Showing papers by "Netherlands Institute for Space Research published in 2008"

The c2d Spitzer spectroscopic survey of ices around low-mass young stellar objects. I. H2O and the 5-8 mu m bands

Abstract: To study the physical and chemical evolution of ices in solar-mass systems, a spectral survey is conducted of a sample of 41 low-luminosity YSOs (L ~ 0.1 - 10 L☉) using 3 - 38 μm Spitzer and ground-based spectra. The sample is complemented with previously published Spitzer spectra of background stars and with ISO spectra of well-studied massive YSOs (L ~ 10⁵ L☉). The long-known 6.0 and 6.85 μm bands are detected toward all sources, with the Class 0-type YSOs showing the deepest bands ever observed. The 6.0 μm band is often deeper than expected from the bending mode of pure solid H₂O. The additional 5 - 7 μm absorption consists of five independent components, which, by comparison to laboratory studies, must be from at least eight different carriers. Much of this absorption is due to simple species likely formed by grain surface chemistry, at abundances of 1%-30% for CH₃OH, 3%-8% for NH₃, 1%-5% for HCOOH, ~6% for H₂CO, and ~0.3% for HCOO⁻ relative to solid H₂O. The 6.85 μm band has one or two carriers, of which one may be less volatile than H₂O. Its carrier(s) formed early in the molecular cloud evolution and do not survive in the diffuse ISM. If an NH4^+-containing salt is the carrier, its abundance relative to solid H₂O is ~7%, demonstrating the efficiency of low-temperature acid-base chemistry or cosmic-ray-induced reactions. Possible origins are discussed for enigmatic, very broad absorption between 5 and 8 μm. Finally, the same ices are observed toward massive and low-mass YSOs, indicating that processing by internal UV radiation fields is a minor factor in their early chemical evolution.

Large tundra methane burst during onset of freezing.

Abstract: Terrestrial wetland emissions are the largest single source of the greenhouse gas methane. Northern high-latitude wetlands contribute significantly to the overall methane emissions from wetlands, but the relative source distribution between tropical and high-latitude wetlands remains uncertain. As a result, not all the observed spatial and seasonal patterns of atmospheric methane concentrations can be satisfactorily explained, particularly for high northern latitudes. For example, a late-autumn shoulder is consistently observed in the seasonal cycles of atmospheric methane at high-latitude sites, but the sources responsible for these increased methane concentrations remain uncertain. Here we report a data set that extends hourly methane flux measurements from a high Arctic setting into the late autumn and early winter, during the onset of soil freezing. We find that emissions fall to a low steady level after the growing season but then increase significantly during the freeze-in period. The integral of emissions during the freeze-in period is approximately equal to the amount of methane emitted during the entire summer season. Three-dimensional atmospheric chemistry and transport model simulations of global atmospheric methane concentrations indicate that the observed early winter emission burst improves the agreement between the simulated seasonal cycle and atmospheric data from latitudes north of 60 degrees N. Our findings suggest that permafrost-associated freeze-in bursts of methane emissions from tundra regions could be an important and so far unrecognized component of the seasonal distribution of methane emissions from high latitudes.

TransCom model simulations of hourly atmospheric CO2 : Analysis of synoptic-scale variations for the period 2002-2003

Abstract: The ability to reliably estimate CO2 fluxes from current in situ atmospheric CO2 measurements and future satellite CO2 measurements is dependent on transport model performance at synoptic and shorter timescales The TransCom continuous experiment was designed to evaluate the performance of forward transport model simulations at hourly, daily, and synoptic timescales, and we focus on the latter two in this paper Twenty-five transport models or model variants submitted hourly time series of nine predetermined tracers (seven for CO2) at 280 locations We extracted synoptic-scale variability from daily averaged CO2 time series using a digital filter and analyzed the results by comparing them to atmospheric measurements at 35 locations The correlations between modeled and observed synoptic CO2 variabilities were almost always largest with zero time lag and statistically significant for most models and most locations Generally, the model results using diurnally varying land fluxes were closer to the observations compared to those obtained using monthly mean or daily average fluxes, and winter was often better simulated than summer Model results at higher spatial resolution compared better with observations, mostly because these models were able to sample closer to the measurement site location The amplitude and correlation of model-data variability is strongly model and season dependent Overall similarity in modeled synoptic CO2 variability suggests that the first-order transport mechanisms are fairly well parameterized in the models, and no clear distinction was found between the meteorological analyses in capturing the synoptic-scale dynamics

Tropical methane emissions: A revised view from SCIAMACHY onboard ENVISAT

Abstract: Methane retrievals from near-infrared spectra recorded by the SCIAMACHY instrument onboard ENVISAT hitherto suggested unexpectedly large tropical emissions Even though recent studies confirm substantial tropical emissions, there were indications for an unresolved error in the satellite retrievals Here we identify a retrieval error related to inaccuracies in water vapor spectroscopic parameters, causing a substantial overestimation of methane correlated with high water vapor abundances We report on the overall implications of an update in water spectroscopy on methane retrievals with special focus on the tropics where the impact is largest The new retrievals are applied in a four-dimensional variational (4D-VAR) data assimilation system to derive a first estimate of the impact on tropical CH_4 sources Compared to inversions based on previous SCIAMACHY retrievals, annual tropical emission estimates are reduced from 260 to about 201 Tg CH_4 but still remain higher than previously anticipated

The c2d Spitzer spectroscopic survey of ices around low-mass young stellar objects. III. CH4

Abstract: CH4 is proposed to be the starting point of a rich organic chemistry. Solid CH4 abundances have previously been determined mostly toward high-mass star-forming regions. Spitzer IRS now provides a unique opportunity to probe solid CH4 toward low-mass star-forming regions as well. Infrared spectra from the Spitzer Space Telescope are presented to determine the solid CH4 abundance toward a large sample of low-mass young stellar objects. A total of 25 out of 52 ice sources in the “Cores to Disks” (c2d) Legacy program have an absorption feature at 7.7 μm, attributed to the bending mode of solid CH4. The solid CH4/H2O abundances are 2%–8%, except for three sources with abundances as high as 11%–13%. The latter sources have relatively large uncertainties due to small total ice column densities. Toward sources with H2O column densities above cm−2, the CH4 abundances (20 out of 25) are nearly constant at . Correlation plots with solid H2O, CH3OH, CO2, and CO column densities and abundances relative to H2O reveal a closer relationship of solid CH4 with CO2 and H2O than with solid CO and CH3OH. The inferred solid CH4 abundances are consistent with models where CH4 is formed through sequential hydrogenation of C on grain surfaces. Finally, the equal or higher abundances toward low-mass young stellar objects compared with high-mass objects and the correlation studies support this formation pathway as well, but not the two competing theories: formation from CH3OH and formation in gas phase with subsequent freezeout.

Detection of hot gas in the filament connecting the clusters of galaxies Abell 222 and Abell 223

Abstract: Context. About half of the baryons in the local Universe are invisible and – according to simulations – their dominant fraction resides in filaments connecting clusters of galaxies in the form of low density gas with temperatures in the range of 10 5 < T < 10 7 K. This warm-hot intergalactic medium has never been detected indisputably using X-ray observations. Aims. We aim to probe the low gas densities expected in the large-scale structure filaments by observing a filament connecting the massive clusters of galaxies A 222 and A 223 (z = 0.21), which has a favorable orientation approximately along our line-of-sight. This filament has been previously detected using weak lensing data and as an over-density of colour-selected galaxies. Methods. We analyse X-ray images and spectra obtained from a deep observation (144 ks) of A 222/223 with XMM-Newton. Results. We present observational evidence of X-ray emission from the filament connecting the two clusters. We detect the filament in the wavelet-decomposed soft-band (0.5–2.0 keV) X-ray image with a 5σ significance. Following the emission down to the 3σ significance level, the observed filament is ≈1.2 Mpc wide. The temperature of the gas associated with the filament, determined from the spectra, is kT= 0.91±0.25 keV, and its emission measure corresponds to a baryon density of (3.4±1.3)×10 −5 (l/15 Mpc) −1/2 cm −3 , where l is the length of the filament along the line-of-sight. This density corresponds to a baryon over-density of ρ/ � ρC �≈ 150. The properties of the gas in the filament are consistent with results of simulations of the densest and hottest parts of the warm-hot intergalactic medium.

TransCom model simulations of hourly atmospheric CO2: Experimental overview and diurnal cycle results for 2002

Abstract: [1] A forward atmospheric transport modeling experiment has been coordinated by the TransCom group to investigate synoptic and diurnal variations in CO2. Model simulations were run for biospheric, fossil, and air-sea exchange of CO2 and for SF6 and radon for 2000-2003. Twenty-five models or model variants participated in the comparison. Hourly concentration time series were submitted for 280 sites along with vertical profiles, fluxes, and meteorological variables at 100 sites. The submitted results have been analyzed for diurnal variations and are compared with observed CO2 in 2002. Mean summer diurnal cycles vary widely in amplitude across models. The choice of sampling location and model level account for part of the spread suggesting that representation errors in these types of models are potentially large. Despite the model spread, most models simulate the relative variation in diurnal amplitude between sites reasonably well. The modeled diurnal amplitude only shows a weak relationship with vertical resolution across models; differences in near-surface transport simulation appear to play a major role. Examples are also presented where there is evidence that the models show useful skill in simulating seasonal and synoptic changes in diurnal amplitude.

Spectropolarimetric signatures of Earth-like extrasolar planets

Abstract: We present results of numerical simulations of flux F and degree of polarization P of light that is reflected by Earth–like extrasolar planets orbiting solar type stars. Our results are presented as functions of the wavelength (from 0.3 to 1.0 ?m, with 0.001 ?m spectral resolution) and as functions of the planetary phase angle. We use different surface coverages for our model planets, including vegetation and a Fresnel reflecting ocean, and clear and cloudy atmospheres. Our adding-doubling radiative transfer algorithm, which fully includes multiple scattering and polarization, only handles horizontally homogeneous planets, so we simulate fluxes and polarization of horizontally inhomogeneous planets by weighting results obtained for homogeneous planets. Like F, P of the reflected starlight is shown to depend strongly on the phase angle, on the composition and structure of the planetary atmosphere, on the reflective properties of the underlying surface, and on the wavelength, in particular in wavelength regions with gaseous absorption bands. The sensitivity of P to a planet’s physical properties appears to be different than that of F. Combining flux with polarization observations thus makes for a strong tool for characterizing extrasolar planets. The calculated total and polarized fluxes are made available through the CDS.

Emissions of CH4 and N2O over the United States and Canada based on a receptor-oriented modeling framework and COBRA-NA atmospheric observations

Abstract: Reference ARVE-ARTICLE-2009-003doi:10.1029/2008GL034031View record in Web of Science Record created on 2009-04-22, modified on 2016-08-08

Four-dimensional variational data assimilation for inverse modelling of atmospheric methane emissions: method and comparison with synthesis inversion

Abstract: A four-dimensional variational (4D-Var) data assimilation system for inverse modelling of atmospheric methane emissions is presented. The system is based on the TM5 atmospheric transport model. It can be used for assimilating large volumes of measurements, in particular satellite observations and quasi-continuous in-situ observations, and at the same time it enables the optimization of a large number of model parameters, specifically grid-scale emission rates. Furthermore, the variational method allows to estimate uncertainties in posterior emissions. Here, the system is applied to optimize monthly methane emissions over a 1-year time window on the basis of surface observations from the NOAA-ESRL network. The results are rigorously compared with an analogous inversion by Bergamaschi et al. (2007), which was based on the traditional synthesis approach. The posterior emissions as well as their uncertainties obtained in both inversions show a high degree of consistency. At the same time we illustrate the advantage of 4D-Var in reducing aggregation errors by optimizing emissions at the grid scale of the transport model. The full potential of the assimilation system is exploited in Meirink et al. (2008), who use satellite observations of column-averaged methane mixing ratios to optimize emissions at high spatial resolution, taking advantage of the zooming capability of the TM5 model.

Detailed high-energy characteristics of AXP 4U 0142+61: Multi-year observations with INTEGRAL, RXTE, XMM-Newton, and ASCA

Abstract: 4U 0142+61 is one of the Anomalous X-ray Pulsars exhibiting hard X-ray emission above 10 keV discovered with INTEGRAL In this paper we present detailed spectral and temporal characteristics both in the hard X-ray (> 10 keV) and soft X-ray (<10 keV) domains obtained using data from INTEGRAL, XMM-Newton, ASCA and RXTE Accumulating data collected over four years with the imager IBIS-ISGRI aboard INTEGRAL, the time-averaged total spectrum shows a power-law like shape with photon index Gamma = 093 +/- 006 4U 0142+61 is detected up to 229 keV and the flux between 20 keV and 229 keV is (1501 +/- 082) x 10(-11) erg cm(-2) s(-1), which exceeds the energy flux in the 2-10 keV band by a factor of similar to 23 Using simultaneously collected data with the spectrometer SPI of INTEGRAL the combined total spectrum yields the first evidence for a spectral break above 100 keV Assuming a logparabolic function for the spectral shape above 20 keV the peak energy of 4U 0142+61 is 228(-41)(+65) keV There is no evidence for significant long-term time variability of the total emission from 4U 0142+61 Both the total flux and the spectral index are stable within the 17% level (1 sigma) Pulsed emission is measured with ISGRI up to 160 keV The 20-160 keV profile shows a broad double-peaked pulse with a 62 sigma detection significance The total pulsed spectrum can be described with a very hard power-law shape with a photon index G = 040 +/- 015 and a 20-150 keV flux of (268 +/- 134) x 10(-11) erg cm(-2) s(-1) To perform accurate phase-resolved spectroscopy over the total X-ray window, we produced pulse profiles in absolute phase for INTEGRAL-ISGRI, RXTE-PCA, XMM-Newton-PN and ASCA-GIS The two known pulses in all soft X-ray profiles below 10 keV are located in the same phases Three XMM-Newton observations in 2003-2004 show statistically identical profiles However, we find a significant profile morphology change between an ASCA-GIS observation in 1999 following a possible glitch of 4U 0142+61 This change can be accounted for by differences in relative strengths and spectral shapes (08-10 keV) of the two pulses The principle peak in the INTEGRAL pulse profile above 20 keV is located at the same phase as one of the pulses detected below 10 keV The second pulse detected with INTEGRAL is slightly shifted with respect to the second peak observed in the soft X-ray band We performed consistent phase-resolved spectroscopy over the total high-energy band and identify at least three genuinely different pulse components with different spectra The high level of consistency between the detailed results from the four missions is indicative of a remarkably stable geometry underlying the emission scenario Finally, we discuss the derived detailed characteristics of the high-energy emission of 4U 0142+61 in relation to three models for the non-thermal hard X-ray emission

X-ray time variability across the atoll source states of 4U 1636-53

Abstract: We have studied the rapid X-ray time variability in 149 pointed observations with the Rossi X-Ray Timing Explorer (RXTE)'s Proportional Counter Array of the atoll source 4U 1636-53 in the banana state and, for the first time with RXTE, in the island state. We compare the frequencies of the variability components of 4U 1636-53 with those in other atoll and Z sources and find that 4U 1636-53 follows the universal scheme of correlations previously found for other atoll sources at (sometimes much) lower luminosities. Our results on the hectohertz QPO suggest that the mechanism that sets its frequency differs from that for the other components, while the amplitude-setting mechanism is common. A previously proposed interpretation of the narrow low-frequency QPO frequencies in different sources in terms of harmonic mode switching is not supported by our data or by previous data on other sources, and the frequency range that this QPO covers is found to be unrelated to spin, angular momentum, or luminosity.

Detailed high-energy characteristics of AXP 4U 0142+61 - Multi-year observations with INTEGRAL, RXTE, XMM-Newton and ASCA

Abstract: We present detailed spectral and temporal characteristics both in the hard X-ray (>10 keV) and soft X-ray (<10 keV) domains, obtained using data from INTEGRAL, XMM-Newton, ASCA and RXTE. The INTEGRAL time-averaged total spectrum shows a power-law like shape with photon index Gamma = 0.93 +/- 0.06. 4U 0142+61 is detected up to 229 keV and the flux between 20 keV and 229 keV is (15.01 +/- 0.82) x 10^(-11) erg/cm^2/s. Using simultaneously collected data with the spectrometer SPI of INTEGRAL the combined total spectrum yields the first evidence for a spectral break with a peak energy of 228 +65/-41 keV. There is no evidence for significant long-term time variability of the total emission. Pulsed emission is measured with ISGRI up to 160 keV. The 20-160 keV profile shows a broad double-peaked pulse with a 6.2 sigma detection significance. The total pulsed spectrum can be described with a very hard power-law shape with a photon index \Gamma = 0.40 +/- 0.15. We performed phase-resolved spectroscopy over the total high-energy band (2.8-300 keV) and identify at least three genuinely different pulse components with different spectra. The high level of consistency between the detailed results from the four missions is indicative for a remarkable stable geometry underlying the emission scenario.

Pressure broadening in the 2ν 3 band of methane and its implication on atmospheric retrievals

Abstract: N 2 -broadened half widths and pressure shifts were obtained for transitions in the 2ν 3 methane band. Laboratory measurements recorded at 0.011 cm −1 resolution with a Bruker 120 HR Fouriertransform spectrometer were analysed from 5860 to 6185 cm −1 . A 140 cm gas cell was filled with methane at room temperature and N 2 as foreign gas at pressures ranging from 125 to 900 hPa. A multispectrum nonlinear constrained least squares approach based on Optimal Estimation was applied to derive the spectroscopic parameters by simultaneously fitting laboratory spectra at different ambient pressures assuming a Voigt line-shape. At room temperature, the half widths ranged between 0.030 and 0.071 cm −1 atm −1 , and the pressure shifts varied from –0.002 to –0.025 cm −1 atm −1 for transitions up to J"=10. Especially for higher rotational levels, we find systematically narrower lines than HITRAN predicts. The Q and R branch of the new set of spectroscopic parameters is further tested with ground based direct sun Fourier transform infrared (FTIR) measurements where systematic fit residuals reduce by about a factor of 3–4. We report the implication of those differences on atmospheric methane measurements using high-resolution ground based FTIR measurements as well as low-resolution spectra from the SCanning Imaging Absorption SpectroMeter for Atmospheric ChartographY (SCIAMACHY) instrument onboard ENVISAT. We find that for SCIAMACHY, a latitudinal and seasonally varying bias of about 1% can be introduced by erroneous broadening parameters.

Magnetic field measurements of O stars with FORS 1 at the VLT.

Abstract: Context. The presence of magnetic fields in O-type stars has been suspected for a long time. The discovery of these fields would explain a wide range of well documented enigmatic phenomena in massive stars, in particular cyclical wind variability, Ha emission variations, chemical peculiarity, narrow X-ray emission lines, and non-thermal radio/X-ray emission. Aims. To investigate the incidence of magnetic fields in O stars, we acquired 38 new spectropolarimetric observations with FORS 1 (FOcal Reducer low dispersion Spectrograph) mounted on the 8-m Kueyen telescope of the VLT. Methods. Spectropolarimetric observations were obtained at different phases for a sample of 13 O stars. Ten stars were observed in the spectral range 348-589 nm, HD36879 and HD148937 were observed in the spectral region 325-621 nm, and HD155806 was observed in both settings. To prove the feasibility of the FORS1 spectropolarimetric mode for the measurements of magnetic fields in hot stars, we present in addition 12 FORS 1 observations of the mean longitudinal magnetic field in theta(1) OriC and compare them with measurements obtained with the MuSiCoS, ESPaDOnS, and Narval spectropolarimeters. Results. Most stars in our sample, which were observed on different nights, show a change of the magnetic field polarity, but a field at a significance level of 3 sigma was detected in only four stars, HD36879, HD148937, HD152408, and HD164794. The largest longitudinal magnetic field, = -276 +/- 88 G, was detected in the Of?p star HD148937. We conclude that large-scale organized magnetic fields with polar field strengths larger than 1 kG are not widespread among O-type stars.

Four-dimensional variational data assimilation for inverse modeling of atmospheric methane emissions: Analysis of SCIAMACHY observations

Abstract: Recent observations from the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) instrument aboard ENVISAT have brought new insights in the global distribution of atmospheric methane. In particular, the observations showed higher methane concentrations in the tropics than previously assumed. Here, we analyze the SCIAMACHY observations and their implications for emission estimates in detail using a four-dimensional variational (4D-Var) data assimilation system. We focus on the period September to November 2003 and on the South American continent, for which the satellite observations showed the largest deviations from model simulations. In this set-up the advantages of the 4D-Var approach and the zooming capability of the underlying TM5 atmospheric transport model are fully exploited. After application of a latitude-dependent bias correction to the SCIAMACHY observations, the assimilation system is able to accurately fit those observations, while retaining consistency with a network of surface methane measurements. The main emission increments resulting from the inversion are an increase in the tropics, a decrease in South Asia, and a decrease at northern hemispheric high latitudes. The SCIAMACHY observations yield considerable additional emission uncertainty reduction, particularly in the (sub-)tropical regions, which are poorly constrained by the surface network. For tropical South America, the inversion suggests more than a doubling of emissions compared to the a priori during the 3 months considered. Extensive sensitivity experiments, in which key assumptions of the inversion set-up are varied, show that this finding is robust. Independent airborne observations in the Amazon basin support the presence of considerable local methane sources. However, these observations also indicate that emissions from eastern South America may be smaller than estimated from SCIAMACHY observations. In this respect it must be realized that the bias correction applied to the satellite observations does not take into account potential regional systematic errors, which - if identified in the future - will lead to shifts in the overall distribution of emission estimates.

Detailed high-energy characteristics of AXP 1RXS J170849-400910 - Probing the magnetosphere using INTEGRAL, RXTE, and XMM-Newton

Abstract: 1RXS J170849-400910 is one of four anomalous X-ray pulsars which emit persistent luminous radiation in soft X-rays (10 keV) as well as in hard X-rays (>10 keV). In this paper we present detailed spectral and temporal characteristics over the whole X-ray band. For this purpose data have been used from INTEGRAL, RXTE and XMM-Newton. The hard X-ray (> 10 keV) time-averaged total spectrum, accumulated over four years with the imager IBIS-ISGRI onboard INTEGRAL adding up to 5.2 Ms net exposure, can be described by a power law with a photon index Gamma = 1.13 +/- 0.06 and extends to similar to 175 keV. The 20-175 keV flux is (7.76 +/- 0.34) x 10(-11) erg cm(-2) s(-1) which exceeds the 2-10 keV (unabsorbed) flux by a factor of similar to 2.3. No evidence for a spectral break is found below 300 keV. Also, no significant long-term time variability has been detected above 20 keV on time scales of 1 and 0.5 year. Pulsed emission is measured with INTEGRAL up to 270 keV, i.e. to much higher energies than the total emission, with a detection significance of 12.3 sigma (20-270 keV). The pulse profiles from 0.5 keV up to 270 keV show drastic morphology changes below similar to 20 keV. Three different pulse components can be recognized in these pulse profiles: 1) a hard pulse peaking around phase 0.8 which contributes to the pulse profiles above similar to 4 keV; 2) a softer pulse which peaks around phase 0.4 not contributing in the hard X-ray domain and 3) a very soft pulse component below 2 keV. A combined time-averaged pulsed spectrum (2.8-270 keV) from INTEGRAL, RXTE-PCA and HEXTE (collected over nine years) can be described with a soft and a hard power-law component: Gamma(s) = 2.79 +/- 0.07 and Gamma(h) = 0.86 +/- 0.16. In the pulsed spectrum extracted from a 25.5 ks net exposure XMM-Newton observation we find a discontinuity between 2 keV and 3 keV. Above these energies the spectrum is consistent with the spectrum taken with RXTE-PCA. The pulse profiles and the total-pulsed spectrum prove to be stable over the whole nine-years time span over which the data have been taken. Also detailed phase-resolved spectroscopy of the pulsed emission confirms the long-term stability as the spectra taken at different epochs connect smoothly. The phase-resolved spectra reveal complex spectral shapes which do not follow the shape of the total-pulsed spectrum. The spectral shape gradually changes with phase from a soft single power law to a complex multi-component shape and then to a hard single power law. The spectrum switches from a very hard (Gamma = 0.99 +/- 0.05) to a very soft (Gamma = 3.58 +/- 0.34) single power-law shape within a 0.1-wide phase interval. The discontinuity measured between 2 keV and 3 keV with XMM-Newton is a result of a curved component. This component which is most apparent within phase interval 0.7-0.9 significantly contributes in the energy range between 4 keV and 20 keV. It has a very steep spectrum below 5 keV with a photon index Gamma similar to - 1.5. From the phase-resolved spectra we identify three independent components with different spectral shapes which together can accurately describe all phase-resolved spectra (2.8-270 keV). The three shapes are a soft power law (Gamma = 3.54), a hard power law (G = 0.99) and a curved shape (described with two logparabolic functions). The phase distributions of the normalizations of these spectral components form three decoupled pulse profiles. The soft component peaks around phase 0.4 while the other two components peak arund phase 0.8. The width of the curved component (similar to 0.25 in phase) is about half the width of the hard component. After 4U 0142+61, 1RXS J1708-40 is the second anomalous X-ray pulsar for which such detailed phase-resolved spectroscopy has been performed. These results give important constraints showing that three dimensional modeling covering both the geometry and different production processes is required to explain our findings.

First superburst from a classical low-mass X-ray binary transient

Abstract: We report the analysis of the first superburst from a transiently accreting neutron star system with the All-Sky Monitor (ASM) on the Rossi X-ray Timing Explorer. The superburst occurred 55 days after the onset of an accretion outburst in 4U 1608-522. During that time interval, the accretion rate was at least 7% of the Eddington limit. The peak flux of the superburst is 22 to 45% of the Eddington limit, and its radiation energy output is between 4 × 10 41 and 9 × 10 41 erg for a distance of 3.2 kpc. Fits of cooling models to the superburst light curve indicate an ignition column depth between 1.5 × 10 12 and 4.1 × 10 12 gc m −2 . Extrapolating the accretion history observed by the ASM, we derive that this column was accreted over a period of 26 to 72. The superburst characteristics are consistent with those seen in other superbursting low-mass X-ray binaries. However, the transient nature of the hosting binary presents significant challenges for superburst theory, requiring additional ingredients for the models. The carbon that fuels the superburst is thought to be produced mostly during the accretion outbursts and destroyed in the frequent type-I X-ray bursts. Mixing and sedimentation of the elements in the neutron star envelope may significantly influence the balance between the creation and destruction of carbon. Furthermore, predictions for the temperature of the neutron star crust fail to reach the values required for the ignition of carbon at the inferred column depth.

Bromine in the tropical troposphere and stratosphere as derived from balloon-borne BrO observations

Abstract: The first tropospheric and stratospheric (4 to 33 km) BrO profile is presented for the inner tropics derived from balloon-borne DOAS (Differential Optical Absorption Spectroscopy) measurements. In combination with photo- chemical modelling, total stratospheric inorganic bromine (Bry) is deduced to be (21.5±2.5) ppt in 4.5-year-old air, probed in 2005. We derive a total contribution of (5.2±2.5) ppt from brominated very short-lived substances and inorganic product gases to stratospheric Bry. Tropo- spheric BrO was found to be <1 ppt. Our results are com- pared to two 3-D CTM SLIMCAT model runs, which differ in the lifetime of the bromine source gases, affecting the ver- tical distribution of Br y in the lower stratosphere. Bromine source gas measurements performed 10 days earlier (Laube et al., 2008), indicate a lower Bry of (17.5±0.4) ppt. Poten- tial reasons for this discrepancy are discussed.

A clumpy-cloud photon-dominated regions model of the global far-infrared line emission of the Milky Way

Abstract: Context. The fractal structure of the interstellar medium suggests that the interaction of UV radiation with the ISM as described in the context of photon-dominated regions (PDR) dominates most of the physical and chemical conditions, and hence the far-infrared and submm emission from the ISM in the Milky Way. Aims. We investigate to what extent the Galactic FIR line emission of the important species CO, C, C + , and O, as observed by the Cosmic Background Explorer (COBE) satellite can be modeled in the framework of a clumpy, UV-penetrated cloud scenario. Methods. The far-infrared line emission of the Milky Way is modeled as the emission from an ensemble of clumps with a power law clump mass spectrum and mass-size relation with power-law indices consistent with the observed ISM structure. The individual clump line intensities are calculated using the KOSMA-τ PDR-model for spherical clumps. The model parameters for the cylindrically symmetric Galactic distribution of the mass density and volume filling factor are determined by the observed radial distributions. A constant FUV intensity, in which the clumps are embedded, is assumed. Results. We show that this scenario can explain, without any further assumptions and within a factor of about 2, the absolute FIR-line intensities and their distribution with Galactic longitude as observed by COBE.

Optical observations of SAX J1808.4−3658 during quiescence

Abstract: We observed the accreting millisecond pulsar SAX J1808.4-3658 with Gemini-South in g' and i' bands, nearly simultaneous with XMM-Newton observations. A clear periodic flux modulation on the system's orbital period is present, consistent with the varying aspect of the donor star's heated face. We model the contributions of a disc and donor star to these optical bands. To produce the observed modulation amplitudes, we conclude that the donor must be irradiated by an external flux two orders of magnitude greater than provided by the measured X-ray luminosity. A possible explanation for this irradiation is that the radio pulsar mechanism becomes active during the quiescent state as suggested by Burderi et al., with relativistic particles heating the donor's day side face. Our modelling constrains the binary inclination to be 36°-67°. We obtain estimates for the pulsar mass of >2.2 M ⊙ (although this limit is sensitive to the source's distance), consistent with the accelerated neutron star cooling in this system indicated by X-ray observations. We also estimate the donor mass to be 0.07-0.11 M ⊙ , providing further indications that the system underwent non-standard binary evolution to reach its current state.

Thermal Radiation Processes

Abstract: We discuss the different physical processes that are important to understand the thermal X-ray emission and absorption spectra of the diffuse gas in clusters of galaxies and the warm-hot intergalactic medium. The ionisation balance, line and continuum emission and absorption properties are reviewed and several practical examples are given that illustrate the most important diagnostic features in the X-ray spectra.

Suzaku Observations of the North Polar Spur: Evidence for Nitrogen Enhancement

Abstract: We present observations of the North Polar Spur (NPS) using the X-ray Imaging Spectrometer (XIS) aboard the Suzaku X-ray satellite. The NPS is a large region of enhanced soft X-ray and radio emission projected above the plane of the Galaxy, likely produced by a series of supernovae and stellar winds from the nearby Sco–Cen OB association. The exceptional sensitivity and spectral resolution of the XIS below 1 keV allow unprecedented probing of low-energy spectral lines, including C VI (0.37 keV) and N VII (0.50 keV), and we have detected highly-ionized nitrogen toward the NPS for the first time. For this single pointing toward the brightest 3/4 keV emission (l = 26: 8, b =+ 22: 0), the best-fit NPS emission model implies a hot (kT � 0:3 keV), collisional ionization equilibrium (CIE) plasma with depleted C, O, Ne, Mg, and Fe abundances of less than 0.5 solar, but an enhanced N abundance, with N=O = .4:0 +0:4 � 0:5 / times solar. The temperature and total thermal energy of the gas suggest heating by one or more supernovae, while the enhanced nitrogen abundance is best explained by enrichment from stellar material that has been processed by the CNO cycle. Due to the time required to develop AGB stars, we conclude that this N=O enhancement cannot be caused by the Sco–Cen OB association, but may result from a previous enrichment episode in the solar neighborhood.

CO emission and export from Asia: an analysis combining complementary satellite measurements (MOPITT, SCIAMACHY and ACE-FTS) with global modeling

Abstract: This study presents the complementary picture of the pollution outflow provided by several satellite observations of carbon monoxide (CO), based on different observation techniques. This is illustrated by an analysis of the Asian outflow during the spring of 2005, through comparisons with simulations by the LMDz-INCA global chemistry transport model. The CO observations from the MOPITT and SCIAMACHY nadir sounders, which provide vertically integrated information with excellent horizontal sampling, and from the ACE-FTS solar occultation instrument, which has limited spatial coverage but allows the retrieval of vertical profiles, are used. Combining observations from MOPITT (mainly sensitive to the free troposphere) and SCIAMACHY (sensitive to the full column) allows a qualitative evaluation of the boundary layer CO. The model tends to underestimate this residual compared to the observations, suggesting underestimated emissions, especially in eastern Asia. However, a better understanding of the consistency and possible biases between the MOPITT and SCIAMACHY CO is necessary for a quantitative evaluation. Underestimated emissions, and possibly too low lofting and underestimated chemical production in the model, lead to an underestimate of the export to the free troposphere, as highlighted by comparisons with MOPITT and ACE-FTS. Both instruments observe large trans-Pacific transport extending from ~20° N to ~60° N, with high upper tropospheric CO observed by ACE-FTS above the eastern Pacific (with values of up to 300 ppbv around 50° N at 500 hPa and up to ~200 ppbv around 30° N at 300 hPa). The low vertical and horizontal resolutions of the global model do not allow the simulation of the strong enhancements in the observed plumes. However, the transport patterns are well captured, and are mainly attributed to export from eastern Asia, with increasing contributions from South Asia and Indonesia towards the tropics. Additional measurements of C 2 H 2 , C 2 H 6 and HCN by ACE-FTS provide further information on the plume history. C 2 H 2 and C 2 H 6 enhancements are well correlated with the CO plumes, indicating common sources and rapid trans-Pacific transport. HCN observations show that the biomass burning contributes mainly at latitudes lower than ~40° N. This study provides a first step towards a full combination of complementary observations, but also highlights the need for a better evaluation of consistency between the datasets in order to allow precise quantitative analyses.

XMM-Newton X-ray study of early type stars in the Carina OB1 association

Abstract: Aims. X-ray properties of the stellar population in the Carina OB1 association are examined with special emphasis on early-type stars. Their spectral characteristics provide some clues to understanding the nature of X-ray formation mechanisms in the winds of single and binary early-type stars. Methods. A timing and spectral analysis of five observations with XMM-Newton is performed using various statistical tests and thermal spectral models. Results. 235 point sources have been detected within the field of view. Several of these sources are probably pre-main sequence stars with characteristic short-term variability. Seven sources are possible background AGNs. Spectral analysis of twenty four sources of type OB and WR 25 was performed. We derived spectral parameters of the sources and their fluxes in three energy bands. Estimating the interstellar absorption for every source and the distance to the nebula, we derived X-ray luminosities of these stars and compared them to their bolometric luminosities. We discuss possible reasons for the fact that, on average, the observed X-ray properties of binary and single early type stars are not very different, and give several possible explanations.

Efficient calculation of intensity and polarization spectra in vertically inhomogeneous scattering and absorbing atmospheres

Abstract: [1] We present a method for the efficient calculation of moderate- and high-resolution spectra of intensity and polarization in absorption bands. This linear-k method explicitly accounts for the vertical distribution of the absorption optical thickness by using a linear approximation. It can be applied to any radiative transfer model that calculates the intensity (vector) and its derivatives with respect to the absorption optical thickness in different altitude layers of the model atmosphere. For the three spectral windows of the Orbiting Carbon Observatory (OCO), the error on the modeled intensity introduced by the linear-k method is below 0.13% for the O 2 -A band, below 0.06% in the weak CO 2 band, and below 0.12% for the strong CO 2 band, for an aerosol type dominated by coarse mode particles with an aerosol optical thickness (AOT) of 0.3 at the O 2 -A band (the maximum AOT for which OCO retrievals are intended to be performed), and a typical aerosol height distribution with most aerosols located in the boundary layer. For these accuracies the computational effort is reduced by two orders of magnitude compared to line-by-line calculations. A sensitivity study with synthetic OCO measurements shows that the linear-k method introduces a small error of 0.025-0.20 ppm on the retrieved profile weighted mean CO 2 mixing ratio, for a boundary layer dominated aerosol distribution, and 0.050-0.50 ppm for an elevated aerosol layer between 5 and 6 km. In most cases the error is considerably smaller than the retrieval noise.

Magnetic field measurements and wind-line variability of OB-type stars

Abstract: Context. The first magnetic fields in O- and B-type stars that do not belong to the Bp-star class, have been discovered. The cyclic UV wind-line variability, which has been observed in a significant fraction of early-type stars, is likely to be related to such magnetic fields. Aims. We attempt to improve our understanding of massive-star magnetic fields, and observe twenty-five carefully-selected, OB-type stars. Methods. Of these stars we obtain 136 magnetic field strength measurements. We present the UV wind-line variability of all selected targets and summarise spectropolarimetric observations acquired using the MUSICOS spectropolarimeter, mounted at the TBL, Pic du Midi, between December 1998 and November 2004. From the average Stokes I and V line profiles, derived using the LSD method, we measure the magnetic field strengths, radial velocities, and first moment of the line profiles. Results. No significant magnetic field is detected in any OB-type star that we observed. Typical 1 sigma errors are between 15 and 200 G. A possible magnetic-field detection for the O9V star 10 Lac remains uncertain, because the field measurements depend critically on the fringe-effect correction in the Stokes V spectra. We find excess emission in UV-wind lines, centred about the rest wavelength, to be a new indirect indicator of the presence of a magnetic field in early B-type stars. The most promising candidates to host magnetic fields are the B-type stars delta Cet and 6 Cep, and a number of O stars. Conclusions. Although some O and B stars have strong dipolar field, which cause periodic variability in the UV wind-lines, such strong fields are not widespread. If the variability observed in the UV wind-lines of OB stars is generally caused by surface magnetic fields, these fields are either weak (less than or similar to few hundred G) or localised.

Zooming in on a sleeping giant: milliarcsecond High Sensitivity Array imaging of the black hole binary V404 Cyg in quiescence

Abstract: Observations of the black hole X-ray binary V404 Cyg with the very long baseline interferometer the High Sensitivity Array (HSA) have detected the source at a frequency of 8.4 GHz, providing a source position accurate to 0.3 mas relative to the calibrator source. The observations put an upper limit of 1.3 mas on the source size (5.2 au at 4 kpc) and a lower limit of 7 × 10 6 K on its brightness temperature during the normal quiescent state, implying that the radio emission must be non-thermal, most probably synchrotron radiation, possibly from a jet. The radio light curves show a short flare, with a rise time of ∼30 min, confirming that the source remains active in the quiescent state.

Observations of the 599 Hz accreting X-ray pulsar IGR J00291+5934 during the 2004 outburst and in quiescence

Abstract: We report on optical and near-infrared observations obtained during and after the 2004 December discovery outburst of the X-ray transient and accretion-powered millisecond pulsar IGR J00291+5934. Our observations monitored the evolution of the brightness and the spectral properties of IGR J00291+5934 during the outburst decay toward quiescence. We also present optical, near-infrared, and Chandra observations obtained during true quiescence. Photometry of the field during outburst reveals an optical and near-infrared counterpart that brightened from R similar or equal to 23 to R similar or equal to 17 and from K = 19 to K similar or equal to 16. Spectral analysis of the RIJHK broadband photometry shows excess in the near-infrared bands that may be due to synchrotron emission. The H alpha emission line profile suggests the orbital inclination is similar or equal to 22 degrees-32 degrees. The preferred range for the reddening toward the source is 0.7 <= E(B - V) <= 0: 9, which is equivalent to 4.06 x 10(21) cm(-2) <= N-H <= 5.22 x 10(21) cm(-2). The Chandra observations of the pulsar in its quiescent state gave an unabsorbed 0.5-10 keV flux for the best-fitting power-law model to the source spectrum of (7.0 +/- 0.9)x10(-14) ergs cm(-2) s(-1) (adopting a hydrogen column of 4.6x10(21) cm(-2)). The fit resulted in a power-law photon index of 2.4(-0.4)(+0.5). The (R - K)(0) color observed during quiescence supports an irradiated donor star and accretion disk. We estimate a distance of 2-4 kpc toward IGR J00291+ 5934 by using the outburst X-ray light curve and the estimated critical X-ray luminosity necessary to keep the outer parts of the accretion disk ionized. Using the quiescent X-ray luminosity and the spin period, we constrain the magnetic field of the neutron star to be <3x10(8) G.

Early anthropogenic CH4 emissions and the variation of CH4 and 13CH4 over the last millennium

Abstract: [1] This study presents a new hypothesis to explain the observed variation of CH4 and δ13C-CH4 over the last millennium. It was originally proposed that the observed minimum of δ13C-CH4 prior to the start of industrialization is caused by a large shift in biomass burning emissions between 1400 and 1700 A.D. According to our new hypothesis, however, the δ13C-CH4 minimum is the first sign of the global rise of anthropogenic CH4 emissions. The main idea is that emissions of isotopically depleted CH4, from, for example, rice cultivation, domestic ruminants, and waste treatment started increasing earlier than the isotopically enriched emissions from fossil fuel, which started with the start of industrialization. However, because the observed increase of atmospheric methane only started around 1750 A.D., these preindustrial anthropogenic emissions must have been accompanied by a net reduction of natural CH4 sources during the Little Ice Age (LIA) compensating for the increase of anthropogenic emissions during that period. Results of transient box model simulations for the last millennium show that under the new hypothesis a close agreement can be obtained between model and measurements. Prior to 1400 A.D., low emissions from anthropogenic biomass burning require a sizable contribution of methane emissions from vegetation to explain the observed high level of δ13C-CH4. During the Little Ice Age, a larger than expected reduction of natural sources is needed, which calls for further verification using a more sophisticated modeling approach and additional constraints from ice core measurements.