Observations of IGR J00291+5934 in quiescence

TL;DR

This paper presents optical and X-ray observations of the millisecond pulsar IGR J00291+5934 in quiescence, revealing optical flaring, orbital modulation, and potential superhump effects, with implications for understanding transient behaviors.

Contribution

It provides the first simultaneous optical and X-ray observations of IGR J00291+5934 in quiescence, identifying orbital modulation and superhump effects, and reports variability features common to soft X-ray transients.

Findings

Detected strong optical flaring in quiescence.

Found evidence for orbital modulation in optical light curve.

Observed changes in quiescent baseline levels over time.

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.