Pulsar Wind Nebulae and Cosmic Rays: A Bedtime Story

Abstract

The role pulsar wind nebulae play in producing our locally observed cosmic ray spectrum remains murky, yet intriguing. Pulsar wind nebulae are born and evolve in conjunction with SNRs, which are favored sites of Galactic cosmic ray acceleration. As a result they frequently complicate interpretation of the gamma-ray emission seen from SNRs. However, pulsar wind nebulae may also contribute directly to the local cosmic ray spectrum, particularly the leptonic component. This paper reviews the current thinking on pulsar wind nebulae and their connection to cosmic ray production from an observational perspective. It also considers how both future technologies and new ways of analyzing existing data can help us to better address the relevant theoretical questions. A number of key points will be illustrated with recent results from the VHE (E > 100 GeV) gamma-ray observatory VERITAS. Keywords: IntroductionThe discovery of cosmic rays in the early twentieth century [1, 2] inaugurated a decades-long, serialized mystery story that has yet to be completed. At the heart of the mystery lies a set of of simple questions. What particles appear in the cosmic ray spectrum that we see from Earth? Where do these particles originate? How are they accelerated? In recent decades direct cosmic ray observations have brought us a wealth of information on the cosmic ray spectrum and its composition, clues to the types of environments in which cosmic rays are born. The interaction of these particles with interstellar magnetic fields, however, prevents them from being traced back to their point of origin.In order to pursue the other half of this mystery-the nature of cosmic ray accelerators-we must use the secondary photon radiation produced in these cosmic ray nurseries. These high-energy (300 MeV -100 GeV) and very high-energy (VHE; E > 100 GeV) gamma rays are not deflected by interstellar magnetic fields and can be used to map cosmic ray populations in and near their parent accelerators. The different processes by which relativistic cosmic ray electrons, protons, and heavier nuclei produce high-energy gamma rays leave characteristic imprints on the gamma-ray energy spectrum of particular astrophysical accelerators. These features may be used to constrain the composition and energy spectrum of accelerated particle populations.The local cosmic ray spectrum is known to be a mix of protons and heavier nuclei, with a smaller admixture of leptons. A mix of astrophysical accelerators within and outside our Galaxy is believed to contribute, including shocks arising in supernova remnants (SNRs), shocks formed by interacting high-velocity winds from massive stars [3], pulsars, and active galactic nuclei (AGN). We focus here on a single subplot of a much larger story-the role played by pulsar wind nebulae (PWNe) in our quest to understand the Galactic cosmic ray spectrum. Dramatis PersonaeNo single gamma-ray observatory provides a complete picture of the gamma-ray sky at all energies and spatial scales. The Fermi Gamma-ray Space Telescope (...

A Very High Energy γ-Ray Survey toward the Cygnus Region of the Galaxy

“…Understanding the relationship between these VHE observations will significantly improve our ability to understand this region in other wavelengths. To do this will require further development of tools such as the "3D Maximum Likelihood Method" currently being developed (Weinstein 2014;Cardenzana 2017) which will allow for observations of significantly extended objects by VERITAS. This will then allow the superior PSF of VERITAS to be used to study the morphology of the Cygnus Cocoon, providing information that could be used to guide the HE analysis.…”

Prospects for detecting galactic sources of cosmic neutrinos with IceCube: An update

“…With the purple band we report the measurements by MAGIC [31]. We report in red and grey the results from the VERITAS detector [34]. With green/magenta lines we show the spectra obtained fixing the parameters to the best fit reported in Table 2 for the case without/with Fermi data.…”

“…The source has been recently studied by the VERITAS collaboration, which has reported a flux comparable to the one reported by MAGIC. In the current picture [34], there are several sources contributing to the emission of MGRO J2031+41: the cocoon, the γ-Cygni SNR, VER J2019+407, and TeV J2032+4130. The latter has been detected by both VERITAS and MAGIC.…”

Pulsar Wind Nebulae and Unidentified Galactic Very High Energy Sources

“…The Galactic Center itself has been proposed (e.g., [41]), and has been proven to be an efficient CR accelerator [42]. Other well-established leptonic accelerators might contribute as well, such as pulsars (e.g., [43]) and pulsar wind nebulae (e.g., [3,44]); these are extensively discussed in this section.…”