It is assumed that the radioactive decay of Ti-44 powers the infrared, optical and UV emission of supernova remnants after the complete decay of Co-56 and Co-57 (the isotopes that dominated the energy balance during the first three to four years afte
r the explosion) until the beginning of active interaction of the ejecta with the surrounding matter. Simulations show that the initial mass of Ti-44 synthesized in core-collapse supernovae is (0.02-2.5) x 10^{-4} solar masses (M_sun). Hard X-rays and gamma-rays from the decay of this Ti-44 have been unambiguously observed from Cassiopeia A only, leading to the suggestion that the values of the initial mass of Ti-44 near the upper bound of the predictions occur only in exceptional cases. For the remnant of supernova 1987A, an upper limit to the initial mass of Ti-44 of < 10^{-3} M_sun has been obtained from direct X-ray observations, and an estimate of (1-2) x 10^{-4} M_sun has been made from infrared light curves and ultraviolet spectra by complex model-dependent computations. Here we report observations of hard X-rays from the remnant of supernova 1987A in the narrow band containing two direct-escape lines of Ti-44 at 67.9 and 78.4 keV. The measured line fluxes imply that this decay provided sufficient energy to power the remnant at late times. We estimate that the initial mass of Ti-44 was (3.1+/-0.8) x 10^{-4} M_sun, which is near the upper bound of theoretical predictions.
We present results of a study of the Galactic ridge X-ray emission (GRXE) in hard X-rays with the IBIS telescope on board INTEGRAL in the region near the Galactic Anticenter (GA) at l=155 deg. We assumed a conservative 2 sigma upper limit on the flux
from the GA in the 25-60 keV energy band of 1.25E-10 erg/s/cm^2 (12.8 mCrab) per IBIS field of view, or 6.6E-12 erg/s/cm^2 (0.7 mCrab) per degree longitude in the 135 deg. < l < 175 deg. region. This upper limit exceeds the expected GRXE intensity in the GA direction by an order of magnitude, given the near-infrared (NIR) surface brightness of the Milky Way in this region and the standard hard X-ray-to-NIR intensity ratio for the GRXE, assuming stellar origin. Based on the CGRO/EGRET surface brightness of the Galaxy above 100 MeV as a tracer of the cosmic-ray (CR) induced gamma-ray background, the expected GRXE flux in GA exceeds the measured 2 sigma upper limit by a factor of 4. Therefore, the non-detection of hard X-ray emission from the GA does not contradict the stellar nature of the GRXE, but is inconsistent with CR origin.
This paper is the second in a series devoted to the hard X-ray (17-60 keV) whole sky survey performed by the INTEGRAL observatory over seven years. Here we present a catalog of detected sources which includes 521 objects, 449 of which exceed a 5 sigm
a detection threshold on the time-averaged map of the sky, and 53 were detected in various subsamples of exposures. Among the identified sources with known and suspected nature, 262 are Galactic (101 low-mass X-ray binaries, 95 high-mass X-ray binaries, 36 cataclysmic variables, and 30 of other types) and 219 are extragalactic, including 214 active galactic nuclei (AGNs), 4 galaxy clusters, and galaxy ESO 389-G 002. The extragalactic (|b|>5 deg) and Galactic (|b|<5 deg) persistently detected source samples are of high identification completeness (respectively ~96% and ~94%) and valuable for population studies.
