Optical and X‐Ray Observations of IGR J00291+5934 in Quiescence

Abstract

We report on optical and X-ray observations of the accretion-poweredmillisecond pulsar IGR J00291+5934 in quiescence. Time-resolved I-bandphotometry has been obtained with the 4.2 m William Herschel Telescope,while a 3 ks Chandra observation provided contemporaneous X-raycoverage. We found an unabsorbed 0.5-10 keV X-ray flux of1{\times}10^{-13} erg cm^{-2} s^{-1}, whichimplies that the source was in quiescence at the time of the opticalobservations. Nevertheless, the optical I-band light curve of IGRJ00291+5934 shows evidence for strong flaring. After removal of thestrongest flares, we find evidence for an orbital modulation in thephase-folded I-band light curve. The overall modulation can be describedby effects resulting from the presence of a superhump. Comparing ourlight curve with that reported recently we find evidence for a change inthe quiescent base level. Similar changes have now been reported forfour soft X-ray transients, implying that they may be a common featureof such systems in quiescence. Furthermore, the maximum in our foldedlight curve occurs at a different phase than observed before.