Investigation of 44Ti decay X-ray lines in CasA

Radioactive 44Ti is of great interest as a diagnostic of the core collapse supernova explosion mechanism and as the source of stable 44Ca in our Galaxy. Its decay gamma-ray lines have been measured several times in the supernova remnant Cassiopeia A, with some inconsistencies among the measurements. This isotope’s decay (44Ti → 44Sc) also produces X-ray fluorescence emission following the electron capture, which might provide an alternate method of direct measurement of the mass of 44Ti ejected, and the location of the 44Ti within the remnant. Here we search for the Sc K-α emission in Chandra ACIS observations of Cassiopeia A. This instrument has sufficient effective area to detect the expected line flux, but because of the large continuum flux, we thus far have obtained only upper limits on the line emission. In two Chandra observations the 2σ upper limit for the 4.09 keV line flux from the entire remnant is ∼3.8 × 10−5 γ/cm2s. We also study more limited parts of the remnant, including apparently iron-rich regions of CasA, and discuss possible complications and limitations of this method.