To appear in “Compact Binaries in the Galaxy and Beyond (2003)” RevMexAA(SC) THERMAL RADIATION FROM AN ACCRETION DISK F.V.Prigara 1 An effect of stimulatedradiation processes on servedinradiobandforcompactandextendedradio thermal radiation from an accretion disk is sourcesrespectively. Comparetheseresultswiththe considered. The radial density waves trig- Lyndell-Bell’s relation r ∝λ4/3 . λ gering flare emission and producing quasi- Some X-ray binaries, e.g. Sco X-1, show weak periodic oscillations in radiation from an ac- emission lines with the variable intensities and ra- 4 cretion disk are discussed. It is argued dial velocities. These features are characteristic for 0 that the observational data suggest the ex- non-saturated lasers. The wavelength of generated 0 istence of the weak laser sources in a two- modeisdeterminedbythesizeofanelementaryres- 2 temperature plasma of an accretion disk. onatorwhichisdependingonthedensity. Theradial n It was shown earlier that thermal radio emission density wave travelling along the radius changes the a J has a stimulated character and that this statement density which results in the variations of the wave- 0 is probably valid for thermal blackbody radiation in length. Convection or advection in the gaseous disk 2 others spectral ranges (Prigara 2003). According to produces a two-temperature plasma in which it is this conception thermal radiation from non-uniform possible to create the inversion of the energy level 3 gas is produced by an ensemble of individual emit- population. This gives rise to the weak lasers simi- v ters. Each of these emitters is an elementary res- lar to the weak molecular masers. 1 onator the size of which has an order of magnitude The radial density wave produces also the de- 9 4 of mean free path of photons, l = 1/(nσ), where lay of flares at low frequencies and high frequency 4 n is the number density of particles, and σ is the quasi-periodic oscillations observed in X-ray bina- 0 absorption cross-section. ries. Density waves can presumably propagate in a 3 The emission of each elementary resonator is co- sufficiently hot plasma of an accretion disk. They 0 herent, with the wavelength λ = l, and thermal ra- differ from the shocks, since the density waves have / h diation from a gaseous layer is incoherent sum of the smooth density and pressure profiles. p radiation produced by individual emitters. This de- The density waves in a hot plasma seem to have - o pendence of the wavelength on the density is con- been observed in a solar microwave burst, causing r sistent with the experimental results by Looney and thestrongcorrelationinthemodulationofradioand t s Brownontheexcitationofplasmaoscillations(Alex- X-rayemissionoveralargedistance(1010cm)inthe a : eev 2003). solar corona (Grechnev et al. 2003). v In the gaseous disk model (Prigara 2003), the i REFERENCES X densitydependsuponthedistancer fromtheenergy r center as follows Alexeev, B.V. 2003, Usp. Fiz. Nauk, 173,145, Physics- a Uspekhi, Vol. 46 n∝(cid:0)v02r2+c2rsr(cid:1)−1/2, (1) Gre5c8h8n,ev11,6V3.V.,White,S.M.&Kundu,M.R.2003,ApJ, Nagar, N.M., Wilson, A.S. & Falcke, H.2001, ApJ, 559, where r is the Schwarzschild radius, c is the speed s L87 of light, and v0 is a constant. This density profile is Prigara, F.V. 2003, Astron. Nachr., 324, No. S1, 425 consistent with the convection dominated accretion flow(CDAF)modelsortheadvectiondominatedac- cretionflow(ADAF) modelswithanoutflow(Nagar et al. 2001). The last equation gives the wavelength depen- dence of the emitting region size r in the form λ r ∝ λ2 at small values of the radius, and r ∝ λ λ λ at the large radii. These relations are indeed ob- 1Institute of Microelectronics and Informatics, Russian Academy of Sciences, 21 Universitetskaya, Yaroslavl 150007, Russia([email protected]). 1