NASA’s artist impression of SN 2006gy, one of the most luminous hypernovae seen. Credit: NASA/CXC/M.Weiss
An international team has spotted a surprising UV rebrightening in a distant superluminous supernova, ASASSN-15lh. The event has baffled the scientists as has no hydrogen emission characteristic of superluminous supernovae and tidal disruption events. Also called hypernovae, these are dozens of times more luminous than normal supernovae. ASASSN-15lh, detected by the All Sky Automated Survey for SuperNovae (ASAS-SN) in 2015, is a real ‘assassin’ among these explosion events. It is about 200X more powerful than the average supernova and approximately 570 billion times brighter than our sun. It is so far the most luminous supernova ever detected.
Now, Brown et al have used data from NASA’s Swift spacecraft and Hubble Space Telescope (HST) to study ASASSN-15lh in detail. They found that the flux of the supernova increased strongly into the ultraviolet, with the luminosity a hundred times greater when compared to the hydrogen-rich, UV-bright SLSN II SN 2008es. This rebrightening is seen about 2 months after the peak brightness, which by itself is as bright as a superluminous supernova. Hubble is much more sensitive than Swift, so the scientists were able to get ultraviolet spectroscopy from a special observation approved after they discovered something unusual with Swift.
Hot, energetic events like ASASSN-15lh produce most of their light in the UV, and it’s at these wavelengths that we can best understand their explosion mechanisms and their nature.
“Hubble and Swift are the only telescopes that can acquire ultraviolet spectroscopy. UV spectroscopy can only be done from space, and these two old telescopes are our only means to get these data, as no future UV telescope is planned anytime in the near future,” said Jeffrey Cooke, Swinburne Uni of Technology, Australia.
They determined the shape of theexplosion the UV/optical flux and the X-ray flux. However, the most puzzling finding was that the observed brightening did not show the hydrogen they would expect if caused by the explosion crashing into hydrogen around it. The optical spectroscopy did not show evidence of broad H-alpha, nor strong or broad Lyman alpha emission in the UV spectra, which would be expected from interaction with hydrogen-rich material. Thus, the team admitted that their research actually raised more new questions than it answered.
“We don’t understand the main peak of the light curve and we don’t understand the rebrightening, though we have some ideas.” Brown concluded. The team will keep following ASASSN-15lh with Swift until it gets too faint. They do not have immediate plans for further studies of this object, though their collaborators are looking deeper into the Hubble spectra and theoretical explanations for the source.
http://phys.org/news/2016-05-asassn-creeda-ultraviolet-rebrightening-superluminous.htmljCp
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