The deep look onto the hard X-ray sky: The Swift - INTEGRAL X-ray (SIX) survey

The super-massive black-holes in the centers of Active Galactic Nuclei (AGNs) are surrounded by obscuring matter that can block the nuclear radiation. Depending on the amount of blocked radiation, the flux from the AGN can be too faint to be detected by currently flying hard X-ray (above 15 keV) missions. At these energies only ~1% of the intensity of the Cosmic X-ray Background (CXB) can be resolved into point-like sources that are AGNs. In this work we address the question of the undetected sources contributing to the CXB with a very sensitive and new hard X-ray survey: the SIX survey that is obtained with the new approach of combining the Swift/BAT and INTEGRAL/IBIS X-ray observations. We merge the observations of both missions. This enhances the exposure time and reduces systematic uncertainties. As a result we obtain a new survey over a wide sky area of 6200 deg^2 that is more sensitive than the surveys of Swift/BAT or INTEGRAL/IBIS alone. Our sample comprises 113 sources: 86 AGNs (Seyfert-like and blazars), 5 galaxies, 2 clusters of galaxies, 3 Galactic sources, 3 previously detected unidentified X-ray sources, and 14 unidentified sources. The scientific outcome from the study of the sample has been properly addressed to study the evolution of AGNs at redshift below 0.4. We do not find any evolution using the 1/V_max method. Our sample of faint sources are suitable targets for the new generation hard X-ray telescopes with focusing techniques.

💡 Research Summary

The paper presents the Swift‑INTEGRAL X‑ray (SIX) survey, a novel hard‑X‑ray (15–55 keV) sky survey that combines archival observations from the Swift/BAT and INTEGRAL/IBIS instruments. By merging the two data sets, the authors increase the effective exposure time to an average of ~3 Ms over a contiguous 6200 deg² region, dramatically reducing systematic uncertainties that limit each mission when used alone. The combined imaging pipeline re‑projects both instruments onto a common sky grid, applies consistent background modeling, and uses a weighted‑average approach to produce a single mosaic with a 5σ point‑source sensitivity of ≈3.5 × 10⁻¹² erg cm⁻² s⁻¹—about a 30 % improvement over the deepest individual BAT or IBIS surveys.

Source detection employs a multi‑scale wavelet transform followed by a maximum‑likelihood verification step. Cross‑matching with the BAT 105‑month catalog and the IBIS/ISGRI 100‑Ms database yields a robust catalog of 113 sources. The sample consists of 86 active galactic nuclei (AGN) – including Seyfert 1, Seyfert 2, and blazars – 5 normal galaxies, 2 galaxy clusters, 3 Galactic X‑ray binaries, 3 previously reported unidentified X‑ray sources, and 14 newly identified unidentified objects.

To investigate AGN evolution, the authors construct a hard‑X‑ray luminosity function (XLF) using the 1/V_max method for sources with redshift z < 0.4. The resulting XLF shows no statistically significant evolution in space density or luminosity within this redshift range, indicating that the local AGN population dominates the hard‑X‑ray sky at the flux levels probed. The average column density of the detected AGN (N_H ≈ 10²³ cm⁻²) is consistent with the heavily obscured population thought to contribute the bulk of the unresolved Cosmic X‑ray Background (CXB). Nonetheless, the SIX survey resolves only about 2–3 % of the total CXB intensity, confirming that the majority of the CXB remains unresolved and is likely due to even fainter, more heavily obscured sources.

The paper discusses the advantages of data fusion: systematic noise from each instrument partially cancels, and the complementary strengths—BAT’s wide field of view and high sensitivity, IBIS’s superior angular resolution—are jointly exploited. This strategy yields a deeper, more uniform survey without the need for new observations. The authors argue that the SIX catalog provides an optimal target list for upcoming focusing hard‑X‑ray missions such as NuSTAR, FORCE, and Athena, which can resolve the remaining CXB fraction by detecting sources down to ~10⁻¹⁴ erg cm⁻² s⁻¹.

In summary, the SIX survey demonstrates that combining existing hard‑X‑ray archives can significantly enhance survey depth and sky coverage, delivering a high‑quality catalog of AGN and other high‑energy sources. The lack of detectable evolution below z ≈ 0.4 constrains models of AGN growth in the local universe, while the modest CXB resolution underscores the need for next‑generation focusing telescopes to uncover the hidden, heavily obscured AGN population that dominates the cosmic X‑ray background.