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

¹ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK e-mail: rm2@mssl.ucl.ac.uk
² University of Amsterdam, Astronomical Institute “Anton Pannekoek”, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands
³ INAF - Osservatorio Astronomico di Roma, via Frascati 33, Monte Porzio Catone, Rome 00040, Italy
⁴ European Southern Observatory, Av. Alonso de Cordova 3107, Vitacura, Santiago, Chile
⁵ INAF, Istituto di Astrofisica Spaziale, via Bassini 15, Milan 20133, Italy
⁶ SRON Netherlands Institute for Space Research, Sorbonnelaan, 2, 3584CA, Utrecht, The Netherlands
⁷ Department of Physics, University of Padua, via Marzolo 8, Padua 35131, Italy
⁸ JILA and Department of Astrophysical and Planetary Sciences, University of Colorado, 440 UCB, Boulder 80309, USA
⁹ AIM-Astrophysique Interactions Multiéchelles (UMR 7158 CEA/CNRS/Université Paris 7 Denis Diderot), CEA Saclay, DSM/IRFU/Service d'Astrophysique, Batiment 709, L'Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France

Received: 31 October 2008
Accepted: 21 January 2009

Abstract

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 (K_s 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 radio position of 1E 1547.0-5408. The faintest of them (object 1) has a brightness , which would yield an unabsorbed X-ray–to–NIR flux ratio 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 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 (at ), computed in our deepest VLT 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.