High-resolution X-ray spectroscopy of the magnetic Of?p star HD148937

High-resolution data of the peculiar magnetic massive star HD148937 were obtained with Chandra-HETGS, and are presented here in combination with a re-analysis of the older XMM-RGS data. The lines of the high-Z elements (Mg, Si, S) were found to be unshifted and relatively narrow (FWHM of about 800km/s), i.e. narrower than the O line recorded by RGS, which possibly indicates that the hot plasma is multi-thermal and has several origins. These data further indicate a main plasma temperature of about 0.6keV and a formation of the X-ray emission at about one stellar radius above the photosphere. From the spectral fits and the H-to-He line ratios, the presence of very hot plasma is however confirmed, though with a smaller relative strength than for the prototype magnetic oblique rotator $\theta^1$,Ori,C. Both stars thus share many similarities, but HD148937 appears less extreme than $\theta^1$,Ori,C despite having also a large magnetic confinement parameter.

💡 Research Summary

This paper presents a comprehensive high‑resolution X‑ray spectroscopic study of the magnetic Of?p star HD 148937, combining new Chandra HETGS observations with a re‑analysis of archival XMM‑Newton RGS data. The authors obtained a 99 ks Chandra exposure on 5 June 2010 using the HETG+ACIS‑S configuration (ObsID 10982). Standard CIAO pipelines (v4.2, CALDB 4.3.1) were employed to extract first‑order MEG and HEG spectra, as well as the 0th‑order image. The spectra were rebinned (HEG by a factor of 4, MEG by 2) to improve signal‑to‑noise while preserving the intrinsic resolution. For global fitting, the three data sets (HEG, MEG, 0th‑order) were grouped to a minimum of 20 counts per bin and fitted simultaneously with χ² statistics.

The archival XMM‑Newton observation (25 Feb 2001, three RGS exposures, total ≈ 38 ks) was reprocessed with SAS 10.0.0, combining the RGS1 and RGS2 first‑order spectra after a careful background assessment. The RGS data were grouped by three channels to enhance the signal‑to‑noise ratio, and only the first‑order spectra were used because the second‑order data are too noisy.

Line properties
The Chandra spectra reveal strong lines from high‑Z ions: Si XIV (6.18 Å), Si XIII (6.74 Å), Mg XI (9.17 Å), Mg XII (8.42 Å), S XV (5.04 Å) and S XVI (4.73 Å). The authors fitted each line with simple Gaussian profiles (triangular fits gave indistinguishable results). The measured centroid shifts are on average +49 ± 32 km s⁻¹, i.e. essentially zero, and the full‑width at half‑maximum (FWHM) averages 827 ± 90 km s⁻¹. These widths are significantly narrower than the ≈ 2000 km s⁻¹ widths previously reported for the O VIII line in the RGS data, indicating that the hotter, high‑Z plasma is confined close to the stellar surface, while the cooler O‑VIII emitting plasma may arise from a more extended region.

Temperature diagnostics
Using the ratios of H‑like to He‑like line fluxes (H/He) and the G‑ratio