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Insights from the first flaring activity of a high synchrotron peaked blazar with X-ray polarization and VHE gamma rays
No full textContext. Blazars exhibit strong variability across the entire electromagnetic spectrum, including periods of high-flux states commonly known as flares. The physical mechanisms in blazar jets responsible for flares remain poorly understood to date.Aims. Our aim is to better understand the emission mechanisms during blazar flares using X-ray polarimetry and broadband observations from the archetypical TeV blazar Mrk 421, which can be studied with higher accuracy than other blazars that are dimmer and/or located farther away.Methods. We studied a flaring activity from December 2023 that was characterized from radio to very high-energy (VHE; E > 0.1 TeV) gamma rays with MAGIC, Fermi-LAT, Swift, XMM-Newton, and several optical and radio telescopes. These observations included, for the first time for a gamma-ray flare of a blazar, simultaneous X-ray polarization measurements with IXPE, in addition to optical and radio polarimetry data. We quantify the variability and correlations among the multi-band flux and polarization measurements, and describe the varying broadband emission within a theoretical scenario constrained by the polarization data.Results. We find substantial variability in both X-rays and VHE gamma rays throughout the campaign, with the highest VHE flux above 0.2 TeV occurring during the IXPE observing window, and exceeding twice the flux of the Crab Nebula. However, the VHE and X-ray spectra are on average softer, and the correlation between these two bands is weaker than those reported in the previous flares of Mrk 421. IXPE reveals an X-ray polarization degree significantly higher than that at radio and optical frequencies, similar to previous results for Mrk 421 and other high synchrotron peaked blazars. Differently to past observations, the X-ray polarization angle varies by ∼100° on timescales of days, and the polarization degree changes by more than a factor of 4. The highest X-ray polarization degree, analyzed in 12 h time intervals, reaches 26 ± 2%, around which an X-ray counter-clockwise hysteresis loop is measured with XMM-Newton. It suggests that the X-ray emission comes from particles close to the high-energy cutoff, hence possibly probing an extreme case of the Turbulent Extreme Multi-Zone model for which the chromatic trend in the polarization may be more pronounced than theoretically predicted. We model the broadband emission with a simplified stratified jet model throughout the flare. The polarization measurements imply an electron distribution in the X-ray emitting region with a very high minimum Lorentz factor (γ′min≳104), which is expected in electron-ion plasma, as well as a variation of the emitting region size of up to a factor of 3 during the flaring activity. We find no correlation between the fluxes and the evolution of the model parameters, which indicates a stochastic nature of the underlying physical mechanism that likely explains the lack of a tight X-ray/VHE correlation during this flaring activity. Such behavior would be expected in a highly turbulent electron-ion plasma crossing a shock front.Key words: acceleration of particles / radiation mechanisms: non-thermal / galaxies: active / BL Lacertae objects: individual: Markarian 421 / gamma rays: general / X-rays: genera
Very high energy observations of the Seyfert galaxy NGC 4151 with MAGIC - Indication of another gamma-ray obscured candidate neutrino source
No full textSeyfert galaxies are emerging as a promising source class of high-energy neutrinos. The Seyfert galaxies NGC 4151 and NGC 1068 have respectively come up as the most promising counterparts of a 3σ and of a 4.2σ neutrino excesses detected by IceCube in the TeV energy range. Constraining the very high energy (VHE) emission associated with the neutrino signal is crucial to unveiling the mechanism and site of neutrino production. In this work, we present the first results of the VHE observations (∼29 hours) of NGC 4151 with the MAGIC telescopes. We detected no gamma-ray excess in the direction of NGC 4151, and we derived constraining upper limits on the VHE gamma-ray flux. The integral flux upper limit (at the 95% confidence level) above 200 GeV is f = 2.3 × 10−12 cm−2 s−1. Comparison of the MAGIC and IceCube measurements suggests the presence of a gamma-ray obscured accelerator, and it allowed us to constrain the gamma-ray optical depth and the size of the neutrino production site.Key words: neutrinos / galaxies: active / galaxies: individual: NGC 4151 / galaxies: Seyfer
Detection of very high-energy gamma-ray emission from Eta Carinae during its 2020 periastron passage
No full textThe colliding-wind binary system η Carinae has been identified as a source of high-energy (HE, below ∼100 GeV) and very high-energy (VHE, above ∼100 GeV) gamma rays in the last decade, making it unique among these systems. With its eccentric 5.5-year-long orbit, the periastron passage, during which the stars are separated by only 1–2 au, is an intriguing time interval to probe particle acceleration processes within the system. In this work, we report on an extensive VHE observation campaign that for the first time covers the full periastron passage carried out with the High Energy Stereoscopic System (H.E.S.S.) in its 5-telescope configuration with upgraded cameras. The VHE gamma-ray emission from η Carinae was detected during the periastron passage with a steep spectrum with the spectral index Γ = 3.3 ± 0.2stat ± 0.1syst. Together with previous and follow-up observations, we derive a long-term light curve sampling one full orbit, showing hints of an increase in the VHE flux towards periastron, but no hint of variability during the passage itself. An analysis of contemporaneous Fermi-LAT data shows that the VHE spectrum represents a smooth continuation of the HE spectrum. From modelling the combined spectrum, we conclude that the gamma-ray emission region is located at distances of ∼10–20 au from the centre of mass of the system and that protons are accelerated up to energies of at least several tera-electronvolts inside the system in this phase.Key words: astroparticle physics / radiation mechanisms: non-thermal / binaries: general / stars: individual: η Carinae / gamma rays: star
Normal tissue sparing with electron beams at PITZ: first irradiation study in zebrafish embryos
No full textFLASH irradiation has emerged as a promising approach to reduce normal tissue toxicity while preserving tumor control, creating a need for sensitive models to study its biological effects. Zebrafish (Danio rerio) embryos provide such a model, offering a vertebrate system with high developmental sensitivity, genetic similarity to humans, and suitability for preclinical radiation biology research. Within the R&D platform FLASHlab@PITZ, a collaboration between the Photo Injector Test Facility at DESY in Zeuthen (PITZ), the Technical University of Applied Sciences Wildau, Charité – Universitätsmedizin Berlin, and the Max-Delbrück Center (MDC), we aim to establish zebrafish as a robust in vivo system to characterize radiation-induced toxicities. In this study, we evaluated the effects of conventional X-rays, electron low dose rate (LDR), and electron high dose rate (HDR, FLASH-like) irradiation on wild-type AB zebrafish embryos at 24 hours post-fertilization (hpf)
Phenomenology of the dark matter sector in the 2HDM extended with complex scalar singlet
No full textThe two-Higgs-doublet model augmented with a complex scalar singlet (2HDMS) is a well-motivated candidate for Beyond Standard Model (BSM) Physics. We investigate the dark matter phenomenology of the 2HDMS with the complex scalar singlet as the dark matter candidate. We perform a study of the parameter space allowed by existing theoretical and experimental constraints from dark matter, flavour physics and collider searches. Further, we discuss a few benchmark scenarios to test the discovery potential for the 2HDMS at the HL-LHC and at future high-energy colliders
Comprehensive Study of Structural and Electrocatalytic Properties of Ni–N Thin Films
No full textThis work presents a systematic and detailed investigation of the structural, electronic, and electrochemical properties of Ni–N thin films grown using a reactive magnetron sputtering at partial nitrogen flow (RN2) of 0, 15, 50, 75, and 100%. Below RN2 = 50%, the phase formed is metallic Ni with some N atoms occupying interstitial sites. However, when the RN2 exceeds 50%, the Ni3N phase sets in, and a fully stoichiometric Ni3N phase is realized at RN2 = 100%. As RN2 increases, the oxidation state of Ni increases and the structural ordering improves, as substantiated by X-ray absorption fine structure and X-ray diffraction analysis. Additionally, hard X-ray photoelectron spectroscopy measurements confirm the formation of a fully stoichiometric Ni3N phase at RN2 = 100%. Finally, the electrocatalytic performance measured through the oxygen evolution reaction clearly demonstrates better performance of Ni3N as compared to pure Ni or other Ni–N phases. This work provides essential building blocks to establish Ni3N as an environmentally friendly, noble metal-free, and earth-abundant catalyst for water splitting reactions
HEGS : Revisiting a decade of H.E.S.S. extragalactic observations
No full textDuring its first phase, from 2004 up to the end of 2012, the H.E.S.S. (High Energy StereoscopicSystem) experiment observed the extragalactic skies for more than 2700 hours. These data have been re-analysed in a single consistent framework, leading to the derivation of a catalog of 23 sources. In total, about 5.7% of the sky was observed, allowing for several additional studies to be conducted: source variability, extragalactic gamma-ray background light, and comparison with the Fermi-LAT catalogues. In this contribution, we discuss these results and present the high-level data (catalogs, maps) released to the astrophysical community
Author Correction: Fundamental physics opportunities with the next-generation Event Horizon Telescope
No full textCorrection to: Living Reviews in Relativity (2025) 28:4https:// doi. org/ 10. 1007/ s41114- 025- 00057-0After this article was published, the Authors requested to change the title andabstract back to their original submission, since the removal of mention of thengEHT project from the title and abstract during revisions does not accurately con-vey nor reflect the fact that this work was initiated and undertaken as part of thengEHT project. The changes made following suggestions by a reviewer had not beenapproved by all authors/project director.Instead of “Fundamental physics opportunities with future ground-based mm/sub-mm VLBI arrays” the title should read “Fundamental physics opportunities with thenext-generation Event Horizon Telescope”