Publication archive

Publication archive

Context. Zeta Pup is the X-ray brightest O-type star of the sky. This object was regularly observed with the RGS instrument onboard XMM-Newton for calibration purposes, which led to an unprecedented set of high-quality spectra.

Aims. We have previously reduced and extracted this data set and integrated it into the most detailed high-resolution X-ray spectrum of any early-type star so far. Here we present the analysis of this spectrum, taking into account for the presence of structures in the stellar wind.

Methods. For this purpose, we used our new modeling tool that allows fitting the entire spectrum with a multi-temperature plasma. We illustrate the impact of a proper treatment of the radial dependence of the X-ray opacity of the cool wind on the best-fit radial distribution of the temperature of the X-ray plasma.

Results. The best-fit of the RGS spectrum of zeta Pup is obtained assuming no porosity. Four plasma components at temperatures between 0.10 and 0.69 keV are needed to adequately represent the observed spectrum. Whilst the hardest emission is concentrated between ~3 and 4 R*, the softer emission starts already at 1.5 R* and extends to the outer regions of the wind.

Conclusions. The inferred radial distribution of the plasma temperatures agrees rather well with theoretical expectations. The mass-loss rate and CNO abundances corresponding to our best-fit model also agree quite well with the results of recent studies of zeta Pup in the UV and optical domain.

Published: 26 February 2013
Photochemically produced aerosols are common among the atmospheres of our solar system and beyond. Observations and models have shown that photochemical aerosols have direct consequences on atmospheric properties as well as important astrobiological ramifications, but the mechanisms involved in their formation remain unclear. Here we show that the formation of aerosols in Titan's upper atmosphere is directly related to ion processes, and we provide a complete interpretation of observed mass spectra by the Cassini instruments from small to large masses. Because all planetary atmospheres possess ionospheres, we anticipate that the mechanisms identified here will be efficient in other environments as well, modulated by the chemical complexity of each atmosphere.
Published: 19 February 2013
Electrons can be accelerated to ultrarelativistic energies at strong (high Mach number) collisionless shock waves that form when stellar debris rapidly expands after a supernova. Collisionless shock waves also form in the flow of particles from the Sun (the solar wind), and extensive spacecraft observations have established that electron acceleration at these shocks is effectively absent whenever the upstream magnetic field is roughly parallel to the shock-surface normal (quasi-parallel conditions). However, it is unclear whether this magnetic dependence of electron acceleration also applies to the far stronger shocks around young supernova remnants, where local magnetic conditions are poorly understood. Here we present Cassini spacecraft observations of an unusually strong solar system shock wave (Saturn's bow shock) where significant local electron acceleration has been confirmed under quasi-parallel magnetic conditions for the first time, contradicting the established magnetic dependence of electron acceleration at solar system shocks. Furthermore, the acceleration led to electrons at relativistic energies (about megaelectronvolt), comparable to the highest energies ever attributed to shock acceleration in the solar wind. These observations suggest that at high Mach numbers, such as those of young supernova remnant shocks, quasi-parallel shocks become considerably more effective electron accelerators.
Published: 17 February 2013

LOFT is an M-class mission candidate for the M3 slot within the Cosmic Vision programme, for a planned launch between 2022 and 2024. LOFT, with 3 other science missions, was recommended by the Space Science Advisory Committee (SSAC) to enter an assessment study (Phase 0), starting by an ESA internal study followed by parallel industrial study activities.

Within the M3 boundary conditions, the readiness for launch by end 2022/2024 is a severe requirement which in practice requires designing the space segment without major technology developments and with minimum developments risks. Therefore, only technologies with estimated Technology Readiness Levels (TRL) of at least 5 by the end of the Phase A (estimated at the end of 2014) may be used.

This document aims at providing a complete and comprehensive list of all high level mission requirements (including S/C and payload, launcher, ground segment and operations) necessary to achieve the science goals detailed in [LOFT Science Requirements Document (SciRD), SRE-SA/LOFT/2011-001, Issue 1, Rev. 7]. Accordingly it is an applicable document that shall be complied with for all mission design activities. The MRD will be further reviewed matching the results of future study phases (e.g. definition phase) to finally evolve into the System Requirements Document at the start of the implementation phase.

Published: 11 February 2013

Context. About half of the baryons of the Universe are expected to be in the form of filaments of hot and low-density intergalactic medium. Most of these baryons remain undetected even by the most advanced X-ray observatories, which are limited in sensitivity to the diffuse low-density medium.

Aims. The Planck satellite has provided hundreds of detections of the hot gas in clusters of galaxies via the thermal Sunyaev-Zel'dovich (tSZ) effect and is an ideal instrument for studying extended low-density media through the tSZ effect. In this paper we use the Planck data to search for signatures of a fraction of these missing baryons between pairs of galaxy clusters.

Methods. Cluster pairs are good candidates for searching for the hotter and denser phase of the intergalactic medium (which is more easily observed through the SZ effect). Using an X-ray catalogue of clusters and the Planck data, we selected physical pairs of clusters as candidates. Using the Planck data, we constructed a local map of the tSZ effect centred on each pair of galaxy clusters. ROSAT data were used to construct X-ray maps of these pairs. After modelling and subtracting the tSZ effect and X-ray emission for each cluster in the pair, we studied the residuals on both the SZ and X-ray maps.

Results. For the merging cluster pair A399-A401 we observe a significant tSZ effect signal in the intercluster region beyond the virial radii of the clusters. A joint X-ray SZ analysis allows us to constrain the temperature and density of this intercluster medium. We obtain a temperature of kT = 7.1 ± 0.9 keV (consistent with previous estimates) and a baryon density of (3.7 ± 0.2) x 10-4 cm-3.

Conclusions. The Planck satellite mission has provided the first SZ detection of the hot and diffuse intercluster gas.

Published: 07 February 2013
We make use of the Planck all-sky survey to derive number counts and spectral indices of extragalactic sources – infrared and radio sources – from the Planck Early Release Compact Source Catalogue (ERCSC) at 100 to 857 GHz (3mm to 350 μm). Three zones (deep, medium and shallow) of approximately homogeneous coverage are used to permit a clean and controlled correction for incompleteness, which was explicitly not done for the ERCSC, as it was aimed at providing lists of sources to be followed up. Our sample, prior to the 80% completeness cut, contains between 217 sources at 100 GHz and 1058 sources at 857 GHz over about 12800 to 16550 deg2 (31 to 40% of the sky). After the 80% completeness cut, between 122 and 452 and sources remain, with flux densities above 0.3 and 1.9 Jy at 100 and 857 GHz. The sample so defined can be used for statistical analysis. Using the multi-frequency coverage of the Planck High Frequency Instrument, all the sources have been classified as either dust-dominated (infrared galaxies) or synchrotron-dominated (radio galaxies) on the basis of their spectral energy distributions (SED). Our sample is thus complete, flux-limited and color-selected to differentiate between the two populations. We find an approximately equal number of synchrotron and dusty sources between 217 and 353 GHz; at 353 GHz or higher (or 217 GHz and lower) frequencies, the number is dominated by dusty (synchrotron) sources, as expected. For most of the sources, the spectral indices are also derived. We provide for the first time counts of bright sources from 353 to 857 GHz and the contributions from dusty and synchrotron sources at all HFI frequencies in the key spectral range where these spectra are crossing. The observed counts are in the Euclidean regime.
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Published: 07 February 2013
The survey of galaxy clusters performed by Planck through the Sunyaev-Zeldovich effect has already discovered many interesting objects, thanks to its full sky coverage. One of the SZcandidates detected in the early months of the mission near to the signal-to-noise threshold, PLCKG214.6+37.0, was later revealed by XMM-Newton to be a triple system of galaxy clusters. We present the results from a deep XMM-Newton re-observation of PLCKG214.6+37.0, part of a multi-wavelength programme to investigate Planck discovered superclusters. The characterisation of the physical properties of the three components has allowed us to build a template model to extract the total SZ signal of this system with Planck data. We have partly reconciled the discrepancy between the expected SZ signal derived from X-rays and the observed one, which are now consistent within 1.2σ. We measured the redshift of the three components with the iron lines in the X-ray spectrum, and confirm that the three clumps are likely part of the same supercluster structure. The analysis of the dynamical state of the three components, as well as the absence of detectable excess X-ray emission, suggests that we are witnessing the formation of a massive cluster at an early phase of interaction.
Published: 07 February 2013
Taking advantage of the all-sky coverage and broadfrequency range of the Planck satellite, we study the Sunyaev-Zeldovich (SZ) and pressure profiles of 62 nearby massive clusters detected at high significance in the 14-month nominal survey. Careful reconstruction of the SZ signal indicates that most clusters are individually detected at least out to R500. By stacking the radial profiles, we have statistically detected the radial SZ signal out to 3 × R500, i.e., at a density contrast of about 50–100, though the dispersion about the mean profile dominates the statistical errors across the whole radial range. Our measurement is fully consistent with previous Planck results on integrated SZ fluxes, further strengthening the agreement between SZ and X-ray measurements inside R500. Correcting for the effects of the Planck beam, we have calculated the corresponding pressure profiles. This new constraint from SZ measurements is consistent with the X-ray constraints from XMM-Newton in the region in which the profiles overlap (i.e., [0.1–1]  R500), and is in fairly good agreement with theoretical predictions within the expected dispersion. At larger radii the average pressure profile is slightly flatter than most predictions from numerical simulations. Combining the SZ and X-ray observed profiles into a joint fit to a generalised pressure profile gives best-fit parameters [P0,c500,γ,α,β ] = [6.41,1.81,0.31,1.33,4.13]. Using a reasonable hypothesis for the gas temperature in the cluster outskirts we reconstruct from our stacked pressure profile the gas mass fraction profile out to 3 R500. Within the temperature driven uncertainties, our Planck constraints are compatible with the cosmic baryon fraction and expected gas fraction in halos.
This article has an erratum.
Published: 07 February 2013
We present the final results from the XMM-Newton validation follow-up of new Planck galaxy cluster candidates. We observed 15 new candidates, detected with signal-to-noise ratios between 4.0 and 6.1 in the 15.5-month nominal Planck survey. The candidates were selected using ancillary data flags derived from the ROSAT All Sky Survey (RASS) and Digitized Sky Survey all-sky maps, with the aim of pushing into the low SZ flux, high-z regime and testing RASS flags as indicators of candidate reliability. Fourteen new clusters were detected by XMM-Newton, ten single clusters and two double systems. Redshifts from X-ray spectroscopy lie in the range 0.2 to 0.9, with six clusters at z > 0.5. Estimated masses (M500) range from 2.5 × 1014 to 8 × 1014 M. We discuss our results in the context of the full XMM-Newton validation programme, in which 51 new clusters have been detected. This includes four double and two triple systems, some of which are chance projections on the sky of clusters at different redshifts. We find thatassociation with a source from the RASS-Bright Source Catalogue is a robust indicator of the reliability of a candidate, whereas association with a source from the RASS-Faint Source Catalogue does not guarantee that the SZ candidate is a bona fide cluster. Nevertheless, most Planck clusters appear in RASS maps, with a significance greater than 2σ being a good indication that the candidate is a real cluster. Candidate validation from association with SDSS galaxy overdensity at z > 0.5 is also discussed. The full sample gives a Planck sensitivity threshold of Y500 ~ 4 × 10-4 arcmin2, with indication for Malmquist bias in the YX–Y500 relation below this threshold. The corresponding mass threshold depends on redshift.
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Published: 07 February 2013
We examine the relation between the galaxy cluster mass M and Sunyaev-Zeldovich (SZ) effect signal D2A Y500 for a sample of 19 objects for which weak lensing (WL) mass measurements obtained from Subaru Telescope data are available in the literature. Hydrostatic X-ray masses are derived from XMM-Newton archive data, and the SZ effect signal is measured from Planck all-sky survey data. We find an MWL–D2A Y500relation that is consistent in slope and normalisation with previous determinations using weak lensing masses; however, there is a normalisation offset with respect to previous measures based on hydrostatic X-ray mass-proxy relations. We verify that our SZ effect measurements are in excellent agreement with previous determinations from Planck data. For the present sample, the hydrostatic X-ray masses at R500 are on average ~20 percent larger than the corresponding weak lensing masses, which is contrary to expectations. We show that the mass discrepancy is driven by a difference in mass concentration as measured by the two methods and, for the present sample, that the mass discrepancy and difference in mass concentration are especially large for disturbed systems. The mass discrepancy is also linked to the offset in centres used by the X-ray and weak lensing analyses, which again is most important in disturbed systems. We outline several approaches that are needed to help achieve convergence in cluster mass measurement with X-ray and weak lensing observations.
Published: 07 February 2013
A comparison is presented of Sunyaev-Zeldovich measurements for 11 galaxy clusters as obtained by Planck and by the ground-based interferometer, the Arcminute Microkelvin Imager. Assuming a universal spherically-symmetric Generalised Navarro, Frenk and White (GNFW) model for the cluster gas pressure profile, we jointly constrain the integrated Compton-Y parameter (Y500) and the scale radius (θ500) of each cluster. Our resulting constraints in the Y500 − θ500 2D parameter space derived from the two instruments overlap significantly for eight of the clusters, although, overall, there is a tendency for AMI to find the Sunyaev-Zeldovich signal to be smaller in angular size and fainter than Planck. Significant discrepancies exist for the three remaining clusters in the sample, namely A1413, A1914, and the newly-discovered Planck cluster PLCKESZ G139.59+24.18. The robustness of the analysis of both the Planck and AMI data is demonstrated through the use of detailed simulations, which also discount confusion from residual point (radio) sources and from diffuse astrophysical foregrounds as possible explanations for the discrepancies found. For a subset of our cluster sample, we have investigated the dependence of our results on the assumed pressure profile by repeating the analysis adopting the best-fitting GNFW profile shape which best matches X-ray observations. Adopting the best-fitting profile shape from the X-ray data does not, in general, resolve the discrepancies found in this subset of five clusters. Though based on a small sample, our results suggest that the adopted GNFW model may not be sufficiently flexible to describe clusters universally.
Published: 07 February 2013
Observations of the high-energy sky, particularly with the INTEGRAL satellite, have quadrupled the number of supergiant X-ray binaries observed in the Galaxy and revealed new populations of previously hidden high-mass X-ray binaries (HMXBs), raising new questions about their formation and evolution. The number of detected HMXBs of different types is now high enough to allow us to carry out a statistical analysis of their distribution in the Milky Way. For the first time, we derive the distance and absorption of a sample of HMXBs using a spectral energy distribution fitting procedure, and we examine the correlation with the distribution of star-forming complexes (SFCs) in the Galaxy. We show that HMXBs are clustered with SFCs with a typical cluster size of 0.3 ± 0.05 kpc and a characteristic distance between clusters of 1.7 ± 0.3 kpc. Furthermore, we present an investigation of the expected offset between the position of spiral arms and HMXBs, allowing us to constrain age and migration distance due to supernova kick for 13 sources. These new methods will allow us to assess the influence of the environment on these high-energy objects with unprecedented reliability.
Published: 06 February 2013
Using soft X-ray measurements from detectors onboard the Geostationary Operational Environmental Satellite (GOES) and simultaneous high-cadence Lyman-± observations from the Large Yield Radiometer (LYRA) onboard the Project for On-Board Autonomy 2 (PROBA2) ESA spacecraft, we study the response of the lower part of the ionosphere, the D region, to seven moderate to medium-size solar flares that occurred in February and March of 2010. The ionospheric disturbances are analyzed by monitoring the resulting sub-ionospheric wave propagation anomalies detected by the South America Very Low Frequency (VLF) Network (SAVNET). We find that the ionospheric disturbances, which are characterized by changes of the VLF wave phase, do not depend on the presence of Lyman-A radiation excesses during the flares. Indeed, Lyman-A excesses associated with flares do not produce measurable phase changes. Our results are in agreement with what is expected in terms of forcing of the lower ionosphere by quiescent Lyman-A emission along the solar activity cycle. Therefore, while phase changes using the VLF technique may be a good indicator of quiescent Lyman-A variations along the solar cycle, they cannot be used to scale explosive Lyman-A emission during flares.
Published: 31 January 2013
We present observational evidence of the variation of the cloud-tracked zonal velocity by ~20 m/s with a timescale of a few hundred days in the southern low latitude region based on an analysis of cloud images taken by the Venus Monitoring Camera on board Venus Express. A spectral analysis suggests that the variation has a periodicity with a period of about 255 days. Although cloud features are not always passive tracers, the periodical variation of the dynamical state is a robust feature. Superposed on this long-term variation of the zonal velocity, Kelvin wave-like disturbances tend to be observed in periods of relatively slow background velocity, while Rossby wave-like disturbances tend to be observed in periods of fast background velocity. Since the momentum deposition by these waves can accelerate and decelerate the mean flow, these waves may contribute to the suggested long-term oscillation.
Published: 30 January 2013
From the masses of the planets orbiting the Sun, and the abundance of elements relative to hydrogen, it is estimated that when the Solar System formed, the circumstellar disk must have had a minimum mass of around 0.01 solar masses within about 100 astronomical units of the star. (One astronomical unit is the Earth-Sun distance.) The main constituent of the disk, gaseous molecular hydrogen, does not efficiently emit radiation from the disk mass reservoir, and so the most common measure of the disk mass is dust thermal emission and lines of gaseous carbon monoxide. Carbon monoxide emission generally indicates properties of the disk surface, and the conversion from dust emission to gas mass requires knowledge of the grain properties and the gas-to-dust mass ratio, which probably differ from their interstellar values. As a result, mass estimates vary by orders of magnitude, as exemplified by the relatively old (3-10 million years) star TW Hydrae, for which the range is 0.0005-0.06 solar masses. Here we report the detection of the fundamental rotational transition of hydrogen deuteride from the direction of TW Hydrae. Hydrogen deuteride is a good tracer of disk gas because it follows the distribution of molecular hydrogen and its emission is sensitive to the total mass. The detection of hydrogen deuteride, combined with existing observations and detailed models, implies a disk mass of more than 0.05 solar masses, which is enough to form a planetary system like our own.
Published: 31 January 2013
Pulsars emit from low-frequency radio waves up to high-energy gamma-rays, generated anywhere from the stellar surface out to the edge of the magnetosphere. Detecting correlated mode changes across the electromagnetic spectrum is therefore key to understanding the physical relationship among the emission sites. Through simultaneous observations, we detected synchronous switching in the radio and x-ray emission properties of PSR B0943+10. When the pulsar is in a sustained radio-"bright" mode, the x-rays show only an unpulsed, nonthermal component. Conversely, when the pulsar is in a radio-"quiet" mode, the x-ray luminosity more than doubles and a 100% pulsed thermal component is observed along with the nonthermal component. This indicates rapid, global changes to the conditions in the magnetosphere, which challenge all proposed pulsar emission theories.
Published: 25 January 2013
Stellar winds are a crucial component of massive stars, but their exact properties still remain uncertain. To shed some light on this subject, we have analyzed an exceptional set of X-ray observations of zeta Puppis, one of the closest and brightest massive stars. The sensitive light curves that were derived reveal two major results. On the one hand, a slow modulation of the X-ray flux (with a relative amplitude of up to 15% over 16 hr in the 0.3-4.0 keV band) is detected. Its characteristic timescale cannot be determined with precision, but amounts from one to several days. It could be related to corotating interaction regions, known to exist in zeta Puppis from UV observations. Hour-long changes, linked to flares or to the pulsation activity, are not observed in the last decade covered by the XMM observations; the 17 hr tentative period, previously reported in a ROSAT analysis, is not confirmed either and is thus transient, at best. On the other hand, short-term changes are surprisingly small (<1% relative amplitude for the total energy band). In fact, they are compatible solely with the presence of Poisson noise in the data. This surprisingly low level of short-term variability, in view of the embedded wind-shock origin, requires a very high fragmentation of the stellar wind, for both absorbing and emitting features (>105 parcels, comparing with a two-dimensional wind model). This is the first time that constraints have been placed on the number of clumps in an O-type star wind and from X-ray observations.
Published: 17 January 2013
Context. Chromospheres and coronae are common phenomena on solar-type stars. Understanding the energy transfer to these heated atmospheric layers requires direct access to the relevant empirical data. Study of these structures has, by and large, been limited to the Sun thus far.

Aims. The region of the temperature reversal can be directly observed only in the far infrared and submillimetre spectral regime. We aim at determining the characteristics of the atmosphere in the region of the temperature minimum of the solar sister star alpha Cen A. As a bonus this will also provide a detailed mapping of the spectral energy distribution, i.e. knowledge that is crucial when searching for faint, Kuiper belt-like dust emission around other stars.

Methods. For the nearby binary system alpha Cen, stellar parameters are known with high accuracy from measurements. For the basic model parameters T_eff, log g and [Fe/H], we interpolate stellar model atmospheres in the grid of Gaia/PHOENIX and compute the corresponding model for the G2 V star alpha Cen A. Comparison with photometric measurements shows excellent agreement between observed photospheric data in the optical and infrared. For longer wavelengths, the modelled spectral energy distribution is compared to Spitzer-MIPS, Herschel-PACS, Herschel-SPIRE, and APEX-LABOCA photometry. A specifically tailored Uppsala model based on the MARCS code and extending further in wavelength is used to gauge the emission characteristics of alpha Cen A in the far infared.
[Abstract abbreviated due to character limitations.]

Published: 10 January 2013
Context. Solar tornados are dynamical, conspicuously helical magnetic structures that are mainly observed as a prominence activity. Aims. We investigate and propose a triggering mechanism for the solar tornado observed in a prominence cavity by SDO/AIA on September 25, 2011. Methods. High-cadence EUV images from the SDO/AIA and the Ahead spacecraft of STEREO/EUVI are used to correlate three flares in the neighbouring active-region (NOAA 11303) and their EUV waves with the dynamical developments of the tornado. The timings of the flares and EUV waves observed on-disk in 195 Å are analysed in relation to the tornado activities observed at the limb in 171 Å. Results. Each of the three flares and its related EUV wave occurred within ten hours of the onset of the tornado. They have an observed causal relationship with the commencement of activity in the prominence where the tornado develops. Tornado-like rotations along the side of the prominence start after the second flare. The prominence cavity expands with the accelerating tornado motion after the third flare. Conclusions. Flares in the neighbouring active region may have affected the cavity prominence system and triggered the solar tornado. A plausible mechanism is that the active-region coronal field contracted by the Hudson effect through the loss of magnetic energy as flares. Subsequently, the cavity expanded by its magnetic pressure to fill the surrounding low corona. We suggest that the tornado is the dynamical response of the helical prominence field to the cavity expansion.
Published: 07 January 2013
Formation and motion (at the initial stage) of six limb CMEs detected in the period June 2010 to June 2011 are investigated using the high-resolution data of the PROBA2 and SDO spacecraft combined with the data of SOHO/LASCO coronagraphs. It is demonstrated that several loop-like structures of enhanced brightness originate in the region of CME formation, and they move one after another with, as a rule, different velocities. These loop-like structures in the final analysis form the frontal structure of CME. Time dependences of the velocity and acceleration of the ejection's front are obtained for all CMEs under consideration. A conclusion is drawn about possible existence of two classes of CMEs depending on their velocity time profiles. Ejections, whose velocity after reaching its maximum sharply drops by a value of more than 100 km/s and then goes over into a regime of slow change, belong to the first class. Another class of CMEs is formed by ejections whose velocity changes slowly immediately after reaching the maximum. It is demonstrated that the CME's angular dimension increases at the initial stage of ejection motion up to a factor of 3 with a time scale of doubling the angular size value within the limits 3.5-11 min since the moment of the first measurement of this parameter of an ejection. For three CMEs it is shown that at the initial stage of their motion for a certain time interval they are stronger expanded than grow in the longitude direction.
Published: 01 January 2013
24-Nov-2020 10:02 UT

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