BLAZ4R and the eROSITA view of z>4 blazars

Abstract

We present BLAZ4R, the first living catalog of confirmed $z>4$ blazars, with a focus on the contribution of eROSITA, on board of the Spectrum Roentgen Gamma (SRG) spacecraft. Blazars at $z>4$ are rare but powerful probes of how active supermassive black holes evolve in connection to relativistic jets, in the first 2 billion years of cosmic history. At these redshifts, X-ray observations are essential for constraining blazars jet power and orientation, enabling effective trace of their parent population. The all-sky surveys provided by eROSITA ensure X-ray detection for BLAZ4R sources and, in some cases, allow spectral and temporal studies of their jetted emission. BLAZ4R includes 54 confirmed blazars, characterized through their X-ray properties, radio spectra and morphology, and multiwavelength profiles. We confirm that jetted sources are significantly more numerous relative to non-jetted counterparts at high-$z$, and that blazars (and therefore the overall jetted population) do not exhibit significantly different features compared to the total active galactic nuclei population in the early Universe. Fast accretion processes that involve relativistic jets are clearly required to justify the existence of fully formed jetted AGN at $z>4$. However, the diverse multiwavelength properties characterizing BLAZ4R do not yet allow us to identify the specific signatures of these processes. We will continue updating BLAZ4R to search for such signatures and ultimately understand the early formation of jetted AGN.

Figures (11)

• Figure 1: Detection likelihood of blazar-associated eRASS sources from the eRASS:5 catalogue as a function of the angular separation between the X-ray source and the blazar optical position, normalized by the X-ray positional uncertainty. The background density shows the distribution of all eRASS:5–LS10 matches, while highlighted points indicate the blazar associations. Full circles denote blazars matched directly to the LS10 counterpart of an eRASS source. Empty points show blazars matched to an eRASS source within 15. Blazar counterparts preferentially occur at higher detection likelihoods compared to typical LS10 matches, while spanning a similar range in normalized separation.
• Figure 2: Sky map of the total sample, including the eROSITA footprint limit (yellow line). Filled red and blue points show the sources originally classified as confirmed and candidate blazars, respectively. Green circles highlights sources detected in eRASS:5 scans. In the eROSITA-DE footprint, only 4 previously classified blazars are undetected.
• Figure 3: eROSITA photon index (top panel) and X-ray flux in the 0.2-2.3 keV range (bottom panel) of the 4 sources with more than 80 counts, as a function of the eRASS scan (i.e. roughly function of time), as labelled. The horizontal dashed lines show the mean value of each source (photon index or flux, in the respective panels) as color-coded, calculated excluding upper limits.
• Figure 4: Radio spectral index distribution of the extended sample, including both confirmed blazars and candidates, color coded for their radio loudness $R$. The data distributed along the identity diagonal show results for sources best fitted with a single power-law, while those best described by double power-laws are located in the first or last quadrants depending on whether they have peaked or convex radio spectra, as labeled.
• Figure 5: Comoving number densities of active black holes hosted in jetted AGN traced by blazars at $z>4$, as a function of redshift. $\Phi(z)$ is derived for the jetted population by assuming a bulk Lorentz factor of $\Gamma=13$ or 8 (circles or squares respectively). The upper figure shows the results for the whole sample, while the lower is limited to $L_{\rm bol}>10^{47}$erg/s. In both figures, differently colored data points and shaded intervals refers to the total $z>4$ sample (orange in the top figure, purple in the bottom one), and the eRASS:5 detected only (green and light blue). The curves show the space densities for the overall massive AGN population as extracted from shen20, integrated starting from a bolometric luminosity of $5\times10^{46}, 10^{47}$ and $3\times10^{47}$ erg/s (dashed blue, solid grey and solid black lines).