No abstract

No abstract

The radioactively powered transient following a binary neutron star (BNS) merger, known as a kilonova (KN), is expected to enter the steady-state nebular phase a few days after merger. Steady-state holds until thermal reprocessing time-scales become long, at which point the temperature and ionisation states need to be evolved time-dependently. We study the onset and significance of time-dependent effects using the non-local thermodynamic equilibrium (NLTE) spectral synthesis code SUMO. We employ a simple single-zone model with an elemental composition of Te, Ce, Pt and Th, scaled to their respective solar abundances. The atomic data are generated using the Flexible Atomic Code (FAC), and consist of energy levels and radiative transitions, including highly forbidden lines. We explore the KN evolution from 5 to 100 days after merger, varying ejecta mass and velocity. We also consider variations in the degree of electron magnetic field trapping, as well as radioactive power generation for alpha and beta decay (but omitting fission products). We find that the transition time, and magnitude of steady-state deviations are highly sensitive to these parameters. For typical KN ejecta, the deviations are minor within the time-frame studied. However, low density ejecta with low energy deposition show significant differences from ∼10 days. Important deviation of the ionisation structure solution impacts the temperature by altering the overall line cooling. Adiabatic cooling becomes important at t ≥ 60 days which, in addition to the temperature and ionisation effects, lead to the bolometric light curve deviating from the instantaneous radioactive power deposited.

We propose a new method to determine magnetic fields, by using the magnetic-field induced electric dipole transition 3p 4 3d 4 D 7/2 → 3p 5 2 P 3/2 in Fe 9+ ions. This ion has a high abundance in astrophysical plasma and is therefore well-suited for direct measurements of even rather weak fields in e.g. solar flares. This transition is induced by an external magnetic field and its rate is proportional to the square of the magnetic field strength. We present theoretical values for what we will label the reduced rate and propose that the critical energy difference between the upper level in this transition and the close to degenerate 3p 4 3d 4 D 5/2 should be measured experimentally since it is required to determine the relative intensity of this magnetic line for different magnetic fields. * Electronic address: Tomas.Brage@fysik.lu.se † Electronic address: rhutton@fudan.edu.cn

We present a measured value for the degree of pseudo-degeneracy between two fine-structure levels in Fe9+ from line intensity ratios involving a transition induced by an external magnetic field. The extracted fine-structure energy difference between the and levels, where the latter is the upper state for the magnetic-field induced line, is needed in our recently proposed method to measure magnetic-field strengths in the solar corona. The intensity of the line at 257.262 Å is sensitive to the magnetic field external to the ion. This sensitivity is in turn strongly dependent on the energy separation in the pseudo-degeneracy through the mixing induced by the external magnetic field. Our measurement, which uses an Electron Beam Ion Trap with a known magnetic-field strength, indicates that this energy difference is 3.5 cm−1. The high abundance of Fe9+ and the sensitivity of the line’s transition probability to field strengths below 0.1 T opens up the possibility of diagnosing coronal magnetic fields. We propose a new method to measure the magnetic field in the solar corona, from similar intensity ratios in Fe9+. In addition, the proposed method to use the line ratio of the blended line with another line from Fe x as the density diagnostic should evaluate the effect of the magnetic-field-induced transition line.