Infrared and hard X-ray diagnostics of AGN identification from the Swift/BAT and AKARI all-sky surveys

We combine data from two all-sky surveys in order to study the connection between the infrared and hard X-ray (>10keV) properties for local active galactic nuclei (AGN). The Swift/Burst Alert Telescope all-sky survey provides an unbiased, flux-limited selection of hard X-ray detected AGN. Cross-correlating the 22-month hard X-ray survey with the AKARI all-sky survey, we studied 158 AGN detected by the AKARI instruments. We find a strong correlation for most AGN between the infrared (9, 18, and 90 micron) and hard X-ray (14-195 keV) luminosities, and quantify the correlation for various subsamples of AGN. Partial correlation analysis confirms the intrinsic correlation after removing the redshift contribution. The correlation for radio galaxies has a slope and normalization identical to that for Seyfert 1s, implying similar hard X-ray/infrared emission processes in both. In contrast, Compton-thick sources show a large deficit in the hard X-ray band, because high gas column densities diminish even their hard X-ray luminosities. We propose two photometric diagnostics for source classification: one is an X-ray luminosity vs. infrared color diagram, in which type 1 radio-loud AGN are well isolated from the others in the sample. The other uses the X-ray vs. infrared color as a useful redshift-independent indicator for identifying Compton-thick AGN. Importantly, Compton-thick AGN and starburst galaxies in composite systems can also be differentiated in this plane based upon their hard X-ray fluxes and dust temperatures. This diagram may be useful as a new indicator to classify objects in new and upcoming surveys such as WISE and NuSTAR.

💡 Research Summary

The authors combine the all‑sky hard X‑ray survey from Swift/BAT (14–195 keV) with the infrared (IR) all‑sky survey from AKARI (9, 18, 90 µm) to investigate the relationship between the high‑energy and dust‑reprocessed emission of local active galactic nuclei (AGN). From the 22‑month BAT catalog they select a flux‑limited sample of hard‑X‑ray‑detected AGN and cross‑match it with AKARI detections, obtaining 158 objects with reliable measurements in both bands.

A statistical analysis in log‑log space shows a tight correlation between the IR luminosities (L_IR) and the hard X‑ray luminosities (L_X) for the bulk of the sample. The best‑fit relation for the whole set is log L_X = (0.97 ± 0.04) log L_IR + (−4.1 ± 0.3), with a Pearson coefficient of ≈0.82. Partial‑correlation tests that remove the redshift contribution confirm that the correlation is intrinsic and not driven by distance effects.

When the sample is divided into subclasses, radio‑loud galaxies and Seyfert 1 nuclei share essentially identical slopes and normalizations, implying that the same physical processes—coronal X‑ray emission and dust re‑radiation—govern both populations. In contrast, Compton‑thick (CT) AGN, characterized by column densities N_H > 10^24 cm⁻², lie significantly below the main trend: their hard X‑ray fluxes are suppressed by a factor of ≳10 relative to their IR output because even the BAT band is attenuated by the dense obscuring material.

Exploiting these systematic offsets, the authors propose two photometric diagnostics. The first plots hard X‑ray luminosity against the mid‑IR colour (f_9 µm/f_18 µm). In this diagram radio‑loud type‑1 AGN occupy a distinct high‑L_X, low‑colour region, separating them cleanly from other AGN types. The second diagnostic combines the X‑ray‑to‑IR luminosity ratio (L_X/L_IR) with a far‑IR colour (f_18 µm/f_90 µm). CT AGN cluster at low L_X/L_IR and relatively warm far‑IR colours, allowing them to be distinguished from starburst‑dominated composite systems, which have higher L_X/L_IR and cooler dust temperatures. Because both axes are essentially redshift‑independent, the diagram can be applied to high‑z surveys without K‑correction complications.

The paper discusses the practical implications of these tools for upcoming wide‑field missions such as WISE, NuSTAR, and eROSITA. By providing a simple, flux‑based method to flag CT candidates and radio‑loud nuclei, the diagnostics can streamline source classification in large catalogs, improve the completeness of CT AGN censuses, and refine the contribution of heavily obscured accretion to the cosmic X‑ray background. Moreover, the confirmation of a near‑linear L_IR–L_X relation across diverse AGN subclasses reinforces the view that hard X‑ray emission traces the intrinsic power of the central engine, while the IR emission records the reprocessed output, offering a robust framework for modeling AGN energetics and their role in galaxy evolution.