eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the primary instrument on the Spectrum-Roentgen-Gamma (SRG) mission, which was successfully launched on July 13, 2019, from the Baikonour cosmodrome. After the commissioning of the instrument and a subsequent calibration and performance verification phase, eROSITA started a survey of the entire sky on December 13, 2019. By the end of 2023, eight complete scans of the celestial sphere will have been performed, each lasting six months. At the end of this program, the eROSITA all-sky survey in the soft X-ray band (0.2-2.3 keV) will be about 25 times more sensitive than the ROSAT All-Sky Survey, while in the hard band (2.3-8 keV) it will provide the first ever true imaging survey of the sky. The eROSITA design driving science is the detection of large samples of galaxy clusters up to redshifts z > 1 in order to study the large-scale structure of the universe and test cosmological models including Dark Energy. In addition, eROSITA is expected to yield a sample of a few million AGNs, including obscured objects, revolutionizing our view of the evolution of supermassive black holes. The survey will also provide new insights into a wide range of astrophysical phenomena, including X-ray binaries, active stars, and diffuse emission within the Galaxy. Results from early observations, some of which are presented here, confirm that the performance of the instrument is able to fulfil its scientific promise. With this paper, we aim to give a concise description of the instrument, its performance as measured on ground, its operation in space, and also the first results from in-orbit measurements.

Context. Do extrasolar planets affect the activity of their host stars? Indications for chromospheric activity enhancement have been found for a handful of targets, but in the X-ray regime, conclusive observational evidence is still missing. Aims. We want to establish a sound observational basis to confirm or reject major effects of Star-Planet Interactions (SPI) in stellar X-ray emissions. Methods. We therefore conduct a statistical analysis of stellar X-ray activity of all known planet-bearing stars within 30 pc distance for dependencies on planetary parameters such as mass and semimajor axis.Results. In our sample, there are no significant correlations of X-ray luminosity or the activity indicator L X /L bol with planetary parameters which cannot be explained by selection effects. Conclusions. Coronal SPI seems to be a phenomenon which might only manifest itself as a strong effect for a few individual targets, but not to have a major effect on planet-bearing stars in general.

Abstract. We present an XMM-Newton observation of the classical T Tauri star BP Tau. In the XMM-Newton RGS spectrum the O  triplet is clearly detected with a very weak forbidden line indicating high plasma densities and/or a high UV flux environment. At the same time concurrent UV data point to a small hot spot filling factor suggesting an accretion funnel shock as the site of the X-ray and UV emission. Together with the X-ray data on TW Hya these new observations suggest such funnels to be a general feature in classical T Tauri stars.

We present Chandra HETGS observations of the classical T Tauri star (CTTS) V4046 Sgr. The He-like triplets of O vii, Ne ix, and Si xiii are clearly detected. Similar to the CTTS TW Hya and BP Tau, the forbidden lines of O vii and Ne ix are weak compared to the intercombination line, indicating high plasma densities in the X-ray emitting regions. The Si xiii triplet, however, is within the low-density limit, in agreement with the predictions of the accretion funnel infall model with an additional stellar corona. V4046 Sgr is the first close binary exhibiting these features. Together with previous high-resolution X-ray data on TW Hya and BP Tau, and in contrast to T Tau, now three out of four CTTS show evidence of accretion funnels.

Context. Classical T Tauri stars (CTTS) are surrounded by actively accreting disks. According to current models material falls along the magnetic field lines from the disk with more or less free-fall velocity onto the star, where the plasma heats up and generates X-rays. Aims. We want to quantitatively explain the observed high energy emission and measure the infall parameters from the data. Absolute flux measurements allow to calculate the filling factor and the mass accretion rate. Methods. We use a numerical model of the hot accretion spot and solve the conservation equations. Results. A comparison to data from XMM-Newton and Chandra shows that our model reproduces the main features very well. It yields for TW Hya a filling factor of 0.3 % and a mass accretion rate 2 × 10 −10 M ⊙ yr −1 .

Abstract. We present a comparative study of X-ray emission from a sample of active M dwarfs with spectral types M3.5-M4.5 using XMM-Newton observations of two single stars, AD Leonis and EV Lacertae, and two unresolved binary systems, AT Microscopii and EQ Pegasi. The light curves reveal frequent flaring during all four observations. We perform a uniform spectral analysis and determine plasma temperatures, abundances and emission measures in different states of activity. Applying multi-temperature models with variable abundances separately to data obtained with the EPIC and RGS detectors we are able to investigate the consistency of the results obtained by the different instruments onboard XMM-Newton. We find that the X-ray properties of the sample M dwarfs are very similar, with the coronal abundances of all sample stars following a trend of increasing abundance with increasing first ionization potential, the inverse FIP effect. The overall metallicities are below solar photospheric ones but appear consistent with the measured photospheric abundances of M dwarfs like these. A significant increase in the prominence of the hotter plasma components is observed during flares while the cool plasma component is only marginally affected by flaring, pointing to different coronal structures. AT Mic, probably a young pre-main-sequence system, has the highest X-ray luminosity and exhibits also the hottest corona. While results of EQ Peg and EV Lac are presented here for the first time, AT Mic and AD Leo have been investigated before with different analysis approaches, allowing a comparison of the results.

The orbital observatory Spectrum-Roentgen-Gamma (SRG), equipped with the grazing-incidence X-ray telescopes Mikhail Pavlinsky ART-XC and eROSITA, was launched by Roscosmos to the Lagrange L2 point of the Sun-Earth system on July 13, 2019. The launch was carried out from the Baikonur Cosmodrome by a Proton-M rocket with a DM-03 upper stage. The German telescope eROSITA was installed on SRG under an agreement between Roskosmos and the DLR, the German Aerospace Agency. In December 2019, SRG started to perform its main scientific task: scanning the celestial sphere to obtain X-ray maps of the entire sky in several energy ranges (from 0.3 to 8 keV with eROSITA, and from 4 to 30 keV with ART-XC). By mid-June 2021, the third six-month all-sky survey had been completed. Over a period of four years, it is planned to obtain eight independent maps of the entire sky in each of the energy ranges. The sum of these maps will provide high sensitivity and reveal more than three million quasars and over one hundred thousand massive galaxy clusters and galaxy groups. The availability of eight sky maps will enable monitoring of long-term variability (every six months) of a huge number of extragalactic and Galactic X-ray sources, including hundreds of thousands of stars with hot coronae. In addition, the rotation of the satellite around the axis directed toward the Sun with a period of four hours enables tracking the faster variability of bright X-ray sources during one day every half year. The chosen strategy of scanning the sky leads to the formation of deep survey zones near both ecliptic poles. The paper presents sky maps obtained by the telescopes on board SRG during the first survey of the entire sky and a number of results of deep observations performed during the flight to the L2 point in the frame of the performance verification program, demonstrating the capabilities of the observatory in imaging, spectroscopy, and timing of X-ray sources. It is planned that in December 2023, the observatory will for at least two years switch to observations of the most interesting sources in the sky in triaxial orientation mode and deep scanning of selected celestial fields with an area of up to 150 square degrees. These modes of operation were tested during the performance verification phase. Every day, data from the SRG observatory are dumped onto the largest antennas of the Russian Deep Space Network in Bear Lakes and near Ussuriysk.

eROSITA (extended ROentgen Survey with an Imaging Telescope Array) is the core instrument on the Russian Spektrum-Roentgen-Gamma (SRG) mission which is scheduled for launch in late 2012. eROSITA is fully approved and funded by the German Space Agency DLR and the Max-Planck-Society.The design driving science is the detection of 50 -100 thousands Clusters of Galaxies up to redshift z ∼1.3 in order to study the large scale structure in the Universe and test cosmological models, especially Dark Energy. This will be accomplished by an all-sky survey lasting for four years plus a phase of pointed observations. eROSITA consists of seven Wolter-I telescope modules, each equipped with 54 Wolter-I shells having an outer diameter of 360 mm. This would provide and effective area at 1.5 keV of ∼ 1500 cm 2 and an on axis PSF HEW of 15 ′′ which would provide an effective angular resolution of 25 ′′ -30 ′′ . In the focus of each mirror module, a fast frame-store pn-CCD will provide a field of view of 1 • in diameter for an active FOV of ∼0.83 deg 2 . At the time of writing the instrument development is currently in phase C/D.