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Post Info TOPIC: LMXBs


L

Posts: 131433
Date:
Low Mass X-ray Binaries
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Title: Spitzer Space Telescope Observations of Low Mass X-ray Binaries
Authors: Stefanie Wachter (Spitzer Science Centre / Caltech)

We present preliminary results from our archival Spitzer Space Telescope program aimed at characterizing the mid-IR properties of compact objects, both isolated and in binary systems, i.e. white dwarfs, X-ray binaries, cataclysmic variables, and magnetars. Most of these sources are too faint at mid-IR wavelengths to be observable from the ground, so this study provides the very first comprehensive look at the mid-IR emission of these objects. Here we present our results for the low mass X-ray binaries. We considered all of the systems listed in the most recent catalogue of Liu et al. (2007) that have known optical counterparts. The particular goals of our projects encompass: to establish the mid-IR spectral energy distribution, to search for the signatures of jets, circumbinary disks, low mass or planetary companions and debris disks, and to study the local environment of these sources.

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L

Posts: 131433
Date:
The gamma-ray universe
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Integral's latest survey of the gamma-ray universe continues to change the way astronomers think of the high-energy cosmos. With over seventy percent of the sky now observed by Integral, astronomers have been able to construct the largest catalogue yet of individual gamma-ray-emitting celestial objects. And there is no end in sight for the discoveries.
Integral is the European Space Agency's latest orbiting gamma-ray observatory. Ever since Integral began scientific operations in 2003, the project team has been devoting a substantial proportion of its observing time to a survey of the gamma-ray universe.

POMFeb2007
Credits: IBIS survey team

This image is a false colour image of the central region of our galaxy. This is a composite image based on all-sky IBIS/ISGRI maps in three energy windows (between 17 and 100 keV) and represents the true 'X-ray colours' of the sources. Red sources are dominated by emission below 30 keV, while blue sources have harder spectra, emitting strongly above 40 keV.


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L

Posts: 131433
Date:
LMXB EXO0748-676
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Title: Discovery of X-ray burst triplets in EXO 0748-676
Authors: L. Boirin (1), L. Keek (2 and 3), M. Mendez (2 and 4), A. Cumming (5), J.J.M. In 't Zand (2 and 3), J. Cottam (6), F. Paerels (7), W.H.G. Lewin (8) ((1) Observatoire Astronomique de Strasbourg, (2) SRON Utrecht, (3) Astronomical Institute Utrecht, (4) Astronomical Institute Amsterdam, (5) McGill University Montreal, (6) NASA GSFC, (7) Columbia, (8) MIT)

Type-I X-ray bursts are thermonuclear flashes that take place on the surface of accreting neutron stars. The wait time between consecutive bursts is set by the time required to accumulate the fuel needed to trigger a new burst; this is at least one hour. Sometimes secondary bursts are observed, approximately 10 min after the main burst. These short wait-time bursts are not yet understood. We observed the low-mass X-ray binary and X-ray burster EXO 0748-676 with XMM-Newton for 158 h, during 7 uninterrupted observations lasting up to 30 h each. We detect 76 X-ray bursts. Most remarkably, 15 of these bursts occur in burst triplets, with wait times of 12 min between the three components of the triplet. We also detect 14 doublets with similar wait times between the two components of the doublet. The characteristics of the bursts indicate that possibly all bursts in this system are hydrogen-ignited, in contrast with most other frequent X-ray bursters in which bursts are helium-ignited, but consistent with the low mass accretion rate in EXO 0748-676. Possibly the hydrogen ignition is the determining factor for the occurrence of short wait-time bursts.

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L

Posts: 131433
Date:
XB1254-690
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Title: Chandra Observation of the Dipping Source XB 1254-690
Authors: R. Iaria, T. Di Salvo, G. Lavagetto, A. D' Ai', N. R. Robba

We present the results of a 53 ks long Chandra observation of the dipping source XB 1254--690. During the observation neither bursts or dips were observed. From the zero-order image we estimated the precise X-ray coordinates of the source with a 90% uncertainty of 0.6“. Since the lightcurve did not show any significant variability, we extracted the spectrum corresponding to the whole observation. We confirmed the presence of the Fe{xxvi} K_alpha absorption lines with a larger accuracy with respect to the previous XMM EPIC pn observation. Assuming that the line width were due to a bulk motion or a turbulence associated to the coronal activity, we estimate that the lines were produced in a photoionised absorber between the coronal radius and the outer edge of the accretion disk.

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L

Posts: 131433
Date:
LMXB EXO0748-676
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Astronomers using a new technique to analyse the mass and radius of a neutron star called EXO 0748-676, have found that the star is made of ordinary neutrons.
Feryal Ozel of the University of Arizona, in Tucson, US, used Europe's XMM-Newton and NASA's Rossi X-ray Timing Explorer space telescopes to determine the amount of shifting in the wavelength of light emitted near the surface of the star. The neutron stars powerful gravitational field stretches light out, to longer wavelengths.
Unprecedented detail in the spectrum of EXO 0748-676, located in the constellation Volans, or Flying Fish, about 30,000 light-years away, allowed the gravitational redshift to be measured.

The calculations put the neutron star's radius at 13.8 kilometres, and its mass as 2.1 solar masses.
These figures suggests the star is made of normal neutrons. It is on the verge of collapsing into a black hole. It is thought that as mass of a neutron star increases, it must become more rigid to avoid forming a black hole .
Exotic matter theories, such as quark stars and Bose-Einstein Condensate-containing neutron stars predict they would collapse into a black hole before reaching a mass as high as 2.1 solar masses.
It seems that these exotic states of matter such as free quarks are not found inside neutron stars

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L

Posts: 131433
Date:
LMXBs
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The European Space Agency’s XMM-Newton has seen vast clouds of superheated gas, whirling around miniature stars and escaping from being devoured by the stars’ enormous gravitational fields - giving a new insight into the eating habits of the galaxy’s ‘cannibal’ stars.

The clouds of gas range in size from a few hundred thousand kilometres to a few million kilometres, ten to one hundred times larger than the Earth. They are composed of iron vapour and other chemicals at temperatures of many millions of degrees.

"This gas is extremely hot, much hotter than the outer atmosphere of the Sun" - Maria Díaz Trigo of ESA’s European Science and Technology Research Centre (ESTEC), who led the research.

ESA’s XMM-Newton x-ray observatory made the discovery when it observed six so-called ‘low-mass X-ray binary’ stars (LMXBs). The LMXBs are pairs of stars in which one is the tiny core of a dead star.
Measuring just 15–20 kilometres across and comparable in size to an asteroid, each dead star is a tightly packed mass of neutrons containing more than 1.4 times the mass of the Sun.
Its extreme density generates a powerful gravitational field that rips gas from its ‘living’ companion star. The gas spirals around the neutron star, forming a disc, before being sucked down and crushed onto its surface, a process known as ‘accretion’.

The newly discovered clouds sit where the river of matter from the companion star strikes the disc. The extreme temperatures have ripped almost all of the electrons from the iron atoms, leaving them carrying extreme electrical charges. This process is known as ‘ionisation’.
The discovery solves a puzzle that has dogged astronomers for several decades. Certain LMXBs appear to blink on and off at X-ray wavelengths. These are ‘edge-on’ systems, in which the orbit of each gaseous disc lines up with Earth.

In previous attempts to simulate the blinking, clouds of low-temperature gas were postulated to be orbiting the neutron star, periodically blocking the X-rays. However, these models never reproduced the observed behaviour well enough.
XMM-Newton solves this by revealing the ionised iron.

"It means that these clouds are much hotter than we anticipated" - Maria Díaz Trigo .

With high-temperature clouds, the computer models now simulate much better the dipping behaviour.

Some 100 known LMXBs populate our galaxy, the Milky Way. Each one is a stellar furnace, pumping X-rays into space. They represent a small-scale model of the accretion thought to be taking place in the very heart of some galaxies. One in every ten galaxies shows some kind of intense activity at its centre.
This activity is thought to be coming from a gigantic black hole, pulling stars to pieces and devouring their remains. Being much closer to Earth, the LMXBs are easier to study than the active galaxies.

"Accretion processes are still not well understood. The more we understand about the LMXBs, the more useful they will be as analogues to help us understand the active galactic nuclei" - Maria Díaz Trigo.

The findings appear in Astronomy & Astrophysics (445, 179–195, 2006). The original article, ‘Spectral changes during dipping in low-mass X-ray binaries due to highly-ionized absorbers’, is by M. Díaz Trigo and A.N. Parmar (ESA, Noordwijk, The Netherlands), L. Boirin (Observatoire Astronomique de Strasbourg, France), M. Méndez and J.S. Kaastra (SRON, National Institute for Space Research, Utrecht, The Netherlands).

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