Artikel zu "X-ray Pulsars"
MIV: X-Ray Pulsars
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XMM-Newton measures the magnetic field of a neutron star for the first time
X-ray Pulsars
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1E 1207.4-5209 at the center of the PKS 1209-51/52 SNR
magnetic field B ~ 3 × 1012 G, 1+3 cyclotron resonance absorption lines at 0.7 keV and 1.4, 2.1 and 2.8 keV. DISCOVERY of X-ray Pulsar 1E 1207.4-5209 — D. J. Helfand, & R.H. Becker, Nature 307 (1984) 215 EXO 0748-676 NS redshift for three lines (Fe XXV and O VIII) is z = 0.35. AXP 1E 2259+586 in the Galactic Supernova Remnant CTB 109 |
K1 
Erste Cyclotron Linie Im X-ray Pulsar 1E 1207.4-5209 (P = 424 ms)
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| 1E 1207.4-5209 - B = (2-4) × 1012 G | |
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Authors: D. Sanwal, G. G. Pavlov, V. E. Zavlin and M. A. Teter |
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Journal-ref: ApJ 574 (2002) L61-L64 [astro-ph/0206195
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Title: Discovery of absorption features in the X-ray spectrum of an isolated neutron star |
| Abstract: We observed 1E 1207.4--5209, a neutron star in the center of the supernova remnant PKS 1209--51/52, with the ACIS detector aboard the Chandra X-ray observatory and detected two absorption features in the source spectrum. The features are centered near 0.7 keV and 1.4 keV, their equivalent widths are about 0.1 keV. | |
[June 6, 2002] Neutron Star (NS) 1E 1207.4-5209
Astronomers Use X-Rays To Probe Gravitational Field Of A Neutron Star
With NASA's Chandra X-ray Observatory, astronomers have detected features that
may be the first direct evidence of the effect of gravity on radiation from a
neutron star. This finding, if confirmed, could enable scientists to measure
the gravitational field of neutron stars and determine whether they contain
exotic forms of matter not seen on Earth.
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Pavlov's group found two dips, or absorption features, in the spectrum of X-rays from the star. If these dips are due to the absorption of X-rays near the star by helium ions in a strong magnetic field, they indicate that the gravitational field reduces the energies of X-rays escaping from near the surface of a neutron star.
"This interpretation is consistent with the data," said Pavlov, "but the features may be a blend of many other features. More precise measurements, preferably with Chandra's grating spectrometer, are needed."
"These absorption features may be the first evidence of the effect of gravity on radiation near the surface of an isolated neutron star," said Pavlov. "This is particularly important because it would allow us to set limits on the type of matter that comprises this star."
Neutron stars are formed when a massive star runs out of fuel and its core collapses. A supernova explosion occurs and the collapsed core is compressed to a hot object about 12 miles in diameter, with a thin atmosphere of hydrogen and possibly heavier ions in a gravitational field 100 billion times as strong as Earth's.
These objects, which have a density of more than 1 billion tons per teaspoonful, are called neutron stars because they have been thought to be composed mostly of neutrons. Although neutron stars have been studied extensively for more than three decades, their exact nature is still unknown.
"We are not even sure that neutron stars are composed of neutrons," said Divas Sanwal, also of Penn State, and lead author on a paper describing the team's results. "They could be largely composed of subatomic particles called pions or kaons, or even free quarks."
One key to narrow the range of possibilities is to measure the strength of gravity on the surface of a neutron star by observing its effect on X-rays from very near the surface of the star. According to Einstein's theory of General Relativity, attraction of photons by a star's gravitational field results in a lower energy of the photon (longer wavelength of radiation) when detected by a distant observer. The measurement of this gravitational redshift relates the mass to the radius of the star, and it will test the theories for the various possible forms of dense matter.
The team, which also includes Slava Zavlin and Marcus Teter, considered several possible explanations for the absorption features observed from 1E 1207. The strength and X-ray energy of the features make it improbable that they are due to intervening interstellar material or absorption due to electrons or ions circling in a strong magnetic field. The most likely hypothesis, they conclude, is that the features are due to absorption by helium ions in a magnetic field about a hundred trillion times more intense than the Earth's magnetic field. In this case, the gravitational redshift reduces the energy of the X-rays by 17 percent.
Pavlov and his colleagues observed 1E 1027 with Chandra's Advanced CCD Imaging Spectrometer on January 6, 2000, and again on January 5, 2002, each time for approximately 30,000 seconds.
an ultra-compact binary with orbital period of
between 0.5 and 3.3 min.
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X-ray pulsar at the center of the SNR PKS 1209-51/52
Zavlin et al. (2000) observed 1E 1207.4-5209 with the Chandra X-ray Observatory and discovered a period of about 424 ms, which proved that the source is a NS. Second observation provided an estimate of the period derivative, (Pavlov et al. 2002). This estimate implies that the characteristic age of the NS, t = 200--1600 kyr, is much larger than the 3--20 kyr age of the SNR (Roger et al. 1998), while the conventional magnetic field, B = 1012 G, is typical for a radio pulsar.
11 Jun 2003
Using the superior sensitivity of ESA's X-ray observatory, XMM-Newton, a team of European astronomers has made the first direct measurement of a neutron star's magnetic field. The results provide deep insights into the extreme physics of neutron stars and reveal a new mystery yet to be solved about the end of this star's life.
A neutron star is a very dense celestial object that usually has something like the mass of our Sun packed into a tiny sphere only 20-30 kilometres across. It is the product of a stellar explosion, known as a supernova, in which most of the star is blasted into space, but its collapsed heart remains in the form of a super-dense, hot ball of neutrons that spins at a incredible rate.
Despite being a familiar class of object, individual neutron stars themselves remain mysterious. Neutron stars are extremely hot when they are born, but cool down very rapidly. Therefore, only few of them emit highly energetic radiation, such as X-rays. This is why they are traditionally studied via their radio emissions, which are less energetic than X-rays and which usually appear to pulse on and off. Therefore, the few neutron stars which are hot enough to emit X-rays can be seen by X-ray telescopes, such as ESA's XMM-Newton.
One such neutron star is 1E1207.4-5209. Using the longest ever XMM-Newton observation of a galactic source (72 hours), Prof. Giovanni Bignami of the Centre d'Étude Spatiale des Rayonnements (CESR) and his team have directly measured the strength of its magnetic field. This makes it the first ever isolated neutron star where this could be achieved. All previous values of neutron star magnetic fields could only be estimated indirectly. This is done by theoretical assumptions based on models that describe the gravitational collapse of massive stars, like those which lead to the formation of neutron stars. A second indirect method is to estimate the magnetic field by studying how the neutron star's rotation slows down, using radio astronomy data.
In the case of 1E1207.4-5209, this direct measurement using XMM-Newton reveals that the neutron star's magnetic field is 30 times weaker than predictions based on the indirect methods.
How can this be explained? Astronomers can measure the rate at which individual neutron stars decelerate. They have always assumed that 'friction' between its magnetic field and its surroundings was the cause. In this case, the only conclusion is that something else is pulling on the neutron star, but what? We can speculate that it may be a small disc of supernova debris surrounding the neutron star, creating an additional drag factor.
The result raises the question of whether 1E1207.4-5209 is unique among neutron stars or the first of its kind. The astronomers hope to target other neutron stars with XMM-Newton to find out.
X-rays emitted by a neutron star like 1E1207.4-5209, have to pass through the neutron star's magnetic field before escaping into space. En route, particles in the star's magnetic field can steal some of the outgoing X-rays, imparting on their spectrum tell-tale marks, known as 'cyclotron resonance absorption lines'. It is this fingerprint that allowed Prof. Bignami and his team to measure the strength of the neutron star's magnetic field.
GF Bignami, PA Caraveo, A De Luca, S Mereghetti - Nature 423 (2003) 725-727
XMM-Newton observations of 1E 1207.4–5209 — Bild:
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Authors: A. De Luca, S. Mereghetti, P.A. Caraveo, M. Moroni, R.P. Mignani, G.F. Bignami |
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Journal-ref: A & A 418 (2004) 625 [astro-ph/0312646
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Title: XMM-Newton and VLT observations of the isolated neutron star 1E 1207.4–5209 |
| Abstract: In August 2002, XMM-Newton devoted two full orbits to the observation of 1E 1207.4-5209, making this isolated neutron star the most deeply scrutinized galactic target of the mission. Thanks to the high throughput of the EPIC instrument, ~360,000 photons were collected from the source, allowing for a very sensitive study of the temporal and spectral behaviour of this object. The spectral data, both time-averaged and phase-resolved, yield one compelling interpretation of the observed features: cyclotron absorption from one fundamental (~0.7 keV) and three harmonics, at ~1.4, ~2.1 and ~2.8 keV. Possible physical consequences are discussed, also on the basis of the obvious phase variations of the features' shapes and depths. We also present deep VLT optical data which we have used to search for a counterpart, with negative results down to ~27. | |
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Timing of the X-ray Pulsar 1E 1207.4-5209
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PSR J1852+0040 (P = 105 ms) — PSR J1210-5226 (P = 424 ms) | |
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Spectrum of EXO 0748-676 - first measurement of a dead star's magnetism
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Authors: Michael T. Wolff, Peter A. Becker, Paul S. Ray, Kent S. Wood |
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Journal-ref: Astrophysical Journal [astro-ph/0506515  ] |
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Title: A Strong X-Ray Burst from the Low Mass X-Ray Binary EXO0748-676 |
| Abstract: We have observed an unusually strong X-ray burst as a part of our regular eclipse timing observations of the low mass binary system EXO0748-676. The burst peak flux was 5.2x10-8 erg cm-2 s-1, approximately five times the normal peak X-ray burst flux observed from this source by RXTE. Spectral fits to the data strongly suggest that photospheric radius expansion occurred during the burst. In this Letter we examine the properties of this X-ray burst, which is the first example of a radius expansion burst from EXO0748-676 observed by RXTE. We find no evidence for coherent burst oscillations. Assuming that the peak burst luminosity is the Eddington luminosity for a 1.4 solar mass neutron star we derive a distance to EXO0748-676 of 7.7 kpc for a helium-dominated burst photosphere and 5.9 kpc for a hydrogen-dominated burst photosphere. | |
[21 Jun 2005]
A Strong X-Ray Burst from the Low Mass X-Ray Binary EXO 0748–676
EXO 0748-676 is a 3.82 hr orbital period system,
normally classified as an “atoll” source, with a neutron star primary accreting matter from
the Roche lobe-filling, low-mass main-sequence secondary star UY Vol.
A possible observation
of gravitationally red-shifted lines in X-ray burst spectra has been reported by Cottam,
et al (2002). Villarreal et al (2004) have recently reported a possible
spin period for the neutron star of 45 Hz.
Literatur zu
[4 July 2002] The new results have been achieved through the remarkable spectroscopic capabilities of the space-borne Chandra X-ray Observatory and X-ray Multi-Mirror (or XMM-Newton) instruments. Sanwal et al.2 report Chandra observations of the young isolated neutron star 1E1207.4-5209. They have found two absorption lines in the spectra, at energies of 700 and 1,400 eV (electron volts), which they interpret as the signature of singly ionized helium in a strong magnetic field. The implied redshift is 0.12-0.23. As well as gravity, the magnetic field (around 1012 gauss) has a large effect on the line energies. But because the field strength is not known accurately, Sanwal et al. cannot be sure that they have correctly identified the lines, and neither can they conclusively rule out other interpretations (such as those based on cyclotron features). This is where the results of Cottam et al. come into play. They obtained XMM-Newton observations of EXO0748-676, a neutron star that is accreting gas from a lower-mass star. For various reasons, the neutron stars in such systems are thought to have surface magnetic fields of around 107 - 109 gauss. Although staggering by terrestrial standards (the Earth's magnetic field is just 1 gauss), these fields are too weak to have a significant effect on atomic spectra at energies higher than 100 eV. So it is easier to determine the redshift from this star's spectrum than it is from that of a strong-field neutron star. Cottam et al. observed EXO0748-676 during a series of 28 X-ray bursts - the flashes from thermonuclear fusion that occur when sufficient hydrogen and helium has piled up on the star's surface. They found three strong spectral lines, which they identify as the signatures of 25-times-ionized iron (Fe XXVI in astronomical nomenclature), Fe XXV and O VIII. The inferred redshift for all three is 0.35, which defines a range of possible values for the mass and radius of this star (Fig. 1 ).
Such identifications should always be handled with care, because there are in principle many atomic species and many transitions to check. But Cottam et al. make a good case that these are the most probable sources of these lines. If the identifications are correct, the redshift is exactly in the range expected for a star made of normal neutron matter, and doesn't fit the models for the most compact strange-matter stars8, although there is still room for exotic components. |
K3.1
Discovery of the Neutron Star Spin Frequency in EXO 0748-676
[] Steckbrief des Neutronensterns EXO 0748-676:
Masse: 1,75 M
– Durchmesser: 23 km – Sternbild: Fliegender Fisch
Bildquelle: Nasa / Strohmayer and Villarreal
Der Kleine hat großen Hunger: Diese Explosionen resultieren daraus, dass der kleine Neutronenstern
seinen riesigen Begleiter langsam "verspeist". Sie ereignen sich
einige Male pro Stunde, wenn Materie des Begleiters mit beinahe Lichtgeschwindigkeit auf den
Neutronenstern fällt und dauern ein bis zwei Minuten.
Bei der Rotation des Neutronensterns machen sich diese Explosionen als Flackern bemerkbar –
so ähnlich, wie ein sich drehendes Licht auf einem Leuchtturm einem Beobachter als Flackern erscheint.
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A neutron star is the core remains of a star once bigger than the Sun. The interior contains matter under forces that perhaps existed at the moment of the Big Bang but which cannot be duplicated on Earth. The neutron star in today's announcement is part of a binary star system named EXO 0748-676, located in the constellation Volans, or Flying Fish, about 30,000 light-years away, visible in southern skies with a large backyard telescope.
In this system, gas from a "normal" companion star plunges onto the neutron star, attracted by gravity. This triggers thermonuclear explosions on the neutron star surface that illuminate the region. Such bursts often reveal the spin rate of the neutron star through a flickering in the X-ray light emitted, called a burst oscillation.
A movie can be found here.
The scientists detected a 45-hertz burst oscillation frequency, which corresponds to a neutron star spin rate of 45 times per second. This is a leisurely pace for neutron stars, which are often seen spinning over 300 times per second.
The scientists next capitalized on EXO 0748-676 observations with the European Space Agency's XMM-Newton satellite from 2002, led by Dr. Jean Cottam of NASA Goddard. Cottam's team had detected spectral lines emitted by hot gas, similar in look to the lines of a cardiogram. These lines had two features. First, they were Doppler shifted. This means the energy detected was an average of the light spinning around the neutron star, moving away from us and then towards us. Second, the lines were gravitationally redshifted. This means that gravity pulled on the light as it tried to escape the region, stealing a bit of its energy.
Strohmayer and Villarreal determined that the 45-hertz frequency and the observed line widths from Doppler shifting are consistent with a neutron star radius between 9.5 and 15 kilometers, with the best estimate at 11.5 kilometers. The relationship among burst frequency, Doppler shifting and radius is that the velocity of gas swirling around the star's surface depends on the star's radius and its spin rate. In essence, a faster spin corresponds to a wider spectral line (a technique similar to how a state trooper can detect speeding cars).
Cottam team's gravitational redshift measurement offered the first measure of a mass-radius ratio, albeit without knowledge of a mass and radius. This is because the degree of redshifting (strength of gravity) depends on the mass and radius of the neutron star. Some scientists had questioned this measurement, for the spectral lines detected seemed too narrow. The new results strengthen the gravitational redshift interpretation of the Cottam team's spectral lines (and thus the mass-radius ratio) because a slower-spinning star can easily produce such relatively narrow lines.
So, ever more confident of the mass-radius ratio and now knowing the radius, the scientists could calculate the neutron star's mass. The value was between 1.5 and 2.3 solar masses, with the best estimate at 1.75 solar masses.
The result supports the theory that matter in the neutron star in EXO 0748-676 is packed so tightly that almost all protons and electrons are squeezed into neutrons, which swirl about as a superfluid, a liquid that flows without friction. Yet the matter isn't packed so tightly that quarks are liberated, a so-called quark star.
"Our results are really starting to put the squeeze on the neutron star equation of state," said Villareal. "It looks like equations of state which predict either very large or very small stars are nearly excluded. Perhaps more exciting is that we now have an observational technique that should allow us to measure the mass-radius relations in other neutron stars."
K3.2
EXO 0748-676 Rules out Soft Equations of State for Neutron Star Matter
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Massive neutron star rules out exotic matter
[28 June 2006] Exotic states of matter such as free quarks do not arise inside neutron stars, according to
a new analysis of one of the super-dense stellar corpses. The result contradicts previous theories and offers
an unprecedented view into the behaviour of matter under extreme pressure.
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Image credit: NASA
The neutron star EXO 0748-676 siphons matter from an ordinary companion star, as in this illustration
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Physicists have speculated that in the dense interiors of especially massive neutron stars, matter might be transformed into exotic states never seen elsewhere.
Some researchers believe the enormous pressure could cause the neutrons to break down, freeing the individual quarks of which they are made. Quarks are never found alone in nature. One group of astronomers reported tentative evidence of such a quark star in 2002 (see Exotic star is made entirely of quarks).
Another theory says the pressure might lead to a form of matter called a Bose-Einstein condensate (BEC). In this weird quantum state, the neutrons do not dissociate into quarks, but their individual identities blur and they behave as a single particle.
Stretched light
In principle, it is possible to distinguish between the regular and exotic states of matter, because both free quarks and BECs would be more easily compressed than neutrons. So a star of a particular mass would have a smaller radius if it were made of squishy exotic matter.
But the exotic matter theories have received a blow from the study by Feryal Ozel of the University of Arizona, in Tucson, US. Using a new technique to analyse the mass and radius of a neutron star called EXO 0748-676, she finds that the star is probably made of ordinary neutrons.
The mass and size were determined from measurements of several other key properties of the star, taken by Europe's XMM-Newton and NASA's Rossi X-ray Timing Explorer space telescopes.
One of these properties was the amount of shifting in the wavelength of light emitted near the surface of the star. The powerful gravitational field near a neutron star stretches light out, towards longer wavelengths.
EXO 0748-676 is the only neutron star with enough detail in its spectrum for this gravitational redshift to be measured. The unprecedented detail allowed Ozel to apply a technique for calculating the mass that has never been practical before.
Massive surprise
Ozel's calculations put the neutron star's radius at 13.8 kilometres. But the real surprise was its mass, which came out to 2.1 times the mass of the Sun.
That mass strongly suggests the star is made of normal neutrons. That is because as the mass of a neutron star increases, it must become more and more rigid to avoid collapsing into a black hole under the force of its own gravity. Most models of quark stars and BEC-containing neutron stars predict they would collapse into a black hole before reaching a mass as high as 2.1 solar masses.
"I think the physical measurement procedure is sound," says Frits Paerels of Columbia University in New York, US. "The number that comes out of it is interesting. The mass is surprisingly large."
Most neutron stars whose masses have been measured in other ways are relatively small, at just 1.4 to 1.5 times the mass of the Sun. But their physical sizes have been difficult to pin down, making it unclear what kind of matter lies inside them.
Ruled out
Ozel says the fact that squishy, exotic states of matter do not seem to occur in a star as massive as EXO 0748-676 suggests that these states do not occur in any neutron stars.
Paerels agrees. He told New Scientist that the study would rule out exotic states of matter in neutron stars, if the results are confirmed by future observations.
"It is interesting, it is titillating, it is suggestive," says Madappa Prakesh of Ohio University, US.
But Prakesh says the star's mass has not been pinned down precisely enough to exclude these exotic states completely. The data show the star's most likely mass is 2.1 solar masses but could be as low as 1.8 solar masses. The lower value would still be compatible with some models of star interiors made of free quarks or BECs.
K3.3
Reprocessing of X-ray Bursts
K3.4
The Burst Spectra of EXO 0748-676
K4 1E 2259+586
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XMM-Newton
Lupe: Intensity map of CTB 109
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The Galactic supernova remnant (SNR) CTB 109 (G109.2--1.0) was discovered in X-rays with Einstein {1980}. In the radio band, it was identified 1981 as an SNR in the Galactic plane survey at l 49 cm with the Westerbork Synthesis Radio Telescope.
CTB 109 is the host remnant of the anomalous X-ray pulsar (AXP) 1E 2259+586 with a spin period of P = 6.98 s and a spin-down rate of 3 - 6 × 10-13 s s-1.
In 2002, an outburst of 1E 2259+586 occurred with bursts similar to Soft Gamma Repeaters and with a sudden spin up of Dn/n = 4 × 10^{-6}.
The distance to the SNR has been estimated from various observations as ~ 3 kpc. The remnant is embedded in a large complex of H II regions which extends over 400 pc along the Galactic plane.
The remnant shell is incomplete in the west, both in radio and X-rays (see Fig.). Bright spots are found around the rim, but the features do not correlate in the two bands. No radio point source is found at the position of the X-ray pulsar, which is displaced by 3'6 from the geometrical center of the radio shell.
CTB 109 is located near a giant molecular cloud (GMC) complex which contains five H II regions.
Sasaki et al.
The distribution of molecular gas in the vicinity of the X-ray Lobe
as seen in the light of the 12CO(1 - 0) line (white contours) overlaid on a Chandra image of
the X-ray emission between 0.35 and 10.0 keV. (Left) The CO data are averaged over the
velocity range from -51 to -44.5 km s-1 and show the absorbing CO in the foreground.
Contours levels are at 0.5, 1.0, 1.5, 2.0, and 2.5 K. The white cross marks the position of the
IR source IRAS 23004+5841. The yellow dashed box shows the extent of the region shown
in the right figure. (Right) The CO data are averaged over the velocity range from -57 to
-52 km s-1. Contours levels are at 0.15, 0.25, 0.5, 0.8, and 1.2 K. Three clouds are seen
Galactic north (up), east (left), and south (down) of the Lobe.
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 | H. Heintzmann | ( Eintrag vom 27.9.2007) |
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