Gamma-Ray Observations of the Crab Nebula: A Study of the Synchro-Compton Spectrum

view Abstract Citations (251) References (46) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gamma-Ray Observations of the Crab Nebula: A Study of the Synchro-Compton Spectrum de Jager, O. C. ; Harding, A. K. ; Michelson, P. F. ; Nel, H. I. ; Nolan, P. L. ; Sreekumar, P. ; Thompson, D. J. Abstract Phase I and II EGRET observations of the Crab Nebula establish the synchro-Compton unpulsed spectrum between approximately 70 MeV and 30 GeV. The soft 70-150 MeV spectrum appears to be a steepened extension of the 1-30 MeV COMPTEL spectrum recently reported by Much et al., indicating that the nebular synchrotron spectrum cuts off with an e-folding energy E0 ∼ 26 MeV. This energy is consistent with the characteristic synchrotron energy hυmax ∼ (3/4π )2hc/r0 = 25 MeV (with r0 being the classical electron radius) expected for the synchrotron burnoff if electrons are accelerated on a timescale equal to the electron gyro- period in the inner nebula. The 70-150 MeV emission in the exponential tail of this cutoff decreased by a factor of about 2 between 1991 and 1993, which is consistent with an approximately 25% reduction in E0 over that time. A steady hard, approximately E-1.85 photon spectrum, added to the synchrotron component, is required for energies up to 10 GeV. This spectrum steepens toward the observed TeV γ-ray spectrum and may represent the expected nebular inverse-Compton (IC) component, which allows a measurement of the mean nebular field strength: from EGRET we obtain barB ∼ 0.13 mG for the radio nebula, whereas the various reported TeV observations correspond to a larger barB ∼ 0.26 mG for the smaller optical nebula. The observation of the IC component also allows us to obtain a lower limit to the time-averaged injection rate Ndot > 4 x 1040 s-1 for all electrons/positrons into the nebula. The combined EGRET/TeV IC spectrum appears to be inconsistent with the assumption of a spherically symmetric particle-dominated pulsar wind with parameter σ ∼ 0.003 (see the work of Kennel ISM: INDIVIDUAL NAME: CRAB NEBULA; ISM: MAGNETIC FIELDS; MAGNETOHYDRODYNAMICS: MHD; RADIATION MECHANISMS: NONTHERMAL; X-RAYS: ISM full text sources ADS | data products SIMBAD (4) HEASARC (1)