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

¹ Università dell’Insubria, Dipartimento di Scienza e Alta Tecnologia, via Valleggio 11, 22100 Como, Italy
² INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
e-mail: cristina.baglio@brera.inaf.it
³ INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00044 Monte Porzio Catone, Italy
⁴ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu km 0.7, 09042 Monserrato, Italy
⁵ Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, via Archirafi 36, 90123 Palermo, Italy
⁶ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁷ Departamento de Astrofísica, Univ. de La Laguna, 38206 La Laguna, Tenerife, Spain
⁸ Institute of Space Sciences (CSIC-IEEC), Campus UAB, Carrer Can Magrans s/n, 08193 Barcelona, Spain
⁹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
¹⁰ Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain

Received: 12 August 2016
Accepted: 29 December 2016

Abstract

We present an optical (gri) study during quiescence of the accreting millisecond X-ray pulsar IGR J00291+5934 performed with the 10.4 m Gran Telescopio Canarias (GTC) in August 2014. Although the source was in quiescence at the time of our observations, it showed a strong optical flaring activity, more pronounced in bluer filters (i.e. the g-band). After subtracting the flares, we tentatively recovered a sinusoidal modulation at the system orbital period in all bands, even when a significant phase shift with respect to an irradiated star, typical of accreting millisecond X-ray pulsars, was 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 the new outburst of the source, which occurred a few months later, in 2015.