Journal
ASTRONOMY & ASTROPHYSICS
Volume 438, Issue 3, Pages 963-972Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361:20042207
Keywords
stars : individual : 4U 2206+54; X-rays : binaries; radio continuum : stars; accretion, accretion disks; magnetic fields; stars : binaries : close
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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 similar to 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 3.6x10(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.
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