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Supernova 1987A at 30 Astronomical Science DOI: doi.org/10.18727/0722-6691/5005 Supernova 1987A at 30 Jason Spyromilio1 ranged across astroparticles and all made famous by HST, Chandra and Bruno Leibundgut1 wavelengths of the electromagnetic ATCA images (see Figure 1, left), is readily Claes Fransson2 spectrum, then one might consider one visible in images from NAOS-CONICA Josefin Larsson2 had found the perfect source. (NACO), and the Spectrograph for INtegral Katia Migotto2 Field Observations in the Near-Infrared Julien Girard1 SN 1987A is just such a heavenly object! (SINFONI) as well (Larsson et al., 2016). That it is circumpolar for the more south- Figure 1 right shows a very recent NACO ern astronomical sites adds to its opti- image at 2.15 μm (Ks-band). The flux in 1 ESO mality. Located in the Large Magellanic the Ks-band is dominated by Brackett-γ 2 Department of Astronomy, The Oskar Cloud (LMC), it is near enough to be emission at 2.165 µm illuminated from the Klein Centre, Stockholm University, resolved, yet far enough to not be a outside. The illumination of the ring at Sweden threat, along an almost unextinguished the earliest times, by the ultraviolet (UV) line of sight, in a relatively uncluttered light emerging as the shock broke the part of its host galaxy. It is evolving on surface of the progenitor star, was used Thirty years on, SN 1987A continues to a human timescale. Many articles on to determine a geometric distance to the develop and, over the last decade in 1987A have appeared in the pages of LMC and remains one of the anchors of particular, has: revealed the presence of The Messenger over the years and there- the extra-galactic distance ladder. The a large centrally concentrated reservoir fore we will not dwell here on the past ring illumination also provided the first of dust; shown the presence of molecu- but rather focus on the current state and evidence of the radiation from the shock lar species within the ejecta; expanded ponder an exciting future. A comprehen- break-out, lasting only a few minutes but such that the ejecta structure is angu- sive review (McCray & Fransson, 2016) with a temperature of about a million larly resolved; begun the destruction of appeared recently and covers SN 1987A degrees. the circumstellar ring and transitioned over the past 10 000 days. to being dominated by energy sources Later, when the fastest ejecta, moving external to the ejecta. We are partici- Despite fading by seven orders of magni- at ~10 % of the speed of light, reached pating in a live experiment in the creation tude from its peak, the supernova and its the ring, the shocked gas emitted brightly of a supernova remnant and here the surroundings remain readily observable. at wavelengths from radio to X-ray. recent progress is briefly overviewed. Lengthy exposure times are still necessary Recently, observations from HST have Exciting developments can be expected to study the details and, critically, the time- been used to show that the ring is begin- as the ejecta and the reverse shock scales over which the supernova changes ning to suffer from the effects of the ejecta continue their interaction, the X-rays remain of order half a year (approximately colliding into it (Fransson et al., 2015). It penetrate into the cold molecular core the light travel time of the ejecta at this will take a while, but the ring is currently and we observe the return of the mate- epoch); therefore continued vigilance is being destroyed. A simple extrapolation rial into the interstellar medium. We needed. ESO, together with the Hubble estimates this process will be complete by anticipate that the nature of the remnant Space Telescope (HST) and the Australia ~ 2025. However, new spots of emission of the leptonisation event in the centre Telescope Compact Array (ATCA), are the outside the ring have appeared and con- will also be revealed. sole observatories that, thanks to the evo- tinued observations may yet provide sur- lution of their observing capabilities, have prises about the surrounding structure. provided the necessary continuous ultra- We get to watch in real time as a shock In the preface to the first SN 1987A con- violet/optical/near-infrared/radio coverage wave with well-defined characteristics ference thirty years ago (Danziger, 1987), of the supernova, and fortunately the fire- impinges on a well-understood structure Lodewijk Woltjer, then Director General of works are still continuing. Together with (both in density and composition); a text ESO, welcomed the participants with the similar monitoring by the Chandra and book illustration of shock theory. prescient statement “It is very well possi- X-ray Multi-Mirror Mission (XMM-Newton) ble that […] SN 1987A will remain observ- space telescopes in X-rays, these facili- able for thousands of years to come.” ties have provided a nearly complete Radioactivities multi-wavelength coverage of the devel- opment of the supernova. These tele- Inside the ejecta, radioactive species Introduction scopes are providing a legacy dataset for freshly synthesised in the explosion pro- this object that cannot be repeated. vide gamma rays and energetic positrons If an observational astronomer was that deposit their energy into the ejecta, allowed to pick the parameters of the The progenitor star (Sanduleak –69°202) provided they do not escape. Which iso- object of study, then ideally the angular of SN 1987A cleared a volume around topes are present, and how much of size would be matched to the resolution itself, sweeping-up material blown off in each, are critical to our understanding of of the telescope, the dimensions of the earlier evolutionary stages (about the emerging spectrum. The isotope mix physical processes would be matched 8000 years ago) into an hour-glass struc- also places limits on the mass cut (the to the angular size and the variability ture dominated by an equatorial ring. This mass coordinate in the proto-neutron star matched to the proposal cycles for tele- structure, first observed with the New where the ejection starts and the collapse scope time. If, in addition, the physics Technology Telescope (NTT) in 1989, and ends) and provides a measure of the 26 The Messenger 167 – March 2017 Figure 1. Left: combined HST (green), Chandra (blue) and ALMA (red) image of SN 1987A. Right: NACO data taken in January 2017 in the Ks-band. The ejecta emission in the centre of the ring is well resolved in both images; the short axis of the ring projects to 1 arcsecond on the sky. The west side of the ring can now be seen to be signifi- cantly brighter than the east side. nucleosynthetic yield of the supernova. ARray (NuSTAR) detection (Boggs et al., core. This distribution is one of the main The presence of 56Ni (source of 56Fe) had 2015) and the INTErnational Gamma-Ray diagnostics of the explosion dynamics long been confirmed in SN 1987A, as Astrophysics Laboratory (INTEGRAL) during the first seconds. had 57Co. Theory predicted that 44Ti detection by Grebenev et al. (2012) of should also be made in the explosion of hard X-ray lines from 44Ti. Observations supernovae. Combining Very Large Tele- with SINFONI (Kjaer et al., 2010; Larsson Shocks scope (VLT) and HST spectra with time- et al., 2016) and HST (Larsson et al., dependent non-local thermodynamic 2011) have revealed a complex structure While the forward shock moves through equilibrium (LTE) radiative transfer calcu- of emission from atomic species that, in the ring, slowing down the ejecta and lations, it was determined that 44Ti had some cases, are collocated with the radi- accelerating the ring material, the reverse taken the role of key energy supplier to oactive species and in others are illumi- shock is formed by the supernova ejecta the supernova eight years after the explo- nated by external sources. In particular, hitting the decelerated medium behind sion (Jerkstrand et al., 2011). the SINFONI observation of the 1.644 µm the forward shock. The reverse shock is [Si I] + [Fe II] line gives a three-dimen- formed in successfully slower and denser It was consequently exciting to see both sional view of the 44Ti distribution in the regions in the supernova ejecta. the Nuclear Spectroscopic Telescope ejecta, responsible for powering the inner The supernova ejecta are being exposed from the outside to X-rays from the ring 120 Figure 2. The light interaction and at some point during the curve of the super- supernova’s teenage years the dominant nova over the past 5000 days. Different source of energy became the conversion 100 components and of kinetic energy from the supernova wavelengths are iden- ejecta with the surroundings. Quite ele- tified. The dimming of gantly, just as the radioactive elements ) 80 the ring in optical –1 whose decay had powered the emission s emission and the g brightening of the of the junior supernova exponentially er emission from the 3 decreased, the X-rays from the interaction –1 60 Ring: 0.5–8 keV ejecta, consequent 0 Ejecta: R-band × 150 with the inner ring provided the energy for (1 on the input of energy from the reverse an outside-in look at the ejecta. These Flux shock, are evident. X-rays are mainly absorbed in the hydro- 40 Ring: [O III] × 300 gen rich envelope and the metal core is still powered by decay of 44Ti. 20 The different emission sites (ionised but Ring: R-band × 5 unshocked ring material, shocked ring gas, reverse shock, inner supernova 0 4000 6000 8000 10 000 ejecta) account for the vastly different Days since explosion velocities, and are easily separated in the The Messenger 167 – March 2017 27 Astronomical Science Spyromilio J.
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