Universities Space Research Association

About: Universities Space Research Association is a nonprofit organization based out in Columbia, Maryland, United States. It is known for research contribution in the topics: Gamma-ray burst & Galaxy. The organization has 1921 authors who have published 5412 publications receiving 255681 citations. The organization is also known as: USRA.

Gamma-Ray Burst Afterglow Broadband Fitting Based Directly on Hydrodynamics Simulations

Abstract: We present a powerful new tool for fitting broadband gamma-ray burst afterglow data, which can be used to determine the burst explosion parameters and the synchrotron radiation parameters. By making use of scale invariance between relativistic jets of different energies and different circumburst medium densities, and by capturing the output of high-resolution two-dimensional relativistic hydrodynamical (RHD) jet simulations in a concise summary, the jet dynamics are generated quickly. Our method calculates the full light curves and spectra using linear radiative transfer sufficiently fast to allow for a direct iterative fit of RHD simulations to the data. The fit properly accounts for jet features that so far have not been successfully modeled analytically, such as jet decollimation, inhomogeneity along the shock front, and the transitory phase between the early-time relativistic and late-time non-relativistic outflow. As a first application of the model we simultaneously fit the radio, X-ray, and optical data of GRB 990510. We find not only noticeable differences between our findings for the explosion and radiation parameters and those of earlier authors, but also an improved model fit when we include the observer angle in the data fit. The fit method will be made freely available on request and online at http://cosmo.nyu.edu/afterglowlibrary. In addition to data fitting, the software tools can also be used to quickly generate a light curve or spectrum for arbitrary observer position, jet, and radiation parameters.

A Cutoff in the X-Ray Fluctuation Power Density Spectrum of the Seyfert 1 Galaxy NGC 3516

Abstract: During 1997 March-July, RXTE observed the bright, strongly variable Seyfert 1 galaxy NGC 3516 once every approx 128 hr for 45 months and nearly continuously (with interruptions due to SAA passage but not Earth occultation) for a 42 day period in the middle These were followed by ongoing monitoring once every approx 43 days These data are used to construct the first well-determined X-ray fluctuation power density spectrum (PDS) of an active galaxy to span more than 4 decades of usable temporal frequency The PDS shows no signs of any strict or quasi-periodicity, but does show a progressive flattening of the power-low slope from -174 at short time scales to -073 at longer time scales This is the clearest observation to date of the long-predicted cutoff in the PDS The characteristic variability time scale corresponding to this cutoff temporal frequency is approx 1 month Although it is unclear how this time scale may be interpreted in terms of a physical size or process, there are several promising candidate models The PDS appears similar to those seen for Galactic black hole candidates such as Cyg X-1, suggesting that these two classes of objects with very different luminosities and putative black hole masses (differing by more than a factor of 10(exp 5)) may have similar X-ray generation processes and structures

Evaluation of GPM IMERG Early, Late, and Final rainfall estimates using WegenerNet gauge data in southeastern Austria

Abstract: The Global Precipitation Measurement (GPM) Integrated Multi-satellite Retrievals for GPM (IMERG) products provide quasi-global (60° N–60° S) precipitation estimates, beginning March 2014, from the combined use of passive microwave (PMW) and infrared (IR) satellites comprising the GPM constellation The IMERG products are available in the form of near-real-time data, ie, IMERG Early and Late, and in the form of post-real-time research data, ie, IMERG Final, after monthly rain gauge analysis is received and taken into account In this study, IMERG version 3 Early, Late, and Final (IMERG-E,IMERG-L, and IMERG-F) half-hourly rainfall estimates are compared with gauge-based gridded rainfall data from the WegenerNet Feldbach region (WEGN) high-density climate station network in southeastern Austria The comparison is conducted over two IMERG 01° × 01° grid cells, entirely covered by 40 and 39 WEGN stations each, using data from the extended summer season (April–October) for the first two years of the GPM mission The entire data are divided into two rainfall intensity ranges (low and high) and two seasons (warm and hot), and we evaluate the performance of IMERG, using both statistical and graphical methods Results show that IMERG-F rainfall estimates are in the best overall agreement with the WEGN data, followed by IMERG-L and IMERG-E estimates, particularly for the hot season We also illustrate, through rainfall event cases, how insufficient PMW sources and errors in motion vectors can lead to wide discrepancies in the IMERG estimates Finally, by applying the method of Villarini and Krajewski (2007), we find that IMERG-F half-hourly rainfall estimates can be regarded as a 25 min gauge accumulation, with an offset of +40 min relative to its nominal time

The LCROSS Cratering Experiment

Abstract: As its detached upper-stage launch vehicle collided with the surface, instruments on the trailing Lunar Crater Observation and Sensing Satellite (LCROSS) Shepherding Spacecraft monitored the impact and ejecta. The faint impact flash in visible wavelengths and thermal signature imaged in the mid-infrared together indicate a low-density surface layer. The evolving spectra reveal not only OH within sunlit ejecta but also other volatile species. As the Shepherding Spacecraft approached the surface, it imaged a 25- to-30-meter-diameter crater and evidence of a high-angle ballistic ejecta plume still in the process of returning to the surface--an evolution attributed to the nature of the impactor.

The Framework for 0-D Atmospheric Modeling (F0AM) v3.1

Abstract: . The Framework for 0-D Atmospheric Modeling (F0AM) is a flexible and user-friendly MATLAB-based platform for simulation of atmospheric chemistry systems. The F0AM interface incorporates front-end configuration of observational constraints and model setups, making it readily adaptable to simulation of photochemical chambers, Lagrangian plumes, and steady-state or time-evolving solar cycles. Six different chemical mechanisms and three options for calculation of photolysis frequencies are currently available. Example simulations are presented to illustrate model capabilities and, more generally, highlight some of the advantages and challenges of 0-D box modeling.