The poor performance of many existing nonpolarizable ion force fields is typically blamed on either the lack of explicit polarizability, the absence of charge transfer, or the use of unreduced Coulomb interactions. However, this analysis disregards the large and mostly unexplored parameter range offered by the Lennard-Jones potential. We use a global optimization procedure to develop water-model-transferable force fields for the ions K + , Na + , Cl – , and Br – in the complete parameter space of all Lennard-Jones interactions using standard mixing rules. No extra-thermodynamic assumption is necessary for the simultaneous optimization of the four ion pairs. After an optimization with respect to the experimental solvation free energy and activity, the force fields reproduce the concentration-dependent density, ionic conductivity, and dielectric constant with high accuracy. The force field is fully transferable between simple point charge/extended and transferable intermolecular potential water models. Our results show that a thermodynamically consistent force field for these ions needs only Lennard-Jones and standard Coulomb interactions.

Preliminary experiments have been performed toward the development of a multi-cell Penning–Malmberg trap for the storage of large numbers of positrons (≥1010 e+). We introduce the master-cell test trap and the diagnostic tools for first experiments with electrons. The usage of a phosphor screen to measure the z-integrated plasma distribution and the number of confined particles is demonstrated, as well as the trap alignment to the magnetic field ( B = 3.1 T) using the m = 1 diocotron mode. The plasma parameters and expansion are described along with the autoresonant excitation of the diocotron mode using rotating dipole fields and frequency chirped sinusoidal drive signals. We analyze the reproducibility of the excitation and use these findings to settle on the path for the next generation multi-cell test device.

A significant fraction of high-mass X-ray binaries are supergiant fast X-ray transients (SFXTs). The prime model for the physics governing their X-ray behaviour suggests that the winds of donor OB supergiants are magnetized. To investigate if magnetic fields are indeed present in the optical counterparts of such systems, we acquired low-resolution spectropolarimetric observations of the two optically brightest SFXTs, IGR J08408−4503 and IGR J11215−5952 with the ESO FORS 2 instrument during two different observing runs. No field detection at a significance level of 3σ was achieved for IGR J08408−4503. For IGR J11215−5952, we obtain 3.2σ and 3.8σ detections ( B z hydr = −978 ± 308 G and B z hydr = 416 ± 110 G) on two different nights in 2016. These results indicate that the model involving the interaction of a magnetized stellar wind with the neutron star magnetosphere can indeed be considered to characterize the behaviour of SFXTs. We detected long-term spectral variability in IGR J11215−5952, while for IGR J08408−4503 we find an indication of the presence of short-term variability on a time scale of minutes.

Funding InformationRFBR, 16-02-00604A.Recent XMM-Newton observations of the B2 type star Oph A indicated a periodicity of 1.205 day, which was ascribed to rotational modulation. Since variability of X-ray emission in massive stars is frequently the signature of a magnetic field, we investigated whether the presence of a magnetic field can indeed be invoked to explain the observed X-ray peculiarity. Two FORS 2 spectropolarimetric observations in different rotation phases revealed the presence of a negative (⟨B z ⟩ all = − 419 ± 101 G) and positive (⟨B z ⟩ all = 538 ± 69 G) longitudinal magnetic field, respectively. We estimate a lower limit for the dipole strength as B d = 1.9 ± 0.2 kG. Our calculations of the Kepler and Alfvén radii imply the presence of a centrifugally supported, magnetically confined plasma around Oph A. The study of the spectral variability indicates a behavior similar to that observed in typical magnetic early-type Bp stars.

This paper discusses thermal equilibrium states of single-species plasmas, such as pure electron plasmas and pure positron plasmas, that are confined in a dipole trap. Thermal equilibrium states for such plasmas are routinely realized in the homogeneous magnetic field of Penning–Malmberg traps. We generalize the theory of these states to include inhomogeneous magnetic dipole fields. The approach to thermal equilibrium takes place in two stages with well separated time scales. On the collision time scale, thermal equilibrium is established along each magnetic field line. On the much longer transport time scale, heat conduction and viscosity bring the plasmas on different flux contours into thermal equilibrium, we call this a state of global thermal equilibrium. We present numerical results for local and global thermal equilibria. These results agree with the analytic predictions for charge collections that are large compared with the Debye length. There is, in principle, no limit to the confinement time of a single-species plasma in a global thermal equilibrium state. Experiments with hot electron–ion plasmas performed in the LDX and RT1 devices give us confidence that, in contrast to a Penning–Malmberg trap, a magnetic dipole field can also confine cold quasi-neutral electron–positron pair plasmas on the time scale of the phenomena of interest. Such pair plasmas are assumed to form in the magnetosphere of neutron stars but have so far not been realized in a laboratory. The creation of an electron–positron pair plasma is the main goal of the APEX collaboration.