On the Potential of Bright, Young Pulsars to Power Ultrahigh Gamma-Ray Sources

Abstract

Abstract The recent discovery of a new population of ultrahigh-energy gamma-ray sources with spectra extending beyond 100 TeV revealed the presence of Galactic PeVatrons—cosmic-ray factories accelerating particles to PeV energies. These sources, except for the one associated with the Crab Nebula, are not yet identified. With an extension of 1° or more, most of them contain several potential counterparts, including supernova remnants, young stellar clusters, and pulsar wind nebulae (PWNe), which can perform as PeVatrons and thus power the surrounding diffuse ultrahigh-energy gamma-ray structures. In the case of PWNe, gamma-rays are produced by electrons, accelerated at the pulsar wind termination shock, through the inverse Compton scattering of 2.7 K cosmic microwave background (CMB)radiation. The high conversion efficiency of pulsar rotational power to relativistic electrons, combined with the short cooling timescales, allow gamma-ray luminosities up to the level of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>0.1</mml:mn> <mml:mover accent="true"> <mml:mrow> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:math> . The pulsar spin-down luminosity, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> </mml:math> , also determines the absolute maximum energy of individual photons: <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>γ</mml:mi> <mml:mo>,</mml:mo> <mml:mo form="prefix" movablelimits="true">max</mml:mo> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>0.9</mml:mn> <mml:msubsup> <mml:mrow> <mml:mover accent="true"> <mml:mi>E</mml:mi> <mml:mo>̇</mml:mo> </mml:mover> </mml:mrow> <mml:mrow> <mml:mn>36</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>0.65</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:mspace width="0.25em"/> <mml:mrow> <mml:mi mathvariant="normal">PeV</mml:mi> </mml:mrow> </mml:math> . This fundamental constraint dominates over the condition set by synchrotron energy losses of electrons for young PWNe with typical magnetic field of ≈100 μ G with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:mi>E</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>≲</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>37</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.33em"/> <mml:mi>erg</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> . We discuss the implications of E γ ,max by comparing it with the highest-energy photons reported by LHAASO from a dozen of ultrahigh-energy sources. Whenever a PWN origin of the emission is possible, we use the LHAASO measurements to set upper limits on the nebular magnetic field.

Topics & Concepts