# The puzzling case of the accreting millisecond X-ray pulsar IGR J00291+5934: flaring optical emission during quiescence

## TL;DR

This study observed optical flaring in the quiescent phase of the accreting millisecond X-ray pulsar IGR J00291+5934, suggesting the presence of an accretion disc and potential superhump activity before a subsequent outburst.

## Contribution

First optical study during quiescence revealing flaring activity and sinusoidal modulation, indicating an accretion disc and superhump presence in IGR J00291+5934.

## Key findings

- Detected strong optical flaring activity in quiescence
- Identified sinusoidal modulation with phase shift
- Suggested presence of an accretion disc and superhump

## Abstract

We present an optical (gri) study during quiescence of the accreting millisecond X-ray pulsar IGR J00291+5934 performed with the 10.4m Gran Telescopio Canarias (GTC) in August 2014. Despite the source being in quiescence at the time of our observations, it showed a strong optical flaring activity, more pronounced at higher frequencies (i.e. the g band). Once the flares were subtracted, we tentatively recovered a sinusoidal modulation at the system orbital period in all bands, even if a significant phase shift with respect to an irradiated star, typical of accreting millisecond X-ray pulsars is detected. We conclude that the observed flaring could be a manifestation of the presence of an accretion disc in the system. The observed light curve variability could be explained by the presence of a superhump, which might be another proof of the formation of an accretion disc. In particular, the disc at the time of our observations was probably preparing to the new outburst of the source, that happened just a few months later, in 2015.