The optical counterpart of IGR J00291+5934 in quiescence^*

¹ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (Lc), Italy e-mail: paolo.davanzo@brera.inaf.it
² Università degli Studi dell'Insubria, Dipartimento di Fisica e Matematica, via Valleggio 11, 22100 Como, Italy
³ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Roma, Italy
⁴ Fundación Galileo Galilei – INAF, Fundación Canaria, C. Alvarez de Abreu, 70, 38700 S/C. de La Palma, Spain

Received: 2 March 2007
Accepted: 18 July 2007

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.