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 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 onto neutron star magnetic poles or cooling of the neutron star crust) to explain the plateau phase.

Quiescent X-ray variability in the neutron star Be/X-ray transient GRO J1750−27

“…This indicates that if the emission mechanism is linked to the periastron passage, it is not always active. This is similar to what has been found for the mini type-I outbursts in 4U 0115+63 and V 0332+53 which are not always present at periastron passages(Wijnands & Degenaar 2016;Rouco Escorial et al 2017).…”

“…The purpose of our campaign was to determine if the crust of the NS in this system was significantly heated during this outburst and, if so, to follow its crust cooling behaviour. Such cooling of an accretion-heated crusts may have been observed for two other Be/X-ray transients (4U 0115+63 and V 0332+53) after the type-II outbursts they exhibited (Wijnands & Degenaar 2016;Rouco Escorial et al 2017). However, contrary to what was found for these two systems, we do not see any strong evidence of such crust heating and cooling behaviour in GRO J1750-27 and, consequently, we infer that the NS crust was not significantly heated during the previous outburst (see Section 3.1).…”

“…This model could also adequately describe our Chandra spectra, however, in the rest of our paper we mainly use the results obtained when using the power-law and blackbody models in order to be able to compare our source with the other two sources (4U 0115+63 and V 0332+53), for which similar studies have been performed. However, only blackbody and power-law fit results have reported for these two sources (see Wijnands & Degenaar 2016;Rouco Escorial et al 2017). Nevertheless, for completeness, we list our NSA model results in Appendix B.…”

Cooling of Accretion-Heated Neutron Stars

“…Important for the interpretation of the cooling emission in Be/X-ray transients is the fact that it is possible that the observed cooling emission might arise from hot spots at the NS magnetic poles (see Rouco Escorial et al, 2017). However, the rest of the surface could contribute (or dominate) significantly to the cooling radiation, but its lower temperature might make this contribution unobservable (see discussion in Elshamouty et al, 2016b).…”

The frequency of Kozai–Lidov disc oscillation driven giant outbursts in Be/X-ray binaries

“…However, sometimes these outbursts are separated by only 1-1.5 yr, and in this case the second outburst is of lower luminosity than the first (e.g. Rouco Escorial et al 2017Escorial et al , 2019. After major outbursts a decrease in the size of the Be star disc is observed (Reig et al , 2016.…”