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|Title:||Characterization of the Inner Knot of the Crab: The Site of the Gamma-Ray Flares?||Authors:||Rudy, Alexander
O'Dell, Stephen L.
Weisskopf, Martin C.
|Issue Date:||2015||Journal:||THE ASTROPHYSICAL JOURNAL||Number:||811||Issue:||1||First Page:||24||Abstract:||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.||Acknowledgments:||The Fermi /LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States, the Commissariat à l’Energie Atomique and the Centre National de la Recherche Scientifique/Institut National de Physique Nucléaire et de Physique des Particules in France, the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy, the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), High Energy Accelerator Research Organization (KEK), and the Japan Aerospace Exploration Agency (JAXA) in Japan, and the K. A. Wallenberg Foundation, the Swedish Research Council, and the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica and the Centre d’Etudes Spatiales in France. The research leading to these results has also received funding from the European Commission Seventh Framework Programme (FP7/2007-2013) under grant agreement n. 267251. Several of the authors would also like to acknowledge both funding and solid support from the Chandra X-ray Center and the Hubble Space Telescope Science Institute under a number of observing proposals: GO3-14054Z, GO3-14057Z, GO4-15058Z, GO4-15059Z, GO-13109, GO-13196, and GO-13348.||URI:||http://hdl.handle.net/20.500.12386/23732||URL:||https://iopscience.iop.org/article/10.1088/0004-637X/811/1/24||ISSN:||0004-637X||DOI:||10.1088/0004-637X/811/1/24||Bibcode ADS:||2015ApJ...811...24R||Fulltext:||open|
|Appears in Collections:||1.01 Articoli in rivista|
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