Blazar Flares Research Articles

Transient Emission from Dissipative Fronts in Magnetized, Relativistic Outflows. I. Gamma‐Ray Flares

The transient emission produced behind internal shocks that are driven by overtaking collisions of a magnetized, relativistic outflow is considered. A self-consistent model capable of describing the structure and dynamics of the shocks and the time evolution of the pair and gamma-ray distribution functions is developed and applied to gamma-ray flares in blazars, in the case in which gamma-ray production is dominated by inverse Compton scattering of external radiation. The dependence of the flare properties on magnetic field dissipation rate, the intensity of ambient radiation, and the thickness of expelled fluid slabs is analyzed. It is shown that (1) the type of gamma-ray flare produced by the model is determined by the ratio of the thickness of ejected fluid slab and the gradient length scale of ambient radiation intensity; (2) the radiative efficiency depends sensitively on the opacity contributed by the background radiation, owing to a radiative feedback, and is typically very high for parameters characteristic to the powerful blazars; and (3) the emitted flux is strongly suppressed at energies for which the pair-production optical depth is initially larger than unity; the time lag and flare duration in this energy range increase with increasing gamma-ray energy. At lower energies, flaring at different gamma-ray bands occurs roughly simultaneously but with possibly different amplitudes. Some observational consequences are discussed.

Electron acceleration and synchrotron radiation in decelerating plasmoids

An equation is derived to calculate the dynamics of relativistic magnetized plasma which decelerates by sweeping up matter from the ISM. Reduction to the non-radiative and radiative regimes is demonstrated for the general case of a collimated plasmoid whose area increases as a power of distance x from the explosion, and in the specific case of a blast-wave geometry where the area increases quadratically with x. An equation for the evolution of the electron momentum distribution function in the comoving fluid frame is used to calculate the observed synchrotron radiation spectrum. The central uncertainty involves the mechanism for transfering energy from the non-thermal protons to the electrons. The simplest prescription is to assume that a fixed fraction of the comoving proton power is instantaneously transformed into a power-law, “shock”-like electron distribution function. This permits an analytic solution for the case of a relativistic plasmoid with a constant internal magnetic field. Effects of parameter changes are presented. Breaks in the temporal behavior of the primary burst emission and long wavelength afterglows occur on two time scales for external ISM density distributions n ext α x − η . The first, as emphasized by Mészáros & Rees, represents the time when the outflow sweeps up ≈ M th/Γ 0 of material from the ISM, where M this the mass in the outflow ejecta and Γ 0 is the initial Lorentz factor of the plasmoid. The second represents the time when synchrotron cooling begins strongly to regulate the number of nonthermal electrons that are producing radiation which is observed at a given energy. Results are applied to GRB and blazar variability. The slopes of the time profiles of the X-ray and optical light curves of the afterglows of GRB 970228 and GRB 970508 are explained by injection of hard electron spectra with indices s < 2 in a deceleration regime near the radiative limit. We also propose that blazar flares are due to the transformation of the directed kinetic energy of the plasmoid when interacting with the external medium, as would occur if relativistic outflows pass through clouds orbiting the central supermassive black hole.

Stochastic Particle Acceleration near Accreting Black Holes

view Abstract Citations (99) References (82) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Stochastic Particle Acceleration near Accreting Black Holes Dermer, Charles D. ; Miller, James A. ; Li, Hui Abstract We consider the stochastic acceleration of particles which results from resonant interactions with plasma waves in black hole magnetospheres. We calculate acceleration rates and escape timescales for protons and electrons resonating with Alfvén waves, and for electrons resonating with whistlers. Assuming either a Kolmogorov or a Kraichnan wave spectrum, accretion at the Eddington limit, magnetic field strengths near equipartition, and turbulence energy densities ∼10% of the total magnetic field energy density, we find that Alfvén waves accelerate protons to Lorentz factors ≲ 104-106 before they escape from the system. Acceleration of electrons by fast-mode and whistler waves can produce a nonthermal population of relativistic electrons whose maximum energy is determined by a competition with radiation losses. Particle energization and outflow is not possible at lower accretion rates, magnetic field strengths, or turbulence levels, owing to dominant Coulomb losses. Increases in the accretion luminosity relative to the Eddington luminosity can trigger particle acceleration out of the thermal background, and this mechanism could account for the differences between radio-quiet and radio-loud active galactic nuclei. Observations of outflowing radio-emitting components following transient X-ray events in Galactic X-ray novae and gamma-ray flares in blazars are in accord with this scenario. Publication: The Astrophysical Journal Pub Date: January 1996 DOI: 10.1086/176631 arXiv: arXiv:astro-ph/9508069 Bibcode: 1996ApJ...456..106D Keywords: GALAXIES: ACTIVE; ACCELERATION OF PARTICLES; ACCRETION; ACCRETION DISKS; BLACK HOLE PHYSICS; RADIATION MECHANISMS: NONTHERMAL; Astrophysics E-Print: 31 pages, 8 figures, uuencoded compressed postscript file. Accepted for publication in The Astrophysical Journal (1 January 1996) full text sources arXiv | ADS | data products SIMBAD (7)