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Post Info TOPIC: 4U 0614+091


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Title: Time-resolved X-Shooter spectra and RXTE light curves of the ultra-compact X-ray binary candidate 4U 0614+091
Authors: O. K. Madej, P. G. Jonker, P. J. Groot, L. M. van Haaften, G. Nelemans, T. J. Maccarone

In this paper we present X-Shooter time resolved spectroscopy and RXTE PCA light curves of the ultra-compact X-ray binary candidate 4U 0614+091. The X-Shooter data are compared to the GMOS data analysed previously by Nelemans et al. (2004). We confirm the presence of C III and O II emission features at ~ 4650 {\AA} and ~ 5000 {\AA}. The emission lines do not show evident Doppler shifts that could be attributed to the motion of the donor star/hot spot around the center of mass of the binary. We note a weak periodic signal in the red-wing/blue-wing flux ratio of the emission feature at ~ 4650 {\AA}. The signal occurs at P = 30.23 0.03 min in the X-Shooter and at P = 30.468 0.006 min in the GMOS spectra when the source was in the low/hard state. Due to aliasing effects the period in the GMOS and X-Shooter data could well be the same. We deem it likely that the orbital period is thus close to 30 min, however, as several photometric periods have been reported for this source in the literature already, further confirmation of the 30 min period is warranted. We compare the surface area of the donor star and the disc of 4U 0614+091 with the surface area of the donor star and the disc in typical hydrogen-rich low-mass X-ray binaries and the class of AM Canum Venaticorum stars and argue that the optical emission in 4U 0614+091 is likely dominated by the disc emission. Additionally, we search for periodic signals in all the publicly available RXTE PCA light curves of 4U 0614+091 which could be associated with the orbital period of this source. A modulation at the orbital period with an amplitude of ~ 10% such as those that have been found in other ultra-compact X-ray binaries (4U 0513-40, 4U 1820-30) is not present in 4U 0614+091.

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XMM-Newton line detection provides new tool to probe extreme gravity

A long-sought-after emission line of oxygen, carrying the imprint of strong gravitational fields, has been discovered in the XMM-Newton spectrum of an exotic binary system composed of two stellar remnants, a neutron star and a white dwarf. Astronomers can use this line to probe extreme gravity effects in the region close to the surface of a neutron star.
Stellar remnants are the last evolutionary step of the life of stars which, after having burned their nuclear fuel, collapse into very compact and exotic objects - white dwarfs, neutron stars and black holes, depending on the mass of the stars. With an enormous mass contained in a very restricted space, these objects are extremely dense; in particular, neutron stars and black holes give rise to very strong gravitational fields and thus prove to be excellent testbeds for Einstein's theory of general relativity.

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Neutron star 'eats' oxygen-rich white dwarf in a peculiar binary system

Astronomers from SRON Netherlands Institute for Space Research and Utrecht University have found blurred oxygen signatures in the X-rays from a neutron star that 'eats' a white dwarf. For the first time the effects of extreme gravity are revealed by oxygen instead of iron atoms.
Although strong gravity near neutron stars and black holes has been studied before in a similar way, this result is unique. Until now, only blurred X-ray signatures of iron atoms have been observed in the X-rays from a neutron star. However, the characteristics of these so called 'iron lines' are disputed, which makes them less suited for extreme gravity field measurements.
The neutron star has been studied before but now Oliwia Madej, PhD student at Utrecht University and SRON, has found blurred oxygen signatures in the X-rays from the star. She made this discovery in an archival observation performed by ESA's XMM-Newton observatory, which is equipped with the SRON reflection grating spectrometer (RGS) that is extremely sensitive in these particular wavelengths. The research was carried out under supervision of SRON researcher Peter Jonker.

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Spitzer Telescope reveals jets of matter around dead star

A team of scientists, including researchers in the University of Southampton's School of Physics and Astronomy, have shown that black holes are not the only known objects in the universe to produce infrared light from beams of particles being shot into space at nearly the speed of light.

Previously, these steady 'relativistic jets' were only seen from black holes which form part of a black hole X-ray binary, a system containing a black hole orbited by a normal star which is so close that the black hole's gravity can peel off the outer part of the normal star and suck in its gas through an accretion disk or disk of matter.
Using the extremely sensitive infrared Spitzer Space Telescope recently launched by NASA, the team discovered one of these steady jets of matter coming from a neutron star (a super-dense type of dead star) in an X-ray binary system. For many years scientists have debated whether there was something unique to black holes that fuelled relativistic jets. It is now clear that the jets must be fuelled by something that both black holes and neutron stars share.
Neutron stars form in the death knells of massive stars, when the pressure at the centre of the star is so large that the electrons and protons of normal matter combine to form a star made almost entirely of neutrons. Not quite dense enough to be black holes, they have masses slightly larger than the Sun's, but diameters about the size of a city, making them as dense as the nuclei of atoms.

"Jets of matter shot off by black holes are usually observed with a radio telescope which enables astronomers to isolate the jet from everything else in the system. However, observing a neutron star's jets with a radio telescope would take many hours because the jets are very faint. The Spitzer Space Telescope sees light which is redder than the reddest colours visible by the human eye and also redder than the light given off by normal stars" - Dr Thomas Maccarone, of the University of Southampton.

Using the Spitzer Telescope, the researchers were therefore able to detect the faint jet of a particular neutron star, 4U 0614+091, in minutes even though it is located about 10,000 light-years away in the constellation Orion. This signal would have taken almost a day to detect on the most powerful radio telescopes on Earth. The Spitzer Telescope also helped the team infer details about the jet's geometry. The team's data indicates that the presence of an accretion disk and an intense gravitational field may be all that is needed to create and fuel a jet of matter.

"For the past 25 years, astronomers have debated the importance of a black hole in jet production. By comparing the behaviour of the relativistic jets seen from neutron star X-ray binaries and from black hole X-ray binaries, astronomers have hoped to compare neutron stars and black holes directly and possibly to see whether these jets are extracting the black holes' rotational energy. This discovery blazes the trail for future studies which should help reveal the nature of relativistic jets" - Dr Thomas Maccarone.

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One of the most mysterious aspects of black holes is their ability to shoot small, steady jets of matter into space near the speed of light. Until the sensitive infrared eyes of NASA's Spitzer Space Telescope recently spotted one of these jets around a nearby neutron star, or super-dense dead star, black holes were the only known objects in the universe with this "talent."

"For years, scientists suspected that something unique to black holes must be fuelling the continuous compact jets because we only saw them coming from black hole systems. Now that Spitzer has revealed a steady jet coming from a neutron star in an X-ray binary system, we know that the jets must be fueled by something that both systems share" - Dr. Simone Migliari of the University of California at San Diego.

Migliari is the lead author of a paper that was published in the May 20, 2006 issue of Astrophysical Journal Letters.
A neutron star X-ray binary system occurs when a companion star orbits a dead star that is so dense all of its atoms have collapsed into neutrons, hence the name "neutron star." The partner circles the neutron star the same way Earth orbits the Sun. Migliari used Spitzer to study a jet in one such system called 4U 0614+091. In this system, the neutron star is more than 14 times the mass of its orbiting companion.
As the smaller object travels around its massive partner, the neutron star's intense gravity collects the material leaving its stellar companion's atmosphere and creates a disk around itself. The disk of matter, or accretion disk, circles the neutron star similar to the way rings circle Saturn. According to Migliari, accretion disks and intense gravitational fields are characteristics that black holes and neutron stars in X-ray binaries share.

Position(2000): RA 94.279 Dec 9.137

"Our data shows that the presence of an accretion disk and an intense gravitational field may be all we need to form and fuel a compact jet" - Dr. Simone Migliari .

Typically, radio telescopes are the tool of choice for observing compact jets around black holes. At radio wavelengths, astronomers can isolate the jet from everything else in the system. However, because the compact jets of a neutron star can be more than 10 times fainter than those of a black hole, using a radio telescope to observe a neutron star's jet would take many hours.
With Spitzer's super-sensitive infrared eyes, Migliari's team detected 4U 0614+091's faint jet in minutes. The infrared telescope also helped astronomers infer details about the jet's geometry. System 4U 0614+091 is located approximately 10,000 light-years away in the constellation Orion.

Other co-authors of this research include: John Tomsick of the University of California at San Diego; Elena Gallo University of California at Santa Barbra, Santa Barbra, Calif.; Gijs Nelemans of the University of Nijmegen in the Netherlands; and Thomas Maccarone, David Russell, and Rob Fender of the University of Southampton in the United Kingdom.

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