The low-luminosity behaviour of the 4U 0115+63 Be/X-ray transient

The Be/X-ray transient 4U 0115+63 exhibited a giant, type-II outburst in 2015 October. The source did not decay to its quiescent state but settled in a meta-stable plateau state (a factor ∼ 10 brighter than quiescence) in which its luminosity slowly decayed. We used XMM-Newton to observe the system during this phase and we found that its spectrum can be well described using a blackbody model with a small emitting radius. This suggests emission from hotspots on the surface, which is confirmed by the detection of pulsations. In addition, we obtained a relatively long (∼7.9 ksec) Swift/X-Ray Telescope observation ∼35 d after our XMM-Newton one. We found that the source luminosity was significantly higher, and although the spectrum could be fitted with a blackbody model, the temperature was higher and the emitting radius smaller. Several weeks later the system started a sequence of type-I accretion outbursts. In between those outbursts, the source was marginally detected with a luminosity consistent with its quiescent level. We discuss our results in the context of the three proposed scenarios (accretion down to the magnestospheric boundary, direct accretion on to neutron star magnetic poles or cooling of the neutron star crust) to explain the plateau phase.