X-ray Burst May Be the First Sign of a Supernova

GRB 080913, a distant supernova detected by Swift. This image merges the view through Swift’s UltraViolet and Optical Telescope, which shows bright stars, and its X-ray Telescope. Credit: NASA/Swift/Stefan Immler

The first moments of a massive star going supernova may be heralded by a blast of x-rays, detectable by space telescopes like Swift, which could then tell astronomers where to look for the full show in gamma rays and optical wavelengths. These findings come from the University of Leicester in the UK where a research team was surprised by the excess of thermal x-rays detected along with gamma ray bursts associated with supernovae.

“The most massive stars can be tens to a hundred times larger than the Sun,” said Dr. Rhaana Starling of the University of Leicester  Department of Physics and Astronomy. “When one of these giants runs out of hydrogen gas it collapses catastrophically and explodes as a supernova, blowing off its outer layers which enrich the Universe.

“But this is no ordinary supernova; in the explosion narrowly confined streams of material are forced out of the poles of the star at almost the speed of light. These so-called relativistic jets give rise to brief flashes of energetic gamma-radiation called gamma-ray bursts, which are picked up by monitoring instruments in space, that in turn alert astronomers.”

Powerful gamma ray bursts — GRBs — emitted from supernovae can be detected by both ground-based observatories and NASA’s Swift telescope. Within seconds of detecting a burst (hence its name) Swift relays its location to ground stations, allowing both ground-based and space-based telescopes around the world the opportunity to observe the burst’s afterglow.

But the actual moment of the star’s collapse, when its collapsing core reacts with its surface, isn’t observed — it happens too quickly, too suddenly. If these “shock breakouts” are the source of the excess thermal x-rays (a.k.a. black body emission) that have been recently identified in Swift data, some of the galaxy’s most energetic supernovae could be pinpointed and witnessed at a much earlier moment in time — literally within the first seconds of their birth.

“This phenomenon is only seen during the first thousand seconds of an event, and it is challenging to distinguish it from X-ray emission solely from the gamma-ray burst jet,” Dr. Starling said. “That is why astronomers have not routinely observed this before, and only a small subset of the 700+ bursts we detect with Swift show it.”

Read more: Finding the Failed Supernovae

More observations will be needed to determine if the thermal emissions are truly from the initial collapse of stars and not from the GRB jets themselves. Even if the x-rays are determined to be from the jets it will provide valuable insight to the structure of GRBs… “but the strong association with supernovae is tantalizing,” according to Dr. Starling.

Read more on the University of Leicester press release here, and see the team’s paper in the Nov. 28 online issue of the Monthly Notices of the Royal Astronomical Society here (Full PDF on arXiv.org here.)

Inset image: An artist’s rendering of the Swift spacecraft with a gamma-ray burst going off in the background. Credit: Spectrum Astro. Find out more about the Swift telescope’s instruments here.

 

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

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

Exoplanet’s Atmosphere Undergoes Dramatic Variations

Since its discovery in 2005, exoplanet HD 189733b has been one of the most-observed extra solar planets, due to its size, compact orbit, proximity to Earth and enticing blue-sky atmosphere. But astronomers using the Hubble Space Telescope and the Swift Telescope have witnessed dramatic changes in the planet’s upper atmosphere following a violent flare from its parent which bathed the planet in intense X-ray radiation. The scientists say being able to watch the action gives a tantalizing glimpse of the changing climates and weather on planets outside our Solar System.

While HD 189733b has a blue sky like Earth, it is one of the many “hot Jupiters” that have been the easiest for exoplanet hunters to find: huge gas planets that orbit extremely close to its star. HD 189733 lies extremely close to its star, called HD 189733A, just one thirtieth the distance Earth is from the Sun, whipping around the star in 2.2 days. Additionally, the system is just 63 light-years away, so close that its star can be seen with binoculars near the famous Dumbbell Nebula.

Even though its star is slightly smaller and cooler than the Sun, this makes the planet’s climate exceptionally hot, at above 1000 degrees Celsius, and the upper atmosphere is battered by energetic extreme-ultraviolet and X-ray radiation.

Even though HD 189733b’s atmosphere wasn’t thought to be evaporating (like a similar exoplanet called Osiris, or HD 209458b) astronomers knew the potential was there. The atmospheric gases extend far beyond the planetary “surface” allowing stellar light to pass through, and in previous observations astronomers were able to get a peek into what chemical compounds surround HD 189733b. From this analysis, scientists deduced that water and methane is contained in the atmosphere; and later, the Spitzer space telescope even mapped the temperature distribution around the globe. Additional research indicated a thin layer of particles exists in the upper atmosphere of HD 189733b, creating thin reflective clouds.

Astronomer Alain Lecavelier des Etangs from at the Paris Institute of Astrophysics in France led a team using Hubble to observe the atmosphere of this planet during two periods in early 2010 and late 2011, as it was silhouetted against its parent star. While backlit in this way, the planet’s atmosphere imprints its chemical signature on the starlight, allowing astronomers to decode what is happening on scales that are too tiny to image directly. They were hoping to observe the atmosphere evaporating away, but were disappointed in 2010.

“The first set of observations were actually disappointing,” Lecavelier said, “since they showed no trace of the planet’s atmosphere at all. We only realized we had chanced upon something more interesting when the second set of observations came in.”

The team’s follow-up observations, made in 2011, showed a dramatic change, with clear signs of a plume of gas being blown from the planet at a rate of at least 1000 tons per second, at speeds of 300,000 mph, giving the planet a comet-like appearance.

“We hadn’t just confirmed that some planets’ atmospheres evaporate,” Lecavelier said, “we had watched the physical conditions in the evaporating atmosphere vary over time. Nobody had done that before.”

So why was the atmosphere’s condition changing?

Despite the extreme temperature of the planet, the atmosphere is not hot enough to evaporate at the rate seen in 2011. Instead the evaporation is thought to be driven by the intense X-ray and extreme-ultraviolet radiation from the parent star, which is about 20 times more powerful than that of our own Sun. Taking into account also that HD 189733b is a giant planet very close to its star, then it must suffer an X-ray dose 3 million times higher than the Earth.

Because X-rays and extreme ultraviolet starlight heat the planet’s atmosphere and likely drive its escape, the team also monitored the star with Swift’s X-ray Telescope (XRT). On Sept. 7, 2011, just eight hours before Hubble was scheduled to observe the transit, Swift was monitoring the star when it unleashed a powerful flare. It brightened by 3.6 times in X-rays, a spike occurring atop emission levels that already were greater than the sun’s.

“The planet’s close proximity to the star means it was struck by a blast of X-rays tens of thousands of times stronger than the Earth suffers even during an X-class solar flare, the strongest category,” said co-author Peter Wheatley, a physicist at the University of Warwick in England.

After accounting for the planet’s enormous size, the team notes that HD 189733b encountered about 3 million times as many X-rays as Earth receives from a solar flare at the threshold of the X class.

“X-ray emissions are a small part of the star’s total output, but it is the part that it is energetic enough to drive the evaporation of the atmosphere,” said co-author Peter Wheatley from the University of Warwick, in the UK. “This was the brightest X-ray flare from HD 189733A of several observed to date, and it seems very likely that the impact of this flare on the planet drove the evaporation seen a few hours later with Hubble.”

The team also said the changes in the star’s output may mean it undergoes a seasonal process similar to the Sun’s 11-year sunspot cycle.

The team hopes to clarify the changes they witnessed using future observations with Hubble and ESA’s XMM-Newton X-ray space telescope, but say there is no question that the planet was hit by a stellar flare, and no question that the rate of evaporation of the planet’s atmosphere shot up.

This research shows the benefits of collaborative research between missions, as Swift saw the flare, and Hubble saw the massive amount of gas stripped out of the planet’s atmosphere. It also gives potential for future research, to watch for changes in both the star and atmospheres of other worlds.

This video from NASA’s Goddard Spaceflight Center provides additional information:

Lead image caption: This artist’s rendering illustrates the evaporation of HD 189733b’s atmosphere in response to a powerful eruption from its host star. NASA’s Hubble Space Telescope detected the escaping gases and NASA’s Swift satellite caught the stellar flare. Credit: NASA’s Goddard Space Flight Center.

Second image caption: Swift’s Ultraviolet/Optical Telescope captured this view of HD 189733b’s star on Sept. 14, 2011. The image is 6 arcminutes across. Credit: NASA/Swift/Stefan Immler

Did a Neutron Star Create the “Christmas Burst”?

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On December 25, 2010, at 1:38 p.m. EST, NASA’s Swift Burst Alert Telescope detected a particularly long-lived gamma-ray burst in the constellation Andromeda. Lasting nearly half an hour, the burst (known as GRB 101225A) originated from an unknown distance, leaving astronomers to puzzle over exactly what may have created such a dazzling holiday display.

Now there’s not just one but two theories as to what caused this burst, both reported in papers by a research team from the Institute of Astrophysics in Granada, Spain. The papers will appear in the Dec. 1 issue of Nature.

Gamma-ray bursts are the Universe’s most luminous explosions. Most occur when a massive star runs out of nuclear fuel. As the star’s core collapses, it creates a black hole or neutron star that sends intense jets of gas and radiation outwards. As the jets shoot into space they strike gas previously shed by the star and heat it, generating bright afterglows.

NASA's Swift observatory is a satellite in low-Earth orbit, scanning the sky for the presence of gamma-ray bursts and gravitational wave forces. (NASA)

If a GRB jet happens to be aimed towards Earth it can be detected by instruments like those aboard the Swift spacecraft.

Luckily GRBs usually come from vast distances, as they are extremely powerful and could potentially pose a danger to life on Earth should one strike directly from close enough range. Fortunately for us the odds of that happening are extremely slim… but not nonexistent. That is one reason why GRBs are of such interest to astronomers… gazing out into the Universe is, in one way, like looking down the barrels of an unknown number of distant guns.

The 2010 “Christmas burst”, as the event also called, is suspected to feature a neutron star as a key player. The incredibly dense cores that are left over after a massive star’s death, neutron stars rotate extremely rapidly and have intense magnetic fields.

One of the new theories envisions a neutron star as part of a binary system that also includes an expanding red giant. The neutron star may have potentially been engulfed by the outer atmosphere of its partner. The gravity of the neutron star would have caused it to acquire more mass and thus more momentum, making it spin faster while energizing its magnetic field. The stronger field would have then fired off some of the stellar material into space as polar jets… jets that then interacted with previously-expelled gases, creating the GRB detected by Swift.

This scenario puts the source of the Christmas burst at around 5.5 billion light-years away, which coincides with the observed location of a faint galaxy.

An alternate theory, also accepted by the research team, involves the collision of a comet-like object and a neutron star located within our own galaxy, about 10,000 light-years away. The comet-like body could have been something akin to a Kuiper Belt Object which, if in a distant orbit around a neutron star, may have survived the initial supernova blast only to end up on a spiraling path inwards.

The object, estimated to be about half the size of the asteroid Ceres, would have broken up due to tidal forces as it neared the neutron star. Debris that impacted the star would have created gamma-ray emission detectable by Swift, with later-arriving material extending the duration of the GRB into the X-ray spectrum… also coinciding with Swift’s measurements.

Both of these scenarios are in line with processes now accepted by researchers as plausible explanations for GRBs thanks to the wealth of data provided by the Swift telescope, launched in 2004.

“The beauty of the Christmas burst is that we must invoke two exotic scenarios to explain it, but such rare oddballs will help us advance the field,” said Chryssa Kouveliotou, a co-author of the study at NASA’s Marshall Space Flight Center in Huntsville, Alabama.

More observations using other instruments, such as the Hubble Space Telescope, will be needed to discern which of the two theories is most likely the case… or perhaps rule out both, which would mean something else entirely is the source of the 2010 Christmas burst!

Read more on the NASA mission site here.

 

Looking Into The Eye Of A Monster – Active Galaxy Markarian 509

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“The world is a vampire, sent to drain… Secret destroyers, hold you up to the flames…” Ah, yes. It’s the biggest vampire of all – the supermassive black hole. In this instance, it’s not any average, garden-variety black hole, but one that’s 300 million times the mass of the Sun and growing. Bullet with butterfly wings? No. This is more a case of butterfly wings with bullets.

An international team of astronomers using five different telescopes set their sites on 460 million light-year distant Markarian 509 to check out the action surrounding its huge black hole. The imaging team included ESA’s XMM-Newton, Integral, NASA/ESA Hubble Space Telescope, NASA’s Chandra and Swift satellites, and the ground-based telescopes WHT and PARITEL. For a hundred days they monitored Markarian 509. Why? Because it is known to have brightness variations which could mean turbulent inflow. In turn, the inner radiation then drives an outflow of gas – faster than a speeding bullet.

“XMM-Newton really led these observations because it has such a wide X-ray coverage, as well as an optical monitoring camera,” says Jelle Kaastra, SRON Netherlands Institute for Space Research, who coordinated an international team of 26 astronomers from 21 institutes on four continents to make these observations.

And the vampire reared its ugly head. Instead of the previously documented 25% changes, it jumped to 60%. The hot corona surrounding the black hole was spattering out cold gas “bullets” at speeds in excess of one million miles per hour. These projectiles are torn away from the dusty torus, but the real surprise is that they are coming from an area just 15 light years away from the center. This is a lot further than most astronomers speculate could happen.

“There has been a debate in astronomy for some time about the origin of the outflowing gas,” says Kaastra.

But there’s more than just bullets here. These new observations at multiple wavelengths are showing the coolest gas in the line of sight toward Markarian 509 has 14 different velocity components – all from different locations at the galaxy’s heart. What’s more, there’s indications the black hole accretion disc may have a shield of gas harboring temperatures ranging in the millions of degrees – the motivating force behind x-rays and gamma rays.

An artist's impression of the central engine of an active galaxy. A black hole is surrounded by matter waiting to fall in. Fearsome radiation from near the black hole drives an outflow of gas. Credits: NASA and M. Weiss (Chandra X-ray Center)

“The only way to explain this is by having gas hotter than that in the disc, a so-called ‘corona’, hovering above the disc,” Jelle Kaastra says. “This corona absorbs and reprocesses the ultraviolet light from the disc, energising it and converting it into X-ray light. It must have a temperature of a few million degrees. Using five space telescopes, which enabled us to observe the area in unprecedented detail, we actually discovered a very hot ‘corona’ of gas hovering above the disc. This discovery allows us to make sense of some of the observations of active galaxies that have been hard to explain so far.”

To make things even more entertaining, the study has also found the signature of interstellar gas which may have been the result of a one-time galaxy collision. Although the evidence may be hundreds of thousands of light years away from Mrk 509, it may have initially triggered this activity.

“The results underline how important long-term observations and monitoring campaigns are to gain a deeper understanding of variable astrophysical objects. XMM-Newton made all the necessary organisational changes to enable such observations, and now the effort is paying off,” says Norbert Schartel, ESA XMM-Newton Project Scientist.

Ah, Markarian 509… “Despite all my rage… I am still just a rat in cage.”

Original Story Source: ESA News. For Further Reading: Multiwavelength Campaign on Mrk 509 VI. HST/COS Observations of the Far-ultraviolet Spectrum.

Swift Survey Finds ‘Missing’ Active Galaxies

From a NASA press release:

Seen in X-rays, the entire sky is aglow. Even far away from bright sources, X-rays originating from beyond our galaxy provide a steady glow in every direction. Astronomers have long suspected that the chief contributors to this cosmic X-ray background were dust-swaddled black holes at the centers of active galaxies. The trouble was, too few of them were detected to do the job.

An international team of scientists using data from NASA’s Swift satellite confirms the existence of a largely unseen population of black-hole-powered galaxies. Their X-ray emissions are so heavily absorbed that little more than a dozen are known. Yet astronomers say that despite the deeply dimmed X-rays, the sources may represent the tip of the iceberg, accounting for at least one-fifth of all active galaxies.

Continue reading “Swift Survey Finds ‘Missing’ Active Galaxies”