Superluminal echo from star explosion detected
Hubble Space Telescope has detected a multiple light echo supernova explosion 2016adj, which arose due to the reflection of radiation from the dust bands of the galaxy Centaurus A. The discovery of the unusual phenomenon is reported in an article published in The Astrophysical Journal Letters.
The supernova SN 2016adj was discovered in February 2016 at a peak brightness of 16 stellar magnitudes. It originated in the giant elliptical galaxy Centaurus A, located 10 to 16 million light-years from Earth. It was classified as a Type Ic supernova, which occurs when the core of a massive star, stripped of its hydrogen and most of its helium envelope, collapses due to a stellar wind or accretion of material onto a companion star. Observations of the supernova and the region around it were made with the wide-angle camera 3 Hubble telescope from February 22, 2016 to July 28, 2021.
A characteristic feature of Centauri A are dark bands that are part of a thick gas-dust disk. The dense interstellar clouds are good conditions for light echo, a phenomenon in which light from the initial bright flash is reflected from distant objects and arrives late to the observer. As the light from a supernova propagates farther and farther away, more and more regions of the gas-dust clouds “light up,” creating a series of rings of light.
The first LE1 light echo was detected just 75 days after the peak of the supernova flare. It appeared as a semicircle with an angular radius of 0.27 arc seconds (for comparison, the angular radius of the Moon is 14-16 arc minutes). The echo expanded radially until 1991 days after the peak of the flare at an average rate of 0.012 arc seconds per month, which is 12 times the speed of light. The illusion of superluminal motion is explained by the fact that the reflected light comes from regions lying in the plane between the supernova and the observer, but these regions are projected to more distant regions, the light from which has not yet had time to reach the observer.
At the end, the light echo took on the character of concentric rings: ring LE2 lay inside LE1, while the other two, LE3 and LE4, were outer rings. From the timing of the rings, as well as the unusual evolution of their color, scientists determined that the first echo was generated by a dense lumpy layer of interstellar medium located in the foreground of the supernova, while the others were dust layers with wide gaps. Although the layer that produced LE2 most likely lies closer to the supernova, its delayed manifestation is due to the presence of a significant gap in the line-of-sight between the flare and the Earth.
The authors of the paper note that a three-dimensional map of the distribution of dust structures can be constructed from the light echo data and approximately learn about their properties. In addition, SN 2016adj generated the earliest light echo known, since in previous cases, the reflected radiation became visible only a few hundred days after the explosion.