorbital variability in PD or PA was found at the period $P_{\rm{so}}$ = 294 d. We also find no correlation between the X-ray and optical polarization; if any, there is a long-term anti-correlation between the X-ray PD and the radio PD.

astro-ph.HE↗

arXiv2025

High optical to X-ray polarization ratio reveals Compton scattering in BL Lacertae's jet

Blazars, supermassive black hole systems (SMBHs) with highly relativistic jets aligned with the line of sight, are the most powerful long-lived emitters of electromagnetic emission in the Universe. We report here on a radio to gamma-ray multiwavelength campaign on the blazar BL Lacertae with unprecedented polarimetric coverage from radio to X-ray wavelengths. The observations caught an extraordinary event on 2023 November 10-18, when the degree of linear polarization of optical synchrotron radiation reached a record value of 47.5%. In stark contrast, the Imaging X-ray Polarimetry Explorer (IXPE) found that the X-ray (Compton scattering or hadron-induced) emission was polarized at less than 7.4% (3sigma confidence level). We argue here that this observational result rules out a hadronic origin of the high energy emission, and strongly favors a leptonic (Compton scattering) origin, thereby breaking the degeneracy between hadronic and leptonic emission models for BL Lacertae and demonstrating the power of multiwavelength polarimetry to address this question. Furthermore, the multiwavelength flux and polarization variability, featuring an extremely prominent rise and decay of the optical polarization degree, is interpreted for the first time by the relaxation of a magnetic "spring" embedded in the newly injected plasma. This suggests that the plasma jet can maintain a predominant toroidal magnetic field component parsecs away from the central engine.

astro-ph.HE↗

arXiv2025

Repeated pattern of $\gamma$-ray flares in the blazar PKS 1502+106 coincident with the IC190730A neutrino event

It has been demonstrated that at least 10 percent of the brightest blazars in the fourth Fermi-LAT catalog of $\gamma$-ray sources exhibit repeating patterns of $\gamma$-ray flares. These events may be associated with the presence of a non-uniform sheath surrounding a fast jet spine in some blazars. Theoretical models suggest that such a sheath could facilitate neutrino production in these structured jets. We aim to test the marginal statistical evidence previously reported for a connection between repeating patterns of $\gamma$-ray flares in blazars and high-energy neutrino events that are positionally consistent with these sources. We identified a repeating pattern of flares in the $\gamma$-ray light curve of the blazar PKS 1502+106, which lies within the 50\% uncertainty region of the IC190730A neutrino candidate event. This occurrence is combined with two other high-energy ($\ge 200$ TeV) neutrino events from ICECAT-1, which arrived in both positional and temporal coincidence with two blazars exhibiting ongoing repeating flare patterns. We conducted a Monte Carlo simulation to evaluate the likelihood of accidental coincidences between the repeating flare patterns and neutrino events, accounting for potential unrecognized systematic uncertainties in the arrival directions of the ICECAT-1 events. Our findings indicate the probability of a random coincidence, in both time and arrival direction for three high-energy neutrino candidates and three blazars with ongoing recurring patterns of $\gamma$-ray flares, is $1.56\times 10^{-3}$ ($3.2\sigma$).

astro-ph.HE↗

arXiv2024

The WALOP-North Instrument I: Optical Design, Filter Design, Calibration

The Wide Area Linear Optical Polarimeter North (WALOP-North) is an optical polarimeter designed for the needs of the PASIPHAE survey. It will be installed on the 1.3m telescope at the Skinakas Observatory in Crete, Greece. After commissioning, it will measure the polarization of millions of stars at high Galactic latitude, aiming to measure hundreds of stars per $deg^2$. The astronomical filter used in the instrument is a modified, polarimetrically-neutral broadband SDSS-r. This instrument will be pioneering one due to its large field-of-view (FoV) of $30\times 30$ $arcmin^2$ and high accuracy polarimetry measurements. The accuracy and sensitivity of the instrument in polarization fraction will be at the 0.1\% and 0.05\% level, respectively. Four separate 4k$\times$4k CCDs will be used as the instrument detectors, each imaging one of the $0\deg{}, 45\deg{}, 90\deg{}$ and $135\deg{}$ polarized FoV separately, therefore making the instrument a four-channel, one-shot polarimeter. Here, we present the overall optical design of the instrument, emphasizing on the aspects of the instrument that are different from WALOP-South. We also present a novel design of filters appropriate for polarimetry along with details on the management of the instrument size and its polarimetric calibration.

astro-ph.IM↗

arXiv2024

IXPE Observation of the Low-Synchrotron Peaked Blazar S4 0954+65 During An Optical-X-ray Flare

The X-ray polarization observations made possible with the Imaging X-ray Polarimetry Explorer (IXPE) offer new ways of probing high-energy emission processes in astrophysical jets from blazars. Here we report on the first X-ray polarization observation of the blazar S4 0954+65 in a high optical and X-ray state. During our multi-wavelength campaign on the source, we detected an optical flare whose peak coincided with the peak of an X-ray flare. This optical-X-ray flare most likely took place in a feature moving along the parsec-scale jet, imaged at 43 GHz by the Very Long Baseline Array. The 43 GHz polarization angle of the moving component underwent a rotation near the time of the flare. In the optical band, prior to the IXPE observation, we measured the polarization angle to be aligned with the jet axis. In contrast, during the optical flare the optical polarization angle was perpendicular to the jet axis; after the flare, it reverted to being parallel to the jet axis. Due to the smooth behavior of the optical polarization angle during the flare, we favor shocks as the main acceleration mechanism. We also infer that the ambient magnetic field lines in the jet were parallel to the jet position angle. The average degree of optical polarization during the IXPE observation was (14.3$\pm$4.1)%. Despite the flare, we only detected an upper limit of 14% (at 3$\sigma$ level) on the X-ray polarization degree; although a reasonable assumption on the X-ray polarization angle results in an upper limit of 8.8% ($3\sigma$). We model the spectral energy distribution (SED) and spectral polarization distribution (SPD) of S4 0954+65 with leptonic (synchrotron self-Compton) and hadronic (proton and pair synchrotron) models. The constraints we obtain with our combined multi-wavelength polarization observations and SED modeling tentatively disfavor hadronic models for the X-ray emission in S4 0954+65.

astro-ph.HE↗

arXiv2024

A Two-Week $IXPE$ Monitoring Campaign on Mrk 421

X-ray polarization is a unique new probe of the particle acceleration in astrophysical jets made possible through the Imaging X-ray Polarimetry Explorer. Here we report on the first dense X-ray polarization monitoring campaign on the blazar Mrk 421. Our observations were accompanied by an even denser radio and optical polarization campaign. We find significant short-timescale variability in both X-ray polarization degree and angle, including a $\sim90^\circ$ angle rotation about the jet axis. We attribute this to random variations of the magnetic field, consistent with the presence of turbulence but also unlikely to be explained by turbulence alone. At the same time, the degree of lower-energy polarization is significantly lower and shows no more than mild variability. Our campaign provides further evidence for a scenario in which energy-stratified shock-acceleration of relativistic electrons, combined with a turbulent magnetic field, is responsible for optical to X-ray synchrotron emission in blazar jets.

astro-ph.HE↗

arXiv2024

Polarization of active galactic nuclei with significant VLBI-Gaia displacements

Numerous studies have reported significant displacements in the coordinates of active galactic nuclei between measurements using radio interferometry techniques and those obtained by the Gaia space observatory. There is a consensus that these discrepancies do indeed manifest astrometrically resolved sub-components of AGN rather than random measurement noise. Among other evidence, it has been reported that AGN with VLBI-to-Gaia displacements (VGD) pointing downstream their parsec-scale radio jets exhibit higher optical polarization compared to sources with the opposite VGD orientation. We aim to verify this previously reported connection between optical polarization and VGD-jet angle using a larger dataset of polarimetric measurements and updated Gaia DR3 positions. We also seek additional evidence supporting the disc-jet dichotomy as an explanation of such a connection by using mm-wave polarization and multi-band optical polarization measurements. We performed optical polarimetric observations of 152 AGN using three telescopes. We cross-match public astrometric data from VLBI and Gaia experiments, obtain corresponding positional displacements, and join this catalog with the polarimetric and jet directions data. AGN with downstream VGD are confirmed to have significantly higher optical fractional polarization than the upstream sample. At the same time, the mm-wavelength polarization of the two samples demonstrates very similar distributions. Our results support the hypothesis that the VLBI-to-Gaia displacements pointing down the radio jet are likely caused by a component in the jet emitting highly polarized synchrotron radiation and dominating in the overall optical emission. The upstream-oriented VGD are likely to be produced by the low-polarized emission of the central engine sub-components dominating in the optical.

astro-ph.GA↗

arXiv2024

Improving polarimetric accuracy of RoboPol to $<$ 0.05 % using a half-wave plate calibrator system

RoboPol is a four-channel, one-shot linear optical polarimeter that has been successfully operating since 2013 on the 1.3 m telescope at Skinakas Observatory in Crete, Greece. Using its unique optical system, it measures the linear Stokes parameters $q$ and $u$ in a single exposure with high polarimetric accuracy of 0.1% - 0.15% and 1 degree in polarization angle in the R broadband filter. Its performance marginally degrades in other broadband filters. The source of the current instrumental performance limit has been identified as unaccounted and variable instrumental polarization, most likely originating from factors such as temperature and gravity-induced instrument flexure. To improve the performance of RoboPol in all broadband filters, including R, we have developed a rotating half-wave plate calibrator system. This calibrator system is placed at the beginning of the instrument and enables modulation of polarimetric measurements by beam swapping between all four channels of RoboPol. Using the new calibrator system, we observed multiple polarimetric standard stars over two annual observing seasons with RoboPol. This has enabled us to achieve a polarimetric accuracy of better than 0.05 % in both $q$ and $u$, and 0.5 degrees in polarization angle across all filters, enhancing the instrument's performance by a factor of two to three.

astro-ph.IM↗