Hard X-Ray Emission and ⁴⁴Ti Line Features of the Tycho Supernova Remnant

Abstract

A deep hard X-ray survey of the International Gamma-Ray Astrophysics Laboratory (INTEGRAL) satellite has detected for the first time non-thermal emission up to 90 keV in the Tycho supernova (SN) remnant. Its 3-100 keV spectrum is fitted with a thermal bremsstrahlung of kT ~ 0.81 ± 0.45 keV plus a power-law model of Γ ~ 3.01 ± 0.16. Based on diffusive shock acceleration theory, this non-thermal emission, together with radio measurements, implies that the Tycho remnant may not accelerate protons up to >PeV but to hundreds TeV. Only heavier nuclei may be accelerated to the cosmic ray spectral "knee." In addition, using INTEGRAL, we search for soft gamma-ray lines at 67.9 and 78.4 keV that come from the decay of radioactive ⁴⁴Ti in the Tycho remnant. A bump feature in the 60-90 keV energy band, potentially associated with the ⁴⁴Ti line emission, is found with a marginal significance level of ~2.6σ. The corresponding 3σ upper limit on the ⁴⁴Ti line flux amounts to 1.5 × 10^-5 photon cm^-2 s^-1. Implications on the progenitor of the Tycho SN, considered to be a Type Ia SN prototype, are discussed.