Posted on by Jason Major

A Star’s Dying Scream May Be a Beacon for Physics

When a star suffered an untimely demise at the hands of a hidden black hole, astronomers detected its doleful, ululating wail — in the key of D-sharp, no less — from 3.9 billion light-years away. The resulting ultraluminous X-ray blast revealed the supermassive black hole’s presence at the center of a distant galaxy in March of 2011, and now that information could be used to study the real-life workings of black holes, general relativity, and a concept first proposed by Einstein in 1915.

Within the centers of many spiral galaxies (including our own) lie the undisputed monsters of the Universe: incredibly dense supermassive black holes, containing the equivalent masses of millions of Suns packed into areas smaller than the diameter of Mercury’s orbit. While some supermassive black holes (SMBHs) surround themselves with enormous orbiting disks of superheated material that will eventually spiral inwards to feed their insatiable appetites — all the while emitting ostentatious amounts of high-energy radiation in the process — others lurk in the darkness, perfectly camouflaged against the blackness of space and lacking such brilliant banquet spreads. If any object should find itself too close to one of these so-called “inactive” stellar corpses, it would be ripped to shreds by the intense tidal forces created by the black hole’s gravity, its material becoming an X-ray-bright accretion disk and particle jet for a brief time.

Such an event occurred in March 2011, when scientists using NASA’s Swift telescope detected a sudden flare of X-rays from a source located nearly 4 billion light-years away in the constellation Draco. The flare, called Swift J1644+57, showed the likely location of a supermassive black hole in a distant galaxy, a black hole that had until then remained hidden until a star ventured too close and became an easy meal.

See an animation of the event below:

The resulting particle jet, created by material from the star that got caught up in the black hole’s intense magnetic field lines and was blown out into space in our direction (at 80-90% the speed of light!) is what initially attracted astronomers’ attention. But further research on Swift J1644+57 with other telescopes has revealed new information about the black hole and what happens when a star meets its end.

(Read: The Black Hole that Swallowed a Screaming Star)

In particular, researchers have identified what’s called a quasi-periodic oscillation (QPO) embedded inside the accretion disk of Swift J1644+57. Warbling at 5 mhz, in effect it’s the low-frequency cry of a murdered star. Created by fluctuations in the frequencies of X-ray emissions, such a source near the event horizon of a supermassive black hole can provide clues to what’s happening in that poorly-understood region close to a black hole’s point-of-no-return.

Einstein’s theory of general relativity proposes that space itself around a massive rotating object — like a planet, star, or, in an extreme instance, a supermassive black hole — is dragged along for the ride (the Lense-Thirring effect.) While this is difficult to detect around less massive bodies a rapidly-rotating black hole would create a much more pronounced effect… and with a QPO as a benchmark within the SMBH’s disk the resulting precession of the Lense-Thirring effect could, theoretically, be measured.

If anything, further investigations of Swift J1644+57 could provide insight to the mechanics of general relativity in distant parts of the Universe, as well as billions of years in the past.

See the team’s original paper here, lead authored by R.C. Reis of the University of Michigan.

Thanks to Justin Vasel for his article on Astrobites.

Image: NASA. Video: NASA/GSFC

Posted on by Nancy Atkinson

Data from Black Hole’s Edge Provides New Test of Relativity

From a NASA press release:

Last year, astronomers discovered a quiescent black hole in a distant galaxy that erupted after shredding and consuming a passing star. Now researchers have identified a distinctive X-ray signal observed in the days following the outburst that comes from matter on the verge of falling into the black hole.

This tell-tale signal, called a quasi-periodic oscillation or QPO, is a characteristic feature of the accretion disks that often surround the most compact objects in the universe — white dwarf stars, neutron stars and black holes. QPOs have been seen in many stellar-mass black holes, and there is tantalizing evidence for them in a few black holes that may have middleweight masses between 100 and 100,000 times the sun’s.

Until the new finding, QPOs had been detected around only one supermassive black hole — the type containing millions of solar masses and located at the centers of galaxies. That object is the Seyfert-type galaxy REJ 1034+396, which at a distance of 576 million light-years lies relatively nearby.

“This discovery extends our reach to the innermost edge of a black hole located billions of light-years away, which is really amazing. This gives us an opportunity to explore the nature of black holes and test Einstein’s relativity at a time when the universe was very different than it is today,” said Rubens Reis, an Einstein Postdoctoral Fellow at the University of Michigan in Ann Arbor. Reis led the team that uncovered the QPO signal using data from the orbiting Suzaku and XMM-Newton X-ray telescopes, a finding described in a paper published today in Science Express.

The X-ray source known as Swift J1644+57 — after its astronomical coordinates in the constellation Draco — was discovered on March 28, 2011, by NASA’s Swift satellite. It was originally assumed to be a more common type of outburst called a gamma-ray burst, but its gradual fade-out matched nothing that had been seen before. Astronomers soon converged on the idea that what they were seeing was the aftermath of a truly extraordinary event — the awakening of a distant galaxy’s dormant black hole as it shredded and gobbled up a passing star. The galaxy is so far away that light from the event had to travel 3.9 billion years before reaching Earth.

Video info: On March 28, 2011, NASA’s Swift detected intense X-ray flares thought to be caused by a black hole devouring a star. In one model, illustrated here, a sun-like star on an eccentric orbit plunges too close to its galaxy’s central black hole. About half of the star’s mass feeds an accretion disk around the black hole, which in turn powers a particle jet that beams radiation toward Earth. Credit: NASA’s Goddard Space Flight Center/Conceptual Image Lab

The star experienced intense tides as it reached its closest point to the black hole and was quickly torn apart. Some of its gas fell toward the black hole and formed a disk around it. The innermost part of this disk was rapidly heated to temperatures of millions of degrees, hot enough to emit X-rays. At the same time, through processes still not fully understood, oppositely directed jets perpendicular to the disk formed near the black hole. These jets blasted matter outward at velocities greater than 90 percent the speed of light along the black hole’s spin axis. One of these jets just happened to point straight at Earth.

Nine days after the outburst, Reis, Strohmayer and their colleagues observed Swift J1644+57 using Suzaku, an X-ray satellite operated by the Japan Aerospace Exploration Agency with NASA participation. About ten days later, they then began a longer monitoring campaign using the European Space Agency’s XMM-Newton observatory.

“Because matter in the jet was moving so fast and was angled nearly into our line of sight, the effects of relativity boosted its X-ray signal enough that we could catch the QPO, which otherwise would be difficult to detect at so great a distance,” said Tod Strohmayer, an astrophysicist and co-author of the study at NASA’s Goddard Space Flight Center in Greenbelt, Md.

As hot gas in the innermost disk spirals toward a black hole, it reaches a point astronomers refer to as the innermost stable circular orbit (ISCO). Any closer to the black hole and gas rapidly plunges into the event horizon, the point of no return. The inward spiraling gas tends to pile up around the ISCO, where it becomes tremendously heated and radiates a flood of X-rays. The brightness of these X-rays varies in a pattern that repeats at a nearly regular interval, creating the QPO signal.

The data show that Swift J1644+57’s QPO cycled every 3.5 minutes, which places its source region between 2.2 and 5.8 million miles (4 to 9.3 million km) from the center of the black hole, the exact distance depending on how fast the black hole is rotating. To put this in perspective, the maximum distance is only about 6 times the diameter of our sun. The distance from the QPO region to the event horizon also depends on rotation speed, but for a black hole spinning at the maximum rate theory allows, the horizon is just inside the ISCO.

“QPOs send us information from the very brim of the black hole, which is where the effects of relativity become most extreme,” Reis said. “The ability to gain insight into these processes over such a vast distance is a truly beautiful result and holds great promise.”

Read our previous article on Swift J1644+57

Lead image caption: This illustration highlights the principal features of Swift J1644+57 and summarizes what astronomers have discovered about it. Credit: NASA’s Goddard Space Flight Center

Posted on by Steve Nerlich

Astronomy Without A Telescope – Light Speed

The effect of time dilation is negligible for common speeds, such as that of a car or even a jet plane, but it increases dramatically when one gets close to the speed of light.

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The recent news of neutrinos moving faster than light might have got everyone thinking about warp drive and all that, but really there is no need to imagine something that can move faster than 300,000 kilometres a second.

Light speed, or 300,000 kilometres a second, might seem like a speed limit, but this is just an example of 3 + 1 thinking – where we still haven’t got our heads around the concept of four dimensional space-time and hence we think in terms of space having three dimensions and think of time as something different.

For example, while it seems to us that it takes a light beam 4.3 years to go from Earth to the Alpha Centauri system, if you were to hop on a spacecraft going at 99.999 per cent of the speed of light you would get there in a matter of days, hours or even minutes – depending on just how many .99s you add on to that proportion of light speed.

This is because, as you keep pumping the accelerator of your imaginary star drive system, time dilation will become increasingly more pronounced and you will keep getting to your destination that much quicker. With enough .999s you could cross the universe within your lifetime – even though someone you left behind would still only see you moving away at a tiny bit less than 300,000 kilometres a second. So, what might seem like a speed limit at first glance isn’t really a limit at all.

The effect of time dilation is negligible for common speeds we are familiar with on Earth, but it increases dramatically and asymptotically as you approach the speed of light.

To try and comprehend the four dimensional perspective on this, consider that it’s impossible to move across any distance without also moving through time. For example, walking a kilometer may be a duration of thirty minutes – but if you run, it might only take fifteen minutes.

Speed is just a measure of how long it takes you reach a distant point. Relativity physics lets you pick any destination you like in the universe – and with the right technology you can reduce your travel time to that destination to any extent you like – as long as your travel time stays above zero.

That is the only limit the universe really imposes on us – and it’s as much about logic and causality as it is about physics. You can travel through space-time in various ways to reduce your travel time between points A and B – and you can do this up until you almost move between those points instantaneously. But you can’t do it faster than instantaneously because you would arrive at B before you had even left A.

If you could do that, it would create impossible causality problems – for example you might decide not to depart from point A, even though you’d already reached point B. The idea is both illogical and a breach of the laws of thermodynamics, since the universe would suddenly contain two of you.

So, you can’t move faster than light – not because of anything special about light, but because you can’t move faster than instantaneously between distant points. Light essentially does move instantaneously, as does gravity and perhaps other phenomena that we are yet to discover – but we will never discover anything that moves faster than instantaneously, as the idea makes no sense.

We mass-laden beings experience duration when moving between distant points – and so we are able to also measure how long it takes an instantaneous signal to move between distant points, even though we could never hope to attain such a state of motion ourselves.

We are stuck on the idea that 300,000 kilometres a second is a speed limit, because we intuitively believe that time runs at a constant universal rate. However, we have proven in many different experimental tests that time clearly does not run at a constant rate between different frames of reference. So with the right technology, you can sit in your star-drive spacecraft and make a quick cup of tea while eons pass by outside. It’s not about speed, it’s about reducing your personal travel time between two distant points.

As Woody Allen once said: Time is nature’s way of keeping everything from happening at once. Space-time is nature’s way of keeping everything from happening in the same place at once.

Posted on by Jean Tate

What is Space?

First, some simple answers: space is everything in the universe beyond the top of the Earth’s atmosphere – the Moon, where the GPS satellites orbit, Mars, other stars, the Milky Way, black holes, and distant quasars. Space also means what’s between planets, moons, stars, etc – it’s the near-vacuum otherwise known as the interplanetary medium, the interstellar medium, the inter-galactic medium, the intra-cluster medium, etc; in other words, it’s very low density gas or plasma (‘space physics’ is, in fact, just a branch of plasma physics!).

But you really want to know what space is, don’t you? You’re asking about the thing that’s like time, or mass.

And one simple, but profound, answer to the question “What is space?” is “that which you measure with a ruler”. And why is this a profound answer? Because thinking about it lead Einstein to develop first the theory of special relativity, and then the theory of general relativity. And those theories overthrew an idea that was built into physics since before the time of Newton (and built into philosophy too); namely, the idea of absolute space (and time). It turns out that space isn’t something absolute, something you could, in principle, measure with lots of rulers (and lots of time), and which everyone else who did the same thing would agree with you on.

Space, in the best theory of physics on this topic we have today – Einstein’s theory of general relativity (GR) – is a component of space-time, which can be described very well using the math in GR, but which is difficult to envision with our naïve intuitions. In other words, “What is space?” is a question I can’t really answer, in the short space I have in this Guide to Space article.

More reading: What is space? (ESA), What is space? (National Research Council of Canada), Ned Wright’s Cosmology Tutorial, and Sean Carroll’s Cosmology Primer pretty much cover this vast topic, from kids’ to physics undergrad’ level.

It’s hard to know just what Universe Today articles to recommend, because there are so many! Space Elevator? Build it on the Moon First illustrates one meaning of the word ‘space’; for meanings closer to what I’ve covered here, try New Way to Measure Curvature of Space Could Unite Gravity Theory, and Einstein’s General Relativity Tested Again, Much More Stringently.

Astronomy Cast episodes Einstein’s Theory of Special Relativity, Einstein’s Theory of General Relativity, Large Scale Structure of the Universe, and Coordinate Systems, are all good, covering as they do different ways to answer the question “What is space?”

Source: ESA