Akkretionsplasmen in Polaren mit ausgeprägter weicher Röntgenstrahlung

Akkretionsprozesse spielen während vielerlei Entwicklungsphasen von Sternen und in vielerlei Größenordnungen eine Rolle. Sie treten beispielsweise während der Sternentstehung auf, in Aktiven Galaxien oder in engen Doppelsternsystemen, deren eine Komponente ins Endstadium seiner Entwicklung eingetreten ist, den so genannten Röntgendoppelsternen und Kataklysmischen Veränderlichen. Magnetische kataklysmische Veränderliche vom Typ der AM-Her-Sterne (Polare) bestehen aus einem Weißen Zwerg mit einem starken Magnetfeld und einem Begleiter geringer Masse, der sein Roche-Volumen ausfüllt. Der Weiße Zwerg akkretiert Materie vom Sekundärstern, die entlang der Magnetfeldlinien auf seine magnetischen Pole geleitet wird. Oberhalb des Weißen Zwergs bildet sich eine Stoßfront aus, und die Materie heizt sich auf hohe Temperaturen auf. Im Bereich zwischen Stoßfront und Photosphäre wird das Plasma in der Akkretionssäule abgebremst und abgekühlt, in erster Linie durch Emission von optischer Zyklotron- und harter Röntgen-Bremsstrahlung. Weicher Röntgen- und ultravioletter Fluss entstehen in der geheizten Photosphäre, in der die Strahlung der Akkretionssäule reprozessiert wird. Weiche und harte Röntgenemission sollten sich folglich in etwa die Waage halten. Einige Systeme weisen jedoch eine deutlich höhere Leuchtkraft im weichen Röntgenbereich auf, die mit einer besonders effektiven Zyklotronkühlung und mit inhomogener Akkretion zusammenhängen dürfte.Zwei magnetische kataklysmische Variable mit ausgeprägtem weichem Röntgenfluss stehen im Mittelpunkt der vorliegenden Arbeit. Für beide Systeme, AI Tri und QS Tel, wurden Röntgendaten mit dem Weltraumteleskop XMM-Newton und optische Daten mit verschiedenen bodengebundenen Teleskopen aufgenommen, die die Grundlage für eine eingehende photometrische und spektroskopische Analyse bilden. Die üblicherweise gebräuchlichen Spektralmodelle, bestehend aus einer Schwarzkörper- und einer Plasmakomponente, wurden erweitert, um einen großen Bereich von Temperaturen abzudecken und somit dem komplexen Aufbau der Emissionsregionen Rechnung zu tragen. Ein deutlicher Überschuss an weicher Röntgenstrahlung kennzeichnet die beobachteten Objekte. Ihre Lichtkurven besitzen eine stark variable Struktur, Zeichen für die zeitlich und räumlich inhomogenen Akkretionsvorgänge. Die Beobachtungsdaten des langperiodischen Polaren AI Tri sind durch die Strahlung aus einer einzelnen Akkretionsregion geprägt, die für ein Zehntel der Orbitalperiode durch den Akkretionsstrom bedeckt wird. Die Spektren lassen sich durch Schwarzkörperkomponenten mit Temperaturen von bis zu $44.0^{+3.3}_{-3.2}$ eV und Emission aus einem Plasma mit solarer Elementhäufigkeit bei Temperaturen zwischen $0.8^{+0.4}_{-0.2}$ keV and $20.3^{+8.7}{-5.1}$ keV wiedergeben. Während zweier Beobachtungen zeigte AI Tri ein "irreguläres" Verhalten: Die Röntgendaten wurden noch wesentlich stärker von weicher Röntgenstrahlung dominiert, die optischen Lichtkurven waren um $\Delta\phi\simeq0.2$ phasenverschoben. Der zweite Polar, QS Tel, wechselt zwischen Phasen von Akkretion auf einen einzelnen und auf beide Pole. Während der Beobachtung mit XMM-Newton wies QS Tel kaum Fluss bei Energien oberhalb von 2 keV auf, und in erster Linie war die weiche Röntgenstrahlung produzierende Akkretionsregion vorherrschend. Die zweite Region mit Flussmaximum im optischen und ultravioletten Bereich zeigte nur geringe Aktivität. Zahlreiche Metalllinien lassen sich in den Röntgenspektren identifizieren, die am besten bei niedrigen Temperaturen unter 5 keV und um $19.5^{+3.7}_{-4.2}$ eV beschrieben werden. Im Vergleich zwischen verschiedenen spektralen Fits wird bei beiden Objekten offensichtlich, dass geeignete Modelle ein hinreichend großes Dichte- und Temperaturintervall überdecken müssen. Dabei beeinflusst die Wahl des Modells deutlich das quantitative Verhältnis zwischen weicher und harter Röntgenstrahlung, das sich daraus ableiten lässt

X-ray and ultraviolet observations of the eclipsing cataclysmic variables OV Bootis and SDSS J103533.02+055158.3 with degenerate donors

The majority of cataclysmic variables are predicted to be post-period minimum systems with degenerate donor stars, the period bouncers. Owing to their intrinsic faintness, however, only a handful of these systems have so far been securely identified. We want to study the X-ray properties of two eclipsing period bouncers, OV Bootis and SDSS J103533.02+055158.3, that were selected for this study due to their proximity to Earth. We have obtained XMM-Newton phase-resolved X-ray and ultraviolet observations of the two objects for spectral and timing analysis. Owing to a recent dwarf nova outburst OV Boo was much brighter than SDSS J103533.02+055158.3 at X-ray and ultraviolet wavelengths and the eclipse could be studied in some detail. An updated eclipse ephemeris was derived. The X-rays were shown to originate close to the white dwarf, the boundary layer, with significant absorption affecting its spectrum. There was no absorption in SDSS J103533.02+055158.3, despite being observed at the same inclination indicating different shapes of the disk and the disk rim. The white-dwarf temperature was re-determined for both objects: the white dwarf in OV Boo was still hot (23,000 K) five months after a dwarf nova outburst, and the white dwarf in SDSS J103533.02+055158.3 hotter than assumed previously (Teff = 11,500 K). All three cataclysmic variables with degenerate donors studied so far in X-rays, including SDSS J121209.31+013627.7, were clearly discovered in X-rays and revealed mass accretion rates dot(M) >= 8 x 10^(-15) Msun/ yr. If their X-ray behavior is representative of the subpopulation of period bouncers, the all-sky X-ray surveys with eROSITA together with comprehensive follow-up will uncover new objects in sufficient number to address the remaining questions concerning late-stage cataclysmic variable evolution.Comment: 12 pages, 13 figures, accepted for publication in A&

XMM-Newton observations of eleven intermediate polars and possible candidates

We aim to identify new intermediate polars (IPs) in XMM-Newton observations from a list of promising candidates. By selecting targets not previously known to be X-ray bright we aim to uncover evidence for an X-ray underluminous IP subpopulation. We performed period searches on the XMM-Newton X-ray and optical data of our targets to seek both the spin and orbital periods, which differ in IPs. We also investigated the X-ray spectra to find the hot plasma emission shown by these objects. With archival Swift data we coarsely investigated the long-term X-ray variability, and with archival optical data from a variety of catalogues, we compared the optical to X-ray luminosity to identify X-ray faint objects. This paper presents the first XMM-Newton observation of the prototype IP, DQ Her. We find firm evidence for HZ~Pup, V349 Aqr, and IGR J18151-1052 being IPs, with likely white dwarf spin periods of 1552, 390, and 390 s, respectively. The former two have luminosities typical of IPs, and the latter is strongly absorbed and with unknown distance. GI~Mon and V1084~Her are apparently non-magnetic CVs with interesting short-term variability unrelated to WD spin. V533~Her is probably a magnetic CV and remains a good IP candidate, while V1039 Cen is possibly a polar. The remaining candidates were too faint to allow for any firm conclusions.Comment: 21 pages, 13 fig

MACHO 311.37557.169: A VY Scl star

Optical surveys, such as theMACHO project, often uncover variable stars whose classification requires follow-up observations by other instruments. We performed X-ray spectroscopy and photometry of the unusual variable starMACHO 311.37557.169 with XMM-Newton in April 2018, supplemented by archival X-ray and optical spectrographic data. The star has a bolometric X-ray luminosity of about 1 × 1032 erg s−1 cm−2 and a heavily absorbed two-temperature plasma spectrum. The shape of its light curve, its overall brightness, its X-ray spectrum, and the emission lines in its optical spectrum suggest that it is most likely a VY Scl cataclysmic variable

The XMM-SERVS Survey: XMM-Newton Point-source Catalogs for the W-CDF-S and ELAIS-S1 Fields

We present the X-ray point-source catalogs in two of the XMM-Spitzer Extragalactic Representative Volume Survey (XMM-SERVS) fields, W-CDF-S (4.6 deg2) and ELAIS-S1 (3.2 deg2), aiming to fill the gap between deep pencil-beam X-ray surveys and shallow X-ray surveys over large areas. The W-CDF-S and ELAIS-S1 regions were targeted with 2.3 and 1.0 Ms of XMM-Newton observations, respectively; 1.8 and 0.9 Ms exposures remain after flare filtering. The survey in W-CDF-S has a flux limit of 1.0 × 10−14 erg cm−2 s−1 over 90% of its area in the 0.5–10 keV band; 4053 sources are detected in total. The survey in ELAIS-S1 has a flux limit of 1.3 × 10−14 erg cm−2 s−1 over 90% of its area in the 0.5–10 keV band; 2630 sources are detected in total. Reliable optical-to-IR multiwavelength counterpart candidates are identified for ≈89% of the sources in W-CDF-S and ≈87% of the sources in ELAIS-S1. A total of 3129 sources in W-CDF-S and 1957 sources in ELAIS-S1 are classified as active galactic nuclei (AGNs). We also provide photometric redshifts for X-ray sources; ≈84% of the 3319/2001 sources in W-CDF-S/ELAIS-S1 with optical-to-near-IR forced photometry available have either spectroscopic redshifts or high-quality photometric redshifts. The completion of the XMM-Newton observations in the W-CDF-S and ELAIS-S1 fields marks the end of the XMM-SERVS survey data gathering. The ≈12,000 pointlike X-ray sources detected in the whole ≈13 deg2 XMM-SERVS survey will benefit future large-sample AGN studies

XMM2ATHENA, the H2020 project to improve XMM-Newton analysis software and prepare for Athena

Webb et al.XMM-Newton, a European Space Agency observatory, has been observing the X-ray, ultra-violet, and optical sky for 23 years. During this time, astronomy has evolved from mainly studying single sources to populations and from a single wavelength, to multi-wavelength/messenger data. We are also moving into an era of time domain astronomy. New software and methods are required to accompany evolving astronomy and prepare for the next-generation X-ray observatory, Athena. Here we present XMM2ATHENA, a program funded by the European Union's Horizon 2020 research and innovation program. XMM2ATHENA builds on foundations laid by the XMM-Newton Survey Science Centre (XMM-SSC), including key members of this consortium and the Athena Science ground segment, along with members of the X-ray community. The project is developing and testing new methods and software to allow the community to follow the X-ray transient sky in quasi-real time, identify multi-wavelength/messenger counterparts of XMM-Newton sources and determine their nature using machine learning. We detail here the first milestone delivery of the project, a new online, sensitivity estimator. We also outline other products, including the forthcoming innovative stacking procedure and detection algorithms, to detect the faintest sources. These tools will then be adapted for Athena and the newly detected/identified sources will enhance preparation for observing the Athena X-ray sky.HThis project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement Number 101004168.Peer reviewe

Serendipitous discovery of the magnetic cataclysmic variable SRGE J075818−612027

We report the discovery of SRGE J075818−612027, a deep stream-eclipsing magnetic cataclysmic variable found serendipitously in SRG/eROSITA calibration and performance verification phase (CalPV) observations of the open cluster NGC 2516 as an unrelated X-ray source. An X-ray timing and spectral analysis of the eROSITA data is presented and supplemented by an analysis of TESS photometry and SALT spectroscopy. X-ray photometry reveals two pronounced dips repeating with a period of 106.144(1) min. The 14-month TESS data reveal the same unique period. A low-resolution identification spectrum obtained with SALT displays hydrogen Balmer emission lines on a fairly blue continuum. The spectrum and the stability of the photometric signal led to the classification of the new object as a polar-type cataclysmic variable. In this context, the dips in the X-ray light curve are explained by absorption in the intervening accretion stream and by a self-eclipse of the main accretion region. The object displays large magnitude differences on long timescales (months) both at optical and X-ray wavelengths, which are interpreted as high and low states and thus support its identification as a polar. The bright phase X-ray spectrum can be reflected with single temperature thermal emission with 9.7 keV and bolometric X-ray luminosity LX ≃ 8 × 1032 erg s−1 at a distance of about 2.7 kpc. The X-ray spectrum lacks the pronounced soft X-ray emission component prominently found in ROSAT-discovered polars