A relativistically smeared spectrum in the neutron star X-ray binary 4U 1705−44: looking at the inner accretion disc with X-ray spectroscopy |
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| Authors: | T Di Salvo A D'Aí R Iaria L Burderi M Dov&#;iak V Karas G Matt A Papitto S Piraino A Riggio N R Robba A Santangelo |
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| Institution: | Dipartimento di Scienze Fisiche ed Astronomiche, Universitàdegli Studi di Palermo, via Archirafi 36, 90123 Palermo, Italy;Universitàdegli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy;Astronomical Institute, Academy of Sciences of the Czech Republic, Bocni II 1401a, CZ-14131 Prague, Czech Republic;Dipartimento di Fisica, Universitàdegli Studi 'Roma Tre', Via della Vasca Navale 84, I-00146 Roma, Italy;INAF –Osservatorio Astronomico di Cagliari, Poggio dei Pini, Strada 54, 09012 Capoterra (CA), Italy;INAF –IASF di Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy;IAAT, University of Tubingen, Sand 1, 72076 Tubingen, Germany |
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| Abstract: | Iron emission lines at 6.4–6.97 keV, identified with fluorescent Kα transitions, are among the strongest discrete features in the X-ray band. These are therefore one of the most powerful probes to infer the properties of the plasma in the innermost part of the accretion disc around a compact object. In this paper, we present a recent XMM–Newton observation of the X-ray burster 4U 1705−44, where we clearly detect a relativistically smeared iron line at about 6.7 keV, testifying with high statistical significance that the line profile is distorted by high-velocity motion in the accretion disc. As expected from disc reflection models, we also find a significant absorption edge at about 8.3 keV; this feature appears to be smeared, and is compatible with being produced in the same region where the iron line is produced. From the line profile, we derive the physical parameters of the inner accretion disc with large precision. The line is identified with the Kα transition of highly ionized iron, Fe xxv , the inner disc radius is R in= 14 ± 2 R g (where R g is the Gravitational radius, GM / c 2 ), the emissivity dependence from the disc radius is r −2.27±0.08 , the inclination angle with respect to the line of sight is i = 39°± 1° . Finally, the XMM–Newton spectrum shows evidences of other low-energy emission lines, which again appear broad and their profiles are compatible with being produced in the same region where the iron line is produced. |
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| Keywords: | line: formation line: identification stars: individual: 4U 1705−44 stars: neutron X-ray: binaries X-rays: general |
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