Artikel zu "SGR 1806-20" - II -"
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SGR1806-20 (MIV)"
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Magnetar (MIV)
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AXP / SGR
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Teil III
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SGR 1806-20
SGR 1806-20 (II)
K1 
The soft gamma-ray repeater SGR1806-20
The discovery of Magnetars (GRB 790107 of SGR 1806-20)
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RXTE detects 7.5 second pulsations
[21. 5. 1998] An international research team led by Chryssa Kouveliotou has used
RXTE to detect 7.5 second pulsations in SGR 1806-20 which, when combined
with earlier ASCA observations of the source imply a magnetic field
strength about 100 times stronger than the typical neutron star.
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Image credit: Robert Mallozzi
Soft Gamma Repeater SGR 1806-20 |(SNR G10.0-0.3 d = 15 kpc?)
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SGR 1806-20, a member of the rare class of object known as Soft Gamma Repeaters, was extremely active between October 1996 and November 1997, as evidenced by more than 40 strong bursts detected by BATSE on board CGRO. The BATSE detection triggered a series of five RXTE observations of the source between November 5 - 18, 1996.
Power Spectrum Analysis showed the detection of a significant period at 7.47665 seconds; the data folded at this period display a typical X-ray pulsar profile. An significant spindown rate was required to fit the pulse arrival times of the 13 day observation period. With knowledge of the period and it's derivative, the researchers searched the archival ASCA data for periods in this range and uncovered a significant pulsation in a 1993 data set. This elongated time baseline allowed the spindown rate to be more accurately measured, at a value of P' = 8.3 ± 0.3 x 10-11 s s-1
A spindown rate of this magnitude implies a superstrong magnetic field, in excess of dex(14) Gauss! An isolated neutron star possessing such a strong magnetic field can be expected to undergo "starquakes", releasing enough energy to power the soft Gamma Ray emissions observed in this class of object. In addition, if up to 10% of neutron stars are formed with such field strengths, and consequently are difficult to observe in radio or X-ray, this might explain the number of supernova remnants without a detectable neutron star at their centers.
K1.1
Sternenbeben enthüllt verborgenen Aufbau eines Neutronensterns
Zum Thema: SGR 1806-20 | |
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Neutronensternbeben
[26. April 2006] Internationalem Forscherteam gelingt es mit seismologischen Methoden erstmals ins
Innere eines extrem kompakten Himmelkörpers zu blicken.
Ein amerikanisch-deutsches Team von Wissenschaftlern des Max-Planck-Instituts für Astrophysik und der NASA hat mit Hilfe von Messungen des "Rossi X-Ray Timing Explorer", eines Röntgensatelliten der NASA, die Dicke der Kruste eines Neutronensterns bestimmt. Neutronensterne sind die dichtesten Objekte, die im Universum existieren, mit bislang nicht bekannten Eigenschaften in ihrem Inneren. Nach den neuen Messungen ist die Kruste von Neutronensternen bis zu 1,5 Kilometer stark und so dicht gepackt, dass ein Teelöffel dieser Materie auf der Erde 10 Millionen Tonnen wiegen würde. |
Bild: Max-Planck-Institut für Astrophysik
Abb. 1: Oberflächenmuster für verschiedene Verwindungsoszillationen,
die möglicherweise durch den Hyperflare angeregt wurden. Die Farbcodierung
und Länge der Pfeile kennzeichnen die Stärke der Schwingungen.
Bild: Max-Planck-Institut für Astrophysik
Abb. 2: Die Röntgenmessung für den Hyperflare, der den Hauptausbruch beim
Zeitnullpunkt zeigt, gefolgt von einer allmählichen Abnahme des Signals.
Die regelmäßigen Pulse stammen von einem Feuerball heißen Plasmas, das
nahe an der Oberfläche des Neutronensterns eingeschlossen ist und sich
durch die Sternrotation periodisch in und aus unserer Beobachtungsrichtung
dreht. Die viel schnelleren seismischen Oszillationen sind viel zu
schnell, um auf diesem Bild sichtbar zu sein. Sie beginnen etwa 50 Sekunden nach dem ersten Ausbruch.
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Das neuartige Verfahren erlaubt es nun, das Innere eines Neutronensterns - eines bisher unerforschten und verborgenen Gebiets - zu untersuchen. Dort sind Druck und Dichte so hoch, dass im Zentrum des Neutronensterns möglicherweise exotische Teilchen zu finden sind, die sonst nur zum Zeitpunkt des Urknalls existiert haben.
"Diese Explosion war die stärkste jemals beobachtete ihrer Art. Wir vermuten, dass sie den Stern durchgeschüttelt und ihn praktisch wie eine Glocke zum Klingen gebracht hat", so Strohmeyer. "Obwohl die durch die Explosion erzeugten Vibrationen schwach sind, geben sie ganz genaue Hinweise darauf, woraus diese merkwürdigen Sterne bestehen. Wie bei einer Glocke hängen die Schwingungen im Neutronenstern davon ab, wie die Wellen durch Schichten verschiedener Dichte laufen, die elastisch oder fest sein können."
Ein Neutronenstern ist der Überrest aus dem Kernbereich eines Sterns, dessen Gesamtmasse einst ein Vielfaches der Masse unserer Sonne betrug. Er enthält ungefähr 1,4M
, allerdings
in einer Kugel von lediglich 20 Kilometern Durchmesser zusammengepresst. Die beiden Wissenschaftler
haben einen Neutronenstern namens SGR 1806-20 untersucht, der etwa 15 kpc von der Erde entfernt im
Sternbild Schütze liegt. Dieses Objekt gehört zu einer bestimmten Art von stark magnetisierten Neutronensternen,
die Magnetare genannt werden.
Am 27. Dezember 2004 ereignete sich auf der Oberfläche von SGR 1806-20 eine Explosion mit noch nie da gewesener Stärke (s. Abb. 2). Sie war die hellste jemals außerhalb unseres Sonnensystems beobachtete Explosion. Die Explosion, auch "Hyperflare" genannt, wurde durch eine plötzliche Veränderung im gewaltigen Magnetfeld des Sterns verursacht, wodurch die Kruste aufgesprengt und wahrscheinlich ein gewaltiges Sternbeben ausgelöst wurde. Dieses Ereignis wurde von einer Vielzahl von Weltraum-Observatorien beobachtet, unter anderem auch vom "Rossi Explorer" der NASA, der das dabei abgestrahlte Röntgenlicht aufzeichnete.
Strohmayer und Watts glauben, dass die Oszillationen auf Torsionsschwingungen der gesamten Sternkruste zurückzuführen sind. Solche Vibrationen sind den bei Beben auf der Erde gemessenen S-Wellen ähnlich, die wie eine Welle entlang eines Seiles laufen (s. Abb. 1). Die beiden Wissenschaftler, die für ihre Studien Messdaten von G.L. Israel
benutzten,
konnten mehrere neue Vibrationsfrequenzen in dem Hyperflare identifizieren.
Watts und Strohmayer bestätigten anschließend ihre Messungen mit Hilfe des "NASA Ramaty High Energy Solar Spectroscopic Imager", einem Satelliten zur Sonnenbeobachtung, der auch den Hyperflare aufgezeichnet hatte. Sie entdeckten dabei erstmals Hinweise auf eine hochfrequente Oszillation von 625 Hertz, die von Wellen stammen könnte, welche sich senkrecht in die Kruste hinein ausbreiten.
Die große Zahl von Frequenzen, die mehr einem Akkord als einem einzelnen Ton gleichen, ermöglichte es den Wissenschaftlern, die Tiefe der Neutronensternkruste abzuschätzen. Dies ist möglich durch den Vergleich der Frequenzen von Wellen, die sich entlang der Sternkruste bewegen, mit jenen, die sich radial durch die Kruste hindurch ausbreiten. Der Durchmesser eines Neutronensterns ist nicht genau bekannt. Wenn man aber den geschätzten Wert von etwa 20 Kilometern annimmt, wäre seine Kruste ungefähr eineinhalb Kilometer dick. Diese aus den gemessenen Frequenzen abgeleitete Zahl stimmt wiederum gut mit theoretischen Modellen überein.
Mit der Sternbeben-Seismologie dürften sich viele weitere Eigenschaften von Neutronensternen bestimmen lassen. Strohmayer und Watts analysierten auch die Daten von "Rossi" zu einem schwächeren Hyperflare eines anderen Magnetars (SGR 1900+14) aus dem Jahr 1998. Sie fanden auch dort die verräterischen Oszillationen. Allerdings waren diese nicht stark genug, um die Krustendicke zu bestimmen.
Mit der Messung der Röntgenstrahlung bei anderen starken Neutronenstern-Explosionen könnten künftig noch weitere Geheimnisse dieser Objekte gelüftet werden, zum Beispiel die Frage nach dem Zustand der Materie in ihrem Innern. Möglicherweise existieren dort nämlich freie Quarks.
Solche Quarks sind die elementarsten Bausteine von Protonen und Neutronen und unter normalen Umständen immer eng aneinander gebunden. Ein Nachweis von ungebundenen Quarks würde helfen, die wahre Natur von Materie und Energie zu verstehen.
Denn bei Experimenten auf der Erde kann man die zur Entdeckung von ungebundenen Quarks notwendigen hohen Energien nicht erzeugen, auch nicht mit den größten Teilchenbeschleunigern,.
"Neutronensterne sind fantastische Laboratorien, um Physik unter Extrembedingungen zu untersuchen.", so Watts. "Wir würden gerne einmal einen solchen Stern aufbrechen, doch da dies wohl leider nicht möglich sein wird, sind Magnetar-Hyperflares vermutlich die beste Möglichkeit, die uns für solche Beobachtungen bleibt."
K1.2
From Prelude to Aftermath — The Giant Flare of 2004 December 27
K2.1
The first giant flare from SGR 1806-20
Zum Thema | |
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| SGR 1806-20 — Etail = 1.6x1044(d/15 kpc)2 erg | |
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Authors: S. Mereghetti, D. Gotz, A. von Kienlin, A. Rau, G. Lichti, G. Weidenspointner, P. Jean |
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Journal-ref: ApJ 624 (2005) L105 [astro-ph/0502577  ] |
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Title: The first giant flare from SGR 1806-20: observations with the INTEGRAL SPI Anti-Coincidence Shield |
| Abstract:
A giant flare from the Soft g-ray Repeater SGR 1806-20 has been detected by several
satellites on 2004 December 27. This tremendous outburst, the first one observed from this
source, was a hundred times more powerful than the two previous giant flares from SGR 0525-66 and SGR 1900+14.
We report the results obtained for this event with the Anticoincidence Shield of the SPI spectrometer on board the INTEGRAL satellite, which provides a high-statistics light curve at E > 80 keV. The initial short peak, which saturated the detector for ~0.7 s, was followed by a ~400 s long tail modulated at the neutron star rotation period of 7.56 s. The tail fluence corresponds to an energy in photons above 3 keV of Etail = 1.6x1044(d/15 kpc)2 erg. This is of the same order of the energy emitted in the pulsating tails of the two giant flares seen from other soft repeaters, despite the hundredfold larger overall emitted energy of the SGR 1806-20 giant flare. Long lasting (~1 hour) hard X-ray emission, decaying in time as t-0.85, and likely associated to the SGR 1806-20 giant flare afterglow has also been detected. INTRODUCTION
Soft g-ray Repeaters (SGRs) are high-energy sources
characterized by sporadic periods of activity in which they emit
short bursts (<1 s) with energy up to ~1041 erg.
They are believed to be highly magnetized (B ~ 1014 - 1015 G) neutron stars, or magnetars. Large soft g-ray flares, reaching peak luminosities above several 1044 erg s-1 and lasting a few minutes, have been observed from two SGRs: on 1979 March 5 from SGR 0525—66 (Mazets et al. 1979) and on 1998 August 27 from SGR 1900+14 (Hurley et al. 1999; Feroci et al. 1999). The detection of only two of such giant outbursts from different sources in about 30 years implies that these events, involving energy releases of more than 1044 ergs, are relatively rare. SGR 1806-20 is currently the most prolific of the four known SGRs. Its level of activity has been increasing in the last few years, during which many bursts have been detected with different satellites, including RXTE and INTEGRAL. On 2004 October 5 two clusters of strong bursts with a total fluence of ~10^{-4} erg cm-2 were emitted within a time span of a few minutes. It was noted that a similar event occurred in SGR 1900+14 three months before its 1998 giant outburst (Aptekar et al. 2001). The increasing level of activity in SGR 1806-20 was also reflected in the properties of its quiescent X—ray emission. XMM-Newton observations showed that its 2-10 keV luminosity doubled and the spectrum became harder in 2004. A similar trend was present in the persistent 20-150 keV emission discovered with INTEGRAL observations carried out in 2003-2004. The energetic activity of SGR 1806-20 culminated on 2004 December 27, when more than twenty satellites recorded the first giant outburst from this source. The hard X-ray flux during the initial pulse was so large to saturate most detectors. Different distinct features, discussed in the next subsections, are visible in the light curve of the flare shown in Fig. 1, where the original data have been rebinned at 2.5 s: • a) a precursor burst 143 s before the flare; • b) a short and very intense initial spike; c) a tail lasting about 400 s, during the first half of which pulsations at the neutron star rotation period are clearly seen. | |
Image credit: INTEGRAL / Mereghetti et al.
Fig. 1 - ACS light curve of the whole flare binned at 2.5 s. The peak of the flare, reaching an observed count
rate > 2·106 counts s-1, is not shown. The inset shows the light curve of the precursor
burst at full resolution (50 ms).
Image credit: INTEGRAL / Mereghetti et al.
Fig. 2 -Declining part of the initial spike. The line is a fit with a power law plus
a constant (fixed at the pre-flare level) to the count rate in the
time interval 1.2—2.2 s. The inset shows the light curve of the
tail binned at the spin period (7.56 s) and fitted with an exponential function.
Image credit: INTEGRAL / Mereghetti et al.
Fig. 3 - Averaged pulse profile obtained by folding the data of the time interval 7.56 - 300 s.
Image credit: INTEGRAL / Mereghetti et al.
Fig. 4 - Individual light curves of the first 24 pulses. Each panel covers
one rotation period of 7.56 s. The top-left panel refers to the time interval
0—7.56 s (the subsequent periods follow from left to right and top to bottom).
The X axes give the phase and the Y axes the normalized intensity (except for the
first panel where a scale factor has been applied).
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The light curve
The light curve of the precursor binned at full resolution (50 ms) is shown in the inset of Fig. 1. The burst lasted slightly longer than 1 s. It had a rapid rise and decline, and there is some evidence for oscillations over a nearly flat topped profile. For a thermal bremsstrahlung spectrum with kT_{br} = 15 keV, as estimated with RHESSI (Hurley et al. 2005), the burst fluence of ~9,280 ACS counts corresponds to 4.4 × 10-6 erg cm-2 at E>80 keV. This implies a fluence above 3 keV a factor ten larger than the RHESSI value. The phase of the event rise time with respect to the 7.56 s pulsations is F ~ 0.1, which does not correspond to those of the peaks seen in the pulsating tail.
The giant flare starts with a short and intense spike during which the ACS count rate remains above 106 counts s-1 for 0.7 s. The time profile at the peak is strongly affected by dead time and saturation effects which cannot be easily modelled. Therefore the timing analysis of the initial spike and an estimate of its energetics are deferred to a future publication.
The pulsating tail
The transition from the initial spike to the pulsating tail is displayed at full resolution in Fig. 2. The light curve in the time interval 1.2—2.2 s is well fitted by a power law function F
t^{-d}
with d = 2.1 (as mentioned above a constant fixed at the pre-flare level is
included in the fit). A narrow burst, lasting ~0.2 s, occurs
at t=2.8 s, while a broader bump is present in the time range from
~2 to 5 s. The latter is in phase (F=0.35—0.6) with the
main peak of the 7.56 s pulsations and can therefore be considered
as the first appearance of the periodicity. Two more bumps
occurring after t=10 s are also due to the 7.56 s pulsations,
which are clearly visible in the tail until t~300 s. The
average profile of the pulsations is shown in
Fig. 3 (phase F=0 corresponds to t=0 s). This
has been obtained by folding the data of
the time interval 7.56—300 s, i.e. excluding the first pulse
which is dominated by the initial spike, at the period of 7.56 s.
The pulse profile evolves
during the flare with the relative intensity of the second peak
increasing with time (Fig. 4).
To characterize the long-term decay profile we have binned the data in time intervals of 7.56 s (see inset of Fig. 1). In this way the short-term variations due to the pulsations are removed. The count rate in the time interval t~15—400 s is well fitted by an exponential function with decay constant t = 138\pm5 s. Note that the first bin (t=7.57—16.4 s) does not contain the initial hard spike. Nevertheless it lies significantly above the fitted exponential function. Assuming a thermal bremsstrahlung spectrum with kT_{br}=30 keV, similar to the tails of the other giant flares, the fluence in the time interval 1—400 s is ~2.6·10^{-4} erg cm-2 for E>80 keV (by extrapolation we find a fluence of ~6.4·10^{-3} erg cm-2 for E>3 keV).
Main features
The main features of the December 27 giant flare, i.e. the presence of a very bright short spike and of a decaying tail modulated at the neutron star spin period, are very similar to those of the two giant flares of SGR 0526-66 and SGR 1900+14. There are, however, a few important differences, the most notable of which is the global energetics of the event. The hard X—ray fluence in the SGR 1806-20 initial spike implies an isotropic-equivalent energy release — we scale our results to a distance of 15 kpc although recent radio measurements indicate a smaller distance — of several 1044(d/15 kpc)2 erg. This is at least two orders of magnitude larger than that of the two other giant flares (1.6·1044 erg for SGR 0526—26; >7·1044 erg for SGR 1900+14). The much higher energy involved in the giant flare from SGR 1806-20 is also reflected in the properties of its transient radio emission which reached a luminosity a factor 500 (Cameron et al. 2005, Gaensler et al. 2005) larger than that seen in SGR 1900+14 after the 27 August 1998 event (Frail et al. 1999).
Energy output in the tail
On the other hand, the energy output in the pulsating tail (1.6·1044(d/15 kpc)2 erg for E>3 keV) is similar to that of SGR 1900+14 (5·1043 erg) and of SGR 0526—66 (4·1044 erg). Note that this value does not change significantly for different spectral assumptions and even considering the additional energy output in the emission after t=400 s would only double the energy budget.
The giant flare of SGR 1806-20
is thus characterized by a much higher spike-to-tail energy ratio (>100) than the previous two giant flares (~1). According to the magnetar model for giant flares essentially all the energy release occurs during the initial ~0.2 s transient phase when a hot relativistic fireball is launched. A fraction of this energy is trapped by closed field lines in the neutron star magnetosphere, forming an optically thick photon-pair plasma which evaporates giving rise to the radiation observed in the pulsating tail.
The magnetic field strength limits the amount of energy that can be confined. The fact that this quantity is similar in the three giant flares, despite the much higher total energy release of SGR 1806-20 , is consistent with a magnetic field of the same order in the three magnetars.
In fact the pulsating tails of the three giant flares are also similar as far as their durations and timing properties are concerned: their envelopes are reasonably well fitted with exponential laws with decay constants of ~70-140 s and are modulated by the neutron star rotation with time-variable pulse profiles (Feroci et al. 2001).
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Literatur zu INTEGRAL | ||||
| Cameron, P.B. et al. | 2005 | Nature |
Discovery of a Radio Afterglow following the Giant Flare from SGR 1806-20 | |
| Aptekar R.L., Frederiks D.D., Golenetskii S.V. et al. | 2001 | ApJSS 137, 227 | ||
| Feroci M. et al. | 1999 | ApJ 515, L9 | ||
| Mazets E.P. et al. | 1979 | Nature 282, | ||
| Hurley K., Cline T., Mazets E. et al. | 1999 | Nature 397, 41 | ||
K2.2
A Giant Flare from The Magnetar SGR 1806-20 (Swift)
| Swift — Eflare > 2x1046 erg | |
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Authors: D. M. Palmer, S. Barthelmy, N. Gehrels, R. M. Kippen, T. Cayton, C. Kouveliotou, D. Eichler, R. A. M. J. Wijers, P. M. Woods, J. Granot, Y. E. Lyubarsky, E. Ramirez-Ruiz, L. Barbier, M. Chester, J. Cummings, E. E. Fenimore, M. H. Finger, B. M. Gaensler, D. Hullinger, H. Krimm, C. B. Markwardt, J. A. Nousek, A. Parsons, S. Patel, T. Sakamoto, G. Sato, M. Suzuki, J. Tueller |
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Journal-ref: Nature 434 (2005) 1107-1109 [astro-ph/0503030 ] |
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Title: Gamma-Ray Observations of a Giant Flare from The Magnetar SGR 1806-20 |
| Abstract:
Magnetars comprise two classes of rotating neutron stars (Soft Gamma
Repeaters (SGRs) and Anomalous X-ray Pulsars), whose X-ray emission is powered
by an ultrastrong magnetic field, Bs ~ 1015 G.
Occasionally SGRs enter into active episodes producing many short X-ray bursts; extremely rarely (about once per 50 years per source), SGRs emit a giant flare, an event with total energy at least 1000 times higher than their typical bursts. Here we report that, on 2004 December 27, SGR 1806-20 emitted the brightest extra-solar transient event ever recorded, even surpassing the full moon brightness for 0.2 seconds. The total (isotropic) flare energy is Eflare > 2x1046 erg, 100 times higher than the only two previous events, making this flare a once in a century event. This colossal energy release likely occurred during a catastrophic reconfiguration of the magnetar's magnetic field. Such an event would have resembled a short, hard Gamma Ray Burst (GRB) if it had occurred within 40 Mpc, suggesting that extragalactic SGR flares may indeed form a subclass of GRBs. INTRODUCTION
Cosmic Explosion the Brightest in Recorded History
Swift: NASA Observes One of the Brightest Cosmic Explosions
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K3
A XMM-Newton View of the Soft g-ray Repeater SGR 1806-20
| SGR 1806-20 — P' = 5.5×10-10 s s-1 | |
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Authors: S. Mereghetti, A. Tiengo, P. Esposito, D. Gotz, L. Stella, G.L. Israel, N. Rea, M. Feroci, R. Turolla, S .Zane |
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Journal-ref: ApJ 628 (2005) 938 [astro-ph/0502417] |
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Title: A XMM-Newton View of the Soft g-ray Repeater SGR 1806—20: Long Term Variability in the pre-Super Giant Flare Epoch |
| Abstract:
The low energy (<10 keV) X-ray emission of the Soft g-ray Repeater SGR1806-20
has been studied by means of four XMM-Newton observations carried out in the last two years,
the latter performed in response to a strong sequence of hard X-ray bursts observed on 2004 October 5.
The source was caught in different states of activity: over the 2003-2004 period the 2-10 keV flux doubled with respect to the historical level observed previously. The long term raise in luminosity was accompanied by a gradual hardening of the spectrum, with the power law photon index decreasing from 2.2 to 1.5, and by a growth of the bursting activity. The pulse period measurements obtained in the four observations are consistent with an average spin-down rate of P' = 5.5×10-10 s s-1, higher than the values observed in the previous years. The long-term behavior of SGR1806-20 exhibits the correlation between spectral hardness and spin-down rate previously found only by comparing the properties of different sources (both SGRs and Anomalous X-ray Pulsars). The best quality spectrum (obtained on 6 September 2003) cannot be fitted by a single power law, but it requires an additional blackbody component (kT=0.79 keV, RBB = 1.9 (d/15 kpc)2 km), similar to the spectra observed in other SGRs and in Anomalous X-ray Pulsars. No evidence for spectral lines was found in the persistent emission, with equivalent width upper limits in the range 30-110 eV. However, a ~3 s deviation at 4.2 keV is present in the cumulative spectrum of 69 bursts detected in September-October 2004. The multi-wavelength spectral energy distribution of SGR1806-20, extending from the IR band to 150 keV as obtained from simultaneous data from the INTEGRAL satellite and the ESO Very Large Telescope, is also presented. | |
K3.1
The calm after the storm: SGR 1806-20 two months after the Giant Flare of 2004
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Authors: A. Tiengo, P. Esposito, S. Mereghetti, N. Rea, L. Stella, G. L. Israel, R. Turolla, S. Zane | |
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Journal-ref: A&A 440 (2005) L63 [astro-ph/0508074 ] | |
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XMM-Newton observation of SGR 1806-20 two months after the Giant Flare of 2004 December 27 Abstract: XMM-Newton observed the soft gamma repeater SGR 1806-20 about two months after its 2004 December 27 giant flare. A comparison with the previous observations taken with the same instrument in 2003-2004 shows that the pulsed fraction and the spin-down rate have significantly decreased and that the spectrum slightly softened. These changes may indicate a global reconfiguration of the neutron star magnetosphere. The spectral analysis confirms that the presence of a blackbody component in addition to the power-law is required. Since this additional component is consistent with being constant with respect to the earlier observations, we explore the possibility of describing the long-term spectral evolution as only due to the power-law variations. In this case, the slope of the power-law does not significantly change and the spectral softening following the giant flare is caused by the increase of the relative contribution of the blackbody over the power-law component. | |
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3. Discussion
Also the spin-down trend of SGR 1806–20 appears to have changed after the flare:
the four spin periods measured by XMM-Newton in 2003–2004 could be linearly fit with P' = 5.5×10-10 s s-1, but the period found in this last observation is smaller than the extrapolation of this trend. It is instead consistent with the slower spin-down rate P' = 1.8×10-10 s s-1, measured during the first RossiXTE observations performed after the giant flare (Woods et al. 2005). Together with the change in pulsed fraction, this result suggests that a substantial reconfiguration of the magnetosphere has occurred, very likely related to the large amount of energy released on 2004 December 27. In the model of magnetar’s twisted magnetosphere (Thompson et al 2002) such a large scale modification is foreseen after a giant flare, since the magnetosphere should relax into a less twisted configuration. As a consequence, the bursting activity should decrease and the spectrum should become softer. Only two bursts are detected by the PN in the ~25 ks observation done after the flare, while in 2004 almost 70 bursts were detected in ~70 ks, with the PN in the same configuration. Therefore, the burst activity had indeed dropped, as already reported in Rea et al. (2005), but not completely stopped (see also, e.g., Palmer el al. 2005). References Palmer, D. M., Barthelmy, S., Gehrels, N., et al. 2005a, Nature 434, 1107 Rea, N., Tiengo, A., Mereghetti, S., et al. 2005, ApJ 627, L133 Thompson, C., Lyutikov, M., Kulkarni, S. R. 2002, ApJ 574, 332 Woods et al. 2007 ApJ 654, 470 | ||
K4
The Brightest Blast
| RHESSI — Giant Flare | |||||||||||
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Authors: S.E. Boggs, A. Zoglauer, E. Bellm, K. Hurley, R.P. Lin, D.M. Smith, C. Wigger, W. Hajdas | ||||||||||
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Journal-ref: ApJ 661 (2007) 458 [astro-ph/0611318 ] | ||||||||||
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Title: The Giant Flare of December 27, 2004 from SGR 1806-20 | ||||||||||
| Abstract:
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.
1. Introduction
The soft gamma repeater SGR1806-20 was discovered in 1979 (Laros et al. 1986), and
has been studied intensively over the intervening two decades at X-ray, g-ray, infrared,
and radio wavelengths. It has emitted over 450 soft g-ray bursts, mostly of short duration,
during sporadic active periods, and has been found to be a quiescent, variable X-ray
source as well, emitting up to ~150 keV (Mereghetti et al. 2005c; Molkov et al. 2005).
Indeed, X-ray observations of its periodic, quiescent component have provided some of the best evidence for a magnetar-strength magnetic field (Kouveliotou et al. 1998), as first proposed by Duncan & Thompson (1992) and Paczynski (1992). The infrared counterpart to SGR1806-20 is a faint, highly obscured source, in keeping with its location towards the Galactic center (Kosugi et al. 2005; Israel et al. 2005). Presumably, it is a lone neutron star, whose infrared intensity varies roughly in concert with bursting activity and its quiescent X-ray flux. There have been numerous attempts to determine the distance to SGR1806-20 by various methods, leading to estimates from 6.4 – 9.8 kpc (Cameron et al. 2005) to 15.1 kpc. In this paper, we will quote all energies and luminosities in terms of d10 = (d/10kpc). Like SGR0525-66 and SGR1900+14, SGR1806-20 has emitted a long duration, hard spectrum giant flare, whose flux at Earth greatly exceeded that of any other known cosmic X-ray source (Hurley et al. 2005; Mazets et al. 2005; Mereghetti et al. 2005a; Palmer et al. 2005). SGRs are not detectable quiescent radio emitters, but giant flares create transient radio nebulae which are observable for weeks (Frail et al. 1999; Gaensler et al. 2005).
X-ray observations carried out several months later, however, revealed a slower spin-down rate, a smaller pulsed fraction for the quiescent emission, a different pulse profile, a softer spectrum, and a decreased flux (Tiengo et al. 2005; Rea et al. 2005). In the magnetar model, these changes are attributed to a major reconfiguration of the neutron star’s magnetic field. In this paper, we present a detailed analysis of the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) data on the giant flare, concentrating on the time-resolved energy spectra of its various phases from 3 keV to 17 MeV. By virtue of the high time and energy resolution and broad spectral coverage of the measurements, we believe that these data present the most complete spectral picture of this, or any other giant flare. References Cameron, P., et al., 2005, Nature 434, 1112 | |||||||||||
The "superflare," from a magnetar named SGR 1806–20, irradiated Earth with more total energy than a powerful solar flare. Yet this object is an estimated 50,000 light-years away in Sagittarius, on the far side of the Milky Way galaxy behind dense interstellar clouds. "This is mind-boggling when you think about how far away it is," says Kevin C. Hurley (University of California, Berkeley), one of the lead investigators. Bryan M. Gaensler, who conducted radio observations of the superflare's afterglow, notes that only the Sun and perhaps a handful of spectacular comets have doused Earth with more total energy than SGR 1806–20's superflare during the two-tenths of a second that it peaked in intensity. During that flicker of time it outshone the full Moon by a factor of two. The magnetar must have let loose as much energy as the Sun generates in 250,000 years, assuming that the distance estimate is accurate.
The burst was so powerful that some of its gamma rays and X-rays reflected off the Moon (a very poor mirror) and were detected by the Russian Helicon-Coronas-F satellite. Amateur radio solar observers with the American Association of Variable Star Observers easily detected the superflare's ionizing effects on Earth's upper atmosphere, even though the radiation smacked into our planet's daylight hemisphere and thus had to compete with the Sun. The superflare has generated intense observational and theoretical research around the world, as the astronomical community has been forced to confront the question of how such a tiny object, about 20 kilometers across, could unleash such unmitigated fury.
Magnetic field lines weaving through the star probably flex its solid crust and heat its interior, leading to stress that is occasionally relieved in sudden "starquakes." Such an event allows the magnetic field to jerk pieces of the crust around and rearrange itself to a lower-energy state. This rearrangement, which is a vastly scaled-up version of a solar flare (a "reconnection event" in the magnetic field), releases a huge amount of magnetic energy in the form of gamma rays, electrons, and positrons (the antimatter counterpart of electrons). It's this radiation that was responsible for the initial spike, which contained 99.7 percent of the superflare's total energy. Electrons and positrons confined by the magnetar's magnetic field annihilate one another over the next several minutes, accounting for a fading tail of emission after the initial 0.2-second spike. This "trapped fireball" model was developed in the mid-1990s by Robert C. Duncan and Christopher Thompson, who also predicted the existence of magnetars in 1992. The SGR superflare might partially explain a long-standing mystery surrounding g-ray bursts (GRBs). These mega-powerful explosions fall into two distinct classes: long events lasting several seconds to several minutes, and short bursts, which last no more than two seconds.
Thanks to the magnetar's great distance, the superflare posed no threat to humanity or Earth's biosphere. The International Space Station was on the opposite side of Earth when the flare hit our planet, but even if the astronauts had faced the full fury of the blast, they would have received a radiation dose less than a dental X-ray. An SGR superflare's pulse of high-energy radiation could seriously damage a planet's atmosphere only if it occurred within about 6 light-years, according to Adrian L. Melott. Numerous papers about the event have already appeared on the preprint server Astro-ph. A number of other papers, including theoretical research that might explain the outburst, are currently being peer-reviewed prior to publication in professional journals. Because of embargoes imposed by some of these journals, astronomers have not been allowed to communicate their results to other scientists, which has hindered progress in understanding this event so far. More details about the superflare, including amateur observations of the atmospheric disturbance, will appear in the May. |
K5
Super-Ausbruch eines SGRs - oder alles nur gebeamt?
Die Gammaquanten waren am 27. Dezember 2004 durch das Sonnensystem geschossen: Der schon länger bekannte
Soft Gamma Repeater SGR 1806-20 hatte bereits die Tage zuvor erhöhte Aktivität gezeigt, aber der Ausbruch
stellte an Intensität alles in den Schatten, was die Gammaastronomie bisher gekannt hatte. Am Erdboden spürte
man zwar direkt nichts, aber die Ionosphäre der Erde reagierte, und mindestens 15 Satelliten und Raumsonden
bekamen etwas mit. Manche wurden von hinten erwischt, doch ihre Detektoren schlugen aus, und fast alle, die
zufällig direkt auf die Quelle schauten, gingen sofort in Sättigung (dafür konnten sie anschließend gut
Variationen der fallenden Strahlung mit 7.6 s Periode verfolgen, offenbar die Rotation eines
Neutronensterns). Die wenigen verläßlichen Aufzeichungen des Ausbruchs selbst stammen von einem
unempfindlichen Detektor auf dem japanischen interplanetaren Satelliten GEOTAIL und von dem russischen
Sonnensatelliten KORONAS-F, dessen Instrument Helicon das Echo des Gammablitzes von der Oberfläche des
Mondes aufzeichnete: Letztere ist ein sehr schlechter Reflektor, und die Dämpfung um eine Million war gerade
richtig für gute Messungen.
Credit: S. Mereghetti [ The Highest Magnetic Fields in the Universe]
|
Das wäre ein Schock, denn hinter den Soft Gamma Repeatern werden heute allgemein Neutronensterne mit besonders starkem Magnetfeld (»Magnetare«) vermutet - und die gesamte in der Magnetosphäre eines Neutronensterns mit 1014 Gauss Oberflächenfeldstärke gespeicherte Energie beträgt ebenfalls rund 1046 erg! Kann man sich überhaupt einen Mechanismus vorstellen, der die gesamte Feldenergie in einem Ruck als Gammablitz freisetzt? Wankt gar die Magnetar-Deutung der Soft Gamma Repeater? Aber vielleicht war alles viel harmloser.
Zum einen gibt es nämlich eine Analyse der Gammalichtkurve des Ausbruchs und des Abfalls der Intensität in den nächsten 0.6 Sekunden, wie sie das Low Energy Particle-Experiment auf GEOTAIL aufzeichnete: Sie läßt sich gut durch einen relativistisch - nahezu lichtschnell - expandierenden Feuerball modellieren, genau wie bei einem klassischen Gamma Ray Burst.
Und der starke Einbruch der Lichtkurve nach 0.6 Sekunden spricht für einen stark kollimierten Jet, mit einem so engen Öffnungswinkel wie bei GRBs. Damit sinkt die Gesamtenergie des Ausbruchs auf unter 2x 1045 erg, und auch die Größe und Entwicklung des Nachglühens im Radiobereich, das seither viele Teleskope verfolgen, paßt besser zu 1043.5 ... 45 erg.
Überdies zeigen Radiospektren dieser Quelle Absorption mehrerer interstellarer Wolken, und daraus ergibt sich eine Distanz von nur 6.4 bis 10 kpc (20 bis 30'000 Lichtjahren), womit der Energiebedarf noch weiter sinkt. Und so erschien der Ausbruch von SGR 1806-20 zwar außergewöhnlich stark auf der Erde, eine Million mal heller als ein typischer Gamma Ray Burst aus den Tiefen des Universums und 100-mal heller als jeder andere Ausbruch eines SGR - aber vermutlich nur, weil wir genau in den Strahl schauten.
K6
SGR 1806-20 after the Giant Flare:
QPOs (RXTE) and a bursting active phase (Chandra)
K6.1 A bursting active phase
| SGR 1806-20 — fX(2-10 keV) ~ 2.2x10-11 erg s-1 cm2 | ||
|
Authors: N. Rea, A. Tiengo, S. Mereghetti, G.L. Israel, S. Zane, R. Turolla, L. Stella | |
|
Journal-ref: ApJ 627 (2005) L133-L136 [astro-ph/0505193 ] | |
|
Title: A first look with Chandra at SGR 1806-20 after the Giant Flare: significant spectral softening and rapid flux decay. | |
We report on the results of a ~30 ks Chandra pointing of the soft g-ray repeater SGR 1806-20, the first X-ray observation with high spectral resolution performed after the 2004 December 27 Giant Flare. The source was found in a bursting active phase and with a significantly softer spectrum than that of the latest observations before the Giant Flare. The observed flux was fX(2-10 keV) ~ 2.2x10-11 erg s-1 cm2, about 20% lower than that measured three months before the event. This indicates that, although its giant flare was ~100 times more intense than those previously observed in two other soft g-ray repeaters, the post flare X-ray flux decay of SGR 1806-20 has been much faster. The pulsed fraction was about 3%, a smaller value than that observed before the flare. We discuss the different properties of the post-flare evolution of SGR 1806-20 in comparison to those of SGR 1900+14 and interpret the results as a strong evidence that a magnetospheric untwisting occurred (or is occurring) after the Giant Flare. | ||
Figure: Long term evolution of the pulse period and energy spectrum of SGR 1806–20.
QPOs (RXTE)
| SGR 1806-20 — QPO: 92.5Hz RXTE | |
|
Authors: G. Israel, et al. |
|
Journal-ref: ApJ 628 (2005) L53-L56
|
|
Title: Discovery of Rapid X-ray Oscillations in the Tail of the SGR 1806-20 Hyperflare |
A superstrong Magnetic Field
| SGR 1806-20 — nQPO = 18, 30, 93, 150, 625 and 1840 Hz | |
|
Authors: Mario Vietri, Luigi Stella, Gian Luca Israel |
|
Journal-ref: ApJ (2007) [astro-ph/0702598 ] |
|
Title: SGR1806-20: evidence for a superstrong Magnetic Field from Quasi Periodic Oscillations |
| Abstract:
Fast Quasi-Periodic Oscillations (QPOs, frequencies of ~ 20 - 1840 Hz) have been recently discovered in the
ringing tail of giant flares from Soft Gamma Repeaters (SGRs), when the luminosity was of order
1041 - 41.5 erg s-1.
These oscillations persisted for many tens of seconds, remained coherent for up to hundreds of cycles and were observed over a wide range of rotational phases of the neutron stars believed to host SGRs. Therefore these QPOs must have originated from a compact, virtually non-expanding region inside the star's magnetosphere, emitting with a very moderate degree of beaming (if at all). The fastest QPOs imply a luminosity variation of DL/Dt ~ 6 × 1043 erg s-2, the largest luminosity variation ever observed from a compact source. It exceeds by over an order of magnitude the usual Cavallo-Fabian-Rees (CFR) luminosity variability limit for a matter-to-radiation conversion efficiency of 100%. We show that such an extreme variability can be reconciled with the CFR limit if the emitting region is immersed in a magnetic field > 1015 G at the star surface, providing independent evidence for the superstrong magnetic fields of magnetars. 1. Introduction
Soft Gamma Repeaters are a small class of galactic sources of X and soft gamma radiation.
They have spin periods of ~5 - 10 s, display a secular spin-down with timescales of ~10 - 100 kyr
and do not possess a companion. Unlike radio pulsars, the rotational energy loss of SGRs is a factor
of 10 - 100 too small to explain their persistent emission, typically
~ 1033 - 34 erg s-1 (see e.g.
Woods & Thompson (2006)). Like Anomalous X-ray Pulsar (AXPs),
with whom they share a number of properties, SGRs are believed to host magnetars, neutron stars
the emission of which is powered by the decay of their superstrong (internal) magnetic field
(B > 1015 G).
2. Quasi Periodic Oscillations in Giant Flares of SGRs
Recent studies led to the discovery that the X-ray flux of the ringing tail of SGRs’ giant
flares is characterized by fast Quasi Periodic Oscillations, QPOs (Israel et al. 2005). Different
QPO modes were detected, some of which were excited simultaneously.
The ringing tail of the December 2004 event from SGR 1806-20 displayed clear QPO signals at about nQPO = 18, 30, 93, 150, 625 and 1840 Hz (Strohmayer & Watts, 2006). Similarly, QPOs around frequencies of nQPO = 28, 54, 84 and 155 Hz were detected during the ringing tail of the 1998 giant flare of SGR 1900+14 (Strohmayer & Watts 2005), while hints for a signal at ~ 43 Hz were found in the March 1979 event from SGR 0526-66 (Barat et al. 1983). These QPOs show large variations of the amplitude with time and, especially, of the phase of the spin modulation in the giant flare’s tail. References Israel, G. L. et al. 2005, ApJ 628, L53 | |
| Literatur zu "SGR 1806-20 (II)"
(I)
(III)
Magnetars
| |||
| C. Kouveliotou, J. van Paradijs, S. Dieters, T. Strohmayer, et al. | 1998 | Nature 393, 235-7 |
"An X-ray pulsar with a superstrong magnetic field in the soft gamma-ray repeater SGR1806-20"
|
| A. Tiengo, P. Esposito, S. Mereghetti, N. Rea, L. Stella, G. L. Israel, R. Turolla, S. Zane | 2005 | A&A 440, L63 |
"The calm after the storm: XMM-Newton observation of SGR 1806-20 two months after ..."
|
| T.E. Strohmayer & A.L. Watts | 2005 | ApJ 632, L111 |
"Discovery of fast X-ray oscillations during the 1998 giant flare from SGR 1900+14"
|
| GianLuca Israel, Tomaso Belloni, Luigi Stella, Yoel Rephaeli, Duane Gruber, et al. | 2005 | ApJ 628, L53–L56 |
"Discovery of Rapid X-ray Oscillations in the Tail of the SGR 1806-20 Hyperflare"
|
| N. Rea, A. Tiengo, S. Mereghetti, G.L. Israel, S. Zane, R. Turolla, L. Stella | 2005 | ApJ 627, L133-6 |
"Chandra: SGR 1806-20 after the Giant Flare: spectral softening and rapid flux decay"
|
| Mereghetti S., Götz D., von Kienlin A. et al. | 2005 | ApJ 624, L105 |
"The first giant flare from SGR 1806-20: observations with the INTEGRAL SPI"
|
| S. Mereghetti, A. Tiengo, P. Esposito, D. Gotz, L. Stella, et al. | 2005 | ApJ 628, 938 |
"A XMM-Newton View of the Soft g-ray Repeater SGR 1806-20"
|
| A.L. Watts & T.E. Strohmayer | 2006 | ApJ 637, L117 |
"RHESSI: X-ray oscillations in the tail of the 2004 hyperflare from SGR 1806-20"
|
 | H. Heintzmann | ( Eintrag vom 19.7.2008) |
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