Sergey Koryagin
senior researcher at Division of Astrophysics and Cosmic Plasma Physics, Ph.D.

Date and place of birth: 08 December 1973, Gorky, USSR.
Radio amateur call RA3TBE.
VKontakte page (http://vk.com/id193170337).

Education:
High school 96 at Nizhni Novgorod with honors (1991), Advanced School of General and Applied Physics at Lobachevsky State University of Nizhni Novgorod with honors (Bachelor of Science - 1995, Master of Science - 1997), postgraduate studies at Institute of Applied Physics (2000), degree of Candiadate of Science in Physics and Mathematics (PhD) for the thesis ``On the theory of the radiative and collisional processes in a magnetized astrophysical plasma'' (2001, scientific supervisor Academician V. V. Zheleznyakov), Associate Professor in Plasma Physics (2010).

Scientific areas of study:
Space plasma physics, plasma under extreme physical conditions, magnetic white dwarfs, pulsars - neutron stars, active galactic nuclei.

Professional career:
• Institute of Applied Physics of the Russian Academy of Sciences (PhD student (1997-2000), junior researcher (2000-2002), researcher (2002-2006), senior researcher (since 2006 up to now));
• Lobachvsky State University of Nizhni Novgorod (senior lecturer 1998-2003, associate professor since 2003 up to now);
• Max Planck Institute for Astrophysics (visitor, 2002).

Membership at professional organizations:
Ordinary member of the European Astronomical Society (since 2011), member of the scientific council of Plasma and High Power Electronics Department at the Institute of Applied Physics (since 2010 up to now), expert at the Russian Foundation for Basic Research (2005-2010).

 Awards:
• State Premium of the Russian Federation for young scientists for outstanding works in science and techniсs (2003);
• The Russian President grants for the state support of the young Russian scientists (2003-2004, 2007-2008);
• Diplomas at the competitions of the young scientists' research works at the Institute of Applied Physics (winner in 1998 and 2000, second place in 2002 and 2007, third place in 2003 and 2006);
• Acknowledgement letter from the Administration of Nizhni Novgorod for the significant personal input at the organization of Nizhni Novgorod city intercollegiate  competition on astronomy, astrophysics and cosmos physics (2009).

Pedagogic work:
Associate Professor at the Advanced School of General and Applied Physics of the Lobachevsky State University of Nizhni Novgorod (practice - solution of problems on mechanics at 1st course, physics laboratory courses at 1-3 courses); supervisor of the Bachelor diploma works (I. I. Bubukina, 2007; S. A. Arsenyev, 2011; I. A. Balandin, 2014); head of the subject committee at Nizhni Novgorod city intercollegiate competitions on astronomy, astrophysics and cosmos physics since 2007; invited lectures ``Basics of plasma physics'' at 8th School of Modern Astrophysics (Puschino, Russia, 2012); supervisor of the scientific research works of high schoolers (1998-2001); member of the jury at the final conferences of Young Researcher School at the Institute of Applied Physics.

Number of publications:
18 scientific papers in refereed journals, 11 articles in conference proceedings, 1 popular science article. In 2015, the Hirsch citation index (h-index) equals 5 (Web of Science).

The key papers and results:

S. A. Koryagin. Electron-ion collision integral in a strong magnetic field // Journal of Experimental and Theoretical Physics. 2000. V. 90, No. 5. P. 741-752 (http://dx.doi.org/10.1134/1.559159).
Boltzmann's collision integral is extended to the case of helical (Larmor) particle trajectories in a magnetic field of arbitrary strength. The main characteristics of collisions of electrons with positively charged ions in strong magnetic fields, where the Larmor radius of electrons becomes less than the characteristic impact parameter of close collisions in the absence of a magnetic field, are investigated. The differential scattering cross section and the corresponding electron-ion collision integral in strong fields are found. The transport collision frequencies are calculated, and the results are compared with the similar quantities for weaker magnetic fields.

V. V. Zheleznyakov, S. A. Koryagin. Polarization spectra of synchrotron radiation and the plasma composition of relativistic jets // Astronomy Letters. 2002. V. 28, No. 11. P. 727-744 (http://dx.doi.org/10.1134/1.1518711).
We investigate the problem of determining the plasma composition of relativistic jets in blazars and microquasars from the polarization frequency spectra of their synchrotron radiation. The effect of plasma composition on this radiation is attributable to a change in the structure of the ordinary and extraordinary waves in plasma, depending on the presence of a nonrelativistic electron-proton component in it and on the type of relativistic particles (electrons, positrons). The structure of the normal waves determines the properties of the observed radiation and primarily the shape of the polarization frequency spectrum. Our analytic calculations of the polarization spectra for simple models of jets with a uniform magnetic field and with a magnetic-field shear revealed characteristic features in the polarization spectra. These features allow us to differentiate between the synchrotron radiation from an admixture of relativistic particles in a cold plasma and the radiation from a relativistic plasma. However, definitive conclusions regarding the relativistic plasma composition (electrons or electron-positron pairs) can be reached only by a detailed analysis of the polarization frequency spectra that will be obtained in future radioastronomical studies with high angular and frequency resolutions.

V. V. Zheleznyakov, S. A. Koryagin, A. V. Serber. Thermal cyclotron radiation by isolated magnetic white dwarfs and constraints on the parameters of their coronas // Astronomy Reports. 2004. V. 48, No. 2. P. 121-135 (http://dx.doi.org/10.1134/1.1648076).
An efficient method for the detection and estimation of the parameters of the coronas of isolated white dwarfs possessing magnetic fields of about 10^7 G is tested. This method is based on the detection of thermal radiation of the coronal plasma at harmonics of the electron gyrofrequency, which is manifest as a polarized infrared excess. The Stokes parameters for the thermal cyclotron radiation from the hot corona of a white dwarf with a dipolar magnetic field are calculated. A new upper limit for the electron density, 10^10 cm^-3, in a corona with a temperature equal or greater than 10^6 K is found for the white dwarf G 99-47 (WD 0553+053). This limit is a factor of 40 lower than the value derived earlier from ROSAT X-ray observations. Recommendations for subsequent infrared observations of isolated magnetic white dwarfs aimed at detecting their coronas or deriving better constraints on their parameters are presented.

V. V. Zheleznyakov, S. A. Koryagin. Quasi-linear stage of synchrotron instability. II. Monoenergetic initial electron distribution // Radiophysics and Quantum Electronics . 2006. V. 49, No. 12. P. 968-978 (http://dx.doi.org/10.1007/s11141-006-0130-1).
We analyze quasi-linear relaxation of an initial monoenergetic quasi-isotropic electron distribution embedded in a cold plasma. It is shown that in this case the radiation energy density increases due to synchrotron instability only at the initial stage of relaxation and after that reabsorption emerges. At the stage of reabsorption, electrons absorb the emitted radiation for the same characteristic time as the time of radiation energy density increase at the maser stage. We calculate the maximum fraction of initial electron energy which is converted into synchrotron radiation energy. Analytical expressions are obtained for the amplitude and width of the frequency spectrum at the instant of the maximum level of synchrotron radiation and for the maximum value and width of the electron distribution over momentum at the end of relaxation.

I. I. Bubukina, S. A. Koryagin. Bremsstrahlung from collisions of low-energy electrons with positive ions in a magnetic field // Journal of Experimental and Theoretical Physics. 2009. V. 108, No. 6. P. 917-927 (http://dx.doi.org/10.1134/S1063776109060028).
Bremsstrahlung from electron-ion collisions in a magnetic field is studied for low energies at which the Larmor radius of an electron is smaller than the characteristic impact parameter of close collisions in zero magnetic field. It is shown that the magnetic field does not qualitatively change the bremsstrahlung power at low frequencies smaller than the reciprocal time of electron transit in the vicinity of an ion in close collision in zero magnetic field. At high frequencies, the radiation intensity decreases in accordance with a power law, attains its minimal value, and then increases in accordance with a power law up to frequencies on the order of the electron cyclotron frequency. At such frequencies, the spectral power attains typical power values in zero magnetic field. At frequencies lower than the cyclotron frequency considered here, bremsstrahlung is polarized predominantly linearly in the plane formed by the magnetic field and the direction of radiation.

S. A. Arsenyev, S. A. Koryagin. Completely bound motion of a positive-energy electron in the Coulomb field of a motionless nucleus and a uniform magnetic field // Radiophysics and Quantum Electronics. 2011. V. 53, No. 11. P. 650-665 (http://dx.doi.org/10.1007/s11141-011-9259-7).
We show that the completely bound classical motion of a positive-energy electron is realized in the Coulomb field of a motionless nucleus and a uniform magnetic field. Such a motion exists due to conservation of the so-called invariant tori in the phase space of the system for not only the negative, but also for the positive energy of an electron. The completely bound trajectories occupy a much larger interval of the velocity directions compared with free trajectories for the same energy in a range of distances from the nucleus in which the typical time of the electron transit near the nucleus is larger than the cyclotron-gyration period, while the negative energy of Coulomb interaction is larger (in absolute value) than the total electron energy. The indicated range of distances is realized in the case of a low electron energy or a strong magnetic field when the Larmor radius of the electron is smaller than the characteristic impact parameter of the close Coulomb collisions in the absence of a magnetic field. The required conditions are realized in the photospheres of isolated magnetic white dwarfs and in the experiments on creation of antihydrogen.