Nine new gamma pulsars brings known gamma-ray pulsars to over 100

ScienceDaily (Nov. 3, 2011) — Pulsars are the lighthouses of the universe. These compact and fast-rotating neutron stars flash many times per second in the radio or gamma-ray band. Pure gamma-ray pulsars are extremely difficult to find despite their high energy because they radiate very few photons per unit of time. Using an improved analysis algorithm, Max Planck scientists and international partners have now discovered a number of previously unknown gamma-ray pulsars with low luminosity in data from the Fermi satellite. These pulsars had been missed using conventional methods. The number of known gamma-ray pulsars has thus grown to over 100.

The paper will be published in the Astrophysical Journal.

These cosmic beacons still pose a few puzzles for scientists: not all radio pulsars can be identified as gamma-ray pulsars and, on the other hand, not all gamma-ray pulsars are "visible" in the radio band. A plausible explanation is the varying width of the light cone over the wavelength range. This may be because emissions at different wavelengths spread out differently. Lower-energy radio waves are bundled more tightly at the magnetic pole of a neutron star field while the cone of high-energy gamma-rays will spread out. Depending on its spatial orientation and intensity of the cone, a pulsar will thus be observed as a radio and/or gamma-ray pulsar. However, other models also describe this phenomenon. To determine the actual cause, as many sources as possible should be examined.

The search for pure gamma-ray pulsars literally starts with the scientists "in the dark." A typical pulsar will rotate several times per second, or at least 108 times per year. Its emission region will sweep across the observer with that frequency. The Large Area Telescope (LAT) on NASA's Fermi satellite however only detects an average of a few thousand photons per year from any given gamma pulsar. This low detection rate makes it extremely difficult for even the fastest supercomputers to assign the individual gamma quanta to an unknown pulsar with a defined rotation period.

For several months, scientists from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute/AEI) in Hanover, the Leibniz University of Hanover and the Max Planck Institute for Radio Astronomy in Bonn analysed data from the LAT in an international collaboration. Using a new method, they have been able to identify nine new pulsars which were "invisible" up to now.

"We used a new kind of hierarchical algorithm which we had originally developed for the search for gravitational waves," explains Bruce Allen, Director at the AEI and professor at the Institute for Gravitational Physics of the Leibniz University Hanover. "It's like digging for diamonds or gold: it's very exciting when you find something," Allen continues.

The nine new pulsars netted by the scientists emit less gamma radiation than those previously known and rotate between three and twelve times per second. Only one of these pulsars was later also found to emit radio waves. The total number of gamma pulsars observed by Fermi has thus risen to over 100.

About three-quarters of the gamma pulsars previously observed by Fermi had first been identified as radio pulsars. In these cases the search for additional gamma radiation is relatively easy. Sky position, rotation period and rotation period change rate are all derived from the radio data, so it takes only a few additional calculations to determine whether detected gamma quanta match a particular set of parameters or not.

A blind search is far more complex as neither position nor pulse period and its change over time are known. At first, each photon from a particular observation sector is assigned a certain probability for a sky position. If a significant accumulation of gamma quanta is observed from a rough direction in the sky, scientists check if the photon's arrival time at Fermi's on-board detector matches an exact sky position and pulse period and its change over time. However, with only a few thousand photons detected over a period of three years but rotation frequencies of only a few hertz, a huge number of prospects have to be tested.

During a first blind search in Fermi data, astronomers found 24 pure gamma pulsars within one year of the launch of the satellite in 2008. A further two were discovered in the following year. After that, the number of new pulsars found stagnated until physicists from Hanover started searching with an algorithm that is 10 times more efficient, and employing additional computing power. They found a further nine gamma-ray pulsars which on average emit only one quarter of the photons compared to previously discovered pure gamma-ray pulsars.

Scientists at the MPI Bonn examined the emission spectra of the corresponding gamma-ray sources to see whether they were candidates for the gamma-ray pulsar blind search. "About one-third of the gamma-ray sources observed by LAT were unknown before the launch of Fermi," says Lucas Guillemot, a member of the research group of Michael Kramer, Director at the MPIfR. "We determined the spectral properties of the Fermi LAT sources, and compared them to those of known gamma-ray sources," he said.

The candidates for a gamma-ray pulsar blind search were then analysed in detail. "The new computing method permits us to evaluate data sets much faster than before," said Holger Pletsch, a member of Allen's group leading the work. The analysis was also run on the ATLAS computing cluster at the AEI in Hanover. ATLAS has one hundred times the computing power that was used in previous blind searches. "Together with the more efficient analysis, this does not only mean that we can analyse the data much faster. We can now search for gamma-ray pulsars which rotate even faster, with periods measured in milliseconds," adds Pletsch. The computing power needed for the search increases proportionally to the cube of the rotation period.

Additionally, part of the computing power of the Einstein@Home project is now allocated to the search for the first pure gamma-ray millisecond-pulsar. This discovery would be a significant contribution to our understanding of pulsars.

Recommend this story on Facebook, Twitter,
and Google +1:

Other bookmarking and sharing tools:

Story Source:

The above story is reprinted from materials provided by Max-Planck-Gesellschaft.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

H. J. Pletsch, L. Guillemot, B. Allen, M. Kramer, C. Aulbert, H. Fehrmann, P. S. Ray, E. D. Barr, A. Belfiore, F. Camilo, P. A. Caraveo, O. Celik, D. J. Champion, M. Dormody, R. P. Eatough, E. C. Ferrara, P. C. C. Freire, J. W. T. Hessels, M. Keith, M. Kerr, A. de Luca, A. G. Lyne, M. Marelli, M. A. McLaughlin, D. Parent, S. M. Ransom, M. Razzano, W. Reich, P. M. Saz Parkinson, B. W. Stappers, M. T. Wolff. Discovery of Nine Gamma-Ray Pulsars in Fermi-LAT Data Using a New Blind Search Method. Astrophysical Journal, 2011 [link]

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Observations of gamma-ray burst reveal surprising ingredients of early galaxies

ScienceDaily (Nov. 2, 2011) — An international team of astronomers led by the Max Planck Institute for Extraterrestrial Physics has used the brief but brilliant light of a distant gamma-ray burst as a probe to study the make-up of very distant galaxies. Surprisingly the new observations revealed two galaxies in the young Universe that are richer in the heavier chemical elements than the Sun. The two galaxies may be in the process of merging. Such events in the early Universe will drive the formation of many new stars and may be the trigger for gamma-ray bursts.

Gamma-ray bursts are the brightest explosions in the Universe. They are first spotted by orbiting observatories that detect the initial short burst of gamma rays. After their positions have been pinned down, they are then immediately studied using large ground-based telescopes that can detect the visible-light and infrared afterglows that the bursts emit over the succeeding hours and days. One such burst, called GRB 090323, was first spotted by the NASA Fermi Gamma-ray Space Telescope. Very soon afterwards it was picked up by the X-ray detector on NASA's Swift satellite and with the GROND system at the MPG/ESO 2.2-metre telescope in Chile. From the GROND observations, the astronomers estimated the minimum rate of star formation, which has to be several times higher than the one in our Galaxy. They could, however, only determine a minimum value because the detected emission could be heavily affected (i.e. absorbed) by the presence of dust in the galaxies. The real rate of star formation, once the (unknown) dust absorption has been taken into account, could easily be 50 times higher than in the Milky Way.

The burst was also studied in great detail using ESO's Very Large Telescope (VLT) just one day after it exploded. These observations show that the brilliant light from the gamma-ray burst had passed through its own host galaxy and another galaxy nearby. These galaxies are being seen as they were about 12 billion years ago. Such distant galaxies are very rarely caught in the glare of a gamma-ray burst.

"When we studied the light from this gamma-ray burst we didn't know what we might find. It was a surprise that the cool gas in these two galaxies in the early Universe proved to have such an unexpected chemical make-up," explains Sandra Savaglio (Max-Planck Institute for Extraterrestrial Physics, Garching, Germany), lead author of the paper describing the new results. "These galaxies have more heavy elements than have ever been seen in a galaxy so early in the evolution of the Universe. We didn't expect the Universe to be so mature, so chemically evolved, so early on."

As light from the gamma-ray burst passed through the galaxies, the gas there acted like a filter, and absorbed some of the light from the gamma-ray burst at certain wavelengths. Without the gamma-ray burst these faint galaxies would be invisible. By carefully analysing the tell-tale fingerprints from different chemical elements the team was able to work out the composition of the cool gas in these very distant galaxies, and in particular how rich they were in heavy elements.

It is expected that galaxies in the young Universe will be found to contain smaller amounts of heavier elements than galaxies at the present day, such as the Milky Way. The heavier elements are produced during the lives and deaths of generations of stars, gradually enriching the gas in the galaxies. Astronomers can use the chemical enrichment in galaxies to indicate how far they are through their lives. But the new observations, surprisingly, revealed that some galaxies were already very rich in heavy elements less than two billion years after the Big Bang. Something unthinkable until recently.

The newly discovered pair of young galaxies must be forming new stars at a tremendous rate, to enrich the cool gas so strongly and quickly. As the two galaxies are close to each other they may be in the process of merging, which would also provoke star formation when the gas clouds collide. The new results also support the idea that gamma-ray bursts may be associated with vigorous massive star formation.

Energetic star formation in galaxies like these might have ceased early on in the history of the Universe. Twelve billion years later, at the present time, the remains of such galaxies would contain a large number of stellar remnants such as black holes and cool dwarf stars, forming a hard to detect population of "dead galaxies," just faint shadows of how they were in their brilliant youths. Finding such corpses in the present day would be a challenge.

"We were very lucky to observe GRB 090323 when it was still sufficiently bright, so that it was possible to obtain spectacularly detailed observations with the VLT. Gamma-ray bursts only stay bright for a very short time and getting good quality data is very hard. We hope to observe these galaxies again in the future when we have much more sensitive instruments, they would make perfect targets for the E-ELT," concludes Savaglio.

Recommend this story on Facebook, Twitter,
and Google +1:

Other bookmarking and sharing tools:

Story Source:

The above story is reprinted from materials provided by Max-Planck-Institut für extraterrestrische Physik (MPE).

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

S. Savaglio et al. Super-solar Metal Abundances in Two Galaxies at z~3.57 revealed by the GRB090323 Afterglow Spectrum. Monthly Notices of the Royal Astronomical Society, 2011

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Gamma-ray flash came from star being eaten by massive black hole

ScienceDaily (June 16, 2011) — A bright flash of gamma rays observed March 28 by the Swift satellite may have been the death rattle of a star falling into a massive black hole and being ripped apart, according to a team of astronomers led by the University of California, Berkeley.

When the Swift Gamma Burst Mission spacecraft first detected the flash within the constellation Draco, astronomers thought it was a gamma-ray burst from a collapsing star. On March 31, however, UC Berkeley's Joshua Bloom sent out an email circular suggesting that it wasn't a typical gamma-ray burst at all, but a high-energy jet produced as a star about the size of our sun was shredded by a black hole a million times more massive.

Careful analysis of the Swift data and subsequent observations by the Hubble Space Telescope and the Chandra X-ray Observatory confirmed Bloom's initial insight. The details are published online on June 16 in Science Express, a rapid publication arm of the journal Science.

"This is truly different from any explosive event we have seen before," Bloom said.

What made this gamma-ray flare, called Sw 1644+57, stand out from a typical burst were its long duration and the fact that it appeared to come from the center of a galaxy nearly 4 billion light years away. Since most, if not all, galaxies are thought to contain a massive black hole at the center, a long-duration burst could conceivably come from the relatively slow tidal disruption of an infalling star, the astronomers said.

"This burst produced a tremendous amount of energy over a fairly long period of time, and the event is still going on more than two and a half months later," said Bloom, an associate professor of astronomy at UC Berkeley. "That's because as the black hole rips the star apart, the mass swirls around like water going down a drain, and this swirling process releases a lot of energy."

Bloom and his colleagues propose in their Science Express paper that some 10 percent of the infalling star's mass is turned into energy and irradiated as X-rays from the swirling accretion disk or as X-rays and higher energy gamma rays from a relativistic jet that punches out along the rotation axis. Earth just happened to be in the eye of the gamma-ray beam.

Bloom draws an analogy with a quasar, which is a distant galaxy that emits bright, high-energy light because of the massive black hole at its center gobbling up stars and sending out a jet of X-rays along its rotation axis. Observed from an angle, these bright emissions are called active galactic nuclei, but when observed down the axis of the jet, they're referred to as blazars.

"We argue that this must be jetted material and we're looking down the barrel," he said. "Jetting is a common phenomenon when you have accretion disks, and black holes actually prefer to make jets."

Looking back at previous observations of this region of the cosmos, Bloom and his team could find no evidence of X-ray or gamma-ray emissions, leading them to conclude that this is a "one-off event," Bloom said.

"Here, you have a black hole sitting quiescently, not gobbling up matter, and all of a sudden something sets it off," Bloom said. "This could happen in our own galaxy, where a black hole sits at the center living in quiescence, and occasionally burbles or hiccups as it swallows a little bit of gas. From a distance, it would appear dormant, until a star randomly wanders too close and is shredded."

Probable tidal disruptions of a star by a massive black hole have previously been seen at X-ray, ultraviolet and optical wavelengths, but never before at gamma-ray energies. Such random events, especially looking down the barrel of a jet, are incredibly rare, "probably once in 100 million years in any given galaxy," said Bloom. "I would be surprised if we saw another one of these anywhere in the sky in the next decade."

The astronomers suspect that the gamma-ray emissions began March 24 or 25 in the uncatalogued galaxy at a redshift of 0.3534, putting it at a distance of about 3.8 billion light years. Bloom and his colleagues estimate that the emissions will fade over the next year.

"We think this event was detected around the time it was as bright as it will ever be, and if it's really a star being ripped apart by a massive black hole, we predict that it will never happen again in this galaxy," he said.

Bloom's colleagues include UC Berkeley theoretical physicist Elliot Quataert, who models the production of jets from accretion disks, and UC Berkeley astronomers S. Bradley Cenko, Daniel A. Perley, Nathaniel R. Butler, Linda E. Strubbe, Antonino Cucchiara, Geoffrey C. Bower and Adam N. Morgan; Dimitrios Giannios and Brian D. Metzger of Princeton University; Andrew J. Levan of the University of Warwick, Coventry, United Kingdom; Nial R. Tanvir, Paul T. O' Brien, Andrew R. King and Sergei Nayakshin of the University of Leicester in the U.K.; Fabio De Colle, Enrico Ramirez-Ruiz and James Guillochon of UC Santa Cruz; William H. Lee of the Universidad Nacional Autonoma de México in Mexico City; Andrew S. Fruchter of the Space Telescope Science Institute in Baltimore, Md.; and Alexander J. van der Horst of the Universities Space Research Association in Huntsville, Ala.

Levan is first author of the companion Science Express paper, and leader of the Chandra and Hubble Space Telescope observation team.

Bloom and his laboratory are supported by grants from NASA and the National Science Foundation.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of California - Berkeley.

Journal Reference:

Joshua S. Bloom, Dimitrios Giannios, Brian D. Metzger, S. Bradley Cenko, Daniel A. Perley, Nathaniel R. Butler, Nial R. Tanvir, Andrew J. Levan, Paul T. O' Brien, Linda E. Strubbe, Fabio De Colle, Enrico Ramirez-Ruiz, William H. Lee, Sergei Nayakshin, Eliot Quataert, Andrew R. King, Antonino Cucchiara, James Guillochon, Geoffrey C. Bower, Andrew S. Fruchter, Adam N. Morgan, Alexander J. Van Der Horst. A Possible Relativistic Jetted Outburst from a Massive Black Hole Fed by a Tidally Disrupted Star. Science, 16 June 2011 DOI: 10.1126/science.1207150

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

View the original article here

Gamma-ray flash came from star being eaten by massive black hole

View the original article here