2,621 research outputs found
Black Hole and Neutron Star Transients in Quiescence
We consider the X-ray luminosity difference between neutron star and black
hole soft X-ray transients (NS and BH SXTs) in quiescence. The current
observational data suggest that BH SXTs are significantly fainter than NS SXTs.
The luminosities of quiescent BH SXTs are consistent with the predictions of
binary evolution models for the mass transfer rate if (1) accretion occurs via
an ADAF in these systems and (2) the accreting compact objects have event
horizons. The luminosities of quiescent NS SXTs are not consistent with the
predictions of ADAF models when combined with binary evolution models, unless
most of the mass accreted in the ADAF is prevented from reaching the neutron
star surface. We consider the possibility that mass accretion is reduced in
quiescent NS SXTs because of an efficient propeller and develop a model of the
propeller effect that accounts for the observed luminosities. We argue that
modest winds from ADAFs are consistent with the observations while strong winds
are probably not.Comment: LateX, 37 pages, 7 figures; Accepted for publication in The
Astrophysical Journa
An Optical Precursor to the Recent X-ray Outburst of the Black Hole Binary GRO J1655-40
The All Sky Monitor on the Rossi X-ray Timing Explorer detected an X-ray
(2-12 keV) outburst from the black hole binary GRO J1655-40 beginning near
April 25, 1996. Optical photometry obtained April 20-24, 1996 shows a steady
brightening of the source in B, V, R, and I beginning about six days before the
start of the X-ray outburst. The onset of the optical brightening was earliest
in I and latest in B. However, the rate of the optical brightening was fastest
in B and slowest in I. The order of the increases in the different optical
filters suggests that the event was an "outside-in" disturbance of the
accretion disk. The substantial delay between the optical rise and the rise of
the X-rays may provide indirect support for the advection-dominated accretion
flow model of the inner regions of the accretion disk.Comment: 8 pages, 2 figures, Latex (uses the standard AAS style file
aas2pp4.sty), accepted for publication in the ApJ Letter
Spallation of Iron in Black Hole Accretion Flows
In the local Galactic interstellar medium there is approximate energy
equipartition between cosmic rays, magnetic fields and radiation. If this holds
in the central regions of AGN, in particular Seyfert galaxies, then consideral
nuclear spallation of Fe occurs, resulting in enhanced abundances of the sub-Fe
elements Ti, V, Cr and Mn. These elements produce a cluster of X-ray
flourescence lines at energies just below the 6.4 keV Fe-K line. It is
suggested that the red wings on the Fe lines observed with ASCA from various
Seyfert AGN are due to the unresolved line emission from these elements. Future
observations with more sensitive X-ray instruments should resolve these lines.
The estimated gamma ray emission from nuclear deexcitation and neutral pion
production is calculated and found to be below the sensitivities of any current
instruments. However, very luminous nearby Seyferts displaying Fe lines with
red wings would have MeV continuum emission detectable by future
instruments such as GLAST.Comment: 11 pages, Latex, requires AASTEX macros, 5 postscript figures,
Astrophysical Journal in pres
Advection-Dominated Accretion and the Spectral States of Black Hole X-Ray Binaries: Application to Nova Muscae 1991
We present a self-consistent model of accretion flows which unifies four
distinct spectral states observed in black hole X-ray binaries: quiescent, low,
intermediate and high states. In the quiescent, low and intermediate states,
the flow consists of an inner hot advection-dominated part extending from the
black hole horizon to a transition radius and an outer thin disk. In the high
state the thin disk is present at all radii. The model is essentially
parameter-free and treats consistently the dynamics of the accretion flow, the
thermal balance of the ions and electrons, and the radiation processes in the
accreting gas. With increasing mass accretion rate, the model goes through a
sequence of stages for which the computed spectra resemble very well
observations of the four spectral states; in particular, the low-to-high state
transition observed in black hole binaries is naturally explained as resulting
from a decrease in the transition radius. We also make a tentative proposal for
the very high state, but this aspect of the model is less secure.
We test the model against observations of the soft X-ray transient Nova
Muscae during its 1991 outburst. The model reproduces the observed lightcurves
and spectra surprisingly well, and makes a number of predictions which can be
tested with future observations.Comment: 68 pages, LaTeX, includes 1 table (forgotten in the previous version)
and 14 figures; submitted to The Astrophysical Journa
Isotopic traits of the Arctic water cycle
The Arctic hydrological cycle undergoes rapid and pronounced changes, including alterations in oceanic and atmospheric circulations, and precipitation patterns. Stable water isotopes (ÎŽ18O, ÎŽ2H, d-excess) can be used to trace these processes including their potential to feedback into the global climate system. The MOSAiC expedition provided a unique opportunity to collect, analyze, and synthesize discrete samples of the different hydrological compartments in the central Arctic, comprising sea ice, seawater, snow, and melt ponds.
Here, we present spatio-temporal variations in the isotopic signatures of more than 1,000 water samples. We found that (i) average seawater ÎŽ18O of -1.7â° conforms to observed and modelled isotopic traits of the Arctic Ocean; (ii) second year ice is relatively depleted compared to first year ice with average ÎŽ18O values of -3.1â° and -0.7â°, respectively. This might be due to post-depositional exchange processes with snow, which has the most depleted isotopic signature among all compartments (mean ÎŽ18O=-15.1â°).
Our dataset provides an unprecedented description of the present-day isotopic composition of the Arctic water covering a complete seasonal cycle. This will ultimately contribute to resolve the linkages between sea ice, ocean, and atmosphere during critical transitions from frozen ocean to open water conditions
West Nile Virus Infection among the Homeless, Houston, Texas1
Among 397 homeless participants studied, the overall West Nile virus (WNV) seroprevalence was 6.8%. Risk factors for WNV infection included being homeless >1 year, spending >6 hours outside daily, regularly taking mosquito precautions, and current marijuana use. Public health interventions need to be directed toward this high-risk population
Susceptibility to tuberculosis is associated with variants in the ASAP1 gene encoding a regulator of dendritic cell migration
Human genetic factors predispose to tuberculosis (TB). We studied 7.6 million genetic variants in 5,530 people with pulmonary TB and in 5,607 healthy controls. In the combined analysis of these subjects and the follow-up cohort (15,087 TB patients and controls altogether), we found an association between TB and variants located in introns of the ASAP1 gene on chromosome 8q24 (P = 2.6 Ă 10â11 for rs4733781; P = 1.0 Ă 10â10 for rs10956514). Dendritic cells (DCs) showed high ASAP1 expression that was reduced after Mycobacterium tuberculosis infection, and rs10956514 was associated with the level of reduction of ASAP1 expression. The ASAP1 protein is involved in actin and membrane remodeling and has been associated with podosomes. The ASAP1-depleted DCs showed impaired matrix degradation and migration. Therefore, genetically determined excessive reduction of ASAP1 expression in M. tuberculosisâinfected DCs may lead to their impaired migration, suggesting a potential mechanism of predisposition to TB
Governmental Context Determines Institutional Value: Independently Certified Performance and Failure in the Spanish Newspaper Industry
Many societies demand that independent professionals (e.g. auditors) certify the performance of firms. The value placed on such certification (i.e. the public perception of reliability/unreliability that may impact on an organization's success/failure) is not uniform, however, but contingent upon changing political contexts. This study presents and analyses data on the entire population of newspapers in Spain from 1966 to 1993, a time of peaceful transition from military dictatorship to capitalist democracy. Our results highlight the contingent nature of institutional life, demonstrating how changes in political contexts are associated with varying understandings of institutions. In particular, our findings support the prediction that, under a dictatorship, independently certified performance is not instrumental in organizational success or failure whereas, in a modern democracy, the certification process has a positive effect on the survival chances of firms.Publicad
International Coercion, Emulation and Policy Diffusion: Market-Oriented Infrastructure Reforms, 1977-1999
Why do some countries adopt market-oriented reforms such as deregulation, privatization and liberalization of competition in their infrastructure industries while others do not? Why did the pace of adoption accelerate in the 1990s? Building on neo-institutional theory in sociology, we argue that the domestic adoption of market-oriented reforms is strongly influenced by international pressures of coercion and emulation. We find robust support for these arguments with an event-history analysis of the determinants of reform in the telecommunications and electricity sectors of as many as 205 countries and territories between 1977 and 1999. Our results also suggest that the coercive effect of multilateral lending from the IMF, the World Bank or Regional Development Banks is increasing over time, a finding that is consistent with anecdotal evidence that multilateral organizations have broadened the scope of the âconditionalityâ terms specifying market-oriented reforms imposed on borrowing countries. We discuss the possibility that, by pressuring countries into policy reform, cross-national coercion and emulation may not produce ideal outcomes.http://deepblue.lib.umich.edu/bitstream/2027.42/40099/3/wp713.pd
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