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HAWC Study on the Ultra-High-Energy Gamma-Ray Emissions from the Pulsar Wind Nebula G32.64+0.53

Based on 2860 days of HAWC Observatory data, this study confirms that the ultra-high-energy gamma-ray emission from the pulsar wind nebula G32.64+0.53 originates from a leptonic PeV accelerator powered by PSR J1849-0001, capable of accelerating electrons to PeV energies within a system approximately 26.8 kyr old.

Original authors: R. Alfaro, C. Alvarez, E. Anita-Rangel, M. Araya, J. C. Arteaga-Velázquez, D. Avila Rojas, H. A. Ayala Solares, R. Babu, P. Bangale, E. Belmont-Moreno, A. Bernal, K. S. Caballero-Mora, T. Capistrán, A

Published 2026-03-23

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: R. Alfaro, C. Alvarez, E. Anita-Rangel, M. Araya, J. C. Arteaga-Velázquez, D. Avila Rojas, H. A. Ayala Solares, R. Babu, P. Bangale, E. Belmont-Moreno, A. Bernal, K. S. Caballero-Mora, T. Capistrán, A. Carramiñana, F. Carreón, S. Casanova, U. Cotti, J. Cotzomi, S. Coutiño de León, E. De la Fuente, P. Desiati, N. Di Lalla, R. Diaz Hernandez, M. A. DuVernois, J. C. Díaz-Vélez, K. Engel, T. Ergin, C. Espinoza, N. Fraija, S. Fraija, J. A. García-González, F. Garfias, N. Ghosh, M. M. González, J. A. González, J. A. Goodman, D. Guevel, J. Gyeong, J. P. Harding, I. Herzog, D. Huang, F. Hueyotl-Zahuantitla, A. Iriarte, S. Kaufmann, D. Kieda, K. Leavitt, W. H. Lee, J. Lee, H. León Vargas, A. L. Longinotti, G. Luis-Raya, K. Malone, O. Martinez, J. Martínez-Castro, J. A. Matthews, P. Miranda-Romagnoli, J. A. Morales-Soto, E. Moreno, M. Mostafá, M. Najafi, A. Nayerhoda, L. Nellen, R. Noriega-Papaqui, N. Omodei, E. Ponce, Y. Pérez Araujo, E. G. Pérez-Pérez, C. D. Rho, A. Rodriguez Parra, D. Rosa-González, M. Roth, H. Salazar, D. Salazar-Gallegos, A. Sandoval, M. Schneider, J. Serna-Franco, M. Shin, A. J. Smith, Y. Son, R. W. Springer, O. Tibolla, K. Tollefson, I. Torres, R. Torres-Escobedo, E. Varela, L. Villaseñor, X. Wang, Z. Wang, I. J. Watson, S. Yu, X. Zhang, H. Zhou, C. de León

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic Firework Factory: A Story of G32.64+0.53

Imagine our galaxy, the Milky Way, as a vast, dark ocean. Usually, it's quiet, but occasionally, you find a lighthouse that doesn't just shine a beam of light—it shoots out a jet of pure energy so powerful it can accelerate tiny particles to speeds almost as fast as light itself.

This paper is about a specific "lighthouse" in our cosmic ocean: a Pulsar Wind Nebula (a glowing cloud of energy) named G32.64+0.53. It's powered by a dead star called a Pulsar (PSR J1849-0001), which is essentially a neutron star spinning so fast it's like a cosmic top, whipping up a storm of particles around it.

Here is the story of how scientists used a giant "net" in the sky to catch the light from this storm and figure out exactly how it works.

1. The Giant Net in the Sky (HAWC)

To see these invisible, high-energy particles, scientists built the HAWC Observatory. Think of HAWC not as a telescope with a lens, but as a giant, 300-pool-sized net made of water tanks sitting high up on a mountain in Mexico.

When a super-fast particle from space hits the atmosphere, it creates a shower of secondary particles, like a pebble splashing into a pond. These "splashes" hit the water tanks in HAWC, creating a flash of blue light (Cherenkov light). By watching these flashes, HAWC can trace the path of the original particle back to its source.

The team looked at 2,860 days of data (almost 8 years of watching the sky) to build a clear picture of what's happening in this specific region.

2. The Mystery of the "Ghost" Nebula

Usually, when you look at a cosmic object, you see it in many colors: radio waves, visible light, X-rays, and gamma rays. But this nebula was a bit of a ghost.

Radio/Optical: Invisible.
X-rays: Visible (a bright, compact core).
Gamma Rays: Very bright and huge (a massive, extended cloud).

This told the scientists a clue: The nebula is made of electrons (tiny charged particles) that are being accelerated to insane speeds. The high-energy electrons glow in X-rays right near the center, but as they drift outward and slow down, they bump into background light and turn that energy into massive gamma rays. It's like a campfire: the hottest coals are in the center (X-rays), but the heat radiating out warms the whole tent (Gamma rays).

3. Catching the Ultra-High-Energy Particles

The big discovery in this paper is that this nebula is a PeVatron.

What is a PeVatron? It's a particle accelerator capable of boosting particles to Peta-electronvolts (PeV). That's a quadrillion electron volts.
The Analogy: If a regular particle accelerator (like the Large Hadron Collider) is a bicycle, a PeVatron is a Formula 1 car. This nebula is one of the few natural "race cars" in our galaxy that can push electrons to these record-breaking speeds.

The HAWC data showed that this nebula is shooting out gamma rays with energies exceeding 100 TeV (Tera-electronvolts), and the signal goes all the way up to 270 TeV. This confirms the nebula is a powerhouse.

4. Solving the Puzzle: The "Time-Travel" Model

The scientists didn't just look at the picture; they built a computer simulation to understand the physics. They treated the nebula like a time machine.

They knew the pulsar was about 43,000 years old (based on how fast it's slowing down), but they wanted to know its true age and how strong its magnetic field was. They fed their model all the data from different telescopes (X-ray, Gamma-ray, etc.) and let the computer run millions of simulations to see which scenario fit best.

The Results of the Simulation:

True Age: The nebula is actually younger than we thought—about 26,800 years old. (The pulsar started spinning faster in the past, so it's slowing down faster than a simple clock would suggest).
The Magnetic Field: The magnetic field holding this storm together is surprisingly weak (only 2.5 micro-Gauss).
- Why does this matter? Imagine a magnetic field as a cage. If the cage is too strong, the particles get trapped and lose energy by glowing in X-rays. Because this cage is weak, the particles escape easily and smash into light particles to create the massive gamma rays we see.
The Speed Limit: The electrons are being accelerated to a maximum energy of 1.5 PeV. This proves the nebula is a true "PeVatron."

5. The Big Picture

This paper is like finding a missing piece of a giant puzzle. For a long time, we knew these "PeVatrons" existed, but we didn't know exactly how they worked or if they were common.

By studying G32.64+0.53, the HAWC team confirmed that Pulsar Wind Nebulae are indeed the factories that create the most energetic particles in our galaxy. They take the rotational energy of a spinning dead star and convert it into a cosmic beam that can accelerate particles to the highest speeds nature allows.

In short: We found a cosmic engine that is 27,000 years old, spinning a weak magnetic field, and accelerating electrons to speeds that make them the champions of our galaxy's energy race. It's a testament to the extreme and violent beauty of our universe.

1. Problem Statement

The paper addresses the origin and physical properties of the ultra-high-energy (UHE) gamma-ray emission associated with the pulsar wind nebula (PWN) G32.64+0.53, powered by the pulsar PSR J1849-0001.

Context: While the nebula is well-detected in X-rays and very-high-energy (VHE) gamma-rays, it lacks counterparts in radio, optical, and GeV ranges. This suggests a non-thermal leptonic origin where high-energy electrons upscatter ambient photons.
Challenge: Previous observations (e.g., by H.E.S.S.) identified the source, but a comprehensive spectral analysis extending into the UHE regime (>100 TeV) using the High Altitude Water Cherenkov (HAWC) Observatory was needed to determine if the system acts as a "PeVatron" (a particle accelerator capable of reaching peta-electronvolt energies).
Goal: To characterize the UHE spectrum up to the highest energies, constrain the physical parameters of the nebula (age, magnetic field, electron spectrum), and verify the leptonic acceleration mechanism.

2. Methodology

The study employs a systematic analysis of 2860 days of HAWC "Pass 5" data, utilizing a sophisticated pipeline to model the complex Galactic environment.

Data Analysis Pipeline:
- Region of Interest (ROI): Defined as Galactic coordinates $l \in [30.0^\circ, 34.5^\circ]$ and $b \in [-3.0^\circ, 3.0^\circ]$ .
- Diffuse Emission Modeling: A physics-based template for Galactic Diffuse Emission (GDE) was generated using HERMES software. This accounts for inverse Compton scattering, bremsstrahlung, and neutral pion decay from cosmic ray interactions with interstellar gas (HI and H $_2$ ).
- Source Finding Algorithm: An iterative maximum likelihood approach using the threeML package and HAWC Accelerated Likelihood (HAL) plugin. The process involves:
  1. Point Source Search: Identifying hotspots in residual maps.
  2. Extension Search: Testing sources for spatial extension (Gaussian vs. Point-like).
  3. Elliptical Morphology Search: A novel step in this pipeline testing for elliptical shapes (2D Gaussian with eccentricity $e$ and position angle $\theta$ ) for extended sources.
- Model Selection: The Bayesian Information Criterion (BIC) was used to select the optimal spectral (Power Law, Cutoff Power Law, Log-Parabola) and morphological models, penalizing overfitting.
Physical Modeling:
- A time-dependent, one-zone leptonic model was implemented using the GAMERA software package.
- The model assumes a single population of electrons injected from the pulsar, losing energy via synchrotron radiation and Inverse Compton Scattering (ICS).
- Free Parameters: Initial spin period ( $P_0$ ), conversion efficiency ( $\eta$ ), electron spectral index ( $\alpha$ ), cutoff energy ( $E_{cut}$ ), and current magnetic field strength ( $B(\tau)$ ).
- Fitting: Parameters were constrained using a Bayesian approach with the emcee Markov Chain Monte Carlo (MCMC) framework against multi-wavelength data (X-ray, VHE, UHE).

3. Key Contributions

Advanced Source Detection: The study successfully resolved the emission region into four distinct sources, including the primary target HAWC J1849-0000. It introduced a specific step to test for elliptical morphology, improving the characterization of extended sources like HAWC J1848-0146.
UHE Spectrum Extension: The analysis extends the measured spectrum of G32.64+0.53 up to 270 TeV, with significant detection ( $\sqrt{TS} \approx 4.7$ ) in the highest energy bin, confirming its status as a powerful UHE accelerator.
Systematic Modeling: The paper provides a rigorous treatment of the Galactic Diffuse Emission and systematic uncertainties (e.g., PMT efficiency, charge uncertainty), ensuring the robustness of the spectral results.
Physical Constraints: It provides the first time-dependent leptonic modeling constraints for this specific PWN using HAWC UHE data, linking the observed spectrum directly to the pulsar's rotational energy evolution.

4. Key Results

Source Properties (HAWC J1849-0000):
- Position: Consistent with HESS J1849-000 and 1LHAASO J1848-0001u (separation < 0.05 $^\circ$ ).
- Morphology: Slightly extended with a size of $0.09^\circ \pm 0.01^\circ$ .
- Spectrum: Best fit by a Log-Parabola model. The flux is detected across nine energy bins from 1.3 TeV to 270 TeV.
- Significance: Total Test Statistic (TS) of 347.
Physical Parameters (Leptonic Model):
- PeVatron Confirmation: The electron spectrum cutoff energy is determined to be $E_{cut} = 1.5^{+1.7}_{-0.6}$ PeV. This confirms the nebula accelerates electrons to PeV energies.
- True Age: The model constrains the true age of the system to $\tau \approx 26.8$ kyr, which is notably younger than the characteristic age ( $\tau_c = 43$ kyr), implying a non-negligible initial spin period ( $P_0 \approx 23.7$ ms).
- Magnetic Field: The current magnetic field is constrained to a low value of $B(\tau) \approx 2.5$ $\mu$ G. This low field supports the dominance of ICS over synchrotron cooling for UHE gamma-ray production.
- Efficiency: The efficiency of converting pulsar spin-down power into relativistic electrons is $\eta \approx 1.2\%$ .
Multi-wavelength Consistency: The model successfully fits data from Chandra, NuSTAR, Fermi-LAT, H.E.S.S., LHAASO, and Tibet-AS $\gamma$ , demonstrating a coherent physical picture from X-rays to UHE gamma-rays.

5. Significance

PeVatron Identification: This work solidifies G32.64+0.53 as a confirmed Galactic PeVatron. The detection of electrons accelerated to $\sim 1.5$ PeV implies that the resulting gamma rays can reach energies up to $\sim 0.6$ PeV via ICS on Cosmic Microwave Background (CMB) photons, consistent with the observed spectrum.
Insight into PWN Evolution: The discrepancy between the true age (26.8 kyr) and characteristic age (43 kyr) provides crucial insights into the spin-down history of PSR J1849-0001, suggesting it was born spinning significantly faster than its current rate.
Magnetic Field Constraints: The derived low magnetic field ($2.5$ $\mu$ G) is critical for understanding the energy budget of PWNe, indicating that a large fraction of the pulsar's energy goes into particle acceleration rather than magnetic field amplification.
Methodological Benchmark: The iterative source-finding pipeline, particularly the inclusion of elliptical morphology testing and sophisticated GDE modeling, sets a new standard for analyzing complex, crowded regions in the Galactic plane using ground-based Cherenkov arrays.