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

We present our Swift monitoring campaign of the slowly rotating neutron star Be/X-ray transient GX 304-1 (spin period of ~275 s) when the source was not in outburst. We found that between its type-I outbursts the source recurrently exhibits a slowly decaying low-luminosity state (with luminosities of 10^(34-35) erg/s). This behaviour is very similar to what has been observed for another slowly rotating system, GRO J1008-57. For that source, this low-luminosity state has been explained in terms of accretion from a non-ionised ('cold') accretion disk. Due to the many similarities between both systems, we suggest that GX 304-1 enters a similar accretion regime between its outbursts. The outburst activity of GX 304-1 ceased in 2016. Our continued monitoring campaign shows that the source is in a quasi-stable low-luminosity state (with luminosities a few factors lower than previously seen) for at least one year now. Using our NuSTAR observation in this state, we found pulsations at the spin period, demonstrating that the X-ray emission is due to accretion of matter onto the neutron star surface. If the accretion geometry during this quasi-stable state is the same as during the cold-disk state, then matter indeed reaches the surface (as predicted) during this latter state. We discuss our results in the context of the cold-disk accretion model.Comment: 5 pages, 2 figures, accepted for A&A Letter

Modeling of granular solids with computational homogenization: Comparison with Biot's theory

peer reviewe

Constraining the properties of dense neutron star cores:the case of the low-mass X-ray binary HETE J1900.1-2455

Measuring the time evolution of the effective surface temperature of neutron stars can provide invaluable information on the properties of their dense cores. Here, we report on a new Chandra observation of the transient neutron star low-mass X-ray binary HETE J1900.1-2455, which was obtained ~2.5 yr after the end of its ~10-yr long accretion outburst. The source is barely detected during the observation, collecting only six net photons, all below 2 keV. Assuming that the spectrum is shaped as a neutron star atmosphere model we perform a statistical analysis to determine a 1-sigma confidence upper range for the neutron star temperature of ~30-39 eV (for an observer at infinity), depending on its mass, radius and distance. Given the heat injected into the neutron star during the accretion outburst, estimated from data provided by all-sky monitors, the inferred very low temperature suggests that either the core has a very high heat capacity or undergoes very rapid neutrino cooling. While the present data do not allow us to disentangle these two possibilities, both suggest that a significant fraction of the dense core is not superfluid/superconductor. Our modeling of the thermal evolution of the neutron star predicts that it may still cool further, down to a temperature of ~15 eV. Measuring such a low temperature with a future observation may provide constraints on the fraction of baryons that is paired in the stellar core.Comment: 14 pages (12 main, 2 appendix), 10 figures, published in MNRA

A strongly changing accretion morphology during the outburst decay of the neutron star X-ray binary 4U 1608−52

It is commonly assumed that the properties and geometry of the accretion flow in transient low-mass X-ray binaries (LMXBs) significantly change when the X-ray luminosity decays below ∼10⁻² of the Eddington limit (L_(Edd)). However, there are few observational cases where the evolution of the accretion flow is tracked in a single X-ray binary over a wide dynamic range. In this work, we use NuSTAR and NICER observations obtained during the 2018 accretion outburst of the neutron star LMXB 4U 1608−52, to study changes in the reflection spectrum. We find that the broad Fe–Kα line and Compton hump, clearly seen during the peak of the outburst when the X-ray luminosity is ∼10³⁷ erg s⁻¹ (∼0.05 L_(Edd)), disappear during the decay of the outburst when the source luminosity drops to ∼4.5 × 10³⁵ erg s⁻¹ (∼0.002 L_(Edd)). We show that this non-detection of the reflection features cannot be explained by the lower signal-to-noise ratio at lower flux, but is instead caused by physical changes in the accretion flow. Simulating synthetic NuSTAR observations on a grid of inner disc radius, disc ionization, and reflection fraction, we find that the disappearance of the reflection features can be explained by either increased disc ionization (log ξ ≳ 4.1) or a much decreased reflection fraction. A changing disc truncation alone, however, cannot account for the lack of reprocessed Fe–Kα emission. The required increase in ionization parameter could occur if the inner accretion flow evaporates from a thin disc into a geometrically thicker flow, such as the commonly assumed formation of a radiatively inefficient accretion flow at lower mass accretion rates

The nature of very-faint X-ray binaries: Near-infrared spectroscopy of 1RXH J173523.7−-354013 reveals a giant companion

Very-faint X-ray binaries (VFXBs) are a sub-class of black holes and neutron stars in binaries that appear to be accreting at a very low rate. In addition to providing interesting constraints on poorly understood forms of accretion, elucidating the nature of VFXBs is particularly interesting for binary evolution and population modeling. Through near-infrared (nIR) spectroscopy, we here investigate the nature of the bursting neutron star and VFXB 1RXH J173523.7$-$354013 (J1735), which persistently accretes at an X-ray luminosity of $L_X \sim 10^{34} - 10^{35}~L_{\odot}$. Our analysis shows that the nIR emission is dominated by that of the companion star, which we find to be a late G or early K-type giant, making this the second neutron star identified as a VFXB found to have a giant companion. We discuss how several of the system properties are difficult to reconcile with a wind-fed symbiotic X-ray binary. We therefore also propose an alternative scenario wherein J1735 is a wide binary system (supported by the discovery of a 7.5 d modulation in the nIR light curves) with a quiescent luminosity of $L_X \sim 10^{34} - 10^{35}~L_{\odot}$, in which the donor star is overflowing its Roche lobe. This raises the possibility that J1735 may, every century or more, exhibit very long and very bright outbursts during which it reaches accretion rates around the Eddington limit like the neutron star Z sources.Comment: 12 pages, 6 figures, accepted for publication in MNRA