Artikel zu "SGR1806-20 (MIV)"

(III)

(II)

(I)

Magnetar (MIV)

The first giant flare from SGR 1806-20
Artikel zu SGR1806-20 (I)
INTEGRAL: The first Gamma-Ray Burst Catalogue
Soft Gamma-Ray Repeater SGR 1806-20
Gamma-Blitz traf die Erde
K1 SGR 1806-20: first evidence of crustal cracking
K2 Gamma-Blitz von Magnetar - Lösung für kosmisches Rätsel?
K3.1 Supergiant Fast X-ray Transients (SFXTs)
K3.2 XTE J1739-285 (n = 1122 Hz)
K3.3 XTE J1739-302
K3.4 Long-term monitoring of XTE J1739-302
K4 A0538-66: a Be/X-ray transient in LMC
K5 A 0535+26 — a Be/Xray binary pulsar
Literatur

Image credit: ESA

Das International Gamma Ray Astrophysics Laboratory - INTEGRAL -

The g-ray giant flare from SGR1806-20

K1 SGR 1806-20: first evidence of crustal cracking

SGR 1806-20 — Crustal Cracking during a giant g-ray flare
	Authors: Steven J. Schwartz, Silvia Zane, Robert J. Wilson, Frank P. Pijpers, Daniel R. Moore, D. O. Kataria, Timothy S. Horbury, Andrew N. Fazakerley, Peter J. Cargill
	Journal-ref: ApJ 627 (2005) L129-L132 [astro-ph/0504056 ]
	Title: The Gamma-Ray Giant Flare from SGR 1806-20: Evidence of Crustal Cracking via Initial Timescales
Abstract: Soft gamma-ray repeaters (SGRs) are neutron stars that emit short (<~1 s) and energetic (<~ 10⁴² erg s^-1) bursts of soft gamma-rays. Only four of them are currently known. Occasionally, SGRs have been observed to emit much more energetic "giant flares'' (~10⁴⁴ - 10⁴⁵ erg s^-1). These are exceptional and rare events. We report here on serendipitous observations of the intense gamma-ray flare from SGR 1806-20 that occurred on 2004 December 27. Unique data from the Cluster and Double Star TC-2 satellites, designed to study the Earth's magnetosphere, provide the first observational evidence of three separate timescales within the early (first 100 ms) phases of this class of events. These observations reveal that in addition to the initial very steep (<0.25 ms) X-ray onset, there is first a 4.9 ms exponential rise timescale followed by a continued exponential rise in intensity on a timescale of 70 ms. These three timescales are a prominent feature of current theoretical models, including the timescale (several milliseconds) for fracture propagation in the crust of the neutron star.

[21 Sep 2005]
On 27 December 2004, radiation from the biggest starquake on a neutron star ever recorded reached Earth. Unique data obtained by Double Star TC-2 and Cluster satellites enabled a group of European scientists to find the first observational evidence of cracks in the neutron star crust, during the initial phase of the starquake. This result, published 16 June 2005 in the Astrophysical Journal, dicriminates between current theories on the physical origin of such massive starquakes.

Credit: NASA/HST/ASU/J. Hester et al., NASA/CXC/ASU/J. Hester et al.

Image 1. The Crab pulsar in a composite view from Hubble (optical in blue) and Chandra (X-ray in red).

Neutron stars and pulsars

Millions of neutron stars populate our Milky Way galaxy. A neutron star is the remaining core of a massive star, once it has exploded. Made almost entirely of neutrons (subatomic particles with no electric charge), this stellar corpse concentrates more than the mass of our Sun within a sphere of ~20 km diameter. It is so dense that a sugar cube of neutron star on Earth would weight as much as all of humanity!
Two other physical properties characterise a neutron star, their fast rotation (or spin) and their high magnetic field.
Astronomers have found different classes of neutron stars based on these properties. Some of them are the fastest spinning stars in the Universe (up to hundreds of revolutions per second); named pulsars (Image 1), as they generate regular pulses of electromagnetic radiation including radio, visible, X-ray and gamma-ray wavelengths.
These pulses are often compared to a spinning lighthouse beacon which appears to flash on and off.

Credit: NASA

Image 2. Artistic impression of cracks on a magnetar in the initial phase of a starquake

Magnetars

Another class of neutron stars is known as magnetars (Image 2), due to their ultra-high magnetic field. Their magnetic field intensity is indeed about 100 gigaTesla (or 10¹¹ T), a thousand times more than an ordinary neutron star. By comparison, the Earth's magnetic field is about 50 microTesla (5×10^-5 T). Most media used for data storage can be erased if they are exposed to a magnetic field of milliTesla (10^-3 T) intensity.
So far, a dozen of magnetars have been found. Four of them are also known as soft gamma repeaters, or SGRs, because they sporadically release large bursts of low energy (soft) gamma rays and (hard) X-rays, usually during short time periods (~ 0.1 s).

Credit: NASA

Image 3. SGR 1806-20 location.

Extreme starquake on 27 December 2004

On 27 December 2004, the radiation from an extremely powerful explosion on the surface of SGR 1806-20 (the numbers indicate its position in the sky) reached Earth and lasted more than 6 minutes. During the first 200 ms, the amount of energy released was equivalent to what our Sun radiates in 250 000 years. It is the brightest event known to have impacted the Earth from an origin outside our solar system.
SGR 1806-20 is located at around 50 000 light-years from Earth on the far side of our Milky Way galaxy, in the direction of the Sagittarius constellation (Image 3). A similar blast within 10 light years would have destroyed the ozone layer and be similar to a major nuclear blast. Fortunately, the closest known magnetar is 13 000 light years away.

Credit: NASA

Video 1. Artist's impression of a giant flare from a magnetar after the cracking of its surface.

Missing link found by Double Star and Cluster

Several scientific satellites observed the giant flare experienced by SGR 1806-20, including ESA γ-ray observatory INTEGRAL (see related publication by Israel, G.L., et al.). The intensity of the emissions received may be roughly described as a major peak followed by a modulated decrease. However, the intensity of this major peak was hundreds of times stronger than any other observed so far (only two other giant flares have been recorded in the past 35 years). "For the first 200 ms it saturated almost all instruments on satellites equipped to observe γ-rays", underlined Prof. Steve J. Schwartz from Imperial College London (UK) in his 16 June 2005 Astrophysical Journal paper.
Although designed to study the Earth's magnetosphere, the thermal electron detectors onboard Double star TC-2 and Cluster satellites performed unsaturated observations of this initial flare rise and decay (Image 4). As explained in his 16 June paper, Professor Schwartz and his co-authors show that these unique data provide the first observational evidence of three separate timescales within the first 100 ms of this event. The characteristics of these timescales (such as the number, duration, shape) play an important role in actual theories of starquakes on magnetars, which allows discriminating between them.
Based on these measured timescales and current theoretical models, the following scenario is confirmed in this paper. The giant flare is produced when the crust of the magnetar can no longer respond almost plastically to internal magnetic stress and finally cracks. One of the three timescales even allows an estimation of the fracture size: about 5 km. This is a significant size considering that SGR 1806-20 has been estimated to be a sphere of few tens of km in diameter.

Credit: Schwartz et al., 2005]

Image 4. Combined Double Star TC-2 (triangles) and Cluster 4 High Energy Electron Analyser (gray circles) count rates. The data have been shifted in time to align features near the peak count rates. Solid lines show exponential fits to the steepest TC-2 rise, and also to the TC-2 determination of the period leading to the main peak

Presence of quasi-periodic oscillations

On 27 June 2005, the Astrophysical Journal published a related study on SGR 1806-20, this time led by Italian astronomer GianLuca Israel from INAF-Osservatorio Astronomico di Roma. His data analysis reveals the presence of quasi-periodic oscillations (or modes) at the end of the 27 December 2004 event.
"These modes are likely to be associated with global seismic oscillations. In particular, the large crustal fracturing inferred by us can easily excite toroidal modes with characteristic frequencies in the observed range", commented Professor Schwartz in his 16 June paper.
Therefore, Double Star TC-2 and Cluster data have not only enabled to directly estimate crustal properties of magnetars, they have also linked interior magnetic processes and their external consequences during giant flares.

"Cluster and Double Star were designed to study the various boundary layers of the Earth's magnetosphere, including the physics of magnetic reconnection. Such boundary layer physics has application throughout the astrophysical plasma universe, and it is therefore appropriate that these missions contribute in a more direct way to the study of magnetic reorganisation in an astrophysical object outside the solar system", concluded Professor Schwartz.
This new result illustrates the complementarity of the Double Star and the Cluster missions. New results from both missions will be discussed this week (19-23 September) at ESTEC during a five day symposium gathering more than 200 researchers coming from Europe, USA, China and Japan.

Related articles
K. Hurley, S. E. Boggs, D. M. Smith, R. C. Duncan, R. Lin, A. Zoglauer, S. Krucker, G. Hurford, H. Hudson, C. Wigger, W. Hajdas, C. Thompson, I. Mitrofanov, A. Sanin, W. Boynton, C. Fellows, A. von Kienlin, G. Lichti, A. Rau, T. Cline, (RHESSI)
An exceptionally bright flare from SGR 1806–20 and the origins of short-duration γ-ray bursts, Nature 434, 1098-1103 (28 April 2005)

Israel, G. L., T. Belloni, L. Stella, Y. Rephaeli, D. E. Gruber, P. Casella, S. Dall'Osso, N. Rea, (RXTE)
Discovery of rapid X-ray oscillations in the tail of the SGR 1806-20 hyperflare,
ApJ, 628:L53–L56, 2005

K2 SGR 1806-20 recorded by INTEGRAL SPI Anti-Coincidence Shield

Zum Thema
Teil 1 — Gamma-Blitz traf die Erde	The first giant flare from SGR 1806-20

Gamma-Blitz von Magnetar trifft Erde — Lösung für mysteriöses kosmisches Rätsel?
[21 Sep 2005] (G. Lichti, A. von Kienlin)
Abb. 1: Magnetar mit extrem starken Magnetfeld. Solch ein Objekt wird als Ort des Gammastrahlenausbruchs vermutet. (Bild: R. Mallozzi/NASA)
Abb. 2: Lichtkurve des Ausbruchs von SGR 1806-20, gemessen mit dem Antikoinzidenzschild des Spektrometers SPI auf dem INTEGRAL-Satelliten. Zu sehen ist der kurze Hauptausbruch bei 0 Sekunden, von dem nur der unterste Teil zu sehen ist. Das Maximum ist etwa 22 mal höher als im Bild gezeigt. Danach setzt das pulsierende Nachleuchten ein, das bis etwa 200 Sekunden deutlich zu sehen ist. (Bild: A. von Kienlin, MPE)
Abb. 3: Überblick und Details der Lichtkurve des Ausbruchs von SGR 1806-20. (Bild: A. von Kienlin, MPE)

Am 27. Dezember 2004 um 22:30:26 MEZ wurde die Erde von einer gewaltigen Wellenfront von Gamma- und Röntgenstrahlung getroffen. Es war der stärkste Fluß von hochenergetischer Strahlung, der bisher auf der Erde gemessen wurde. Er war sogar stärker als der stärkste jemals gemessene Flare (Strahlungsausbruch) von der Sonne. Das interessante an dieser Entdeckung ist die Entstehung dieser Strahlung. Sie stammt nämlich von einem winzigen Himmelskörper, einem extrem dichten Neutronenstern mit einem äußerst starken Magnetfeld, der sich auf der anderen Seite unserer Milchstraße in etwa 50 000 Lichtjahren Entfernung befindet. Dieser Magnetar, der einen Durchmesser wie eine mittlere Großstadt und eine Masse vergleichbar mit der Sonne hat, erlitt eine gewaltige magnetische Instabilität, wobei sich sein starkes Magnetfeld in einen niedrigeren Energiezustand umorientierte. In den ersten 0.2 s wurde von diesem Objekt die gleiche Energiemenge emittiert wie von der Sonne in etwa einer Viertelmillion Jahren. Dieser Ausbruch war etwa 100-mal stärker als der bisher stärkste Ausbruch von einem Magnetar. Verständlich, daß dieses Ereignis Wissenschaftler auf der ganzen Welt, darunter auch Wissenschaftler des Max-Planck-Institut für extraterrestrische Physik in Garching bei München, in Aufregung versetzte und sie die Daten schnell auswerteten. Erste Ergebnisse sind in einem Artikel enthalten, der vor kurzem bei Nature zur Veröffentlichung eingereicht wurde. Sie werfen ein neues Licht auf die Physik von Magnetaren und tragen dazu bei, ein seit langem existierendes Rätsel um kosmische Gamma-Strahlenbursts lösen zu können.
Dieses seltene Ereignis wurde von einem Detektor gemessen und sofort weltweit bekannt gemacht, der von einer Gruppe am Max-Planck-Institut für extraterrestrische Physik für die INTEGRAL-Mission der ESA gebaut wurde. Es handelt sich dabei um ein Antikoinzidenzschild des INTEGRAL-Spektrometers SPI, das aus 91 Blöcken aus sehr schweren und somit gut für den Nachweis von Gammastrahlung geeigneten Wismutgermanat-Kristallen (BGO) besteht, die insgesamt 512 kg wiegen. Dieses Schild, dessen Hauptaufgabe darin besteht, den Hauptdetektor von SPI von Hintergrundstrahlung abzuschirmen, wird auch als Gammastrahlendetektor verwendet. Es handelt sich dabei um einen der sensitivsten Gammaburstdetektoren, der zur Zeit die Erde umkreist. "Allein durch die Messung dieses Strahlenausbruches hat sich die Entwicklung dieses Burstdetektors gelohnt", sagt Giselher Lichti, unter dessen Führung dieser Detektor entwickelt und gebaut wurde. Später wurde dann festgestellt, daß dieser Burst auch noch von 13 anderen Röntgen- und Gammadetektoren gemessen wurde, die im interplanetaren Raum zwischen Erde und Saturn Messungen durchführen. Sogar der russische Coronas-F Satellit sah diesen Burst, obwohl er sich zur Zeit des Ereignisses hinter der Erde befand, er die direkte Strahlung von der Quelle also gar nicht messen konnte. Eine Analyse der Ankunftszeiten ergab, daß dieses Instrument Gammastrahlen von diesem Burst gemessen hatte, die von der Mondoberfläche reflektiert worden waren.
Wegen der Stärke des Bursts und seiner durchdringenden Strahlung konnten auch Detektoren diesen Burst messen, die nicht auf die Stelle am Himmel gerichtet waren, von dem die Strahlung kam. Die Gammastrahlung durchdrang nämlich die Abschirmungen aus Metall oder Kristallen und brachte die Detektoren kurzzeitig in die Sättigung. Zum Glück ist die Atmosphäre für diese Strahlung undurchdringlich, da sie die Atome der Hochatmosphäre ionisiert und dabei absorbiert wird, so daß das Leben auf der Erde nicht in Gefahr war. Lediglich die auf dem Satelliten Integral angebrachten Zähler des vom Max-Planck-Institut für extraterrestrische Physik gebauten Burstdetektors sind so ausgelegt, daß sie nicht gesättigt wurden. "Das Verhalten der vom MPE gebauten Detektoren unter so hohem Strahlungsfluß muß noch genauer untersucht werde, um eine direkte Abschätzung der gesamten Energieabstrahlung dieses Ereignisses zu erlauben", sagt Andreas von Kienlin, ebenfalls Wissenschaftler am MPE, der das Instrument am Boden und im Orbit kalibriert und eingestellt hat. Mit einer Zeitauflösung von 50 ms liefert das vom Max-Planck-Institut für extraterrestrische Physik gebaute Instrument auch eine hochaufgelöste Lichtkurve.

Very-Large Array in Socorro, New Mexico,

Die Position des Objektes, das den Ausbruch zeigte, befindet sich in der Konstellation Sagittarius (zur Zeit des Ausbruchs nur etwa 5º von der Sonne entfernt), in der Nähe des galaktischen Zentrums. Mit Hilfe des interplanetaren Netzwerkes, einem Zusammenschluß von mehreren Satellitenmissionen, gelang es Kevin Hurley von der Universität Berkeley in Kalifornien mittels Triangulation die Position des Objektes so genau zu bestimmen, daß man es mit dem bekannten Magnetar SGR 1806-20 identifizieren konnte (SGR steht für Soft Gamma-Ray Repeater, weil diese Sterne gelegentlich Bursts von weicher Gammastrahlung emittieren). Diese Position wurde sehr genau von Radioastronomen des Very-Large Array in Socorro, New Mexico, durch Messung eines verglimmenden Nachleuchtens bei Radiowellen bestätigt. Die Beobachtung dieses Nachleuchtens liefert außerdem wichtige Informationen über den Explosionsmechanismus und wird zu einem besseren Verständnis des beobachteten Phänomens beitragen.
Der Ausbruch begann mit der Emission von harter Gammaemission, die nur einen Bruchteil einer Sekunde dauerte, aber den Großteil der emittierten Energie enthielt. Dieser Ausbruch war gefolgt von einer schwächeren Röntgenemission, die mehr als 6 Minuten andauerte und deren Intensität mit einer Periode von 7.56 s oszillierte. Diese Oszillation wird mit der Rotationsperiode des Neutronensterns in Verbindung gebracht.
Die Messungen zeigten, daß die Energieverteilung der Gammaquanten des Hauptausbruches charakteristisch für ein ultra-heißes thermisches Glühen (Plasma) ist. Genau das, was die Wissenschaftler von einem Magnetar erwarten, der leichte hochenergetische Teilchen ausstößt (im wesentlichen Elektronen und Positronen, den Antiteilchen der Elektronen). Die meisten dieser Teilchen zerstrahlten offensichtlich in reine Gammastrahlen, die dann in den interstellaren Raum entwichen. Radiobeobachtungen erzählen uns jedoch, daß wenig Strahlung von diesen Teilchen selbst im Radiobereich erzeugt wurde. Die oszillierende Röntgenemission stammt offenbar von übriggebliebenen Elektronen, Positronen und Gammastrahlen, die im Magnetfeld des Magnetars eingeschlossen sind. Die Theorie sagt vorher, daß solch ein heißer eingeschlossener Feuerball (trapped fireball) innerhalb von Minuten schrumpfen und verdampfen sollte. Seine Helligkeit scheint zu oszillieren, weil der Feuerball über das Magnetfeld on die Oberfläche des rotierenden Neutronensterns gebunden ist.

Bild: A. von Kienlin, MPE

Lichtkurve des Ausbruchs von SGR 1806-20, gemessen mit dem Antikoinzidenzschild des Spektrometers SPI auf dem INTEGRAL-Satelliten. Zu sehen ist der kurze Hauptausbruch bei 0 Sekunden, von dem nur der unterste Teil zu sehen ist. Das Maximum ist etwa 22 mal höher als im Bild gezeigt. Danach setzt das pulsierende Nachleuchten ein, das bis etwa 200 Sekunden deutlich zu sehen ist.

Die riesige Energiemenge des Ausbruchs vom 27. Dezember 2004 legt eine neue Lösung für ein altes Problem der Gammastrahlen-Burstastronomie nahe. Es handelt sich um die Frage, was die Quellen der so genannten "Short-Duration Gamma-Ray Bursts" sind. In den letzten 35 Jahren hat man Hunderte von kurzen (<2 s) mysteriösen Blitzen von hochenergetischer Strahlung aus den Tiefen des Raumes gemessen, ohne daß man weiß, woher diese gemessene Strahlung kommt. Eine Hypothese besagt, daß diese Strahlung bei der Verschmelzung von zwei kompakten Objekten (z. B. von zwei Neutronensternen oder einem Neutronensternmit einem Schwarzen Loch) entstehen könnte. Die neuen Beobachtungen lassen nun eine weitere Interpretation der Beobachtungen zu. Es könnte sich dabei nämlich zum Teil um Ausbrüche wie dem am 27. Dezember beobachteten handeln. Danach können solche kurzen Ausbrüche auf Grund ihrer Intensität von sehr fernen Galaxien beobachtet werden. Ein Ereignis mit der vor kurzem gemessen Stärke könnte bis zu Entfernungen von einigen Hundertmillionen Lichtjahren beobachtet werden. Da sich in diesem Entfernungsbereich viele Galaxien befinden müßte man solche Ereignisse häufig sehen. Man könnte damit also die Beobachtungen zu einem beträchtlichen Teil, wenn nicht sogar ganz, erklären.
Wie kann man sich nun den enormen Energieausstoß von einem solchen Magnetar erklären? Die Erfinder des Magnetarmodells, die Theoretiker Robert Duncan und Christopher Thompson, schlagen folgendes neue Szenario vor, um den gigantischen Energieausstoß bei einem solchen Flare erklären zu können: um ihre Idee verstehen zu können, muß man sich erst einmal das ungeheuer starke Magnetfeld eines Magnetars bewußt machen, das um einen Faktor 1000 stärker ist als das eines normalen Neutronensterns (das bereits eine Billion (10¹²) mal stärker ist als das Magnetfeld der Erde). In solchen starken Feldern wird z. B. ein Wasserstoffatom so stark deformiert, daß es nadelförmig wird (~200 mal schmäler als lang). So ein Stern hat tief in seinem Inneren ein stark verdrilltes Magnetfeld, dessen Magnetfeldlinien sich wie eine Uhrfeder um die Rotationsachse winden. Sein äußeres Magnetfeld jedoch ähnelt mehr oder weniger dem eines Dipols eines Stabmagneten (vergleichbar dem Erdmagnetfeld).
Man glaubt, daß das verdrillte innere Magnetfeld das Überbleibsel der schnellen Rotation ist, die der Stern bei seiner Geburt mitbekam. Es enthält den größten Teil der magnetischen Energie des Sterns. Dieses Magnetfeld übt unvermeidlicherweise eine Kraft von unten auf die 1 km dicke Kruste des Sterns mit einem Radius von 10 km aus. Mit der Zeit wird diese Kraft die Kruste so verschieben, daß die nördliche magnetische Hemisphäre sich gegenüber der südlichen verschiebt. Das hat zum einen zur Folge, daß sich das äußere Magnetfeld verdrillt und zum anderen, daß starke Ladungsströme um den Stern fließen. Wenn sich die Magnetfelder immer stärker verdrillen, dann lassen diese Ströme den Stern hell im hochenergetischen Röntgenbereich aufscheinen. Die Verdrillung des äußeren Magnetfeldes beeinflußt auch die Rotation des Sterns und führt zu einer stärkeren Abbremsung.
Das scheint mit dem Magnetar SGR 1806-20 im Jahr 2004 passiert zu sein. Von März 2004 bis zum Ausbruch im Dezember hat SGR 1806-20 viele einzelne schwache Ausbrüche gezeigt, die auf eine Verschiebung der Kruste hindeuteten (diese Ausbrüche waren weniger stark als der Riesenausbruch vom Dezember, aber einzelne Bursts emittierten in einem Bruchteil einer Sekunde immer noch soviel Energie wie die Sonne in einem Jahr). SGR 1806-20 wurde also immer heller im Röntgenlicht, mit Emission von immer mehr harten Röntgenphotonen und einer stärkeren Abbremsung. Alle diese Messungen deuteten darauf hin, daß sich das äußere Magnetfeld mehr und mehr verdrillte. In dem Modell für den Ausbruch vom 27. Dezember von Duncan und Thompson wurde die Verdrillung so stark, daß der Stern mit seiner Kruste instabil wurde. Die Spannung des äußeren Magnetfelds hat sich dann in einem enormen Ausbruch entladen und das Magnetfeld hat sich dann in einem niedrigeren und unverdrillten Zustand neu angeordnet.

K3 Supergiant Fast X-ray Transients (SFXTs)

Zum Thema
»Superbursts« IGR J11215-5952: Supergiant Fast X-ray Transient	X-ray Transients X-Ray Pulsars — single and binary — with and without SNR

[16 November 2005] Eine neue Klasse von Röntgendoppelsternen
mit Superriesen als Partner eines kompakten Objekts (vermutlich eines Neutronensterns) hat der ESA-Satellit Integral entdeckt - und von solchen »supergiant fast X-ray transients« könnte es sogar eine Menge in der Milchstraße geben,

Supergiant Fast X-ray Transients — A new class
	Authors: Ignacio Negueruela, David M. Smith, Pablo Reig, Sylvain Chaty, Jose Miguel Torrejon
	Journal-ref: ESA-SP 604 (2005) Proceedings of "The X-ray Universe 2005" [astro-ph/0511088 ]
	Title: Supergiant Fast X-ray Transients: A new class of high mass X-ray binaries unveiled by INTEGRAL
Abstract: INTEGRAL monitoring of the Galactic Plane is revealing a growing number of recurrent X-ray transients, characterised by short outbursts with very fast rise times (~ tens of minutes) and typical durations of a few hours. Here we show that several of these transients are associated with OB supergiants and hence define a new class of massive X-ray binaries which we call Supergiant Fast X-ray Transients (SFXTs). Many other transient X-ray sources display similar X-ray characteristics, suggesting that they belong to the same class. Since they are difficult to detect and their number is growing fast and steadily, they could represent a major class of X-ray binaries.

Integral reveals new class of ‘supergiant’ X-ray binary stars
ESA’s Integral gamma-ray observatory has discovered a new, highly populated class of X-ray fast ‘transient’ binary stars, undetected in previous observations. With this discovery, Integral confirms how much it is contributing to revealing a whole hidden Universe.
The new class of double star systems is characterised by a very compact object that produces highly energetic, recurrent and fast-growing X-ray outbursts, and a very luminous ‘supergiant’ companion.

INTEGRAL / Blondin

Interaction between compact stellar object and wind of a supergiant

The compact object can be an accreting body such as a black hole, a neutron star or a pulsar. Scientists have called such class of objects ‘supergiant fast X-ray transients’. ‘Transients’ are systems which display periods of enhanced X-ray emission.
Before the launch of Integral, only a dozen X-ray binary stars containing supergiants had been detected. Actually, scientists thought that such high-mass X-ray systems were very rare, assuming that only a few of them would exist at once since stars in supergiant phase have a very short lifetime.
However, Integral’s data combined with other X-ray satellite observations indicate that transient supergiant X-ray binary systems are probably much more abundant in our Galaxy than previously thought.
In particular, Integral is showing that such ‘supergiant fast X-ray transients’, characterised by fast outbursts and supergiant companions, form a wide class that lies hidden throughout the Galaxy.

Supergiant fast X-ray transients

IBIS / INTEGRAL

The figure shows an artist's impression of the stellar wind and mass transfer around a supergiant with a light curve observed with IBIS onboard INTEGRAL overlayed.

Due to the transitory nature, in most cases these systems were not detected by other observatories because they lacked the combination of sensitivity, continuous coverage and wide field of view of Integral.
They show short outbursts with very fast rising times – reaching the peak of the flare in only a few tens of minutes – and typically lasting a few hours only. This makes the main difference with most other observed transient X-ray binary systems, which display longer outbursts, lasting typically a few weeks up to months.
In the latter case, the long duration of the outburst is consistent with a ‘viscous’ mass exchange between the star and an accreting compact object.
In ‘supergiant fast X-ray transients’, associated with highly luminous supergiant stars, the short duration of the outburst seems to point to a different and peculiar mass exchange mechanism between the two bodies.
This may have something to do with the way the strong radiative winds, typical of highly massive stars, feed the compact object with stellar material.

Interaction between compact stellar object and wind of a supergiant
Scientists are now thinking about the reasons for such short outbursts. It could be due to the supergiant donor ejecting material in a non-continuous way. For example, a clumpy and intrinsically variable nature of a supergiant’s radiative winds may give rise to sudden episodes of increased accretion rate, leading to the fast X-ray flares.
Alternatively, the flow of material transported by the wind may become, for reasons not very well understood, very turbulent and irregular when falling into the enormous gravitational potential of the compact object.
“In any case, we are pretty confident that the fast outbursts are associated to the mass transfer mode from the supergiant star to the compact object,” says Ignacio Negueruela.
“We believe that the short outbursts cannot be related to the nature of the compact companion, as we observed fast outbursts in cases where the compact objects were very different - black holes, slow X-ray pulsars or fast X-ray pulsars.”
Studying sources such as ‘supergiant fast X-ray transients’, and understanding the reasons for their behaviour, is very important to increase our knowledge of accretion processes of compact stellar objects. Furthermore, it is providing valuable insight into the evolution paths that lead to the formation of high-mass X-ray binary systems.
Though the role of Integral has been key in the discovery of this class of objects, there have been important contributions from other X-ray satellites.
In particular, the work that led to this discovery was started by US collaborator David Smith, using Rossi XTE data on XTE J1739-302, appearing in the article ‘XTE J1739-302 as a supergiant fast X-ray transient’ by

D.M. Smith, et al. 2006.

K3.2 XTE J1739-285 (n = 1122 Hz)

Zum Thema
J1748-2446ad — Spinning Pulsar Smashes Record	Gravitationsstrahlung

XTE J1739-285 — n = 1122 Hz
	Authors: P. Kaaret, Z. Prieskorn, J.J.M. in 't Zand, S. Brandt, N. Lund, S. Mereghetti, D. Gotz, E. Kuulkers, J.A. Tomsick
	Journal-ref: ApJ 657 (2007) L97 [astro-ph/0611716 ]
	Title: Discovery of 1122 Hz X-Ray Burst Oscillations from the Neutron-Star X-Ray Transient XTE J1739-285
Abstract: We report the discovery of oscillations at 1122 Hz in an X-ray burst from the X-ray transient XTE J1739-285. The signal has a peak Leahy power of 42.8 and, after consideration of the number of trials, has a chance probability of occurrence of 4 × 10^-5 equivalent to a 4.2 sigma detection. The oscillation frequency suggests that XTE J1739-285 contains the fastest rotating neutron star yet found. We also found millisecond quasiperiodic oscillations in the persistent emission with frequencies ranging from 757 Hz to 862 Hz. We detected seven X-ray bursts and derive an upper limit on the source distance of about 10.6 kpc from the brightest burst. 1. Introduction Weakly magnetized neutron stars can be spun up to rates of several 100 Hz by accretion in low-mass X-ray binaries (Alpar et al. 1982). The first direct measurements of millisecond spin rates in actively accreting neutron stars in low-mass X-ray binaries (LMXBs) came from the discovery of oscillations in thermonuclear X-ray bursts occurring on the neutron star surface (Strohmayer et al. 1996). The burst oscillation frequencies were later found to be nearly equal to those of coherent pulsations implying that the burst oscillation frequency indicates the neutron star spin rate. This X-ray technique has no known biases against the detection of very high spin rates, unlike radio pulsation searches, and the sample of X-ray burst oscillation frequencies has been exploited to constrain the neutron star spin rate distribution. Analysis of a sample of 11 X-ray measured spin frequencies in the range 270-619 Hz suggests a limiting spin rate near 760 Hz if the distribution is uniform and bounded (Chakrabarty et al. 2003). This is below the expected maximum spin frequency possible without centrifugal breakup and has been interpreted as evidence that some physical process, possibly gravitational radiation, limits the maximum possible spin rate. However, the discovery of a radio pulsar spinning at 716 Hz, a frequency above any previously measured in X-rays, suggests that the true maximum spin rate is higher (Hessels et al. 2006). Here, we describe observations made with the Rossi X-Ray Timing Explorer (RXTE) following the detection of X-ray bursts from the transient source XTE J1739-285 as part of a program to search for millisecond oscillations in both X-ray bursts and persistent emission from neutron star X-ray binaries which are newly discovered or found to be active. We detected seven X-ray bursts and found oscillations at a frequency of 1122 Hz in the brightest burst. This suggests that XTE J1739-285 contains the most rapidly rotating neutron star yet discovered. 2. OBSERVATIONS OF XTE J1739-285 XTE J1739-285 was discovered during RXTE PCA scans of the Galactic bulge on 1999 October 19 (Markwardt et al. 1999) and underwent short outbursts in May 2001 and October 2003. The source became active again in August 2005 (Bodaghee et al. 2005) and two X-ray bursts were detected with the JEM-X instrument on INTEGRAL on 2005 September 30 and October 4 (Brandt et al. 2005). Triggered by the detection of the X-ray bursts, we obtained 19 observations using RXTE in the period beginning 2005 October 12 and ending 2005 November 16. References Alpar, M.A., A.F. Cheng, M.A. Ruderman, J. Shaham, 1982 Nature 300, 728 Chakrabarty, D., Morgan, E.H., Muno, M.P., Galloway, D.K., et al. 2003, Nature 424, 42 Hessels, J.W.T, Ranson, S.M., Stairs, I.H., Freire, C.C., Kaspi, V.M., Camilo, F. 2006, Science 311, 1901 Strohmayer, T.E., et al., 1996, ApJ 469, L9 Strohmayer, T.E. & Markwardt, C.B. 2002, ApJ 577, 337 Strohmayer, T.E. & Markwardt, C.B., Swank, J.H., in ’t Zand, J. 2003, ApJ 596, L67

Integral points to the fastest spinning neutron star

[16 February 2007] Astronomers using ESA's gamma-ray observatory, Integral, have detected what appears to be the fastest spinning neutron star yet. This tiny stellar corpse is spinning 1122 times every second. If confirmed, the discovery gives astronomers the chance to glimpse the insides of the dead star.

The neutron star, known by its catalogue number XTE J1739-285, was discovered during one of its active phases on 19 October 1999 by NASA's Rossi X-Ray Timing Explorer (RXTE) satellite. In August 2005, while Integral was monitoring the bulge of the Galaxy, XTE J1739-285 started to come back to life. About a month later Integral discovered the first short bursts of X-rays from the object.

Image credit: NASA/Dana Berry

Fig. .— This artist's impression shows a spinning neutron star (pulsar) approximately 10 kilometres in diameter. When a neutron star orbits another star, its strong gravitational field can pull gas from the other star. This coats the surface of the neutron star. When the coating reaches a height of between 5-10 metres, the gas ignites in a thermonuclear explosion. This massive release of energy generally lasts from between several seconds to several minutes and a burst of X-rays is released.

Erik Kuulkers, who leads the Galactic bulge monitoring programme, informed Philip Kaaret, that things were still hotting up near the end of October. Kaaret arranged for the RXTE satellite to observe XTE J1739-285 between 31 October and 16 November. Together the two satellites recorded about twenty bursts between September and November.
Just because a star dies, it doesn't mean its life is over. A neutron star is the tiny heart of a collapsed star. Measuring about 10 kilometres across, yet containing something like the mass of the Sun, the interior of a neutron star is the most exotic realm that astronomers can imagine. According to their calculations a thimbleful of neutron star material weighs a hundred million tonnes.
When a neutron star orbits another star, its strong gravitational field can pull gas from the other star. This coats the surface of the neutron star. When the coating reaches a height of between 5-10 metres, the gas ignites in a thermonuclear explosion. This massive release of energy generally lasts from between several seconds to several minutes and a burst of X-rays is released.
Previous observations of other neutron stars have shown that the X-rays emitted during bursts display oscillations that correspond to the rotation rate of the neutron stars. So the team began analysing the XTE J1739-285 bursts for oscillations. What they found was astounding. In the brightest burst, which RXTE recorded on 4 November, there were indeed oscillations but they were nearly twice as fast as any previously observed.
"It was quite a surprise to us," admits Kuulkers. However, after running a series of checks, the team satisfied themselves that the oscillations were indeed taking place 1122 times a second (1122 Hz).
Previously, the fastest neutron stars were known to spin with frequencies between 270-619 Hz. This had led some astronomers to estimate, using statistical arguments, that the fastest a neutron star could spin was 760 Hz. If the new observations are confirmed, XTE J1739-285 smashes this limit.
"Our detection is just above the level where we think there is something real. We definitely need more observations. If we see the signal again, then everyone will believe it," says Kuulkers.
This doesn't mean that neutron stars can spin as fast as they like. If the spin is too fast, even the crushing gravity of the star will be unable to hold matter to the surface and the star will break up. The exact break-up speed depends on the internal conditions of a neutron star and as yet, astronomers do not know these precisely.
"Our putative 1122 Hz detection places a serious constraint on neutron star models. If we can find more stars that spin in this range, it will certainly allow us to exclude some models of their interior structure," says Kuulkers.
So, now it is just a matter of patience. The astronomers will keep watch, not only for XTE J1739-285 to burst again, but also for other fast-spinning X-ray neutron stars.

—
	Authors: C. M. Zhang, H.X. Yin, Y.H. Zhao, Y.C. Wei, X.D. Li
	Journal-ref: PASP (2007) [0708.3566 ]
	Title: Does Sub-millisecond Pulsar XTE J1739-285 Contain a Low Magnetic Neutron Star or Quark Star?
Abstract: With the possible detection of the fastest spinning nuclear-powered pulsar XTE J1739-285 of frequency 1122 Hz (0.8913 ms), it arouses us to constrain the mass and radius of its central compact object and to imply the stellar matter compositions: neutrons or quarks. Spun-up by the accreting materials to such a high rotating speed, the compact star should have either a small radius or short innermost stable circular orbit. By the empirical relation between the upper kHz quasi-periodic oscillation frequency and star spin frequency, a strong constraint on mass and radius is obtained as 1.51 solar masses and 10.9 km, which excludes most equations of states (EOSs) of normal neutrons and strongly hints the star promisingly to be a strange quark star. Furthermore, the star magnetic field is estimated to be about 4 × 10⁷ G < B < 10⁸ G , which reconciles with those of millisecond radio pulsars, revealing the clues of the evolution linkage of two types of astrophysical objects. 1. Introduction The most rapidly spinning astrophysical object in the universe, named XTE J1739-285, rotating 1122 times per second, has been declaimed to be detected recently in the accreting X-ray binary system (Kaaret et al. 2007), which is the first sub-millisecond pulsar (spin period 0.89 ms) coming into our view since the discovery of the first radio pulsar by Jocelyn Bell and Anthony Hewish forty years ago (Hewish, Bell & Pilkingston et al. 1968) if the 1122 Hz is a spin frequency. If the declared spin frequency 1122 Hz is fully confirmed, then this compact object breaks through the record of the fastest radio pulsar with the spinning frequency of 716 Hz (1.39 ms) discovered recently (Hessels et al. 2006). References Hessels, J. W. T., Ransom, S. M., Stairs, I. H., et al. 2006, Science, 311, 1901 Hewish, A., Bell, J., & Pilkington, J. D. et al.1968, Nature, 217, 709 Kaaret, P., Prieskorn, Z., & in’t Zand, J., et al. 2007, ApJ, 657, L97

—

Authors: Bejger, M.; Haensel, P.; Zdunik, J.L.

Journal-ref: A&A (2008) [astro-ph/0612216

]

Title: Rotation at 1122 Hz and the neutron star structure

Image credit:

Fig. 1.— Gravitational mass, M, vs. circumferential equatorial radius, R_eq, for neutron stars stably rotating at f = 1122 Hz, for ten EOSs.
Small-radius termination by filled circle: setting-in of instability with respect to the axi-symmetric perturbations.
Dotted segments to the left of the filled circles: configurations unstable with respect to those perturbations.
Large-radius termination by an open circle: the mass-shedding instability. The mass-shedding points are very well fitted by the dashed curve R_min = 15.5 (M/1.4M)^1/3 km.

Abstract:
  • Aims. Recent observations of XTE J1739-285 suggest that it contains a neutron star rotating at 1122 Hz. Such rotation imposes bounds on the structure of neutron star in XTE J1739-285. These bounds may be used to constrain poorly known equation of state of dense matter at densities > 10¹⁵ g cm^-3.
  • Methods. One-parameter families of stationary configurations rotating rigidly at 1122 Hz are constructed, using a precise 2-D code solving Einstein equations. Hydrostatic equilibrium solutions are tested for stability with respect to axi-symmetric perturbations. A set of ten diverse EOSs of neutron stars is considered. Hypothetical strange stars are also studied.
  • Results. For each EOS, the family of possible neutron star models is limited by the mass shedding limit, corresponding to maximum allowed equatorial radius, R_max, and by the instability with respect to the axi-symmetric perturbations, reached at the minimum allowed equatorial radius, R_min.
We get R_min ~ 10 − 13 km, and R_max ~ 16 − 18 km, with allowed mass 1.4−2.3 M.
Allowed stars with hyperonic or exotic-phase core are supramassive and have a very narrow mass range. Quark star with accreted crust might be allowed, provided such a model is able to reproduce X-ray bursts from XTE J1739-285.

1. Introduction

Because of their strong gravity, neutron stars can be very rapid rotators; theoretical studies indicate that they could rotate at submillisecond periods, i.e., at frequency n_max = 1/period > 1000 Hz (e.g., Cook et al. 1994a, Salgado et al. 1994).

References
Cook G. B., Shapiro S.L., Teukolsky S.A. 1994a ApJ, 424, 823
Cook G. B., Shapiro S.L., Teukolsky S.A. 1994b ApJ, 423, L117
Salgado M., Bonazzola S., Gourgoulhon E., Haensel P., 1994, A&A 108, 455

K3.3 XTE J1739-302

XTE J1739-302 — fast X-ray transient
	Authors: D.M. Smith, W.A. Heindl, C.B. Markwardt, J.H. Swank, I. Negueruela, T.E. Harrison, L. Huss
	Journal-ref: ApJ 638 (2006) 974-981 [astro-ph/0510658 ]
	Title: XTE J1739-302 as a supergiant fast X-ray transient
Abstract: XTE J1739-302 is a transient X-ray source with unusually short outbursts, lasting on the order of hours. Here we give a summary of X-ray observations we have made of this object in outburst with the Rossi X-ray Timing Explorer (RXTE) and at a low level of activity with the Chandra X-ray Observatory, as well as observations made by other groups. Visible and infrared spectroscopy of the mass donor of XTE J1739-302 are presented in a companion paper. The X-ray spectrum is hard both at low levels and in outburst, but somewhat variable, and there is strong variability in the absorption column from one outburst to another. Although no pulsation has been observed, the outburst data from multiple observatories show a characteristic timescale for variability on the order of 1500-2000 s. The Chandra localization (right ascension 17h 39m 11.58s, declination -30o 20' 37.6'', J2000) shows that despite being located less than 2 degrees from the Galactic Center and highly absorbed, XTE J1739-302 is actually a foreground object with a bright optical counterpart. The combination of a very short outburst timescale and a supergiant companion is shared with several other recently-discovered systems, forming a class we designate as Supergiant Fast X-ray Transients (SFXTs). Three persistently bright X-ray binaries with similar supergiant companions have also produced extremely short, bright outbursts: Cyg X-1, Vela X-1, and 1E 1145.1-6141. INTRODUCTION

XTE J1739-302 — d ~ 2.3 kpc — O8 Iab(f)
	Authors: Negueruela, Ignacio; Smith, David M.; Harrison, Thomas E.; Torrejón, José Miguel
	Journal-ref: ApJ 638 (2006) 982 [astro-ph/0510675 ]
	Title: The Optical Counterpart to the Peculiar X-Ray Transient XTE J1739-302
Abstract: The weak X-ray transient XTE J1739-302, characterized by extremely short outbursts, has recently been identified with a reddened star. Here we present spectroscopy and photometry of the counterpart, identifying it as a O8 Iab(f) supergiant at a distance of ~2.3 kpc. XTE J1739-302 thus becomes the prototype of the new class of supergiant fast X-ray transients (SFXTs). The optical and infrared spectra of the counterpart to XTE J1739-302 do not reveal any obvious characteristics setting it apart from other X-ray binaries with supergiant companions, which display a very different type of X-ray light curve. 1. Introduction The X-ray transient XTE J1739−302 = AX J1739.1−3020 = IGR J17391−3021 is highly unusual because of its extremely short outbursts. The only outburst that was monitored over its full extent was found to last less than one day. A pointed Chandra observation led to the unambiguous identification of the optical counterpart to XTE J1739−302. As discussed in Paper I, the interest of XTE J1739−302 stems from the fact that there seems to be a large number of similar objects, characterized by supergiant donors, low quiescence X-ray luminosities and short (< 1d) outbursts, which we term Supergiant Fast X-ray Transients (SFXTs). 4. Discussion Until recently, it was widely believed that High-Mass X-ray Binaries (HMXBs) with supergiant donors were always persistent X-ray sources, while Be/X-ray binaries were mostly transients, though some of them were persistent low-luminosity X-ray sources. Supergiant HMXBs are believed to be powered by direct accretion from the strong radiative wind of the supergiant. They display typical X-ray luminosities in the L_X ~ 10^35-36 erg s^-1 range, which are relatively constant over the long run, though presenting strong short-term stochastic variability. The behavior of XTE J1739−302 differs from this pattern in two important respects. On the one hand, its quiescent X-ray luminosity is rather lower than this (L_X < 10³⁴ erg s^-1) and the lightcurve shows abrupt “outbursts” lasting only several hours. A third interesting property of XTE J1739−302 is the fact that the absorption column density derived from the fits to X-ray spectra is not only larger than the expected interstellar absorption, but also highly variable (N_H = (4 - 38)× 10²² cm^-2).

K3.4 Long-term monitoring of XTE J1739-302

XTE J1739-302 — P_orb ~ 8 d
	Authors: P. Blay, S. Martínez-Núñez, I. Negueruela, K. Pottschmidt, D.M. Smith, J.M. Torrejón, P. Reig, P. Kretschmar, I. Kreykenbohm
	Journal-ref: (2008) [0806.4097 ]
	Title: INTEGRAL long-term monitoring of the Supergiant Fast X-ray Transient XTE J1739-302
Abstract: • Context. In the past few years, a new class of High Mass X-Ray Binaries (HMXRB) has been claimed to exist, the Supergiant Fast X-ray Transients (SFXT). These are X-ray binary systems with a compact companion orbiting a supergiant star which show very short and bright outbursts in a series of activity periods overimposed on longer quiescent periods. Only very recently the first attempts to model the behaviour of these sources have been published, some of them within the framework of accretion from clumpy stellar winds. • Aims. Our goal is to analyze the properties of XTE J1739-302/IGR J17391-3021 within the context of the clumpy structure of the supergiant wind. • Methods. We have used INTEGRAL and RXTE/PCA observations in order to obtain broad band (1 – 200 keV) spectra and light curves of XTE J1739-302 and investigate its X-ray spectrum and temporal variability. • Results. We have found that XTE J1739-302 follows a much more complex behaviour than expected. Far from presenting a regular variability pattern, XTE J1739-302 shows periods of high, intermediate, and low flaring activity. 1. Introduction Wind-fed Supergiant X-Ray Binaries (SGXRBs) display high energy emission arising from the accretion of material in the wind of an OB supergiant onto the compact component of the system (a neutron star -NS- or black hole in orbit around the supergiant). SGXRBs are persistent X-ray sources, displaying an X-ray luminosity L_X ~ 10³⁶ erg s^-1. Because of the physical characteristics of wind accretion, their emission is variable on short timescales, with frequent flares, but relatively stable on the long term (for example, the long-term RXTE/ASM lightcurve of Vela X-1, averaged and smoothed with a running window of 30 d length, shows variations by only a factor of ~ 4; Ribo et al. 2006). If the orbit is eccentric, the luminosity is modulated on the orbital period of the system (e.g., Leahy 2002). Stronger short flares, with a fast rise and a typical timescale of the order of a few hours, have been observed from several systems, such as Vela X-1 (Laurent et al. 1995; Krivonos et al. 2003) or 4U 1907+09 (Fritz et al. 2006). Recently, thanks to the improved sensitivity of high energy missions, many new SGXRBs have been discovered, leading to the suggestion of new classes of X-ray sources. On the one hand, there is a number of highly absorbed SGXRBs, invisible to previous missions due to high absorption in the softer X-ray bands (e.g., Chaty & Filliatre 2005). On the other hand, Supergiant Fast X-ray Transients (SFXTs) display fast outbursts, with a typical duration of a few hours, but stay in quiescence most of the time (Smith et al. 2006; Negueruela et al. 2006a; Sguera et al. 2006). Unlike in classical SGXRBs, the Xray luminosity of SFXTs goes down below the sensitivity limit of the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) and they remain undetectable for long time spans. They can only be observed during an outburst or flare, for a short time. Though several models have been proposed for this difference in behaviours, it seems to be a natural consequence of the clumpy nature of OB star winds (Walter & Zurita Heras 2007; Negueruela et al. 2008). Sidoli et al. (2007) propose an alternative hypothesis, based on observations of IGR J11215−5952, in which the observed flaring activity is due to the interaction of the compact object with an extended equatorial decretion disc around the supergiant star. Although the definition of SFXTs as a putative new class of objects was only possible when the optical counterparts to these systems started to be identified, INTEGRAL has contributed decisively to the characterization of the high energy behaviour of these sources. So far, ~ 12 SFXTs or related objects have been detected by INTEGRAL (Walter & Zurita Heras 2007; Sguera et al. 2006). Among them, the best characterized system is XTE J1739-302 = IGR J17391-3021, generally taken as the prototype of the class (Smith et al. 2006; Negueruela et al. 2006b). XTE J1739-302 was discovered by RXTE during a short outburst in 1997 (Smith et al. 1998), when it was detected only for a period of a few hours. The optical counterpartwas identified thanks to a Chandra localization as an O8 Iab(f) supergiant at a distance of ≈ 2.3 kpc (Negueruela et al. 2006b). References Negueruela, I., Smith, D.M., Reig, P., et al. 2006a, ESA SP-604, 165 Negueruela, I., Smith, D.M., Harrison, T.E., & Torrejon, J.M. 2006b, ApJ 638, 982 Negueruela, I., Torrejon, J.M., Reig, P. et al. 2008, Sguera, V., Bazzano, A., Bird, A. J., et al. 2006, ApJ 646, 452 Sidoli, L., Romano, P., et al. 2007, A&A 476, 1307 Sidoli, L., Romano, P., Mangano, V., et al. 2008, Smith, D. M., Main, D., Marshall, F., et al. 1998, ApJ 501, L181 Smith, D. M., Heindl, W.A., Markwardt, C.A., et al. 2006, ApJ 638, 974 Walter, R., & Zurita Heras, J.A. 2007, A&A 476, 335

K4 A0538-66: a Be/X-ray transient in LMC

Zum Thema: A 0535-66 — HMXB in LMC — P_rot = 69 ms — P_orbt = 16.65 d — B ~ 10¹¹G
N 63A in LMC X-ray binaries - theory and observation	ULX transient in Cen A

A0538-66 in quiescence — L_X ~ 6 × 10³³ erg s^-1
	Authors: P. Kretschmar, J. Wilms, R. Staubert, I. Kreykenbohm, W. A. Heindl
	ref: 5th INTEGRAL Workshop (2004) [astro-ph/0406015 ]
	Title: XMM-Newton Observations of the Be/X-ray transient A0538-66 in quiescence
Abstract: We present XMM-Newton observations of the recurrent Be/X-ray transient A0538-66, situated in the Large Magellanic Cloud, in the quiescent state. Despite a very low luminosity state of L_X = (5 — 8) × 10³³ erg s^-1 in the range 0.3 — 10 keV, the source is clearly detected up to ~8 keV. and can be fitted using either a power law with photon index alpha=1.9 or a bremsstrahlung spectrum with kT=3.9 keV. The spectral analysis confirms that the off-state spectrum is hard without requiring any soft component, contrary to the majority of neutron stars observed in quiescence up to now. INTRODUCTION The recurrent Be/X-ray transient A0538–66 (X0535−668) was discovered in 1977 when two outbursts were observed with the Ariel 5 satellite (White & Carpenter 1978). In the following years several other outbursts were observed with HEAO 1 and Einstein with coincident X-ray and optical flares showing a recurrence of 16.65 days (Skinner 1980) interpreted as the period of an eccentric binary orbit. This period has been confirmed as a by-product of the MACHO project by Alcock et al. (2001), who also found a longer optical modulation of 421 days. The optical counterpart, identified by Johnston et al. (1980), was found to be a member of the Large Magellanic Cloud (LMC) (Pakull & Parmar 1981). Optical and UV spectroscopy (Charles et al. 1983) classify the counterpart as B2 IIIe star. Based on the distance to the LMC, the luminosity of the outbursts observed in the first years after detection can be estimated as around 10³⁹ ergs/s making this a super-Eddington source and one of the most powerful X-ray binaries known. An observation with Einstein (HEAO 2) 1980/81 during a strong outburst found 69ms pulsations (Skinner et al. 1982; Ponman et al. 1984); up to this day this remains the only measure of the pulse period of this accreting pulsar. From the rapid spin period and the luminosity Skinner et al. (1982) inferred an upper limit to the magnetic field of B ~ 10¹¹ G assuming that matter accreted onto the polar caps uninhibited by a centrifugal barrier. In later years the source has been mostly quiescent. Weak outburst activity at two to three orders of magnitude below the early observations was found in the ROSAT All-Sky Survey in 1990 (Mavromatakis & Haberl 1993) and in an ASCA observation in 1995 (Corbet et al. 1997); Alcock C., Allsman R.A., Alves D.R., et al. 2001, MNRAS, 321, 678 Charles P.A., Booth L., Densham R.H., et al. 1983, MNRAS, 202, 657 Corbet R.H.D., Charles P.A., Southwell K.A., Smale A.P. 1997, ApJ, 476, 833 Johnston M.D., Griffiths R.E., Ward M.J. 1980, Nature, 285, 26 Mavromatakis F., Haberl F. 1993, A&A, 274, 304 Ponman T.J., SkinnerG.K., Bedford D.K. 1984, MNRAS, 207, 621 Skinner G.K. 1980, Nature, 288, 141 Skinner G.K., Bedford D.K., Elsner R.F., et al. 1982, Nature, 297, 568 White N.E., Carpenter G.F. 1978, MNRAS, 183, 11P

K5 A 0535+26 — A Be/Xray binary pulsar

Zum Thema: A 0535-66 — d = 2 kpc — P_rot = 103 s — P_orb = 111 d — e ~ 0.47 — B = 4.5 × 10¹²G
The binary X-ray pulsar A0535+262 X-ray binaries - theory and observation	Accreting X-ray Pulsars: Cyclotron Lines

A 0535+26 — d = 2 kpc — a major X-ray outburst in May/June 2005 — optical and IR flux

Authors: M.J. Coe, P. Reig, V.A. McBride, J.L. Galache, J. Fabregat

Journal-ref: MNRAS 368 (2006) 447-453 [astro-ph/0602115

]

Title: A 0535+26: Back in business

Image credit:

Figure 8. Estimate of the radius of the Ha emitting region of the circumstellar disk compared to the neutron star orbit (solid black line). The three sizes of disk shown correspond to the following radii values :
(a) 4.8×10¹² cm, (b) 6.8×10¹² cm and (c) 10.7×10¹² cm.
The dashed circles are 4 truncation radii: from inside out they are 7:1, 5:1, 4:1 and 3:1.

Abstract: In May/June 2005, after 10 years of inactivity, the Be/X-ray binary system A 0535+26 underwent a major X-ray outburst. In this paper data are presented from 10 years of optical, IR and X-ray monitoring showing the behaviour of the system during the quiescent epoch and the lead up to the new outburst. The results show the system going through a period when the Be star in the system had a minimal circumstellar disk and then a dramatic disk recovery leading, presumably, to the latest flare up of X-ray emission. The data are interpreted in terms of the state of the disk and its interaction with the neutron star companion.

INTRODUCTION AND BACKGROUND

The Be/X-ray systems represent the largest sub-class of massive X-ray binaries. A survey of the literature reveals that of the 115 identified massive X-ray binary pulsar systems (identified here means exhibiting a coherent X-ray pulse period), most of the systems fall within this Be counterpart class of binary. The orbit of the Be star and the compact object, presumably a neutron star, is generally wide and eccentric. X-ray outbursts are normally associated with the passage of the neutron star close to the circumstellar disk.
A 0535+26 was discovered by Ariel V during a Type II outburst in 1975. The primary is an O9.7IIIe star and its optical and infrared (IR) emission has been the subject of many papers in an attempt to decode the behavioural patterns of this system.
Throughout the last decade (1995-2004) there have been no reported detections of the source at X-ray wavelengths. However, in May/June 2005 the source was detected undergoing significant X-ray activity by the SWIFT and RHESSI observatories.
In this paper we report on the recent developments in the optical and IR fluxes in the context of the re-emergence of A 0535+26 as an active X-ray source.

A 0535+26 — giant outburst in 1994 — L_X = 10³⁸ erg s^-1
	Authors: M. Maisack, J.E. Grove, E. Kendziorra, P. Kretschmar, R. Staubert, M.S. Strickman
	Journal-ref: A&A 325 (1997) 212 [astro-ph/9703135 ]
	Title: Pulse Phase Spectroscopy of A 0535+26 during its 1994 giant outburst observed with OSSE
Abstract: We present pulse phase spectroscopy of A 0535+26 in the energy range 35-200 keV from OSSE observations of its giant outburst in 1994. We discuss the phase dependence of the continuum parameters and the cyclotron resonance feature (CRF) at 110 keV already found in the phase averaged spectrum. We find that a CRF is required at every phase. The behaviour of the line parameters with phase and the pulse shape indicate that the emission occurs in a pencil-beam geometry. INTRODUCTION In Be X-ray binaries (BeXRB) with a neutron star as the compact object, mass transfer is mediated via a circumstellar disk around the companion. The companions are O or B stars with high rotational velocities which are responsible for the loss of material in the equatorial plane. The material in this disk, which may be present or absent for extended periods, flows out rather slowly (velocities of several tens of km/s) compared to the fast B star wind outside the equatorial plane. In fact, the dynamics of the material in this circumstellar disk may be dominated by Keplerian motion. Be-XRB undergo dramatic increases in X-ray flux when the neutron star passes through the dense regions of the circumstellar disk near periastron passage and enhanced mass accretion sets in. Outbursts with luminosities in excess of L_X = 10³⁸ erg s^-1 can be observed in these systems at these times, while the X-ray luminosity during quiescence is L_X = 10³³ erg s^-1 and below.

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H. Heintzmann

( Eintrag vom 15.8.2008)

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