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Title: Characterization of the inner knot of the crab: The site of the gamma-ray flares?

A particularly intriguing recent result from γ-ray astronomy missions is the detection of powerful flares from the Crab Nebula, which challenges the current understanding of pulsar wind nebulae and acceleration mechanisms. In order to search for the production site(s) of these flares, we conducted a multi-wavelength observing campaign using Keck, the Hubble Space Telescope (HST), and the Chandra X-ray Observatory. As the short timescales of the γ-ray flares ($$\lesssim 1$$ day) suggest a small emitting region, the Crab's inner knot (about 0.6 arcsec from the pulsar) is a candidate site for such flaring. Our paper describes observations of the inner knot, seeking to understand its nature and possible relationship with γ-ray flares. Using singular-value decomposition, analysis of the HST images yielded results consistent with traditional methods while substantially reducing some uncertainties. These analyses show that the knot's intrinsic properties (especially size and brightness) are correlated with its (projected) separation from the pulsar. This characterization of the inner knot helps in constraining standard shock model parameters, under the assumption that the knot lies near the shocked surface. While the standard shock model gives good agreement in several respects, two puzzles persist: (a) the observed angular size of the knot relative to the pulsar–knot separation is much smaller than expected; and (b) the variable high degree of polarization (reported by others) is difficult to reconcile with a highly relativistic downstream flow. But, the IR–optical flux of the inner knot is marginally consistent with the shock accelerating most of the Nebula's optical-emitting particles.
 [1] ;  [2] ;  [3] ;  [4] ;  [4] ;  [5] ;  [6] ;  [7] ;  [8] ;  [3] ;  [9] ;  [5] ; ORCiD logo [10] ;  [11] ;  [1] ;  [6] ;  [12] ;  [4] ; ORCiD logo [5] ; ORCiD logo [3] more »; ORCiD logo [4] « less
  1. Univ. of California, Santa Cruz, CA (United States). Dept. of Astronomy and Astrophysics
  2. Univ. of Hamburg (Germany). Inst. for Experimental Physics
  3. National Inst. for Astrophysics (INAF), Milano (Italy); National Inst. for Astrophysics (INAF), Pavia (Italy)
  4. NASA Marshall Space Flight Center (MSFC), Huntsville, AL (United States)
  5. Stanford Univ, CA (United States). Kavli Inst. for Particle Astrophysics and Cosmology
  6. Deutsches Elektronen-Synchrotron (DESY), Zeuthen (Germany)
  7. Univ. of California, Berkeley, CA (United States). Astronomy Dept. and Theoretical Astrophysics Center
  8. Stanford Univ., CA (United States). Kavli Inst. for Particle Astrophysics and Cosmology
  9. National Inst. of Nuclear Physics (INFN), Roma (Italy)
  10. NASA Goddard Space Flight Center (GSFC), Greenbelt, MD (United States). Astrophysics Science Division
  11. Max Planck Inst. for Radio Astronomy, Bonn (Germany)
  12. National Inst. for Astrophysics (INAF), Milano (Italy); Univ. of Zielona Gora (Poland). Kepler Inst. of Astronomy
Publication Date:
Grant/Contract Number:
Accepted Manuscript
Journal Name:
The Astrophysical Journal (Online)
Additional Journal Information:
Journal Name: The Astrophysical Journal (Online); Journal Volume: 811; Journal Issue: 1; Journal ID: ISSN 1538-4357
Institute of Physics (IOP)
Research Org:
SLAC National Accelerator Lab., Menlo Park, CA (United States)
Sponsoring Org:
Contributing Orgs:
Fermi LAT Collaboration
Country of Publication:
United States
79 ASTRONOMY AND ASTROPHYSICS; ISM: jets and outflows; ISM: supernova remnants; pulsars: individual (Crab); techniques: image processing
OSTI Identifier: