Meta-stable low-level accretion rate states or neutron star crust cooling in the Be/X-ray transients V0332+53 and 4U 0115+63

Abstract

The Be/X-ray transients V0332+53 and 4U 0115+63 exhibited giant, type-II outbursts in 2015. Here we present Swift/XRT follow-up observations at the end of those outbursts. Surprisingly, the sources did not decay back to their known quiescent levels but stalled at a (slowly decaying) meta-stable state with luminosities a factor ∼10 above that observed in quiescence. The spectra in these states are considerably softer than the outburst spectra and appear to soften in time when the luminosity decreases. The physical mechanism behind these metastable states is unclear and they could be due to low-level accretion (either directly onto the neutron stars or onto their magnetospheres) or due to cooling of the accretion-heated neutron star crusts. Based on the spectra, the slowly decreasing luminosities, and the spectral softening, we favour the crust cooling hypothesis but we cannot exclude the accretion scenarios. On top of this meta-stable state, weak accretion events were observed that occurred at periastron passage and may thus be related to regular type-I outbursts.

Discovery of accretion-driven pulsations in the prolonged low X-ray luminosity state of the Be/X-ray transient GX 304–1

“…However, we observed GX 304-1 at luminosities of ∼ 10 34 erg s −1 which are, at least, one order of magnitude higher than the ones observed in systems that might indeed have exhibited such crust cooling behaviour (L X ∼10 32−33 erg s −1 ; e.g. Degenaar 2016 andRouco Escorial et al 2017; see the review by Wijnands et al 2017). In addition, we observed short term variability (on time scales of days) for GX 304-1 which is not expected in the cooling scenario.…”

GRO J1750–27: A neutron star far behind the Galactic Center switching into the propeller regime

“…It can be described by a black body model with the temperature of about 1 keV. Formally this spectrum looks softer in a comparison with the spectra before the transition, but its temperature is somewhat higher than expected for the propeller regime (∼ 0.5 keV with the subsequent cooling, Wijnands & Degenaar 2016). It can be associated either with insufficient statistics (formally, the source spectrum can be also described by the absorbed powerlaw model with the photon index of Γ ≃ 3), or with a possible continuation of the accretion after the transition into the propeller regime (see, e.g., a discussion of different mechanisms of the low state emission in Tsygankov et al 2017a).…”

Study of the X-ray pulsar IGR J19294+1816 with NuSTAR: Detection of cyclotron line and transition to accretion from the cold disk

“…As can be seen, this simple spectral model fits the data in XRT range at all observed flux levels. Although some spectral variability is observed, the transition to the thermal spectrum, expected in the case of cooling NS surface (see e.g., Wijnands & Degenaar 2016), is never observed in IGR J19294+1816, strongly indicating the continuation of accretion in the quiescent state.…”