2008A&A...482..607T

We report on near-infrared (IR) observations of the three anomalous X-ray pulsars XTEJ1810-197, 1RXSJ1708-4009, and 1E1841-045, and the soft gamma-ray repeater SGR1900+14, taken with the ESO-VLT, the Gemini, and the CFHT telescopes. This work aims at identifying and/or confirming the IR counterparts of these magnetars, as well as at measuring their possible IR variability. To perform photometry of objects as faint as K_s∼20, 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. We confirm with high confidence identification of the IR counterpart to XTEJ1810-197 and its IR variability. For 1E1841-045 and SGR1900+14, we propose two candidate IR counterparts based on the detection of IR variability. For 1RXSJ1708-4009, we show that none of the potential counterparts within the source X-ray error circle can as yet be convincingly associated with this AXP. The IR variability of the AXP XTEJ1810-197 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 1E1841-045 and SGR1900+14, follow-up observations are needed to confirm our proposed candidates with higher confidence.