Interaction With Circumstellar Material
The three bright rings around SN 1987A (or SNR1987A) are material from the stellar wind of the progenitor. These rings were ionized by the ultraviolet flash from the supernova explosion, and consequently began emitting in various emission lines. These rings did not "turn on" until several months after the supernova; the turn-on process can be very accurately studied through spectroscopy. The rings are large enough that their angular size can be measured accurately: the inner ring is 0.808 arcseconds in radius. Using the distance light must have traveled to light up the inner ring as the base of a right angle triangle and the angular size as seen from the Earth for the local angle, one can use basic trigonometry to calculate the distance to SN1987A, which is about 168,000 light-years. The material from the explosion is catching up with the material expelled during its red giant phase and heating it, so we observe ring structures around the star.
Around 2001, the expanding (>7000 km/s) supernova ejecta collided with the inner ring. This caused its heating and the generation of x-rays — the x-ray flux from the ring increased by 3 times between 2001 and 2009. A part of the x-ray radiation, which is absorbed by the dense ejecta close to the center, is responsible for a comparable increase in the optical flux from the supernova remnant in 2001–2009. This increase of the brightness of the remnant reversed the trend observed before 2001, when the optical flux was decreasing due to the decaying of 44Ti isotope.
Read more about this topic: SN 1987A
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