Publication archive

Publication archive

We report on the X-ray observation of a strong lensing selected group, SL2S J08544-0121, with a total mass of 2.4 ± 0.6 × 1014 MSun which revealed a separation of 124 ± 20 kpc between the X-ray emitting collisional gas and the collisionless galaxies and dark matter (DM), traced by strong lensing. This source allows to put an order of magnitude estimate to the upper limit to the interaction cross-section of DM of 10 cm2 g−1. It is the lowest mass object found to date showing a DM–baryons separation, and it reveals that the detection of bullet-like objects is not rare and confined to mergers of massive objects opening the possibility of a statistical detection of DM–baryons separation with future surveys.
Published: 07 June 2014
X-ray emission from stars much more massive than the Sun was discovered only 35 years ago. Such stars drive fast stellar winds where shocks can develop, and it is commonly assumed that the X-rays emerge from the shock-heated plasma. Many massive stars additionally pulsate. However, hitherto it was neither theoretically predicted nor observed that these pulsations would affect their X-ray emission. All X-ray pulsars known so far are associated with degenerate objects, either neutron stars or white dwarfs. Here we report the discovery of pulsating X-rays from a non-degenerate object, the massive B-type star Xi1 CMa. This star is a variable of beta Cep-type and has a strong magnetic field. Our observations with the X-ray Multi-Mirror (XMM-Newton) telescope reveal X-ray pulsations with the same period as the fundamental stellar oscillations. This discovery challenges our understanding of stellar winds from massive stars, their X-ray emission and their magnetism.
Published: 04 June 2014
Anomalous microwave emission (AME) is believed to be due to electric dipole radiation from small spinning dust grains. The aim of this paper is a statistical study of the basic properties of AME regions and the environment in which they emit. We used WMAP and Planck maps, combined with ancillary radio and IR data, to construct a sample of 98 candidate AME sources, assembling SEDs for each source using aperture photometry on 1°-smoothed maps from 0.408 GHz up to 3000 GHz. Each spectrum is fitted with a simple model of free-free, synchrotron (where necessary), cosmic microwave background (CMB), thermal dust, and spinning dust components. We find that 42 of the 98 sources have significant (>5σ) excess emission at frequencies between 20 and 60 GHz. An analysis of the potential contribution of optically thick free-free emission from ultra-compact H II regions, using IR colour criteria, reduces the significant AME sample to 27 regions. The spectrum of the AME is consistent with model spectra of spinning dust. Peak frequencies are in the range 20−35 GHz except for the California nebula (NGC 1499), which appears to have a high spinning dust peak frequency of (50 ± 17) GHz. The AME regions tend to be more spatially extended than regions with little or no AME. The AME intensity is strongly correlated with the sub-millimetre/IR flux densities and comparable to previous AME detections in the literature. AME emissivity, defined as the ratio of AME to dust optical depth, varies by an order of magnitude for the AME regions. The AME regions tend to be associated with cooler dust in the range 14−20 K and an average emissivity index, βd, of +1.8, while the non-AME regions are typically warmer, at 20−27 K. In agreement with previous studies, the AME emissivity appears to decrease with increasing column density.
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Published: 21 May 2014
We report on the first observation of a single hybrid magnetic structure that contains both a pseudostreamer and a double streamer. This structure was originally observed by the SWAP instrument on board the PROBA2 satellite between 2013 May 5 and 10. It consists of a pair of filament channels near the south pole of the Sun. On the western edge of the structure, the magnetic morphology above the filaments is that of a side-by-side double streamer, with open field between the two channels. On the eastern edge, the magnetic morphology is that of a coronal pseudostreamer without the central open field. We investigated this structure with multiple observations and modeling techniques. We describe the topology and dynamic consequences of such a unified structure.
Published: 21 May 2014
This document was submitted by the Athena team in response to the Call for mission concepts for the large-size 'L2' mission opportunity in ESA's Science Programme issued in January 2014. This document is provided for information to the science community.
Published: 02 April 2014
We report on two regularly rotating galaxies at redshift z ~ 2, using high-resolution spectra of the bright [C II] 158 μm emission line from the HIFI instrument on the Herschel Space Observatory. Both SDSS090122.37+181432.3 ("S0901") and SDSSJ120602.09+514229.5 ("the Clone") are strongly lensed and show the double-horned line profile that is typical of rotating gas disks. Using a parametric disk model to fit the emission line profiles, we find that S0901 has a rotation speed of vsin (i) ~ 120 ± 7 km s-1 and a gas velocity dispersion of sigmag < 23 km s-1 (1 sigma). The best-fitting model for the Clone is a rotationally supported disk having vsin (i) ~ 79 ± 11 km s-1 and sigmag 4 km s-1 (1 sigma). However, the Clone is also consistent with a family of dispersion-dominated models having sigmag = 92 ± 20 km s-1. Our results showcase the potential of the [C II] line as a kinematic probe of high-redshift galaxy dynamics: [C II] is bright, accessible to heterodyne receivers with exquisite velocity resolution, and traces dense star-forming interstellar gas. Future [C II] line observations with ALMA would offer the further advantage of spatial resolution, allowing a clearer separation between rotation and velocity dispersion.
Published: 30 April 2014
Galaxy mergers play a key role in the evolution of galaxies and the growth of their central supermassive black holes (SMBHs). A search for (active) SMBH binaries (SMBHBs) at the centers of the merger remnants is currently ongoing. Perhaps the greatest challenge is to identify the inactive SMBHBs, which might be the most abundant, but are also the most difficult to identify. Liu et al. predicted characteristic drops in the light curves of tidal disruption events (TDEs), caused by the presence of a secondary SMBH. Here, we apply that model to the light curve of the optically inactive galaxy SDSS J120136.02+300305.5, which was identified as a candidate TDE with XMM-Newton. We show that the deep dips in its evolving X-ray light curve can be well explained by the presence of a SMBHB at its core. A SMBHB model with a mass of the primary of MBH = 107 Msun, a mass ratio q=0.08, and a semimajor axis ab = 0.6 mpc is in good agreement with the observations. Given that primary mass, introducing an orbital eccentricity is needed, with eb = 0.3. Alternatively, a lower mass primary of MBH = 106 Msun in a circular orbit fits the light curve well. Tight binaries like this one, which have already overcome the "final parsec problem," are prime sources of gravitational wave radiation once the two SMBHs coalesce. Future transient surveys, which will detect TDEs in large numbers, will place tight constraints on the SMBHB fraction in otherwise non-active galaxies.
Published: 23 April 2014
We use Planck HFI data combined with ancillary radio data to study the emissivity index of the interstellar dust emission in the frequency range 100–353 GHz, or 3–0.8 mm, in the Galactic plane. We analyse the region l = 20°–44° and |b| ≤ 4° where the free-free emission can be estimated from radio recombination line data. We fit the spectra at each sky pixel with a modified blackbody model and two opacity spectral indices, βmm and βFIR, below and above 353 GHz, respectively. We find that βmm is smaller than βFIR, and we detect a correlation between this low frequency power-law index and the dust optical depth at 353 GHz, τ353. The opacity spectral index βmm increases from about 1.54 in the more diffuse regions of the Galactic disk, |b| = 3°–4° and τ353 ~ 5 × 10-5, to about 1.66 in the densest regions with an optical depth of more than one order of magnitude higher. We associate this correlation with an evolution of the dust emissivity related to the fraction of molecular gas along the line of sight. This translates into βmm ~ 1.54 for a medium that is mostly atomic and βmm ~ 1.66 when the medium is dominated by molecular gas. We find that both the two-level system model and magnetic dipole emission by ferromagnetic particles can explain the results. These results improve our understanding of the physics of interstellar dust and lead towards a complete model of the dust spectrum of the Milky Way from far-infrared to millimetre wavelengths.
Published: 05 April 2014
The small and active Saturnian moon Enceladus is one of the primary targets of the Cassini mission. We determined the quadrupole gravity field of Enceladus and its hemispherical asymmetry using Doppler data from three spacecraft flybys. Our results indicate the presence of a negative mass anomaly in the south-polar region, largely compensated by a positive subsurface anomaly compatible with the presence of a regional subsurface sea at depths of 30 to 40 kilometers and extending up to south latitudes of about 50°. The estimated values for the largest quadrupole harmonic coefficients (106J2 = 5435.2 ± 34.9, 106C22 = 1549.8 ± 15.6, 1 sigma) and their ratio (J2/C22 = 3.51 ± 0.05) indicate that the body deviates mildly from hydrostatic equilibrium. The moment of inertia is around 0.335MR2, where M is the mass and R is the radius, suggesting a differentiated body with a low-density core.
Published: 04 April 2014
We construct the depth profile – the bathymetry – of Titan's large sea Ligeia Mare from Cassini RADAR data collected during the 23 May 2013 (T91) nadir-looking altimetry flyby. We find the greatest depth to be about 160 m and a seabed slope that is gentler toward the northern shore, consistent with previously imaged shoreline morphologies. Low radio signal attenuation through the sea demonstrates that the liquid, for which we determine a loss tangent of 3 ± 1∙10−5, is remarkably transparent, requiring a nearly pure methane-ethane composition, and further that microwave absorbing hydrocarbons, nitriles, and suspended particles be limited to less than the order of 0.1% of the liquid volume. Presence of nitrogen in the ethane-methane sea, expected based on its solubility and dominance in the atmosphere, is consistent with the low attenuation, but that of substantial dissolved polar species or suspended scatterers is not.
Published: 16 March 2014
Published online 24 February 2014

The nature and origin of the cold interstellar medium (ISM) in early-type galaxies are still a matter of debate, and understanding the role of this component in galaxy evolution and in fuelling the central supermassive black holes requires more observational constraints. Here, we present a multiwavelength study of the ISM in eight nearby, X-ray and optically bright, giant elliptical galaxies, all central dominant members of relatively low-mass groups. Using far-infrared spectral imaging with the Herschel Photodetector Array Camera & Spectrometer, we map the emission of cold gas in the cooling lines of [C II] 157 μm, [O I] 63 μm and [O Ib] 145 μm. Additionally, we present H-alpha+[N II] imaging of warm ionized gas with the Southern Astrophysical Research (SOAR) telescope, and a study of the thermodynamic structure of the hot X-ray emitting plasma with Chandra. All systems with extended H-alpha emission in our sample (6/8 galaxies) display significant [C II] line emission indicating the presence of reservoirs of cold gas. This emission is cospatial with the optical H-alpha+[N II] emitting nebulae and the lowest entropy soft X-ray emitting plasma. The entropy profiles of the hot galactic atmospheres show a clear dichotomy, with the systems displaying extended emission-line nebulae having lower entropies beyond r >= 1 kpc than the cold-gas-poor systems. We show that while the hot atmospheres of the cold-gas-poor galaxies are thermally stable outside of their innermost cores, the atmospheres of the cold-gas-rich systems are prone to cooling instabilities. This provides considerable weight to the argument that cold gas in giant ellipticals is produced chiefly by cooling from the hot phase. We show that cooling instabilities may develop more easily in rotating systems and discuss an alternative condition for thermal instability for this case.
[Remainder of abstract truncated due to character limitations]

Published: 25 February 2014
The paper presents first results of the data-based modeling of the geomagnetospheric magnetic field, using the data of Polar, Geotail, Cluster, and Time History of Events and Macroscale Interactions during Substorms satellites, taken during the period 1995-2012 and covering 123 storm events with SYM-H > -200 nT. The most important innovations in the model are (1) taking into account the interplanetary magnetic field (IMF)-dependent shape of the model magnetopause, (2) a physically more consistent global deformation of the equatorial current sheet due to the geodipole tilt, (3) symmetric and partial components of the ring current are calculated based on a realistic background magnetic field, instead of a purely dipolar field, used in earlier models, and (4) the validity region on the nightside is extended to ~ 40-50 RE. The model field is confined within a magnetopause, based on Lin et al. (2010) empirical model, driven by the dipole tilt angle, solar wind pressure, and IMF Bz. A noteworthy finding is a significant dependence of the magnetotail flux connection across the equatorial plane on the model magnetopause flaring rate, controlled by the southward component of the IMF.
Published: 23 January 2014
The 'snowline' conventionally divides Solar System objects into dry bodies, ranging out to the main asteroid belt, and icy bodies beyond the belt. Models suggest that some of the icy bodies may have migrated into the asteroid belt. Recent observations indicate the presence of water ice on the surface of some asteroids, with sublimation a potential reason for the dust activity observed on others. Hydrated minerals have been found on the surface of the largest object in the asteroid belt, the dwarf planet (1) Ceres, which is thought to be differentiated into a silicate core with an icy mantle. The presence of water vapour around Ceres was suggested by a marginal detection of the photodissociation product of water, hydroxyl, but could not be confirmed by later, more sensitive observations. Here we report the detection of water vapour around Ceres, with at least 1026 molecules being produced per second, originating from localized sources that seem to be linked to mid-latitude regions on the surface. The water evaporation could be due to comet-like sublimation or to cryo-volcanism, in which volcanoes erupt volatiles such as water instead of molten rocks.
Published: 22 January 2014
The Euclid spacecraft industry day was held on 15 January 2014 at the European Space Research and Technology Centre (ESTEC), the Netherlands. ESA and the Prime Contractor for Euclid, Thales Alenia Space, presented the mission, including its procurement plan and related business opportunities. The book of presentations is available to download here.
Published: 15 January 2014
Using Planck data combined with the Meta Catalogue of X-ray detected Clusters of galaxies (MCXC), we address the study of peculiar motions by searching for evidence of the kinetic Sunyaev-Zeldovich effect (kSZ). By implementing various filters designed to extract the kSZ generated at the positions of the clusters, we obtain consistent constraints on the radial peculiar velocity average, root mean square (rms), and local bulk flow amplitude at different depths. For the whole cluster sample of average redshift 0.18, the measured average radial peculiar velocity with respect to the cosmic microwave background (CMB) radiation at that redshift, i.e., the kSZ monopole, amounts to 72 ± 60 km s-1. This constitutes less than 1% of the relative Hubble velocity of the cluster sample with respect to our local CMB frame. While the linear ΛCDM prediction for the typical cluster radial velocity rms at z = 0.15 is close to 230 km s-1, the upper limit imposed by Planck data on the cluster subsample corresponds to 800 km s-1 at 95% confidence level, i.e., about three times higher. Planck data also set strong constraints on the local bulk flow in volumes centred on the Local Group. There is no detection of bulk flow as measured in any comoving sphere extending to the maximum redshift covered by the cluster sample. A blind search for bulk flows in this sample has an upper limit of 254 km s-1 (95% confidence level) dominated by CMB confusion and instrumental noise, indicating that the Universe is largely homogeneous on Gpc scales. In this context, in conjunction with supernova observations, Planck is able to rule out a large class of inhomogeneous void models as alternatives to dark energy or modified gravity. The Planck constraints on peculiar velocities and bulk flows are thus consistent with the ΛCDM scenario.
Published: 13 January 2014

Available online 24 September 2013

High resolution images of Venus Northern hemisphere obtained with the Venus Monitoring Camera (VMC/VEx) allow studying small-scale dynamical phenomena at the cloud tops (~62-70 km altitude) including features like wave trains. A systematic visual search of these waves was performed; more than 1500 orbits were analyzed and wave patterns were observed in more than 300 images. Four types of waves were identified in VMC images on the base of their morphology: long, medium, short and irregular type waves. With the aim to characterize the wave types and their possible excitation source, we retrieved wave properties such as location (latitude and longitude), local time, solar zenith angle, packet length and width, orientation, and wavelength of each wave. The long type waves appear as long and narrow straight features extending more than a few hundreds kilometers and with wavelengths between 7 and 17 km. Medium type waves exhibit irregular wavefronts extending more than 100 km and with wavelengths in the range 8-21 km. Short wave packets have a width of several tens of kilometers and extend to few hundreds kilometers and are characterized by smaller wavelengths (3-16 km). Irregular wave fields appear to be the result of wave interference. The waves are often identified in all VMC filters and are mostly found in the cold collar region at high latitudes (60-80°N) and are concentrated above Ishtar Terra, a continental size highland that includes the highest mountain belts of the planet. The high speed of the Venus Express spacecraft close to the pericentre does not allow to measure phase speed of waves due to the short temporal interval between image pairs. [Remainder of abstract truncated due to character limitation]

Published: 01 January 2014

Reference: ESA/SRE(2013)7

This Definition Study Report (also known as the Red Book) presents the outcome of the CHEOPS Definition study. It describes the resulting mission concept that will fulfil the mission science requirements, and therefore allow us to achieve the science objectives presented in the original proposal, and further detailed and elaborated in this document.

Published: 01 November 2013

Reference: ECHO-SRE-SA-PHASEA-010, Issue 2

A Design Reference Mission, hereafter the EChO Core Survey, has been constructed for the Exoplanet Characterisation Observatory in order to determine the mission lifetime required to fulfil the science requirements. The EChO science requirements call for the observation of at least 100 exoplanets of diverse type and environment to provide a “Chemical Census” of exoplanet atmospheres. A proportion of these are required to be observed at high signal to noise ratio to provide an Origin sample and “Rosetta Stones” giving a deeper understanding of the physics and chemistry of their atmospheres. Two target samples lists have been derived: one using catalogues of real targets as known today, and a second using a statistical approach to predict how many targets will be available by the time EChO is operational in the 2020’s. These lists have been evaluated using mission performance models to test the observing time required to fulfil the EChO Core Survey. We find that a nominal mission lifetime of four years is sufficient to fulfil the science requirements and a mission of six years will fulfil the ambitious goals for EChO. The use of separate target lists and performance models gives confidence that the Core Survey can be undertaken within the mission lifetime and that will result in a revolution in understanding the origin and evolution of planets.

Published: 18 December 2013

Reference: SRE-PA/2011.040/, Issue 3, Revision 2

This document provides a detailed description of the ESA EChO radiometric model. This model will be used to validate the mission requirements, which have, in turn, been derived from the science requirements. The model provides the means to calculate, for a given host star/exoplanet target:

  • The SNR that can be achieved in a single primary transit
  • The SNR that can be achieved in a single occultation
  • The number of transit/occultation revisits necessary to achieve a specified SNR
  • The total number of revisits that could be achieved during the proposed mission lifetime.

The radiometric model will be used to establish whether proposed samples of known or model targets can be observed to the signal-to-noise ratio (SNR) called for in the Science Requirements Document, with mission requirements given in the Mission Requirements Document (i.e. within the mission lifetime, with the observation efficiency required and the minimum design requirements), and to confirm the minimum design requirements for the mission.

Published: 17 December 2013

Reference: ESA-STEQ-RP-0001, Issue 1, Revision 1

STE-QUEST, a Space-Time Explorer and Quantum Equivalence Principle Space Test, is one of five candidate missions for the M3 launch opportunity. A Preliminary Requirements Review (PRR) of all candidate missions has been performed to review their status in support of the M3 selection. This document reports the results of the technical and programmatic review for the STE-QUEST mission candidate.

Published: 16 December 2013
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