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

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.

8 pages, 4 figures, 2 tables, accepted for publication in MNRAS

References in corpus (11)

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Cited by in corpus (9)

• The X-ray properties of Be/X-ray pulsars in quiescence
• Spectrum formation in X-ray pulsars at very low mass accretion rate: Monte-Carlo approach
• The Swift Bulge Survey: Motivation, Strategy, and First X-ray Results
• The frequency of Kozai-Lidov disc oscillation driven giant outbursts in Be/X-ray binaries
• On obtaining neutron-star mass and radius constraints from quiescent low-mass X-ray binaries in the Galactic plane
• Discovery of accretion-driven pulsations in the prolonged low X-ray luminosity state of the Be/X-ray transient GX 304-1
• Quiescent X-ray variability in the neutron star Be/X-ray transient GRO J1750-27
• Recurrent low-level luminosity behaviour after a giant outburst in the Be/X-ray transient 4U 0115+63
• Flux variability of the "10 keV feature" of 4U 0115+63