Following the recent discovery of X-ray quasi-periodic eruptions (QPEs) coming from the nucleus of the galaxy GSN 069, here we report on the detection of QPEs in the active galaxy named RX J1301.9+2747. QPEs are rapid and recurrent increases of the X-ray count-rate by more than one order of magnitude with respect to a stable quiescent level. During a XMM-Newton observation lasting 48 ks that was performed on 30 and 31 May 2019, three strong QPEs lasting about half an hour each were detected in the light curves of RX J1301.9+2747. The first two QPEs are separated by a longer recurrence time (about 20 ks) compared to the second and third (about 13 ks). This pattern is consistent with the alternating long-short recurrence times of the GSN 069 QPEs, although the difference between the consecutive recurrence times is significantly smaller in GSN 069. Longer X-ray observations will better clarify the temporal pattern of the QPEs in RX J1301.9+2747 and will allow a detailed comparison with GSN 069 to be performed. The X-ray spectral properties of QPEs in the two sources are remarkably similar, with QPEs representing fast transitions from a relatively cold and likely disk-dominated state to a state that is characterized by a warmer emission similar to the so-called soft X-ray excess, a component that is almost ubiquitously seen in the X-ray spectra of unobscured, radiatively efficient active galaxies. Previous X-ray observations of RX J1301.9+2747 in 2000 and 2009 strongly suggest that QPEs have been present for at least the past 18.5 yr. The detection of QPEs from a second galactic nucleus after GSN 069 rules out contamination by a Galactic source in both cases, such that QPEs ought to be considered a novel extragalactic phenomenon associated with accreting supermassive black holes.

/pdf/x-ray-quasi-periodic-eruptions-from-the-galactic-nucleus-of-23b1a070ff.pdf

X-ray quasi-periodic eruptions from the galactic nucleus of RX J1301.9+2747

We present the result of a projected cross-correlation analysis of AGNs and galaxies at redshifts from 0.3 to 3.0. The Japanese Virtual Observatory (JVO) was used to obtain Subaru Suprime-Cam images and UKIDSS catalog data around AGNs. We investigated 1809 AGNs, which is about a ten-times larger sample than that used in previous studies on AGN–galaxy clustering at redshifts larger than 0.6; 90% of the AGN samples were optically-selected AGN from the SDSS and 2dF catalogs. The galaxy samples at low redshift include many redder objects from the UKIDSS survey, while at higher redshift they are mainly blue galaxies from Suprime-Cam. We found a significant excess of galaxies around the AGNs at redshifts from 0.3 to 1.8. For the low-redshift samples (z < 0.9), we obtained a correlation length of r0 = 5–6 h � 1 Mpc (� = 1.8), which indicates that the AGNs at this redshift range reside in a similar environment around typical local galaxies. We also found that AGNs at higher redshift ranges reside in a denser environment than lower redshift AGNs. For z = 1.3–1.8 AGNs, the cross-correlation length was measured as 11 +6 � 3 h � 1 Mpc (� = 1.8). Considering that our sample of galaxies is based on optical observations with SuprimeCam at the redshift range, it is expected that blue star-forming galaxies comprise the majority of objects that are observed to be clustered around the AGNs. It has been successfully demonstrated that using the archive through the Virtual Observatory system can provide a powerful tool for investigating the small-scale environment of intermediate redshift AGNs.

/pdf/early-science-result-from-the-japanese-virtual-observatory-1y1wo494pa.pdf

Early Science Result from the Japanese Virtual Observatory: AGN and Galaxy Clustering at z = 0.3 to 3.0

We have analyzed all available archival XMM-Newton observations of X Comae, a bright X-ray quasar behind the Coma cluster, to study the properties of the warm-hot intergalactic medium in the vicinity of the nearest massive galaxy cluster. The RGS observations confirm the possible presence of a Ne IX K-alpha absorption line at the redshift of Coma, although with a limited statistical significance. This analysis is therefore in line with the earlier analysis by Takei et al. (2007) based on a sub-set of these data. Its large column density and optical depth, however, point to implausible conditions for the absorbing medium, thereby casting serious doubts to its reality. Chandra has never observed X Comae and therefore cannot provide additional information on this source. We combine upper limits to the presence of other X-ray absorption lines (notably from O VII and O VIII) at the redshift of Coma with positive measurements of the soft excess emission from Coma measured by ROSAT (Bonamente et al. 2003). The combination of emission from warm-hot gas at kT~1/4 keV and upper limits from absorption lines provide useful constraints on the density and the sightline length of the putative WHIM towards Coma. We conclude that the putative warm-hot medium towards Coma is consistent with expected properties, with a baryon overdensity >=10 and a sightline extent of order of tens of Mpc.

Characterization of the warm-hot intergalactic medium near the Coma cluster through high-resolution spectroscopy of X Comae

Background has played an important role in X-ray missions, limiting the exploitation of science data in several and sometimes unexpected ways. In this presentation I review past X-ray missions focusing on some important lessons we can learn from them. I then go on discussing prospects for overcoming background related limitations in future ones.

The role of the background in past and future X-ray missions

The various classes of extragalactic X-ray source and their possible contributions to the X-ray Background are reviewed. Particular attention is paid to recent limits to the source density from fluctuation analysis and to the problem of spectral mismatch. A new class of X-ray source is probably needed to explain the Background. Bremsstrahlung from a hot Intergalactic Medium is a strong possibility if enough energy can be found.

The Evolution of Extragalactic X-Ray Sources

A ∼7 hour observation of the central part of the Coma Cluster of galaxies has been performed with the EXOSAT LE telescopes and CMA detectors. Five serendipitous sources are detected within the inner 35 arcmin radius of the field. Optical spectroscopy demonstrates that at least three of these are background AGN not associated with the cluster. At the sensitivity level of the EXOSAT exposure, we would have expected to see only 0.01 background sources based on the Einstein Medium Sensitivity Survey. The EXOSAT and Einstein results may be reconciled if these AGN have a much softer average X-ray spectrum than previously assumed.

/pdf/optical-observations-of-serendipitous-exosat-sources-in-the-1je7gsw9v9.pdf

Optical Observations of Serendipitous Exosat Sources in the Coma Cluster

EXOSAT observations of two gamma-ray burst sources

On rapporte la decouverte de quatre noyaux galactiques actifs dans un champ EXOSAT. Cela est difficile a comprendre a moins que leur spectre RX ne soit plus aigu que le spectre «canonique» (en supposant que leurs deplacements vers le rouge soient cosmologiques). On decrit les observations RX puis le travail d'identification optique et on discute de l'interpretation des resultats

Serendipitous EXOSAT sources in the region of the Coma cluster: Active Galactic Nuclei with steep X-ray spectra

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Optical Observations of Serendipitous Exosat Sources in the Coma Cluster

EXOSAT observations of two gamma-ray burst sources

Serendipitous EXOSAT sources in the region of the Coma cluster: Active Galactic Nuclei with steep X-ray spectra