CDHY PeVatron Workshop

8 Oct 2025, 08:00 → 10 Oct 2025, 18:00 America/New_York

Nevis Labs, Columbia University

Unveiling Galactic PeVatrons with frontier multi-messenger observations and theoretical models

The 3rd CDHY workshop “Unveiling Galactic PeVatrons with frontier multi-messenger observations and theoretical models,” will be held in-person at Columbia University Nevis Labs, near New York City, on Oct 8-10, 2025. The recent discoveries of over 50 ultra-high-energy (UHE; > 100 TeV) gamma-ray sources and neutrino emissions in the Galactic Plane have marked a paradigm shift in the century-long history of cosmic-ray astrophysics. While the UHE telescopes (LHAASO, HAWC, and Tibet AS gamma) operate as PeVatron search engines, IACTs (H.E.S.S, VERITAS, and MAGIC) and X-ray observations are crucial for identifying the most energetic particle acceleration sites. Taking advantage of the wealth of observational data from current multi-messenger instruments, we would like to address the questions of what are the Galactic PeVatrons, and how do they accelerate particles to PeV energies. 

This workshop aims to bring the latest observational results, theoretical models, and the experts to Columbia University and/or New York. We will discuss the future visions for understanding the Galactic PeVatron phenomena most comprehensively with the upcoming gamma-ray, X-ray, neutrino and radio telescopes over the next decade.

About CDHY: The Columbia-DIAS-Heidelberg-Yale (CDHY) Initiative is a series of seminars/meetings established by Columbia University (New York), Dublin Institute for Advanced Studies (Dublin), MPIK Heidelberg, and Yale University (New Haven). The initiative hosts several activities including focused mini-workshops such as the CDHY PeVatron workshop in New York. 

 

Confirmed Speakers Include:

• Felix Aharonian (Remote)
• Hongjun An
• Zhen Cao
• Silvia Celli
• Luca Comisso
• Rebecca Diesing
• Ke Fang
• Andrea Giuliani
• Gwenael Giacinti
• Jamie Holder
• Oleg Kargaltsev
• Alison Mitchell
• Kaya Mori
• Kohta Murase
• Laura Oliviera Nieto
• Angela Olinto
• Lorenzo Sironi
• Tea Temim
• Naomi Tsuji
• Xiaojie Wang
• Ruizhi Yang
• Shuo Zhang

 

Scientific Organizing Committee:

Felix Aharonian, Paolo Coppi, Jim Hinton, Yuri Levin, Kaya Mori, Reshmi Mukherjee

Local Organizing Committee:

Kaya Mori, Reshmi Mukherjee, Matthew Lundy, Svanik Tandon, Jooyun Woo

 

 

Image Credit: Hudson River and view of the Manhattan skyline from the region around Nevis Labs - Ariana Ray. Overlaid, is the image of the central region of our galaxy, the Milky Way, that contains an exotic collection of objects. Image Credit: X-Ray:NASA/CXC/UMass/D. Wang et al.; Radio:NRF/SARAO/MeerKAT.

 

Wednesday, 8 October

Introduction and Highlights above 10 TeV

1 Welcome and Logistics

Speakers: Kaya Mori (Columbia University), Prof. Reshmi Mukherjee (Barnard College, Columbia University)

2 The High-Altitude View of the Most Extreme Phenomena in the Universe

Speaker: Angela Olinto

3 Highlights at > 1 TeV from LHAASO (placeholder title)

Speaker: Zhen Cao

4 Hunting PeVatrons with HAWC and SWGO

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 TeV, revealing candidate sources that may accelerate particles to PeV energies. Looking ahead, the planned Southern Wide-field Gamma-ray Observatory (SWGO) will extend this search to the Southern Hemisphere, enabling access to the Galactic Center region and complementing the sky coverage of HAWC. In this talk, I will review recent HAWC results relevant to the hunt for PeVatrons, discuss their implications for cosmic-ray acceleration, and outline the prospects and discovery potential of SWGO in the coming decade

Speaker: Xiaojie Wang

10:30 Coffee break

SNRs

5 Ongoing Particle Acceleration in Young Supernova Remnants Revealed by Multi-Epoch NuSTAR Observations

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 between gain and loss emerges observationally as year-scale variability in hard X-ray synchrotron emission. Tracking the evolution of hard X-ray flux and spectral shape, therefore, provides direct insight into ongoing particle acceleration.

In this talk, I present multi-epoch NuSTAR observations of four historic young SNRs—Cassiopeia A, G1.9+0.3, Kepler, and Tycho. I report spectral changes in the hard X-ray band and the interpretation of these results in the context of ongoing electron acceleration. Finally, I discuss the implications for proton acceleration in young SNRs and the possibility of corresponding variability in their gamma-ray emission.

Speaker: Jooyun Woo

6 The Maximum Energy of Shock-Accelerated Cosmic Rays

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 acceleration (DSA). However, it remains unclear whether SNRs can accelerate particles to PeV energies, particularly after the very early stages of their evolution. This uncertainty has prompted searches for other source classes and necessitates comprehensive theoretical modeling of the maximum proton energy accelerated by an arbitrary shock. In this talk I will review the current theory of DSA, and introduce a semi-analytic formalism based on kinetic simulations that accounts for the complex interplay between particle acceleration, magnetic field amplification, and shock evolution. I will use this formalism to parameterize the conditions under which SNRs can be PeVatrons, and to make generalized predictions for the maximum energy accelerated by shock-powered phenomena.

Speaker: Rebecca Diesing (Columbia University & Institute for Advanced Study)

12:00 Lunch

SNRs

7 Maximum energy and spectra of particles accelerated at supernova remnants

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 observed spectrum of cosmic rays can be achieved or whether SNRs are able to accelerate protons to the ‘knee’ at about 3 PeV.
Most SNRs are created in core-collapse explosions and expand into the wind bubble of their progenitor stars. This circumstellar medium (CSM) features a complex spatial distribution of gas and magnetic field that strongly affects and modifies key ingredients for particle acceleration, such as particle injection and confinement, shock conditions, and acceleration efficiency. In this talk we specifically discuss the prospects for reaching the PeV scale and spectral variations between various elements that derive from abundance gradients in the circumstellar medium.

Speaker: Martin Pohl

8 Two Paths Toward Testing SNRs as Galactic PeVatrons

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 theory-motivated catalog of the most promising Galactic SNRs for a stacking analysis with IceCube. Second, we use hybrid kinetic simulations with a faux-shock setup to probe the maximum energies achievable by CRs, finding that Emax(t) depends on both shock Mach number and transverse box size, and may involve physics beyond the Bell instability. Together, these two approaches help constrain the viability of SNRs as the sources of Galactic PeV cosmic rays.

Speaker: Emily Simon

9 Discussion of SNRs

PWNe

10 Evolution of Pulsar Wind Nebulae Inside Supernova Remnants

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 expansion into the unshocked SN ejecta to its late-phase interaction with the SNR reverse shock. I will present an overview of the various evolutionary stages of composite SNRs and show how the multi-wavelength signatures of the PWN/SNR interaction can reveal important information about the SNR and PWN dynamics, the SN progenitor and explosion asymmetry, particle cooling and acceleration processes, and the eventual escape of energetic particles into the ISM.

Speaker: Tea Temim

11 Constraints on particle acceleration from multiwavelength properties of UHE/TeV sources associated with pulsars

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.

Speaker: Oleg Kargaltsev (The George Washington University)

15:45 Coffee break

PWNe

12 Going Beyond One-Zone: A multi-zone SED model for PWNe

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 distributions (SEDs) have been useful for estimating key parameters like magnetic field strength and maximum electron energy, they cannot explain several important observations. These include the spatially varying X-ray emission spectra and the detection of VHE/UHE emissions extending far beyond the X-ray-emitting regions. Such findings suggest a more complex internal structure and evolution than a one-zone model can account for.

To address these limitations, we have developed a multi-zone SED model. Our model incorporates particle diffusion and prescribes spatial variations in both the magnetic field and particle flow, providing a more realistic representation of PWN structure. In this presentation, we will discuss our application of this model to a sample of PWNe of various ages, present new insights gained from our analysis, and outline our methodology for ultimately incorporating both temporal and spatial evolution into our model --- a crucial next step toward a comprehensive understanding of these fascinating systems.

Speaker: Hongjun An (Chungbuk National University)

13 Pulsar Wind Nebulae as Galactic PeVatrons: A Comparative Study of CTA 1, PSR J1849−0001, and PSR J1740+1000

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 (114 kyr). Despite their diverse evolutionary stages and environments, all three nebulae emit photons beyond 100 TeV, posing important questions about particle acceleration and transport over pulsar lifetimes. We compare their X-ray properties measured with Chandra X-ray Observatory (CXO) and their UHE properties measured by LHAASO, also computing estimates for the magnetic field and maximum particle energy based on these measurements. We then perform multi-wavelength (MW) SED modeling for these systems and compare the results with our estimates, assessing the applicability and limitations of the single-zone framework to PWNe spanning a wide range of ages.

Speaker: Seth Gagnon (The George Washington University)

14 Detections, constraints, and future aspect of TeV halos

Speaker: Ruo-Yu Shang

15 Discussion of PWNe

18:00 Reception

Thursday, 9 October

Microquasars

16 Microquasars as PeV accelerators: the mounting observational evidence from very-high-energy gamma-ray observations.

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 observations made using pointing instruments. In particular, I will discuss the results of the recent H.E.S.S. observation campaign of V4641 Sgr, which stands out as the brightest and most extreme source in its class to date. Finally, I will outline what to expect from future observations of the population from both current and future gamma-ray facilities.

Speaker: Laura Olivera Nieto

17 Recent X-ray studies of microquasars (TBD)

TBD

Speaker: Naomi Tsuji (University of Tokyo/ICRR)

18 Microquasars and pulsar wind nebulae as PeVatrons

Speaker: Gwenael Giacinti

10:30 Coffee break

Microquasars

19 MHD simulations for SS 433 that model the recent IXPE polarization detection

Speaker: Haocheng Zhang

20 Discussion of Microquasars

11:50 Group photo + Lunch

Acceleration Theory

21 Reconnection-powered multi-messenger emission from black hole coronae

We argue that magnetic reconnection—a process by which opposite field lines annihilate, releasing their energy to the plasma—plays a major role for dissipation of the available free energy in black hole magnetospheres. Inverse Compton scattering within the chain of magnetic islands / flux ropes self-consistently created by reconnection in black hole magnetospheres can power the mysterious hard X-ray “coronal” emission of X-ray binaries, and generate GeV-to-TeV flares in M87. We will also argue that reconnection-driven hadronic acceleration in the coronal regions of active Seyfert galaxies may be the source of the TeV neutrinos detected by IceCube.

Speaker: Lorenzo Sironi

22 Ion acceleration in 3D hybrid simulations of perpendicular shocks

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 self-consistent kinetic simulation has previously reported the emergence of energetic ions in such shocks, our 3D simulations demonstrate that protons spontaneously develop a nonthermal tail—unlike in 2D—undergoing rapid acceleration via shock drift acceleration. We explore the implications of these findings for heliospheric shocks, supernova remnants, and radio supernovae. Furthermore, the growing interest in reproducing collisionless shocks in laboratory experiments using high-power lasers offers a unique opportunity to study these systems in controlled environments. We discuss the implications of our results for conditions relevant to laboratory experiments.

Speaker: Luca Orusa (Columbia University)

23 Cosmic Ray Acceleration in Turbulent Magnetized Plasmas

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 magnetized turbulence drives efficient nonthermal acceleration, generating power-law energy distributions for ions and electrons, along with pronounced pitch-angle anisotropies in the electrons. Particular emphasis will be placed on the interplay between turbulence intermittency, magnetic reconnection, and the role of magnetic field curvature in energizing cosmic rays. These results offer a pathway toward interpreting high-energy astrophysical observations through the lens of kinetic plasma processes.

Speaker: Luca Comisso (Columbia University)

24 Discussion of Acceleration Theory

15:00 Coffee break

Neutrinos

25 Traces of the Galactic Center PeVatron

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 the Galactic center hadronic PeVatron using magnetic filaments and molecular clouds, aiming for a unified picture to explain several high-energy phenomena in the central a few hundred parsecs.

Speaker: Shuo Zhang (Michigan State University)

26 Galactic PeVatrons through the Neutrino Lens

Dozens of ultrahigh-energy gamma-ray sources have been detected to date. However, it remains largely unknown which of these sources accelerate hadronic cosmic rays and thus contribute to the mysterious cosmic-ray knee. High-energy neutrinos offer a unique and unambiguous probe of hadronic PeVatrons, as they are most likely produced through interactions of hadronic cosmic rays. In this talk, I will present observations of the Galactic plane using neutrinos detected by the IceCube Observatory. I will also explore theoretical interpretations that shed light on these findings and discuss their broader implications for our understanding of Galactic PeVatrons

Speaker: Ke Fang

27 Science with Cherenkov neutrino telescopes

Neutrino astronomy is starting to unveil new features of the extreme non-thermal Universe, thanks to the operation of several large volume Cherenkov neutrino telescopes, as IceCube, Baikal-GVD, and KM3NeT. These instruments benefit of extended and complementary fields of view, coupled to high duty cycle and sub-degree angular resolution, allowing the study of cosmic neutrinos and their sources. While the existence of a diffuse flux of high-energy neutrinos has been proved more than a decade ago, two major discoveries have occurred in the last couple of years: i) the first detection of an ultra-high-energy neutrino, KM3230213A, and ii) the emergence of the Galactic neutrino component. In this contribution, I will report on both these observations, providing insights into ongoing analyses and future studies.

Speaker: Silvia Celli

18:00 Gala Dinner

Friday, 10 October

Emerging Pev Sources

28 Studying PeVatron candidates with VERITAS and PANOSETI

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 will summarize some of the key VERITAS results and current projects. We will also discuss a new approach to PeVatron searches, which aims to achieve the large collection area required for ultra-high-energy gamma-ray astronomy by using small-aperture, low-cost, imaging atmospheric Cherenkov telescopes.

Speaker: Jamie Holder

29 Investigating the distribution and propagation of Galactic Cosmic rays using gamma-rays

Speaker: Ruizhi Yang

30 Contribution of young massive stellar clusters to the Galactic diffuse gamma-ray and neutrino emissions

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 most of the Galactic YMSCs could be unresolved, thus contributing to the diffuse gamma-ray and neutrino radiation observed along the Galactic Plane. In this study, we estimate the possible contribution of the population of YMSCs to the Galactic diffuse radiative emissions, by simulating synthetic samples of these sources resembling the observed properties of local clusters. We compute the resulting secondary emission from hadronic interactions occurring in each cluster by particles accelerated at the cluster’s collective wind termination shock and at the supernovae exploded in the core, and compare them with diffuse gamma-ray and neutrino observations by different experiments, including LHAASO and IceCube.

Speaker: Silvia Celli

10:30 Coffee break

Emerging Pev Sources

31 Revealing the origin of UHE gamma-rays from massive-star environments with 3D (M)HD simulations and detailed modelling: the case of the Cygnus region.

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 physical modelling of these environments is necessary to pin-point acceleration sites and assess the role of massive-star environments in the Galactic cosmic-ray ecosystem, regarding the origin of PeV cosmic rays and Galactic particle transport. In this talk, I present our recent efforts in both gamma-ray modelling and 3D (M)HD simulations of massive-star environments. In particular, I demonstrate that a tens-of-kyr old, powerful supernova expanding rapidly in a low-density superbubble can account for the recent detection of PeV photons from the Cygnus region (Härer+ 25b, accepted). We fit the broadband gamma-ray spectrum and morphology with a lepto-hadronic model, which includes the above mentioned supernova (hadronic emission, 10 TeV-PeV) and stellar-wind shocks (leptonic emission, GeV-TeV). I discuss these results in the context of background from Galactic diffuse emission and other source candidates. To recover the energy dependence of the morphology, we construct a 3D molecular cloud model and solve the transport equation to obtain the radial particle distribution. We estimate the supernova maximum energy from dedicated a 3D HD simulation of the stellar association Cygnus OB2 (Vieu et al. 2024). I emphasise how we use 3D HD and MHD simulations to constrain ambient conditions in massive-star environments more generally. For example, we demonstrated in Härer+ 25a that stellar-wind interaction results in highly non-trivial magnetic field morphologies and non-uniform shocks, which can result in steep, curved spectra, as typically observed from massive-star environments.

Speaker: Lucia Haerer (MPIK, Heidelberg, Germany)

32 Cosmic ray escape from Galactic PeVatrons

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 and are transported into the surrounding medium. Subsequent interactions with molecular clouds may yield passive UHE sources; gamma-ray and neutrino emitters only indirectly associated with a nearby PeVatron.
In this contribution I will present examples where modelling applied to data can probe cosmic ray transport on different scales, and discuss candidate “passive” UHE sources.

Speaker: Alison Mitchell (ECAP, FAU Erlangen-Nürnberg)

33 Discussion on Alternative PeVatrons

12:15 Barbeque Lunch

Galactic Accelerators at all Energies

34 A spectromorphological perspective of CTA 1 at VHE energies

Speaker: Svanik Tandon (Columbia University)

35 Evidence of an Energetic Magnetar Powering 1LHAASO J0500+4454

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 scenarios, respectively.
If a nebula of inverse Compton scattering $e^{\pm}$ powers 1LHAASO J0500+4454,
then SED modeling indicates that the current particle energy in the nebula is $\sim 10^{48}$~erg.
If the coincident magnetar \psr's rotational energy powered 1LHAASO J0500+4454, then a conservative energy budget calculation requires an initial magnetar spin period $P_{0} \lesssim 10$~ms and a spin-down timescale $\tau_{\rm sd} \lesssim 30$~yr, which has implications for the origins of magnetars.
Furthermore, if 1LHAASO J0500+4454's age is similar to the magnetar's characteristic age, then it was likely a PeVatron at birth.

Speaker: Jason Alford

36 TeV Binary HESS J0632+057: Multi-Wavelength Insights into Pulsar–Disk Interaction

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 campaign conducted in 2023-2024, combining data from Swift/XRT, NuSTAR, SALT, UVOT, and VERITAS. Our observations target the orbital phase $\phi \approx 0.3 \textrm{-} 0.4$, a critical interval during which the compact object is expected to intersect the Be star's disk. During this phase, we identify a clear transition between two distinct X-ray flux states, possibly induced by structural variability or clumpiness in the disk. This is supported by contemporaneous optical H $\alpha$ spectroscopy, which reveals changes in the disk size. The X-ray and TeV data show strong variability, including both concurrent and independent behaviors, indicative of complex, dynamic interactions between the compact object and the disk.

We discuss these findings within the context of intrabinary shock (IBS) models and pulsar–disk interaction scenarios, highlighting the role of variable shock geometry and evolving disk conditions in shaping the observed emission.

Speaker: Jaegeun Park (Chungbuk National University)

37 Probing PeVatron Candidates with CO/H I and IRจCmm Data: V4641 Sgr & LHAASO J2108+5157.

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 association with V4641 Sgr.

The LHAASO observatory discovered J2108+5157 at ultra--high energies, and HAWC with subsequent observations measure significant 3--146~TeV emission, favor an extended morphology, and deliver a power-law spectral fit that further constraints and refines its PeVatron-candidate nature. In the absence of a confirmed counterpart, and indications of hadronic illumination of molecular/atomic gas, the source remains observationally enigmatic.

In this contribution we present pioneering APEX $^{12,13}$CO (J=2$\rightarrow$1) data for V4641 Sgr and new Nobeyama $^{12,13}$CO (J=1$\rightarrow$0) data for LHAASO~J2108+5157, combined with DRAO/HI4PI H\,I, to test hadronic scenarios via target--gas constraints and to propose counterparts toward LHAASO J2108+5157.

Speakers: Eduardo De La Fuente Acosta, Montserrat Montiel Dücker Romero

38 Topic Talk

Speaker: Marco Tavani

15:30 Coffee break

Galactic Accelerators at all Energies

39 Following up dark LHAASO Sources with VERITAS

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 constrain their morphology and spectra, and to search for emission bridging the gap between GeV and PeV energies. Using a combination of archival and new observations, VERITAS is conducting a focused campaign on this class of sources. I will summarize the challenges these objects pose for Imaging Atmospheric Cherenkov Telescopes—such as their extreme hardness, faint sub-10 TeV flux, and often extended morphology and present the status and new results from our ongoing VERITAS follow-up program.

Speaker: Matthew Lundy (Columbia University)

40 Unraveling the Complex Gamma-Ray Emission from the eHWC J1825-134 Region with HAWC

The eHWC J1825-134 region, located in the southern field of view of the High-Altitude Water Cherenkov (HAWC) observatory, is among the most complex gamma-ray emission sites on the Galactic plane. This region hosts several potential PeVatron candidates capable of accelerating particles to PeV energies. Disentangling the overlapping gamma-ray emission and linking it to specific accelerators is essential for understanding the mechanisms of gamma-ray production and transport near these sources. In this talk, I will present HAWC’s observations of ultra-high-energy (UHE) gamma-ray emission from this rich region and discuss the potential PeVatron sources associated with the UHE gamma rays.

Speaker: Dezhi Huang

41 High Synchrotron Peaked Pevatron Sources in the Galaxy

Extreme particle acceleration usually involves magnetic fields and thus is inevitably accompanied by synchrotron emission from the accelerated particles.
Measurement of this emission, including its polarization, can break model degeneracies and place important constraints on the physical mechanisms operating in a pevatron source.
I will briefly review some of the constraints and then specialize to galactic sources, which have typical magnetic fields in the micro- to milli-Gauss range. For a PeV electron gyrating in such fields, its synchrotron emission lands approximately in the ~0.1 - 10 MeV range, unfortunately known as the "MeV gap" due to the current lack of space instrumentation that works in this energy range. I will discuss future missions and concepts such as COSI, GRAMS, and MEGAT that attempt to fill this energy coverage gap, and the sensitivity levels they need to reach in order to provide meaningful constraints.

Speaker: Paolo Coppi

42 Energy Partition in Relativistic Magnetic Reconnection

Speaker: Nilay Mancini

43 Discussion: what's next for Galactic PeVatrons observations and theories

17:45 Closing Remarks