A
network of small, ground-based telescopes stalking the night-time skies for
transient supernovas fished out a whopper -- a one-of-a-kind cosmic explosion
that at its peak blasted out more light than 50 times all the stars in the
Milky Way galaxy.
The
object, which takes its name, ASASSN-15lh from the All-Sky Automated Survey for
SuperNovae, was first discovered on June 14, 2015. A week later, astrophysicist
Subo Dong, with the Kavli Institute for Astronomy and Astrophysics at Peking
University, was looking at the object’s spectra, a chemical breakdown of its
light, and realized something strange was going on.
The
measurements were so diverse, the survey's automated software didn't even distinguish
it as a supernova, Dong wrote in an email to Discovery News.
“We
believed about various non-supernova, exotic situations, but none appeared to
work,” Dong said.
Working
with associates at observatories worldwide, Dong started thinking the object
may belong to a uncommon class of so-called superluminous supernovae, a
suspicion bolstered by follow-up measurements taken by the 10-meter South
African Large Telescope .
“Seeing
the SALT spectrum was the moment we knew for sure that we were on to something
big,” Dong said.
ASASSN-15lh
is situated about 3.8 billion light-years away, but is so bright that if were
as near as Sirius, a bright star 8.6 light-years away from Earth, the supernova
would seem to be almost as bright as the sun. It
is 200 times more powerful than the average supernova and more than double as
bright as any supernova formerly discovered.
Researchers
are virtually in the dark about what generated the blast. One theory is that a
dense, swiftly spinning neutron star, known as a magenetar, is driving the
supernova. The star would have to be spinning at least 1,000 times a second, a
speed that challenges the laws of physics. It also would have to be approximately
100 percent proficient at converting its spin energy into light, another
theoretical limit.
Another
option is that ASASSN-15lh is driven by some nuclear reactions related to a
supermassive black hole at the center of its host galaxy, but researchers don’t
know of any such phenomenon, nor have they pinpointed the object’s precise position
in its galaxy. Additional information is expected this year from Hubble Space
Telescope observations.
“The
Hubble observations are meant to show whether or not the event occurred right
at the nucleus of the host galaxy, or whether it is offset from the center. If
it is offset, this would rule out the explanation of this event as somehow
related to the host galaxy's central supermassive black hole -- assuming it has
one, as most massive galaxies do,” Ohio State astrophysicist Todd Thompson
wrote in an email to Discovery News.
“What
kind of event from a supermassive black hole could yield such an event? We
don't know since we've never perceived something like it, but it has been proposed
that it might be a so-called ‘tidal disruption event’ -- a bright explosion rising
when a star gets torn apart and then consumed by a supermassive black hole. The
problem is that we've grasped these events before, and they don't look like
ASASSN-15lh. In particular, most stars are composed of generally hydrogen and
helium, and yet the spectra of ASASSN-15lh show no signs of either,” Thompson
said.
The
supernova’s host galaxy itself presents another puzzle. Most superluminous
supernovae are found in small, dwarf galaxy where lots of stars are forming.
ASASSN-15lh, in contrast, seems to be in a large, but comparatively quiescent
galaxy about three times more massive than the Milky Way.
Researchers
have rejected the likelihood that the supernova appears brighter than it
actually is due to any magnifying effects of intervening galaxies, a phenomena
known as gravitational lensing.
“The good news is that ASASSN-15lh is bright, so it is comparatively easy to get high-quality observations. Many groups of astrophysicists are using some of the most advanced telescopes, ground- and space-based, to study ASASSN-15lh. I am sure that in the near future, we will understand it much better,” Dong said.
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