Measuring the initial mass of ⁴⁴Ti in SN 1987A through the ⁴⁴Sc emission line

¹ Dipartimento di Fisica e Chimica E. Segrè, Università degli Studi di Palermo, Via Archirafi 36, 90123 Palermo, Italy
² INAF-Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ AIM, CEA, CNRS, Université Paris-Saclay, Université de Paris, 91191 Gif sur Yvette, France
⁴ Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 106319, Taiwan
⁵ Astrophysical Big Bang Laboratory (ABBL), RIKEN Cluster for Pioneering Research, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
⁶ Institute for Applied Problems in Mechanics and Mathematics, Naukova Street 3-b, 79060 Lviv, Ukraine
⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁸ RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
⁹ Astrophysical Big Bang Laboratory (ABBL), RIKEN Pioneering Research Institute (PRI), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
¹⁰ Astrophysical Big Bang Group (ABBG), Okinawa Institute of Science and Technology Graduate University (OIST), 1919-1 Tancha, Onna-son, Kunigami-gun, Okinawa 904-0495, Japan

^★ Corresponding author.

Received: 20 May 2025
Accepted: 7 July 2025

Abstract

Context. Deriving the mass and large-scale asymmetries of radioactive isotopes offers valuable insights into the complex phases of a supernova explosion. Important examples include ⁵⁶Ni, with its decay products ⁵⁶Co and ⁵⁶Fe, and ⁴⁴Ti, which are studied through their X-ray emission lines, and provide a powerful diagnostic tool to probe the explosive nucleosynthesis processes in the inner layers of the exploding star.

Aims. In this framework, SN 1987A provides a privileged laboratory, being the youngest supernova remnant from which the mass of Ti has been estimated. However, some uncertainty remains in determining the initial mass of ⁴⁴Ti. Previous analyses, relying on NuSTAR and INTEGRAL data, report M₄₄ = (1.5 ± 0.3) × 10⁻⁴ M_⊙ and M₄₄ = (3.1 ± 0.8) × 10⁻⁴M_⊙, respectively. In this paper, we estimate the initial mass of ⁴⁴Ti via its decay product, the ⁴⁴Sc emission line at 4.09 keV, using Chandra observations.

Methods. We performed multi-epoch spectral analysis focusing on the inner part of the remnant, to minimize the contamination from the X-ray emission stemming from the shocked plasma. As a result, we report the detection of ⁴⁴Sc emission line in the central part of SN 1987A with a ~99.7% (3 σ) significance.

Results. The simultaneous fit of the spectra extracted from observations between 2016 and 2021 yields a line flux of 6.8_−2.3^+2.2 × 10⁻⁷ photons s⁻¹ cm⁻², corresponding to a ⁴⁴Ti mass of M₄₄ = (1.6 ± 0.5) × 10⁻⁴M_⊙ (errors at the 90% confidence level). The results obtained with our spectral analysis appear to align with those derived with NuSTAR.