HARD X-RAY EMISSIONS FROM CASSIOPEIA A OBSERVED BY INTEGRAL

Abstract

ABSTRACT Cassiopeia A (Cas A), as the nearby young remnant of a core-collapse supernova, is the best candidate for astrophysical studies in supernova explosion and its environment. We studied the hard X-ray emission from Cas A using the 10 year data of INTEGRAL observations, and first detected non-thermal continuum emission from the source up to 220 keV. The 44Ti line emission at 68 and 78 keV is confirmed by our observations with a mean flux of ∼(2.2 ± 0.4) × 10−5 ph cm−2 s−1, corresponding to a 44Ti yield in Cas A of (1.3 ± 0.4) × 10−4 M ⊙. The continuum emission from 3 to 500 keV can be fit with a thermal bremsstrahlung of kT ∼ 0.79 ± 0.08 keV plus a power-law model of Γ ∼ 3.13 ± 0.03. The non-thermal emission from Cas A is well fit by a power-law model without a cutoff up to 220 keV. This radiation characteristic is inconsistent with diffusive shock acceleration models with a remnant shock velocity of only 5000 km s−1. The central compact object in Cas A cannot significantly contribute to the emission above 80 keV. Some possible physical origins of the non-thermal emission above 80 keV from the remnant shock are discussed. We deduce that the asymmetrical supernova explosion scenario of Cas A is a promising scenario for the production of high-energy synchrotron radiation photons, where a portion of the ejecta with a velocity of ∼0.1c and opening angle of ∼10° can account for the 100 keV emission, as is consistent with the “jet” observed in Cas A.