An Extreme Particle Accelerator Fueled by Pulsar PSR J1849−0001

Astrophysicists have made significant strides in understanding the role of pulsar wind nebulae as extreme particle accelerators. Recent findings from the Large High Altitude Air Shower Observatory (LHAASO) reveal that the Crab Nebula, powered by the most energetic pulsar in the Milky Way, emits gamma rays in the PeV range (10^15 eV), solidifying its status as a particle accelerator of extreme capability. The LHAASO collaboration has now identified another point source of ultra-high-energy gamma rays (E > 100 TeV) closely associated with the pulsar wind nebula driven by PSR J1849-0001, a pulsar with a braking power approximately fifty times less than that of the Crab pulsar. Their findings are documented in a recent publication in Nature Astronomy.

In cosmic ray energy spectra measured on Earth, there exists a notable feature referred to as the "knee," which occurs around 3 PeV. This feature is typically interpreted as the threshold for proton acceleration by ordinary galactic sources, while extragalactic accelerators dominate beyond a second breakpoint known as the "ankle." The existence of galactic sources capable of achieving energies significantly exceeding the PeV threshold remains an open question. The detection of gamma photons near the PeV scale, generated by electrons of several PeV or protons of tens of PeV, serves as direct evidence of such extreme acceleration capabilities.

The Crab Nebula has been confirmed as a gamma emitter up to 1.1 PeV by LHAASO, demonstrating that pulsar wind nebulae can act as exceptionally efficient particle accelerators. However, despite its near-theoretical acceleration efficiency, the Crab's luminosity alone cannot account for the cosmic ray flux observed beyond the knee.

The energy loss of electrons due to synchrotron radiation imposes an upper limit on their maximum energy, which is a function of the square root of the efficiency and inversely proportional to the square root of the magnetic field. To overcome the intense magnetic field of 100 μG in the Crab Nebula, the acceleration efficiency must reach at least 16% of the theoretical limit to explain the observations. Regardless of the specific particle acceleration mechanism, with such efficiency, protons could be accelerated to the scale of 10 PeV without experiencing radiative losses like electrons, provided they are introduced into the particle acceleration zone. Nevertheless, even if the Crab Nebula operates as an extremely effective proton accelerator, the deduced luminosity of PeV energy protons is insufficient to explain the cosmic ray flux measured beyond the ankle. Furthermore, its central pulsar exhibits extreme properties that are difficult to generalize: it is less than 1,000 years old and possesses the highest braking power of any detected pulsar in our galaxy (4.5×10^38 erg.s-1).

The question of whether other, less energetic pulsar wind nebulae could also produce particles beyond the PeV threshold has thus become increasingly relevant. This is the focus of the particle astrophysicists from the LHAASO collaboration.

PSR J1849−0001 is a young and rapidly rotating pulsar (with a rotation period of 38.5 ms), located approximately 7 kpc away. Its braking power is about fifty times less than that of the Crab pulsar, and it is surrounded by a pulsar wind nebula detected in X-rays and gamma rays. Successive observations by HESS, ASγ, and LHAASO have revealed that its gamma spectrum extends to several hundred TeV, including the detection of a photon reaching 2 PeV. The likelihood that this event is due to diffuse gamma background is negligible, indicating a robust astrophysical origin. The flux of photons near the PeV scale is comparable to, if not greater than, that measured in the Crab Nebula when corrected for distance.

Astroparticle physicists indicate that the measured gamma spectrum extends to the PeV range following a power-law distribution, with luminosity several times that of the Crab Nebula. Combined X-ray observations constrain the average magnetic field within the nebula to about 3 μG (significantly lower than that of the Crab), yet they reveal an extreme particle acceleration efficiency, close to or exceeding unity.

This finding challenges the particle acceleration theory in pulsar wind nebulae and suggests non-ideal magnetohydrodynamic conditions within the accelerator, potentially involving magnetic reconnection upstream of the terminal shock wave.

The emission of a 2 PeV gamma photon via inverse Compton scattering necessitates that electrons are first accelerated to energies at least comparable to that photon energy. This requirement imposes stringent constraints on the acceleration efficiency: it must simultaneously overcome synchrotron radiative losses and limits imposed by the pulsar's braking power (its magnetic field).

According to the researchers, if acceleration occurs at the termination shock, observations suggest a very low magnetic equipartition coefficient, implying an acceleration efficiency significantly higher than expected in ideal MHD scenarios. Such a condition is difficult to reconcile with first-order Fermi acceleration mechanisms; however, the researchers propose it could be explained by non-ideal processes, such as magnetic reconnection in a striped pulsar wind.

Alternative scenarios are also considered, placing the acceleration zone upstream or downstream of the termination shock. These would partially relax the constraints but still require exceptionally high efficiencies.

These results highlight a significant tension between LHAASO observations and current theoretical models of pulsar wind nebulae. Achieving acceleration efficiencies on the order of unity or greater represents a major challenge, even within non-ideal MHD frameworks.

The pulsar wind nebula associated with PSR J1849−0001 thus stands as a new example of a galactic source capable of accelerating electrons to PeV energies. This discovery suggests that extreme acceleration conditions may be relatively common in young pulsar wind nebulae and could significantly contribute to the galactic population of cosmic ray accelerators known as PeVatrons, provided there is efficient proton loading.

The search for cosmic accelerators continues...