On September 14, 2015 at 09:50:45 UTC advanced LIGO observed GW150914, a chirp of gravitational waves caused by the merging of two stellar-mass black holes. Just 0.4 seconds later, the Fermi gamma ray telescope observed a faint burst of gamma rays lasting about a second. While there is a small chance this is simply due to chance, it looks like the gamma ray burst was triggered by the merging black holes. 

A gamma ray burst (GRB) is a transient emission of gamma rays typically lasting less than two seconds. On average, about one gamma ray burst occurs every day. They appear randomly in all directions of the sky, and this means they aren’t produced in our galaxy. If they were, then GRBs would mostly be found along the plane of the Milky Way. They are thought to be caused by things like colliding neutron stars, or possibly the capture of a neutron star by a black hole.

This particular GRB (named GW150914-GBM) was observed by the Fermi Gamma-ray Burst Monitor, which can observe 70% of the sky at any given time. That’s great for observing these short-lived events, but it means that locating the source of a particular event is a bit imprecise. The most likely location of GW150914-GBM falls within the likely location of the gravity wave source GW150914. There are other aspects of the GRB that would tend to support a simultaneous event. While the burst was faint it had a hard x-ray spectrum, which would seem to rule out known sources within our own galaxy. There is still a possibility that some extragalactic event such as the collision of neutron stars just happened to occur in the same general direction 0.4 seconds after the gravitational wave event, but that doesn’t seem likely. Given its faintness there’s also a small chance that this could be a false-alarm.

If we assume the two events have the same cause that would mean the burst also occurred 1.3 billion light years away. From its apparent peak brightness we can calculate its peak luminosity. It turns out the peak luminosity of this event is an order of magnitude dimmer than any previous short GRB event. This would support the idea that it was not caused by a neutron star collision.

If this GRB was caused by merging black holes, it would be quite surprising. Stellar mass black hole binaries aren’t expected to have a disk of material around them that could emit gamma rays. We’ll need more data to be sure. Fortunately there will be plenty of opportunity to observe similar events over the next few years.

Paper: V. Connaughton, et al. Fermi GBM Observations of LIGO Gravitational Wave event GW150914 (preprint)

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Yesterday there was a flurry of news about a gamma ray burst (GRB) appearing in the Andromeda galaxy.  This would make it the closest observed gamma ray burst, which would be a boon for astronomers.  News of this discovery travelled fast, particularly on Twitter and other social networks.  Pretty soon a few news sites had picked up the story. But it turns out it wasn’t a gamma ray burst.  

It all started with a space telescope known as Swift.  Swift is designed to study gamma ray bursts, and one of its detectors is a wide field telescope known as the Burst Array Telescope (BAT).  The BAT is designed to look for bursts of high energy radiation from unknown sources.  If it detects one, it “triggers” and sends an alert so that other telescopes can be position to observe the event.  Gamma ray bursts can be short lived, so time is of the essence.

Normally the level needed for the BAT to trigger is pretty high (6.5 sigma for you statisticians) so that it doesn’t cry wolf all the time (what are known as spurious events).  But the bar is set a bit lower if the energy burst seems to be from a nearby galaxy.  So Tuesday night (EDT) BAT detected a burst, and Swift’s x-ray telescope also observed a burst of x-rays.  The burst also happened to be in the direction of the Andromeda galaxy.  So it triggered and the alert went out.

This is what a possible GRB looks like.
Credit: Goddard Space Flight Center/NASA

Naturally, some of the astronomers working in this area of research use Twitter, and started tweeting about a possible GRB in Andromeda.  This was picked up by their fans and other astronomers, and the whole thing cascaded.  It turns out it was a known x-ray source, probably an x-ray binary.  So while it initially looked promising, it turned out to be a spurious event.  Sometimes this happens, and it is better to have the occasional false alarm rather than miss an important event.

Of course all of this played out in the social media circles, and its seems rather chaotic at the time. It also means sometimes things get reported as far more certain than they actually are.  If you actually look at what is being said, however, you’ll see something rather interesting.  If you go back and look at the comments, such as those tagged with #GRBM31 on Twitter, you’ll notice that the astronomers are pretty careful about saying things like “possible” GRB.  They spread the tentative news, and start looking for evidence to confirm or deny the event.  As they learn things from a clear source, they start tweeting that as well.  You’ll also note there is a great deal of excitement.

This is part of what makes science interesting.  Cool things happen, even if we later find out it isn’t as cool as we thought.

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When you look up into the night sky, you are seeing into the past. Cosmic distances are so vast that it takes time for light to travel them. Light from the closest star to Earth—the Sun—takes more than 8 minutes to reach us. After several hours that same light reaches the outer region of our solar system, where Pluto and other dwarf planets reside, but it will take more than four years for the light to reach the next nearest star. After 100 years the light from our sun has reached less than 20,000 stars. The brightest star in the night sky is Sirius, more than 8 light-years away, which means we see Sirius not as it is now, but as it was 8 years ago. 

Most of the stars we see are even further away. The constellation Orion has several bright stars. Betelgeuse, the red star at the upper shoulder of Orion, is about 640 light-years away. Rigel, the bright blue star at Orion’s foot, is about 800 light-years away. Even further away is the Orion Nebula, more than 1,300 light years away. When you look at that fuzzy patch in Orion’s sword, the light that strikes your eyes began its journey when the Roman Empire was reaching its end, Islam was just beginning, and the Tang Dynasty was at its height.

The farthest object we can see with the naked eye is a faint object in the constellation Andromeda. On a clear dark night it looks almost like a smudge of chalk dust against the night sky. This faint smudge is the Andromeda Galaxy, the nearest spiral galaxy to our own Milky Way Galaxy, and is 2.5 million light-years away. To look at Andromeda is to look back to a time when early human ancestors, such as Homo habilis were walking the plains of Africa.

With modern telescopes we can see much further than the Andromeda Galaxy. In recent years astronomers have observed stars and galaxies billions of light years away.  Just a few years ago the farthest object ever observed was neither a galaxy nor a star, but rather a gamma ray burst (GRB). A gamma ray is a kind of light with an extremely short wavelength. Visible light, such as the color green, has a wavelength of about 500 nanometers, which is the length of 5,000 hydrogen atoms placed end to end, or about the size of a virus. A gamma ray is about a picometer in length. A single hydrogen atom is about 60 picometers wide. Because of their short wavelengths, gamma rays have a very high energy. A gamma ray burst is a short, intense blast of gamma rays produced by the death of a star as it goes supernova, or by the collision of two neutron stars. They last only from a few seconds to several minutes, but they are incredibly bright. It is thought that Betelgeuse will produce a gamma ray burst when it becomes a supernova sometime in the next few thousand years. (We don’t need to worry, though, since the burst won’t be directed toward Earth. We will only see the supernova as a star which becomes brighter than the moon for a while.)

GRB 090423 as seen by the Swift satellite. The image is a composite of data from Swift’s UV/Optical and X-Ray telescopes.
Credit: NASA/Swift/Stefan Immler

In 2009 astronomers were able to observe a gamma ray burst named GRB 090423, seen above. When they measured its distance they found the light from GRB 090423 had been traveling for about 13 billion years. The universe is only about 13.5 to 14 billion years old, so this burst occurred between the time when atoms had formed—about 400,000 years after the big bang—and the time when stars and galaxies start forming—about 900 million years after the big bang—which is a period known as the Cosmic Dark Ages. By studying the light from GRB 090423, astronomers found that it was caused by the death of an early star. They were also able to show that this star was not a first generation star. In other words it was not a star formed from the hydrogen and helium created by the big bang, it was formed by the gas and dust remains of earlier stars which had exploded, just as our sun was formed from older, long dead stars. This means some stars must have lived and died even before this star formed more than 13 billion years ago, which is much earlier than once thought possible.

Since then we’ve observed a few slightly more distant objects.  These seem to be protogalaxies, but determining their exact distance is a bit of a challenge.  We will likely find even more distant objects when the James Webb telescope is launched.

So the next time you are out at night, take a trip back in time and look up at the stars.

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