A Magnetar at the Heart of Our Milky Way

This is an artist's impression of PSR J1745-2900, a pulsar with a very high magnetic field ("magnetar") in direct vicinity of the central source of our Galaxy, a supermassive black hole of approximately 4 million times the mass of our sun. Measurements of the pulsar imply that a strong magnetic field exists in the vicinity around the black hole. (Credit: MPIfR/Ralph Eatough.)

Astronomers have discovered a magnetar at the centre of our Milky Way. This pulsar has an extremely strong magnetic field and enables researchers to investigate the direct vicinity of the black hole at the heart of the galaxy. An international team of scientists headed by the Max Planck Institute for Radio Astronomy in Bonn have, for the first time, measured the strength of the magnetic field around this central source and were able to show that the latter is fed by magnetic fields. These control the inflow of mass into the black hole, also explaining the x-ray emissions of this gravity trap.

The discovery of a pulsar closely orbiting the candidate supermassive black hole at the centre of the Milky Way (called Sagittarius A*, or Sgr A* in short) has been one of the main aims of pulsar astronomers for the last 20 years. Pulsars, those extremely precise cosmic clocks, could be used to measure the properties of space and time around this object, and to see if Einstein's theory of General Relativity could hold up to the strictest tests.

Shortly after the announcement of a flaring X-ray source in the direction of the Galactic centre by NASA's Swift telescope, and the subsequent discovery of pulsations with a period of 3.76 seconds by NASA's NuSTAR telescope, a radio follow-up program was started at the Effelsberg radio observatory of the Max Planck Institute for Radio Astronomy (MPIfR).

"As soon as we heard about the discovery of regular pulsations with the NuSTAR telescope we pointed the Effelsberg 100-m dish in the direction of the Galactic centre," says Ralph Eatough from MPIfR's Fundamental Physics Research department, the lead author of the study. "On our first attempt the pulsar was not clearly visible, but some pulsars are stubborn and require a few observations to be detected. The second time we looked, the pulsar had become very active in the radio band and was very bright. I could hardly believe that we had finally detected a pulsar in the Galactic centre!" Because this pulsar is so special, the research team spent a lot of effort to prove that it was a real object in deep space and not due to human-made radio interference created on Earth.

Additional observations were performed in parallel and subsequently with other radio telescopes around the world (Jodrell Bank, Very Large Array, Nançay). "We were too excited to sleep in between observations! We were calculating flux densities at 6am on Saturday morning and we could not believe that this magnetar had just turned on so bright." says Evan Keane from the Jodrell Bank Observatory. Other collaborations worked at different telescopes (Australia Telescope/ATCA, Parkes and Green Bank Telescope). A research paper on the ATCA results by Shannon & Johnston appears in this week's issue of the British journal MNRAS.

"The Effelsberg radio telescope was built such that it could observe the Galactic centre. And 40 years later it detects the first radio pulsar there," explains Heino Falcke, professor at Radboud Universiteit Nijmegen. "Sometimes we have to be patient. It was a laborious effort, but finally we succeeded."

The newly found pulsar, labelled PSR J1745-2900, belongs to a specific subgroup of pulsars, the so-called magnetars. Magnetars are pulsars with extremely high magnetic fields of the order of 100 million (108) Tesla, about 1000 times stronger than the magnetic fields of ordinary neutron stars, or 100,000 billion times Earth's magnetic field. The emission from these objects is also known to be highly polarized. Measurements of the rotation of the plane of polarization caused by an external magnetic field (the so-called Faraday effect) can be used to infer the strength of the magnetic field along the line-of-sight to the pulsar.

The magnetic field strength in the vicinity of the black hole at the centre of the Galaxy is an important property. The black hole is gradually swallowing its surroundings (mainly hot ionized gas) in a process of accretion. Magnetic fields caused by this in-falling gas can influence the structure and dynamics of the accretion flow, helping or even hindering the process. The new pulsar has allowed measurements of the strength of the magnetic field at the beginning of the accretion flow to the central black hole, indicating there is indeed a large-scale and strong magnetic field.

"In order to understand the properties of Sgr A*, we need to comprehend the accretion of gas into the black hole," says Michael Kramer, director at MPIfR and head of its Fundamental Physics research department. "However, up to now, the magnetization of the gas, which is a crucial parameter determining the structure of the accretion flow, remains unknown. Our study changes that by using the discovered pulsar to probe the strength of the magnetic field at the start of this accretion flow of gas into the central object."

Source : Max-Planck-Gesellschaft.

By Science and Universe

Monster Galaxies Lose Their Appetite With Age

This image shows two of the galaxy clusters observed by NASA's Wide-field Infrared Survey Explorer (WISE) and Spitzer Space Telescope missions. Galaxy clusters are among the most massive structures in the universe. The central and largest galaxy in each grouping, called the brightest cluster galaxy or BCG, is seen at the center of each image. Image credit: NASA/JPL-Caltech/SDSS/NOAO  

Our universe is filled with gobs of galaxies, bound together by gravity into larger families called clusters. Lying at the heart of most clusters is a monster galaxy thought to grow in size by merging with neighboring galaxies, a process astronomers call galactic cannibalism. 

New research from NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer (WISE) is showing that, contrary to previous theories, these gargantuan galaxies appear to slow their growth over time, feeding less and less off neighboring galaxies. 

"We've found that these massive galaxies may have started a diet in the last 5 billion years, and therefore have not gained much weight lately," said Yen-Ting Lin of the Academia Sinica in Taipei, Taiwan, lead author of a study published in the Astrophysical Journal. 

Peter Eisenhardt, a co-author from NASA's Jet Propulsion Laboratory in Pasadena, Calif., said, "WISE and Spitzer are letting us see that there is a lot we do understand -- but also a lot we don't understand -- about the mass of the most massive galaxies." Eisenhardt identified the sample of galaxy clusters studied by Spitzer, and is the project scientist for WISE. 

The new findings will help researchers understand how galaxy clusters -- among the most massive structures in our universe -- form and evolve. 

Galaxy clusters are made up of thousands of galaxies, gathered around their biggest member, what astronomers call the brightest cluster galaxy, or BCG. BCGs can be up to dozens of times the mass of galaxies like our own Milky Way. They plump up in size by cannibalizing other galaxies, as well as assimilating stars that are funneled into the middle of a growing cluster. 

To monitor how this process works, the astronomers surveyed nearly 300 galaxy clusters spanning 9 billion years of cosmic time. The farthest cluster dates back to a time when the universe was 4.3 billion years old, and the closest, when the universe was much older, 13 billion years old (our universe is presently 13.8 billion years old). 

"You can't watch a galaxy grow, so we took a population census," said Lin. "Our new approach allows us to connect the average properties of clusters we observe in the relatively recent past with ones we observe further back in the history of the universe." 

Spitzer and WISE are both infrared telescopes, but they have unique characteristics that complement each other in studies like these. For instance, Spitzer can see more detail than WISE, which enables it to capture the farthest clusters best. On the other hand, WISE, an infrared all-sky survey, is better at capturing images of nearby clusters, thanks to its larger field of view. Spitzer is still up and observing; WISE went into hibernation in 2011 after successfully scanning the sky twice. 

The findings showed that BCG growth proceeded along rates predicted by theories until 5 billion years ago, or a time when the universe was about 8 billion years old. After that time, it appears the galaxies, for the most part, stopped munching on other galaxies around them. 

The scientists are uncertain about the cause of BCGs' diminished appetites, but the results suggest current models need tinkering. 

"BCGs are a bit like blue whales -- both are gigantic and very rare in number. Our census of the population of BCGs is in a way similar to measuring how the whales gain their weight as they age. In our case, the whales aren't gaining as much weight as we thought. Our theories aren't matching what we observed, leading us to new questions," said Lin. 

Another possible explanation is that the surveys are missing large numbers of stars in the more mature clusters. Clusters can be violent environments, where stars are stripped from colliding galaxies and flung into space. If the recent observations are not detecting those stars, it's possible that the enormous galaxies are, in fact, continuing to bulk up. 

Future studies from Lin and others should reveal more about the feeding habits of one of nature's largest galactic species. 

JPL 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 in Pasadena. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA. For more information about Spitzer, visithttp://spitzer.caltech.edu and http://www.nasa.gov/spitzer . 

JPL managed and operated WISE for NASA's Science Mission Directorate. Edward Wright is the principal investigator and is at UCLA. The mission was selected competitively under NASA's Explorers Program managed by the agency's Goddard Space Flight Center in Greenbelt, Md. The science instrument was built by the Space Dynamics Laboratory in Logan, Utah. The spacecraft was built by Ball Aerospace & Technologies Corp. in Boulder, Colo. Science operations and data processing take place at the Infrared Processing and Analysis Center at the California Institute of Technology in Pasadena. Caltech manages JPL for NASA. More information is online at http://www.nasa.gov/wise and http://wise.astro.ucla.edu and http://jpl.nasa.gov/wise .

Article Source : http://www.jpl.nasa.gov

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Spitzer Discovers Young Stars with a 'Hula Hoop'

In this artist's impression, a disk of dusty material leftover from star formation girds two young stars like a hula hoop. As the two stars whirl around each other, they periodically peek out from the disk, making the system appear to "blink" every 93 days. Image credit: NASA/JPL-Caltech

Astronomers using NASA's Spitzer Space Telescope have spotted a young stellar system that "blinks" every 93 days. Called YLW 16A, the system likely consists of three developing stars, two of which are surrounded by a disk of material left over from the star-formation process. 

As the two inner stars whirl around each other, they periodically peek out from the disk that girds them like a hula hoop. The hoop itself appears to be misaligned from the central star pair, probably due to the disrupting gravitational presence of the third star orbiting at the periphery of the system. The whole system cycles through bright and faint phases, with the central stars playing a sort of cosmic peek-a-boo as the tilted disk twirls around them. It is believed that this disk should go on to spawn planets and the other celestial bodies that make up a solar system. 

Spitzer observed infrared light from YLW 16A, emitted by the warmed gas and dust in the disk that still swathes the young stars. Other observations came from the ground-based 2MASS survey, as well as from the NACO instrument at the European Southern Observatory's Very Large Telescope in Chile. 

YLW 16A is the fourth example of a star system known to blink in such a manner, and the second in the same star-forming region Rho Ophiuchus. The finding suggests that these systems might be more common than once thought. Blinking star systems with warped disks offer scientists a way to study how planets form in these environments. The planets can orbit one or both of the stars in the binary star system. The famous science fictional planet Tatooine in "Star Wars" orbits two stars, hence its double sunsets. Such worlds are referred to as circumbinary planets. Astronomers can record how light is absorbed by planet-forming disks during the bright and faint phases of blinking stellar systems, which in turn reveals information about the materials that comprise the disk. 

"These blinking systems offer natural probes of the binary and circumbinary planet formation process," said Peter Plavchan, a scientist at the NASA Exoplanet Science Institute and Infrared Processing and Analysis Center at the California Institute of Technology, Pasadena, Calif., and lead author of a new paper accepted for publication in Astronomy & 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 Caltech. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center. Caltech manages JPL for NASA. For more information about Spitzer, visithttp://spitzer.caltech.edu and http://www.nasa.gov/spitzer .

Source : JPL/ NASA

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