The origin of the high energy gamma-ray emission from the Crab nebula - clues from the spatial extension and energy spectrum

• Applicant: Professor Dr. Dieter Horns
• Subject Area: Astrophysics and Astronomy ; Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 360780919

• Final Report Year: 2021

Final Report Abstract

The supernova remnant SN1054 (Crab Nebula and Pulsar) is a comparably young remnant of a nearby (d = 2 kpc towards the outer Galaxy) core collapse supernova. A remarkable feature of the system is its apparent brightness across the entire accessible spectrum of electromagnetic radiation. This is one of the reasons that the Crab nebula has been widely used as a standard candle. Given the moderate angular extension of about 4 arc minutes across, the system has been carefully scrutinized with high resolution imaging telescopes like the Hubble space telescope in the optical as well as with the Chandra X-ray telescope. The dynamics of spatial structures visible from the radio to X-ray wavelengths have been observed. Different to the sub arc second angular resolution achievable in the X-ray, optical, and radio bands, gamma-ray observations have only recently achieved sufficient angular resolution to attempt to resolve the Crab nebula at the highest observable energies. At energies larger than approximately 109 eV, the emission of the nebula is predominantly generated via inverse Compton scattering of electrons on the ambient photon field. In this project, we have investigated the spectral, spatial, and temporal characteristics of the gamma-ray emission from the Crab Nebula as well as from the Crab Pulsar. The observational data were obtained with the large area telescope (LAT) on board of the Fermi satellite. The LAT has been observing the Crab Nebula since its first light in 2008. We have also combined our observational results with the ones obtained through the observations with the H.E.S.S. telescopes. The newly found results provide important clues on the processes powering the nebula and the distribution of particles and fields in the nebula and in the vicinity of the pulsar. • The inverse Compton nebula at 10^10 GeV is approximately of the same size as the radio nebula (roughly 4 arc minutes diameter). • The apparent size of the nebula decreases with increasing energy: At an energy of 10^12 eV, the diameter is reduced by half. Even though the decrease has been expected, it turns out to be more dramatic than predicted. • The synchrotron nebula at energies of 6 × 10^7 eV up to 6 × 10^8 eV is more compact than previously thought: From our observations of the dimming of the nebula’s emission in this energy band, we conclude that the bulk of the emission is produced in a volume limited to be only a few light days across which corresponds to about 0.8 arc second angular size. • The pulsar’s energy spectrum shows distinctly different features when considering the different phase intervals, indicating that different parts of the magnetosphere generate the pulsed emission. The highest energy tail is very likely produced in the transition region between magnetosphere and wind launching zone.

Publications

• Fermi Large Area Telescope observations of the fast-dimming Crab Nebula in 60-600 MeV Astronomy & Astrophysics 638, id.A147, (2020)
  Yeung, Paul K.H. und Horns, D.
  (See online at https://doi.org/10.1051/0004-6361/201936740)
• Inferring the origins of the pulsed γ-ray emission from the Crab pulsar with ten-year Fermi-LAT data, Astronomy & Astrophysics 640, id.A43, (2020). Erratum ib. 649, id.C1, (2020)
  Yeung, Paul. K.H.
  (See online at https://doi.org/10.1051/0004-6361/202038166 https://doi.org/10.1051/0004-6361/202038166e)
• Resolving the Crab pulsar wind nebula at teraelectronvolt energies Nature Astronomy 4, 167 (2020)
  H.E.S.S. collaboration
  (See online at https://doi.org/10.1038/s41550-019-0910-0)
• The Energy-dependent γ-ray Morphology of the Crab Nebula Observed with the Fermi Large Area Telescope Astrophys. J. 875, 123 (2019). Erratum ib. 909, 221 (2020)
  Yeung, Paul K.H. und Horns, D.
  (See online at https://doi.org/10.3847/1538-4357/ab107a https://doi.org/10.3847/1538-4357/abea1b)
• Studies of Gamma-rays from the Crab Pulsar/Nebula Complex: Spatial Morphology, Temporal Behaviour and Spectroscopy, Dissertation at the Universität Hamburg, 2021
  K.H. Yeung