Podcaster: Richard Drumm
Organization: 365 Days Of Astronomy
Description: What is the space assassination thing that happened in 2015?
Bio: Richard Drumm is President of the Charlottesville Astronomical Society and President of 3D – Drumm Digital Design, a video production company with clients such as Kodak, Xerox and GlaxoSmithKline Pharmaceuticals. He was an observer with the UVa Parallax Program at McCormick Observatory in 1981 & 1982. He has found that his greatest passion in life is public outreach astronomy and he pursues it at every opportunity.
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What is the space assassination thing that happened in 2015?
Wait, what? An assassin is a murderer, especially one who kills a politically important person.
So what does this have to do with astronomy? You got some ‘splainin’ to do Richard!
Well buckle your seat belts loyal listeners and I’ll tell you!
In 2015, astronomers detected a bright flash of light, brighter than any seen before. It happened in a galaxy that’s 2.7 billion light years away so it was dim only because it was really far away.
OK. A star dies in a supernova explosion. Fine. So that helps explain things a little bit. That’s the victim of the supposed assassination in today’s title.
Who is the assassin? I mean, a supernova is where a star runs out of fuel to hold itself up and collapses under it’s own weight in a titanic explosion.
No assassination’s needed.
The 2015 explosion was 20 times brighter than the Milky Way galaxy, outshining a few hundred billion stars all at once. It was so bright one team of astronomers called it a Superluminous Supernova or SLSN.
But there was another team.
This team had a different idea for the back story on what they called a superluminous transient, also named ASASSN-15lh. They said it wasn’t created by a supernova at all but was a Tidal Disruption Event or TDE.
Two teams, two ideas. The SLSN team and the TDE team. They both can’t be right. They can both be wrong, too. Or one of them can be right and the other wrong.
Science progresses by making and re-making theories about things that we see happening. If one theory has trouble explaining our observations we modify it or come up with a completely new one and start over again until we have it right.
Science is not a democracy. It’s a meritocracy. Only ideas that have merit survive. It can be a bit rough and tumble at times, with arguments going back & forth over this theory or that one. But eventually the competitors come to a consensus about what idea is correct and we move on from there.
In this case, the assassin isn’t a murderer. ASASSN is the All Sky Automated Survey for Supernovae, operated by Ohio State University.
ASASSN found the one event that the two groups, the SLSN group and the TDE group, are both looking at.
The ASASSN project has two telescopes at this point with plans to expand to 16 telescopes in the future. Each telescope is actually 4 telescopes of 14CM or 5 1/2 inches in diameter.
They use 4 commercially available 400mm Nikon lenses that you or I could easily order through Amazon. Each clear night the two telescope arrays cover about 20,000 square degrees of sky!
The automated part of ASASSN relies on the subtraction method of image comparison. An image from last week is superimposed on the current image, but one of them is positive and the other is negative.
All of this is handled by a computer, nobody has to tell it what to do, it’s all automatic.
If there has been no change in the brightness of a section of the night sky over that time period the resulting image mix of negative & positive will be an even field of grey.
If there has been a change in the brightness of any star or galaxy, as you’d have with a supernova, then the resulting image will have a spot on it that sticks out like a sore thumb! It’s super obvious and the computer can spot it easily and the discovery is made.
Then other, larger telescopes around the world are directed to the spot to take a confirmation image of the supernova.
Unit 1 of ASASSN is called “Brutus” after Caesar’s assassin Marcus Junius Brutus, and it’s located in Hawaii.
Unit 2 is located at the Las Cumbres Observatory site at the Cerro Tololo Observatory in Chile. It’s named “Cassius” for Brutus’s brother-in-law and co-conspirator Gaius Cassius Longinus. He, Brutus and 20 other Roman Senators murdered Roman dictator Julius Caesar in 44BC.
This triggered a round of civil wars that ultimately led to the downfall of the Roman Republic and the establishment of the Roman Empire. The first Emperor was Julius Caesar’s adopted son Octavian. So the whole affair backfired!
Now where was I? Oh yeah, astronomy. This is a show about astronomy…
The TDE team said there’s 3 reasons they don’t buy the SLSN theory. They cite:
1. the temperature evolution of the event,
2. the presence of highly ionized CNO gas and
3. the location of the event.
The event happened in the core of a galaxy, where we know that supermassive black holes reside. In this galaxy the SMBH had a mass of 100 million times the mass of our Sun.
Close to the SMBH of a galaxy is a place where a massive star is highly unlikely, so a supernova, either normally luminous or superluminous, is unlikely too.
So the TDE team says that 15lh was probably created by an even more extreme event – a spinning supermassive black hole ripping apart a passing star that came too close. The fact that the black hole is spinning means it will rip the star apart instead of simply swallowing it whole.
The TDE team in their scientific paper says “The rapid spin and high black hole mass can explain the high luminosity of this event.”
The remains of the star become an intensely bright accretion disk around the SMBH. TDEs are rare and have only been observed a handful of times before.
The team says that a TDE has in this case been used to probe the spin of a black hole, a property which is very difficult to measure.
But the SLSN team says ‘Not so fast!’ In their scientific paper they tell us “No optically bright TDE to date shows an absence of broad emission lines like ASASSN-15lh.”
Broad spectral emission lines tell us that the source of the light is rotating fast, with part of the accretion disk, in this case, blue-shifted as it rotates toward us and part red-shifted as it rotates away from us.
15lh brightened and then dimmed, as you’d expect a supernova to do, but it then had a second episode of brightening in the ultraviolet part of the spectrum.
The SLSN team says that this makes it easier to reconcile 15lh with the properties of an SLSN than than it would for a TDE. They continue to favor the SLSN interpretation over the TDE.
The upshot of all this is we need more data. We need to know more about both of these possible explanations. We need to study more Tidal Disruption Events and more Superluminous Supernovae.
Both of these things are fairly rare. We need to characterize them better. Only then will we have enough of a handle on what these things are like and which of them are likely what’s happening with 15lh.
More observations, more papers, more discussion at scientific conferences. That’s science for ya!
Thank you for listening to the 365 Days of Astronomy Podcast!
End of podcast:
365 Days of Astronomy
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