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

## TL;DR

This study investigates the low luminosity behavior of the Be/X-ray transient 4U 0115+63 during a plateau phase following a giant outburst, revealing surface hot spots, pulsations, and variable accretion states through X-ray observations.

## Contribution

The paper provides detailed X-ray spectral and timing analysis of 4U 0115+63 during its plateau phase, offering insights into accretion processes and neutron star surface emission in this state.

## Key findings

- Spectrum fits a black-body model indicating hot spots.
- Detected pulsations confirming surface emission.
- Luminosity varies with outburst activity.

## Abstract

The Be/X-ray transient 4U 0115+63 exhibited a giant, type-II outburst in October 2015. 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 black-body model with a small emitting radius. This suggests emission from hot spots on the surface, which is confirmed by the detection of pulsations. In addition, we obtained a relatively long (~7.9 ksec) Swift/XRT observation ~35 days after our XMM-Newton one. We found that the source luminosity was significantly higher and, although the spectrum could be fitted with a black-body 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 onto neutron star magnetic poles or cooling of the neutron star crust) to explain the plateau phase.