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Post Info TOPIC: Giant magnetar flare
Blobrana


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SGR 1806-20
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Title: XMM-Newton observations of SGR 1806-20 over seven years following the 2004 Giant Flare
Author: G. Younes, C. Kouveliotou, V. M. Kaspi

We report on the study of 14 XMM-Newton observations of the magnetar SGR 1806-20 spread over a period of 8 years, starting in 2003 and extending to 2011. We find that in mid 2005, a year and a half after a giant flare (GF), the torques on the star increased to the largest value yet seen, with a long term average rate between 2005 and 2011 of dot{\nu} approx 1.35 x 10-11 Hz s-1, an order of magnitude larger than its historical level measured in 1995. The pulse morphology of the source is complex in the observations following the GF, while its pulsed-fraction remained constant at about 7% in all observations. Spectrally, the combination of a black-body (BB) and power-law (PL) components is an excellent fit to all observations. The BB and PL fluxes increased by a factor of 2.5 and 4, respectively, while the spectra hardened, in concordance with the 2004 major outburst that preceded the GF. The fluxes decayed exponentially back to quiescence with a characteristic time-scale of tau~1.5 yrs, although they did not reach a constant value until at least 3.5 years later (2009). The long-term timing and spectral behaviour of the source point to a decoupling between the mechanisms responsible for their respective behaviour. We argue that low level seismic activity causing small twists in the open field lines can explain the long lasting large torques on the star, while the spectral behaviour is due to a twist imparted onto closed field lines after the 2004 large outburst.

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Blobrana


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RE: Giant magnetar flare
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On December 27, 2004, the radiation from an explosion on the surface of SGR 1806-20 reached Earth.

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Blobrana


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Posts: 131433
Date:
SGR 1806-20
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On December 27, 2004, the radiation from an explosion on the surface of SGR 1806-20 reached Earth. In terms of gamma rays the burst was brighter than a full moon and had an absolute magnitude of around -291. It was the brightest event known to have been sighted on this planet from an origin outside our solar system.

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Blobrana


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Posts: 131433
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RE: Giant magnetar flare
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Title: The first Suzaku observation of SGR 1806-20
Authors: P. Esposito, A. Tiengo, S. Zane, R. Turolla, S. Mereghetti, D. Gotz, G.L. Israel, N. Rea

The soft gamma-ray repeater SGR 1806-20 has been attracting a lot of attention owing to the fact that in December 2004 it emitted the most powerful giant flare ever observed. Here we present the results of the first Suzaku observation of SGR 1806-20, that seems to have reached a state characterized by a flux close to the pre-flare level and by a relatively soft spectrum. Despite this, the source remained quite active, as testified by several short bursts observed by Suzaku. We discuss the broadband spectral properties of SGR 1806-20 in the context of the magnetar model, considering its recent theoretical developments.

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Blobrana


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Posts: 131433
Date:
SGR 1900+14
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Title: GEOTAIL observation of SGR 1900+14 giant flare on 27 August 1998
Authors: Y. T. Tanaka, et al
Comments: Proceedings of the 363. WE-Heraeus Seminar on: Neutron Stars and Pulsars (Posters and contributed talks) Physikzentrum Bad Honnef, Germany, May.14-19, 2006, eds. W.Becker, H.H.Huang, MPE Report 291, pp.205-207

The soft gamma repeater (SGR) 1900+14 emitted the giant flare on 27 August 1998. Most gamma-ray detectors saturated during the initial spike of the giant flare because of the intense flux. However the plasma particle detector onboard GEOTAIL observed the first 300 ms time profile with a time resolution of 5.577 ms and the initial spike of the giant flare was first resolved. The time profile shows some similarities to that of the SGR 1806-20 giant flare in 2004: the clear exponential decay and the small hump in the decay phase around 300 or 400 ms.

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Blobrana


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Posts: 131433
Date:
SGR 1806-20
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Title: The Giant Flare of December 27, 2004 from SGR 1806-20
Authors: Steven E. Boggs, A. Zoglauer, E. Bellm, K. Hurley, R. P. Lin, D. M. Smith, C. Wigger, W. Hajdas

The giant flare of December 27, 2004 from SGR 1806-20 represents one of the most extraordinary events captured in over three decades of monitoring the gamma-ray sky. One measure of the intensity of the main peak is its effect on X- and gamma-ray instruments. RHESSI, an instrument designed to study the brightest solar flares, was completely saturated for ~0.5 s following the start of the main peak. A fortuitous alignment of SGR 1806-20 near the Sun at the time of the giant flare, however, allowed RHESSI a unique view of the giant flare event, including the precursor, the main peak decay, and the pulsed tail. Since RHESSI was saturated during the main peak, we augment these observations with Wind and RHESSI particle detector data in order to reconstruct the main peak as well. Here we present detailed spectral analysis and evolution of the giant flare. We report the novel detection of a relatively soft fast peak just milliseconds before the main peak, whose timescale and sizescale indicate a magnetospheric origin. We present the novel detection of emission extending up to 17 MeV immediately following the main peak, perhaps revealing a highly-extended corona driven by the hyper-Eddington luminosities. The spectral evolution and pulse evolution during the tail are presented, demonstrating significant magnetospheric twist and evolution during this phase. Blackbody radii are derived for every stage of the flare, which show remarkable agreement despite the range of luminosities and temperatures covered. Finally, we place significant upper limits on afterglow emission in the hundreds of seconds following the giant flare.

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Blobrana


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A rare spectacular moment is sometimes millions of years in the making - such as cataclysmic death of a massive star.

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-- Edited by Blobrana at 01:10, 2006-10-14

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Blobrana


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Astronomers have measured the thickness of the crust of a neutron star for the first time. The technique, which involves studying how the dense stellar corpse reverberates during a "starquake", may one day reveal the nature of the exotic matter thought to lie at the star's core.

Neutron stars form when stars up to 40 times the mass of the Sun explode at the end of their lives and leave behind super-dense, spinning corpses. The corpses are thought to be made of neutrons, although their incredible densities have led some researchers to propose their centres contain a state of matter found nowhere else in the universe.
Now, a team led by Tod Strohmayer of NASA's Goddard Space Flight Center in Greenbelt, Maryland, US, has begun to probe the actual structure of the stars. Using data from NASA's Rossi X-ray Timing Explorer, the researchers measured the frequencies of vibrations on a star called SGR 1806-20 during a starquake in December 2004. The observed 0.25-second flash was so bright it overwhelmed the detectors on many satellites - making an energy measurement impossible - and disrupted some radio communication on Earth.
The brief flash was followed by a fainter afterglow of gamma rays lasting for about 500 seconds, which showed a recurring signal every 7.5 seconds.
That signal led scientists using Europe's INTEGRAL spacecraft to trace the source of the "superflare" to a dead star - called a neutron star - known to spin at that rate.
The starquake probably occurred when magnetic fields inside the star, which are attached to the crust, got so twisted up that they ripped the crust open. This released a fireball of particles and radiation that astronomers observed as the brightest flash of high-energy photons ever seen from beyond the solar system.

"It was so bright, it came right through the body of the Swift satellite, even though Swift wasn't pointed at the object" - John Nousek, mission director for NASA's Swift spacecraft - launched especially to detect gamma-ray bursts (GRBs) - at Pennsylvania State University, US.

"We think this explosion, the biggest of its kind ever observed, really jolted the star and literally started it ringing like a bell. The vibrations created in the explosion, although faint, provide very specific clues about what makes up these bizarre objects." - Tod Strohmayer

The team found a high-frequency oscillation of 625 Hz, which suggested the waves were ringing through the crust vertically. This, along with the other frequencies measured, led the researchers to estimate the crust's thickness at about 1.5 kilometres. The entire neutron star is only thought to span about 20 kilometres across.

"That's pretty thick, but not ridiculous. So far, all we had for understanding the structure of neutron stars is really just theoretical models. So having the measurement is very exciting." - Stephen Eikenberry, astrophysicist at the University of Florida in Gainesville, US.

The team says it could probe even deeper into the structure of neutron stars if it could analyse the vibrations from an even more powerful starquake.

"A bigger event might let us peer all the way to the centre of the neutron star and see what it's made of." - Stephen Eikenberry.

Such an event could reveal whether the stars are composed simply of neutrons or whether they contain free-roaming quarks or other exotic particles. Quarks are subatomic particles that make up protons and neutrons and have so far only been observed bound together in groups. But some theorists think that the incredible densities inside neutron stars may allow the quarks to break free of each other.

"They're such extreme objects, we don't have any other way in the universe to probe how matter behaves in these conditions." - Stephen Eikenberry.

Measurements of the distance to SGR 1806-20, range from 30,000 to 50,000 light years from Earth.

Strohmayer presented the results this week at a meeting of the American Physical Society in Dallas, Texas, US.

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Blobrana


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Posts: 131433
Date:
Giant magnetar flare
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On November 3rd, 2005, no fewer than six spacecraft detected a powerful burst of gamma rays coming from the constellation Ursa Major in the general direction of the two galaxies M81 and M82. Both galaxies are about 12 million light-years away.

The Swift, HETE-2, RHESSI, Mars Odyssey spacecraft, the European Space Agency's Integral satellite, and the Russian Konus-Wind satellite all detected the burst.

The main GBR pulse lasting just one-tenth of a second. If the burst originated in M81 or M82, the total energy and spectrum closely resemble the December 2004 giant flare from SGR 1806–20.


This light curve from the Russian Konus-Wind satellite shows that most of the gamma rays from the November 3rd event arrived in less than 0.1 second. This pattern closely resembles that of a giant magnetar flare observed on December 27, 2004. Courtesy Konus-Wind Team / Ioffe Physical Technical Institute.

Robert C. Duncan along with Christopher Thompson of the Canadian Institute of Theoretical Astrophysics first conceived of magnetars in the early 1990s.

"Nearby galaxies like M81 and M82 are so rare in the sky, and the energetics of the flare are so well suited to it being at M81's distance, that we are definitely taking the M81/M82 possibility very seriously" - Daniel A. Perley, University of California, Berkeley.

Daniel A. Perley has calculated that there is only a 3 percent probability that an event like the November 3rd burst would occur so near a bright galaxy such as M81 or M82 merely by chance.

"All evidence is consistent with the interpretation that this was a magnetar flare in the M81 group of galaxies" - Robert C. Duncan, University of Texas, Austin, US.
However the area of uncertainty also includes much more distant galaxies.

"While the burst profile does resemble a magnetar giant flare, we still can't prove that this is an extragalactic magnetar flare. I do believe that extragalactic magnetar flares are out there, and I'm as anxious as anyone to find the first one. But I think we need to be cautious about claiming victory" - Kevin C. Hurley.


This wide-field image from the Sloan Digital Sky Survey includes the relatively nearby galaxies M81 (slightly above centre) and M82 (top centre). On November 3, 2005, six spacecraft detected a short burst of high-energy gamma rays coming from somewhere near or within the marked area of uncertainty. This zone is so close to M81 and M82 that the source is probably in one of these galaxies or in a dwarf galaxy in the same cluster. Courtesy SDSS Collaboration.

M81
Position(2000): RA: 09 : 55.6 (h:m) Dec: +69 : 04 (deg:m)
M82
Position(2000): RA: 09 : 55.8 (h:m) Dec +69 : 41 (deg:m)

If it turns out that it was a giant magnetar flare in M81 or M82, it will narrow down the time scale of these events.
On December 27, 2004, astronomers saw a powerful burst of gamma rays from the other side of our galaxy. For two-tenths of a second, the highly magnetised neutron star , SGR 1806–20,blasted Earth with a higher rate of energy than any previous observed object outside the solar system.
These two separate flares suggest that such blasts occur once every few decades in a large spiral galaxy such as the Milky Way.

They also provide a connection to gamma-ray bursts (GRBs), powerful explosions emanating from deep space.
Short GRBs have durations of less than 2 seconds and account for about one in six GRBs. Recently it was discovered that most short GRBs are triggered by the merger of two neutron stars or a neutron star and a black hole in distant galaxies. (Long GRBs are thought to occur when massive star cores collapse to form black holes)

But magnetar giant flares, with their short but intense pulses of gamma rays, appear to be another class of short GRBs. After the December 2004 giant flare, astronomers immediately realised that it would have looked similar to a typical short GRB in a nearby galaxy,. The occurrence of these two events in less than a year suggests that about 10 to 20 percent of short GRBs could be extragalactic magnetar giant flares.
But more data is needed to say either way.

"It's all very speculative at this point"- Kevin C. Hurley.

Magnetar giant flares are maybe triggered when the magnetic-field lines within the star builds up stress in the thin crust, this then eventually leads to a powerful "starquake".
The sudden, large-scale crustal shift allows the field lines outside the star to rearrange themselves into a lower-energy state, and transforming themselves into gamma rays and a shower of subatomic particles.

Astronomers are also studying data from a short GRB picked up on September 6th, which might have been a magnetar giant flare in the much more distant galaxy IC 328 in Eridanus.

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