Goddard Space Flight Center

About: Goddard Space Flight Center is a facility organization based out in Greenbelt, Maryland, United States. It is known for research contribution in the topics: Galaxy & Solar wind. The organization has 19058 authors who have published 63344 publications receiving 2786037 citations. The organization is also known as: GSFC & Space Flight Center.

Gaia Early Data Release 3: The astrometric solution

Abstract: Gaia Early Data Release 3 (Gaia EDR3) contains results for 1.812 billion sources in the magnitude range G = 3 to 21 based on observations collected by the European Space Agency Gaia satellite during the first 34 months of its operational phase. We describe the input data, the models, and the processing used for the astrometric content of Gaia EDR3, as well as the validation of these results performed within the astrometry task. The processing broadly followed the same procedures as for Gaia DR2, but with significant improvements to the modelling of observations. For the first time in the Gaia data processing, colour-dependent calibrations of the line- and point-spread functions have been used for sources with well-determined colours from DR2. In the astrometric processing these sources obtained five-parameter solutions, whereas other sources were processed using a special calibration that allowed a pseudocolour to be estimated as the sixth astrometric parameter. Compared with DR2, the astrometric calibration models have been extended, and the spin-related distortion model includes a self-consistent determination of basic-angle variations, improving the global parallax zero point. Gaia EDR3 gives full astrometric data (positions at epoch J2016.0, parallaxes, and proper motions) for 1.468 billion sources (585 million with five-parameter solutions, 882 million with six parameters), and mean positions at J2016.0 for an additional 344 million. Solutions with five parameters are generally more accurate than six-parameter solutions, and are available for 93% of the sources brighter than G = 17 mag. The median uncertainty in parallax and annual proper motion is 0.02-0.03 mas at magnitude G = 9 to 14, and around 0.5 mas at G = 20. Extensive characterisation of the statistical properties of the solutions is provided, including the estimated angular power spectrum of parallax bias from the quasars.

A High Spatial Resolution X-Ray and Hα Study of Hot Gas in the Halos of Star-forming Disk Galaxies. I. Spatial and Spectral Properties of the Diffuse X-Ray Emission

Abstract: We present arcsecond resolution Chandra X-ray and ground-based optical Hα imaging of a sample of 10 edge-on star-forming disk galaxies (seven starburst and three "normal" spiral galaxies), a sample that covers the full range of star formation intensity found in disk galaxies. The X-ray observations make use of the unprecedented spatial resolution of the Chandra X-ray observatory to more robustly than before remove X-ray emission from point sources and hence obtain the X-ray properties of the diffuse thermal emission alone. We have combined the X-ray observations with existing, comparable-resolution, ground-based Hα and R-band imaging and present a mini-atlas of images on a common spatial and surface brightness scale to aid cross-comparison. In general, the morphology of the extraplanar diffuse X-ray emission is very similar to the extraplanar Hα filaments and arcs, on both small and large scales (scales of tens of parsecs and kiloparsecs, respectively). The most spectacular cases of this are found in NGC 1482 (for which we provide the first published X-ray observation) and NGC 3079. We provide a variety of quantitative measures of how the spectral hardness and surface brightness of the diffuse X-ray emission varies with increasing height z above the plane of each galaxy. Of the eight galaxies in which diffuse X-ray emitting halos are found (the starbursts and the normal spiral NGC 891), significant spatial variation in the spectral properties of the extraplanar emission (|z| ≥ 2 kpc) is only found in two cases: NGC 3628 and NGC 4631. In general, the vertical distribution of the halo-region X-ray surface brightness is best described as an exponential, with the observed scale heights of the sample galaxies lying in the range Heff ~ 2-4 kpc. The presence of extraplanar X-ray emission is always associated with the presence of extraplanar optical line emission of similar vertical extent. No X-ray emission was detected from the halos of the two low-mass normal spiral galaxies NGC 6503 and NGC 4244. Active galactic nuclei, where present, appear to play no role in powering or shaping the outflows from the starburst galaxies in this sample. The Chandra ACIS X-ray spectra of extraplanar emission from all these galaxies can be fitted with a common two-temperature spectral model with an enhanced α-to-iron element ratio. This is consistent with the origin of the X-ray emitting gas being either metal-enriched merged SN ejecta or shock-heated ambient halo or disk material with moderate levels of metal depletion onto dust. Our favored model is that SN feedback in the disks of star-forming galaxies create, via blow-out and venting of hot gas from the disk, tenuous exponential atmospheres of density scale height Hg ~ 4-8 kpc. The soft thermal X-ray emission observed in the halos of the starburst galaxies is either this preexisting halo medium, which has been swept up and shock-heated by the starburst-driven wind, or wind material compressed near the walls of the outflow by reverse shocks within the wind. In either case, the X-ray emission provides us with a powerful probe of the properties of gaseous halos around star-forming disk galaxies.

Physics of Solar Prominences: I-Spectral Diagnostics and Non-LTE Modelling

Abstract: This review paper outlines background information and covers recent advances made via the analysis of spectra and images of prominence plasma and the increased sophistication of non-LTE (i.e. when there is a departure from Local Thermodynamic Equilibrium) radiative transfer models. We first describe the spectral inversion techniques that have been used to infer the plasma parameters important for the general properties of the prominence plasma in both its cool core and the hotter prominence-corona transition region. We also review studies devoted to the observation of bulk motions of the prominence plasma and to the determination of prominence mass. However, a simple inversion of spectroscopic data usually fails when the lines become optically thick at certain wavelengths. Therefore, complex non-LTE models become necessary. We thus present the basics of non-LTE radiative transfer theory and the associated multi-level radiative transfer problems. The main results of one- and two-dimensional models of the prominences and their fine-structures are presented. We then discuss the energy balance in various prominence models. Finally, we outline the outstanding observational and theoretical questions, and the directions for future progress in our understanding of solar prominences.

Greenland Ice Sheet: High-Elevation Balance and Peripheral Thinning

Abstract: Aircraft laser-altimeter surveys over northern Greenland in 1994 and 1999 have been coupled with previously reported data from southern Greenland to analyze the recent mass-balance of the Greenland Ice Sheet. Above 2000 meters elevation, the ice sheet is in balance on average but has some regions of local thickening or thinning. Thinning predominates at lower elevations, with rates exceeding 1 meter per year close to the coast. Interpolation of our results between flight lines indicates a net loss of about 51 cubic kilometers of ice per year from the entire ice sheet, sufficient to raise sea level by 0.13 millimeter per year-approximately 7% of the observed rise.

Non‐linear X‐ray variability in X‐ray binaries and active galaxies

Abstract: We show that the rms-flux relation recently discovered in the X-ray light curves of Active Galactic Nuclei (AGN) and X-ray binaries (XRBs) implies that the light curves have a formally non-linear, exponential form, provided the rms-flux relation applies to variations on all time-scales (as it appears to). This phenomenological model implies that stationary data will have a log normal flux distribution. We confirm this result using an observation of Cyg X-1, and further demonstrate that our model predicts the existence of the powerful millisecond flares observed in Cyg X-1 in the low/hardstate, and explains the general shape and amplitude of the bicoherence spectrum in that source. Our model predicts that the most variable light curves will show the most extreme non-linearity. This result can naturally explain the apparent non-linear variability observed in some highly variable Narrow Line Seyfert 1 (NLS1) galaxies, as well as the low states observed on long time-scales in the NLS1 NGC 4051, as being nothing more than extreme manifestations of the same variability process that is observed in XRBs and less variable AGN. That variability process must be multiplicative (with variations coupled together on all time-scales) and cannot be additive (such as shot-noise), or related to self-organised criticality, or result from completely independent variations in many separate emitting regions. Successful models for variability must reproduce the observed rms-flux relation and non-linear behaviour, which are more fundamental characteristics of the variability process than the power spectrum or spectral-timing properties. Models where X-ray variability is driven by accretion rate variations produced at different radii remain the most promising.