The flux variability of blazars at very high energies does not have a clear origin. Flux variations on time scales down to the minute suggest that variability originates in the jet, where a ...relativistic boost can shorten the observed time scale, while the linear relation between the flux and its rms or the skewness of the flux distribution suggests that the variability stems from multiplicative processes, which are associated in some models with the accretion disk. We study the rms-flux relation and emphasize its link to Pareto distributions, characterized by a power-law probability density function. Such distributions are naturally generated within a minijets-in-a-jet statistical model, in which boosted emitting regions are isotropically oriented within the bulk relativistic flow of a jet. We prove that, within this model, the flux of a single minijet is proportional to its rms. This relation still holds when considering a large number of emitting regions, for which the distribution of the total flux is skewed and could be interpreted as being log-normal. The minijets-in-a-jet statistical model reconciles the fast variations and the statistical properties of the flux of blazars at very high energies.
Context. The characterization of a time series is a powerful tool for investigating the nature of mechanisms that generate variability in astrophysical objects. Blazar variability across the entire ...electromagnetic spectrum is a long-standing puzzle, and it has been difficult to ascertain the mechanisms at play. Aims. Lognormal variability in X-ray light curves, probably related to accretion disk activity, has been discovered in various compact systems, such as Seyfert galaxies and X-ray binaries. Identifying a similar behaviour in blazars would establish a link between them. Methods. Public X-ray data from the blazar BL Lac are used to investigate the nature of its variability, and more precisely the flux dependency of the variability and the distribution of fluxes. Results. The variations in the flux are found to have a lognormal distribution and the average amplitude of variability is proportional to the flux level. Conclusions. BL Lac is the first blazar in which lognormal X-ray variability is clearly detected. The light curve is orders of magnitude less variable than other blazars, with few bursting episodes. If this defines a specific state of the source, then the lognormality might be the imprint of the accretion disk on the jet, linking for the first time accretion and jet properties in a blazar.
We update the list of GeV-TeV extragalactic γ-ray sources using the two-year catalog from the Fermi Large-Area Telescope (LAT) and recent results from ground-based γ-ray telescopes. Breaks in the ...spectra between the high-energy (100 MeV < E < 300 GeV) and the very high-energy (E > 200 GeV) ranges, as well as their dependence on distance, are discussed in the context of absorption on the extragalactic background light (EBL). We calculate the size of the expected break using a model for the EBL and compare it to the data, taking into account systematic uncertainties in the measurements. We develop a novel Bayesian model to describe this dataset and use it to constrain two simple models for the EBL-induced breaks.
ABSTRACT We report the first hard X-ray observations with NuSTAR of the BL Lac-type blazar PKS 2155-304, augmented with soft X-ray data from XMM-Newton and γ-ray data from the Fermi Large Area ...Telescope, obtained in 2013 April when the source was in a very low flux state. A joint NuSTAR and XMM spectrum, covering the energy range 0.5-60 keV, is best described by a model consisting of a log-parabola component with curvature and a (local) photon index 3.04 0.15 at photon energy of 2 keV, and a hard power-law tail with photon index 2.2 0.4. The hard X-ray tail can be smoothly joined to the quasi-simultaneous γ-ray spectrum by a synchrotron self-Compton component produced by an electron distribution with index p = 2.2. Assuming that the power-law electron distribution extends down to γmin = 1 and that there is one proton per electron, an unrealistically high total jet power of Lp ∼ 1047 erg s−1 is inferred. This can be reduced by two orders of magnitude either by considering a significant presence of electron-positron pairs with lepton-to-proton ratio , or by introducing an additional, low-energy break in the electron energy distribution at the electron Lorentz factor γbr1 ∼ 100. In either case, the jet composition is expected to be strongly matter-dominated.
The High Energy Stereoscopic System (H.E.S.S.) is one of the three arrays of imaging atmospheric Cherenkov telescopes (IACTs) currently in operation. It is composed of four 12-meter telescopes and a ...28-meter one, and is sensitive to gamma rays in the energy range ~ 30 GeV – 100 TeV. The cameras of the 12-m telescopes recently underwent a substantial upgrade, with the goal of improving their performance and robustness. The upgrade involved replacing all camera components except for the photomultiplier tubes (PMTs). This meant developing new hardware for the trigger, readout, power, cooling and mechanical systems, and new software for camera control and data acquisition. Several novel technologies were employed in the cameras: the readout is built around the new NECTAr digitizer chip, developed for the next generation of IACTs; the camera electronics is fully controlled and read out via Ethernet using a combination of FPGA and embedded ARM computers; the software uses modern libraries such as Apache Thrift, ØMQ and Protocol buffers. This work describes in detail the design and the performance of the upgraded cameras.
Context.High-energy γ-rays propagating in the intergalactic medium can interact with background infrared photons to produce e+ e- pairs, resulting in the absorption of the intrinsic γ-ray spectrum. ...TeV observations of the distant blazar 1ES 1101-232 were thus recently used to put an upper limit on the infrared extragalactic background light density. Aims.The created pairs can upscatter background photons to high energies, which in turn may pair produce, thereby initiating a cascade. The pairs diffuse on the extragalactic magnetic field (EMF) and cascade emission has been suggested as a means for measuring its intensity. Limits on the IR background and EMF are reconsidered taking into account cascade emissions. Methods.The cascade equations are solved numerically. Assuming a power-law intrinsic spectrum, the observed 100 MeV–100 TeV spectrum is found as a function of the intrinsic spectral index and the intensity of the EMF. Results.Cascades emit mainly at or below 100 GeV. The observed TeV spectrum appears softer than for pure absorption when cascade emission is taken into account. The upper limit on the IR photon background is found to be robust. Inversely, the intrinsic spectra needed to fit the TeV data are uncomfortably hard when cascade emission makes a significant contribution to the observed spectrum. An EMF intensity around 10-8 nG leads to a characteristic spectral hump in the GLAST band. Higher EMF intensities divert the pairs away from the line-of-sight and the cascade contribution to the spectrum becomes negligible.
Aims.Our goal was to find important clues concerning particle acceleration in relativistic jets through the analysis of multi-wavelength observations of TeV blazars. Methods.Simultaneous observations ...of Mkn 421 were taken in very high energy γ-rays (>200 GeV, CAT experiment), X-rays (RXTE), and optical (KVA). Multi-day RXTE observations are also presented, allowing for detailed modelling of the spectral variability. Results.Short timescale (≈30 mn) variations in VHE γ-rays are found, correlated with X-rays, but not with the optical. The X-ray spectrum hardens with flux until the photon indices saturate above a threshold flux ≈10-9erg s-1 cm-2. The fractional variability decreases from X-rays to optical as a power law with $F_{\rm var}\propto E^{0.24\pm0.01}$. The full spectral energy distribution is well-fitted by synchrotron self-Compton emission from cooling electrons injected with a Maxwellian distribution of characteristic energy $\gamma_b$. Fluctuations in the injected power with $P_{\rm inj}\propto \gamma_b^4$ explain the observed variability. Conclusions.The spectral saturation and the power-law dependence of the fractional variability are novel results that may extend to other TeV blazars. The ability of Maxwellian injections to reproduce the observed features suggest second-order Fermi acceleration or magnetic reconnection may play the dominant role in particle acceleration.
Following the discovery of high-energy (HE; E > 10 MeV) and very-high-energy (VHE; E > 100 GeV) γ-ray emission from the low-frequency-peaked BL Lac (LBL) object AP Librae, its electromagnetic ...spectrum is studied over 60 octaves in energy. Contemporaneous data in radio, optical and UV together with the (non-simultaneous) γ-ray data are used to construct the most precise spectral energy distribution of this source. The data have been found to be modelled with difficulties with single-zone homogeneous leptonic synchrotron self-Compton (SSC) radiative scenarios due to the unprecedented width of the HE component when compared to the lower-energy component. The two other LBL objects also detected at VHE appear to have similar modelling difficulties. Nevertheless, VHE γ-rays produced in the extended jet could account for the VHE flux observed by HESS.
Very high energy (>100 GeV) gamma-ray emission has been detected for the first time from the composite supernova remnant G 0.9+0.1 using the HESS instrument. The source is detected with a ...significance of ≈$13\sigma$, and a photon flux above 200 GeV of ($5.7\pm0.7_{\rm stat}\pm1.2_{\rm sys})\times10^{-12}$ cm-2 s-1, making it one of the weakest sources ever detected at TeV energies. The photon spectrum is compatible with a power law (${\rm d}N/{\rm d}E \propto E^{-\Gamma}$) with photon index $\Gamma = 2.40\pm0.11_{\rm stat}\pm0.20_{\rm sys}$. The gamma-ray emission appears to originate in the plerionic core of the remnant, rather than the shell, and can be plausibly explained as inverse Compton scattering of relativistic electrons.
We presented in 2014 the very first data from a polarised gamma-ray beam between 1.7 and 74MeV. We now show the results of their analysis, and in particular the polarimetry measurements. With these ...results, we are establishing a new, high-performance way to do gamma-ray astronomy and, for the first time, polarimetry, in the e+e- pair regime.
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