Context. The detection of quasi-periodic oscillations (QPOs) in the light curves of active galactic nuclei (AGNs) can provide insights into the physics of the super-massive black holes (SMBHs) powering these systems and could represent a signature of the existence of SMBH binaries, setting fundamental constraints on SMBH evolution in the Universe. Aims. The identification of long-term QPOs, characterized by periods on the order of several months to years, is particularly challenging and can only be achieved via all-sky monitoring instruments that provide unbiased, continuous light curves of astrophysical objects. The Fermi-LAT satellite, thanks to its monitoring observing strategy, is an ideal instrument for such a goal. Here, we aim to identify QPOs in the γ-ray light curves of the brightest AGNs within the Fermi-LAT catalog. Methods. We analyzed the light curves of the 35 brightest Fermi-LAT AGNs, including data from the beginning of the Fermi mission (August 2008) to April 2021, with energies from 100 MeV to 300 GeV. Two time binnings were investigated: 7 and 30 days. The search for quasi-periodic features was then performed using the continuous wavelet transform. The significance of the result was tested via Monte Carlo simulations of artificial light curves with the same power spectral density and probability distribution function as the original light curves. The significances were then corrected for the look-elsewhere effect and provided as post-trials. Results. We identified 24 quasars with candidate QPOs. Several of our candidates coincide with previous claims in the literature, namely: PKS 0537−441, S5 0716+714, Mrk 421, B2 1520+31, and PKS 2247−131. All our candidates are transient. The most significant multi-year QPO, with a period of about 1100 days, was observed in the quasar S5 1044+71. It is reported here for the first time.
HESS J1813-178 is one of the brightest sources detected during the first HESS Galactic Plane survey. The compact source, also detected by MAGIC, is believed to be a pulsar wind nebula powered by one of the most powerful pulsars known in the Galaxy, PSR J1813-1749 with a spindown luminosity of E = 5.6 • 10 37 erg s −1 . With its extreme physical properties, as well as the pulsar's young age of 5.6 kyrs, the 𝛾-rays detected in this region allow us to study the evolution of a highly atypical system. Previous studies of the region in the GeV energy range show emission extended beyond the size of the compact H.E.S.S. source. Using the archival H.E.S.S. data with improved background methods, we perform a detailed morphological and spectral analysis of the region. Additionally to the compact, bright emission component, we find significantly extended emission, whose position is coincident with HESS J1813-178. We reanalyse the region in GeV and derive a joint-model in order to find a continuous description of the emission in the region from GeV to TeV. Using the results derived in this analysis, as well as X-ray and radio data of the region, we perform multi-wavelength spectral modeling. Possible hadronic or leptonic origins of the 𝛾-ray emission are investigated, and the diffusion parameters necessary to explain the extended emission are examined.
Since the last H.E.S.S. publication on the stellar cluster Westerlund 2 in 2011, the H.E.S.S. dataset on this region has increased more than three-fold in exposure to ∼ 220 h of total observation time. By applying a novel approach to correct for atmospheric variations in IACT data, the commonly applied data quality selection criteria can be adapted to exploit as much of this dataset as possible. In combination with current analysis techniques, it is furthermore possible to disentangle and better characterise this complex region of the gamma-ray sky. Applying an extensive 3D fitting procedure, we find three distinct VHE gamma-ray sources in the vicinity of Westerlund 2, adding a new emission region to the previously reported sources HESS J1023−575 and HESS J1026−582. Even though the sources partly overlap, their spectral indices differ from one another, providing new clues on the relativistic particle acceleration and propagation in the region around the massive star cluster. The new source component shows an elongated morphology that seems to emerge from the star cluster, following the multi-parsec-scale CO jet cloud initially found in NANTEN data as reported in 2009.
GRB 221009A is the brightest gamma-ray burst ever detected. To probe the very-high-energy (VHE, >100 GeV) emission, the High Energy Stereoscopic System (H.E.S.S.) began observations 53 hours after the triggering event, when the brightness of the moonlight no longer precluded observations. No significant VHE emission from the gamma-ray burst is detected in an analysis of the obtained H.E.S.S. data. Differential and integral upper limits are computed using data from the third, fourth, and ninth nights after the initial detection of the GRB. The constraints derived from the H.E.S.S. observations complement the available multi-wavelength data. For example, the afterglow of GRB 221009A presents a novel opportunity to explore extreme Klein-Nishina effects, for which upper limits in the VHE band are valuable. We will present and discuss the results of the analysis and highlight the important role of IACTs in following up on such powerful events.
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