Missing Titanium in the Asymmetric Supernova Remnant W 49B

The progenitor of the W 49B supernova remnant is still under debate. One of the candidates is a jet-driven core-collapse supernova. In such a highly asymmetric explosion, a strong α -rich freeze-out is expected in local high-entropy regions, which should enrich elements synthesized by the capture of α -particles such as 44 Ti and 48 Cr (decaying to 44 Ca and 48 Ti, respectively). In the present work, in order to infer the progenitor of the W 49B remnant, we constrain the amount of stable Ti ( 48 Ti) synthesized, using Suzaku observations. We found no firm evidence for the Ti line and set the upper limit of M Ti / M Fe < 9.9 × 10 −4 (99% limit using Xspec/AtomDB) and M Ti / M Fe < 5.8 × 10 −4 (99% limit using SPEX). Our limits can exclude almost all core-collapse supernova models, suggesting that a Type Ia supernova from a near-Chandrasekhar-mass white dwarf is the most favorable candidate for the origin of W 49B. Future observations with X-ray microcalorimeter missions, such as XRISM, will give us a stronger constraint on the progenitor.