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Hohle M.M.,Universitats Sternwarte Jena | Haberl F.,Max Planck Institute for Extraterrestrial Physics | Vink J.,University of Amsterdam | de Vries C.P.,SRON Netherlands Institute for Space Research | And 4 more authors.
Monthly Notices of the Royal Astronomical Society

RXJ0720.4-3125 is the most peculiar object among a group of seven isolated X-ray pulsars (the so-called Magnificent Seven), since it shows long-term variations of its spectral and temporal properties on time-scales of years. This behaviour was explained by different authors either by free precession (with a 7 or 14 yr period) or possibly a glitch that occurred around MJD = 52866 ± 73d. We analysed our most recent XMM-Newton and Chandra observations in order to further monitor the behaviour of this neutron star. With the new data sets, the timing behaviour of RXJ0720.4-3125 suggests a single (sudden) event (e.g. a glitch) rather than a cyclic pattern as expected by free precession. The spectral parameters changed significantly around the proposed glitch time, but more gradual variations occurred already before the (putative) event. Since MJD ≈ 53000d the spectra indicate a very slow cooling by ∼2eV over 7 yr. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source

Hohle M.M.,Max Planck Institute for Extraterrestrial Physics | Hohle M.M.,Universitats Sternwarte Jena | Haberl F.,Max Planck Institute for Extraterrestrial Physics | Vink J.,University Utrecht | And 5 more authors.
Astronomy and Astrophysics

Aims. Since the last phase coherent timing solution of the nearby radio-quiet isolated neutron star RX  J0720.4-3125 six new XMM-Newton and three Chandra observations were carried out. The phase coherent timing solutions from previous authors were performed without restricting to a fixed energy band. However, we recently showed that the phase residuals are energy dependent, and thus phase coherent solutions must be computed referring always to the same energy band. Methods. We updated the phase coherent timing solution for RX J0720.4-3125 by including the recent XMM-Newton EPIC-pn, MOS1, MOS2 and Chandra ACIS data in the energy range 400-1000 eV. Altogether these observations cover a time span of almost 10 yrs. A further timing solution was obtained including the ROSAT pointed data. In this case, observations cover a time span of ≈16 yrs. To illustrate the timing differences between the soft band (120-400 eV) and the hard band (400-1000 eV) a timing solution for the soft band is also presented and the results are verified using a Zn 2 test. Results. In contrast to previous work, we obtain almost identical solutions whether or not we include the ROSAT or Chandra data. Thanks to the restriction to the hard band, the data points from EPIC-pn are in better agreement with those from MOS1, MOS2 and Chandra than in previous works. In general the phase residuals are still large and vary with time. In particular, the latest XMM-Newton and Chandra data show that the phase residuals have attained relatively large and negative values. Using this and previous timing solutions, the residuals indicate a cyclic behaviour with a period ≈ 7-9 yrs if the variations follow a sinusoid, or twice this value in case the residuals are modulated by an abs(sine) probably approaching a new minimum around MJD = 55 240 days (February 2010). As an alternative interpretation, the phase residuals can be fitted with a glitch that occured around MJD = 53 000 days. © 2010 ESO. Source

Tachihara K.,Joint Alma Observatory | Tachihara K.,Japan National Astronomical Observatory | Saigo K.,Japan National Astronomical Observatory | Higuchi A.E.,Joint Alma Observatory | And 6 more authors.
Astrophysical Journal

In order to investigate the origin of the interstellar turbulence, detailed observations in the CO J = 1-0 and 3-2 lines have been carried out in an interacting region of a molecular cloud with an H II region. As a result, several 1000-10,000AU scale cloudlets with small velocity dispersion are detected, whose systemic velocities have a relatively large scatter of a fewkms-1. It is suggested that the cloud is composed of small-scale dense and cold structures and their overlapping effect makes it appear to be a turbulent entity as a whole. This picture strongly supports the two-phase model of a turbulent medium driven by thermal instability proposed previously. On the surface of the present cloud, the turbulence is likely to be driven by thermal instability following ionization shock compression and UV irradiation. Those small-scale structures with line widths of 0.6kms-1 have a relatively high CO line ratio of J = 3-2 to 1-0, 1 ≲ R 3 - 2/1 - 0 ≲ 2. The large velocity gradient analysis implies that the 0.6kms-1 width component cloudlets have an average density of 103-10 4 cm-3, which is relatively high at cloud edges, but their masses are only ≲ 0.05 M. © 2012. The American Astronomical Society. All rights reserved.. Source

Masda S.G.,Universitats Sternwarte Jena | Al-Wardat M.A.,Al al-Bayt University | Neuhauser R.,Universitats Sternwarte Jena | Al-Naimiy H.M.,University of Sharjah
Research in Astronomy and Astrophysics

We present the physical and geometrical parameters of the individual components of the close visual double-lined spectroscopic binary system Gliese 762.1, which were estimated using Al-Wardat's complex method for analyzing close visual binary systems. The estimated parameters of the individual components of the system are as follows: radius RA = 0.845 ± 0.09 R o, RB = 0.795 ± 0.10 R o, effective temperature TA eff = 5300 ± 50 K, TB eff = 5150 ± 50 K, surface gravity log gA = 4.52 ± 0.10, log gB = 4.54±0.15 and luminosity LA = 0.51±0.08 L o, LB = 0.40±0.07 L o. New orbital elements are presented with a semi-major axis of 0.0865 ± 0.010 arcsec using the Hippracos parallax π = 58.96 ± 0.65 mas, and an accurate total mass and individual masses of the system are determined as M = 1.72 ± 0.60 Mo, MA = 0.89 ± 0.08 M o and MB = 0.83 ± 0.07 Mo. Finally, the spectral types and luminosity classes of both components are assigned as K0V and K1.5V for the primary and secondary components respectively, and their positions on the H-R diagram and evolutionary tracks are given. © 2016 National Astronomical Observatories, Chinese Academy of Sciences and IOP Publishing Ltd. Source

Suleimanov V.,University of Tubingen | Suleimanov V.,Kazan Federal University | Hambaryan V.,Universitats Sternwarte Jena | Potekhin A.Y.,Ecole Normale Superieure de Lyon | And 4 more authors.
Astronomy and Astrophysics

Context. In the X-ray spectra of most X-ray dim isolated neutron stars (XDINSs), absorption features with equivalent widths (EWs) of 50-200 eV are observed. These features are usually connected with the proton cyclotron line, but their nature is not yet well known. Aims. We theoretically investigate different models to explain these absorption features and compare their properties with observations to obtain a clearer understanding of the radiation properties of magnetized neutron star surfaces. Based on these models, we create a fast and flexible code to fit observed spectra of isolated neutron stars. Methods. We consider various theoretical models of the magnetized neutron star surface, including naked condensed iron surfaces and partially ionized hydrogen model atmospheres, with semi-infinite and thin atmospheres above a condensed surface. Spectra of condensed iron surfaces are represented by a simple analytical approximation. The condensed surface radiation properties are considered as the inner atmosphere boundary condition for the thin atmosphere. The properties of the absorption features (especially equivalent widths) and the angular distributions of the emergent radiation are described for all models. A code for computing light curves and integral emergent spectra of magnetized neutron stars is developed. We assume a dipole surface magnetic field distribution with a possible toroidal component and corresponding temperature distribution. A model with two uniform hot spots at the magnetic poles may also be employed. Results. Light curves and spectra of highly magnetized neutron stars with parameters typical of XDINSs are computed using different surface temperature distributions and various local surface models. Spectra of magnetized model atmospheres are approximated by diluted black-body spectra with one or two Gaussian lines having parameters, which allow us to describe the model absorption features. The EWs of the absorption features in the integral spectra cannot significantly exceed 100 eV, if a local surface model assumes either a semi-infinite magnetic atmosphere or a naked condensed surface. A thin atmosphere above a condensed surface can have an absorption feature whose EW exceeds 200 eV in the integrated spectrum. If the toroidal component of the magnetic field on the neutron star atmosphere is 3-7 times higher than the poloidal component, the absorption feature in the integral spectrum is too wide and shallow to be detectable. Conclusions. To explain the prominent absorption features in the soft X-ray spectra of XDINSs, we infer that a thin atmosphere above the condensed surface must be present, whereas a strong toroidal magnetic field component on the XDINS surfaces can be excluded. © 2010 ESO. Source

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