Thermal Emission and Radioactive Lines, but No Pulsar, in the Broadband X-Ray Spectrum of Supernova 1987A

Abstract

Supernova 1987A offers a unique opportunity to study an evolving supernova in unprecedented detail over several decades. The X-ray emission is dominated by interactions between the ejecta and the circumstellar medium, primarily the equatorial ring (ER). We analyze 3.3 Ms of NuSTAR data obtained between 2012 and 2020, and two decades of XMM-Newton data. Since $\sim$2013, the flux below 2 keV has declined, the 3-8 keV flux has increased, but has started to flatten, and the emission above 10 keV has remained nearly constant. The spectra are well described by a model with three thermal shock components. Two components at 0.3 and 0.9 keV are associated with dense clumps in the ER, and a 4 keV component may be a combination of emission from diffuse gas in the ER and the surrounding low-density H II region. We disfavor models that involve non-thermal X-ray emission and place constraints on non-thermal components, but cannot firmly exclude an underlying power law. Radioactive lines show a ${}^{44}$Ti redshift of $670_{-380}^{+520}$ km s${}^{-1}$, ${}^{44}$Ti mass of $1.73_{-0.29}^{+0.27}\times 10^{-4}$ solar masses, and ${}^{55}$Fe mass of $<4.2\times 10^{-4}$ solar masses. The 35-65 keV luminosity limit on the compact object is $2\times 10^{34}$ erg s${}^{-1}$, and $<15$% of the 10-20 keV flux is pulsed. Considering previous limits, we conclude that there are currently no indications of a compact object, aside from a possible hint of dust heated by a neutron star in recent ALMA images.

39 pages, 15 figures, 12 tables, no pulsar. Accepted for publication in ApJ. Updated to match the accepted version. We have made a number of clarifications and significantly expanded the discussion about instrumental uncertainties in Appendices C and D. The scientific conclusions remain unchanged

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• Investigating Time Evolution of Thermal Emission from the Putative Neutron Star in SN 1987A for 50+ Years
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• Unusual X-ray Oxygen Line Ratios of SN 1987A Arising From the Absorption of Galactic Hot Interstellar Medium
• Measuring the initial mass of 44Ti in SN 1987A through the 44Sc emission line
• X-ray Measurements of ${}^{44}$Ti in Four Supernova Remnants