Multiwavelength observations of the Be/X-ray binary IGR J01217-7257 (=SXP 2.16) during outburst

Abstract

We present simultaneous, multiwavelength observations of the Small Magellanic Cloud Be/XRB IGR J01217-7257 (=SXP 2.16) during outbursts in 2014, 2015 and 2016. We also present the results of the RXTE observations of the Small Magellanic Cloud during which the source was initially discovered with a periodicity of 2.1652 ± 0.0001 s that we associate with the spin period of the neutron star. A systematic temporal analysis of long-term Swift/BAT data reveals a periodic signal of 82.5 ± 0.7 d, in contrast with a similar analysis of long base-line Optical Gravitational Lensing Experiment (OGLE) I-band light curves that reveals 83.67 ± 0.05 d also found in this work. Interpreting the longer X-ray periodicity as indicative of binary motion of the neutron star, we find that outbursts detected by INTEGRAL and Swift between 2014 and 2016 are consistent with type I outbursts seen in Be/XRBs, occurring around periastron. Comparing these outbursts with the OGLE data, we see a clear correlation between outburst occurrence and increasing I-band flux. A periodic analysis of subdivisions of OGLE data reveals three epochs during which short periodicities of ~1 d are significantly detected which we suggest are non-radial pulsations (NRPs) of the companion star. These seasons immediately precede those exhibiting clear outburst behaviour, supporting the suggested association between the NRPs, decretion disc growth and the onset of type I outbursts.