A&A 482, 607-615 (2008)
DOI: 10.1051/0004-6361:20078692

Adaptive optics, near-infrared observations of magnetars

V. Testa¹, N. Rea^{2, 3}, R. P. Mignani⁴, G. L. Israel¹, R. Perna⁵, S. Chaty⁶, L. Stella¹, S. Covino⁷, R. Turolla^{8, 4}, S. Zane⁴, G. Lo Curto⁹, S. Campana⁷, G. Marconi⁹, and S. Mereghetti<sup>10

1</sup>  INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy
    e-mail: testa@mporzio.astro.it
²  University of Amsterdam, Astronomical Institute "Anton Pannekoek", Kruislaan 403, 1098 SJ, The Netherlands
³  SRON Netherlands Institute for Space Research, Sorbonnelaan, 2, 3584 CA, Utrecht, The Netherlands
⁴  MSSL, University College London, Holmbury St. Mary, Dorking Surrey RH5 6NT, UK
⁵  JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, 440 UCB, Boulder, CO, 80309, USA
⁶  Laboratoire AIM, CEA/DSM - CNRS - Université Paris Diderot, DAPNIA/Service d'Astrophysique, Bât.709, CEA-Saclay, 91191 Gif-sur-Yvette, France
⁷  INAF - Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (Lc), Italy
⁸  University of Padua, Physics Department, via Marzolo 8, 35131, Padova, Italy
⁹  ESO - European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile
¹⁰  INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica "G.Occhialini", via Bassini 15, 20133, Milano, Italy

(Received 17 September 2007 / Accepted 17 January 2008 )

Abstract
Context. We report on near-infrared (IR) observations of the three anomalous X-ray pulsars XTE J1810-197, 1RXS J1708-4009, and 1E 1841-045, and the soft gamma-ray repeater SGR 1900+14, taken with the ESO-VLT, the Gemini, and the CFHT telescopes.
Aims. This work aims at identifying and/or confirming the IR counterparts of these magnetars, as well as at measuring their possible IR variability.
Methods. To perform photometry of objects as faint as , we used data taken with the largest telescopes, equipped with the most advanced IR detectors and in most of the cases with adaptive optics devices. The latter are critical for achieving the sharp spatial accuracy required to pinpoint faint objects in crowded fields.
Results. We confirm with high confidence identification of the IR counterpart to XTE J1810-197 and its IR variability. For 1E 1841-045 and SGR 1900+14, we propose two candidate IR counterparts based on the detection of IR variability. For 1RXS J1708-4009, we show that none of the potential counterparts within the source X-ray error circle can as yet be convincingly associated with this AXP.
Conclusions. The IR variability of the AXP XTE J1810-197 does not follow the same monotonic decrease in its post-outburst X-ray emission. Instead, the IR variability appears more like the one observed in radio band, although simultaneous IR and radio observations are crucial for any conclusion in this respect. For 1E 1841-045 and SGR 1900+14, follow-up observations are needed to confirm our proposed candidates with higher confidence.

Key words: stars: pulsars: individual: XTE J1810-197 -- stars: pulsars: individual: 1RXS J1708-4009 -- stars: pulsars: individual: 1E 1841-045 -- stars: pulsars: individual: SGR 1900+14 -- stars: pulsars: general -- stars: neutron

© ESO 2008

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