The most UV-bright supernova is powered by an internal engine

16apd_lccomp
The observed luminosity of Gaia16apd as seen through red (r) and ultraviolet (UVM2) colour filters. Compared to another nearby superluminous supernova, it shows the same brightness in the red but is initially much brighter in the ultraviolet!

Earlier this year, the Gaia satellite, in the process of mapping out our Milky Way in unprecedented detail, happened to discover its first superluminous supernova (SLSN). This was a great discovery not only because it turned out to be one of the most nearby such events we’ve ever found, but because it was about 5 times brighter in the ultraviolet than any other SLSN (and even more so compared to a normal SN)! Using UV data from the Swift and Hubble space telescopes, and optical data from Whipple Observatory, we show that this UV excess can only be explained by a powerful central engine. The key evidence is that despite the much higher flux level, the depths of the UV absorption lines in the spectrum are the same as in more typical SLSNs, which tells us that all the extra light had to pass through the same layer of absorbing material. If the extra energy was coming from the outside, for example a collision with dense external gas around the star, we would have seen relatively weaker absorption lines than in other SLSNs. We made a model for the luminosity, powered by a rapidly rotating central magnetar, and found that this model predicted the UV excess for exactly the duration over which we saw it. Pretty cool! See also this nice paper from Yan et al., who show that the overall greater UV flux in SLSNs compared to normal SNs likely indicates a low metal content in the outer envelope.


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