The MAGIC telescopes are conducting a long-term observational program aimed at discovering new extreme blazars emitting at TeV energies. These sources - also referred to as extremely high-peaked BL Lac objects (EHBLs) - belong to the most energetic subclass of blazars. They are capable of accelerating particles through complex mechanisms within highly efficient relativistic jets that are...
Blazars, a highly energetic subclass of jetted active galactic nuclei (AGN), exhibit a broad band spectral energy distribution (SED) characterized by two broad bumps, commonly interpreted as non-thermal emission produced by relativistic particles in the jet. Over the past decades, an anticorrelation between the peak frequencies of these components and the total SED luminosity has been...
Blazars, among the most extreme classes of active galactic nuclei, are powered by relativistic jets whose energy dissipation mechanisms remain poorly understood. The flat radio spectrum and the core-shift effect trace the spatial distributions of magnetic fields and relativistic particles, while variability encodes dynamical information about dissipation processes. However, a unified framework...
Blazars, a subclass of active galactic nuclei, possess relativistic plasma jets aligned close to the line of sight of the observer and exhibit broadband emission from radio to gamma-ray energies. However, the jet launching mechanism, particle acceleration processes, and the origin of high-energy emission are still not fully understood. In this study, we focus on OP 313, a flat-spectrum radio...
LHAASO has reported 43 sub-PeV gamma-ray sources, which are promising candidates for cosmic-ray (CR) accelerators above the PeV energy, commonly called as PeVatrons. Multi-wavelength observations are crucial for identifying the underlying particle species and estimating the CR energy content of these sources. In this work we investigate the region around LHAASO J1912+1014u (and HESS J1912+101)...
The origin of Galactic sources capable of accelerating cosmic rays up to PeV energies (PeVatrons) remains a central question in high-energy astrophysics. Recently, LHAASO has detected ultra-high-energy ($E > 100$ TeV) gamma rays from approximately 50 sources, pinpointing locations of PeVatron candidates; however, about half of them remain unidentified due to the lack of observations at other...
The recent detection of ultra-high-energy (UHE; >100 TeV) ฮณ-rays by LHAASO has advanced our understanding of galactic particle accelerators, providing strong evidence for PeV particle acceleration. While sources such as supernova remnants (SNRs), microquasars, and young massive clusters are the main PeVatron candidates, the origin of their UHE emission remains a subject of intense debate. A...
Supernova remnants (SNRs) have been considered as a source of the bulk of Galactic cosmic rays (CR). Recently, some evidence of extended gamma-ray emission have been reported by e.g. LHAASO and H.E.S.S. and this emission often has a high maximum energy, suggesting the existence of particles escaping from acceleration regions of SNRs. Although some analytical works have explained based on the...
Initially unidentified gamma-ray source HESS J1303-631 has been believed to be an evolved Pulsar Wind Nebula (PWN) associated with the pulsar PSR J1301-6305 on account of its energy-dependent morphology that features shrinking of the emission region with energy towards the location of the pulsar. Primarily detected by H.E.S.S. above 1 TeV, it was subsequently also detected by Fermi-LAT at GeV...
Recent discoveries of diffuse gamma rays and neutrinos from Galactic plane provide strong evidence of Galactic PeVatrons, but what kind of astrophysical objects are accelerating PeV cosmic rays are still unknown. In addition, LHAASO has identified mysterious "dark" sources with no clear counterparts at lower gamma-ray energies, posing a new astrophysical puzzle. In this talk, I will introduce...
Recent detection of very-high-energy gamma-rays exceeding 10 TeV from several microquarsars suggests that microquasars are accelerating particles with the energies reaching PeV. Given that particles are indeed accelerated around microquasars, synchrotron X-rays from primary or secondary electrons are also expected. We have observed the second sub-PeV-brightest microquasar, SS433/W50 with the...
Gamma-ray binaries exhibit variable high-energy emission driven by interactions between a compact object and its massive stellar companion. Their orbitally modulated emission provides an opportunity to investigate particle acceleration under dynamically changing astrophysical conditions. HESS J0632+057, discovered by the High Energy Stereoscopic System (H.E.S.S.) in 2007, is a Galactic...
Microquasars are a subclass of X-ray binaries that host relativistic jets and are considered potential Galactic emitters of very-high-energy (VHE; 0.1โ100 TeV) gamma rays. Based on the mass of the companion star, they are categorized into high-mass and low-mass systems, with the latter remaining relatively unexplored in the TeV regime. GRS 1915+105 is one of the most well-known low-mass...
The LHAASO โPeanutโ is a composite and off-Galactic plane ultra-high-energy gamma-ray source that is among the most intriguing sources released in the 1LHAASO catalogue. The region, which includes a diffuse strip and point-like sources LHAASO J0216+4239, LHAASO J0207+4300, spans nearly five degrees in extension. Currently, there exists no identified multiwavelength counterpart to the Peanut,...
Explosive transients, such as core-collapse supernovae (CCSNe) and tidal disruption events (TDEs), are among the most energetic phenomena in the universe and are prime candidates for efficient particle acceleration up to very-high-energies (VHE; E>100 GeV).
The joint configuration of the Cherenkov Telescope Array Observatoryโs (CTAO) first Large-Sized Telescope (LST-1) and the Major...
Among the diverse population of astrophysical transients, those powered by the interaction between explosive ejecta and circumstellar material (CSM) are of particular interest, not only as electromagnetic transients but also as potential sites of high-energy particle acceleration. Understanding how CSM interaction converts kinetic energy of the ejecta into electromagnetic radiation through...
Supernova remnants (SNRs) are generally considered as the main contributors to the Galactic sea of cosmic rays (CRs) on the account of their energy budget. Indeed, they are the only known sources in the Galaxy that are capable of explaining the energy density of CRs at lowest energies. Acceleration of CRs at the shock fronts of SNRs is confirmed by detection of non-thermal emission of radio...
In ground-based gamma-ray astronomy, discoveries and measurements are often challenged by systematic uncertainties of the applied Instrument Response Functions. In most cases, both existing and planned future instruments rely on pre-generated simulations, where the actual observation and detector conditions are taken into account only to a certain, limited degree. Furthermore, they often...
The search for beyond the standard model physics with gamma-ray telescopes has traditionally been dominated by searches that leverage the extreme astrophysical environments of high-energy sources. These include searches for unexpected signal deviations (i.e., photon absorption or delay) and/or searches for predictable features (i.e., excesses from dark matter decay/annihilation). Both methods...
The Cherenkov Telescope Array Observatory (CTAO) will be the first open, proposal-driven research infrastructure for very-high-energy gamma-ray astronomy. With an order-of-magnitude improvement in sensitivity over existing instruments and an energy range extending from a few tens of GeV to several hundred TeV, CTAO will enable transformative studies of the non-thermal Universe.
Operating...
Despite the diversity and energy dependence of $\gamma$-ray pulse morphologies in Crab, Vela and Dragonfly, the phaseograms of these three canonical young pulsars can be organised within a single four-beam geometric template. Using \textit{Fermi} Large Area Telescope data, we fit the 60~MeV--3~GeV phaseograms with a mechanism-agnostic, geometry-first parametric model that incorporates...
The sunspot butterfly diagram is a well known pattern, where the location of sunspots is shown to drift from higher latitudes to lower latitudes over a solar cycle, correlating with solar activity. The Fermi-LAT has been operational since 2008, collecting a full solar cycle of gamma photons from the sun over the time period of 2008-2021. We take a first look at the distribution of these...
The Sun exhibits a TeV flux that significantly exceeds theoretical expectations, challenging conventional particle interaction models. In this Letter, we demonstrate that secondary electron-positron ($e^{\pm}$) pairs constitute a crucial and irreducible component of the observed solar signal. Due to their Extensive Air Shower (EAS) morphology being virtually indistinguishable from that of...
The Large High Altitude Air Shower Observatory (LHAASO) has recently released the highest-energy measurements of the diffuse gamma-ray flux, offering the opportunity to study the spatial distribution and energy spectrum of Galactic cosmic rays (CRs) in the TeV-PeV sky through a multi-messenger approach. In addition, the high-precision CR observations by the same collaboration have revealed a...
Shock interaction in classical novae occurs when a fast outflow from the white dwarf collides with slower matter ejected earlier in the outburst. These shocks radiate across the electromagnetic spectrum: from radio synchrotron, to thermal optical/UV/X-ray emission, to gamma-rays. We present a parameterized one-dimensional toy model for shock interaction in classical novae that uses...
Andes Large-area PArticle detector for Cosmic-ray physics and Astronomy (ALPACA) is an air shower array experiment that aims to observe ultra-high-energy (UHE) gamma rays in the southern sky in 2027. It consists of a surface air shower array with a geometrical area of 83,000 ${\rm m}^2$ and a water Cherenkov-type muon detector array with a 2.0 ${\rm m}$ soil overburden covering an area of...
In Andes Large-area PArticle detector for Cosmic-ray physics and Astronomy (ALPACA), an air-shower array designed for sub-PeV $\gamma$-ray observations in the Southern Hemisphere, the accuracy of primary energy reconstruction is a key factor that determines the energy spectrum of individual $\gamma$-ray sources.
In this study, we compare the performance of $\gamma$-ray energy estimators...
The High Altitude Water Cherenkov (HAWC) observatory is a wide-field gamma-ray detector located at an altitude of 4100 m in Mexico, sensitive to gamma rays in the energy range from 300 GeV to several hundred TeV. HAWC consists of 300 water Cherenkov detectors (WCDs) in the main array and 345 smaller WCDs in the surrounding outrigger array. The current air shower reconstruction uses only the...
Water Cherenkov Detectors play a central role in high-energy astroparticle physics, with key applications in neutrino physics, gamma-ray astronomy, and cosmic-ray research. The performance of these detectors crucially relies on photomultiplier tubes (PMTs) to capture Cherenkov radiation. In recent years, employing multiPMT modules has emerged as a promising strategy to enhance the capabilities...
The Probing Extreme PeVatron Sources (PEPS) project is designed to explore the largely uncharted energy range between $10^{15}$ eV and $5 \times 10^{16}$ eV in gamma rays. PEPS will search for the most extreme Galactic accelerators, probe super-heavy dark matter scenarios, and investigate the transition from Galactic to extragalactic cosmic rays. The experiment will be deployed at the site of...
The Cherenkov Telescope Array Observatory (CTAO) is an upcoming very-high-energy gamma-ray observatory which will have an order of magnitude better sensitivity than the current generation of imaging atmospheric Cherenkov telescope arrays. The Schwarzschild-Couder Telescope is a candidate design for a CTAO medium-sized telescope featuring a finely pixelated camera populated with silicon...
In the 1960s, Third Cambridge Catalogue (3C) provided the first demonstration of radio galaxies (RGs). Radio galaxies with active nuclei were soon classified as Fanaroff-Riley (FR) type I or II. Most of our knowledge on RGs have come through these powerful sources. However, advances in observational techniques have revealed that the majority of the jetted AGN population in the local universe...
In this work study of possible correlations between very high-energy Fermi LAT photons and active galactic nuclei is presented. Two main issues related to this field of studies are are background modeling and the nonโuniform sky distribution of sources. Approach used in this analysis is dataโdriven: background is estimated empirically and associations between photons and candidate sources are...
Identifying the origin of TeV-PeV neutrinos requires understanding both how much target material is available for hadronic interactions and where high-energy emission originates in blazar jets. While excess neutrino emission toward the Galactic Center suggests that dense gas environments play an important role, the association of a high-energy neutrino with the blazar TXS 0506+056 indicates...
VERITAS is one of the worldโs most sensitive very-high-energy (VHE; E > 100 GeV) gamma-ray observatories. Approximately half of its observations are targeted on active galactic nuclei (AGN). We will describe our recent efforts to reprocess the historical archive of these AGN observations and produce a standardized, first VERITAS TeV AGN Catalog. We will present the current status of catalog,...
The acceleration mechanisms powering the emission of high-energy gamma-rays within the jets of active galactic nuclei are continuously the target of investigations by the astroparticle physics community. While not yet fully understood, the community has advanced its knowledge of the classification of the acceleration processes, among other things, through the analysis of data from Imaging Air...
Gamma-ray Bursts (GRBs) generate powerful relativistic jets that inject a large amount of energy into their surrounding environment, producing blast waves that accelerate particles to high energies. The GRB afterglow radiation provides a powerful means to investigate the microphysics of relativistic shocks and to probe the medium surrounding the progenitor of the burst. In this study, we...
The Einstein Probe (EP), launched in 2024, was designed to explore the dynamic X-ray sky and advance our understanding of gamma-ray bursts (GRBs) and stellar explosions. With its sensitivity to soft X-rays and rapid localization capability, EP, together with extensive multi-wavelength follow-up campaigns, has uncovered a growing sample of GRB-related events. Observations from the earliest...
The MeV afterglow in gamma-ray bursts (GRBs) represents a largely unexplored window into the transition from prompt emission to forward-shock-dominated afterglow. At MeV energies, the emission could arise from synchrotron radiation of the decelerating forward shock or from lingering prompt emission components. Each of these mechanisms carries distinct signatures of jet physics, magnetic field...
On 2026 February 26 at 10:37:55 UTC, the Fermi Gamma-ray Burst Monitor (GBM) triggered on the exceptionally bright, long-duration GRB 260226A. Owing to its extreme intensity, the burst also triggered the Large Area Telescope (LAT) onboard approximately 20 s later, marking only the second time during the Fermi mission that an autonomous onboard trigger was initiated from a LAT seed...
Gamma-ray bursts (GRBs) are among the most extreme transients in the universe, but their explosion and emission mechanisms remain unclear. To investigate the nature of GRB jets, we focus on X-ray flares (XFs) and extended emissions (EEs), which are X-ray emissions occurring 100 to 1000 seconds after the main burst. These signals can be observed with recently developed multi-wavelength...
