VLT/NACO near-infrared observations of the transient radio magnetar 1E 1547.0-5408

Context: Despite about a decade of observations, very little is known about the optical and infrared (IR) emission properties of the soft gamma-ray repeaters (SGRs) and of the anomalous X-ray pulsars (AXPs), the magnetar candidates, and about the physical processes which drive their emission at these wavelengths. This is mainly due to the limited number of identifications achieved so far, five in total, and to the sparse spectral coverage obtained from multi-band optical/IR photometry. Aims: The aim of this work is to search for a likely candidate counterpart to the recently discovered transient radio AXP 1E 1547.0-5408. Methods: We performed the first deep near-IR (NIR) observations (Ks band) of 1E 1547.0-5408 with the Very Large Telescope (VLT) on three nights (July 8th, 12th, and August 17th), after the X-ray source rebrightening and during the subsequent decay reported around June 2007. Results: We detected four objects within, or close to, the 3σ radio position of 1E 1547.0-5408. The faintest of them (object 1) has a brightness Ks = 20.27 ± 0.05, which would yield an unabsorbed X-ray-to-NIR flux ratio F_X/FKs ˜ 800 for 1E 1547.0-5408, i.e. on average lower than those derived for other magnetars. The non-detection of object 1 on the nights of July 8th and August 17th only allowed us to set an upper limit of Δ Ks ˜ 0.2 on its NIR variability, which prevented us from searching for correlations with the radio or X-ray flux. We detected no other object at the radio position down to a limit of Ks ˜ 21.7 (at 5 σ), computed in our deepest <sl>VLT</sl> image (July 12th). Conclusions: From our observations we cannot confidently propose a NIR counterpart to 1E 1547.0-5408. More NIR observations of object 1, e.g. to determine its colors and to monitor variability, would be conclusive to determine whether or not it can be considered a plausible candidate.