Pulkovo Observatory

Abstract During the last 40–50 years, the TeV blazar Ton 599 has systematically experienced major outbursts detected in a wide wavelength range from radio to γ-rays. In this work, we present an analysis of Ton 599 quasi-periodic variability across multiple wavelengths using an observing baseline from 1983 to 2025. The γ-ray, optical, and radio emissions are found to be highly correlated with time lags ∼0–360 days, which indicates that they are triggered by the same population of particles. Using the Lomb–Scargle periodogram and the Weighted Wavelet Z-transform, we revealed several periodic components with characteristic periods of 1.4, 1.7, 2.3, 6.5, and 7.5 yrs. The result is consistent with the detection of periodic components in the 1997–2011 light curves, which means that we observe the same mechanism causing long-term periodic variability. A model of a binary supermassive black hole (SMBH) with a precessing jet, applied to the radio light curves of Ton 599, yields frequency-dependent best-fitting parameters with orbital periods ranging from ∼1.2 to 1.7 yrs and precession periods from ∼5.8 to 7.7 yrs. This result implies the existence of an SMBH system modulating emission through both the orbiting and jet precession effects, with differing observed periods possibly reflecting frequency-dependent emission regions along a structured, stratified jet. Nonetheless, the short-term periodicity and exceptionally strong flares likely arise from internal jet shocks, which aligns with typical blazar behavior. We suggest that the multiband quasi-periodicity of Ton 599 is reasonably described by a combination of geometric effects (orbital motion and precession) and stochastic processes.

Abstract Solar superflares of S‐class (>X10 in soft X‐rays) pose extreme space weather hazards, yet their prediction remains a fundamental challenge owing to their rapid and transient natures and the limitations of conventional event‐based forecasts. We introduce for the first time, a probabilistic spatiotemporal framework designed to identify extended epochs and heliographic zones with heightened superflare likelihood, rather than predicting individual events. Our methodology analyzes nearly five decades of Geostationary Operational Environmental Satellites SXR observations (1975–2025) and introduces two novel components: (a) a cross‐Laplacian diagnostic, derived from the butterfly diagrams of sunspot area and SXR flare density, which identifies photosphere‐corona coupling and pinpoints magnetic energy accumulation sites as preferential zones for S‐flare occurrence; and (b) a temporal model based on the coupled phase states of 1.7‐year and 7‐year oscillations, extracted via wavelet analysis and projected forward using a Bayesian Least‐Squares Support Vector Machine method. Integrating these spatial and temporal diagnostics yields a unified forecast, we identify 2025.7–2026.6 and 2027.2–2027.9 years, as the primary high‐probability windows for S‐class flare activity during Solar Cycle 25, corresponding preferentially to latitudinal bands in the southern (S–S) and northern (N–N) hemispheres, respectively. Subsequent windows are projected for Cycle 26. This physics‐informed, probabilistic approach provides a robust strategy for anticipating periods of extreme solar activity by characterizing the inherent patterns of magnetic energy storage and release in the solar atmosphere.

Abstract We present a new two-dimensional (2D) map of total Galactic extinction, $A_\mathrm{V}$, across the entire dust half-layer from the Sun to extragalactic space for Galactic latitudes $|b| > 13\degr$, as well as a three-dimensional (3D) map of $A_\mathrm{V}$ within 2~kpc of the Sun.
These maps are based on $A_\mathrm{V}$ and distance estimates derived from a dataset, which utilizes {\it Gaia} Data Release 3 parallaxes and multi-band photometry for nearly 100 million dwarf stars. We apply our own corrections to account for significant systematics in this dataset. Our 2D map achieves an angular resolution of 6.1~arcmin, while the 3D map offers a transverse resolution of 3.56~pc --- corresponding to variable angular resolution depending on distance --- and a radial resolution of 50~pc. In constructing these maps, we pay particular attention to the solar neighborhood (within 200~pc) and to high Galactic latitudes.
The 3D map predicts $A_\mathrm{V}$ from the Sun to any extended object within the Galactic dust layer with an accuracy of $\sigma(A_\mathrm{V}) = 0.1$~mag. The 2D map provides $A_\mathrm{V}$ estimates for the entire dust half-layer up to extragalactic distances with an accuracy of $\sigma(A_\mathrm{V}) = 0.07$~mag. We provide $A_\mathrm{V}$ estimates from our maps for various classes of extended celestial objects \textbf{with angular sizes primarily in the range of 2--40~arcmin}, including 19,809 galaxies and quasars, 170 Galactic globular clusters, 458 open clusters, \textbf{and several hundreds molecular clouds from two lists}. We also present extinction values for 8,293 Type Ia supernovae. Comparison of our extinction estimates with those from previous maps and literature sources reveals systematic differences, indicating large-scale spatial variations in the extinction law and suggesting that earlier 2D reddening maps based on infrared dust emission tend to underestimate low extinction values.

Abstract A new daily composite of the solar flare index (SFI) and the hemispherically‐resolved versions (hSFI) are presented for 1937 to 2024. The data set confirms that the northern hemisphere (NH) dominated solar flare activity during Solar Cycles 17 to 21, but that the southern hemisphere has dominated from Solar Cycle 22 to present. That said, the highest SFI value occurred in the NH during the recent superstorm of May 2024. In sunspot activity, the “Gnevyshev‐Ohl rule” shows that the sum of sunspot numbers during even‐numbered cycles is related to those of adjacent odd‐numbered cycles. A similar rule appears to apply to SFI. The “Gnevyshev gap” phenomenon where solar maximum activity sometimes has two peaks separated by up to 1–2 years of a gap is confirmed for SFI. Although our data set represents the longest continuous daily data set for solar flare activity to‐date, it is known that stronger solar flare events occurred before 1937. Therefore, a brief discussion of earlier solar flare events in the historical record is also provided for context. The statistics of the SFI and hSFI series are compared to other solar and geomagnetic activity indices, including the May and October 2024 solar storms. Statistical analysis of past geomagnetic storms confirms they are more frequent during active cycles and less frequent during solar minima. Strong geomagnetic storms are also more likely to occur during the positive phase of a 1.7 year's quasi‐biennial oscillation in solar activity. The likelihood of low‐magnetic latitude aurorae seems to have a 30 year periodicity component.

It is shown how the violation of the invariance of the Z-component of the orbital angular momentum Lz in the axially symmetric potential of the Galaxy with a bar can serve as an indicator of the degree of orbital chaos of globular clusters in the central region of the Galaxy. In this case, the higher the variations of Lz of the orbit over a certain period of time, the higher the chaos of the orbit. In essence, a new method for analyzing orbital dynamics regular or chaotic is proposed. A high level of correlation between the results of orbit classication by the proposed method and the results of classication by other methods is shown. As a result, a sample of 45 globular clusters in the central region of the Galaxy with a radius of 3.5 kpc is divided into regular, chaotic, and weakly chaotic.

The parameters of an axisymmetric model for the gravitational potential of the Galaxy have been refined. The basic curve of the Galaxy’s rotation in a distance interval of R ~ 0–190 kpc was constructed using the velocities of masers, classical Cepheids, Red Clump stars, Blue Horizontal Branch stars, halo stars, globular clusters, and dwarf satellite galaxies of the Milky Way. The rotation curve was selected in such a way that there would be no dominant burst of circular velocities in the central ( $$R < 2$$ kpc) region of the Galaxy. As a result, we constructed two two-component models of the galactic potential, which include contributions from the disk and the halo of invisible matter, as well as a three-component model with a small-mass bulge added in advance. These models can be useful in studying the long-term orbital evolution of stars and open and globular star clusters in the central ( $$R < 4$$ kpc) region of the Galaxy. The constructed models were tested for self-consistency by comparing their rotation curves with a set of model curves generated with the Illustris TNG50 software package.

Abstract The radiation mechanism of decimetric wideband and pulsating radio bursts from the Sun (in terms of decimetric type-IV (t-IVdm) burst) and other flaring stars is a long-standing problem. Early investigations were based on the leading-spot hypothesis for the Sun and yielded contradictory results. Here, we analyzed the flare-associated t-IVdm burst on 20110924 with medium-strong levels of polarization and from sources near a sunspot. We found that the emission is intermittent and the maximum T B exceeds 1011 K, with well-defined upper and lower frequency cutoffs. The radio sources are left-handed polarized, located above the sunspot with a negative polarity. The sources align well with the sites of the second harmonic of the local electron gyrofrequency. These findings provide essential evidence that the burst is induced by the electron cyclotron maser emission (ECME) in the harmonic X mode. We further modeled the transport of downward-streaming energetic electrons along a coronal loop and found that most electrons get mirrored within the specific altitude range of 20–100 Mm. This explains why such bursts tend to have well-defined spectral ranges. We also found the ECME-radiating energetic electrons exhibit a shell-like velocity distribution function instead of the generally presumed loss-cone distribution. The study greatly expands the application of ECME in solar radio astronomy and provides solar samples for similar bursts from other flaring stars.

Observational aspects of the formation of near-surface toroidal magnetic fields on the Sun were considered. Based on the analysis of magnetic field observations in the period 2010–2024, diagrams of the distribution of horizontal magnetic fields and magnetic bipoles of ephemeral regions in the solar cycle were obtained. The obtained diagrams are close to the concept of an extended cycle of solar activity. The formation of surface azimuthal magnetic fields was modeled based on the evolution of U-shaped magnetic bipoles of active regions. The modeling took into account near-equatorial bipoles with a magnetic field propagating through the solar equator; this made it possible to achieve good agreement with the duration of the azimuthal field polarity at low latitudes.

Four samples of open star clusters (OSCs) with average ages of 5.2, 18.6, 40, and 61 Myr have been analyzed. The selection of these OSCs was carried out from a narrow region inclined to the Galactic $$Y$$ axis at an angle of $$25^{\circ}$$ . Spectral analysis of the vertical positions and velocities of the selected clusters showed that the Radcliffe wave is associated with OSCs no older than 30 Myr. The following estimates of the Radcliffe wave characteristics were obtained for the OSCs with an average age of 5.2 Myr: $$z_{\textrm{max}}=117\pm 12$$ pc with the wavelength $$\lambda=4.55\pm 0.14$$ kpc, and the vertical velocity disturbance amplitude $$W_{\textrm{max}}=4.86\pm 0.19$$ km s $${}^{-1}$$ with the wavelength $$\lambda=1.74\pm 0.08$$ kpc. For the OSCs with an average age of 18.6 Myr, the estimates are as follows: $$z_{\textrm{max}}=54\pm 5$$ pc and $$\lambda=6.30\pm 0.12$$ kpc, and the vertical velocity disturbance amplitude $$W_{\textrm{max}}=7.90\pm 0.16$$ km s $${}^{-1}$$ and $$\lambda=0.83\pm 0.11$$ kpc. The radial motion of the Radcliffe wave away from the Galactic center has been confirmed. The velocity of such movement is 10 pc Myr $${}^{-1}$$ . In our opinion, the spatial distribution of OSCs younger than 30 Myr does not contradict the hypothesis of the association of the Radcliffe wave with the impact of shock waves from supernova explosions that arose on an extended front comparable in scale to the entire wave, that is, about 2 kpc in size.

Abstract We present an analysis of the cross-correlation between optical brightness and polarization degree in different types of blazars. The aim is to identify objects with simultaneous and consistent changes in characteristics and to determine if this behavior relates to the types of objects studied. The analysis includes 23 objects: 11 Flat Spectrum Radio Quasars (FSRQs), 11 BL Lacs, and 1 radio galaxy. Dense overlapping observation series in the R band were used, collected over more than 10 yr as part of a monitoring program for bright blazars at St. Petersburg State University. The cross-correlation analysis procedure is detailed, including a method for assessing significance based on Monte Carlo simulations of synthetic light curves modeled using a Damped Random Walk. Significant correlations were found for 5 FSRQs and 1 BL Lac. No significant correlation was detected for 10 BL Lacs and 6 FSRQs. One object did not yield a reliable estimate. Based on the current results, we cannot claim that the observed difference in the behavior of these emission characteristics for different classes of blazars is significant. It is possible that observed correlations may be explained by the contribution of simultaneous flare events to the changes in flux and polarization degree curves, which occur more frequently in FSRQs.