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Register a new userParticle-acceleration timescales in TeV blazar flares
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Observations of minute-scale flares in TeV Blazars place constraints on particle acceleration mechanisms in those objects. The implications for a variety of radiation mechanisms have been addressed in the literature; in this paper we compare four different acceleration mechanisms: diffusive shock acceleration, second-order Fermi, shear acceleration and the converter mechanism. When the acceleration timescales and radiative losses are taken into account, we can exclude shear acceleration and the neutron-based converted mechanism as possible acceleration processes in these systems. The first-order Fermi process and the converter mechanism working via SSC photons are still practically instantaneous, however, provided sufficient turbulence is generated on the timescale of seconds. We propose stochastic acceleration as a promising candidate for the energy-dependent time delays in recent gamma-ray flares of Markarian 501.
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The jets of blazars are renowned for their multi-wavelength flares and rapid extreme variability; however, there are still some important unanswered questions about the physical processes responsible for these spectral and temporal changes in emission properties. In this paper, we develop a time-dependent particle evolution model for the time-varying emission spectrum of blazars. In the model, we introduce time-dependent electric and magnetic fields, which consistently include the variability of relevant physical quantities in the transport equation. The evolution on the electron distribution is numerically solved from a generalized transport equation that contains the terms describing the electrostatic, first-order and second-order emph{Fermi} acceleration, escape of particles due to both advection and spatial diffusion, as well as energy losses due to the synchrotron emission and inverse-Compton scattering of both synchrotron and external ambient photon fields. We find that the light curve profiles of blazars are consistent with the particle spectral evolution resulting from time-dependent electric and magnetic fields, rather than the effects of the acceleration or the cooling processes. The proposed model is able to simultaneously account for the variability of both the energy spectrum and the light curve profile of the BL Lac object Mrk 421 with reasonable assumptions about the physical parameters. The results strongly indicate that the magnetic field evolution in the dissipated region of a blazar jet can account for the variabilities.
We report the results of our optical (VRI) photometric observations of the TeV blazar 1ES 0806$+$524 on 153 nights during 2011-2019 using seven optical telescopes in Europe and Asia. We investigated the variability of the blazar on intraday as well as on long-term timescales. We examined eighteen intraday light curves for flux and color variations using the most reliable power-enhanced F-test and the nested ANOVA test. Only on one night was a small, but significant, variation found, in both $V$ band and $R$ band light curves. The $V-R$ color index was constant on every one of those nights. Flux density changes of around 80 % were seen over the course of these eight years in multiple bands. We found a weighted mean optical spectral index of 0.639$pm$0.002 during our monitoring period by fitting a power law ($F_{ u} propto u^{-alpha}$) in 23 optical ($VRI$) spectral energy distributions of 1ES 0806$+$524. We discuss different possible mechanisms responsible for blazar variability on diverse timescales.
We present a new time-dependent leptonic code that we developed to model the varying multi-wavelength (MWL) emission during blazar flares. In our modeling, we assume that the blazar emission originates from a plasma blob located in the jet, and that relativistic electrons are injected into the blob and may undergo stochastic (Fermi II) or shock (Fermi I) acceleration. We numerically solve the kinetic equation for electron evolution in the blob, taking into account particle injection, escape, acceleration and radiative cooling. In order to calculate the spectral energy distribution (SED) of the blob emission we assume a synchrotron self-Compton (SSC) scenario, including also synchrotron self absorption and gamma-gamma absorption processes. Our code computes the evolution of the electron spectrum and of the associated broad-band SED. As a first application, we attempt to connect the continuous, steady-state emission from the blazar Mrk 421 with a flare observed in February 2010, using a minimal number of free parameters in a two-zone scenario in which a turbulent region is present around the emitting zone. Mrk 421 is a high-synchrotron-peaked (HSP) BL Lac, and one of the brightest extragalactic gamma-ray sources in the Very High Energy (VHE) gamma-ray band. It is also the closest TeV emitting blazar to the Earth (redshift z=0.031).
The mechanism that accelerates particles to the energies required to produce the observed high-energy impulsive emission in solar flares is not well understood. Drake et al. (2006) proposed a mechanism for accelerating electrons in contracting magnetic islands formed by kinetic reconnection in multi-layered current sheets. We apply these ideas to sunward-moving flux ropes (2.5D magnetic islands) formed during fast reconnection in a simulated eruptive flare. A simple analytic model is used to calculate the energy gain of particles orbiting the field lines of the contracting magnetic islands in our ultrahigh-resolution 2.5D numerical simulation. We find that the estimated energy gains in a single island range up to a factor of five. This is higher than that found by Drake et al. for islands in the terrestrial magnetosphere and at the heliopause, due to strong plasma compression that occurs at the flare current sheet. In order to increase their energy by two orders of magnitude and plausibly account for the observed high-energy flare emission, the electrons must visit multiple contracting islands. This mechanism should produce sporadic emission because island formation is intermittent. Moreover, a large number of particles could be accelerated in each magnetohydrodynamic-scale island, which may explain the inferred rates of energetic-electron production in flares. We conclude that island contraction in the flare current sheet is a promising candidate for electron acceleration in solar eruptions.
Following the detection of strong TeV gamma-ray flares from the BL Lac object 1ES 1959+650 with the Whipple 10 m Cherenkov telescope on May 16 and 17, 2002, we performed intensive Target of Opportunity (ToO) radio, optical, X-ray and TeV gamma-ray observations from May 18, 2002 to August 14, 2002. Observations with the X-ray telescope RXTE and the Whipple and HEGRA gamma-ray telescopes revealed several strong flares, enabling us to sensitively test the X-ray/gamma-ray flux correlation properties. Although the X-ray and gamma-ray fluxes seemed to be correlated in general, we found an ``orphan gamma-ray flare that was not accompanied by an X-ray flare. After describing in detail the radio (UMRAO, VLA), optical (Boltwood, Abastumani), X-ray (RXTE) and gamma-ray (Whipple, HEGRA) light curves and Spectral Energy Distributions (SEDs) we present initial modeling of the SED with a simple Synchrotron Self-Compton (SSC) model. With the addition of another TeV blazar with good broadband data, we consider the set of all TeV blazars to begin to look for a connection of the jet properties to the properties of the central accreting black hole thought to drive the jet. Remarkably, the temporal and spectral X-ray and gamma-ray emission characteristics of TeV blazars are very similar, even though the masses estimates of their central black holes differ by up to one order of magnitude.
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