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- ItemThe eROSITA X-ray telescope on SRG(Les Ulis : EDP Sciences, 2021) Predehl, P.; Andritschke, R.; Arefiev, V.; Babyshkin, V.; Batanov, O.; Becker, W.; Böhringer, H.; Bogomolov, A.; Boller, T.; Borm, K.; Bornemann, W.; Bräuninger, H.; Brüggen, M.; Brunner, H.; Brusa, M.; Bulbul, E.; Buntov, M.; Burwitz, V.; Burkert, W.; Clerc, N.; Churazov, E.; Coutinho, D.; Dauser, T.; Dennerl, K.; Doroshenko, V.; Eder, J.; Emberger, V.; Eraerds, T.; Finoguenov, A.; Freyberg, M.; Friedrich, P.; Friedrich, S.; Fürmetz, M.; Georgakakis, A.; Gilfanov, M.; Granato, S.; Grossberger, C.; Gueguen, A.; Gureev, P.; Haberl, F.; Hälker, O.; Hartner, G.; Hasinger, G.; Huber, H.; Ji, L.; Kienlin, A. v.; Kink, W.; Korotkov, F.; Kreykenbohm, I.; Lamer, G.; Lomakin, I.; Lapshov, I.; Liu, T.; Maitra, C.; Meidinger, N.; Menz, B.; Merloni, A.; Mernik, T.; Mican, B.; Mohr, J.; Müller, S.; Nandra, K.; Nazarov, V.; Pacaud, F.; Pavlinsky, M.; Perinati, E.; Pfeffermann, E.; Pietschner, D.; Ramos-Ceja, M. E.; Rau, A.; Reiffers, J.; Reiprich, T. H.; Robrade, J.; Salvato, M.; Sanders, J.; Santangelo, A.; Sasaki, M.; Scheuerle, H.; Schmid, C.; Schmitt, J.; Schwope, A.; Shirshakov, A.; Steinmetz, M.; Stewart, I.; Strüder, L.; Sunyaev, R.; Tenzer, C.; Tiedemann, L.; Trümper, J.; Voron, V.; Weber, P.; Wilms, J.; Yaroshenko, V.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.
- ItemDispersion measure variability for 36 millisecond pulsars at 150MHz with LOFAR(Les Ulis : EDP Sciences, 2020) Donner, J.Y.; Verbiest, J.P.W.; Tiburzi, C.; Osłowski, S.; Künsemöller, J.; Bak Nielsen, A.-S.; Grießmeier, J.-M.; Serylak, M.; Kramer, M.; Anderson, J.M.; Wucknitz, O.; Keane, E.; Kondratiev, V.; Sobey, C.; McKee, J.W.; Bilous, A.V.; Breton, R.P.; Brüggen, M.; Ciardi, B.; Hoeft, M.; van Leeuwen, J.; Vocks, C.Context. Radio pulses from pulsars are affected by plasma dispersion, which results in a frequency-dependent propagation delay. Variations in the magnitude of this effect lead to an additional source of red noise in pulsar timing experiments, including pulsar timing arrays (PTAs) that aim to detect nanohertz gravitational waves. Aims. We aim to quantify the time-variable dispersion with much improved precision and characterise the spectrum of these variations. Methods. We use the pulsar timing technique to obtain highly precise dispersion measure (DM) time series. Our dataset consists of observations of 36 millisecond pulsars, which were observed for up to 7.1 yr with the LOw Frequency ARray (LOFAR) telescope at a centre frequency of ~150 MHz. Seventeen of these sources were observed with a weekly cadence, while the rest were observed at monthly cadence. Results. We achieve a median DM precision of the order of 10−5 cm−3 pc for a significant fraction of our sources. We detect significant variations of the DM in all pulsars with a median DM uncertainty of less than 2 × 10−4 cm−3 pc. The noise contribution to pulsar timing experiments at higher frequencies is calculated to be at a level of 0.1–10 μs at 1.4 GHz over a timespan of a few years, which is in many cases larger than the typical timing precision of 1 μs or better that PTAs aim for. We found no evidence for a dependence of DM on radio frequency for any of the sources in our sample. Conclusions. The DM time series we obtained using LOFAR could in principle be used to correct higher-frequency data for the variations of the dispersive delay. However, there is currently the practical restriction that pulsars tend to provide either highly precise times of arrival (ToAs) at 1.4 GHz or a high DM precision at low frequencies, but not both, due to spectral properties. Combining the higher-frequency ToAs with those from LOFAR to measure the infinite-frequency ToA and DM would improve the result.
- ItemFirst constraints on the AGN X-ray luminosity function at z 6 from an eROSITA-detected quasar(Les Ulis : EDP Sciences, 2021) Wolf, J.; Nandra, K.; Salvato, M.; Liu, T.; Buchner, J.; Brusa, M.; Hoang, D. N.; Moss, V.; Arcodia, R.; Brüggen, M.; Comparat, J.; de Gasperin, F.; Georgakakis, A.; Hotan, A.; Lamer, G.; Merloni, A.; Rau, A.; Rottgering, H. J. A.; Shimwell, T. W.; Urrutia, T.; Whiting, M.; Williams, W. L.Context. High-redshift quasars signpost the early accretion history of the Universe. The penetrating nature of X-rays enables a less absorption-biased census of the population of these luminous and persistent sources compared to optical/near-infrared colour selection. The ongoing SRG/eROSITA X-ray all-sky survey offers a unique opportunity to uncover the bright end of the high-z quasar population and probe new regions of colour parameter space. Aims. We searched for high-z quasars within the X-ray source population detected in the contiguous 140 deg2 field observed by eROSITA during the performance verification phase. With the purpose of demonstrating the unique survey science capabilities of eROSITA, this field was observed at the depth of the final all-sky survey. The blind X-ray selection of high-redshift sources in a large contiguous, near-uniform survey with a well-understood selection function can be directly translated into constraints on the X-ray luminosity function (XLF), which encodes the luminosity-dependent evolution of accretion through cosmic time. Methods. We collected the available spectroscopic information in the eFEDS field, including the sample of all currently known optically selected z > 5.5 quasars and cross-matched secure Legacy DR8 counterparts of eROSITA-detected X-ray point-like sources with this spectroscopic sample. Results. We report the X-ray detection of eFEDSU J083644.0+005459, an eROSITA source securely matched to the well-known quasar SDSS J083643.85+005453.3 (z = 5.81). The soft X-ray flux of the source derived from eROSITA is consistent with previous Chandra observations. The detection of SDSS J083643.85+005453.3 allows us to place the first constraints on the XLF at z > 5.5 based on a secure spectroscopic redshift. Compared to extrapolations from lower-redshift observations, this favours a relatively flat slope for the XLF at z 6 beyond L∗, the knee in the luminosity function. In addition, we report the detection of the quasar with LOFAR at 145 MHz and ASKAP at 888 MHz. The reported flux densities confirm a spectral flattening at lower frequencies in the emission of the radio core, indicating that SDSS J083643.85+005453.3 could be a (sub-) gigahertz peaked spectrum source. The inferred spectral shape and the parsec-scale radio morphology of SDSS J083643.85+005453.3 indicate that it is in an early stage of its evolution into a large-scale radio source or confined in a dense environment. We find no indications for a strong jet contribution to the X-ray emission of the quasar, which is therefore likely to be linked to accretion processes. Conclusions. Our results indicate that the population of X-ray luminous AGNs at high redshift may be larger than previously thought. From our XLF constraints, we make the conservative prediction that eROSITA will detect 90 X-ray luminous AGNs at redshifts 5.7 < z < 6.4 in the full-sky survey (De+RU). While subject to different jet physics, both high-redshift quasars detected by eROSITA so far are radio-loud; a hint at the great potential of combined X-ray and radio surveys for the search of luminous high-redshift quasars.