PeV Emission of the Crab Nebula: Constraints on the Proton Content in Pulsar Wind and Implications

Recently, two photons from the Crab Nebula with energy of approximately 1 PeV were detected by the Large High Altitude Air Shower Observatory (LHAASO), opening an ultrahigh-energy window for studying pulsar wind nebulae (PWNe). Remarkably, the LHAASO spectrum at the highest-energy end shows a possible hardening, which could indicate the presence of a new component. A two-component scenario with a main electron component and a secondary proton component has been proposed to explain the whole spectrum of the Crab Nebula, requiring a proton energy of 1046–1047 erg remaining in the present Crab Nebula. In this paper, we study the energy content of relativistic protons in pulsar winds using the LHAASO data of the Crab Nebula, considering the effect of diffusive escape of relativistic protons. Depending on the extent of the escape of relativistic protons, the total energy of protons lost in the pulsar wind could be 10–100 times larger than that remaining in the nebula presently. We find that the current LHAASO data allow up to (10–50)% of the spindown energy of pulsars being converted into relativistic protons. The escaping protons from PWNe could make a considerable contribution to the cosmic-ray flux of 10–100 PeV. We also discuss the leptonic scenario for the possible spectral hardening at PeV energies.

ALMA observations of PSR B1259−63/LS 2883 in an inactive period: Variable circumstellar disk?

We report Atacama Large Millimeter/submillimeter Array (ALMA) observations of the gamma-ray binary system containing the pulsar PSR B1259−63 orbiting around a massive star LS 2883 in an inactive period between the 2017 and 2021 periastron passages. We detected radio continuum emission from the binary system at 97 GHz (Band 3) and 343 GHz (Band 7). Compared with our previous ALMA observations performed soon after the 2017 periastron passage, the fluxes have decreased by a factor of six at 97 GHz and of two at 343 GHz. The flux at 343 GHz is large relative to that at 97 GHz and appears to be thermal emission from the circumstellar disk around LS 2883. The decrease of the 343 GHz flux may indicate that the disk has expanded and become partially optically thin since the disk is no longer affected by pulsar winds. The flux at 97 GHz is consistent with that expected from the pulsed emission from the pulsar, which indicates that the unpulsed emission that had been produced through pulsar-disk or pulsar-stellar wind interaction has disappeared. The image of the system is consistent with a point source and shows no sign of ejecta.

Diagnosing pulsar winds in black-widow, redback, and other binary millisecond pulsar systems

ABSTRACT Binary systems composed of a recycled millisecond pulsar and a stellar companion in close orbit could be excellent sites to diagnose pulsar winds. In such systems, the pulsar outflow irradiates and heats up the companion atmosphere, which can lead to the observation of strong day/night modulations in temperature. We demonstrate with particle shower simulations that the particle energy of the wind affects the heating depth in the atmosphere: the wind heat can be deposited above or below the photosphere, leading to different signatures in the observed spectra. We apply our method to four specific systems: We find that systems with cool night-side companions showing strong temperature variations can give interesting lower limits on the particle energy in the winds. In particular, if the companion night side of PSR B1957+20 were to be suddenly irradiated, deep heating would only take place if particles with energy >100 TeV were present. Observational evidence of deep heating in this system thus suggests that (i) such particles exist in the pulsar wind and/or (ii) binary evolution non-trivially takes the companion to the observed temperature asymmetry. Besides, the observed temperature difference can be maintained only with particle energies of the order of 100 MeV.

Modelling gas evacuation mechanisms in present-day globular clusters: stellar winds from evolved stars and pulsar heating

ABSTRACT We employ hydrodynamical simulations to investigate the underlying mechanism responsible for the low levels of gas and dust in globular clusters. Our models examine the competing effects of energy and mass supply from the various components of the evolved stellar population for globular clusters 47 Tucanae, M15, NGC 6440, and NGC 6752. Ignoring all other gas evacuation processes, we find that the energy output from the stars that have recently turned off the main sequence are capable of effectively clearing the evolved stellar ejecta and producing intracluster gas densities consistent with current observational constraints. This result distinguishes a viable gas and dust evacuation mechanism that is ubiquitous among globular clusters. In addition, we extend our analysis to probe the efficiency of pulsar wind feedback in globular clusters. We find that if the energy supplied by the pulsar winds is effectively thermalized within the intracluster medium, the material would become unbound. The detection of intracluster ionized gas in 47 Tucanae allows us to place particularly strict limits on pulsar wind thermalization efficiency, which must be extremely low in the cluster’s core in order to be in accordance with the observed density constraints.