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We study simultaneous soft (0.7–7 keV) and hard (7–20 keV) X-ray light curves at a total of eight epochs during 2016–2019 of two TeV blazars Mrk 421 and 1ES 1959+650 observed by the SXT and LAXPC instruments on-board AstroSat. The light curves are 45–450 ks long and may be sampled with time bins as short as 600–800 s with high signal-to-noise ratio. The blazars show a harder when brighter trend at all epochs. Discrete cross-correlation functions indicate that the hard and soft X-ray variability are strongly correlated. The time lag is consistent with zero in some epochs, and indicates hard or soft lag of a few hours in the rest. In the leptonic model of blazar emission, soft lag may be due to slower radiative cooling of lower energy electrons while hard lag may be caused by gradual acceleration of the high energy electrons emitting at the hard X-ray band. Assuming the above scenario and the value of the Doppler factor (δ) to be 10–20, the hard and soft lags may be used to estimate the magnetic field to be ∼0.1 Gauss and the acceleration parameter to be ∼104 in the emission region. Due to the availability of the high time resolution (∼ minutes to hours) light curves from AstroSat , the value of the illusive acceleration parameter could be estimated, which provides a stringent constraint on the theories of particle acceleration in blazar jets. [ABSTRACT FROM AUTHOR]

The power spectral density (PSD) of the X-ray emission variability from the accretion disc-corona region of black hole X-ray binaries and active galactic nuclei has a broken power-law shape with a characteristic break time-scale T B. If the disc and the jet are connected, the jet variability may also contain a characteristic time-scale related to that of the disc-corona. Recent observations of the blazar Mrk 421 have confirmed the broken power-law shape of the PSD of its jet X-ray variability. We model the time variability of a blazar, in which emitting particles are assumed to be accelerated by successive shock waves flowing down the jet with a varying inter-shock time-scale (T IS). We investigate the possible relation between the characteristic time-scales in the disc and jet variability based on the above model, along with mathematically and physically simulated disc variability. We find that both the PSD of the jet and disc variability may have a broken power-law shape but the break time-scales are not related in general except only in systems with a small range of BH mass. The break in the jet and the disc PSD are connected to the interval between large amplitude outbursts in the jet (T IS) and to the viscous time-scale in the disc, respectively. In frequency bands where multiple emission processes are involved or emission is from lower energy particles, the break in the PSD may not be prominent enough for detection. [ABSTRACT FROM AUTHOR]

We compare the rise and decay time-scales of ∼200 long-term (∼weeks–months) GeV and R -band outbursts and ∼25 short-term (∼hour–day) GeV flares in a sample of 10 blazars from the Fermi -LAT and the Yale/SMARTS monitoring programme. We find that most of the long-term outbursts are symmetric, indicating that the observed variability is dominated by the crossing time-scale of a disturbance. A larger fraction of short-term flares are asymmetric with an approximately equal fraction of longer and shorter decay than rise time-scale. We employ the MUlti-ZOne Radiation Feedback (MUZORF) model to interpret the above results. We find that the outbursts with slow rise times indicate a gradual acceleration of the particles to GeV energy. A change in the bulk Lorentz factor of the plasma or the width of the shocked region can lead to an increase of the cooling time causing a faster rise than decay time. Luminosity or the distance of the broad line region (BLR) affect the cooling time strongly if a single emission mechanism, e.g. external Compton scattering of BLR photons, is considered but may not if other mechanisms, e.g. synchrotron self-Compton and external Compton scattering of the torus photon, are included. This work carries out an extensive and systematic study, which can be used to estimate relevant geometric and physical parameters of the jet from the symmetry property of any observed short-term flares in the context of the MUZORF model. [ABSTRACT FROM AUTHOR]

We present the time variability properties of a sample of six blazars, AO0235+164, 3C 273, 3C 279, PKS 1510-089, PKS 2155-304, and 3C 454.3, at optical-IR frequencies as well as γ-ray energies. These observations were carried out as a part of the Yale/SMARTS program during 2008-2010 that has followed the variations in emission of the bright Fermi Large Area Telescope monitored blazars in the southern sky with closely spaced observations at BVRJK bands. We find that the optical-near IR variability properties are remarkably similar to those at the γ-ray energies. The discrete auto-correlation functions of the variability of these six blazars at optical-IR and γ-ray energies do not show any periodicity or characteristic timescale. The power spectral density (PSD) functions of the R-band variability of all six blazars are fit well by simple power-law functions with negative slopes such that there is higher amplitude variability on longer timescales. No clear break is identified in the PSD of any of the sources. The average slope of the PSD of R-band variability of these blazars is similar to what was found by the Fermi team for the γ-ray variability of a larger sample of bright blazars. This is consistent with leptonic models where the optical-IR and γ-ray emission is generated by the same population of electrons through synchrotron and inverse Compton processes, respectively. The prominent flares present in the optical-IR as well as the γ-ray light curves of these blazars are predominantly symmetric, i.e., have similar rise and decay timescales, indicating that the long-term variability is dominated by the crossing time of radiation or a disturbance through the emission region rather than by the acceleration or energy-loss timescales of the radiating electrons. For the blazar 3C 454.3, which has the highest-quality light curves, the total energy output, the ratio of γ-ray to optical energy output, and the γ-ray versus optical flux relation differ in the six individual flares observed between 2009 August and December. The results are consistent with the location of a large γ-ray outburst in 3C 454.3 during 2009 December being in the jet at ~18 pc from the central engine. This poses strong constraints on the models of high-energy emission in the jets of blazars. [ABSTRACT FROM AUTHOR]

We present multiwavelength data for 12 blazars observed from 2008 to 2010 as part of an ongoing optical-infrared photometric monitoring project. Sources were selected to be bright, southern (δ < 20°) blazars observed by the Fermi Gamma-Ray Space Telescope. Light curves are presented for the 12 blazars in BVRJK at near-daily cadence. We find that optical and infrared fluxes are well correlated in all sources. Gamma-ray bright flat spectrum radio quasars (FSRQs) in our sample have optical/infrared emission correlated with gamma-rays consistent with inverse Compton-scattering models. In FSRQs, variability amplitude increases toward IR wavelengths, consistent with the presence of a thermal accretion disk varying on significantly longer timescales than the jet. In BL Lac objects, variability is mainly constant, or increases toward shorter wavelength. FSRQs have redder optical-infrared colors when they are brighter, while BL Lac objects show no such trend. Several objects show complicated color-magnitude behavior: AO 0235+164 appears in two different states depending on its gamma-ray intensity. OJ 287 and 3C 279 show some hysteresis tracks in their color-magnitude diagrams. Individual flares may be achromatic or otherwise depart from the trend, suggesting different jet components becoming important at different times. We present a time-dependent spectral energy distribution of the bright FSRQ 3C 454.3 during its 2009 December flare, which is well fit by an external Compton model in the bright state, although day-to-day changes pose challenges to a simple one-zone model. All data from the SMARTS monitoring program are publicly available on our Web site. [ABSTRACT FROM AUTHOR]