Context.
We present a ~130 ks observation of the prototypical wind-accreting, high-mass X-ray binary Vela X-1 collected with
XMM-Newton
at orbital phases between 0.12 and 0.28. A strong flare took ...place during the observation that allows us to investigate the reaction of the clumpy stellar wind to the increased X-ray irradiation.
Aims.
To examine the wind’s reaction to the flare, we performed both time-averaged and time-resolved analyses of the RGS spectrum and examined potential spectral changes.
Methods.
We focused on the high-resolution
XMM-Newton
RGS spectra and divided the observation into pre-flare, flare, and post-flare phases. We modeled the time-averaged and time-resolved spectra with phenomenological components and with the self-consistent photoionization models calculated via
CLOUDY
and
XSTAR
in the pre-flare phase, where strong emission lines due to resonant transitions of highly ionized ions are seen.
Results.
In the spectra, we find emission lines corresponding to K-shell transitions in highly charged ions of oxygen, neon, magnesium, and silicon as well as radiative recombination continua (RRC) of oxygen. Additionally, we observe potential absorption lines of magnesium at a lower ionization stage and features identified as iron L lines. The
CLOUDY
and
XSTAR
photoionization models provide contradictory results, either pointing towards uncertainties in theory or possibly a more complex multi-phase plasma, or both.
Conclusions.
We are able to demonstrate the existence of a plethora of variable narrow features, including the firm detection of oxygen lines and RRC that RGS enables to observe in this source for the first time. We show that Vela X-1 is an ideal source for future high-resolution missions, such as
XRISM
and
Athena
.
We demonstrate a widely applicable technique to absolutely calibrate the energy scale of x-ray spectra with experimentally well-known and accurately calculable transitions of highly charged ions, ...allowing us to measure the K-shell Rydberg spectrum of molecular O_{2} with 8 meV uncertainty. We reveal a systematic ∼450 meV shift from previous literature values, and settle an extraordinary discrepancy between astrophysical and laboratory measurements of neutral atomic oxygen, the latter being calibrated against the aforementioned O_{2} literature values. Because of the widespread use of such, now deprecated, references, our method impacts on many branches of x-ray absorption spectroscopy. Moreover, it potentially reduces absolute uncertainties there to below the meV level.
A key requirement for the correct interpretation of high-resolution X-ray spectra is that transition energies are known with high accuracy and precision. We investigate the K-shell features of
,
, ...and
gases, by measuring their photo ion-yield spectra at the BESSY II synchrotron facility simultaneously with the 1s-
p fluorescence emission of He-like ions produced in the Polar-X EBIT. Accurate
calculations of transitions in these ions provide the basis of the calibration. While the
result agrees well with previous measurements, the
spectrum appears shifted by
0.5 eV, about twice the uncertainty of the earlier results. Our result for
shows a large departure from earlier results, but may suffer from larger systematic effects than our other measurements. The molecular spectra agree well with our results of time-dependent density functional theory. We find that the statistical uncertainty allows calibrations in the desired range of 1-10 meV, however, systematic contributions still limit the uncertainty to
40-100 meV, mainly due to the temporal stability of the monochromator energy scale. Combining our absolute calibration technique with a relative energy calibration technique such as photoelectron energy spectroscopy will be necessary to realize its full potential of achieving uncertainties as low as 1-10 meV.
A key requirement for the correct interpretation of high-resolution X-ray spectra is that transition energies are known with high accuracy and precision. We investigate the K-shell features of
Ne
,
...CO
2
, and
SF
6
gases, by measuring their photo ion-yield spectra at the BESSY II synchrotron facility simultaneously with the 1s–
n
p fluorescence emission of He-like ions produced in the Polar-X EBIT. Accurate
ab initio
calculations of transitions in these ions provide the basis of the calibration. While the
CO
2
result agrees well with previous measurements, the
SF
6
spectrum appears shifted by
∼
0.5 eV, about twice the uncertainty of the earlier results. Our result for
Ne
shows a large departure from earlier results, but may suffer from larger systematic effects than our other measurements. The molecular spectra agree well with our results of time-dependent density functional theory. We find that the statistical uncertainty allows calibrations in the desired range of 1–10 meV, however, systematic contributions still limit the uncertainty to
∼
40–100 meV, mainly due to the temporal stability of the monochromator energy scale. Combining our absolute calibration technique with a relative energy calibration technique such as photoelectron energy spectroscopy will be necessary to realize its full potential of achieving uncertainties as low as 1–10 meV.
Graphical abstract
We demonstrate a widely applicable technique to absolutely calibrate the energy scale of x-ray spectra with experimentally well-known and accurately calculable transitions of highly charged ions, ...allowing us to measure the K -shell Rydberg spectrum of molecular O2 with 8 meV uncertainty. We reveal a systematic ~450 meV shift from previous literature values, and settle an extraordinary discrepancy between astrophysical and laboratory measurements of neutral atomic oxygen, the latter being calibrated against the aforementioned O2 literature values. Because of the widespread use of such, now deprecated, references, our method impacts on many branches of x-ray absorption spectroscopy. Moreover, it potentially reduces absolute uncertainties there to below the meV level.
We demonstrate a widely applicable technique to absolutely calibrate the energy scale of x-ray spectra with experimentally well-known and accurately calculable transitions of highly charged ions, ...allowing us to measure the K-shell Rydberg spectrum of molecular O2 with 8 meV-uncertainty. We reveal a systematic ~450meV shift from previous literature values, and settle an extraordinary discrepancy between astrophysical and laboratory measurements of neutral atomic oxygen, the latter being calibrated against the aforementioned O2 literature values. Because of the widespread use of such, now deprecated, references, our method impacts on many branches of x-ray absorption spectroscopy. Moreover, it potentially reduces absolute uncertainties there to below the meV level.
Abstract
A key requirement for the correct interpretation of high-resolution X-ray spectra is that transition energies are known with high accuracy and precision. We investigate the K-shell features ...of
$$\mathrm {Ne}$$
Ne
,
$$\mathrm {CO}_2$$
CO
2
, and
$$\mathrm {SF}_6$$
SF
6
gases, by measuring their photo ion-yield spectra at the BESSY II synchrotron facility simultaneously with the 1s–
n
p fluorescence emission of He-like ions produced in the Polar-X EBIT. Accurate
ab initio
calculations of transitions in these ions provide the basis of the calibration. While the
$$\mathrm {CO}_2$$
CO
2
result agrees well with previous measurements, the
$$\mathrm {SF}_6$$
SF
6
spectrum appears shifted by
$$\sim $$
∼
0.5 eV, about twice the uncertainty of the earlier results. Our result for
$$\mathrm {Ne}$$
Ne
shows a large departure from earlier results, but may suffer from larger systematic effects than our other measurements. The molecular spectra agree well with our results of time-dependent density functional theory. We find that the statistical uncertainty allows calibrations in the desired range of 1–10 meV, however, systematic contributions still limit the uncertainty to
$${\sim }$$
∼
40–100 meV, mainly due to the temporal stability of the monochromator energy scale. Combining our absolute calibration technique with a relative energy calibration technique such as photoelectron energy spectroscopy will be necessary to realize its full potential of achieving uncertainties as low as 1–10 meV.
Graphical abstract
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