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Bamberg, Germany

Doroshenko V.,Institute For Astronomie Und Astrophysik | Santangelo A.,Institute For Astronomie Und Astrophysik | Kreykenbohm I.,Dr. Karl Remeis Sternwarte | Kreykenbohm I.,Erlangen Center for Astroparticle Physics | Doroshenko R.,Institute For Astronomie Und Astrophysik
Astronomy and Astrophysics | Year: 2012

We present an analysis of the spectral properties of the peculiar X-ray pulsar X Per based on INTEGRAL observations. We show that the source exhibits an unusually hard spectrum and is confidently detected by ISGRI up to more than 100 keV. We find that two distinct components may be identified in the broadband 4-200 keV spectrum of the source. We interpret these components as the result of thermal and bulk Comptonization in the vicinity of the neutron star and describe them with several semi-phenomenological models. The previously reported absorption feature at ≈30a keV is not required in the proposed scenario and therefore its physical interpretation must be taken with caution. We also investigated the timing properties of the source in the framework of existing torque theory, concluding that the observed phenomenology can be consistently explained if the magnetic field of the neutron star is ∼10 14G. © 2012 ESO. Source

Bozzo E.,University of Geneva | Stella L.,National institute for astrophysics | Van Der Klis M.,University of Amsterdam | Watts A.,University of Amsterdam | And 6 more authors.
EPJ Web of Conferences | Year: 2014

LOFT, the Large Observatory For X-ray Timing, is one of five ESA M3 candidate missions. It will address the Cosmic Vision theme: "Matter under Extreme Conditions". By coupling for the first time a huge collecting area for the detection of X-ray photons with CCD-quality spectral resolution (15 times bigger in area than any previously flown X-ray instrument and >100 times bigger for spectroscopy than any similar-resolution instrument), the instruments onboard LOFT have been designed to (i) determine the properties of ultradense matter by reconstructing its Equation of State through neutron star mass and radius measurements of unprecedented accuracy; (ii) measure General Relativity effects in the strong field regime in the stationary spacetimes of neutron stars and black holes of all masses down to a few gravitational radii. Besides the above two themes, LOFT's observations will be devoted to "observatory science", providing new insights in a number of research fields in high energy astrophysics (e.g. Gamma-ray Bursts). The assessment study phase of LOFT, which ended in September 2013, demonstrated that the mission is low risk and the required Technology Readiness Level can be easily reached in time for a launch by the end of 2022. © 2014 Owned by the authors. Source

Doroshenko V.,Institute For Astronomie Und Astrophysik | Santangelo A.,Institute For Astronomie Und Astrophysik | Suleimanov V.,Institute For Astronomie Und Astrophysik | Staubert R.,Institute For Astronomie Und Astrophysik | And 5 more authors.
AIP Conference Proceedings | Year: 2010

The angular momentum of matter accreting onto the neutron star produces significant spin-up torque. Effective braking mechanism must exist to balance it in order to explain the existence of slowly-rotating X-ray pulsars. The efficiency of breaking steeply decreases with the rotational frequency and the magnetic field strength. Slowly rotating sources like GX 301-2 must therefore be highly magnetized (B∼1014G), which is in apparent contradiction with the field estimate from the position of a cyclotron line observed in GX 301-2 (B∼3×1012G). We suggest that this contradiction may be resolved if the line forming region resides in an accretion column of significant height [1]. We investigate this hypothesis using INTEGRAL and BATSE observations and conclude, that the field at the top of the column shall be weak enough to explain the observed cyclotron line energy. © 2010 American Institute of Physics. Source

Nieva M.-F.,Max Planck Institute for Astrophysics | Przybilla N.,Dr. Karl Remeis Sternwarte | Irrgang A.,Dr. Karl Remeis Sternwarte
Journal of Physics: Conference Series | Year: 2011

The derivation of high precision/accuracy parameters and chemical abundances of massive stars is of utmost importance to the fields of stellar evolution and Galactic chemical evolution. We concentrate on the study of OB-type stars near the main sequence and their evolved progeny, the BA-type supergiants, covering masses of ∼6 to 25 solar masses and a range in effective temperature from ∼8000 to 35 000 K. The minimization of the main sources of systematic errors in the atmospheric model computation, the observed spectra and the quantitative spectral analysis play a critical role in the final results. Our self-consistent spectrum analysis technique employing a robust non-LTE line formation allows precise atmospheric parameters of massive stars to be derived, achieving 1σ-uncertainties as low as 1% in effective temperature and ∼0.05-0.10 dex in surface gravity. Consequences on the behaviour of the chemical elements carbon, nitrogen and oxygen are discussed here in the context of massive star evolution and Galactic chemical evolution, showing tight relations covered in previous work by too large statistical and systematic uncertainties. The spectral analysis of larger star samples, like from the upcoming Gaia-ESO survey, may benefit from these findings. Source

Przybilla N.,Dr. Karl Remeis Sternwarte | Nieva M.-F.,Dr. Karl Remeis Sternwarte | Nieva M.-F.,Max Planck Institute for Astrophysics | Butler K.,Universitatssternwarte
Journal of Physics: Conference Series | Year: 2011

It is generally accepted that the atmospheres of cool/lukewarm stars of spectral types A and later are described well by LTE model atmospheres, while the O-type stars require a detailed treatment of NLTE effects. Here model atmosphere structures, spectral energy distributions and synthetic spectra computed with ATLAS9/SYNTHE and TLUSTY/SYNSPEC, and results from a hybrid method combining LTE atmospheres and NLTE line-formation with DETAIL/SURFACE are compared. Their ability to reproduce observations for effective temperatures between 15 000 and 35 000 K are verified. Strengths and weaknesses of the different approaches are identified. Recommendations are made as to how to improve the models in order to derive unbiased stellar parameters and chemical abundances in future applications, with special emphasis on Gaia science. Source

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