Hubble Watches Spinning Black Hole Swallow a Star

In 2015, the All-Sky Automated Survey for Supernovae (aka. ASAS-SN, or Assassin) detected something rather brilliant in a distant galaxy. At the time, it was thought that the event (named ASASSN-15lh) was a superluminous supernova – an extremely bright explosion caused by a massive star reaching the end of its lifepsan. This event was thought to be brightest supernova ever witnessed, being twice as bright as the previous record-holder.

But new observations provided by an international team of astronomers have provided an alternative explanation that is even more exciting. Relying on data from several observatories – including the NASA/ESA Hubble Space Telescope – they have proposed that the source was a star being ripped apart by a rapidly spinning black hole, an event which is even more rare than a superluminous supernova.

According to the ASAS-SN’s findings – which were published in January of 2016 in Science – the superluminous light source appeared in a galaxy roughly 4 billion light-years from Earth. The luminous source was twice as bright as the brightest superluminous supernova observed to date, and its peak luminosity was 20 times brighter than the total light output of the entire Milky Way.

Credit: ESA/Hubble, ESO, M. Kornmesser
This artist’s impression depicts a rapidly spinning supermassive black hole surrounded by an accretion disc. Credit: ESA/Hubble, ESO, M. Kornmesse

What seemed odd about it was the fact that the superluminous event appeared within a massive, red (i.e. “quiescent”) galaxy, where star formation has largely ceased. This was in contrast to most super-luminous supernovae that have been observed in the past, which are typically located in blue, star-forming dwarf galaxies. In addition, the star (which is Sun-like in size) is not nearly massive enough to become an extreme supernova.

As such, the international team of astronomers – led by Giorgos Leloudas of the Weizmann Institute of Science in Israel and the Dark Cosmology Center in Denmark – conducted follow-up observations using space-based and Earth-based observatories. These included the Hubble Space Telescope, the Very Large Telescope (VLT) at the ESO’s Paranal Observatory and the New Technology Telescope (NTT) at the La Silla Observatory.

With information from these facilities, they arrived at a much different conclusion. As Dr. Leloudas explained in a Hubble press release:

“We observed the source for 10 months following the event and have concluded that the explanation is unlikely to lie with an extraordinary bright supernova. Our results indicate that the event was probably caused by a rapidly spinning supermassive black hole as it destroyed a low-mass star.”

The process is colloquially known as “spaghettification”, where an object is ripped apart by the extreme tidal forces of a black hole. In this case, the team postulated that the star drifted too close to the supermassive black hole (SMBH) at the center of the distant galaxy. The resulting heat and the shocks created by colliding debris led to a massive burst of light – which was mistakenly believed to be a very bright supernova.

Multiple lines of evidence support this theory. As they explain in their paper, this included the fact that over the ten-months that they observed it, the star went through three distinct spectroscopic phases. This included a period of substanial re-brightening, where the star emitted a burst of UV light that accorded with a sudden increase in its temperature.

Combined with the unlikely location and the mass of the star, this all pointed towards tidal disruption rather than a massive supernova event. But as Dr. Leloudas admits, they cannot be certain of this just yet. “Even with all the collected data we cannot say with 100% certainty that the ASASSN-15lh event was a tidal disruption event.” he said. “But it is by far the most likely explanation.”

As always, additional observations are necessary before anyone can say for sure what caused this record-breaking luminous event. But in the meantime, the mere fact that something so rare was witnessed should be enough to cause some serious excitement! Speaking of which, be sure to check out the simulation videos (above and below) to see what such an event would look like:

Further Reading: Hubble Space Telescope

Superluminous Supernova Puzzles Astronomers

Supernovae are surprisingly dependable. These brilliant and powerful explosions that mark the end of massive stars’ lives tend to shine anywhere from one hundred million to a few billion times brighter than the Sun for weeks on end. And their intrinsic brightness is always well known.

But in recent years a rare class of cosmic explosions, which are tens to hundreds of times more luminous than ordinary supernovae, has been discovered. And now one of these odd superluminous supernovae is mystifying astronomers further, with characteristics that simply don’t add up.

The Dark Energy Survey (DES) came online in August 2013 in order to investigate millions of galaxies for the subtle effects of weak lensing, the phenomenon where intervening invisible matter causes distant galaxies to appear minutely sheared and stretched.

The survey started off with a bang; its first images revealed a rare superluminous supernova, dubbed DES13S2cmm, 7.8 billion light-years away.

“Fewer than forty such supernovae have ever been found and I never expected to find one in the first DES images,” said Andreas Papadopoulos from the University of Portsmouth in a press release. “As they are rare, each new discovery brings the potential for greater understanding  or more surprises.”

The problem is this: DES13S2cmm doesn’t easily match the typical characteristics of a superluminous supernova. The stellar explosion could be seen in the data six months later, much longer than most other superluminous supernovae observed to date.

“Its unusual, slow decline was not apparent at first,” said Mark Sullivan from Southampton University. “But as more data came in and the supernova stopped getting fainter, we would look at the light curve and ask ourselves, ‘what is this?’ ”

So Sullivan decided to investigate further. But understanding its origins are proving difficult.

For some supernovae, the optical light we see is actually created by radioactivity. In fact, supernovae tend to create large amounts of radioactive elements, which don’t occur naturally on Earth. Nickel-56, with a half-life of roughly six days, is a common example.

As the nickel decays into cobalt, it releases gamma rays, which are trapped by the other material ejected by the supernova. These trapped rays heat up the surrounding material until it radiates in the optical. In this case, the peak magnitude of the supernova is directly proportional to the amount of nickel-56 created in the explosion.

“We have tried to explain the supernova as a result of the decay of the radioactive isotope nickel-56,” said coauthor Dr Chris D’Andrea of the University of Portsmouth. “But to match the peak brightness, the explosion would need to produce more than three times the mass of our Sun of the element. And even then the behavior of the light curve doesn’t match up.”

So the team is now investigating other explanations. In one intriguing scenario the supernova was relatively normal but created a magnetar — an extremely dense and highly magnetic neutron star that’s millions of times more powerful than the strongest magnets on Earth — whose energy made the explosion exceptionally bright.

But this explanation doesn’t match the data either.

A few months ago a team of astronomers led by Robert Quimby explained a superluminous supernovae, PS1-10afx, by a chance cosmic alignment, where intervening matter worked like a lens to deflect and intensify the background light for a typical Type Ia supernova. D’Andrea, however, doesn’t believe this is the case here.

“DES13S2cmm looks nothing like a normal type of supernova, either in its photometric evolution or its spectroscopy,” D’Andrea told Universe Today. “So while we can never be sure that a very faint but very massive galaxy lies between us and another object and is serendipitously brightening the object, there is no need to adopt that assumption in the case of DES13S2cmm.”

chance cosmic alignment — where intervening matter worked like a lens to deflect and intensify the background light – See more at: http://www.skyandtelescope.com/astronomy-news/stellar-science/mysteriously-bright-supernova-explained/#sthash.m7Z8PJ3k.dpuf
chance cosmic alignment — where intervening matter worked like a lens to deflect and intensify the background light – See more at: http://www.skyandtelescope.com/astronomy-news/stellar-science/mysteriously-bright-supernova-explained/#sthash.m7Z8PJ3k.dpuf

So astronomers are heading back to the drawing board.

“With so few known, it’s hard to really understand their properties in detail,” said Bob Nichol from the University of Portsmouth. “DES should find enough of these objects to allow us to understand superluminous supernovae as a population. But if some of these discoveries prove as difficult to interpret as DES13S2cmm, we’re prepared for the unusual.”

The results will be presented today at the National Astronomy Meeting 2014 in Portsmouth.