No abstract

Blazars are a class of jet-dominated active galactic nuclei with a typical double-humped spectral energy distribution. It is of common consensus that the synchrotron emission is responsible for the low frequency peak, while the origin of the high frequency hump is still debated. The analysis of X-rays and their polarization can provide a valuable tool to understand the physical mechanisms responsible for the origin of high-energy emission of blazars. We report the first observations of BL Lacertae (BL Lac) performed with the Imaging X-ray Polarimetry Explorer, from which an upper limit to the polarization degree Π X < 12.6% was found in the 2–8 keV band. We contemporaneously measured the polarization in radio, infrared, and optical wavelengths. Our multiwavelength polarization analysis disfavors a significant contribution of proton-synchrotron radiation to the X-ray emission at these epochs. Instead, it supports a leptonic origin for the X-ray emission in BL Lac.

The average shower depth ⟨ max ⟩ of Ultra-high Energy Cosmic Rays is observed to flatten with energy at the highest energies. The standard interpretation of these data is that the composition is getting heavier; an alternative interpretation is the existence of new effects in proton interactions above ∼ 50 TeV center-of-mass energy. We have used CORSIKA to study, through air-shower simulations, observational signatures of a possible increase in cross-section and multiplicity in collisions exceeding this threshold. We have simulated hadronic collisions for primaries with energies in the range 10 8 − 10 11 GeV. We have used two different high energy models for the simulations, QGSJETII-04 and EPOS LHC, with Fluka for low energy interactions on both. A smooth transition from Galactic to extragalactic cosmic rays was implemented, by fitting a Galactic component with an exponential suppression at ∼ 10 9 GeV. The remaining flux in Auger data was interpreted as extragalactic protons. Above 10 9 GeV, the proton-air cross-section and the multiplicity of secondary particles were altered, so as to bring the simulated ⟨ max ⟩ in agreement with Auger data. The parameter space of the viable cross-section and multiplicity in the scenario where the composition of Auger cosmic rays at the highest energies remains unchanged and light, places constraints on the phenomenology of any new physics affecting the interactions for high energy protons that may be probed by √ > 50 TeV collisions. We found out that if new physics indeed sets in, the cross-section of proton-Air interactions has to be ∼ 800-900 mb at 140 TeV center-of-mass energy, accompanied with an increase of the number of secondary particles by a factor between 2-3.