ASTROPHYSICAL PROCESSES
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- Versione italiana
- Academic year
- 2022/2023
- Teacher
- CRISTIANO GUIDORZI
- Credits
- 6
- Didactic period
- Primo Semestre
- SSD
- FIS/01
Training objectives
- The objectives of the course are supplying solid knowledge and mastering of the fundamental physics ruling several astrophysical processes, that include concepts of special relativity applied to astrophysics, radiative transfer, advanced electromagnetism, radiative processes, dynamics of astrophysical fluids and shock waves, plasma effects on electromagnetic waves, particle acceleration mechanisms. The acquired knowledge will enable the student to lay the foundations for more focused studies on most of the main arguments of cutting-edge research in the field of high-energy and multi-messenger astrophyics, as well as other disciplines, such as dynamics of fluids and shock physics.
This course addresses the main radiative processes in astrophysics that are relevant in the field of high energy astrophysics and of explosive phenomena. To this aim, it addresses concepts of advanced electromagnetisms, that are the basic theory of radiation fields and radiation from moving charges. The basic physics of explosive phenomena, which is at the core of several classes of astrophysical transients, is addressed through different aspects: the astrophysical fluid dynamics, the jump conditions for both relativistic and non-relativistic shocks, particle acceleration and the so-called Fermi first and second order mechanisms.
Another topic that is key in the study of explosive transients is the imprint left by cold plasma on crossing electromagnetic waves.
The student will be supplied with the formal rigour and will show their abilities in properly formulating and addressing problems concerning the treated subjects, required to undertake any research activity in the related fields. Prerequisites
- The course requires familiarity with the basics of special relativity, basic electromagnetism, general physics, classical and quantum mechanics, and basic astropysics, in particular elements of stellar structure and evolution.
Course programme
- Main topics: special relativity in astrophysics (4 hours). Radiative transfer (8 hours). Advanced electromagnetism (10). Radiative processes (14 hours). Fluid dynamics and shocks (8 hours). Electromagnetic waves through plasmas (6 hours). Fermi acceleration mechanisms (4 hours).
Detailed topics: elements of special relativity in astrophysics: Doppler boosting, aberration of light, Lorentz invariance of distribution function, specific intensity. Radiative transfer: spontaneous and stimulated emission, absorption, radiative transfer equation, mean free path, thermal and blackbody radiation, optical depth, Einstein coefficients. Advanced electromagnetism: Lienard-Wiechert potentials, velocity and radiation fields, radiation from moving charged particles, Larmor formula, dipole approximation, polarisation of radiation and Stokes parameters. Radiative processes: Thomson scattering, bremsstrahlung, synchrotron radiation, coherent curvature radiation, Compton and inverse Compton scattering, Comptonisation and Kompaneets equation, along with several astrophysical examples. Basic equations of fluid dynamics for an ideal fluid in astrophysics: non-relativistic dynamics and stress tensor; relativistic dynamics and stress-energy tensor. Jump conditions for both non-relativistic (Rankine-Hugoniot) and relativistic (Taub) shocks. Self-similar solutions (Sedov-Taylor, Blandford-McKee). Electromagnetic waves through astrophysical plasmas: dispersion measure and Faraday rotation. Cherenkov radiation. Razin effect. Scattering caused by plasma irregularities. Fermi first and second order mechanisms in the context of cosmic ray acceleration. Didactic methods
- The lectures are delivered through slides that the teacher makes available on the web soon after they have been presented and discussed. During classes the teacher very often intersperses slides with calculations of the relevant quantities, which are estimated by means of specific examples and exercises. This way, the student becomes very familiar with the kind of exam and the sort of questions he/she will be expected to address in the oral exam, as well as with the evaluation criteria adopted by the teacher.
Learning assessment procedures
- During classes the teacher occasionally assigns exercises along with the numerical values of the solutions to motivate the students to practice and test the knowledge they are expected to acquire on the topics presented by the teacher. A detailed step-by-step solution is available on request to the students in the following weeks. The final exam consists of a unique oral session with a typical duration of 45 to 60 minutes, during which the student is asked both general questions about theory and more specific problems. These, in particular, will test the student's capability of properly addressing the problems. To this aim, the student will have to know the values of the fundamental constants and how to use them to estimate the requested astrophysical quantities.
The exam aims at assessing the expertise acquired by the student, their ability to establish connections between the different topics and aspects of the course, as well as the formal mathematical rigour demanded by each topic. Reference texts
- Slides are made available. The first four textbooks listed below are the main source of reference for the course. The remaining textbooks are additional sources for a more in-depth study.
1) H. Bradt, "Astrophysics Processes", Cambridge
2) G.B. Rybicki, A.P. Lightman, "Radiative Processes in Astrophysics", Wiley
3) M. Vietri, "Astrofisica delle Alte Energie", Bollati Boringhieri.
4) K. Thorne, R. Blandford, "Modern Classical Physics", Princeton.
5) M. Longair, "High Energy Astrophysics", Cambridge University Press.
6) G. Ghisellini, "Radiative Processes in High Energy Astrophysics", Springer.
7) S. Weinberg, "Lectures on Astrophysics", Cambridge University Press.
8) Burke, B.F., Graham-Smith, F.: An Introduction to Radio Astronomy. Third Edition. Cambridge University Press (2010).
9) Marr, J.M., Snell, R.L., Kurtz, S.E.: Fundamentals of Radio Astronomy. Observational Methods. CRC Press, Taylor & Francis Group (2015).
