Abstract
The study of multi-wavelength flux variability in BL Lacertae objects is very important to discern unstable processes and emission mechanisms underlying their extreme observational features. While the innermost regions of these objects are not accessible from direct observations, we may draw conclusions about their internal structure via the detection of flux variations on various timescales, based on the light-travel argument. In this paper, we review the sub-hour X-ray variability in high-energy peaked BL Lacertae sources (HBLs) that are bright at X-rays and provide us with an effective tool to study the details related to the physics of the emitting particles. The X-ray emission of these sources is widely accepted to be a synchrotron radiation from the highest-energy electrons, and the complex spectral variability observed in this band reflects the injection and radiative evolution of freshly-accelerated particles. The detection of sub-hour X-ray flux variability is very important since it can be related to the small-scale jet turbulent structures or triggered by unstable processes occurring in the vicinity of a central supermassive black hole. We summarize the fastest X-ray variability instances detected in bright HBLs and discuss their physical implications.
Highlights
The BL Lacertae objects (BL Lacs) are active nuclei of elliptical galaxies [1] showing a rapid variability at all frequencies, high and variable radio-optical polarization, compact and flat-spectrum radio emission, superluminal motion of some radio-components, smooth and broad non-thermal continuum covering the entire electromagnetic spectrum from the radio range to γ-rays and the absence of emission lines in the optical band [2].Along with the flat-spectrum radio quasars (FSRQs), BL Lacertae objects form one of the extreme classes of active galactic nuclei (AGNs), blazars, which can be powered by the accretion onto supermassive black holes (SMBHs) and have relativistic jets pointed along the observer’s line-of-sight [3]
From the past X-ray observations of Mrk 501, an extreme intra-day variability (IDV) was reported by [45] when an increase of the 2–10 keV flux by 60 percent in 200 s was followed by a drop by 40 percent in 600 s during the observation performed with the Photon Counting Array (PCA) onboard the satellite Rossi X-ray
The relativistic magnetohydrodynamic simulations of [51] showed that the shock propagation through an inhomogeneous jet medium can lead to the amplification of turbulent magnetic fields in a shocked area
Summary
The BL Lacertae objects (BL Lacs) are active nuclei of elliptical galaxies [1] showing a rapid variability at all frequencies, high and variable radio-optical polarization, compact and flat-spectrum radio emission, superluminal motion of some radio-components, smooth and broad non-thermal continuum covering the entire electromagnetic spectrum from the radio range to γ-rays and the absence of emission lines in the optical band [2]. The Doppler boosting is related to the combination of relativistic speed and a small jet inclination angle that amplifies the observed flux, shifts the observed emission to higher frequencies and decreases the variability timescale This effect is crucial to preserve the γ-ray photons from the strong attenuation by the surrounding low-energy radiation field (via the photon-photon pair production mechanism). There is a variety of models explaining its origin: (1) an inverse Compton (IC) scattering of synchrotron photons by their “parent” electron-positron plasma (the so-called synchrotron self-Compton model (SSC); Marscher and Gear [7]); (2) hadronic models incorporating the generation of hard X-ray–γ-ray photons by relativistic protons, either directly (proton synchrotron; [8]) or indirectly (e.g., synchrotron emission from a secondary electron population; Mannheim [9]) Among these scenarios, a valid model can be selected by means of the intensive multi-wavelength (MWL) flux variability and inter-band cross-correlation studies. The detection and detailed investigation of sub-hour X-ray instances is very important since they can be related to the smallest scale turbulent structures in the jet or triggered by unstable processes occurring in the vicinity of the central supermassive black hole
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