1,995 research outputs found
General Relativistic Simulations of Magnetized Plasmas around Merging Supermassive Black Holes
Coalescing supermassive black hole binaries are produced by the mergers of
galaxies and are the most powerful sources of gravitational waves accessible to
space-based gravitational observatories. Some such mergers may occur in the
presence of matter and magnetic fields and hence generate an electromagnetic
counterpart. In this Letter, we present the first general relativistic
simulations of magnetized plasma around merging supermassive black holes using
the general relativistic magnetohydrodynamic code Whisky. By considering
different magnetic field strengths, going from non-magnetically dominated to
magnetically dominated regimes, we explore how magnetic fields affect the
dynamics of the plasma and the possible emission of electromagnetic signals. In
particular we observe a total amplification of the magnetic field of ~2 orders
of magnitude which is driven by the accretion onto the binary and that leads to
much stronger electromagnetic signals, more than a factor of 10^4 larger than
comparable calculations done in the force-free regime where such amplifications
are not possible.Comment: 7 pages, 5 figures. Minor changes to match version accepted for
publication on The Astrophysical Journal Letter
Iron fluorescence from within the innermost stable orbit of black hole accretion disks
The fluorescent iron Ka line is a powerful observational probe of the inner
regions of black holes accretion disks. Previous studies have assumed that only
material outside the radius of marginal stability can contribute to the
observed line emission. Here, we show that fluorescence by material inside the
radius of marginal stability, which is in the process of spiralling towards the
event horizon, can have a observable influence on the iron line profile and
equivalent width. For concreteness, we consider the case of a geometrically
thin accretion disk, around a Schwarzschild black hole, in which fluorescence
is excited by an X-ray source placed at some height above the disk and on the
axis of the disk. Fully relativistic line profiles are presented for various
source heights and efficiencies. It is found that the extra line flux generally
emerges in the extreme red wing of the iron line, due to the large
gravitational redshift experienced by photons from the region within the radius
of marginal stability. We apply our models to the variable iron line seen in
the ASCA spectrum of the Seyfert nucleus MCG-6-30-15. It is found that the
change in the line profile, equivalent width, and continuum normalization, can
be well explained as being due to a change in the height of the source above
the disk. We discuss the implications of these results for distinguishing
rapidly-rotating black holes from slowly rotating holes using iron line
diagnostics.Comment: 20 pages, LaTeX. Accepted for publication in Astrophysical Journal.
Figures 3 to 7 replaced with corrected versions (previous figures affected by
calculational error). Some changes in the best fitting parameter
XMM-Newton Archival Study of the ULX Population in Nearby Galaxies
We present the results of an archival XMM-Newton study of the bright X-ray
point sources (L_X > 10^38 erg/s) in 32 nearby galaxies. From our list of
approximately 100 point sources, we attempt to determine if there is a
low-state counterpart to the Ultraluminous X-ray (ULX) population, searching
for a soft-hard state dichotomy similar to that known for Galactic X-ray
binaries and testing the specific predictions of the IMBH hypothesis. To this
end, we searched for "low-state" objects, which we defined as objects within
our sample which had a spectrum well fit by a simple absorbed power law, and
"high-state" objects, which we defined as objects better fit by a combined
blackbody and a power law. Assuming that ``low-state'' objects accrete at
approximately 10% of the Eddington luminosity (Done & Gierlinski 2003) and that
"high-state" objects accrete near the Eddington luminosity we further divided
our sample of sources into low and high state ULX sources. We classify 16
sources as low-state ULXs and 26 objects as high-state ULXs. As in Galactic
black hole systems, the spectral indices, Gamma, of the low-state objects, as
well as the luminosities, tend to be lower than those of the high-state
objects. The observed range of blackbody temperatures for the high state is
0.1-1 keV, with the most luminous systems tending toward the lowest
temperatures. We therefore divide our high-state ULXs into candidate IMBHs
(with blackbody temperatures of approximately 0.1 keV) and candidate stellar
mass BHs (with blackbody temperatures of approximately 1.0 keV). A subset of
the candidate stellar mass BHs have spectra that are well-fit by a
Comptonization model, a property similar of Galactic BHs radiating in the
"very-high" state near the Eddington limit.Comment: 54 pages, submitted to ApJ (March 2005), accepted (May 2006); changes
to organization of pape
Assessing the effect of dynamics on the closed-loop protein-folding hypothesis
The closed-loop (loop-n-lock) hypothesis of protein folding suggests that loops of about 25 residues, closed through interactions between the loop ends (locks), play an important role in protein structure. Coarse-grain elastic network simulations, and examination of loop lengths in a diverse set of proteins, each supports a bias towards loops of close to 25 residues in length between residues of high stability. Previous studies have established a correlation between total contact distance (TCD), a metric of sequence distances between contacting residues (cf. contact order), and the log-folding rate of a protein. In a set of 43 proteins, we identify an improved correlation (
r
2
= 0.76), when the metric is restricted to residues contacting the locks, compared to the equivalent result when all residues are considered (
r
2
= 0.65). This provides qualified support for the hypothesis, albeit with an increased emphasis upon the importance of a much larger set of residues surrounding the locks. Evidence of a similar-sized protein core/extended nucleus (with significant overlap) was obtained from TCD calculations in which residues were successively eliminated according to their hydrophobicity and connectivity, and from molecular dynamics simulations. Our results suggest that while folding is determined by a subset of residues that can be predicted by application of the closed-loop hypothesis, the original hypothesis is too simplistic; efficient protein folding is dependent on a considerably larger subset of residues than those involved in lock formation.
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Filling the Gaps with Public Policy: The Application of the Hague Convention Protocol in US Courts in the Absence of a Co-Signing State
This article will consider a possible avenue for filling \u27gaps\u27 when the 1980 Hague Abduction Convention or the 1996 Child Protection Convention do not apply in child custody/abduction cases. Specifically, it will explore utilizing internal US domestic relations law to facilitate the return of a child who has been abducted to the USA from a non-signatory country. To better illustrate the potential effects, the article will explore this \u27gap filler\u27 through the lens of a case study involving Japan, the most prominent first world country that is not yet a signatory to the Convention. The article also considers the implications for the international community
Photoionization of Galactic Halo Gas by Old Supernova Remnants
We present new calculations on the contribution from cooling hot gas to the
photoionization of warm ionized gas in the Galaxy. We show that hot gas in
cooling supernova remnants (SNRs) is an important source of photoionization,
particularly for gas in the halo. We find that in many regions at high latitude
this source is adequate to account for the observed ionization so there is no
need to find ways to transport stellar photons from the disk. The flux from
cooling SNRs sets a floor on the ionization along any line of sight. Our model
flux is also shown to be consistent with the diffuse soft X-ray background and
with soft X-ray observations of external galaxies.
We consider the ionization of the clouds observed towards the halo star HD
93521, for which there are no O stars close to the line of sight. We show that
the observed ionization can be explained successfully by our model EUV/soft
X-ray flux from cooling hot gas. In particular, we can match the H alpha
intensity, the S++/S+ ratio, and the C+* column. From observations of the
ratios of columns of C+* and either S+ or H0, we are able to estimate the
thermal pressure in the clouds. The slow clouds require high (~10^4 cm^-3 K)
thermal pressures to match the N(C+*)/N(S+) ratio. Additional heating sources
are required for the slow clouds to maintain their ~7000 K temperatures at
these pressures, as found by Reynolds, Hausen & Tufte (1999).Comment: AASTeX 5.01; 34 pages, 2 figures; submitted to Astrophysical Journa
Structure and functional motifs of GCR1, the only plant protein with a GPCR fold?
Whether GPCRs exist in plants is a fundamental biological question. Interest in deorphanizing new G
protein coupled receptors (GPCRs), arises because of their importance in signaling. Within plants, this
is controversial as genome analysis has identified 56 putative GPCRs, including GCR1 which is
reportedly a remote homologue to class A, B and E GPCRs. Of these, GCR2, is not a GPCR; more
recently it has been proposed that none are, not even GCR1. We have addressed this disparity
between genome analysis and biological evidence through a structural bioinformatics study, involving
fold recognition methods, from which only GCR1 emerges as a strong candidate. To further probe
GCR1, we have developed a novel helix alignment method, which has been benchmarked against the
the class A – class B - class F GPCR alignments. In addition, we have presented a mutually consistent
set of alignments of GCR1 homologues to class A, class B and class F GPCRs, and shown that GCR1
is closer to class A and /or class B GPCRs than class A, class B or class F GPCRs are to each other.
To further probe GCR1, we have aligned transmembrane helix 3 of GCR1 to each of the 6 GPCR
classes. Variability comparisons provide additional evidence that GCR1 homologues have the GPCR
fold. From the alignments and a GCR1 comparative model we have identified motifs that are common
to GCR1, class A, B and E GPCRs. We discuss the possibilities that emerge from this controversial
evidence that GCR1 has a GPCR fol
Possible X-ray diagnostic for jet/disk dominance in Type 1 AGN
Using Rossi X-ray Timing Explorer Seyfert 1 and 1.2 data spanning 9 years, we
study correlations between X-ray spectral features. The sample consists of 350
time-resolved spectra from 12 Seyfert 1 and 1.2 galaxies. Each spectrum is
fitted to a model with an intrinsic powerlaw X-ray spectrum produced close to
the central black hole that is reprocessed and absorbed by material around the
black hole. To test the robustness of our results, we performed Monte Carlo
simulations of the spectral sample. We find a complex relationship between the
iron line equivalent width (EW) and the underlying power law index (Gamma). The
data reveal a correlation between Gamma and EW which turns over at Gamma <~ 2,
but finds a weak anti-correlation for steeper photon indices. We propose that
this relationship is driven by dilution of a disk spectrum (which includes the
narrow iron line) by a beamed jet component and, hence, could be used as a
diagnostic of jet-dominance. In addition, our sample shows a strong correlation
between the reflection fraction (R) and Gamma, but we find that it is likely
the result of modeling degeneracies. We also see the X-ray Baldwin effect (an
anti-correlation between the 2-10 keV X-ray luminosity and EW) for the sample
as a whole, but not for the individual galaxies and galaxy types.Comment: Accepted for publication in ApJ, 14 page
Freeform Deposition Method for Coolant Channel Closeout
A method is provided for fabricating a coolant channel closeout jacket on a structure having coolant channels formed in an outer surface thereof. A line of tangency relative to the outer surface is defined for each point on the outer surface. Linear rows of a metal feedstock are directed towards and deposited on the outer surface of the structure as a beam of weld energy is directed to the metal feedstock so-deposited. A first angle between the metal feedstock so-directed and the line of tangency is maintained in a range of 20-90.degree.. The beam is directed towards a portion of the linear rows such that less than 30% of the cross-sectional area of the beam impinges on a currently-deposited one of the linear rows. A second angle between the beam and the line of tangency is maintained in a range of 5-65 degrees
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