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V. V. Sokolov

Publications and source records attributed to V. V. Sokolov.

At least 19 recordsLinked to original sources
arXiv2026

Vector systems of Painlevé type

The group reduction procedure is applied to vector generalizations of the NLS, mKdV, and KdV equations. The resulting ODE systems admit isomonodromic Lax representations and are multicomponent generalizations of the Painlevé equations P$_1$, P$_2$, P$_{34}$, and P$_4$. Some of them can be interpreted as nonautonomous deformations of well-known systems integrable in the Liouville sense, in particular, the Garnier and Hénon--Heiles systems. In one case, an unexpected connection with the equations of quasiperiodic dressing chain for the Schrödinger operator is established.

nlin.SI↗
arXiv2025

Novikov equations for commuting differential operators of orders 3,4,5

We consider Novikov equations for commutative ring generated by differential operators of orders 3,4,5. We present an explicit Hamiltonian form of these equations. Using the method of compatible Poisson brackets, we find a separation of variables on a hyperelliptic curve of genus 2 for the Novikov equations.

nlin.SI↗
arXiv2023

Revealing characteristics of dark GRB 150309A: dust extinguished or high-z?

Dark GRBs constitute a significant fraction of the GRB population. In this paper, we present the multiwavelength analysis of an intense two-episodic GRB 150309A observed early on to ~114 days post-burst. Despite the strong gamma-ray emission, no optical afterglow was detected for this burst. However, we discovered near-infrared afterglow ($K_{\rm S}$-band), ~5.2 hours post burst, with the CIRCE instrument mounted at the 10.4m GTC. We used Fermi observations of GRB 150309A to understand the prompt emission mechanisms and jet composition. We performed the early optical observations using the BOOTES robotic telescope and late-time afterglow observations using the GTC. A potential faint host galaxy is also detected at optical wavelength using the GTC. We modelled the potential host galaxy of GRB 150309A in order to explore the environment of the burst. The time-resolved spectral analysis of Fermi data indicates a hybrid jet composition consisting of a matter-dominated fireball and magnetic-dominated Poynting flux. GTC observations of the afterglow revealed that the counterpart of GRB 150309A was very red, with H-$K_{\rm S}$ > 2.1 mag (95 $\%$ confidence). The red counterpart was not discovered in any bluer filters of Swift UVOT, indicative of high redshift origin. This possibility was discarded based on multiple arguments, such as spectral analysis of X-ray afterglow constrain z < 4.15 and a moderate redshift value obtained using spectral energy distribution modelling of the potential galaxy. The broadband afterglow SED implies a very dusty host galaxy with deeply embedded GRB (suggesting $A_{\rm V}$ $\gtrsim$ 35 mag). The environment of GRB 150309A demands a high extinction towards the line of sight, demanding dust obscuration is the most probable origin of optical darkness and the very red afterglow of GRB 150309A. This result makes GRB 150309A the highest extinguished GRB known to date.

astro-ph.HE↗
arXiv2022

Probing into emission mechanisms of GRB 190530A using time-resolved spectra and polarization studies: Synchrotron Origin?

Multi-pulsed GRB 190530A, detected by the GBM and LAT onboard \fermi, is the sixth most fluent GBM burst detected so far. This paper presents the timing, spectral, and polarimetric analysis of the prompt emission observed using \AstroSat and \fermi to provide insight into the prompt emission radiation mechanisms. The time-integrated spectrum shows conclusive proof of two breaks due to peak energy and a second lower energy break. Time-integrated (55.43 $\pm$ 21.30 \%) as well as time-resolved polarization measurements, made by the Cadmium Zinc Telluride Imager (CZTI) onboard \AstroSat, show a hint of high degree of polarization. The presence of a hint of high degree of polarization and the values of low energy spectral index ($α_{\rm pt}$) do not run over the synchrotron limit for the first two pulses, supporting the synchrotron origin in an ordered magnetic field. However, during the third pulse, $α_{\rm pt}$ exceeds the synchrotron line of death in few bins, and a thermal signature along with the synchrotron component in the time-resolved spectra is observed. Furthermore, we also report the earliest optical observations constraining afterglow polarization using the MASTER (P $<$ 1.3 \%) and the redshift measurement ($z$= 0.9386) obtained with the 10.4m GTC telescopes. The broadband afterglow can be described with a forward shock model for an ISM-like medium with a wide jet opening angle. We determine a circumburst density of $n_{0} \sim$ 7.41, kinetic energy $E_{\rm K} \sim$ 7.24 $\times 10^{54}$ erg, and radiated $γ$-ray energy $E_{\rm γ, iso} \sim$ 6.05 $\times 10^{54}$ erg, respectively.

astro-ph.HE↗
arXiv2021

Photometric, polarimetric, and spectroscopic studies of the luminous, slow-decaying Type Ib SN 2012au

Optical, near-infrared (NIR) photometric and spectroscopic studies, along with the optical imaging polarimetric results for SN 2012au, are presented in this article to constrain the nature of the progenitor and other properties. Well-calibrated multiband optical photometric data (from $-$0.2 to +413 d since $B$-band maximum) were used to compute the bolometric light curve and to perform semi-analytical light-curve modelling using the $\texttt{MINIM}$ code. A spin-down millisecond magnetar-powered model explains the observed photometric evolution of SN 2012au reasonably. Early-time imaging polarimetric follow-up observations ($-$2 to +31 d) and comparison with other similar cases indicate signatures of asphericity in the ejecta. Good spectral coverage of SN 2012au (from $-$5 to +391 d) allows us to trace the evolution of layers of SN ejecta in detail. SN 2012au exhibits higher line velocities in comparison with other SNe Ib. Late nebular phase spectra of SN 2012au indicate a Wolf$-$Rayet star as the possible progenitor for SN 2012au, with oxygen, He-core, and main-sequence masses of $\sim$1.62 $\pm$ 0.15 M$_\odot$, $\sim$4$-$8 M$_\odot$, and $\sim$17$-$25 M$_\odot$, respectively. There is a clear absence of a first overtone of carbon monoxide (CO) features up to +319 d in the $K$-band region of the NIR spectra. Overall analysis suggests that SN 2012au is one of the most luminous slow-decaying Type Ib SNe, having comparatively higher ejecta mass ($\sim$4.7$-$8.3 M$_\odot$) and kinetic energy ($\sim$[4.8 $-$ 5.4] $\times$ 10$^{51}$ erg). Detailed modelling using $\texttt{MESA}$ and the results obtained through $\texttt{STELLA}$ and $\texttt{SNEC}$ explosions also strongly support spin-down of a magnetar with mass of around 20 M$_\odot$ and metallicity Z = 0.04 as a possible powering source of SN 2012au.

astro-ph.HE↗
arXiv2021

GRB 140102A: Insight into Prompt Spectral Evolution and Early Optical Afterglow Emission

We present and perform a detailed analysis of multi-wavelength observations of \thisgrb, an optical bright GRB with an observed reverse shock (RS) signature. Observations of this GRB were acquired with the BOOTES-4 robotic telescope, the \fermi, and the \swift missions. Time-resolved spectroscopy of the prompt emission shows that changes to the peak energy (\Ep) tracks intensity and the low-energy spectral index seems to follow the intensity for the first episode, whereas this tracking behavior is less clear during the second episode. The fit to the afterglow light curves shows that the early optical afterglow can be described with RS emission and is consistent with the thin shell scenario of the constant ambient medium. The late time afterglow decay is also consistent with the prediction of the external forward shock (FS) model. We determine the properties of the shocks, Lorentz factor, magnetization parameters, and ambient density of \thisgrb, and compare these parameters with another 12 GRBs, consistent with having RS produced by thin shells in an ISM-like medium. The value of the magnetization parameter ($R_{\rm B} \approx 18$) indicates a moderately magnetized baryonic dominant jet composition for \thisgrb. We also report the host galaxy photometric observations of \thisgrb obtained with 10.4m GTC, 3.5m CAHA, and 3.6m DOT telescopes and find the host (photo $z$ = $2.8^{+0.7}_{-0.9}$) to be a high mass, star-forming galaxy with a star formation rate of $20 \pm 10 \msun$ $\rm yr^{-1}$.

astro-ph.HE↗
arXiv2020

On matrix Painlevé II equations

The Painlevé--Kovalevskaya test is applied to find three matrix versions of the Painlevé II equation. All these equations are interpreted as group-invariant reductions of integrable matrix evolution equations, which makes it possible to construct isomonodromic Lax pairs for them.

nlin.SI↗
arXiv2020

Non-Abelian evolution systems with conservation laws

We find noncommutative analogs for well-known polynomial evolution systems with higher conservation laws and symmetries. The integrability of obtained non-Abelian systems is justified by explicit zero curvature representations with spectral parameter.

nlin.SI↗
arXiv2020

10.4m GTC observations of the nearby VHE-detected GRB 190829A/SN 2019oyw

Aims. GRB 190829A (z = 0.0785), detected by Fermi and Swift with two emission episodes separated by a quiescent gap of ~40 s, was also observed by the H.E.S.S. telescopes at Very-High Energy (VHE). We present the 10.4m GTC observations of the afterglow of GRB 190829A and underlying supernova and compare it against a similar GRB 180728A and discuss the implications on underlying physical mechanisms producing these two GRBs. Methods. We present multi-band photometric data along with spectroscopic follow-up observations taken with the 10.4m GTC telescope. Together with the data from the prompt emission, the 10.4m GTC data are used to understand the emission mechanisms and possible progenitor. Results. A detailed analysis of multi-band data of the afterglow demands cooling frequency to pass between the optical and X-ray bands at early epochs and dominant with underlying SN 2019oyw later on. Conclusions. Prompt emission temporal properties of GRB 190829A and GRB 180728A are similar, however the two pulses seem different in the spectral domain. We found that the supernova (SN) 2019oyw associated with GRB 190829A, powered by Ni decay, is of Type Ic-BL and that the spectroscopic/photometric properties of this SN is consistent with those observed for SN 1998bw but evolved comparatively early.

astro-ph.HE↗
arXiv2020

THESEUS$-$BTA cosmological tests using Multimessenger Gamma-Ray Bursts observations

Modern Multimessenger Astronomy is a powerful instrument for performing cosmological crucial tests of the Standard Cosmological Model in the wide redshift interval up to $z \sim 10$. This is principally important for discussion related to discrepancies between local and global measurements of cosmological parameters. We present a review of multimessenger gamma-ray burst observations currently conducted and planed for THESEUS$-$BTA cooperative program. Such observations give a unique opportunity to test the fundamental foundations of cosmological models: gravitation theory; cosmological principle of homogeneity and isotropy of large-scale distribution of matter; and space expansion paradigm. Important role of various selection effects leading to systematic distortions of true cosmological relations is discussed.

astro-ph.CO↗
arXiv2020

High Redshift Long Gamma-Ray Bursts Hubble Diagram as a Test of Basic Cosmological Relations

We examine the prospects of the high redshift Long Gamma Ray Bursts (LGRB) Hubble Diagram as a test of the basic cosmological principles. Analysis of the Hubble Diagram allows us to test several fundamental cosmological principles using the directly observed flux-distance-redshift relation. Modern LGRB data, together with the correlation between the spectral peak energy and the isotropic-equivalent radiated energy (the so-called Amati relation) can be used for construction of the Hubble Diagram at the model-independent level. We emphasise observational selection effects, which inevitably exist and distort the theoretically predicted relations. An example is the weak and strong gravitational lensing bias effect for high redshift LGRB in the presence of limited observational sensitivity (Malmquist bias). After bias correction, there is a tendency to vacuum dominated models with $Ω_Λ\rightarrow 0.9$, $Ω_\text{m} \rightarrow 0.1$. Forthcoming gamma-ray observations by the space THESEUS mission together with ground and space based multimessenger facilities will allow us to improve essentially the restrictions on alternative basic principles of cosmological models.

astro-ph.CO↗
arXiv2020

S-stars motion around relativistic compact object Sgr A*

A review of modern VLTI observations of the orbital motion of closest stars to the relativistic compact object Sgr A* and its ability to test gravitation theories in the conditions of Post-Newtonian approximation. The observed orbital parameters, second order Doppler effect and gravitational redshift, measured for several S-stars, are compared with theoretical PN predictions.

astro-ph.GA↗
arXiv2019

Multiwavelength observations of GRB 140629A. A long burst with an achromatic jet break in the optical and X-ray afterglow

We investigate the long GRB140629A through multiwavelength observations, which cover optical, infrared and X-rays between 40s and 3yr after the burst, to derive the properties of the dominant jet and its host galaxy. Polarisation observations by the MASTER telescope indicate that this burst is weakly polarised. The optical spectrum contains absorption features, from which we confirm the redshift of the GRB as originating at z=2.276. We performed spectral fitting of the X-rays to optical afterglow data and find there is no strong spectral evolution. We determine the hydrogen column density to be 7.2x10^21cm^-2 along the line of sight. The afterglow in this burst can be explained by a blast wave jet with a long-lasting central engine expanding into a uniform medium in the slow cooling regime. At the end of energy injection, a normal decay phase is observed in both the optical and X-ray bands. An achromatic jet break is also found in the afterglow light curves 0.4d after trigger. We fit the multiwavelength data simultaneously with a model based on a numerical simulation and find that the observations can be explained by a narrow uniform jet in a dense environment with an opening angle of 6.7deg viewed 3.8deg off-axis, which released a total energy of 1.4x10^54erg. Using the redshift and opening angle, we find GRB 140629A follows both the Ghirlanda and Amati relations. From the peak time of the light curve, identified as the onset of the forward shock (181s after trigger), the initial Lorentz factor is constrained in the range 82-118. Fitting the host galaxy photometry, we find the host to be a low mass, star-forming galaxy with a star formation rate of logSFR=1.1^+0.9_-0.4Myr^-1. We obtain a value of the neutral hydrogen density by fitting the optical spectrum, logN(HI)=21.0+-0.3, classifying this host as a damped Lyman-alpha. High ionisation lines are also detected in the spectrum.

astro-ph.HE↗
arXiv2019

The afterglow and kilonova of the short GRB 160821B

GRB 160821B is a short duration gamma-ray burst (GRB) detected and localized by the Neil Gehrels Swift Observatory in the outskirts of a spiral galaxy at z=0.1613, at a projected physical offset of 16 kpc from the galaxy's center. We present X-ray, optical/nIR and radio observations of its counterpart and model them with two distinct components of emission: a standard afterglow, arising from the interaction of the relativistic jet with the surrounding medium, and a kilonova, powered by the radioactive decay of the sub-relativistic ejecta. Broadband modeling of the afterglow data reveals a weak reverse shock propagating backward into the jet, and a likely jet-break at 3.5 d. This is consistent with a structured jet seen slightly off-axis while expanding into a low-density medium. Analysis of the kilonova properties suggests a rapid evolution toward red colors, similar to AT2017gfo, and a low nIR luminosity, possibly due to the presence of a long-lived neutron star. The global properties of the environment, the inferred low mass (M_ej < 0.006 Msun) and velocities (v > 0.05 c) of lanthanide-rich ejecta are consistent with a binary neutron star merger progenitor.

astro-ph.HE↗
arXiv2019

A multi-wavelength analysis of a collection of short-duration GRBs observed between 2012-2015

We investigate the prompt emission and the afterglow properties of short duration gamma-ray burst (sGRB) 130603B and another eight sGRB events during 2012-2015, observed by several multi-wavelength facilities including the GTC 10.4m telescope. Prompt emission high energy data of the events were obtained by INTEGRAL/SPI/ACS, Swift/BAT and Fermi/GBM satellites. The prompt emission data by INTEGRAL in the energy range of 0.1-10 MeV for sGRB 130603B, sGRB 140606A, sGRB 140930B, sGRB 141212A and sGRB 151228A do not show any signature of the extended emission or precursor activity and their spectral and temporal properties are similar to those seen in case of other short bursts. For sGRB130603B, our new afterglow photometric data constraints the pre jet-break temporal decay due to denser temporal coverage. For sGRB 130603B, the afterglow light curve, containing both our new as well as previously published photometric data is broadly consistent with the ISM afterglow model. Modeling of the host galaxies of sGRB 130603B and sGRB 141212A using the LePHARE software supports a scenario in which the environment of the burst is undergoing moderate star formation activity. From the inclusion of our late-time data for 8 other sGRBs we are able to; place tight constraints on the non-detection of the afterglow, host galaxy or any underlying kilonova emission. Our late-time afterglow observations of the sGRB 170817A/GW170817 are also discussed and compared with the sub-set of sGRBs.

astro-ph.HE↗
arXiv2018

The Excess Density of Field Galaxies near z=0.56 around the Gamma-Ray Burst GRB021004 Position

We test for reliability any signatures of field galaxies clustering in the GRB 021004 line of sight. The first signature is the GRB 021004 field photometric redshifts distribution based on the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences observations with a peak near z = 0.56 estimated from multicolor photometry in the GRB direction. The second signature is the Mg II 2796, 2803AA absorption doublet at z = 0.56 in VLT/UVES spectra obtained for the GRB 021004 afterglow. The third signature is the galaxy clustering in a larger (of about 3 sq.deg.) area around GRB 021004 with an effective peak near z = 0.56 for both the spectral and photometric redshifts from a few catalogs of clusters based on the Sloan Digital Sky Survey (SDSS) and Baryon Oscillation Spectroscopic Survey (BOSS) as a part of SDSS-III. From catalog data the size of the whole inhomogeneity in distribution of the galaxy cluster with the peak near z = 0.56 was also estimated as about 6--8 deg. or 140--190 Mpc. A possibility of inhomogeneity (a galaxy cluster) near the GRB 021004 direction can be also confirmed by an inhomogeneity in cosmic microwave background related with the Sunyaev-Zeldovich effect.

astro-ph.GA↗
arXiv2018

Unveiling the enigma of ATLAS17aeu

Aim. The unusual transient ATLAS17aeu was serendipitously detected within the sky localisation of the gravitational wave trigger GW170104. The importance of a possible association with gravitational waves coming from a binary black hole merger led to an extensive follow-up campaign, with the aim of assessing a possible connection with GW170104. Methods. With several telescopes, we carried out both photometric and spectroscopic observations of ATLAS17aeu, for several epochs, between $\sim 3$ and $\sim 230$ days after the first detection. Results. We studied in detail the temporal and spectroscopic properties of ATLAS17aeu and its host galaxy. We detected spectral features similar to those of a broad lined supernova superposed to an otherwise typical long-GRB afterglow. Based on analysis of the optical light curve, spectrum and host galaxy SED, we conclude that the redshift of the source is probably $z \simeq 0.5 \pm 0.2$. Conclusions. While the redshift range we have determined is marginally compatible with that of the gravitational wave event, the presence of a supernova component and the consistency of this transient with the E$_{\rm p}$-E$_{\rm iso}$ correlation support the conclusion that ATLAS17aeu was associated with the long gamma-ray burst GRB170105A. This rules out the association of the GRB170105A/ATLAS17aeu transient with the gravitational wave event GW170104, which was due to a binary black hole merger.

astro-ph.HE↗