HARD X-RAY EMISSIONS FROM CASSIOPEIA A OBSERVED BY INTEGRAL

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 Ti-44 line emission at 68 and 78 keV is confirmed by our observations with a mean flux of similar to(2.2 +/- 0.4) x 10(-5) ph cm(-2) s(-1), corresponding to a Ti-44 yield in Cas A of (1.3 +/- 0.4) x 10(-4)M(circle dot). The continuum emission from 3 to 500 keV can be fit with a thermal bremsstrahlung of kT similar to 0.79 +/- 0.08 keV plus a power-law model of Gamma similar to 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 similar to 0.1c and opening angle of similar to 10 degrees can account for the 100 keV emission, as is consistent with the "jet" observed in Cas A.ESA member states; National Natural Science Foundation of China (NSFC) [11273005]; SRFDP [20120001110064]; 973 Program [2014CB845800]SCI(E)ARTICLEwangwei@bao.ac.cn; zhuo.li@pku.edu.cn282