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Abstract:

The detection of gravitational waves (GWs) from a binary neutron star (BNS) merger by Advanced LIGO and Advanced Virgo (GW170817), together with its electromagnetic counterpart, the short gamma-ray burst GRB 170817A, heralded the birth of multimessenger astronomy. The detection of TeV emission from GRBs motivates follow-up observations with the Cherenkov Telescope Array Observatory (CTAO), which is ideal for detecting such signals due to its unprecedented sensitivity, rapid response, and wide-field survey capabilities. The aim of this work is to evaluate GeV–TeV GW follow-up strategies for CTAO using a multistep simulation pipeline and to estimate the expected rate of joint GW–GRB detections during observing run O5. Using a simulated sample of BNS systems with corresponding GW detections, gamma-ray emission is simulated through phenomenological prescriptions based on the observed population of short GRBs, including off-axis jet scenarios. CTAO observations are simulated to account for instrument response, sky tiling strategies, integration times, and varying observing conditions. Strategies with variable and constant integration times are investigated. We find that, via an optimized follow-up strategy, about 5% of simulated GW-associated short GRBs produce GeV–TeV radiation detectable by CTAO. Detectability is strongly influenced by the jet opening angle and viewing angle, suggesting that even rough estimates of the viewing angle in GW alerts could enhance targeting. This framework motivates future follow-ups of GW-detectable events, including neutron star–black hole mergers, and further supports the development of advanced strategies incorporating galaxy distributions and synergies with future detectors such as the Einstein Telescope.

Abstract:

Context. Composite galaxies that contain both Seyfert and starburst components may produce very high-energy (VHE; > 100 GeV) γ -ray emission at a wide range of spatial scales, from a few Schwarzschild radii of a supermassive black hole (SMBH; R S = 10 −6 pc for M SMBH = 10 7 M ⊙ ) to dimensions of kiloparsec-size jet-driven outflows. In addition to supernova remnants, various sources have been suggested to explain data collected on composite galaxies, including multi-messenger neutrino and ultra-high-energy cosmic-ray data. Aims. The closest composite Seyfert–starburst galaxies (NGC 1068, the Circinus galaxy, and NGC 4945) are observed with the High Energy Stereoscopic System (H.E.S.S.) to provide constraints on cosmic-ray populations in these systems. Methods. Data obtained in H.E.S.S. observations have been analyzed to search for VHE γ -ray counterparts to the GeV γ -ray signals detected with Fermi -LAT and for potential spectral components in the VHE range. Results. No significant signals have been found in these H.E.S.S. data. Upper limits on the VHE γ -ray fluxes were applied to constrain theoretical models involving different spectral components.

Abstract:

The blazar PKS 0903 - 57 has recently been classified as a flat spectrum radio quasar at a redshift of z = 0.2621 . In March and April 2020, Fermi-LAT and AGILE reported tremendous activity in high-energy γ rays with the flux increasing by  ∼ 2 orders of magnitude compared to quiescence. The flare was observed with H.E.S.S. in very-high-energy γ rays for six nights with a total observation time of 13.1 h, resulting in the discovery of PKS 0903 - 57 in this energy band with an average flux of 1.5 × 10 − 10 ph cm − 2 s − 1 above an energy threshold of  ∼ 180 GeV corresponding to 60% of the Crab Nebula flux above the same threshold. The very-high-energy γ-ray flux was strongly variable. X-ray and optical data were collected with Swift and ATOM, and also indicate significant variability. The observed multiwavelength flux and spectral variability during the H.E.S.S. observation window suggest variability time scales on the order of a few hours and reveal complex correlation patterns. The lack of absorption beyond that of the extragalactic background light in the γ-ray domain suggests that the emission region was located outside of the broad-line region. A leptonic one-zone modeling of the six H.E.S.S. observation nights using the dusty torus as seed photons for the inverse-Compton scattering, results in a low magnetization of the emission region. This implies that shock acceleration is likely the main driver during the event.