EDP Sciences
Free access
Volume 438, Number 3, August II 2005
Page(s) 963 - 972
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20042207

A&A 438, 963-972 (2005)
DOI: 10.1051/0004-6361:20042207

Further evidence for the presence of a neutron star in  4U 2206+54 . INTEGRAL and VLA observations

P. Blay1, M. Ribó2, 3, I. Negueruela4, J. M. Torrejón4, P. Reig5, 6, A. Camero1, I. F. Mirabel7 and V. Reglero1

1  Institut de Ciència dels Materials, Universitat de València, PO Box 22085, 46071 Valencia, Spain
    e-mail: [pere.blay;ascension.camero;victor.reglero]@uv.es
2  DSM/DAPNIA/Service d'Astrophysique, CEA/Saclay, Bât. 709, L'Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France
3  AIM - Unité Mixte de Recherche CEA - CNRS - Université Paris VII - UMR 7158, France
    e-mail: mribo@discovery.saclay.cea.fr
4  Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Escuela Politécnica Superior, Universitat d'Alacant, Ap. 99, 03080 Alicante, Spain
    e-mail: [ignacio;jmt]@dfists.ua.es
5  IESL, Foundation for Research and Technology, 71110 Heraklion, Crete, Greece
6  University of Crete, Physics Department, PO Box 2208, 710 03 Heraklion, Crete, Greece
    e-mail: pau@physics.uoc.gr
7  European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
    e-mail: fmirabel@eso.org

(Received 19 October 2004 / Accepted 29 March 2005 )

The majority of High Mass X-ray Binaries (HMXBs) behave like X-ray pulsars, revealing that they contain a magnetised neutron star. Among the four HMXBs showing neither pulsations nor the characteristics of accreting black holes, there is the unusual HMXB 4U 2206+54 . Here we present contemporaneous high-energy and radio observations of this system conducted with INTEGRAL and the VLA, in order to unveil its nature. The high-energy spectra show clear indications of the presence of an absorption feature at ~32 keV. This is the third high-energy observatory to reveal marginal evidence of this feature, giving strong support to the existence of a cyclotron resonance scattering feature, which implies a magnetic field of  $3.6\times 10^{12}$ G. On the other hand, the source is not detected at centimetre radio wavelengths with a 3$\sigma$ upper limit of 0.039 mJy. The expected radio emission for an accreting black hole in the low/hard state, inferred from X-ray flux measurements, would be at least 60 times greater than the measured upper limit. Both results firmly indicate that, in spite of the absence of pulsations, 4U 2206+54 hosts a magnetic accreting neutron star, the first one not to be observed as an X-ray pulsar.

Key words: stars: individual: 4U 2206+54 -- X-rays: binaries -- radio continuum: stars -- accretion, accretion disks -- magnetic fields -- stars: binaries: close

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© ESO 2005

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