Adaptive Optics Near-Infrared Observations of Magnetars: XTE J1810-197, 1RXS J1708-4009, 1E 1841-045 and SRG 1900+14

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 {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. 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.