The position of SN 1987A

Department of Physics, KTH Royal Institute of Technology, The Oskar Klein Centre, AlbaNova, 106 91 Stockholm, Sweden

^★ Corresponding author: cteg@kth.se

Received: 12 May 2025
Accepted: 25 October 2025

Abstract

Aims. The accurate positional measurement of Supernova (SN) 1987A is important for determining the kick velocity of its compact object and the velocities of the ejecta and various shock components. In this work, we perform absolute astrometry to determine the position of SN 1987A.

Methods. We used multi-epoch Hubble Space Telescope imaging to model the early ejecta and the equatorial ring (ER). We combined our measurements and obtained the celestial coordinates in the International Celestial Reference System (ICRS) by registering the observations onto Gaia Data Release 3.

Results. The final average position of the different measurements is α = 5^h35^m27_⋅^s9884(30),δ = −69∘16′11_⋅′′1134(136) (ICRS J2016). The early ejecta position is located 14 mas south and 16 mas east of the ER center, with the offset being significant at 96% confidence. The offset may be due to instrument and/or filter-dependent systematics and registration uncertainties, though an intrinsic explosion offset relative to the ER remains possible. Image registration with proper motion corrections yields similar astrometry and a source proper motion of μ_east(≡ PM_α*) = 1.60 ± 0.15 mas yr⁻¹ and μ_north(≡ PM_δ) = 0.44 ± 0.09 mas yr⁻¹, in agreement with the typical local motion of the Large Magellanic Cloud.

Conclusions. The absolute positional uncertainty of 21 mas adds a systematic uncertainty to the sky-plane kick velocity of 123 (t/40 yr⁻¹ km s⁻¹, where t is the time since the explosion. Comparing the location of the compact source observed with JWST to our updated position implies a sky-plane kick of 399 ± 148 km s⁻¹ and a 3D kick of 472 ± 126 km s⁻¹, which is consistent with previous estimates.