Observations of IGR J00291+5934 in quiescence

Abstract

We report on optical and X-ray observations of the accretion powered ms pulsar IGR J00291+5934 in quiescence. Time resolved I-band photometry has been obtained with the 4.2 m William Herschel Telescope, while a 3 ks Chandra observation provided contemporaneous X-ray coverage. We found an unabsorbed 0.5-10 keV X-ray flux of 1x10^-13 erg cm-2 s-1 which implies that the source was in quiescence at the time of the optical observations. Nevertheless, the optical I-band light curve of IGR J00291+5934 shows evidence for strong flaring. After removal of the strongest flares, we find evidence for an orbital modulation in the phase folded I-band light curve. The overall modulation can be described by effects resulting from the presence of a superhump. Comparing our lightcurve with that reported recently we find evidence for a change in the quiescent base level. Similar changes have now been reported for 4 soft X-ray transients implying that they may be a common feature of such systems in quiescence. Furthermore, the maximum in our folded lightcurve occurs at a different phase than observed before.