Fragmentation In The First Galaxies

Using a one-zone model, we study the gravitational fragmentation of baryonic cold streams flowing into a typical first galaxy at high redshift to predict the expected fragmentation mass scale and thus a characteristic mass of the first population of stars to form by shock fragmentation. In this simplified model, we bypass the resolution barrier in simulations of progalaxy formation and directly probe the fragmentation mass scale of accreting gas streams. We find the fragmentation mass scale to be most strongly dependent on the metallicity of the gas and the background Lyman-Werner radiation field, and that either a large cluster of solar mass fragments or one large (10(5) M(circle dot)) fragment forms.Astronom

Environmental Racism and Biased Methods of Risk Assessment

Based on analysis of a risk assessment for a proposed Louisiana uranium enrichment facility, the authors argue that environmental injustice occurs when assessors\u27 scientific methods cause de facto discrimination

X-ray spectrum of the high polarization quasar PKS 1510-089

We present results on the X-ray spectra of the radio-loud, high-polarization quasar, PKS 1510-089, based on new data obtained using ASCA, and from archival ROSAT data. The X-ray spectrum obtained by ASCA is unusually hard, with the photon index=1.30+-0.06, while the (non-simultaneous) ROSAT data indicate a steeper spectrum (1.9+-0.3). The X-ray flux at 1 keV is within 10% during both observations. A break in the underlying continuum at about 0.7 keV is suggested. Flat X-ray spectra seem to be the characteristic of high polarization quasars, and their spectra also appear to be harder than that of the other radio-loud but low-polarization quasars. The multiwavelength spectrum of PKS 1510-089 is similar to many other gamma-ray blazars, suggesting the emission is dominated by that from a relativistic jet. A big blue-bump is also seen in its multiwavelength spectrum, suggesting the presence of a strong thermal component as well.Comment: 19 pages (Latex + 5 ps figures), Accpeted for publication in the Astrophysical Journal, December 20, 199

The X-ray transient XTE J1118+480: Multiwavelength observations of a low-state mini-outburst

We present multiwavelength observations of the newly discovered X-ray transient XTE J1118+480 obtained in the rising phase of the 2000 April outburst. This source is located at unusually high Galactic latitude and in a very low absorption line of sight. This made the first EUVE spectroscopy of an X-ray transient outburst possible. Together with our HST, RXTE, and UKIRT data this gives unprecedented spectral coverage. We find the source in the low hard state. The flat IR-UV spectrum appears to be a combination of optically thick disk emission and flat spectrum emission, possibly synchrotron, while at higher energies, including EUV, a typical low hard state power-law is seen. EUVE observations reveal no periodic modulation, suggesting an inclination low enough that no obscuration by the disk rim occurs. We discuss the nature of the source and this outburst and conclude that it may be more akin to mini-outbursts seen in GRO J0422+32 than to a normal X-ray transient outburst.Comment: 4 pages, 3 figures, replaced with accepted version. Uses emulateapj5.st

The Hard X-Ray Emission from Scorpius X-1 Seen by INTEGRAL

We present the results of our hard X-ray and gamma-ray study of the LMXB Sco X-1 utilizing INTEGRAL data as well as contemporaneous RXTE PCA data. We have investigated the hard X-ray spectral properties of Sco X-1 including the nature of the high-energy, nonthermal component and its possible correlations with the location of the source on the soft X-ray color-color diagram. We find that Sco X-1 follows two distinct spectral tracks when the 20-40 keV count rate is greater than 130 counts/second. One state is a hard state which exhibits a significant high-energy, powerlaw tail to the lower energy thermal spectrum. The other state shows a much less significant high-energy component. We found suggestive evidence for a correlation of these hard and soft high-energy states with the position of Sco X-1 on the low-energy X-ray color-color diagram. We have searched for similar behavior in 2 other Z sources: GX 17+2 and GX 5-1 with negative results

Mass Determination of Black Holes in LMC X-1 and Nova Muscae 1991 from their High-Energy Spectra

We offer a brief description of the bulk-motion Comptonization (BMC) model for accretion onto black holes, illustrated by its application to observational data for LMC X-1, and Nova Muscae 1991. We then extract some physical parameters of these systems from observables (within the context of the BMC model}, drawing from results on GRO J1655-40, for which we presented extensive analysis previously. We derive estimates of the mass, (16 +/- 1) solar masses and mass accretion rate in the disk in Eddington units around 2 for LMC X-1, and (24 +/- 1)d_{5.5} and the disk mass acretion rate around 3 for Nova Muscae 1991 [where d_{5.5} stands for the distance in 5.5 kpc units]. Differences between these estimates and previous estimates based on dynamical studies are discussed. It is further shown that the disk inner radius increases with the high-to-low state transition in Nova Muscae 1991. Specifically, our analysis suggests that the inner-disk radius increases to 17 Scwarzschild radii as the transition to the low-hard state occurs.Comment: 14 pages, 3 figures, The paper is accepted for publication in the Astrophysical Journal Letter