The position of SN 1987A

TL;DR

This work provides a precise absolute position for SN 1987A by performing multi-epoch HST astrometry of the early ejecta and the equatorial ring, tied to Gaia DR3 within the ICRS frame. By independently locating the explosion center from the ejecta and the ER, and by robustly registering images with proper-motion corrections, the authors obtain $\alpha = 5^{\mathrm{h}} 35^{\mathrm{m}} 27^{\mathrm{s}}.9884(30)$ and $\delta = -69^{\circ} 16' 11''.1134(136)$ (ICRS J2016), with a 21 mas absolute positional uncertainty that propagates to the sky-plane kick velocity. The measured proper motion is $\mu_{\rm east} = 1.60 \pm 0.15$ mas yr$^{-1}$ and $\mu_{\rm north} = 0.44 \pm 0.09$ mas yr$^{-1}$, and the inferred kick velocities from JWST-detected emission nearby the compact object are $v_{\rm kick,sky} = 399 \pm 148$ km s$^{-1}$ and $v_{\rm kick,3D} = 472 \pm 126$ km s$^{-1}$, which are consistent with prior estimates. The analysis also notes a potential intrinsic offset between the explosion center and ER and highlights that the dominant remaining uncertainty is the explosion center, with future observations needed to resolve the compact object's nature and refine the kick estimate.

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 ${α= 5^{\mathrm{h}}~ 35^{\rm{m}}~ 27^{\rm{s}}.9884(30)}$, ${δ= -69^{\circ}~ 16'~ 11''.1134(136)}$ (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 ${μ_{\rm east} (\equiv \rm{PM_{α}*}) = 1.60 \pm 0.15 ~\rm{mas ~ yr^{-1}}}$ and ${μ_{\rm{north}} (\equiv \rm{PM_δ}) = 0.44 \pm 0.09~\rm{mas ~ yr^{-1}}}$, 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.

Figures (5)

• Figure 1: HST/FOC imaging observations of SN 1987A. The color scale of the images is inverse hyperbolic sine and arbitrarily scaled to highlight the ejecta in the center. For visualization only, the images are resampled to a common grid where north is up and east is left. The field of view (FOV) of each full image is $11" \times 11"$; zoomed-in insets focus on the expanding ejecta and cover $1\overset{\prime\prime}{.}950 \times 1\overset{\prime\prime}{.}525$.
• Figure 2: HST image of the ER at day 7226. The red annulus represents the fitted diffuse ring model, plotted with a width of 2$\sigma$. The black points represent the fitted hot spot centroids, while the dashed black ellipse represents the fitted ellipse around the hot spot centroids. Both methods give an estimate of the center of the ER, showcased as a black and red cross mark for the hot spot ellipse and diffuse ring, respectively.
• Figure 3: SN 1987A position estimates and astrometric trajectory. Left panel: Celestial coordinates of SN 1987A determined from the HST images. The gray hollow stars correspond to the diffuse ring positions after image registration onto the Gaia DR3 frame, without correcting for the proper motion of the reference stars. The gray diamond shows the weighted average of the diffuse ring positions. The red "$\times{}$" marker and its ellipse are the weighted average of the hot spot ellipse positions and the 68.3% confidence region, respectively (individual points omitted). The pink and cyan squares show the F175W (X0C80102T, X0C80103T) and F501N (X0C80106T, X0C80107T) ejecta positions, respectively. The cyan hexagons are the F501N (X0C80106T, X0C80107T) diffuse ring positions with their registration and fitting uncertainties. The upside--down purple triangle is the average of the four early ejecta positions. The black cross marks the final position, derived as the average of the mean diffuse ring and early ejecta position. The black ellipse shows the total uncertainty, corresponding to their positional discrepancy. Right panel: Time evolution of the average diffuse ring position at each epoch. The blue hollow stars show the diffuse ring center at each observational epoch, which are obtained after registering the images by accounting for the proper motion of the reference stars. For clarity, the early WFPC2 points are omitted due to their larger uncertainties and small contribution to the fit. The blue triangle marks the J2016 position from the best--fit astrometric trajectory, together with its 68.3% confidence region. The gray diamond and its ellipse are the same as in the left panel but appear different due to the differing aspect ratios. The blue circles show the predicted positions from the best-fit trajectory of SN 1987A annually (from J2003 to J2023). The solid segments connect these points and represent the predicted proper motion from the best--fit trajectory.
• Figure 4: HST/WFC3/F625W image of the ER and ejecta at day 10,698. The image is registered onto the Gaia DR3 frame (J2016). Left panel: Full view of the ER showing the favored final position of SN 1987A and the ALMA submillimeter position from 2019ApJ...886...51C. The FOV is $2\overset{\prime\prime}{.}025 \times 1\overset{\prime\prime}{.}450$. The white rectangle outlines the region shown in the right panel. Right panel: Zoom-in on the ejecta with the final position and the ALMA estimate overlaid. The FOV is $0\overset{\prime\prime}{.}625 \times 0\overset{\prime\prime}{.}600$.
• Figure 5: Individual positions relative to final explosion center. The black cross shows the favored final position $\alpha=5^{\rm{h}}~ 35^{\rm{m}}~ 27^{\rm{s}}.9884(30)$, $\delta=-69^{\circ}~ 16'~ 11".1134(136)$ (ICRS J2016). The black circle indicates the total uncertainty, defined as the offset between the mean early ejecta and mean diffuse ring positions. The upside-down purple triangle marks the average early ejecta position; the red "'$\times{}$" marker shows the average hot spot ellipse position; the gray diamond indicates the average diffuse ring position. Their corresponding ellipses represent 68.3% confidence regions. The right orange triangle shows the position from 2018ApJ...864..174A, corrected from J2015 ($5^{\rm{h}}~ 35^{\rm{m}}~ 27^{\rm{s}}.9875(11)$, $-69^{\circ}~ 16'~ 11".1070(40)$) to J2016 ($5^{\rm{h}}~ 35^{\rm{m}}~ 27^{\rm{s}}.9878$, $-69^{\circ}~ 16'~ 11".1066$) using the estimated proper motion of SN 1987A.