The optical counterpart of IGR J00291+5934 in quiescence*

Abstract

Aims. The recent (December 2004) discovery of the sixth accretion-powered millisecond X-ray pulsar IGR J00291+5934 provides a very good chance to deepen our knowledge of such systems. Although these systems are well studied at high energies, poor informations are available for their optical/NIR counterparts during quiescence. Up to now, only for SAX J1808.4-3658, the first discovered system of this type, we have a secure multiband detection of its optical counterpart in quiescence. Among the seven known system IGR J00291+5934 is the one that resembles SAX J1808.4-3658 more closely. Methods. With the Italian 3.6 m TNG telescope, we have performed deep optical and NIR photometry of the field of IGR J00291+5934 during quiescence in order to look for the presence of a variable counterpart. Results. We present here the first multiband (VRIJH) detection of the optical and NIR counterpart of IGR J00291+5934 in quiescence as well as a deep upper limit in the k-band. We obtain an optical light curve that shows variability consistent with a sinusoidal modulation at the known 2.46 h orbital period and present evidence for a strongly irradiated companion. © ESO 2007.