When Black Holes Feast on Screaming Stars

We don’t normally think of black holes as erupting — they have more of a reputation for gobbling up anything that crosses the event horizon in the popular imagination. But they do give off gamma-ray bursts sometimes.

And those in the middle-weight range, between 100 and 100,000 solar masses, also occasionally exhibit distinctive x-ray emissions when they feed on ill-fated stars. Think of it as a star’s final scream, ripping across light-years of spacetime at a very distinct frequency, akin to a very low D-sharp.

It’s called a quasi-periodic oscillation (QPO), and it’s the result of matter from the dying star forming an accretion disk as it torn apart, just outside the black hole’s event horizon.”The disk gets heated up and we can see emissions from the disk very close to the black hole in x-rays,” University of Michigan astronomer Rubens Reis explained via press release. “As this matter is falling in, it gives a quasiperiodic wobble.”

Such a signal has only been detected around one supermassive black hole — the type with solar masses ranging in the millions — lurking at the center of a nearby galaxy known as REJ 1034+396, some 576 million light years away.

In a paper published last month in Science, Reis and his fellow astronomers announced that they have detected a QPO signal from a supermassive black hole lurking in the Draco constellation, a whopping 3.9 billion light years away.

That distance means the signal from this x-ray source — dubbed Swift J1644+57 — gives astronomers a chance to learn more about black holes and general relativity at a much earlier era in our universe’s history, when conditions were far more extreme.

NASA’s Swift Gamma-Ray Burst Telescope first spotted the signal last year, and initially astronomers thought it was a gamma-ray burst, until the signal faded away. They concluded that it was the result of a black hole waking up after a quiet phase when a star wandered a bit too close and got gobbled up.

It took subsequent analysis of data collected by two orbiting observatories (first with Suzaku and then with XMM Newton) before Reis and his colleagues were able to detect the telltale oscillation demonstrating that it was, indeed, a QPO.

They were helped by the fact that in addition to the x-rays, the rare event also produced jets that blasted matter outward at very high speeds — and one of those jets just happened to be pointing Earthward. This boosted the QPO signal sufficiently for astronomers to pick it out of all the noise in the data.

So now Reis et al have access to information from the edge of a supermassive black hole, in a galaxy far, far, away — and they fully expect to learn a great deal about the physics under such extreme conditions. Will the principles of general relativity hold firm? We’ll have to wait and see.

Images: (top) Swift J1644+57. Credit: NASA’s Goddard Space Flight Center. (bottom) Swift Gamma-Ray Burst Telescope. Credit: NASA. Both public domain.

This is awesome.

What would a gamma-ray burst sound like? No one really knows, but members of the team that work with the Fermi Large Area Telescope (LAT) have translated gamma-ray measurements into musical notes and have created a “song” from the photons from one of the most energetic of these powerful explosions, GRB 080916C which occurred in September of 2008.

“In translating the gamma-ray measurements into musical notes we assigned the photons to be “played” by different instruments (harp, cello, or piano) based on the probabilities that they came from the burst,” the team wrote in the Fermi blog. “By converting gamma rays into musical notes, we have a new way of representing the data and listening to the universe.”

In the beginning of the song, before the burst starts, the harp plucks out a few lonely notes. After about half a minute, the piano joins in on top of the harp background, and the notes begin to pile on more and more rapidly. The cello enters the scene as the burst begins in earnest, creating a full, energetic sound.

The Fermi team created an accompanying animation to help see what is happening:

The top panel shows each individual gamma-ray. The colors refer to low (red), medium (blue) and high (green) quality gamma-rays (played by harp, cello and piano respectively). The energy of the gamma-ray is on the y-axis (higher energy gamma-rays are towards the top of the plot) and the arrival time of the gamma-rays are on the x-axis (later arriving gamma-rays are further to the right). The vertical white line tells you where the music is currently playing. The bottom panel shows the number of gamma-rays (which is the number of notes played) in each time slice.

Beautiful.

Source: NASA Blogs/GLAST

Gamma-Ray Burst: A Milestone Explosion

Credit: R. Klebesadel, I. Strong & R. Olson (LANL), Vela Project

Explanation: Gamma-Ray Bursts (GRBs) were discovered by accident. Thirty three years ago today, satellites first recorded a GRB. The data plotted here show that the count rate of the satellite gamma-ray instrument abruptly jumped indicating a sudden flash of gamma-rays. The Vela satellites that detected this and other GRBs were developed to test technology to monitor nuclear test ban treaties. With on board sensors they watched for brief x-ray and gamma-ray flashes, the telltale signatures of nuclear explosions. As intended, the Velas found flashes of gamma-rays - but not from nuclear detonations near Earth. Instead, the flashes were determined to come from deep space! Dubbed “cosmic gamma-ray bursts” they are now known to be the most powerful explosions originating in distant galaxies. What could power a gamma-ray burst?

n-a-s-a:

GRB 090423: The Farthest Explosion Yet Measured

Credit: Gemini Observatory / NSF / AURA, D. Fox & A. Cucchiara (Penn State U.), and E. Berger (Harvard Univ.)

Explanation: An explosion so powerful it was seen clear across the visible universe was recorded in gamma-radiation by NASA’s orbiting Swift Observatory. Farther than any known galaxy, quasar, or optical supernova, the gamma-ray burst recorded was clocked at redshift 8.2, making it the farthest explosion of any type yet detected.

Short Gamma-Ray Bursts Localized

Illustration Credit : Dana Berry, NASA

Explanation: What causes gamma-ray bursts? The most energetic type of explosions known in the cosmos has been an enigma since discovered over 30 years ago. It now appears that there may not be one unique type of progenitor. Long duration gamma-ray bursts (GRBs) have been localized, over the past few years, to blue regions in the universe rich in star formation. Massive young stars nearing the end of their short lives commonly explode in these regions. Astronomers associate these long duration GRBs, that can last from seconds to minutes, with a type of stellar explosion common in young massive stars. Over the past few months, short duration GRBs have finally been localized and found to occur in different types of regions — not only blue regions rich in star formation. Many astronomers therefore now theorize that short GRBs, which typically last less than one second, are the result of a different progenitor process than long GRBs. A leading model is that a short GRB will occur when a neutron star either impacts another neutron star or a black hole. Such collisions may occur well after star-forming regions have otherwise burned out. Pictured in the above illustration, two energized neutrons stars finally approach each other in their orbits, a death spiral that might end with a short GRB.