Shortly magnetism in stellar odd couple

Thanks to the observations of NASA’s satellite NuSTAR it was possible to derive the intensity of the magnetic field of the neutron star that with his partner consists of a Supergiant Fast X-ray Transient source type. Information is vital to understanding the mechanisms that produce the different behavior observed in these unpredictable high energy sources. The team that conducted the study was formed by Patrizia Romano and Lorenzo Natalucci.

This artist’s impression shows a high-mass binary system, composed of a supergiant luminous star (in blue) and a compact stellar object, such as a neutron star.



Objects that manage to rip part of the outer layers of the supergiant. In plunging to its attractors, in spiraling orbits ever closer, this material becomes hot to the point of issuing a large amount of radiation in the X-band happens, however, that among these sources, defined Supergiant X-Ray Binary (SGXRB), or binary systems with giant X-ray emission, there is someone even more unusual. Those who, suddenly, within hours, they see massively increase their X-ray emission, even a thousand times, then return to a more moderate and continuous flow: those that astrophysicists call with the initials SFXT (Supergiant Fast Transient X-ray).
Even the founder of this strange class of X-ray sources from the name source IGR J17544-2619, discovered more than ten years ago, exactly in 2003 by ESA’s INTEGRAL satellite, escapes these uncertainties. Since then, this source was sieved by all virtually space observers for the high-energy astrophysics, including XMM-Newton, Chandra and Swift. More than seven years now, this satellite, notes with methodical precision not only IGR J17544-2619 but also the other dozen sources that today make up the class of SFXT, studying systematically both their ‘eruptions’ that continued emissions in the X-ray.
Because these sources behave so bizarre and are still far from clear, and the various theoretical models that try to rebuild their property, sometimes even antagonistic, are still incomplete or even inconsistent with some observational evidence. Some of them suggest that the process of accretion of matter onto the compact object is governed by the structure of the gas, which can reach ‘gushing’ and then intermittently from the companion star, others propose that the magnetic field of the attractor and its speed of rotation need to regulate the speed of growth of matter and therefore its brightness in the X-ray.
To clarify some obscure points of the properties and behavior of these sources now the NuStar NASA satellite observations are made on IGR J17544-2619. The unprecedented accuracy of these measures has enabled the team of researchers led by Varun Bhalerao Inter University Center for Astronomy and Astrophysics (India) and attended by researchers INAF Patrizia Romano and Lorenzo Natalucci, to identify the spectrum of X-rays the radiation emitted by the source, the unmistakable signature of the interaction between the stellar matter and the magnetic field of the object that is drawing compact: what is called “line of the cyclotron.”
“The observation of NuStar has allowed for the first time to detect and then to measure the energy of a row in a cyclotron SFXT. The immediate consequence of this discovery is the direct measurement of the magnetic field of the celestial body on which it is falling matter from the companion. Information is vital for understanding the mechanisms that produce the different behavior observed in SFXT than other normal binary X”, explains Patrizia Romano INAF-IASF of Palermo, study coordinator of SFXT with Swift and second author of the paper describing the measure, for publication in the journal Monthly Notices of the Royal Astronomical Society.



“This is another demonstration of the incredible ability of NuStar to reveal rows of cyclotron in binary stars. On the other hand, it’s just one of the purposes of the programmatic mission”, adds Lorenzo Natalucci, researcher IAPS-INAF in Rome and member of the scientific team of NuStar. “This result follows major studies with INTEGRAL and Swift satellites and it has seen a fundamental contribution from Italian researchers.”
The revelation of the cyclotron line in IGR J17544-2619 marks, then, a turning point. The SFXT have in common with other high-mass binary is the companion (a massive star-type OB), is the compact object, probably a neutron star. All things being equal, for example, the mutual orbit of the two bodies in the binary system, one would expect to see the same phenomena, the same behavior. Instead the first emit intense pulses (flare) radiation X, the second in an almost constant. As if to say: If we make a cake for the same ingredients in the same proportions to two chefs working in the same kitchen with the same recipe, then it is expected to produce very similar cakes, if not indistinguishable. Unless the way in which mixing the ingredients, for example, is different. “So is for SFXT, and the models most accredited tend to show that the cooks, so to speak, are pouring flour (i.e. the gas that is deposited on the neutron star) in a different way, one freely, one more irregular”, says again Romano.

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