Wednesday, March 25, 2009

An Erratic Black Hole Regulates Itself


Micro-quasar GRS 1915+105, located near the plane of the Milky Way galaxy. 
Image credit: X-ray: NASA/CXC/Harvard/J. Neilsen et al. 
Optical: Palomar DSS2

New results from NASA's Chandra X-ray Observatory have made a major advance in explaining how a special class of black holes may shut off the high-speed jets they produce. These results suggest that these black holes have a mechanism for regulating the rate at which they grow. 

Black holes come in many sizes: the supermassive ones, including those in quasars, which weigh in at millions to billions of times the mass of the Sun, and the much smaller stellar-mass black holes which have measured masses in the range of about 7 to 25 times the Sun's mass. Some stellar-mass black holes launch powerful jets of particles and radiation, like seen in quasars, and are called "micro-quasars". 

The new study looks at a famous micro-quasar in our own Galaxy, and regions close to its event horizon, or point of no return. This system, GRS 1915+105 (GRS 1915 for short), contains a black hole about 14 times the mass of the Sun that is feeding off material from a nearby companion star. As the material swirls toward the black hole, an accretion disk forms. 

This system shows remarkably unpredictable and complicated variability ranging from timescales of seconds to months, including 14 different patterns of variation. These variations are caused by a poorly understood connection between the disk and the radio jet seen in GRS 1915. 

Chandra, with its spectrograph, has observed GRS 1915 eleven times since its launch in 1999. These studies reveal that the jet in GRS 1915 may be periodically choked off when a hot wind, seen in X-rays, is driven off the accretion disk around the black hole. The wind is believed to shut down the jet by depriving it of matter that would have otherwise fueled it. Conversely, once the wind dies down, the jet can re-emerge. 

"We think the jet and wind around this black hole are in a sort of tug of war," said Joseph Neilsen, Harvard graduate student and lead author of the paper appearing in the journal Nature. "Sometimes one is winning and then, for reasons we don't entirely understand, the other one gets the upper hand." 

The latest Chandra results also show that the wind and the jet carry about the same amount of matter away from the black hole. This is evidence that the black hole is somehow regulating its accretion rate, which may be related to the toggling between mass expulsion via either a jet or a wind from the accretion disk. Self-regulation is a common topic when discussing supermassive black holes, but this is the first clear evidence for it in stellar-mass black holes. 

"It is exciting that we may be on the track of explaining two mysteries at the same time: how black hole jets can be shut down and also how black holes regulate their growth," said co-author Julia Lee, assistant professor in the Astronomy department at the Harvard-Smithsonian Center for Astrophysics. "Maybe black holes can regulate themselves better than the financial markets!" 

Although micro-quasars and quasars differ in mass by factors of millions, they should show a similarity in behavior when their very different physical scales are taken into account. 

"If quasars and micro-quasars behave very differently, then we have a big problem to figure out why, because gravity treats them the same," said Neilsen. "So, our result is actually very reassuring, because it's one more link between these different types of black holes." 

The timescale for changes in behavior of a black hole should vary in proportion to the mass. For example, an hour-long timescale for changes in GRS 1915 would correspond to about 10,000 years for a supermassive black hole that weighs a billion times the mass of the Sun. 

"We cannot hope to explore at this level of detail in any single supermassive black hole system," said Lee. "So, we can learn a tremendous amount about black holes by just studying stellar-mass black holes like this one." 

It is not known what causes the jet to turn on again once the wind dies down, and this remains one of the major unsolved mysteries in astronomy. 

"Every major observatory, ground and space, has been used to study this black hole for the past two decades," said Neilsen. "Although we still don't have all the answers, we think our work is a step in the right direction." 

This was work made using Chandra’s High Energy Transmission Gratings Spectrometer. These results appear in the March 26th issue of Nature. NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory controls Chandra's science and flight operations from Cambridge, Mass. 

Additional information and images about this discovery can be found at: Chadra .Harvard

Janet Anderson, 256-544-6162
Marshall Space Flight Center, Huntsville, Ala.

Janet.L.Anderson@nasa.gov

Megan Watzke 617-496-7998
Chandra X-ray Center, Cambridge, Mass.

m.watzke@cfa.harvard.edu

Fast Facts for GRS 1915+105:

Credit: X-ray (NASA/CXC/Harvard/J.Neilsen); Optical & IR (Palomar DSS2)
Scale: Image is 5 degrees across.
Category: Black Holes, Neutron Stars/X-ray Binaries
Coordinates (J2000): RA 19h 15m 11.60s | Dec +10° 56' 44.00''
Constellation: Aquila
Observation Date: August 14th, 2007
Observation Time:  14 hours
Obs. ID: 7485
Color Code: X-ray (Purple); Optical & IR (Red, Green, Blue)
Instrument: ACIS / HETG
References: Neilsen, J. and Lee, J., 2009 Nature, Accepted
Distance Estimate: About 40,000 light years

Tuesday, March 24, 2009

Around the World in 80 Telescopes


Live 24-hour webcast from astronomical observatories

Organised by ESO, the European Southern Observatory, from its HQ in Garching, Germany

Date: 3 April 2009, 09:00 UT (Universal Time/GMT) to 4 April 2009, 09:00 UT (Universal Time/GMT).

Duration: 24 hours

"Around the World in 80 Telescopes" is a unique live 24-hour webcast, following night and day around the globe to some of the most advanced observatories both on and off the planet. You can watch it right here on the 100HA website, and on the 100HA channel on Ustream.tv.

Find out what's happening at a research observatory in your country, or on the other side of the planet, and discover what astronomers are doing right now! Who is observing? What are they researching? What do they hope to discover?

You'll see a snapshot of life at many different observatories where astronomers will present exclusive images and talk about their work. Some will be observing distant galaxies, searching for extrasolar planets around other stars or studying our own Solar System. Some will be studying the Universe in visible light, others in radio waves or other wavelengths. Some may be working at solar observatories or with telescopes out in space. All of them will have a different story to tell.

Questions?: Check out the "Around the World in 80 Telescopes" FAQ. Get in touch!

Keep up with the latest news from the team by following us on Twitter: @telescopecast

Got a question (astronomical or about the webcast) for the "Around the World in 80 Telescopes" team, or just want to tell us where and when you'll be watching? Send us email to 80t-msg@eso.org and we'll read out as many as we can during the webcast. You can also contact some observatories directly: check out their individual pages from the schedule below.

Webcast schedule

"Around the World in 80 Telescopes" begins with the telescopes on Mauna Kea in Hawaii, before moving westwards around the planet. As this is a live event, the schedule may be subject to change in the event of technical problems.
All dates and times are in Universal Time (UT), or "GMT". Click on the times below to see the equivalent local time in cities around the world.

Date / Time (UT)Observatory
3 April 09:00Gemini North telescope (Hawaii, USA)
3 April 09:20Subaru Telescope, National Astronomical Observatory of Japan (NAOJ) (Hawaii, USA)
3 April 09:40United Kingdom Infrared Telescope (UKIRT) (Hawaii, USA)
3 April 10:00W. M. Keck Observatory (Hawaii, USA)
3 April 10:20James Clerk Maxwell Telescope (JCMT) (Hawaii, USA)
3 April 10:40Canada-France-Hawaii Telescope (CFHT) (Hawaii, USA)
3 April 11:00Submillimeter Array (Hawaii, USA)
3 April 11:20Caltech Submillimeter Observatory (CSO) (Hawaii, USA)
3 April 11:40MOA Telescope (New Zealand)
3 April 12:00Anglo-Australian Telescope (AAT) (Australia)
3 April 12:20GEO600, the German-British Gravitational Wave Detector (Germany)
3 April 12:40NAOJ Nobeyama, Nobeyama Radio Observatory (NRO) (Japan)
3 April 13:00Gunma Astronomical Observatory (Japan)
3 April 13:20Okayama Astrophysical Observatory (OAO) (Japan)
3 April 13:40Themis (Observatorio del Teide) (Spain)
3 April 13:50SolarLab (Observatorio del Teide) (Spain)
3 April 14:00Quijote (Observatorio del Teide) (Spain)
3 April 14:10ESA's XMM-Newton X-ray observatory & INTEGRAL gamma-ray observatory(Space/Spain)
3 April 14:40Atacama Pathfinder Experiment (APEX) (Chile)
3 April 15:00Atacama Large Millimeter/submillimeter Array (ALMA) (Chile)
3 April 15:20European VLBI Network (EVN) (Netherlands)
3 April 15:40ASTRON Westerbork Synthesis Radio Telescope (WSRT) (Netherlands)
3 April 16:00LOFAR, the LOW Frequency Array of ASTRON (Netherlands)
3 April 16:20Virgo Gravitational Wave Detector at the European Gravitational Observatory (Italy)
3 April 16:40Plateau de Bure Interferometer (France)
3 April 17:00The University of Manchester's Jodrell Bank Observatory (United Kingdom)
3 April 17:20The Hubble Space Telescope (Space/USA)
3 April 17:40The Swift Gamma Ray Burst Explorer (Space/USA)
3 April 18:00The Fermi Gamma-ray Space Telescope (Space/USA)
3 April 18:20The Very Large Array (VLA) (USA)
3 April 18:40Himalayan Chandra Telescope (Indian Astronomical Observatory, Hanle) (India)
3 April 19:00The Robert C. Byrd Green Bank Telescope (USA)
3 April 19:20SOHO (Solar and Heliospheric Observatory) and TRACE (Transition Region and Coronal Explorer) (Space/USA)
3 April 19:40STEREO (Solar TErrestrial RElations Observatory) (Space/USA)
3 April 20:00Laser Interferometer Gravitational-Wave Observatory (LIGO) (USA)
3 April 20:20Galaxy Evolution Explorer (GALEX) (Space/USA)
3 April 20:40NASA's Chandra X-ray Observatory (Space/USA)
3 April 21:00The Southern African Large Telescope (SALT) (South Africa)
3 April 21:20NASA's Spitzer Space Telescope (Space/USA)
3 April 21:40Observatoire de Haute-Provence (France)
3 April 22:00Calar Alto Observatory (Centro Astronómico Hispano Alemán) (Spain)
3 April 22:20IRAM 30-metre telescope (Spain)
3 April 22:40Hinode (SOLAR-B) (Space/Japan)
3 April 23:00Gran Telescopio Canarias (Observatorio del Roque de los Muchachos, La Palma)(Spain)
3 April 23:10William Herschel Telescope (Observatorio del Roque de los Muchachos, La Palma)(Spain)
3 April 23:20Telescopio Nazionale Galileo (Observatorio del Roque de los Muchachos, La Palma)(Spain)
3 April 23:30Swedish Solar Telescope (Observatorio del Roque de los Muchachos, La Palma)(Spain)
3 April 23:40Allen Telescope Array (USA)
4 April 00:00Telescope Bernard Lyot (TBL), Pic du Midi (France)
4 April 00:20CSIRO Australia Telescope National Facility - Parkes Observatory (Australia)
4 April 00:40Space Sciences Laboratory - UC Berkeley (Space/USA)
4 April 01:00University of Tasmania Hobart 26m Radiotelescope (Mount Pleasant Observatory)(Australia)
4 April 01:20Australian International Gravitational Wave Observatory (AIGO) Research Facility(Australia)
4 April 01:40Shanghai Radio Telescope (Shanghai Astronomical Observatory) (China)
4 April 02:00Arecibo Observatory (Puerto Rico)
4 April 02:20ESO Very Large Telescope (VLT) (Chile)
4 April 02:40Concordia station, Dome C, Antarctica (Antarctica)
4 April 03:00Las Campanas Observatory (Chile)
4 April 03:20ESO La Silla Observatory (Chile)
4 April 03:40Rothney Astrophysical Observatory (Canada)
4 April 04:00Gemini South telescope (Chile)
4 April 04:20NOAO South - Cerro Tololo Inter-American Observatory (Chile)
4 April 04:40Molonglo Observatory Synthesis Telescope (Australia)
4 April 05:00McDonald Observatory (Hobby-Eberly Telescope) (USA)
4 April 05:20Apache Point Observatory ARC 3.5-meter Telescope (USA)
4 April 05:40Large Binocular Telescope Observatory (USA)
4 April 06:00TAMA 300 (Japan)
4 April 06:20Arizona Radio Observatory's Submillimeter Telescope, Mt Graham (USA)
4 April 06:35Vatican Telescope, Mt Graham (USA)
4 April 06:50MMT Observatory (USA)
4 April 07:05Kepler Mission (Space/USA)
4 April 07:25The 10-meter South Pole Telescope/IceCube Neutrino Telescope (South Pole, Antarctica)
4 April 07:40Kitt Peak National Observatory (USA)
4 April 08:00Lick Observatory (USA)
4 April 08:20CHARA (Mount Wilson) (USA)
4 April 08:40Palomar Observatory / Hale Telescope (USA)

Contact: Douglas Pierce-Price, dpiercep@eso.org

Disclaimer: All opinions expressed in "Around the World in 80 Telescopes" are those of the speakers and/or authors, and do not necessarily reflect the position of ESO, the International Year of Astronomy 2009, International Astronomical Union, UNESCO or other organisations involved in this project.

Sunday, March 22, 2009

Hubble Uncovers an Unusual Stellar Progenitor to a Supernova

About this image: Archival photographs from NASA's Hubble Space Telescope have been used to uncover the progenitor star to a supernova that exploded in 2005. To the surprise of astronomers, the progenitor is a rare class of ultra-bright star that, according to theory, shouldn't explode so early in its evolution.

[Top Center] This is a 2005 ground-based photograph of the supernova as seen in host galaxy NGC 266, located in the constellation Pisces.
Credit: Puckett Observatory

[Bottom Left] This is a 1997 Hubble archival visible-light image of the region of the galaxy where the supernova exploded. The white circle marks a star that Hubble measured to have an absolute magnitude of -10.3. This corresponds to the brightness of 1 million suns (at the galaxy's distance of 215 million light-years).
Credit: NASA, ESA, and A. Gal-Yam (Weizmann Institute of Science, Israel)

[Bottom Center] This is a near-infrared light photo of the supernova explosion taken on Nov. 11, 2005, with the Keck telescope, using adaptive optics. The blast is centered on the position of the progenitor.
Credit: NASA, ESA, and A. Gal-Yam (Weizmann Institute of Science, Israel), D. Leonard (San Diego State University), and D. Fox (Penn State University)

[Bottom Right] This is a visible-light Hubble follow-up image taken on September 26, 2007. Note that a bright source near the site of the supernova can be seen in all three panels, but the progenitor star is gone. The Hubble pictures from both epochs were taken with the Wide Field Planetary Camera 2.
Credit: NASA, ESA, and A. Gal-Yam (Weizmann Institute of Science, Israel)

NASA's Hubble Space Telescope has identified a star that was one million times brighter than the sun before it exploded as a supernova in 2005. According to current theories of stellar evolution, the star should not have self-destructed so early in its life. "This might mean that we are fundamentally wrong about the evolution of massive stars, and that theories need revising," says Avishay Gal-Yam of the Weizmann Institute of Science, Rehovot, Israel.

The doomed star, which is estimated to have weighed about 100 times our sun's mass, was not mature enough, according to theory, to have evolved a massive iron core of nuclear fusion ash. This is the prerequisite for a core implosion that triggers a supernova blast.

The finding appears today in the online version of Nature Magazine.

The explosion, called supernova SN 2005gl, was seen in the barred-spiral galaxy NGC 266 on October 5, 2005. Pre-explosion pictures from the Hubble archive, taken in 1997, reveal the progenitor as a very luminous point source with an absolute visual magnitude of -10.3.

The progenitor was so bright that it probably belonged to a class of stars called Luminous Blue Variables (LBVs), "because no other type of star is as intrinsically brilliant," says Gal-Yam. As an LBV-class star evolves it sheds much of its mass through a violent stellar wind. Only at that point does it develop a large iron core and ultimately explodes as a core-collapse supernova.

Extremely massive and luminous stars topping 100 solar masses, such as Eta Carinae in our own Milky Way Galaxy, are expected to lose their entire hydrogen envelopes prior to their ultimate explosions as supernovae. "These observations demonstrate that many details in the evolution and fate of LBVs remain a mystery. We should continue to keep an eye on Eta Carinae, it may surprise us yet again," says supernova expert Mario Livio of the Space Telescope Science Institute, Baltimore, Md.

"The progenitor identification shows that, at least in some cases, massive stars explode before losing most of their hydrogen envelope, suggesting that the evolution of the core and the evolution of the envelope are less coupled than previously thought, a finding which may require a revision of stellar evolution theory," says co-author Douglas Leonard from San Diego State University, Calif.

One possibility is that the progenitor to SN 2005gl was really a pair of stars, a binary system that merged. This would have stoked nuclear reactions to brighten the star enormously, making it look more luminous and less evolved than it really is. "This also leaves open the question that there may be other mechanisms for triggering supernova explosions," says Gal-Yam. "We may be missing something very basic in understanding how a superluminous star goes through mass loss."

Gal-Yam reports that the observation revealed that only a small part of the star's mass was flung off in the explosion. Most of the material, says Gal-Yam, was drawn into the collapsing core that has probably become a black hole estimated to be at least 10 to 15 solar masses.

Gal-Yam and Leonard located the progenitor in archival images of NGC 266 taken in 1997. It was easily identifiable only because it is so superluminous. Only Hubble could clearly resolve it at such a great distance.

The team then used the Keck telescope to precisely locate the supernova on the outer arm of the galaxy. A follow-up observation with Hubble in 2007 unequivocally showed that the superluminous star was gone. To make sure the new observation was consistent with the 1997 archival image, the astronomers used the same Hubble camera used in 1997, the Wide Field Planetary Camera 2.

Finding archival images of stars before the stars explode as supernovae isn't an easy task. Several other supernova progenitor candidates have been reported prior to the Hubble observation. The only other absolutely indisputable progenitor, however, was the blue supergiant progenitor to SN 1987A. In the case of SN 1987A, it was thought that the progenitor star was once a red supergiant and at a later stage evolved back to blue supergiant status. This led to a major reworking of supernova theory. The progenitor star observed by Gal-Yam is too massive to have gone through such an oscillation to the red giant stage, so yet another new explanation is required, he says.

CONTACT
Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4514
villard@stsci.edu

Avishay Gal-Yam
Weizmann Institute of Science, Rehovot, Israel
011-972-8-934-2063
avishay.gal-yam@weizmann.ac.il

Thursday, March 19, 2009

Finding Twin Earths: Harder Than We Thought!

Credit: David A. Aguilar (CfA)
This artist's conception shows a hypothetical twin Earth orbiting a Sun-like star. A new study shows that characterizing a distant Earth's atmosphere will be difficult, even using next-generation technology like the James Webb Space Telescope. If an Earth-like world is nearby, though, then by adding observations of a number of transits, astronomers should be able to detect biomarkers like methane or ozone.

Does a twin Earth exist somewhere in our galaxy? Astronomers are getting closer and closer to finding an Earth-sized planet in an Earth-like orbit. NASA's Kepler spacecraft just launched to find such worlds. Once the search succeeds, the next questions driving research will be: Is that planet habitable? Does it have an Earth-like atmosphere? Answering those questions will not be easy.

Due to its large mirror and location in outer space, the James Webb Space Telescope (scheduled for launch in 2013) will offer astronomers the first real possibility of finding those answers. In a new study, Lisa Kaltenegger (Harvard-Smithsonian Center for Astrophysics) and Wesley Traub (Jet Propulsion Laboratory) examined the ability of JWST to characterize the atmospheres of hypothetical Earth-like planets during a transit, when part of the light of the star gets filtered through the planet's atmosphere. They found that JWST would be able to detect certain gases called biomarkers, such as ozone and methane, only for the closest Earth-size worlds.

"We'll have to be really lucky to decipher an Earth-like planet's atmosphere during a transit event so that we can tell it is Earth-like," said Kaltenegger. "We will need to add up many transits to do so - hundreds of them, even for stars as close as 20 light-years away."

"Even though it's hard, it will be an incredibly exciting endeavor to characterize a distant planet's atmosphere," she added.

In a transit event, a distant, extrasolar planet crosses in front of its star as seen from Earth. As the planet transits, gases in its atmosphere absorb a tiny fraction of the star's light, leaving fingerprints specific to each gas. By splitting the star's light into a rainbow of colors or spectrum, astronomers can look for those fingerprints. Kaltenegger and Traub studied whether those fingerprints would be detectable by JWST.

Their study has been accepted for publication in The Astrophysical Journal.

The transit technique is very challenging. If Earth were the size of a basketball, the atmosphere would be as thin as a sheet of paper, so the resulting signal is incredibly tiny. Moreover, this method only works when the planet is in front of its star, and each transit lasts for a few hours at most.

Kaltenegger and Traub first considered an Earth-like world orbiting a Sun-like star. To get a detectable signal from a single transit, the star and planet would have to be extremely close to Earth. The only Sun-like star close enough is Alpha Centauri A. No such world has been found yet, but technology is only now becoming capable of detecting Earth-sized worlds.

The study also considered planets orbiting red dwarf stars. Such stars, called type M, are the most abundant in the Milky Way - far more common than yellow, type G stars like the Sun. They are also cooler and dimmer than the Sun, as well as smaller, which makes finding an Earth-like planet transiting an M star easier.

An Earth-like world would have to orbit close to a red dwarf to be warm enough for liquid water. As a result, the planet would orbit more quickly and each transit would last a couple of hours to mere minutes. But it would undergo more transits in a given amount of time. Astronomers could improve their chances of detecting the atmosphere by adding the signal from several transits, making red dwarf stars appealing targets because of their more frequent transits.

An Earth-like world orbiting a star like the Sun would undergo a 10-hour transit once every year. Accumulating 100 hours of transit observations would take 10 years. In contrast, an Earth orbiting a mid-sized red dwarf star would undergo a one-hour transit once every 10 days. Accumulating 100 hours of transit observations would take less than three years.

"Nearby red dwarf stars offer the best possibility of detecting biomarkers in a transiting Earth's atmosphere," said Kaltenegger.

"Ultimately, direct imaging - studying photons of light from the planet itself - may prove a more powerful method of characterizing the atmosphere of Earth-like worlds than the transit technique," said Traub.

Both NASA's Spitzer and Hubble Space Telescopes have studied the atmospheric compositions of extremely hot, gas-giant extrasolar planets. The characterization of a "pale blue dot" is the next step from there, whether by adding up hundreds of transits of one planet or by blocking out the starlight and analyzing the planet's light directly.

In a best-case scenario, Alpha Centauri A may turn out to have a transiting Earth-like planet that no one has spotted yet. Then, astronomers would need only a handful of transits to decipher that planet's atmosphere and possibly confirm the existence of the first twin Earth.

This research was partially funded by NASA.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.

For more information, contact:

David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu

Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu

NASA's Fermi Mission, Namibia's HESS Telescopes Explore a Blazar

Credit: NASA/Goddard Space Flight Center Conceptual Image Lab

About this image: In the heart of an active galaxy, matter falling into a supermassive black hole somehow creates jets of particles traveling near the speed of light. For active galaxies classified as blazars, one of these jets beams right toward Earth.
Click to view animation

The four identical telescopes of the High Energy Stereoscopic System in Namibia detect faint atmospheric flashes caused by the absorption of ultrahigh-energy gamma rays. Credit: H.E.S.S

An international team of astrophysicists using telescopes on the ground and in space have uncovered surprising changes in radiation emitted by an active galaxy. The picture that emerges from these first-ever simultaneous observations with optical, X-ray and new-generation gamma-ray telescopes is much more complex than scientists expected and challenges current theories of how the radiation is generated.

The galaxy in question is PKS 2155-304, a type of object known as a "blazar." Like many active galaxies, a blazar emits oppositely directed jets of particles traveling near the speed of light as matter falls into a central supermassive black hole; this process is not well understood. In the case of blazars, the galaxy is oriented such that we're looking right down the jet.

PKS 2155-304 is located 1.5 billion light-years away in the southern constellation of Piscis Austrinus and is usually a detectable but faint gamma-ray source. But when its jet undergoes a major outburst, as it did in 2006, the galaxy can become the brightest source in the sky at the highest gamma-ray energies scientists can detect -- up to 50 trillion times the energy of visible light. Even from strong sources, only about one gamma ray this energetic strikes a square yard at the top of Earth's atmosphere each month.

Atmospheric absorption of one of these gamma rays creates a short-lived shower of subatomic particles. As these fast-moving particles rush through the atmosphere, they produce a faint flash of blue light. The High Energy Stereoscopic System (H.E.S.S), an array of telescopes located in Namibia, captured these flashes from PKS 2155-304.

Gamma rays at lower energies were detected directly by the Large Area Telescope (LAT) aboard NASA's orbiting Fermi Gamma-ray Space Telescope. "The launch of Fermi gives us the opportunity to measure this powerful galaxy across as many wavelengths as possible for the first time," says Werner Hofmann, spokesperson for the H.E.S.S. team at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany.

With the gamma-ray regime fully covered, the team turned to NASA's Swift and Rossi X-ray Timing Explorer (RXTE) satellites to provide data on the galaxy's X-ray emissions. Rounding out the wavelength coverage was the H.E.S.S. Automatic Telescope for Optical Monitoring, which recorded the galaxy's activity in visible light.

Between August 25 and September 6, 2008, the telescopes monitored PKS 2155-304 in its quiet, non-flaring state. The results of the 12-day campaign are surprising. During flaring episodes of this and other blazars, the X- and gamma-ray emission rise and fall together. But it doesn't happen this way when PKS 2155-304 is in its quiet state -- and no one knows why.

What's even stranger is that the galaxy's visible light rises and falls with its gamma-ray emission. "It's like watching a blowtorch where the highest temperatures and the lowest temperatures change in step, but the middle temperatures do not," says Berrie Giebels, an astrophysicist at France's École Polytechnique who works with both the H.E.S.S. and Fermi LAT teams.

"Astronomers are learning that the various constituents of the jets in blazars interact in fairly complicated ways to produce the radiation that we observe," says Fermi team member Jim Chiang at Stanford University, Calif. "These observations may contain the first clues to help us untangle what's really going on deep in the heart of a blazar."

The findings have been submitted to The Astrophysical Journal.

The H.E.S.S. team includes scientists from Germany, France, the United Kingdom, Poland, the Czech Republic, Ireland, Armenia, South Africa and Namibia. The Fermi mission is an astrophysics and particle physics partnership, developed by NASA in collaboration with the U.S. Department of Energy, along with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the United States.


Francis Reddy
NASA's Goddard Space Flight Center

Tuesday, March 17, 2009

Quadruple Saturn Moon Transit Snapped by Hubble

Saturn's comparatively paper-thin rings are tilted edge on to Earth every 15 years. Because the orbits of Saturn's major satellites are in the ring plane, too, this alignment gives astronomers a rare opportunity to capture a truly spectacular parade of celestial bodies crossing the face of Saturn. Leading the parade is Saturn's giant moon Titan – larger than the planet Mercury. The frigid moon’s thick nitrogen atmosphere is tinted orange with the smoggy byproducts of sunlight interacting with methane and nitrogen. Several of the much smaller icy moons that are closer in to the planet line up along the upper edge of the rings. Hubble’s exquisite sharpness also reveals Saturn's banded cloud structure.

Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Acknowledgment: M.H. Wong (STScI/UC Berkeley) and C. Go (Philippines)

About this image: On February 24, 2009, the Hubble Space Telescope took a photo of four moons of Saturn passing in front of their parent planet. In this view, the giant orange moon Titan casts a large shadow onto Saturn's north polar hood. Below Titan, near the ring plane and to the left is the moon Mimas, casting a much smaller shadow onto Saturn's equatorial cloud tops. Farther to the left, and off Saturn's disk, are the bright moon Dione and the fainter moon Enceladus.

These rare moon transits only happen when the tilt of Saturn's ring plane is nearly "edge on" as seen from the Earth. Saturn's rings will be perfectly edge on to our line of sight on August 10, 2009, and September 4, 2009. Unfortunately, Saturn will be too close to the sun to be seen by viewers on Earth at that time. This "ring plane crossing" occurs every 14-15 years. In 1995-96 Hubble witnessed the ring plane crossing event, as well as many moon transits, and even helped discover several new moons of Saturn.

The banded structure in Saturn's atmosphere is similar to Jupiter's.

Early 2009 was a favorable time for viewers with small telescopes to watch moon and shadow transits crossing the face of Saturn. Titan, Saturn's largest moon, crossed Saturn on four separate occasions: January 24, February 9, February 24, and March 12, although not all events were visible from all locations on Earth.

These pictures were taken with Hubble's Wide Field Planetary Camera 2 on February 24, 2009, when Saturn was at a distance of roughly 775 million miles (1.25 billion kilometers) from Earth. Hubble can see details as small as 190 miles (300 km) across on Saturn. The dark band running across the face of the planet slightly above the rings is the shadow of the rings cast on the planet.

Annotated Image of Saturn's Rings and Moons
Illustration Credit: NASA, ESA, and Z. Levay (STScI)
Photo Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Acknowledgment: M.H. Wong (STScI/UC Berkeley) and C. Go (Philippines)

Photo Sequence of Saturn: February 24, 2009
Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Acknowledgment: M.H. Wong (STScI/UC Berkeley) and C. Go (Philippines)

Saturn's Rings Viewed from Earth (1)
Illustration Credit: NASA, ESA, and Z. Levay (STScI)
Photo Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)
Acknowledgment: M.H. Wong (STScI/UC Berkeley) and C. Go (Philippines)

About this Image (1) : As Saturn travels around its orbit, Hubble sees a different view of the tilted rings from a position near Earth. The rings nearly disappear twice during Saturn's approximately 30-year orbit, because we see them edge on and they are extremely thin relative to their diameter.

Monday, March 16, 2009

A Curious Pair of Galaxies

ESO PR Photo 11a/09
A Curious Pair of Galaxies

ESO PR Video 11a/09
Arp 261 zoom in

video
Pan over Arp 261

The ESO Very Large Telescope has taken the best image ever of a strange and chaotic duo of interwoven galaxies. The images also contain some surprises — interlopers both far and near.

Sometimes objects in the sky that appear strange, or different from normal, have a story to tell and prove scientifically very rewarding. This was the idea behind Halton Arp’s catalogue of Peculiar Galaxies that appeared in the 1960s. One of the oddballs listed there is Arp 261, which has now been imaged in more detail than ever before using the FORS2 instrument on ESO’s Very Large Telescope. The image proves to contain several surprises.

Arp 261 lies about 70 million light-years distant in the constellation of Libra, the Scales. Its chaotic and very unusual structure is created by the interaction of two galaxies that are engaged in a slow motion, but highly disruptive close encounter. Although individual stars are very unlikely to collide in such an event, the huge clouds of gas and dust certainly do crash into each other at high speed, leading to the formation of bright new clusters of very hot stars that are clearly seen in the picture. The paths of the existing stars in the galaxies are also dramatically disrupted, creating the faint swirls extending to the upper left and lower right of the image. Both interacting galaxies were probably dwarfs not unlike the Magellanic Clouds orbiting our own galaxy.

The images used to create this picture were not actually taken to study the interacting galaxies at all, but to investigate the properties of the inconspicuous object just to the right of the brightest part of Arp 261 and close to the centre of the image. This is an unusual exploding star, called SN 1995N, that is thought to be the result of the final collapse of a massive star at the end of its life, a so-called core collapse supernova. SN 1995N is unusual because it has faded very slowly — and still shows clearly on this image more than seven years after the explosion took place! It is also one of the few supernovae to have been observed to emit X-rays. It is thought that these unusual characteristics are a result of the exploding star being in a dense region of space so that the material blasted out from the supernova ploughs into it and creates X-rays.

Apart from the interacting galaxy and its supernova the image also contains several other objects at wildly different distances from us. Starting very close to home, two small asteroids, in our Solar System between the orbits of Mars and Jupiter, happened to cross the images as they were being taken and show up as the red-green-blue trails at the left and top of the picture. The trails arise as the objects are moving during the exposures and also between the exposures through different coloured filters. The asteroid at the top is number 14670 and the one to the left number 9735. They are probably less than 5 km across. The reflected sunlight from these small bodies takes about fifteen minutes to get to the Earth.

The next closest object is probably the apparently bright star at the bottom. It may look bright, but it is still about one hundred times too faint to be seen with the unaided eye. It is most likely a star rather like the Sun and about 500 light-years from us — 20 million times further away than the asteroids. Arp 261 itself, and the supernova, are about 140 000 times further away again than this star, but still in what astronomers would regard as our cosmic neighbourhood. Much more distant still, perhaps some fifty to one hundred times further away than Arp 261, lies the cluster of galaxies visible on the right of the picture. There is no doubt, however, that a much more remote object lies, unrecognised, amongst the faint background objects seen in this marvellous image.

Contact
Henri Boffin
ESO education and Public Outreach Department

Phone: +49 89 3200 6222

E-mail:
hboffin@eso.org

ESO Press Officer in Chile: Valentina Rodriguez - +56 2 463 3123 - vrodrigu@eso.org

Hearts of Galaxies Close in for Cosmic Train Wreck

A new image from NASA's Spitzer Space Telescope offers a rare view of an imminent collision between the cores of two merging galaxies, each powered by a black hole with millions of times the mass of the sun.

The galactic cores are in a single, tangled galaxy called NGC 6240, located 400-million light years away in the constellation Ophiuchus. Millions of years ago, each core was the dense center of its own galaxy before the two galaxies collided and ripped each other apart. Now, these cores are approaching each other at tremendous speeds and preparing for the final cataclysmic collision. They will crash into each other in a few million years, a relatively short period on a galactic timescale.

The spectacular image combines visible light from NASA's Hubble Space Telescope and infrared light from Spitzer. It catches the two galaxies during a rare, short-lived phase of their evolution, when both cores of the interacting galaxies are still visible but closing in on each other fast.

"One of the most exciting things about the image is that this object is unique," said Stephanie Bush of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., lead author of a new paper describing the observation in an upcoming issue of the Astrophysical Journal. "Merging is a quick process, especially when you get to the train wreck that is happening. There just aren't many galactic mergers at this stage in the nearby universe."

NGC 6240 is already putting out huge amounts of infrared light, an indication that a burst of star formation is underway. The extra infrared radiation is common in interacting galaxies; as the two galaxies interact, dust and gas swept up by the collision form a burst of new stars that give off infrared light. Such galaxies are called luminous infrared galaxies. Spitzer's infrared array camera can image the extra heat from newly formed stars, even though their visible light is obscured by thick dust clouds around them.

The blob-like shape of the galaxy is due to the sustained violence of the collision. Streams of millions of stars are being ripped off the galaxy, forming wispy "tidal tails" that lead off NGC 6240 in several directions. But things are about to get even more violent as the main event approaches and the two galactic cores meld into one.

In the center of NGC 6240, the two black holes in the cores will whip up a frenzy of radiation as they careen towards one another head-on, likely transforming the galaxy into a monster known as an ultra-luminous infrared galaxy, thousands of times as bright in infrared as our Milky Way.

Another fascinating aspect of this rare object is that no two galactic mergers are the same. "Not only are there few objects at this stage, but each object is unique because it came from different progenitor galaxies," said Bush. "These observations give us another layer of information about this galaxy, and galactic mergers in general."

Infrared light taken by Spitzer's infrared array camera at 3.6 and 8.8 microns (red) shows cold dust and radiation from star formation; visible light from Hubble (green and blue) shows hot gas and stars.

Other authors of this paper include Zhong Wang, Margarita Karovska and Giovanni Fazio, all of the Harvard-Smithsonian Center for Astrophysics. NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

Whitney Clavin 818-354-4673
Jet Propulsion Laboratory, Pasadena, Calif.

whitney.clavin@jpl.nasa.gov
ssc2009-06
jpl2009-050


About the Object
  • Object name: NGC 6240
  • Object type: Galaxy
  • Position (J2000): RA: 16h 52m 58.97s Dec: 2° 24' 1.70"
  • Distance: 400,000,000 Light Years
  • Constellation: Ophiuchus

About the Data - Spitzer Data
  • Image Credit: NASA/JPL-Caltech/STScI-ESA
  • Instrument: IRAC
  • Wavelength: 3.6 and 8.0 microns (red), visible light (green and blue)