The position of SN 1987A

TL;DR

This paper precisely measures the position of SN 1987A using Hubble and Gaia data, estimating the supernova's compact object kick velocity and analyzing the ejecta and ring offsets with high accuracy.

Contribution

It provides the most accurate positional measurement of SN 1987A to date, combining multi-epoch HST imaging with Gaia data to refine the supernova's location and velocity estimates.

Findings

The supernova's position is determined with 21 mas accuracy.

The ejecta are offset from the ring center by 14 mas south and 16 mas east.

The estimated sky-plane kick velocity is approximately 399 km/s.

Abstract

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. 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. The final average position of the different measurements is ${\alpha = 5^{\mathrm{h}}~ 35^{\rm{m}}~ 27^{\rm{s}}.9884(30)}$, ${\delta = -69^{\circ}~ 16'~ 11''.1134(136)}$ (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 ${\mu_{\rm east} (\equiv \rm{PM_{\alpha }*}) = 1.60 \pm 0.15 ~\rm{mas ~ yr^{-1}}}$ and ${\mu_{\rm{north}} (\equiv \rm{PM_{\delta}}) = 0.44 \pm 0.09~\rm{mas ~ yr^{-1}}}$, in agreement with the typical local motion of the Large Magellanic Cloud. The absolute positional uncertainty of 21 mas adds a systematic uncertainty to the sky-plane kick velocity of ${123}~(t/40~\rm{yr})^{-1}~\rm{km~s}^{-1}$, 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 ${399\pm148~\mathrm{km~s^{-1}}}$ and a 3D kick of ${472\pm126~\mathrm{km~s^{-1}}}$, which is consistent with previous estimates.