Characterization of the inner knot of the Crab: The site of the gamma-ray flares?
- Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064 (United States)
- Institut für Experimentalphysik, Universität Hamburg, Luruper Chaussee 149, D-22761 Hamburg (Germany)
- INAF–IASF Milano, via E. Bassini 15, I-20133 Milano (Italy)
- NASA Marshall Space Flight Center, Astrophysics Office (ZP12), Huntsville, AL 35812 (United States)
- W. W. Hansen Experimental Physics Laboratory, Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305 (United States)
- DESY, Platanenallee 6, D-15738 Zeuthen (Germany)
- Astronomy Department and Theoretical Astrophysics Center, University of California, Berkeley, 601 Campbell Hall, Berkeley, CA 94720 (United States)
- INFN Roma Tor Vergata, via della Ricerca Scientifica 1, I-00133 Roma (Italy)
- NASA Goddard Space Flight Center, Astrophysics Science Division, Greenbelt, MD 20771 (United States)
- Max-Planck-Institut für Radioastronomie, auf dem Hügel 69, D-53121 Bonn (Germany)
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. 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 (≲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. This 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. However, the IR–optical flux of the inner knot is marginally consistent with the shock accelerating most of the Nebula’s optical-emitting particles.
- OSTI ID:
- 22882627
- Journal Information:
- Astrophysical Journal, Vol. 811, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); Since 2009, the country of publication for this journal is the UK.; ISSN 0004-637X
- Country of Publication:
- United Kingdom
- Language:
- English
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