The origin of Galactic cosmic rays up to the knee at a few PeV remains one of the central open questions in astroparticle physics. Identifying and characterizing “PeVatrons,” astrophysical accelerators capable of reaching PeV energies, is key to solving this puzzle. The High-Altitude Water Cherenkov Observatory (HAWC) has provided unprecedented sensitivity from hundreds of GeV to beyond 100...
Young supernova remnants (SNRs), only a few hundred years old, are among the most energetic and dynamic environments in the Galaxy. Their fast shocks (travelling at thousands of km/s) and amplified magnetic fields (0.1–1 G) create ideal conditions for cosmic-ray acceleration. At the same time, high-energy electrons undergo rapid synchrotron cooling, which counteracts acceleration. This balance...
Identifying the accelerators of Galactic cosmic ray (CR) protons with energies up to a few PeV remains a theoretical and observational challenge. Supernova remnants (SNRs) represent strong candidates because they provide sufficient energetics to reproduce the CR flux observed at Earth. Moreover, their strong forward shocks provide an efficient means of accelerating particles: diffusive shock...
Observations have established supernova remnants (SNRs) as efficient accelerators of cosmic rays, in particular electrons. Whereas the acceleration process is known in principle, many aspects are poorly understood. In fact, a number of central points that were considered certain twenty years ago have since had to be discarded. It is still unclear how the soft production spectrum required by...
The idea of supernova remnants (SNRs) as Galactic PeVatrons is increasingly challenged by the lack of observational evidence, with current theory suggesting that only the most energetic SNRs in unusually dense environments can reach PeV energies. We approach this problem in two complementary ways. First, we rely on neutrinos as smoking-gun signatures of hadronic interactions and compile a...
Composite supernova remnants (SNRs) are those consisting of both a central pulsar that produces a wind of synchrotron-emitting relativistic particle and a supernova (SN) blast wave that expands into the surrounding interstellar medium (ISM). The evolution of the pulsar wind nebula (PWN) is coupled to the evolution of its host SNR and characterized by distinct stages, from the PWN’s early...
I will review several cases which provide the strongest constraints on particle acceleration efficiency in pulsar winds. I will also discuss constraints on particle transport from a multiwavelength data standpoint with an emphasis on recently discovered misaligned outflows (a.k.a. kinetic jets or pulsar filaments) and prospects of detecting/resolving them in the TeV energy range with CTAO.
Pulsar wind nebulae (PWNe) are powerful cosmic accelerators, acting as a primary source of energetic leptons in our galaxy. Their high-energy emissions, spanning from X-ray to ultra-high-energy (UHE; >100 TeV), provide a critical diagnostic for understanding acceleration and evolution of ultra-relativistic partlcles within these dynamic sources.
While one-zone models of PWN spectral energy...
Recent LHAASO observations have revealed ultra-high-energy (UHE) gamma-ray emission extending into the PeV range from several pulsar wind nebulae (PWNe), establishing them as prime candidates for Galactic “PeVatrons.” We conducted a comparative study of three PWNe powered by pulsars with markedly different spin-down ages: PSR J0007+7303 (14 kyr), PSR J1849−0001 (43 kyr), and PSR J1740+1000...
X-ray binary systems exhibiting jets (microquasars) have emerged as promising candidates to explain the observed cosmic-ray flux in the PeV energy range. A handful of microquasars are now known to emit multi-TeV radiation, implying in-situ acceleration of >100 TeV particles. In my talk, I will go over the existing very-high-energy (VHE) gamma-ray observations of these systems, with a focus on...
The recent diffuse gamma-ray and neutrino data support the multimessenger connection expected in hadronic scenarios. Microquasars have been of interest as very-high-energy gamma-ray emitters. We discuss implications of cosmic-ray ion acceleration in such objects as well as possible connections to active galactic nuclei.
Understanding the mechanisms that drive particle acceleration at collisionless, non-relativistic shocks—such as those in supernova remnants—is essential for unveiling the origin of cosmic rays. We use hybrid simulations (kinetic ions–fluid electrons) to study particle acceleration and magnetic field amplification at non-relativistic, weakly magnetized, perpendicular shocks. While no...
Understanding how cosmic rays attain their very high energies remains one of the key open problems in astrophysics. In this talk, I will discuss recent progress on particle acceleration in turbulent, magnetized plasmas, focusing on environments such as magnetized winds, jets, and black hole coronae. I will present results from first-principles particle-in-cell simulations that reveal how...
TeV emission from the Galactic center strongly suggests the existence of a hadronic Pevatron located within ~10 pc of Sgr A*. If there is indeed a PeV proton/ion source and abundant molecular clouds in the surrounding, we expect several different observable effects such as multi-TeV neutrino, gamma-ray photons and electrons. In this presentation, I will discuss our efforts finding traces of...
One of the original motivations for the construction of VERITAS was to search for the origin of the highest energy Galactic cosmic rays. The locations explored in this search include the remnants of historical supernovae, middle-aged supernova remnants interacting with dense molecular clouds, and potential cosmic ray “PeVatrons”, identified by their ultra-high-energy gamma-ray emission. We...
Young massive stellar clusters (YMSCs) have emerged as energetic non-thermal sources, after the recent observation of extended gamma-ray emission by a dozen YMSCs. The large size of their gamma-ray halos, of the order of the excavated bubble from the collective wind, makes the detection of individual YMSCs rather challenging because of the low surface brightness. As a result, the emission from...
Recently, massive-star environments have been established as a new class of gamma-ray sources, which can show degree-scale diffuse emission up to ultra-high energies. They are shaped by feedback from massive stars and harbour a variety of particle accelerators, such as supernova remnants, stellar-wind shocks, and compact objects. A concerted effort of both detailed gamma-ray analyses and...
Cosmic ray acceleration to ~PeV energies has recently emerged as being far more common-place than previously understood. The galactic population of known “ultra high energy” (≥ 100 TeV) gamma-ray emitters encompasses a variety of source classes, including pulsar environments, stellar clusters and dark sources. At these energies, however, cosmic rays may more readily escape their accelerator...
We investigate the origin of unidentified, extended TeV source 1LHAASO J0500+4454, considering three possible origins: cosmic rays interacting with a molecular cloud (MC), particles accelerated in a currently undetected supernova remnant (SNR), and an energetic outflow powered by a pulsar.
Upper limits on the CO and X-ray emission from the $\gamma$-ray emitting region disfavor the MC and SNR...
TeV gamma-ray binaries (TGBs) are rare, high-mass systems capable of accelerating particles to energies >10 TeV. Among them, HESS J0632+057, a candidate pulsar–Be star binary, stands out as a promising source, exhibiting phase-dependent X-ray and TeV flares. These phase-dependent flares can provide new insights into pulsar–disk interactions.
We present results from a new multi-wavelength...
The HAWC observatory detected ฆร-ray emission from the microquasar V4641 Sgr ($\gt$ 200 TeV) and revealed a $\sim$ 100 pc bubble, indicating PeV--scale particle acceleration; LHAASO subsequently extended the spectrum to ~0.8 PeV. In IR--mm data, we identify a millimeter clump $\sim$ 20$'$ from the microquasar, report its position and basic properties, and assess any spatial/kinematic...
The recent LHAASO survey has revealed a new population of >25 TeV sources in the Northern Hemisphere, including several PeVatron candidates without clear counterparts at lower energies. These “TeV-only” sources present a critical test for models of Galactic particle acceleration. VERITAS, with its superior angular resolution and sensitivity between 100 GeV and 25 TeV, is well positioned to...
TeV emission from the Galactic center strongly suggests the existence of a hadronic Pevatron located within ~10 pc of Sgr A*. If there is indeed a PeV proton/ion source and abundant molecular clouds in the surrounding, we expect several different observable effects such as multi-TeV neutrino, gamma-ray photons and electrons. In this presentation, I will discuss our efforts finding traces of...
