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

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

Abstract

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 G. On the other hand, the source is not detected at centimetre radio wavelengths with a 3σ 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.