New Insights from eROSITA's All-Sky Survey

The eROSITA satellite, part of the Spectrum Roentgen Gamma mission, conducted its first all-sky survey from December 2019 to June 2020. This satellite collects Hard X-ray emissions from different celestial sources, primarily from Active Galactic Nuclei (AGN) – extremely bright regions at the center of some galaxies powered by supermassive black holes. This paper discusses the findings from the survey, focusing on the hard X-ray sources detected and their characteristics.

#eROSITA and AGN

eROSITA is designed to map the X-ray emissions from large structures in the universe, particularly galaxy clusters. However, it detects a significant number of AGN, which radiate energy across various wavelengths. These sources can outshine the galaxies they reside in. By observing AGN in X-rays, researchers can investigate the environment near supermassive black holes. The primary source of these X-rays is thought to be a region called the corona, filled with fast-moving electrons that scatter light from the accretion disk around the black hole.

AGNs are divided into two main types:

1. Seyfert 1: These AGN are not obscured and show broad spectral lines along with strong soft X-ray emissions.
2. Seyfert 2: These AGN are obscured by clouds of gas and dust, making soft X-ray emissions weaker or non-existent.

#Survey Findings

During the eROSITA survey, over 5,000 hard X-ray sources were detected. The data allowed for the classification of these sources based on their properties and the identification of counterparts in other wavelengths. Out of the total detected sources, many were classified as AGN, and a significant number were found to be heavily obscured, lacking soft X-ray emissions.

Statistical methods were employed to identify possible counterparts for each X-ray source. A code based on Bayesian statistics helped determine with high confidence which optical sources matched with the X-ray detections.

The research utilized additional data from the Swift BAT and HEAO-1 AGN sample for comparison, enhancing the understanding of eROSITA's capabilities in detecting hard X-ray sources.

#Characteristics of the Sample

From the survey, 5,466 hard X-ray sources were recorded. The sample consisted mainly of point-like sources, with a small portion being extended sources like galaxy clusters. Researchers paid particular attention to the hard-only sources, which were characterized by their lack of soft X-ray emissions.

A key element of the analysis involved determining the likelihood of these sources being real detections versus being spurious. After applying various flags and checks, 2,547 sources were identified as having strong counterparts, and 111 of these were categorized as hard-only sources.

#The Importance of Counterparts

The identification of counterparts is crucial for understanding the nature of celestial sources. In this study, counterparts were identified using data from the Legacy Surveys, which provided optical and infrared observations. The NWAY method was employed to match X-ray sources with those identified in the optical surveys, leading to the understanding of their properties.

The majority of X-ray sources had optical counterparts within the surveyed area, indicating that the data gathered from eROSITA is valuable for studying AGN. The NWAY analysis allowed for robust identification of sources with varying levels of certainty.

#Testing the Sample

To assess the quality of counterparts, researchers compared the findings with known catalogs. They matched the eROSITA data with results from the Swift BAT catalog, examining overlapping sources. This comparison showed a significant number of matched sources, reinforcing the reliability of the eROSITA survey.

The analysis included various classifications of AGN based on their optical characteristics and spectral properties. The results indicated a clear distinction between obscured and unobscured AGN types based on their X-ray and optical emissions.

#Spectroscopic Redshifts

The findings also included the collection of spectroscopic redshifts for many sources. By matching the X-ray counterparts with existing redshift catalogs, researchers populated a catalog with over a thousand sources where redshifts were determined. The redshift distribution showed that many of the sources are relatively nearby, with a peak around a certain value.

In total, a significant portion of the sample with spectroscopic redshifts was classified as extragalactic, further emphasizing the presence of AGN in the surveyed area.

#Source Classification

Following the identification of counterparts, researchers employed a classification methodology to categorize the sources. The classifications were based on various criteria, including the availability of redshift data and the nature of the optical emissions.

Sources were classified into extragalactic and Galactic categories. The optical nature of the sources was analyzed to determine their types more accurately. Many sources with no available redshift were classified based on their optical features. The classification results provided a comprehensive understanding of the sample's composition.

#Hard X-ray Only Sources

A significant focus was on hard-only sources, which are characterized by their absence of soft X-ray emissions. These sources were identified through careful statistical analysis, and close examination revealed that many of these sources are likely to be heavily obscured AGN.

These hard-only sources were analyzed further through their optical properties, and many showed weak optical emissions, suggesting they are found in edge-on galaxies or are heavily obscured by gas and dust. The study indicated that a substantial fraction of the identified hard-only sources are in fact AGN, highlighting their importance in understanding the population of obscured AGN in the universe.

#Variable Sources

The analysis also touched upon variable sources, with a portion of X-ray detected sources showing evidence of variability over short time scales. Variability can be an important characteristic of certain AGN, as it often indicates changes in the accretion processes occurring near the black holes.

The study found multiple sources exhibiting significant variability, which opens up further avenues for research into the dynamics of AGN and their behavior over time.

#Conclusion

The findings from the eROSITA all-sky survey offer new insights into the nature of AGN, particularly heavily obscured sources. The robust identification of counterparts and the classification of sources enhance the overall understanding of X-ray emissions in the universe.

This work highlights the capabilities of the eROSITA mission to probe deeper into hard X-ray selected samples, revealing a diverse population of AGN. Future studies and follow-up observations will help refine these findings and contribute to an improved understanding of the relationships between AGN, their host galaxies, and the universe's large-scale structure.

As more data from future surveys becomes available, the wealth of information gathered will continue to shed light on the complexities surrounding AGN and their roles within the cosmos.