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We used the monochromatic soft-x-ray beamline P04 at the synchrotron-radiation facility PETRA III to resonantly excite the strongest $2p\text{−}3d$transitions in neonlike $\mathrm{Ni}$ions, ${\left[2p^6\right]}_{J=0}\to {\left[{\left(2p^5\right)}_{1/2}3d_{3/2}\right]}_{J=1}$and ${\left[2p^6\right]}_{J=0}\to {\left[{\left(2p^5\right)}_{3/2}3d_{5/2}\right]}_{J=1}$, respectively dubbed $3C$and $3D$, achieving a resolving power of 15 000 and signal-to-background ratio of 30. We obtain their natural linewidths, with an accuracy of better than 10%, as well as the oscillator-strength ratio $f\left(3C\right)/f\left(3D\right)=2.51\left(11\right)$from analysis of the resonant fluorescence spectra. These results agree with those of previous experiments, earlier predictions, and our own advanced calculations. Published by the American Physical Society2024 

ABSTRACT New long Chandra grating observations of the O supergiant ζ Pup show not only a brightening of the X-ray emission line flux of 13 per cent in the 18 yr since Chandra’s first observing cycle, but also clear evidence – at more than 4σ significance – of increased wind absorption signatures in its Doppler-broadened line profiles. We demonstrate this with non-parametric analysis of the profiles as well as Gaussian fitting and then use line-profile model fitting to derive a mass-loss rate of 2.47 ± 0.09 × 10−6$${\mathrm{M_{\odot }~{\mathrm{y}r^{-1}}}}$$, which is a 40 per cent increase over the value obtained from the cycle 1 data. The increase in the individual emission line fluxes is greater for short-wavelength lines than long-wavelength lines, as would be expected if a uniform increase in line emission is accompanied by an increase in the wavelength-dependent absorption by the cold wind in which the shock-heated plasma is embedded.