EDP Sciences
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Volume 488, Number 1, September II 2008
Page(s) 267 - 270
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:200809926
Published online 01 July 2008

A&A 488, 267-270 (2008)
DOI: 10.1051/0004-6361:200809926

Research Note

Near infrared VLT/MAD observations of the isolated neutron stars RX J0420.0-5022 and RX J1856.5-3754

R. P. Mignani1, R. Falomo2, A. Moretti2, A. Treves3, R. Turolla4, 1, N. Sartore3, S. Zane1, C. Arcidiacono2, M. Lombini5, J. Farinato2, A. Baruffolo2, R. Ragazzoni2, and E. Marchetti6

1  Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK
    e-mail: rm2@mssl.ucl.ac.uk
2  INAF, Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, Padua 35122, Italy
3  Department of Physics and Mathematics, University of Insubria, Via Valleggio 11, Como 22100, Italy
4  Department of Physics, University of Padua, via Marzolo 8, Padua 35131, Italy
5  Osservatorio Astronomico di Bologna, INAF, via Ranzani 1, Bologna 40127, Italy
6  European Southern Observatory, Karl-Schwarzschild-Strasse 2, Garching 85748, Germany

Received 7 April 2008 / Accepted 16 June 2008

Context. High-energy observations have unveiled peculiar classes of isolated neutron stars which, at variance with radio pulsars, are mostly radio silent and not powered by the star rotation. Among these objects are the magnetars, hyper-magnetized neutron stars characterized by transient X-ray/$\gamma$-ray emission, and neutron stars with purely thermal, and in most cases stationary, X-ray emission (a.k.a., X-ray dim isolated neutron stars or XDINSs). While apparently dissimilar in their high-energy behavior and age, both magnetars and XDINSs have similar periods and unusually high magnetic fields. This suggests a tantalizing scenario where the former evolve into the latter.
Aims. Discovering so far uninvestigated similarities between the multi-wavelength properties of these two classes would be a further step forward to establish an evolutionary scenario. A most promising channels is the near infrared (NIR) one, where magnetars are characterized by a distinctive spectral flattening with respect to the extrapolation of the soft X-ray spectrum.
Methods. We observed the two XDINSs RX J0420.0-5022 and RX J1856.5-3754 with the Multi-Conjugate Adaptive Optics Demonstrator (MAD) at the Very Large Telescope (VLT), as part of the instrument guaranteed time observations program, to search for their NIR counterparts.
Results. Both RX J1856.5-3754 and RX J0420.0-5022 were not detected down to $K_{\rm s} \sim20$ and $K_{\rm s} \sim 21.5$, respectively.
Conclusions. In order to constrain the relation between XDINSs and magnetars it would be of importance to perform deeper NIR observations. A good candidate is 1RXS J214303.7+065419 which is the XDINS with the highest inferred magnetic field.

Key words: stars: neutron

© ESO 2008

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