Archive for the ‘Astrophysics’ Category

Why did a black hole blast star stuff into space at a quarter of light-speed on June 3, 2009? Here is what happened

January 10, 2012 3 comments

On June 3, 2009, in an X-ray binary star system far, far away. . .

We know the what of the extraordinary event that occurred in May 2009 around a distant black hole; we just don’t know the why of it, although the possibilities are pretty amazing.

At the 2012 American Astronomical Society (AAS) meeting in Austin, Texas, Gregory Sivakoff of the University of Alberta in Canada reported some astounding observations he and his colleagues accomplished using a globe-spanning array of radio telescopes and two NASA satellites.

The whole episode was a cosmic stroke of luck: The light from an event that happened some 28,000 years ago reached Earth just days before the global collaboration was scheduled to open for business. Goddard astrophysics writer Francis Reddy explains the details of the science today in a web feature story and animation.

Let’s start with the what: On or about June 3, 2009, enormous blobs of hot electrically charged matter were ejected from a black hole at about a quarter of the speed of light — roughly 75 million meters per second.

Next, the where: These black-hole “bullets,” as Reddy calls them in his web feature, were ejected from a binary star system. Called H1743–322, the  system lies about 28,000 light-years from Earth. NASA’s HEAO-1 satellite discovered it in 1977

In H1743–322, a black hole and a star orbit each other at close quarters, every few days. They are close enough that the black hole’s massive gravity draws a steady stream of material off its companion’s wispy surface. The hot electrically charged gas swirls around the edge of the black hole, forming a whirlpool-like “accretion disc.” As the gas accelerates to high speed, it radiates X-rays that satellites at Earth can detect.

“Some of the infalling matter becomes re-directed out of the accretion disk as dual, oppositely directed jets,” Reddy writes. “Most of the time, the jets consist of a steady flow of particles. Occasionally, though, they morph into more powerful outflows that hurl massive gas blobs at significant fractions of the speed of light.”

Years ago, Sivakoff’s colleague James Miller-Jones, currently based at the International Center for Radio Astronomy Research at Curtin University in Perth, Australia, conceived of a plan to mount a “multiwavelength campaign” to study the periodic outbursts that astronomers observe from X-ray binaries like H1743–322. They got their chance on May 22, 2009.

On that date, renewed activity around the black hole triggered the Burst Alert Telescope on NASA’s Swift satellite. Miller-Jones, Sivakoff, and the other members of the international team of observers were able to marshal three radio telescopes: the Very Long Baseline Array, the Very Large Array, and the Australia Telescope Compact Array. The team also drew on data from NASA’s Rossi X-ray Timing Explorer (RXTE) satellite (which was just switched off this week, by the way, after 16 years of meritorious service).

Using information from the telescopes and satellites, the scientists were able to reconstruct the events leading up to and following the ejection of black-hole bullets from the binary system. Sivakoff reported those findings today at the AAS meeting.

Now, finally, what about the “how” of this outburst? That’s not very clear yet.

In similar black hole binaries, Miller-Jones says, astronomers have measured ejections traveling 92 percent of the speed of light!  What process can shoot giant blobs of stuff out of the accretion zone of a black hole at such incredible speeds?

Sivakoff sketches out one possible explanation: Imagine knots of mass in the accretion disc, swirling around, getting closer and closer to the black hole. The disc is looped by powerful magnetic fields, which twist and tangle together as the disc rotates. When magnetic flux lines cross and connect, it could release enough energy to boost the black-hole bullets up and out of the disk.

“I think of a fairly stiff rope that is firmly to attached to the accretion disc,” Sivakoff explains. “As the disc spins, the rope is wound up, forming a sort of helix. Of course, there’s not one but many such ropes in an accretion disc. If two of those ropes touch — what astronomers call magnetic reconnection — lots of energy can be released. I like to envision ‘crossing the streams,’ a la Ghost Busters. This energy can accelerate particles, launching the bullet.”

There is another scenario, Miller-Jones says. Some scientists have proposed that what actually happens is that the inner edge of the accretion disc constricts, edging closer to the black hole’s “event horizon,” beyond which matter cannot escape. The magnetic and gravitational forces at this border region are extremely intense.

The forces could unleash a surge of material into the black hole’s paired jets, with a wavelike shock front ahead of it. “This causes particle acceleration,” Miller-Jones says, “and hence bright radio emission at this shock front.” So the bullets may actually be sudden surges in the jets, not discrete blobs.

But these explanations are just informed speculation at the moment. Additional multi-telescope observations could eventually provide enough clues to untangle the extreme physics that power black-hole bullets.

The team can only hope their recent stroke of luck holds out. Sivakoff says that the H1743–322  system conveniently started to flare up in late May 2009 — just as the team was preparing for the official opening of their observing window.

“Technically our observing was supposed to start in June 2009,” Sivakoff says. “But when this outburst went off a few days before our window was supposed to open up, we actually got permission to start observing earlier.”

So the discovery was the team’s inaugural run. “This was quite a trial by fire,” Sivakoff says.

OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center. And while we’re at it, links to websites posted on this blog do not imply endorsement of those websites by NASA.

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Swift Detects Most Distant Object In The Universe! AGAIN!

May 25, 2011 2 comments

Now where have we heard THAT news before? For aficionados of NASA’s Swift satellite, or even space science and astronomy in general, this headline probably rings a few bells. Like this one for example, announced on April 28, 2009:

New Gamma-Ray Burst Smashes Cosmic Distance Record

But what many of you may not be aware of is that, within 24 hours of the April 28 headline, Swift detected yet another gamma-ray burst (the death-throes of a massive star), which was even more distant. Why didn’t you know? Well, because we didn’t either!

image of GRB 090429b
A Gemini Observatory color image of the afterglow of GRB 090429B, a candidate for the most distant object in the universe. This “izH” image has been constructed from three images taken at the Gemini Observatory North telescope through different optical and infrared filters. The red color results from the absence of all optical light, which has been absorbed by hydrogen gas in the distant universe. Without that absorption, the afterglow color would be bluer than any of the galaxies and stars seen here. (Credit: Credit: Gemini Observatory/AURA/NASA/ Levan, Tanvir, Cucchiara, Fox)

The explosion, termed GRB 090429B, was detected on April 29, 2009, by Swift. Nino Cucchiara and his then-PhD supervisor Derek Fox, along with collaborators including Nial Tanvir and Andrew Levan from the UK, observed the GRB with the 8-meter Gemini telescope in Hawai’i, and found that it was red. Very red.

Now this can mean two things: either it’s a really long way away, or it went off in a really dusty galaxy. So Nino and collaborators asked the Gemini operators to take a spectrum of the source, which would provide a measurement of the object’s distance.

Unfortunately, even on Hawai’i, astronomers are at the mercy of the weather. And just as Gemini prepared to take the spectrum, the weather turned and observing was impossible. By the next observing opportunity, the GRB was too faint to take a usable spectrum.

Fortunately, that’s not the end of the story, but it made the job much harder. Now, after two years of hard graft, and observations with Gemini and with the Hubble Space Telescope, Nino and collaborators have released their findings. And the cosmic record holder has fallen!

Well, probably. Their result shows, based on analysis of the images, that there is a 99.3 percent likelihood that this object was more distant that GRB 090423 — the object being trumpeted just before this star exploded. The precise distance is not known because of the lack of spectrum, but there is a 98.9 percent chance that is lies further away than a galaxy discovered in 2010 — 13.07 billion light years away — which surpassed April 2009’s GRB 090423 as the most distant known object. Whether it is the farthest object ever seen is not entirely clear: a galaxy detected in 2011 may lie a little further away…. or may actually not be a distant object at all.

Either way, this new result is another triumph for GRB science, for Swift and the optical and infrared facilities like Gemini, and above all for the hard-working determination of the scientists studying these enigmatic phenomena.

Follow Phil Evans on twitter: @swift_phil

Has-been: In 2008, GRB 080319b had it's 15 minutes of fame as the farthest known object in the universe.

A gamma-ray burst is a tremendous release of energy triggered by the collapse of a massive star.

OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center. And while we’re at it, links to websites posted on this blog do not imply endorsement of those websites by NASA.


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Hubble hits the Red Limit. Next up: Webb Telescope

January 26, 2011 Leave a comment

hubble space telescope in orbit
The day had to come, and we all knew it. Hubble Space Telescope has been squinting for years, and now it’s reached the limit of its power to see back to the earliest epochs of cosmic time. As in Cosmic Time, or the amount of time elapsed since the Big Bang.

Today, a team of scientists made this exciting announcement:

SANTA CRUZ, CA–Astronomers studying ultra-deep imaging data from the Hubble Space Telescope have found what may be the most distant galaxy ever seen, about 13.2 billion light-years away. The study pushed the limits of Hubble’s capabilities, extending its reach back to about 480 million years after the Big Bang, when the universe was just 4 percent of its current age.

A story on Bad Astronomy explains the details, as does a NASA press release and one from the University of California, Santa Cruz. And the First Galaxies website provides even deeper scientific background in plain English.

As light from a distant galaxy speeds toward us, it gets stretched, or “redshifted,” by the expansion of space itself. Astronomers measure redshift with a quantity called “z.” The paper in Nature reports a redshift of z=10. 3. The first galaxies probably formed 200 to 300 million years post-Big Bang, which is more z’s than Hubble can deliver. To get to that redshift, Hubble would need instruments that can see even redder — more redshifted — light than it can now. So, in short, Hubble is at the “red limit” of what it can see.

I asked Jason Tumlinson, a galaxy researcher at the Space Telescope Science Institute to explain:

“My opinion is that we’re very near the limits of what HST can do in terms of pushing back the redshift frontier, and in fact have been operating at HST’s limits for several years. Everything depends on the performance of the cameras, and the major upgrade provided by the new Wide Field Camera 3 (WFC3) in 2009 has made a big difference.”

The upgraded WFC3 was installed on Hubble during the final servicing mission in May 2009.

I asked Amber Straughn, a Goddard astrophysicist in the Observational Cosmology Laboratory and a member of the James Webb Space Telescope (JWST) team, to explain why Hubble has reached the “red limit” of its seeing ability:

“The short answer is, at z~10, we are AT the limit of HST’s ability to look back in time. The reason for this is simply due to HST’s wavelength coverage. The light from these very distant galaxies is very, very red — and HST’s (Wide Field Camera 3) filters cut off at around 1.7 microns. . . .That’s the ‘red limit’ of HST.”

See Dr. Straughn talk to a TV reporter about the Webb Telescope.

Another issue, Tumlinson says, is the amount of Hubble telescope time available. The light-sensing detectors on Hubble contribute a certain amount of electronic “noise” that can swamp the signal from whatever you happen to be observing. To overcome this, astronomers have to schedule enough “Hubble time” to make sure the signal from the astronomical target is sufficiently stronger than the background noise – sort of like the way you have to raise your voice to be heard in a noisy room.

Tumlinson explains:

“The detector itself adds noise to the measurement — called readout noise, generally — which is an important factor in setting the faintest observable source. Of course, HST users could go deeper and push further with longer observations so that they collect more source counts relative to this noise term, but only so much time is available. “

NASA and the scientific community saw Hubble’s red limit coming. So they invented the James Webb Space Telescope. With its huge collecting mirror — 6.5 meters (21.3 feet) in diameter — and ultrasensitive infrared detectors, Webb can see longer, redder wavelengths of light, and “redder” translates to “more distant.”

Tumlinson explains:

“Discovering galaxies at high redshift is one of the top reasons NASA is building JWST. Being much larger and optimized for this sort of work, Webb should make z ~ 10 detections routine, and could push the frontier to z = 12, 15, or even higher.”

Z=15 is around 275 million years after the Big Bang — the sweet spot for observing the first stars and galaxies forming. Stay tuned!

OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center. And while we’re at it, links to websites posted on this blog do not imply endorsement of those websites by NASA.

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Phil Evans' Swift Universe: Contemplating the inconstancy of the Crab

January 12, 2011 Leave a comment

New results from NASA space observatories have revealed something surprising about the Crab Nebula: This famous supernova remnant — long considered a veritable “old faithful” of X-ray sources for the constancy of it energy output — appears to be dimming over time. We asked Phil Evans, gogblog’s on-call X-ray scientist and a member of the NASA Swift Observatory science team, to tell us why the inconstancy of the Crab is so important to astronomers.

image of crab nebulaThe Crab Nebula has a prestigious history. It formed when a star exploded in a supernova, and was first observed and recorded by Chinese observers in 1054 AD. The glow of the supernova was so bright, people could see it during the day for more than 3 weeks!

The material which was blown off the star has been expanding since then in a complex structure with leg-like filaments that earned it its name. It’s also a very bright source of X-rays, and — particularly usefully — its brightness and spectrum don’t change; so astronomers can (and do) use it to calibrate their X-ray instruments. In fact, “a Crab” is an internationally recognized unit of measurement.

The problem is, these new results suggest that the Crab is not constant after all, according to a press release issued today by NASA’s Goddard Space Flight Center. The measurements taken over the last few years by the Fermi, Swift, RXTE and INTEGRAL satellites show that the Crab actually varies by a few percent every year. This is not too disastrous right now: It’s pretty hard to calibrate high-energy instruments to an accuracy of 1 percent or so, and the definition of “a Crab” as a unit of measurement has a fixed definition. But as technology advances, we will probably find that the Crab is no longer the ideal calibration source.

This type of finding, by the way, is not unusual. It is often the case that an object described as the “protoype” of its class turns out to be atypical! Indeed the star Vega, long used as a standard in optical astronomy, was recently found not to be standard. The exciting thing about all of this is it shows us how much we still have to learn. The Crab is among the brightest X-ray sources in the sky, and yet it is able to jump out and surprise us.

In a related point under the same press release, recently published work from the NASA’s Fermi Gamma-ray Space Telescope and the Italian Space Agency’s AGILE satellite have found large gamma-ray flares from the Crab Nebula. Investigation is ongoing, but this may indicate a really strong electric field. As study coauthor Stefan Funk said, “The strength of the gamma-ray flares shows us they were emitted by the highest-energy particles we can associate with any discrete astrophysical object,” which in themselves present plenty of challenges.

The Crab nebula: exciting and enigmatic? Yes! Constant and well understood? No! A fantastic natural laboratory? You bet.

— Phil Evans

Follow Phil on Twitter to get updates on his life and work in X-ray astronomy.

chart of declining crab nebula x-ray output

OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center. And while we’re at it, links to websites posted on this blog do not imply endorsement of those websites by NASA.

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Planes, trains, bikes, and automobiles: Goddard engineer Kevin Boyce hits the road to make sure that a space observatory "made in Japan" makes it to space and sends home a pay-off for science

November 15, 2010 Leave a comment

KEVIN BOYCEAn earlier post featured the scary “spacecraft house of horrors” video about the testing torments suffered by our satellites before we send them to orbit. The video was hosted by our own Kevin Boyce, a spacecraft systems engineer. These days, Kevin is part of the international team working on the Japanese Astro-H mission. Here’s an account of his recent trip to Japan to help design an X-ray instrument.

How do you say in Japanese, “If you don’t succeed, try, try again”?

ASTRO-E was to be Japan’s fifth X-ray astronomy mission, but unfortunately the spacecraft was lost during launch on February 10, 2000.

Ok, try again. A follow-on mission, Astro-E2, launched successfully on July 10, 2005 from the Uchinoura Space Center in Japan. Soon after launch, the mission was renamed Suzaku.

The ill-fated Astro-E spaceraft

The ill-fated Astro-E spacecraft

Kevin Boyce can tell you all about it. Recently, as he was landing at Tokyo’s Narita Airport, it (almost) felt like coming home. “I’ve been here almost 40 times now,” he says. That started in the late 90’s with the ill-fated Astro-E project. Then he worked on the Astro E2/Suzaku mission that followed.

Now he’s an instrument systems engineer on one of the instruments on a new spacecraft called Astro-H. As he disembarks from the plane, he wonders if he should take the usual trains to the hotel, or take the bus this time. He decides on the bus option, and gets some cash from the ATM and buys a Matcha Creme Frappuccino from the Starbucks. Yes, America has left its mark here too.

artist concept of astro h

Artist's concept of Astro-H

Astro-H is Kevin’s third go-round with Japan’s space agency, JAXA, and Japan’s 8th space-based astronomy mission. It will launch into low-Earth orbit intending to trace the growth history of the largest structures in the universe, reveal the behavior of matter in extreme gravitational fields, determine the spin of black holes and study neutron stars, trace shock acceleration structures in clusters of galaxies, and investigate the detailed physics of galactic jets.

Um, is THAT all?

To do all that requires a gadget called a Soft X-ray Spectrometer (SXS), and Kevin is here in Japan to help shepherd the design of the instrument through a complex and high-stakes process that is difficult to carry out effectively solely by email or phone. It take as bunch of long plane rides and as many Matcha Creme Frappuccinos.

He’s in Japan for a week to participate in one of the quarterly Astro-H design meetings. “At these meetings all the various instrument teams report on their status, along with the spacecraft systems team,” he explains. “This generally lasts for two days.”

The rest of the time, the scientists and engineers pick apart the various sub-systems of the SXS. The devil is in the details, as the cliché goes. Miss a detail, and possibly buy lots of (expensive) trouble. Space missions take years and years and millions and millions of dollars.

SXS pushes X-ray observing technology. “Many of the people on both sides of the Pacific who are working on Astro-H, myself included, have been trying to get this technology operating on orbit since 1995,” he explains. “So it’s not just the trains and locations that make it feel like home. Some of my best old friends are here.”

This particular trip included a “hole.” Meeting took up Tuesday and Thursday, but Wednesday was a Japanese holiday, with no meetings scheduled. But you can’t fly home for a day. So what to do?

“Happily, some of our Japanese colleagues scheduled a bike trip into the mountains, and rented me a bike so I could join them,” he says. “We rode 50 kilometers up toward lake Yamanaka, climbing 700 meters in the process. And then back..”

[Read Kevin’s account of the bike trip on the NASA Blueshift blog.]

Snow-capped Mt. Fuji forms part of the background for a bike trip in Japan.

Snow-capped Mt. Fuji forms the background for a bike ride into the mountains.

After that ride, the design meeting was almost anticlimactic. But very important! The reason the X-Ray Spectrometer failed on Astro-E2 was basically due to incomplete communication between Goddard Space Flight Center and the Japanese during the design of the instrument. “This time we’re meeting much more often, and exchanging far more information, so that doesn’t happen again,” Boyce explains. “It’s not enough to exchange drawings and requirements documents. Each side really has to understand the whole instrument, and indeed the whole spacecraft system.”

So this time, Boyce attends the Japanese design meetings and reviews, and they attend the NASA reviews, and they all spend a lot more time on airplanes. But it’s still worth it, because Japan gets an instrument they don’t have the expertise to build at this point, and the US gets access to a whole mission’s worth of scientific data for just the cost of an instrument. Everyone wins.

“But only if we make it work,” Boyce says. “So four, five, six, or more times each year several of us hop on a plane for a week in our other homes here in Japan. Kampai!”

OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center. And while we’re at it, links to websites posted on this blog do not imply endorsement of those websites by NASA.

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Phil Evans Swift Universe: how nature's strongest magnets power some of nature's brightest blasts

November 4, 2010 Leave a comment

A magnetar formed inside a collapsing massive star

A magnetar formed inside a collapsing massive star

Today “Swift Universe” guest blogger Phil Evans brings us some breaking news from the Gamma Ray Bursts 2010 conference in Annapolis, Maryland.

You’re all familiar with magnets. Well, two of my colleagues at the University of Leicester — Professor Paul O’Brien and his graduate students Antonia Rowlinson and Nicola Lyons — have announced evidence that some gamma-ray bursts (GRBs) are powered by stars called magnetars — super-strong magnets in space, if you like.

The idea is that, when the GRB goes off, the core of the dying star may not collapse straight to a black hole but instead could live for a couple of minutes as a rapidly rotating, magnetic neutron star called a magnetar. The magnetic field acts like a brake slowing the magnetar down and pumping its energy into the GRB, until after a few minutes the star has slowed down and collapses into a black hole.

Using data from the Swift satellite, my colleagues found that some GRBs show a period of constant brightness and then suddenly get really faint: just as the magnetar model predicts.

“So what?” you may ask. Well, GRBs are pretty much unique tools to study the early universe, and it’s the deaths of massive stars, some of which die as GRBs, which gives the universe the chemicals that you are I are made from. Getting a handle on the processes by which a star dies, and how it gives off its energy, is fundamental to using GRBs to study these matters. Showing that some GRBs are powered by magnetars is a big step forward.

One note of caution though: this isn’t “the” answer. While it seems to be the only explanation for some GRBS, in this same conference scientists from Berkeley university have shown using data from the Fermi satellite that the brightest GRBs can’t be powered by magnetars, but need a black hole right from the word go. Life’s never straightforward… but it’s often interesting!

Follow Phil as a Swift scientist on Twitter:  @Swift_Phil

Ron Cowen at Science News published a detailed write-up on the research.

OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center. And while we’re at it, links to websites posted on this blog do not imply endorsement of those websites by NASA.

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Gogblogcast #3: Chatting with NASA's Holly Gilbert about solar prominences

November 1, 2010 Leave a comment

I recently spoke to NASA solar physicist Holly Gilbert about a solar prominence eruption caught by the Solar Dynamics Observatory. These great looping eruptions of hot plasma are one of Gilbert’s main research interests. Here is what she had to say.


OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center. And while we’re at it, links to websites posted on this blog do not imply endorsement of those websites by NASA.

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Chillaxing in deep space with the James Webb Space Telescope: Stay frosty, little mercury cadmium telluride infrared detector chip!

September 14, 2010 2 comments

webb art
With a 6.5 meter diameter mirror, the James Webb Space Telescope will operate at a temperature of only 35 degrees above absolute zero in a deep space orbit.

Imagine you are a mercury cadmium telluride infrared detector chip, chillaxing in deep space nearly a million miles from Earth.

(OK, I know it’s a stretch, but give it a try.)

You are riding on the James Webb Space Telescope, the most high-tech space observatory ever attempted. But in the end, mission success rides on you, little infrared photon detector chip! You are the retinas of this great observatory. It sees only as well as YOU see.

So you are sitting there, peering out at the darkness, waiting for a photon from the dawn of the universe to pierce your semiconductor crystal matrix and knock an electron out of its comfy valence shell and into the conduction band. This generates the faintest of electric currents. This means data: the stuff of astronomical discoveries.

But 12 times an hour, the humming of electrons in your crystal guts is an illusion, a lie. Infrared telescope scientists call it dark current.

I learned about dark current at a recent colloquium here at Goddard. Three heavy hitters in the Webb Telescope project gave an update on the fabrication and testing of this multi-billion dollar space telescope.

Mark Clampin, the Webb Telescope project scientist, brought us up to speed on the observatory as a whole. He mentioned that 17 percent of Webb’s “flight mass” — the stuff that will blast into space on a giant Arianne launch vehicle — is now built. That represents about a billion dollars in gear.

Randy Kimble also spoke. He is the Webb mission’s Integration & Test Project Scientist. He gave us a progress report on the telescope’s instruments. And it was all good news: The flight versions of the instruments — again, the ones that will actually go into space — will begin arriving at Goddard next summer to be shaken, frozen, and irradiated during a series of grueling tests.

But the first speaker — the one who introduced “dark current” to my vocabulary — was John Mather. He gave a concise summary of the observatory’s science mission.

At one point, he bragged up Webb’s detectors. These are the chips, like the CCD in your digital camera, that turn infrared photons from farthest cosmos into trickley little electric currents and, ultimately, astronomical data.

(You may recall that John C. Mather shared the 2006 the Nobel Prize in Physics with George F. Smoot for their discoveries related to the microwave background glow from the Big Bang, using the COBE satellite.)

The detectors are kept cold by a combination of the ambient chill of deep space and a lot of clever engineering to block the sun’s rays and isolate the instruments from 220 watts of heat radiating from the telescope’s on-board electronics. Mather bragged that Webb’s infrared detectors have “dark currents measured in a few electrons per hour per pixel.”

Huh? Dark what? The phrase “dark current” and the idea of measuring single electrons per hour set off my cool-o-meter.

As the colloquium ended, I hurried to the head of the room hoping to grab Mather’s attention before someone else got to him. “The statistic you mentioned about dark currents. Can you explain?”

He said that each pixel of each detector spontaneously generates about a dozen electrons per hour — that’s the dark current — simply staring off into empty space. Hence the term: dark current. A perfect detector would produce nothing, just total silence, unless an actual infrared photon came in and bonked one of its atoms.

But, hey, 12 electrons per hour is pretty dark! And out of that darkness, we hope, will come the data to illuminate our understanding of the birth of the first stars, the evolution of galaxies, and the nature of planetary systems around other stars.

OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center. And while we’re at it, links to websites posted on this blog do not imply endorsement of those websites by NASA.

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And then the universe said 'Hah!' NASA's Swift satellite can't believe its eyes when it spots the brightest X-ray glow from a gamma ray burst outside our galaxy

July 14, 2010 14 comments

Credit: NASA/Swift/Mary Pat Hrybyk-Keith and John JonesThree weeks ago, a distant point in space in another galaxy released a blast of X-rays so bright even the satellite that saw it first didn’t believe its eyes. Then Phil Evans came home from vacation and got very, very lucky.

“One of the things I personally like most about doing research,” he says, “is when you discover something brand new — even if it’s ‘just’ the brightest X-ray object that we think we’ve ever seen — there’s a moment when there is only one person in the universe who knows about this. And sometimes you get to be that one person.”

It went down this way: On June 21, NASA’s orbiting Swift observatory was on sentry duty for Earth’s astronomers, watching the universe for Gamma ray bursts. A GRB went off on June 21, later catalogued GRB 100621A. (GRBs are violent eruptions of energy from the explosion of a massive star turning into a black hole.)

Gamma ray bursts announce their appearance as, well, a burst of gamma rays. Since gammas are the most energetic form of electromagnetic radiation, GRBs are the most powerful beacons in the universe.

When Swift detects a burst, it radios the coordinates to Earth. Astronomers and robotic observatories scramble to aim their instruments at the GRB. Swift also slews its instruments, such as the X-Ray Telescope (XRT), to the target.

Swift's X-Ray Telescope captured this image of GRB 100621A

Swift's X-Ray Telescope captured this image of GRB 100621A

Ideally, astronomers want to observe both the immediate or “prompt” emissions and, as time passes, the fading afterglow of X-rays, ultraviolet light, and (rarely) visible light.

Swift beams to Earth a record of when it detected each photon, and then software on the ground turns this into a “light curve” — literally a record of how the GRB’s brightness changes over time in various wavelengths.

Meanwhile, back at the lab…
OK, enough about Swift; back to Phil. He’s is a post-doctoral research assistant in the X-ray and Observational Astronomy group at the University of Leicester in England, and part of the Swift team. He wrote the software that converts the photo detections from Swift into light curves.

So he got home from holiday on June 29. The next morning, he examined the light curves that his software had created while he was in the Lake District in North West England, camping with his wife and two young sons.

And he saw something very puzzling: For one event, GRB 100621A, the record of its earliest X-ray emission was missing. He’d also received an email from another astronomer who had also noticed the gap.

Swift beauty 1 202“I looked at this and thought that’s odd, I’ll have to come back to it. I was looking at it and thinking, this is very strange.”

By noon, he had the data gap plugged. It took him a few more hours to check it, and to appreciate what had actually happened. The next day he announced it to the rest of the Swift community around the globe.

It turned out that GRB 100621A had been so bright early on, it had temporarily blinded Swift’s detectors. At the center of the image, which is the brightest part of the image, X-rays streamed in at a peak rate of 143,000 per second — well, for 0.2 seconds, anyway! But the X-ray camera literally could not count that fast. It was like a lone soccer goalkeeper being fired at by a dozen World Cup strikers.

Correct me if I’m wrong
Phil’s light-curve-making software has a way of dealing with this situation. It counts the X-ray photons streaming in around the edges of the image, where it’s not so bright and intense. Then it multiplies that by a correction factor to estimate how bright it must be in the glaring center of the image.

This correction was used in the famous “naked eye” GRB 080319B of 2008, which was so bright you could have seen it without a telescope, briefly, in a dark location on Earth. The correction: 32 times.

Correction for the June 21 GRB: 168!

Phil designed the software so that if the correction factor exceeds a certain expected threshold, the software just doesn’t report the data to astronomers on the ground. In a sense, Swift didn’t believe its own eyes.

“When it did the correction, it saw the size of it and said, no, that’s got to be nonsense, there’s got to be some sort of mistake,” Phil explains. “It just said, ‘This can’t be real. I’m not publishing this to the world because I’m going to look like an idiot.”

Or to put it more politely, Phil’s software refuses to report what it determines to be bad data. But it wasn’t bad; it was spot-on correct.

Swift beauty 2 202A new (X-ray) world record

“I didn’t totally register at first how bright it is, and then I mentioned it to a few people, and they went ‘What!’ And then we started to get a better feel for the fact that this was something to put in your record books.”

What kind of record are we talking about?

When the system was designed, astronomers weren’t expecting to see anything as bright as what Swift saw on June 21. The next brightest such object in X-rays was 2008’s naked-eye GRB 080319B.

This GRB was seven times fainter but twice as far away as the June 21 event. But move the new record holder to the same distance, and it would still be 1.5 times as bright as GRB 080319B.

Both of these events happened outside our Milky Way Galaxy. For example, the June 21 GRB happened 5 billion light years away, which means the light from it left about the time our solar system formed. There are brighter X-ray sources in our own galaxy, but that’s like comparing your next-door neighbor’s porch light to the blinking aircraft beacon atop a radio tower 10 miles away.

Relative to Swift, the latest GRB is a clear record-breaker. It is definitely the brightest thing, GRB or otherwise, Swift’s X-ray eye has ever seen.

And that is, perhaps, the most striking thing about this whole episode: the way in which this latest GRB has confounded expectations.

Barbara Kennedy said it best recently on a teleconference with Phil and a couple of other scientists, including Dave Burrows of Penn State University (PSU), the lead scientist for Swift’s X-ray telescope. Barbara is a press officer for PSU, which has issued a release about the event.

We — the scientists, Barbara, and a couple other media people who cover Swift — were discussing what approach to use to explain this GRB to the public. Biggest? Brightest? First?

I thought Barbara nailed it when she said:  “The best scientists in the world thought ‘We’ll never see anything that bright, so we don’t have to design the software to handle it.’ And then the universe said ‘Hah! Look what I can throw at you!'”


Phil_40Follow Phil Evans in his role as a Swift Scientist on Twitter: @Swift_Phil, where news of this discovery was first announced!]

See Barbara Kennedy’s press release on the X-ray GRB on the PSU website.

OH AND DID I MENTION? All opinions and opinionlike objects in this blog are mine alone and NOT those of NASA or Goddard Space Flight Center.

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