Villanueva de la Cañada, Spain
Villanueva de la Cañada, Spain

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Jordan C.,University of Oxford | Ness J.-U.,Newton Science Operations Center
Monthly Notices of the Royal Astronomical Society | Year: 2012

Emission line fluxes from cool stars are widely used to establish an apparent emission measure distribution, Emd app(T e), between temperatures characteristic of the low transition region and the low corona. The true emission measure distribution, Emd t(T e), is determined by the energy balance and geometry adopted and, with a numerical model, can be used to predict Emd app(T e), to guide further modelling. The scaling laws that exist between coronal parameters arise from the dimensions of the terms in the energy balance equation. Here, analytical approximations to numerical solutions for Emd t(T e) are presented, which show how the constants in the coronal scaling laws are determined. The apparent emission measure distributions show a minimum value at some T o and a maximum at the mean coronal temperature T c (although in some stars, emission from active regions can contribute). It is shown that, for the energy balance and geometry adopted, the analytical values of the emission measure and electron pressure at T o and T c depend on only three parameters: the stellar surface gravity and the values of T o and T c. The results are tested against full numerical solutions for ε Eri (K2 V) and are applied to Procyon (α CMi, F5 IV/V). The analytical approximations can be used to restrict the required range of full numerical solutions, to check the assumed geometry and to show where the adopted energy balance may not be appropriate. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Crowther P.A.,University of Sheffield | Barnard R.,Open University Milton Keynes | Carpano S.,Newton Science Operations Center | Clark J.S.,Open University Milton Keynes | And 2 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2010

We present Very Large Telescope/FORS2 time-series spectroscopy of the Wolf-Rayet (WR) star #41 in the Sculptor group galaxy NGC 300. We confirm a physical association with NGC 300 X-1, since radial velocity variations of the He II λ4686 line indicate an orbital period of 32.3 ± 0.2 h which agrees at the 2σ level with the X-ray period from Carpano et al. We measure a radial velocity semi-amplitude of 267 ± 8 kms-1, from which a mass function of 2.6 ± 0.3 M⊙ is obtained. A revised spectroscopic mass for the WN-type companion of 26+7 -5 M⊙ yields a black hole mass of 20 ± 4M⊙ for a preferred inclination of 60°-75°. If the WR star provides half of the measured visual continuum flux, a reduced WR (black hole) mass of 15+4 -2.5 M⊙ (14.5+3 -2.5 M.) would be inferred. As such, #41/NGC 300 X-1 represents only the second extragalactic WR plus black hole binary system, after IC 10 X-1. In addition, the compact object responsible for NGC 300 X-1 is the second highest stellar-mass black hole known to date, exceeded only by IC 10 X-1. © 2010 The Authors. Journal compilation © 2010 RAS.

Done C.,Durham University | Trigo M.D.,Newton Science Operations Center
Monthly Notices of the Royal Astronomical Society | Year: 2010

The detection of an extremely broad iron line in XMM-Newton MOS data from the low/hard state of the black hole binary GX339-4 is the only piece of evidence which unambiguously conflicts with the otherwise extremely successful truncated disc interpretation of this state. However, it also conflicts with some aspect of observational data for all other alternative geometries of the low/hard state, including jet models, making it very difficult to understand. We re-analyse these data and showthat they are strongly affected by pile-up evenwith extensive centroid removal as the source is ~200 times brighter than the recommended maximum count rate. Instead, we extract the simultaneous PN timing-mode data which should not be affected by pile-up. These show a line which is significantly narrower than in the MOS data. Thus these data are easily consistent with a truncated disc, and indeed, strongly support such an interpretation. © 2010 The Authors. Journal compilation. © 2010 RAS.

Gomez de Castro A.I.,Complutense University of Madrid | Lopez-Santiago J.,Complutense University of Madrid | Talavera A.,Newton Science Operations Center
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2013

Pre-main sequence (PMS) binaries are surrounded by circumbinary discs from which matter falls on to both components. The material dragged from the circumbinary disc flows on to each star through independent streams channelled by the variable gravitational field. The action of the bar-like potential is most prominent in high eccentricity systems made of two equal mass stars. AK Sco is a unique PMS system composed of two F5 stars in an orbit with e = 0.47. Henceforth, it is an ideal laboratory to study matter infall in binaries and its role inorbit circularization. In this Letter, we report the detection of a 1.3 mHz ultra low-frequency oscillation in the ultraviolet light curveat periastron passage. This oscillation lasts 7 ks being most likely fed by the gravitational energy released when the stream's tails spiralling on to each star get in contact at periastron passage enhancingthe accretion flow; this unveils a new mechanism for angular momentumloss during PMS evolution and a new type of interacting binary. © 2012 The Authors,© 2012 RAS.

Dobrotka A.,Slovak University of Technology in Bratislava | Mineshige S.,Kyoto University | Ness J.-U.,Newton Science Operations Center
Monthly Notices of the Royal Astronomical Society | Year: 2015

The stochastic variability (flickering) of the nova-like system (subclass of cataclysmic variable) MV Lyr yields a complicated power density spectrum with four break frequencies. Scaringi et al. analysed high-cadence Kepler data of MV Lyr, taken almost continuously over 600 d, giving the unique opportunity to study multicomponent Power Density Spectra (PDS) over a wide frequency range. We modelled this variability with our statistical model based on disc angular momentum transport via discrete turbulent bodies with an exponential distribution of the dimension scale. Two different models were used, a full disc (developed from the white dwarf to the outer radius of ~1010 cm) and a radially thin disc (a ring at a distance of ~1010 cm from the white dwarf) that imitates an outer disc rim. We succeed in explaining the two lowest observed break frequencies assuming typical values for a disc radius of 0.5 and 0.9 times the primary Roche lobe and an α parameter of 0.1-0.4. The highest observed break frequency was also modelled, but with a rather small accretion disc with a radius of 0.3 times the primary Roche lobe and a high α value of 0.9 consistent with previous findings by Scaringi. Furthermore, the simulated light curves exhibit the typical linear rms-flux proportionality linear relation and the typical log-normal flux distribution. As the turbulent process is generating fluctuations in mass accretion that propagate through the disc, this confirms the general knowledge that the typical rms-flux relation is mainly generated by these fluctuations. In general, a higher rms is generated by a larger amount of superposed flares which is compatible with a higher mass accretion rate expressed by a larger flux. © 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Dobrotka A.,Slovak University of Technology in Bratislava | Ness J.-U.,Newton Science Operations Center
Monthly Notices of the Royal Astronomical Society | Year: 2015

An optical light curve of SU UMa type dwarf nova V1504 Cyg taken by Kepler was analysed in order to study fast optical variability (flickering). We calculated power density spectra and rms-flux relations for two different stages of activity, i.e. quiescence and regular outbursts. A multicomponent power density spectrum with two break frequencies was found during both activity stages. The rms-flux relation is obvious only in the quiescent data. However, while the collection of all outburst data do not show this variability, every individual outburst does show it in the majority of cases keeping the rms value approximately in the same interval. Furthermore, the same analysis was performed for light-curve subsamples taken from the beginning, middle and the end of the supercycle both for quiescence and regular outbursts. Every light-curve subsample shows the same multicomponent power density spectrum. The stability of the break frequencies over the supercycle can be confirmed for all frequencies except for the high break frequency during outburst, which shows variability, but with rather low confidence. Finally, the low break frequency can be associated with the geometrically thin disc or its inner edge, while the high break frequency can originate from the inner geometrically thick hot disc. Furthermore, with our statistical method to simulate flickering light curves, we show that the outburst flickering light curve of V1504 Cyg needs an additional constant flux level to explain the observed rms-flux behaviour. Therefore, during the outbursts another non-turbulent radiation source should be present. © 2015 The Authors.

Schartel N.,Newton Science Operations Center
Frascati Workshop 2013 - 10th International Workshop on Multifrequency Behaviour of High Energy Cosmic Sources | Year: 2013

XMM-Newton is one of the most successful science missions of the European Space Agency. Since 2003 every year about 300 articles are published in refereed journals making directly use of XMM-Newton data. All XMM-Newton calls for observing proposals are highly oversubscribed by factors of six and more. In the following some scientific highlights of XMM-Newton observations of black holes are summarized.

Schartel N.,Newton Science Operations Center
Astronomische Nachrichten | Year: 2012

The historical development of ground based astronomical telescopes leads us to expect that space-based astronomical telescopes will need tobe operational for many decades. The exchange of scientific instruments in space will be a prerequisite for the long lasting scientific success of such missions. Operationally, the possibility to repair or replace key spacecraft components in space will be mandatory. We argue that these requirements can be fulfilled with robotic missions and see the development of the required engineering as the main challenge. Ground based operations, scientifically and technically, will require a low operational budget of the running costs. These can be achieved through enhanced autonomy of the spacecraft and mission independent concepts for the support of the software. This concept can be applied to areas where the mirror capabilities do not constrain the lifetime of the mission. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Read A.M.,University of Leicester | Guainazzi M.,Newton Science Operations Center | Sembay S.,University of Leicester
Astronomy and Astrophysics | Year: 2014

Aims. We aim to examine the relative cross-calibration accuracy of the on-axis effective areas of the XMM-Newton EPIC pn and MOS instruments. Methods. Spectra from a sample of 46 bright, high-count, non-piled-up, isolated on-axis point sources are stacked together, and model residuals are examined to characterize the EPIC MOS-to-pn inter-calibration. Results. The MOS1-to-pn and MOS2-to-pn results are broadly very similar. The cameras show the closest agreement below 1 keV, with MOS excesses over pn of 0-2% (MOS1/pn) and 0-3% (MOS2/pn). Above 3 keV, the MOS/pn ratio is consistent with energy-independent (or only mildly increasing) excesses of 7-8% (MOS1/pn) and 5-8% (MOS2/pn). In addition, between 1-2 keV there is a "silicon bump"-an enhancement at a level of 2-4% (MOS1/pn) and 3-5% (MOS2/pn). Tests suggest that the methods employed here are stable and robust. Conclusions. The results presented here provide the most accurate cross-calibration of the effective areas of the XMM-Newton EPIC pn and MOS instruments to date. They suggest areas of further research where causes of the MOS-to-pn differences might be found, and allow the potential for corrections and possible rectification of the EPIC cameras to be made in the future. © 2014 ESO.

Ness J.U.,Newton Science Operations Center
Bulletin of the Astronomical Society of India | Year: 2012

The 21st century X-ray observatories XMM-Newton, Chandra, and Swift gave us completely new insights into the X-ray behaviour of nova outbursts. These new-generation X-ray observatories provide particularly high spectral resolution and high density in monitoring campaigns, simultaneously in X-rays and UV/optical. The entire evolution of several nova outbursts has been observed with the Swift XRT and UVOT instruments, allowing studies of the gradual shift of the peak of the SED from UV to X-rays, time scales to the onset and duration of the X-ray brightest supersoft source (SSS) phase, and pre- and post-SSS X-ray emission. In addition, XMM-Newton and Chandra observations can effciently be scheduled, allowing deeper studies of strategically chosen evolutionary stages. Before Swift joined in 2005, Chandra and XMM-Newton blind shots in search of SSS emission unavoidably led to some underexposed observations taken before and/or after the SSS phase. More systematic Swift studies reduced this number while increasing the number of novae. Pre- and post-SSS spectra at low and high spectral resolution were successfully modelled with collisional plasma models. Pre-SSS emission arises in shocks and post-SSS emission in radiatively cooling thin ejecta. In contrast, the grating spectra taken during the SSS phase are a lot more complex than expected and have not yet been successfully modeled. Available hot white dwarf (WD) radiation transport models give only approximate reproduction of the observations, and make some critical assumptions that are only valid in isolated WDs. More grating spectra would be important to search for systematic trends between SSS spectra and system parameters. Summary of well-established discoveries with Swift, XMM-Newton, and Chandra: • About 50% of novae display faint X-ray emission before the start of the SSS phase • The start of the SSS phase is not a smooth process. High-amplitude variations during the early SSS phase were seen that disappear close to the time when the optical plateau phase begins. • The end of the SSS phase is in most cases a smooth process. • The SSS grating spectra contain continuum spectra that roughly resemble a blackbody shape • The SSS X-ray grating spectra of systems with known high inclination angles contain emission lines on top of the continuum • The SSS X-ray spectra of systems with unknown or low inclination angles contain deep absorption lines from the interstellar medium and local, high-ionisation absorption lines that are blue shifted.

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