Abstract James Webb Space Telescope (JWST) observations have been demonstrated to be efficient in detecting multiple stellar populations in globular clusters (GCs) in the low-mass regime of M dwarfs. ...We present an overview, and first results, of different projects that can be explored by using the JWST observations gathered under program GO2560 for 47 Tucanae, the first program entirely devoted to the investigation of multiple populations in very-low-mass stars, which includes spectroscopic data for the faintest GC stars for which spectra are available. Our color–magnitude diagram (CMD) shows some substructures for ultracool stars, including gaps and breaks in slope. In particular, we observe both a gap and a minimum in the F322W2 luminosity function less than 1 mag apart, and discuss which it could be associated with the H-burning limit. We detect stars fainter than this minimum, very likely brown dwarfs. We corroborate the ubiquity of the multiple populations across different masses, from ∼0.1 M ⊙ up to red giants (∼0.8 M ⊙ ). The oxygen range inferred for the M dwarfs, both from the CMD and from the spectra of two M dwarfs associated with different populations, is similar to that observed for giants. We have not detected any difference between the fractions of stars in distinct populations across stellar masses ≳ 0.1 M ⊙ . This work demonstrates the JWST's capability in uncovering multiple populations within M dwarfs and illustrates the possibility to analyze very-low-mass stars in GCs approaching the H-burning limit and the brown-dwarf sequence.
We search the literature for reports on the spectral properties of neutron star low-mass X-ray binaries when they have accretion luminosities between 1034 and 1036 erg s−1, corresponding to roughly ...0.01–1 per cent of the Eddington accretion rate for a neutron star. We found that in this luminosity range the photon index (obtained from fitting a simple absorbed power law in the 0.5–10 keV range) increases with decreasing 0.5–10 keV X-ray luminosity (i.e. the spectrum softens). Such behaviour has been reported before for individual sources, but here we demonstrate that very likely most (if not all) neutron star systems behave in a similar manner and possibly even follow a universal relation. When comparing the neutron star systems with black hole systems, it is clear that most black hole binaries have significantly harder spectra at luminosities of 1034–1035 erg s−1. Despite a limited number of data points, there are indications that these spectral differences also extend to the 1035–1036 erg s−1 range, but above a luminosity of 1035 erg s−1 the separation between neutron star and black hole systems is not as clear as below. In addition, the black hole spectra only become softer below luminosities of 1034 erg s−1 compared to 1036 erg s−1 for the neutron star systems. This observed difference between the neutron star binaries and black hole ones suggests that the spectral properties (between 0.5 and 10 keV) at 1034–1035 erg s−1 can be used to tentatively determine the nature of the accretor in unclassified X-ray binaries. More observations in this luminosity range are needed to determine how robust this diagnostic tool is and whether or not there are (many) systems that do not follow the general trend. We discuss our results in the context of properties of the accretion flow at low luminosities and we suggest that the observed spectral differences likely arise from the neutron star surface becoming dominantly visible in the X-ray spectra. We also suggest that both the thermal component and the non-thermal component might be caused by low-level accretion on to the neutron star surface for luminosities below a few times 1034 erg s−1.
Abstract
We continue to investigate two-dimensional laterally propagating flames in type I X-ray bursts using fully compressible hydrodynamics simulations. In the current study we relax previous ...approximations where we artificially boosted the flames. We now use more physically realistic reaction rates, thermal conductivities, and rotation rates, exploring the effects of neutron star rotation rate and thermal structure on the flame. We find that at lower rotation rates the flame becomes harder to ignite, whereas at higher rotation rates the nuclear burning is enhanced by increased confinement from the Coriolis force and the flame propagates steadily. At higher crustal temperatures, the flame moves more quickly and accelerates as it propagates through the atmosphere. If the temperature is too high, instead of a flame propagating across the surface the entire atmosphere burns uniformly. Our findings could have implications for the relationship between observed burst rise times and neutron star rotation and accretion rates. All of the software used for these simulations is freely available.
ABSTRACT
We measured the thermonuclear burning efficiency as a function of accretion rate for the Type I X-ray bursts of five low-mass X-ray binary systems. We chose sources with measured neutron ...star spins and a substantial population of bursts from a large observational sample. The general trend for the burst rate is qualitatively the same for all sources; the burst rate first increases with the accretion rate up to a maximum, above which the burst rate declines, despite the increasing accretion rate. At higher accretion rates, when the burst rate decreases, the α-value (the ratio of accretion energy and burst energy) increases by up to a factor of 10 above that in the rising burst rate regime. These observations are contrary to the predictions of 1D numerical models, but can be explained as the consequence of a zone of stable burning on the neutron star surface, which expands with increasing accretion rate. The stable burning also ‘pollutes’ the unstable burning layer with ashes, contributing to the change in burst properties measured in the falling burst rate regime. We find that the mass accretion rate at which the burst rate begins to decrease is anticorrelated with the spin of the neutron star. We conclude that the neutron star spin is a key factor, moderating the nuclear burning stability, via the local accretion rate and fuel composition over the star.
ABSTRACT
We report on multiwavelength observations during quiescence and of the first detected outburst of the ≈60 min orbital period AM CVn SDSS J113732+405458. Using X-ray and UV observations, we ...determined an upper limit duration of the event of about 1 yr. The amplitude of the outburst was remarkably small, of around 1 mag in r and 0.5 mag in g. We have also investigated the colour variations of SDSS J113732+405458 and other long-period AM CVns in outbursts and identified a track on the colour–magnitude diagram that is not compatible with the predictions of the disc instability model, suggesting that some outbursts in long-period AM CVns are caused by enhanced mass-transfer. To our knowledge, these are the first studies of the colour evolution in AM CVns. During quiescence we measured an X-ray luminosity for SDSS J113732+405458 of ≈3 × 1029 erg s−1 in the 0.5–10 keV band. This indicates a very low accretion rate, in agreement with the disc instability model for long-period systems. However, such a model predicts stable discs at somewhat long periods. The discovery of this system outburst, along with similarities to the long-period system SDSS J080710+485259 with a comparably long, weak outburst, indicates that these enhanced mass-transfer events may be more common in long-period AM CVns. A larger sample would be needed to determine empirically at what period, if any, the disc instability stops functioning entirely. Finally, we identified an infrared excess in the quiescence spectrum attributable to the donor. This makes SDSS J113732+405458 the second AM CVn to have a directly detected donor.
Abstract
The paradigm in which magnetic fields play a crucial role in launching/collimating outflows in many astrophysical objects continues to gain support. However, semi-analytical models including ...the effect of magnetic fields on the dynamics and morphology of jets are still missing due to the intrinsic difficulties in integrating the equations describing a collimated, relativistic flow in the presence of gravity. Only few solutions have been found so far, due to the highly non-linear character of the equations together with the need to blindly search for singularities. These numerical problems prevented a full exploration of the parameter space. We present a new integration scheme to solve r-self-similar, stationary, axisymmetric magnetohydrodynamic (MHD) equations describing collimated, relativistic outflows crossing smoothly all the singular points (Alfvén point and modified slow/fast points). For the first time, we are able to integrate from the disc mid-plane to downstream of the modified fast point. We discuss an ensemble of jet solutions, emphasizing trends and features that can be compared to observables. We present, for the first time with a semi-analytical MHD model, solutions showing counter-rotation of the jet for a substantial fraction of its extent. We find diverse jet configurations with bulk Lorentz factors up to 10 and potential sites for recollimation between 103 and 107 gravitational radii. Such extended coverage of the intervals of quantities, such as magnetic-to-thermal energy ratios at the base or the heights/widths of the recollimation region, makes our solutions suitable for application to many different systems where jets are launched.
ABSTRACT
Rapid bursts at optical wavelengths have been reported for several accreting white dwarfs. In these bursts, the optical luminosity can increase by up to a factor of 30 in less than an hour, ...before fading on time-scales of several hours, and the energy release can reach ~1039 erg (‘micronovae’). Several systems have also shown these bursts to be semirecurrent on time-scales of days to months, and the temporal profiles of these bursts strongly resemble those observed in Type-I X-ray bursts in accreting neutron stars. It has been suggested that the observed micronovae may be the result of localized thermonuclear runaways in the surface layers of accreting white dwarfs. Here we propose a model in which the magnetic confinement of accretion streams on to the accreting magnetic white dwarf may trigger localized thermonuclear runaways. The model proposed to trigger micronovae appears to favour magnetic systems with both a high white dwarf mass and a high mass-transfer rate.
ABSTRACT
We report the detection of an infrared burst lagging a thermonuclear Type I X-ray burst from the accreting neutron star (NS) 4U 1728-34 (GX 354-0). Observations were performed simultaneously ...with XMM–Newton (0.7–12 keV), NuSTAR (3–79 keV), and HAWK-I@VLT (2.2 $\, \mu$m). We measure a lag of 4.75 ± 0.5 s between the peaks of the emission in the two bands. Due to the length of the lag and the shape of the IR burst, we found that the most plausible cause for such a large delay is reprocessing of the Type I burst X-rays by the companion star. The inferred distance between the NS and the companion can be used to constrain the orbital period of the system, which we find to be larger than ∼66 min (or even ≳2 h, for a realistic inclination <75°). This is much larger than the current tentatively estimated period of ∼11 min. We discuss the physical implications on the nature of the binary and conclude that most likely the companion of 4U 1728-34 is a helium star.
ABSTRACT
Hotspots on the surface of accreting neutron stars have been directly observed via pulsations in the light curves of X-ray pulsars. They are thought to occur due to magnetic channelling of ...the accreted fuel to the neutron star magnetic poles. Some X-ray pulsars exhibit burst oscillations during Type I thermonuclear X-ray bursts that are thought to be caused by asymmetries in the burning. In rapidly rotating neutron stars, it has been shown that the lower gravity at the equator can lead to preferential ignition of X-ray bursts at this location. These models, however, do not include the effect of accretion hotspots at the the neutron star surface. There are two accreting neutron star sources in which burst oscillations have been observed to track exactly the neutron star spin period. We analyse whether this could be due to the X-ray bursts igniting at the magnetic pole of the neutron star, because of heating in the accreted layers under the hotspot causing ignition conditions to be reached earlier. We investigate heat transport in the accreted layers using a 2D model and study the prevalence of heating down to the ignition depth of X-ray bursts for different hotspot temperatures and sizes. We perform calculations for accretion at the pole and at the equator, and infer that ignition could occur away from the equator at the magnetic pole for hotspots with temperature $T_{\mathrm{HS}}\gtrsim 1\times 10^8\, \mathrm{K}$. However, current observations have not identified such high temperatures in accretion-powered X-ray pulsars.
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