Vital clues about the devastating ends to the lives of massive stars can be found by studying the aftermath of their explosions. In its more than twelve years of science operations, NASA's Chandra X-ray Observatory has studied many of these supernova remnants sprinkled across the Galaxy. Credit: X-ray: NASA/CXC/SAO/I.Lovchinsky et al, IR: NASA/JPL-Caltech
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Located some 14,700 light years from the Earth toward the center of our galaxy, a newly photographed supernova remnant cataloged as G350.1+0.3 is making astronomers scratch their heads. The star which created this unusual visage is suspected to have blown its top some 600 to 1,200 years ago. Although it would have been as bright as the event which created the “Crab”, chances are no one saw it due to the massive amounts of gas and dust at the Milky Way’s heart. Now NASA’s Chandra X-ray Observatory and the ESA’s XMM-Newton telescope has drawn back the curtain and we’re able to marvel at what happens when a supernova imparts a powerful X-ray “kick” to a neutron star!
Credit: X-ray: NASA/CXC/SAO/I.Lovchinsky et al, IR: NASA/JPL-CaltechPhotographic proof from Chandra and XMM-Newton are full of clues which give rise to the possibility that a compact object located in the influence of G350.1+0.3 may be the core region of a shattered star. Since it is off-centered from the X-ray emissions, it must have received a powerful blast of energy during the supernova event and has been moving along at a speed of 3 million miles per hour ever since. This information agrees with an “exceptionally high speed derived for the neutron star in Puppis A and provides new evidence that extremely powerful ‘kicks’ can be imparted to neutron stars from supernova explosions.”
As you look at the photo, you’ll notice one thing in particular… the irregular shape. The Chandra data in this image appears as gold while the infrared data from NASA’s Spitzer Space Telescope is colored light blue. According to the research team, this unusual configuration may have been caused by the stellar debris field imparting itself into the surrounding cold molecular gas.
These results appeared in the April 10, 2011 issue of The Astrophysical Journal. The scientists on this paper were Igor Lovchinsky and Patrick Slane (Harvard-Smithsonian Center for Astrophysics), Bryan Gaensler (University of Sydney, Australia), Jack Hughes (Rutgers University), Stephen Ng (McGill University), Jasmina Lazendic (Monash University Clayton, Australia), Joseph Gelfand (New York University, Abu Dhabi), and Crystal Brogan (National Radio Astronomy Observatory).
An artist's concept of NuSTAR in space. Image credit: NASA/JPL-Caltech/Orbital
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On March 14, NASA will launch the Nuclear Spectroscopic Telescope Array or NuSTAR. This is the first time a telescope will focus on high energy X-rays, effectively opening up the sky for more sensitive study. The telescope will target black holes, supernova explosions, and will study the most extreme active galaxies. NuSTAR’s use of high-energy X-rays have an added bonus: it will be able to capture and compose the most detailed images ever taken in this end of the electromagnetic spectrum.
NuSTAR’s eyes are two Wolter-I optic units; once in orbit each will ‘look’ at the same patch of sky. The Wolter-I mirror works by reflecting an X-ray twice, once off of an upper mirror shaped like a parabola and again off a lower mirror shaped like a hyperbola. The mirrors are nearly parallel to the direction of the incoming X-ray, reflecting most of the X-ray instead of absorbing it, but the slight angle allows for a very small collection area per surface. To get a full picture, mirrors of varying size are nested together.
Technicians work on NuSTAR this month at the Orbital Science Corporation in Dulles, Virginia. Image credit: NASA/JPL-Caltech/Orbital
Each of NuSTAR’s eyes, each unit, are made of 133 concentric shells of mirrors shaped from flexible glass like that found in laptop computer screens. This is an improvement over past missions like Chandra and XMM-Newton that both used high density materials such as Platinum, Iridium and Gold as mirror coatings. These materials achieve great reflectivity for low energy X-rays but can’t capture high energy X-rays.
Like human eyes, NuSTAR’s optical units are co-aligned to give the telescope a wider field of view and enable the capture of more sensitive images. These images will be made into detailed composites by scientists on the ground.
Also like human eyes, NuSTAR’s optical units need to be distanced from one another since X-ray telescopes require long focal lengths. In other words, the optics must be separated by several meters from the detectors. NuSTAR does this with a 33 foot (10 metre) long mast or boom between units.
Previous X-ray missions have accommodated these long focal lengths by launching fully deployed observatories on large rockets. NuSTAR won’t. It has a unique deployable mast that will extend once the payload is in orbit. This allows for a launch on the small Pegasus rocket. Undeployed, the telescope measures just 2 metres in length and one metre in diameter.
During its two-year primary mission, NuSTAR will map the celestial sky focussing on black holes, supernova remnants, and particle jets traveling near the speed of light. It will also look at the Sun. Observations of microflares could explain the temperature of the Sun’s corona. It will also search the Sun for evidence of a hypothesized dark matter particle to test a theory about dark matter.
“NuSTAR will provide an unprecedented capability to discover and study some of the most exotic objects in the universe, from the corpses of exploded stars in the Milky Way to supermassive black holes residing in the hearts of distant galaxies,” said Lou Kaluzienski, NuSTAR program scientist at NASA Headquarters in Washington.
The telescope shipped from the Orbital Sciences Corporation in Dulles, Virginia to Vandenberg Air Force Base in California on January 27. There, it will be mated to its Pegasus launch vehicle on February 17. It will launch from underneath the L-1011 “Stargazer” aircraft on March 14 after taking off near the equator from Kwajalein Atoll in the Pacific.
A new look at M16, the Eagle Nebula in this composite from the Herschel telescope in far-infrared and XMM-Newton in X-ray. Credits: far-infrared: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium; X-ray: ESA/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger
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Here’s a stunning new look deep inside the iconic “Pillars of Creation.” As opposed to the famous Hubble Space Telescope image (below) — which shows mainly the surface of the pillars of gas and dust — this composite image from ESA’s Herschel Space Observatory in far-infrared and XMM-Newton telescope in X-rays allows astronomers to peer inside the pillars and see more detail of the structures in this region. It shows how the hot young stars detected by the X-ray observations are carving out cavities, sculpting and interacting with the surrounding ultra-cool gas and dust.
But enjoy the view while you can. The sad part is that likely, this beautiful region has already been destroyed by a supernova 6,000 years ago. But because of the distance, we haven’t seen it happen yet.
Gas Pillars in the Eagle Nebula, as seen by the Hubble Space Telescope. Credit: NASA/ESA/STScI, Hester & Scowen (Arizona State University)
The Eagle Nebula is 6,500 light-years away in the constellation of Serpens. It contains a young hot star cluster, NGC6611, which is visible with modest back-yard telescopes. This cluster is sculpting and illuminating the surrounding gas and dust, resulting in a huge hollowed-out cavity and pillars, each several light-years long.
The Hubble image hinted at new stars being born within the pillars, deep inside small clumps known as ‘evaporating gaseous globules’ or EGGs, but because of the obscuring dust, Hubble’s visible light picture was unable to see inside and prove that young stars were indeed forming.
The new image shows those hot young stars are responsible for carving the pillars.
The new image also uses data from near-infrared images from the European Southern Observatory’s (ESO’s) Very Large Telescope at Paranal, Chile, and visible-light data from its Max Planck Gesellschaft 2.2m diameter telescope at La Silla, Chile. All the individual images are below:
M16 seen in several different wavelengths. Credits: far-infrared: ESA/Herschel/PACS/SPIRE/Hill, Motte, HOBYS Key Programme Consortium; ESA/XMM-Newton/EPIC/XMM-Newton-SOC/Boulanger; optical: MPG/ESO; near-infrared/VLT/ISAAC/McCaughrean & Andersen/AIP/ESO
Earlier mid-infrared images from ESA’s Infrared Space Observatory and NASA’s Spitzer, and the new XMM-Newton data, have led astronomers to suspect that one of the massive, hot stars in NGC6611 may have exploded in a supernova 6,000 years ago, emitting a shockwave that destroyed the pillars. But we won’t see the destruction for several hundred years yet.
This 327-MHz radio view of the center of our galaxy highlights the position of the black hole system H1743-322, as well as other features. (Credit: J. Miller-Jones, ICRAR-Curtin Univ.; C. Brogan, NRAO)
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In mid-2009 a binary star system cataloged as H H1743–322 shot off something very unusual. Poised about 28,000 light years distant in the direction of the constellation of Scorpius, this rather ordinary system made up of a normal star and unknown mass black hole was busy exchanging mass. The pair orbits in mere days with a stream of material flowing continuously between them. This gas causes a flat accretion disk measuring millions of miles across to form and it is centered on the black hole. As the matter twirls toward the center, it becomes compressed and heats to tens of millions of degrees, spitting out X-rays… and bullets.
Utilizing data from NASA’s Rossi X-ray Timing Explorer (RXTE) satellite and the National Science Foundation’s (NSF) Very Long Baseline Array (VLBA) radio telescope, an international team of astronomers were able to confirm the moment a black hole located within our galaxy fired a super speedy clump of gas into surrounding space. Blasting forth at about one-quarter the speed of light, these “bullets” of ionized gas are hypothesized to have originated from an area just outside the black hole’s event horizon.
“Like a referee at a sports game, we essentially rewound the footage on the bullets’ progress, pinpointing when they were launched,” said Gregory Sivakoff of the University of Alberta in Canada. He presented the findings today at the American Astronomical Society meeting in Austin, Texas. “With the unique capabilities of RXTE and the VLBA, we can associate their ejection with changes that likely signaled the start of the process.”
As we have learned, some of the matter headed toward the center of a black hole can be ejected from the accretion disk as opposing twin jets. For the most part, these jets are a constant stream of particles, but can sometimes form into strong “outflows” which get spit out – rapid fire – as gaseous blobs. In early June 2009, H1743–322 did just that… and astronomers were on hand observing with RXTE, the VLBA, the Very Large Array near Socorro, N.M., and the Australia Telescope Compact Array (ATCA) near Narrabri in New South Wales. During this time they were able to confirm the happenings through X-ray and radio data. From May 28 to June 2, things were nominal “though RXTE data show that cyclic X-ray variations, known as quasi-periodic oscillations or QPOs, gradually increased in frequency over the same period” and by June 4th, ATCA verified that activity had pretty much sloughed off. By June 5th, even the QPOs were gone.
Then it happened…
On the same day that everything went totally quiet, H1743–322 fired off a bullet! Radio emissions jumped and a highly accurate and detailed VLBA image disclosed a energetic missile of gas blasting forth along a jet trajectory. The very next day a second bullet took out in the opposite direction. But this wasn’t the curious part of the event… It was the timing. Up to this point, researchers speculated that a radio outburst accompanied the firing of the gas bullet, but VLBA information showed they were launched around 48 hours in advance of the major radio flare. This information will be published in the Monthly Notices of the Royal Astronomical Society.
Radio imaging by the Very Long Baseline Array (top row), combined with simultaneous X-ray observations by NASA's RXTE (middle), captured the transient ejection of massive gas "bullets" by the black hole binary H1743-322 during its 2009 outburst. By tracking the motion of these bullets with the VLBA, astronomers were able to link the ejection event to the disappearance of X-ray signals seen in RXTE data. These signals, called quasi-periodic oscillations (QPOs), vanished two days earlier than the onset of the radio flare that astronomers previously had assumed signaled the ejection. (Credit: NRAO and NASA's Goddard Space Flight Center)
“This research provides new clues about the conditions needed to initiate a jet and can guide our thinking about how it happens,” said Chris Done, an astrophysicist at the University of Durham, England, who was not involved in the study.
These are just mini-ammo compared to what happens in the center of an active galaxy. They don’t just fire bullets – they blast off cannons. A massive black hole weighing in a millions to billions of times the mass of the Sun can shoot off its load across millions of light years!
“Black hole jets in binary star systems act as fast-forwarded versions of their galactic-scale cousins, giving us insights into how they work and how their enormous energy output can influence the growth of galaxies and clusters of galaxies,” said lead researcher James Miller-Jones at the International Center for Radio Astronomy Research at Curtin University in Perth, Australia.
Technicians work on RXTE in 1995. Credit: NASA/Goddard
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For more than 16 years, 2,200 papers in refereed journals, 92 doctoral theses, and more than 1,000 rapid notifications alerting astronomers around the globe to new astronomical activity, the NASA Rossi X-Ray Timing Explorer is now retired. It sent the last of its data on January 4th of this year and on January 5th the plucky little satellite was decommissioned. If you’re not familiar with Rossi’s activities, then picture sending back images and data on the extreme environments around white dwarfs, neutron stars and black holes… because that’s what made the mission famous.
On December 30, 1995, the mission was launched as XTE from Cape Canaveral, Florida on board a Delta II 7920 rocket. Within weeks it was named in honor of Bruno Rossi, an MIT astronomer and a pioneer of X-ray astronomy and space plasma physics who died in 1993. However, the mission itself didn’t die – it excelled with honors. The entire scientific community recognized the importance of RXTE research and bestowed it with five awards – four Rossi Prizes (1999, 2003, 2006 and 2009) from the High Energy Astrophysics Division of the AAS and the 2004 NWO Spinoza prize, the highest Dutch science award, from the Netherlands Organization for Scientific Research.
On board, the Rossi was three scientific instruments housed in one unit. The first was the Proportional Counter Array (PCA), which was centered on the lower end of the energy band and was crafted by Goddard. The second instrument was the High Energy X-Ray Timing Experiment (HEXTE) that could be aimed at very specific targets and was manufactured by the University of California at San Diego for exploring the upper energy range. The last of the trio was the All-Sky Monitor developed by the Massachusetts Institute of Technology (MIT) in Cambridge. It took in about 80% of the sky during each orbit, delivering astronomers with an unprecedented amount of data on the wide variances of X-Ray sky and allowing them to record bright sources over a period of time as short as a few microseconds up to months. All of this information was taken in over a broad span of energy ranging from 2,000 to 250,000 electron volts.
The Rossi X-Ray Timing Explorer asked little and returned much. Over its operating lifetime it gave us new insight in the life cycles of neutron stars and black holes. Through its eyes we learned about magnetars and discovered the first accreting millisecond pulsar. But that’s not all. The RXTE provided hard evidence which supported Einstein’s theory by observing “frame dragging” in the neighborhood of a black hole. Even though the instrumentation would be considered antique by today’s standards, it certainly served its purpose. “The spacecraft and its instruments had been showing their age, and in the end RXTE had accomplished everything we put it up there to do, and much more,” said Tod Strohmayer, RXTE project scientist at Goddard.
According to the NASA news release, the decision to decommission RXTE followed the recommendations of a 2010 review board tasked to evaluate and rank each of NASA’s operating astrophysics missions. The three and a half ton satellite is expected to return to Earth sometime between the years 2014 and 2023, depending on solar activity. It will have a fiery end… burning out like the superstar that it was. To celebrate its career, the scientific community will hold a special session on RXTE during the 219th meeting of the American Astronomical Society (AAS) in Austin, Texas. The session is scheduled for Tuesday, January 10, at 3 p.m. CST. A press conference on new RXTE results will also be held at the meeting on January 10 at 1:45 p.m. EST. The decision to decommission RXTE followed the recommendations of a 2010 review board tasked to evaluate and rank each of NASA’s operating astrophysics missions. “After two days we listened to verify that none of the systems we turned off had autonomously re-activated, and we’ve heard nothing,” said Deborah Knapp, RXTE mission director at Goddard.
A pulsar within a supernova remnant in the Small Magellanic Cloud. X-rays are blue; optical data is red and green. (NASA/CXC/Univ.Potsdam/L.Oskinova et al.)
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For the first time astronomers have located a pulsar – the super-dense, spinning remains of a star – nestled within the remnants of a supernova in the Small Magellanic Cloud. The image above, a composite of x-ray and optical light data acquired by NASA’s Chandra Observatory and ESA’s XMM-Newton, shows the pulsar shining brightly on the right surrounded by the ejected outer layers of its former stellar life.
The optically-bright area on the left is a large star-forming region of dust and gas nearby SXP 1062.
A pulsar is a neutron star that emits high-energy beams of radiation from its magnetic poles. These poles are not always aligned with its axis of rotation, and so the beams swing through space as the neutron star spins. If the Earth happens to be in direct line with the beams at some point along their path, we see them as rapidly flashing radiation sources… sort of like a cosmic lighthouse on overdrive.
What’s unusual about this pulsar – called SXP 1062 – is its slow rate of rotation. Its beams spin around at a rate of about once every 18 minutes, which is downright poky for a pulsar, most of which spin several times a second.
X-ray image of SXP 1062
This makes SXP 1062 one of the slowest known pulsars discovered within the Small Magellanic Cloud, a dwarf galaxy cruising alongside our own Milky Way about 200,000 light-years distant.
The supernova that presumably created the pulsar and its surrounding remnant wrapping is estimated to have taken place between 10,000 and 40,000 years ago – relatively recently, by cosmic standards. To see a young pulsar spinning so slowly is extra unusual since younger pulsars have typically been observed to have rapid rotation rates. Understanding the cause of its leisurely pace will be the next goal for SXP 1062 researchers.
Artist's rendering showing the jet fully established. Credit: NASA/Goddard Space Flight Center/CI Lab
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Is everything quiet in deep space? Not hardly. It’s a place jammed with noises of all kinds. So much noise, in fact, that it could be quite difficult to pick up a faint signature of something small… something like the smallest black hole known. Thanks to NASA’s Rossi X-ray Timing Explorer (RXTE) , an international team of astronomers have found the pulse they were looking for and it’s a pattern that’s only been seen in one other black hole system.
Its name is IGR J17091-3624 and it’s a binary system which consists of a normal star and a black hole with a mass that measures only about three times solar. In theoretical terms, that’s right at the edge where possibility of being a black hole begins.
Here’s the picture… In this binary system, escaping gas from the “normal” star flows across space in the direction of the black hole. This action creates a disk where friction heats it to millions of degrees – releasing X-rays. Periodic changes in the strength of the X-ray emissions point towards the actions taking place within the gas disk. Scientists theorize that fast changes occur at the event horizon… the point of no return.
IGR J17091-3624 was discovered when it went into outburst in 2003. Current observations have it becoming active every few years and its most recent flare began in February of this year and has been kicking up cosmic dust ever since. Observations place it in the general direction of Scorpius, but astronomers aren’t sure of an exact distance – somewhere between 16,000 light years to more than 65,000. However, IGR J17091-3624 isn’t absolutely alone in its unique changes. Black hole binary, GRS 1915+105, also displays a number of well-ordered rhythms, too.
This animation compares the X-ray ‘heartbeats’ of GRS 1915 and IGR J17091, two black holes that ingest gas from companion stars. GRS 1915 has nearly five times the mass of IGR J17091, which at three solar masses may be the smallest black hole known. A fly-through relates the heartbeats to hypothesized changes in the black hole’s jet and disk. Credit: NASA/Goddard Space Flight Center/CI Lab
“We think that most of these patterns represent cycles of accumulation and ejection in an unstable disk, and we now see seven of them in IGR J17091,” said Tomaso Belloni at Brera Observatory in Merate, Italy. “Identifying these signatures in a second black hole system is very exciting.”
Binary GRS 1915 has some very cool characteristics. Right now astronomers have observed jets blasting out in opposite directions cruising along at 98% the speed of light. These originate at the event horizon where strong magnetic fields fuel them and each pulsation matches the occurrence of the jets. By observing the X-ray spectrum with RXTE, researchers have discovered the interior of the disk creates enough radiation to halt the gas flow – an outward wind which negates the inward flow – and shuts down activity. As a result, the inner disk glows hot and bright, eliminating itself as it flows toward the black hole and kick starts the jet activity again. It’s a process that happens in as little as 40 seconds!
Right now astronomers aren’t able to prove that IGR J17091 has a particle jet, but the regular pulsations indicate it. Records show this “heartbeat” occurs about every five seconds – about 8 times faster than its counterpart and some 20 times more faint. Numbers like this would make it a very tiny black hole.
“Just as the heart rate of a mouse is faster than an elephant’s, the heartbeat signals from these black holes scale according to their masses,” said Diego Altamirano, an astrophysicist at the University of Amsterdam in The Netherlands and lead author of a paper describing the findings in the November 4 issue of The Astrophysical Journal Letters. It’s just the beginning of a full scale program involving RXTE to compare information from both black holes. Even more detailed data will be added from NASA’s Swift satellite and XMM-Newton, too.
“Until this study, GRS 1915 was essentially a one-off, and there’s only so much we can understand from a single example,” said Tod Strohmayer, the project scientist for RXTE at NASA’s Goddard Space Flight Center in Greenbelt, Md. “Now, with a second system exhibiting similar types of variability, we really can begin to test how well we understand what happens at the brink of a black hole.”
Don’t like spiders? Well, here’s one that will grow on you! Located about 160,000 light years in the web of the Large Magellanic Cloud, star-forming region 30 Doradus is best known as the “Tarantula Nebula”. But don’t let it “bug” you… this space-born arachnid is home to giant stars whose intense radiation causes stellar winds to blast through surrounding gases to give us an incredible view!
When seen through the eyes of the Chandra X-ray Observatory, these huge shockwaves of x-ray energy heat the encompassing gaseous environment up to multi-millions of degrees and show up as blue. The supernovae detonations blast their way outward… gouging out “bubbles” in the cooler gas and dust. They show up hued as orange when observed through infra-red emissions and recorded by the Spitzer Space Telescope.
What’s so special about the Tarantula? Because it is so close, it’s a prime candidate for studying an active HII region. This stellar nursery is the largest in our Local Group and a perfect laboratory for monitoring stellar evolution. Right now astronomers are intensely interested in what causes growth on such a large scale – and their curent findings show it doesn’t have anything to do with pressure and radiation from the massive stars. However, an earlier study had opposing conclusions when it came to 30 Doradus’ central regions. By employing the Chandra Observatory observations, we may just find different opinions!
“Observations show that star formation is an inefficient and slow process. This result can be attributed to the injection of energy and momentum by stars that prevents free-fall collapse of molecular clouds. The mechanism of this stellar feedback is debated theoretically; possible sources of pressure include the classical warm H II gas, the hot gas generated by shock heating from stellar winds and supernovae, direct radiation of stars, and the dust-processed radiation field trapped inside the H II shell.” says Laura Lopez (et al). “By contrast, the dust-processed radiation pressure and hot gas pressure are generally weak and not dynamically important, although the hot gas pressure may have played a more significant role at early times.”
This is an artist's concept of a pulsar (blue-white disk in center) pulling in matter from a nearby star (red disk at upper right). The stellar material forms a disk around the pulsar (multicolored ring) before falling on to the surface at the magnetic poles. The pulsar's intense magnetic field is represented by faint blue outlines surrounding the pulsar. Credit: NASA
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Neutron stars have been classed as “undead”… real zombie stars. Even though technically defunct, the neutron star continues to shine – and occasionally feed on a neighbor if it gets too close. They are born when a massive star collapses under its gravity and its outer layers are blown far and wide, outshining a billion suns, in a supernova event. What’s left is a stellar corpse… a core of inconceivable density… where one teaspoon would weigh about a billion tons on Earth. How would we study such a curiosity? NASA has proposed a mission called the Neutron Star Interior Composition Explorer (NICER) that would detect the zombie and allow us to see into the dark heart of a neutron star.
The core of a neutron star is pretty incredible. Despite the fact that it has blown away most of its exterior and stopped nuclear fusion, it still radiates heat from the explosion and exudes a magnetic field which tips the scales. This intense form of radiation caused by core collapse measures out at over a trillion times stronger than Earth’s magnetic field. If you don’t think that impressive, then think of the size. Originally the star could have been a trillion miles or more in diameter, yet now is compressed to the size of an average city. That makes a neutron star a tiny dynamo – capable of condensing matter into itself at more than 1.4 times the content of the Sun, or at least 460,000 Earths.
“A neutron star is right at the threshold of matter as it can exist – if it gets any denser, it becomes a black hole,” says Dr. Zaven Arzoumanian of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We have no way of creating neutron star interiors on Earth, so what happens to matter under such incredible pressure is a mystery – there are many theories about how it behaves. The closest we come to simulating these conditions is in particle accelerators that smash atoms together at almost the speed of light. However, these collisions are not an exact substitute – they only last a split second, and they generate temperatures that are much higher than what’s inside neutron stars.”
If approved, the NICER mission will be launched by the summer of 2016 and attached robotically to the International Space Station. In September 2011, NASA selected NICER for study as a potential Explorer Mission of Opportunity. The mission will receive $250,000 to conduct an 11-month implementation concept study. Five Mission of Opportunity proposals were selected from 20 submissions. Following the detailed studies, NASA plans to select for development one or more of the five Mission of Opportunity proposals in February 2013.
This is an artist's concept of the NICER instrument on board the International Space Station. NICER is the cube in the foreground on the left. The circular objects protruding from the cube are telescopes that focus X-rays from the pulsar on to the detector. Credit: NASA
What will NICER do? First off, an array of 56 telescopes will gather X-ray information from a neutron stars magnetic poles and hotspots. It is from these areas that our zombie stars release X-rays, and as they rotate create a pulse of light – thereby the term “pulsar”. As the neutron star shrinks, it spins faster and the resultant intense gravity can pull in material from a closely orbiting star. Some of these pulsars spin so fast they can reach speeds of several hundred of rotations per second! What scientists are itching to understand is how matter behaves inside a neutron star and “pinning down the correct Equation Of State (EOS) that most accurately describes how matter responds to increasing pressure. Currently, there are many suggested EOSs, each proposing that matter can be compressed by different amounts inside neutron stars. Suppose you held two balls of the same size, but one was made of foam and the other was made of wood. You could squeeze the foam ball down to a smaller size than the wooden one. In the same way, an EOS that says matter is highly compressible will predict a smaller neutron star for a given mass than an EOS that says matter is less compressible.”
Now all NICER will need to do is help us to measure a pulsar’s mass. Once it is determined, we can get a correct EOS and unlock the mystery of how matter behaves under intense gravity. “The problem is that neutron stars are small, and much too far away to allow their sizes to be measured directly,” says NICER Principal Investigator Dr. Keith Gendreau of NASA Goddard. “However, NICER will be the first mission that has enough sensitivity and time-resolution to figure out a neutron star’s size indirectly. The key is to precisely measure how much the brightness of the X-rays changes as the neutron star rotates.”
So what else does our zombie star do that’s impressive? Because of their extreme gravity in such small volume, they distort space/time in accordance with Einstein’s theory of General Relativity. It is this space “warp” that allows astronomers to reveal the presence of a companion star. It also produces effects like an orbital shift called precession, allowing the pair to orbit around each other causing gravitational waves and producing measurable orbital energy. One of the goals of NICER is to detect these effects. The warp itself will allow the team to determine the neutron star’s size. How? Imagine pushing your finger into a stretchy material – then imagine pushing your whole hand against it. The smaller the neutron star, the more it will warp space and light.
Here light curves become very important. When a neutron star’s hotspots are aligned with our observations, the brightness increases as one rotates into view and dims as it rotates away. This results in a light curve with large waves. But, when space is distorted we’re allowed to view around the curve and see the second hotspot – resulting in a light curve with smoother, smaller waves. The team has models that produce “unique light curves for the various sizes predicted by different EOSs. By choosing the light curve that best matches the observed one, they will get the correct EOS and solve the riddle of matter on the edge of oblivion.”
Two examples of galaxy pairs in the COSMOS survey (courtesy of the Chandra X-ray Center). The Hubble Space Telescope images show galaxies undergoing a close encounter (shown in gold). X-rays, as detected by Chandra, indicate which of the two galaxies hosts an AGN. In addition, diffuse X-ray emission from hot gas is present thus highlighting that such galaxy associations tend to reside in galaxy groups, an environment of rapid galaxy and black hole growth.
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Yep. It’s true. Almost all galaxies are guilty of having a supermassive black hole in their centers. Some even tip the scales at millions – or even billions – of times more mass than the Sun. However, how they came to be so weighty is a true enigma. Thanks to research done by Dr. John Silverman (IPMU) and the international COSMOS team, the Chandra X-Ray Observatory and the European Southern Observatory’s Very Large Telescope have revealed that galaxy interactions may be responsible for the growth of supermassive black holes – and they’ve left behind some very important clues…
If you’re big – you’re big. As a general rule, supermassive black holes like to hang out in massive galaxies. Their mass is usually directly related to the central bulge. Now the consensus is that massive galaxies gained their girth (at least in part) by mergers and interactions with smaller galaxies. This act of cannibalism in galactic evolution has been postulated to explain how matter gathers toward the middle, eventually resulting in a supermassive black hole.
How do we determine this? One way is to take a closer look at galaxies currently in merger as compared to ones in isolation. While the concept is easy, carrying out the test hasn’t been. A supermassive black hole leaves visual observations “blinded by the light” while a quasar can effectively “outshine” an entire host galaxy, leaving an interactor almost impossible to detect. But, like a bulging waistline, such interactions should distort the overall contours of the galaxy.
Now the COSMOS team might have an answer to the riddle.. by assuming a galaxy is interacting if it has a nearby neighbor. It’s a test that can happen without needing to know if distortion is present in optical images. What makes it possible are accurate distance measurements of about 20,000 galaxies in the COSMOS field as provided by the zCOSMOS redshift survey with the European Southern Observatory’s Very Large Telescope. Isolated galaxies are used to give a comparison sample to lay the foundation as to whether an active galactic nucleus is common to interacting galaxies. With help from NASA’s Chandra Observatory, X-ray observations pinpoint galaxies which host an AGN. The X-ray emission signature dominates in growing SMBHs and X-rays are capable of cutting through the gas and dust of star-forming regions.
In their report to The Astrophysical Journal the team states that galaxies in close pairs are twice as likely to harbor AGNs as compared to galaxies in isolation. This answer may prove that beginning galaxy interactions can lead to “enhanced black hole growth”. Because it’s not a drastically common occcurrance, it means that only about 20% of SMBHs that break the scale happen via a merger event and that “final coalescence” might also play a role.
One thing we do know is that galaxies and their black holes, like people and their waistlines, all get a little heavier with time.
