by Danielle Dickinson
Danielle Dickinson is a fourth-year graduate student at Purdue University studying astrophysics. They are interested in mass loss in massive stars and supernova remnants; they use ground-based and JWST observations of extragalactic supernovae and Cassiopeia A to answer: What do massive stars do before they die? How do supernova explosion dynamics impact the morphology of supernova remnants? She enjoys rock climbing, running, and playing with her two cats, Minerva and Pepe. You can find more of them on their website and their Bluesky.
Title: Emission lines due to ionizing radiation from a compact object in the remnant of Supernova 1987A
Authors: Claes Fransson, M. Barlow, P. J. Kavanagh, J. Larsson, O. C. Jones, B. Sargent, M. Meixner, P. Bouchet, T. Temim, G. S. Wright, J. Blommaert, N. Habel
First Author’s Institution: Department of Astronomy, Stockholm University, Sweden
Status: Published in Science [open access]
The Large Magellanic Cloud’s most recent Supernova
Supernova (SN) 1987A exploded almost 38 years ago in the Large Magellanic Cloud, making it the youngest supernova in our galactic neighborhood. SN 1987A is a continuously evolving object, and its proximity to Earth makes it the optimal laboratory to study SN dynamics in high resolution. It was detected on February 24, 1987, one day after the explosion. We know the exact time of explosion because of a neutrino burst caused by the explosion that was detected by three neutrino facilities across the globe. The explosion resulted from the death of a Blue Supergiant star, Sanduleak −69 202, with a mass 15 – 20 times the mass of our sun, and likely formed a Neutron Star (NS) after its explosion. Previous studies have hinted at the presence of this NS, but there is not a decisive detection yet. Today’s authors seek to answer: What lies at the center of SN 1987A and how does it impact the surrounding ejecta?
Observations: JWST Observations of SN 1987A
Today’s authors obtained JWST MIRI MRS Integral Field Unit (IFU) Spectroscopy of the inner region of SN 1987A to study the inner ejecta and the location of the neutron star remnant. Each pixel in the field of view in an IFU has an associated spectrum, making a data “cube.” Thus, the IFU Spectrographs mounted on JWST are powerful tools in obtaining the spectra of resolved targets while providing spatial information. Past observations in the infrared (IR) with Spitzer could not resolve the complex distribution of the inner ejecta, but JWST observations resolve these structures for the first time in IR.
SN 1987A is shown in Figure 1 as observed by JWST and a side view as described by an artist’s impression. The center is outlined by the Equatorial Ring/Inner Region that contains clumpy hotspots. The ring structures are thought to be Circumstellar Material (CSM) lost by Sanduleak -69 202 at least 20,000 years before the explosion. The blue central region of the SN is filled with SN ejecta that is diffuse and has a large structure, shaped like a keyhole. The artist’s rendition highlights the hourglass shaped distribution of CSM surrounding the explosion. As the shock propagates outward, it excites a slice of the CSM, which then appears as rings in observations.
Results: Something lies at the heart of SN 1987A
SN 1987A is an expanding explosion, and the velocity of the emitting gas is imprinted on the line emission via Doppler shifting. The IFU spectroscopy showed strong emission in two Argon lines ([Ar VI] and [Ar II]) at a velocity, 416 ± 206 km/s, faster than the rest frame of the SN. This is a different velocity from the clumpy, inner ring, which has a velocity of -12 km/s with respect to the SN rest frame. This suggests this central emission is unrelated to the inner ring. Velocity slices of these emission lines are shown in Figure 2; the central source is best seen in Panels E-H and is circled in Panel F. Analysis of the radial profiles of the central emission in these spectral lines represent an unresolved point source (the neutron star) rather than emission from an extended source. With MIRI, this central emission is also observed in Sulfur ([S III] and [S IV]) with slow velocities.
What drives the central Ar and S emission?
Ar and S are produced by the burning of O and Si at the core of the star. Thus, this emission originates from the inner core of the ejecta. These lines must be excited by a shock wave or an ionizing source. Simulations that model how gas is ionized show that the ionizing light source could be synchrotron radiation from a Pulsar Wind Nebula (PWN) or a cooling Neutron Star. But, there are some uncertainties in the modeling, including the ionizing spectrum, density, elemental abundances, and dust properties, which make it hard to conclusively point to the exact source. Nonetheless, it is clear that the mechanism requires the presence of a NS. The authors explore other mechanisms unrelated to a NS, but these are excluded due to which lines are present in the central object’s IR spectrum and the shape of the emission line profiles. Not only does this analysis show further evidence for the remnant NS of SN 1987A, but it also begins to constrain the motion of the NS following the explosion, called a natal kick. Translating the spectral measurements to the velocity the NS was kicked is dependent on whether one assumes a PWN or cooling NS model, leaving the natal kick velocity of the NS formed by SN 1987A unknown.
SN 1987A left a remnant compact object behind in the wake of its explosion and has been elusive for 38 years. New observations presented by today’s authors show a central source buried in the ejecta of SN 1987A, strong in Argon and Sulfur emission lines. The excitation of these lines is consistent with the presence of a NS, which invites a new era of studying Cooling NS and Pulsar Wind Nebulae with JWST.
Astrobite edited by Jessie Thwaites
Featured image credit: NidhiYashwanth (Pixabay) and Canva Image Library, edited by Danielle Dickinson
