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.

Error analysis for CO and CH 4 total column retrievals from SCIAMACHY 2.3 μm spectra

Abstract: A detailed sensitivity analysis of the Iterative Maximum Likelihood Method (IMLM) algorithm and its application to the SCIAMACHY 2.3 ?m spectra is presented. The sensitivity analysis includes a detailed assessment of the impact of aerosols in the 2.3 ?m range. Results show that near strong aerosol sources mineral dust and biomass aerosols can have an effect of ~7?10% on the CH4 total columns retrieved from this wavelength range. Similar but somewhat larger effects are found for CO, but due to the larger variability of CO these errors are less important. Away from strong sources much smaller effects of a few percent are found. Spectroscopic uncertainties are mostly negligible except for uncertainties in the CH4 intrinsic line intensities, which can be important. Application of the IMLM algorithm to the SCIAMACHY 2.3 ?m spectra shows that the quality of the retrieved CO and CH4 total columns is good, except for a bias for large instrument-noise errors which is partly due to remaining calibration issues. Polarization sensitivity of the SCIAMACHY instrument has a negligible effect on the retrieved CO and CH4 total columns. The H2O total columns, which have to be retrieved simultaneously with CO and CH4 due to overlapping absorption lines, agree well with H2O total columns from ECMWF data. This ensures that the fit to the H2O absorptions is of sufficient quality not to hamper the retrieved CO and CH4 total columns from SCIAMACHY spectra.

A deep survey of the AKARI north ecliptic pole field: I. WSRT 20 cm radio survey description, observations and data reduction

Abstract: Aims. The Westerbork Radio Synthesis Telescope, WSRT, has been used to make a deep radio survey of an ∼1.7 degree 2 field coinciding with the AKARI north ecliptic pole deep field. The observations, data reduction and source count analysis are presented, along with a description of the overall scientific objectives. Methods. The survey consisted of 10 pointings, mosaiced with enough overlap to maintain a similar sensitivity across the central region that reached as low as 21 μJy beam −1 at 1.4 GHz. Results. A catalogue containing 462 sources detected with a resolution of 17.0 �� × 15.5 �� is presented. The differential source counts calculated from the WSRT data have been compared with those from the shallow VLA − NEP survey of Kollgaard et al. 1994, and show a pronounced excess for sources fainter than ∼1 mJy, consistent with the presence of a population of star forming galaxies at sub-mJy flux levels. Conclusions. The AKARI north ecliptic pole deep field is the focus of a major observing campaign conducted across the entire spectral region. The combination of these data sets, along with the deep nature of the radio observations will allow unique studies of a large range of topics including the redshift evolution of the luminosity function of radio sources, the clustering environment of radio galaxies, the nature of obscured radio-loud active galactic nuclei, and the radio/far-infrared correlation for distant galaxies. This catalogue provides the basic data set for a future series of paper dealing with source identifications, morphologies, and the associated properties of the identified radio sources.

Determining the mid-plane conditions of circumstellar discs using gas and dust modelling: a study of HD 163296

Abstract: The mass of gas in protoplanetary discs is a quantity of great interest for assessing their planet formation potential. Disc gas masses are, however, traditionally inferred from measured dust masses by applying an assumed standard gas-to-dust ratio of g/d = 100. Furthermore, measuring gas masses based on CO observations has been hindered by the effects of CO freeze-out. Here we present a novel approach to study the mid-plane gas by combining C18O line modelling, CO snowline observations and the spectral energy distribution and selectively study the inner tens of au where freeze-out is not relevant. We apply the modelling technique to the disc around the Herbig Ae star HD 163296 with particular focus on the regions within the CO snowline radius, measured to be at 90 au in this disc. Our models yield the mass of C18O in this inner disc region of MC18O(<90au)∼2×10−8 MC18O(<90au)∼2×10−8 M⊙. We find that most of our models yield a notably low g/d < 20, especially in the disc mid-plane (g/d < 1). Our only models with a more interstellar medium (ISM)-like g/d require C18O to be underabundant with respect to the ISM abundances and a significant depletion of sub-micron grains, which is not supported by scattered light observations. Our technique can be applied to a range of discs and opens up a possibility of measuring gas and dust masses in discs within the CO snowline location without making assumptions about the gas-to-dust ratio.

Terahertz Superconducting Hot Electron Bolometer Heterodyne Receivers

Abstract: We highlight the progress on NbN hot electron bolometer (HEB) mixers achieved through fruitful collaboration between SRON Netherlands Institute for Space Research and Delft University of Technology, the Netherlands. This includes the best receiver noise temperatures of 700 K at 1.63 THz using a twin-slot antenna mixer and 1050 K at 2.84 THz using a spiral antenna coupled HEB mixer. The mixers are based on thin NbN films on Si and fabricated with a new contact-process and-structure. By reducing their areas HEB mixers have shown an LO power requirement as low as 30 nW. Those small HEB mixers have demonstrated equivalent sensitivity as those with large areas provided the direct detection effect due to broadband radiation is removed. To manifest that a HEB based heterodyne receiver can in practice be used at arbitrary frequencies above 2 THz, we demonstrate a 2.8 THz receiver using a THz quantum cascade laser (QCL) as local oscillator.

The double-peaked 2008 outburst of the accreting milli-second X-ray pulsar, IGR J00291+5934

Abstract: Context. In August 2008, the accreting milli-second X-ray pulsar (AMXP), IGR J00291+5934, underwent an outburst lasting ∼100 days, the first since its discovery in 2004. Aims. We present data from the 2008 double-peaked outburst of IGR J00291+5934 from Faulkes Telescope North, the Isaac Newton Telescope, the Keck Telescope, PAIRITEL, the Westerbork Synthesis Radio Telescope and the Swift, XMM-Newton and RXTE X-ray missions. We study the outburst’s evolution at various wavelengths, allowing us to probe accretion physics in this AMXP. Methods. We study the light curve morphology, presenting the first radio-X-ray spectral energy Distributions (SEDs) for this source and the most detailed UV-IR SEDs for any outbursting AMXP. We show simple models that attempt to identify the emission mechanisms responsible for the SEDs. We analyse short-timescale optical variability, and compare a medium resolution optical spectrum with those from 2004. Results. The outburst morphology is unusual for an AMXP, comprising two peaks, the second containing a “plateau” of ∼10 days at maximum brightness within 30 days of the initial activity. This has implications on duty cycles of short-period X-ray transients. The X-ray spectrum can be fitted by a single, hard power-law. We detect optical variability of ∼0.05 mag, on timescales of minutes, but find no periodic modulation. In the optical, the SEDs contain a blue component, indicative of an irradiated disc, and a transient near-infrared (NIR) excess. This excess is consistent with a simple model of an optically thick synchrotron jet (as seen in other outbursting AMXPs), however we discuss other potential origins. The optical spectrum shows a double-peaked Hα profile, a diagnostic of an accretion disc, but we do not clearly see other lines (e.g. He I, II) that were reported in 2004. Conclusions. Optical/IR observations of AMXPs appear to be excellent for studying the evolution of both the outer accretion disc and the inner jet, and may eventually provide us with tight constraints to model disc-jet coupling in accreting neutron stars.