PACHA: Probing AGN Coronae with High-redshift AGN

Abstract

The X-ray emission of active galactic nuclei (AGN) is generally attributed to inverse Compton scattering of accretion-disk photons by hot electrons in a compact corona. In local AGN, directly constraining coronal properties is challenging because the high-energy cutoff often lies beyond the NuSTAR bandpass. High-redshift, luminous quasars enable systematic constraints on the high-energy cutoff, as cosmological redshift shifts the spectal cutoff into the observable hard X-ray band. We present first results from the ``Probing the AGN Coronae with High-redshift AGN'' (PACHA) project, based on quasi-simultaneous NuSTAR and XMM-Newton observations of 13 radio-quiet AGN at $z>1$. We constrain the high-energy cutoff and coronal temperature at 90\% confidence level for 10 and 9 sources, respectively. The sample exhibits a mean cutoff energy of $E_{\rm cut}=80.8\pm8.1$ keV and a mean coronal temperature of $kT_{\rm e}=18.4\pm1.6$ keV, both significantly lower than those measured in local {\it Swift}-BAT AGN, while the mean optical depth ($τ=4.8\pm0.3$) is significantly higher. The uncertainties are at 1~$σ$. Combining our high-redshift sample with local AGN, we find a potential anti-correlation between cutoff energy and both X-ray luminosity and black hole mass, with no significant dependence on Eddington ratio. Within a hybrid coronal framework, the inferred temperatures lie well below the pair-production limits for purely thermal coronae, indicating a substantial efficient Compton cooling and/or non-thermal electron component. The detection of low coronal temperatures in high-luminosity AGN is broadly consistent with predictions from recent radiation MHD simulations that consider purely thermal electron populations, implying that non-thermal electrons may not be the primary drivers of the observed coronal properties in these systems.

Figures (13)

• Figure 1: AGN spectra computed assuming a Comptonization model with different coronal temperatures. For $z\sim0$ sources with high $kT_{\rm e}$, the high-energy cutoff is shifted above the NuSTAR bandpass (3--78 keV; yellow shaded region), limiting our ability to constrain the coronal temperature.
• Figure 2: Photon index as a function of the coronal temperature of the AGN coronae from our PACHA sample (blue) and the Akylas2021 sample (grey). The purple lines indicate different optical depth calculated using Eq. \ref{['eq:tau']}.
• Figure 3: Cutoff energy as a function of redshift for high-luminosity sources in our sample (blue) and local, low luminosity AGN (grey) adopted from Akylas2021. The lower limits are calculated at the 90% confidence level for both samples (open symbols).
• Figure 4: Cutoff energy as a function of 2--10 keV luminosity (upper), black hole mass (middle), and Eddington ratio (bottom) of sources in our PACHA sample (blue) and Swift-BAT selected AGN (grey) from Akylas2021. The means and their 1$\sigma$ uncertainties in different luminosity, Eddington ratio, and black hole mass bins are plotted in shaded region.
• Figure 5: Illustration of the coronal properties derived in this work. Sources with higher X-ray luminosities and larger black hole masses exhibit systematically lower coronal temperatures. The sizes of the various components are not to scale.