Netherlands Institute for Space Research

About: Netherlands Institute for Space Research is a facility organization based out in Utrecht, Netherlands. It is known for research contribution in the topics: Galaxy & Neutron star. The organization has 737 authors who have published 3026 publications receiving 106632 citations. The organization is also known as: SRON & Space Research Organisation Netherlands.

The science of EChO

Abstract: The science of extra-solar planets is one of the most rapidly changing areas of astrophysics and since 1995 the number of planets known has increased by almost two orders of magnitude. A combination of ground-based surveys and dedicated space missions has resulted in 560-plus planets being detected, and over 1200 that await confirmation. NASA's Kepler mission has opened up the possibility of discovering Earth-like planets in the habitable zone around some of the 100,000 stars it is surveying during its 3 to 4-year lifetime. The new ESA's Gaia mission is expected to discover thousands of new planets around stars within 200 parsecs of the Sun. The key challenge now is moving on from discovery, important though that remains, to characterisation: what are these planets actually like, and why are they as they are? In the past ten years, we have learned how to obtain the first spectra of exoplanets using transit transmission and emission spectroscopy. With the high stability of Spitzer, Hubble, and large ground-based telescopes the spectra of bright close-in massive planets can be obtained and species like water vapour, methane, carbon monoxide and dioxide have been detected. With transit science came the first tangible remote sensing of these planetary bodies and so one can start to extrapolate from what has been learnt from Solar System probes to what one might plan to learn about their faraway siblings. As we learn more about the atmospheres, surfaces and near-surfaces of these remote bodies, we will begin to build up a clearer picture of their construction, history and suitability for life. The Exoplanet Characterisation Observatory, EChO, will be the first dedicated mission to investigate the physics and chemistry of Exoplanetary Atmospheres. By characterising spectroscopically more bodies in different environments we will take detailed planetology out of the Solar System and into the Galaxy as a whole. EChO has now been selected by the European Space Agency to be assessed as one of four M3 mission candidates.

Statistics of velocity centroids: effects of density–velocity correlations and non‐Gaussianity

Abstract: We continue with our previous work on statistics of velocity centroids, to retrieve information about the scaling properties of an underlying turbulent velocity field from spectroscopic observations. We use synthetic data sets with extreme effects of velocity-density correlations that we create artificially, which also have a non-Gaussian distribution of fluctuations. We confirm that centroids can be used to obtain the scaling properties of the turbulent velocity when the ratio of the density dispersion to the mean density is less than unity, regardless of velocity-density correlations and non-Gaussianity. It was found that extreme velocity-density correlations can distort the statistics of velocity centroids, impeding the recovery of the turbulent velocity spectral index from centroids. We show that such correlations introduce high-order moments to the maps of centroids, which we disregarded in previous work, but that they are only important when the density dispersion is large in comparison with the mean density. It was also found that non-Gaussian velocity and/or density distort the statistics of centroids too, but to a lower degree than extreme cross-correlations.

Constraints on relativistic jets in quiescent black hole X-ray binaries from broad-band spectral modelling

Abstract: The nature of black hole jets at the lowest detectable luminosities remains an open question, largelyduetoadearthofobservationalconstraints.Here,wepresentanew,nearlysimultaneous broad-band spectrumoftheblackholeX-raybinary(BHXB)XTEJ1118+480 atanextremely low Eddington ratio (LX ∼ 10 −8.5 LEdd). Our new spectral energy distribution (SED) includes the radio, near-infrared, optical, ultraviolet, and X-ray wavebands. XTE J1118+480 is now the second BHXB at such a low Eddington ratio with a well-sampled SED, thereby providing new constraints on highly sub-Eddington accretion flows and jets, and opening the door to begin comparison studies between systems. We apply a multizone jet model to the new broad-band SED, and we compare our results to previous fits to the same source using the same model at 4–5decadeshigherluminosity.WefindthatafteraBHXBtransitionstotheso-calledquiescent spectral state, the jet base becomes more compact (by up to an order of magnitude) and slightly cooler (by at least a factor of 2). Our preferred model fit indicates that jet particle acceleration is much weaker after the transition into quiescence. That is, accelerated non-thermal particles no longer reach high enough Lorentz factors to contribute significant amounts of synchrotron X-ray emission. Instead, the X-ray waveband is dominated by synchrotron self-Compton emission from a population of mildly relativistic electrons with a quasi-thermal velocity distribution that are associated with the jet base. The corresponding (thermal) synchrotron component from the jet base emits primarily in the infrared through ultraviolet wavebands. Our results on XTE J1118+480 are consistent with broad-band modelling for A0620-00 (the only other comparably low Eddington ratio BHXB with a well-sampled SED) and for Sgr A* (the quiescent supermassive black hole at the Galactic centre). The above could therefore represent a canonical baseline geometry for accreting black holes in quiescence. We conclude with suggestions for future studies to further investigate the above scenario.

Discovery of X-ray burst triplets in EXO 0748-676

Abstract: Type-I X-ray bursts are thermonuclear flashes that take place on the surface of accreting neutron stars. The wait time between consecutive bursts is set by the time required to accumulate the fuel needed to trigger a new burst; this is at least one hour. Sometimes secondary bursts are observed, approximately 10 min after the main burst. These short wait-time bursts are not yet understood. We observed the low-mass X-ray binary and X-ray burster EXO 0748-676 with XMM-Newton for 158 h, during 7 uninterrupted observations lasting up to 30 h each. We detect 76 X-ray bursts. Most remarkably, 15 of these bursts occur in burst triplets, with wait times of 12 min between the three components of the triplet, T1, T2, and T3. We also detect 14 doublets with similar wait times between the two components of the doublet, D1 and D2. We characterize this behavior to try and obtain a better understanding of bursts with short wait times. We measure the burst peak flux, fluence, wait time and time profile, and study correlations between these parameters and with the persistent flux representing the mass accretion rate. (i) For all bursts with a long wait time, the fluence is tightly correlated with the wait time, whereas burst with short wait times generally have higher fluences than expected from this relationship; (ii) wait times tend to be longer after doublets and triplets; (iii) the time profile of single bursts, S1, and of the first burst in a double or triple burst, D1 and T1, always contains a slow component which is generally absent in the D2, T2 and T3 bursts; (iv) the peak flux is highest for S1, D1 and T1 bursts, but this is still a factor of 7 lower than the highest peak flux ever seen for a burst in this system; (v) the persistent flux, representing the mass accretion rate onto the neutron star, is about 1% of Eddington, which is among the lowest value so far measured for this system. The amount of energy per gram of accreted mass liberated during bursts is consistent with a fuel mixture of hydrogen-rich material. The characteristics of the bursts indicate that possibly all bursts in this system are hydrogen-ignited, in contrast with most other frequent X-ray bursters in which bursts are helium-ignited, but consistent with the low mass accretion rate in EXO 0748-676. Possibly the hydrogen ignition is the determining factor for the occurrence of short wait-time bursts. For example the 12 min wait time may be associated with a nuclear beta decay timescale. Appendix is only available in electronic form at http://www.aanda.org

ALMA and ROSINA detections of phosphorus-bearing molecules: the interstellar thread between star-forming regions and comets

Abstract: To understand how phosphorus (P)-bearing molecules are formed in star-forming regions, we have analysed the Atacama Large Millimeter/Submillimeter Array (ALMA) observations of PN and PO towards the massive star-forming region AFGL 5142, combined with a new analysis of the data of the comet 67P/Churyumov–Gerasimenko taken with the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) instrument onboard Rosetta. The ALMA maps show that the emission of PN and PO arises from several spots associated with low-velocity gas with narrow linewidths in the cavity walls of a bipolar outflow. PO is more abundant than PN in most of the spots, with the PO/PN ratio increasing as a function of the distance to the protostar. Our data favour a formation scenario in which shocks sputter phosphorus from the surface of dust grains, and gas-phase photochemistry induced by UV photons from the protostar allows efficient formation of the two species in the cavity walls. Our analysis of the ROSINA data has revealed that PO is the main carrier of P in the comet, with PO/PN > 10. Since comets may have delivered a significant amount of prebiotic material to the early Earth, this finding suggests that PO could contribute significantly to the phosphorus reservoir during the dawn of our planet. There is evidence that PO was already in the cometary ices prior to the birth of the Sun, so the chemical budget of the comet might be inherited from the natal environment of the Solar system, which is thought to be a stellar cluster including also massive stars.