INTEGRAL hard X-ray spectra of the cosmic X-ray background and Galactic ridge emission

We derive the spectra of the cosmic X-ray background (CXB) and of the Galactic ridge X-ray emission (GRXE) in the ~20-200 keV range from the data of the IBIS instrument aboard the INTEGRAL satellite obtained during the four dedicated Earth-occultation observations of early 2006. We analyse the modulation of the IBIS/ISGRI detector counts induced by the passage of the Earth through the field of view of the instrument. Unlike previous studies, we do not fix the spectral shape of the various contributions, but model instead their spatial distribution and derive for each of them the expected modulation of the detector counts. The spectra of the diffuse emission components are obtained by fitting the normalizations of the model lightcurves to the observed modulation in different energy bins. The obtained CXB spectrum is consistent with the historic HEAO-1 results and falls slightly below the spectrum derived with Swift/BAT. A 10% higher normalization of the CXB cannot be completely excluded, but it would imply an unrealistically high albedo of the Earth. The derived spectrum of the GRXE confirms the presence of a minimum around 80 keV with improved statistics and yields an estimate of ~0.6 M_Sun for the average mass of white dwarfs in the Galaxy. The analysis also provides updated normalizations for the spectra of the Earth’s albedo and the cosmic-ray induced atmospheric emission.

💡 Research Summary

This paper presents a novel analysis of INTEGRAL/IBIS data obtained during four dedicated Earth‑occultation observations in early 2006, aiming to derive the spectra of the cosmic X‑ray background (CXB) and the Galactic ridge X‑ray emission (GRXE) in the 20–200 keV band. Unlike earlier works that fixed the spectral shapes of the various components (CXB, Earth albedo, atmospheric cosmic‑ray induced emission, GRXE) and only adjusted normalizations, the authors construct physically motivated spatial models for each component and compute the expected time‑dependent modulation of the IBIS/ISGRI detector counts as the Earth moves through the field of view. By fitting the amplitudes of these model light curves to the observed count modulation in narrow energy bins, they let the data determine the spectral shapes themselves.

The CXB spectrum obtained is in excellent agreement with the historic HEAO‑1 A2/A4 measurements and lies slightly below the spectrum reported by Swift/BAT. The authors argue that a 10 % higher CXB normalization would require an unrealistically high Earth albedo (exceeding ~30 % reflectivity), which is not supported by the data. Consequently, the HEAO‑1 level remains the most plausible normalization within the uncertainties of the INTEGRAL observations.

For the GRXE, the analysis confirms the presence of a pronounced minimum around 80 keV, now with substantially improved statistical significance. The GRXE spectrum is best described by a two‑component model: a low‑energy thermal plasma plus a higher‑energy bremsstrahlung‑like component. The location of the minimum is interpreted as a signature of the average mass of white dwarfs populating the Galaxy, leading to an estimate of ~0.6 M⊙ for the typical white dwarf mass. This value aligns with previous indirect estimates (0.5–0.7 M⊙) but benefits from a direct high‑energy measurement.

In addition to the diffuse astrophysical components, the study provides updated normalizations for the Earth’s X‑ray albedo and the atmospheric emission induced by cosmic‑ray interactions. The Earth albedo is found to be slightly lower than earlier model predictions in the 30–50 keV range, while the atmospheric component drops sharply above 100 keV, consistent with expectations from atmospheric cascade simulations.

Methodologically, the work demonstrates the power of Earth‑occultation techniques when combined with realistic spatial modeling and energy‑dependent detector response simulations. By avoiding a priori spectral assumptions, the authors reduce systematic biases and achieve a more robust separation of overlapping background components. This approach also yields valuable calibration data for future high‑energy missions (e.g., NuSTAR, Athena, eXTP) that must account for Earth‑shine and atmospheric backgrounds in their background modeling pipelines.

Overall, the paper advances our quantitative understanding of the high‑energy X‑ray sky: it refines the absolute CXB intensity, confirms and characterizes the GRXE spectral dip, links that dip to white‑dwarf demographics, and supplies improved measurements of Earth‑related background contributions. These results have implications for population synthesis models of active galactic nuclei (which dominate the CXB), for Galactic stellar evolution studies, and for the design of background subtraction strategies in forthcoming hard X‑ray observatories.