On the quenching of LRD X-ray emission by both Compton-thick gas and high accretion rates

Albert Sneppen, Darach Watson, James H. Matthews, Stuart A. Sim

First listed 2026-05-14 | Last updated 2026-05-14

Abstract

Little Red Dots (LRDs), candidate high-redshift supermassive black holes accreting in dense gas, remain undetected in X-rays. In previous work, we provided the first quantitative models that reproduce the optical and near-infrared spectra of LRDs with the Sirocco radiative transfer code, thereby constraining the properties of the surrounding gas. Here, we use those constraints to predict the X-ray attenuation produced by dense gas cocoons, and explore its dependence on Balmer-break strength, metallicity, intrinsic X-ray SED, and observed bandpass as a function of redshift. We find that the X-ray constraints are very tight, requiring both extinction by a Compton-thick gas column $N_{\rm H}\sim10^{25}\,{\rm cm}^{-2}$ with moderate metallicity ($0.05$-$0.1\,Z_\odot$) and intrinsically weak X-ray emission (bolometric to X-ray luminosity ratio, $k_{\rm bol,X}\gtrsim 30$) as observed in high accretion rate, narrow-line AGN, to make LRDs sufficiently faint to evade detection. Intrinsically bright X-ray emitters as seen in typical broad-line AGN would be detected even behind the typical modest metallicity, Compton-thick gas columns inferred from the optical spectra. Very low metallicity objects could be detected in X-rays even with low intrinsic X-ray luminosities, suggesting that LRDs are not (currently) chemically pristine.

Short digest

This paper takes the Sirocco cocoon models that already match Little Red Dot optical and near-IR spectra and asks whether those same dense envelopes can also suppress the missing X-ray signal. It finds that current non-detections are only consistent if LRDs combine Compton-thick gas columns of roughly N_H~10^25 cm^-2 and moderate metallicity of about 0.05-0.1 Z_sun with intrinsically X-ray-weak coronae, corresponding to k_bol,X ≳ 30 and more closely resembling high-accretion narrow-line AGN than typical broad-line quasars. The key point is that obscuration alone does not hide a normal hard, X-ray-bright AGN template, while very low-metallicity cocoons would also leak too much X-ray flux, so the observed LRD population is unlikely to be chemically pristine. ([arxiv.org](https://arxiv.org/abs/2605.15263))

Key figures to inspect

• Figure 1. This is the paper’s clearest bottom-line figure because it puts the modelled LRD X-ray spectra directly against the observational upper limits. It shows that after cocoon attenuation, a typical broad-line AGN template still remains too X-ray bright, whereas a softer, intrinsically weaker narrow-line Seyfert 1-like template can fall below the limits for LRD-like columns and metallicities. Use this figure to make the paper’s main claim concrete: both heavy obscuration and a weak intrinsic corona are required.
• Figure 2. This is the best physical-diagnostic figure in the paper. The left panel links stronger Balmer breaks to higher inferred columns and a more pronounced Compton-processed X-ray spectrum, while the right panel shows that metallicity is the key regulator of soft-X-ray photoelectric absorption even when the UV to NIR spectrum changes only modestly. It is the figure that carries the chemically non-pristine implication, because very low-metallicity cocoons transmit substantially more X-ray flux and are therefore much harder to reconcile with the non-detections.