2,188 research outputs found
Pre-Merger Localization of Gravitational-Wave Standard Sirens With LISA I: Harmonic Mode Decomposition
The continuous improvement in localization errors (sky position and distance)
in real time as LISA observes the gradual inspiral of a supermassive black hole
(SMBH) binary can be of great help in identifying any prompt electromagnetic
counterpart associated with the merger. We develop a new method, based on a
Fourier decomposition of the time-dependent, LISA-modulated gravitational-wave
signal, to study this intricate problem. The method is faster than standard
Monte Carlo simulations by orders of magnitude. By surveying the parameter
space of potential LISA sources, we find that counterparts to SMBH binary
mergers with total mass M~10^5-10^7 M_Sun and redshifts z<~3 can be localized
to within the field of view of astronomical instruments (~deg^2) typically
hours to weeks prior to coalescence. This will allow targeted searches for
variable electromagnetic counterparts as the merger proceeds, as well as
monitoring of the most energetic coalescence phase. A rich set of astrophysical
and cosmological applications would emerge from the identification of
electromagnetic counterparts to these gravitational-wave standard sirens.Comment: 29 pages, 12 figures, version accepted by Phys Rev
Machine learning applied to enzyme turnover numbers reveals protein structural correlates and improves metabolic models.
Knowing the catalytic turnover numbers of enzymes is essential for understanding the growth rate, proteome composition, and physiology of organisms, but experimental data on enzyme turnover numbers is sparse and noisy. Here, we demonstrate that machine learning can successfully predict catalytic turnover numbers in Escherichia coli based on integrated data on enzyme biochemistry, protein structure, and network context. We identify a diverse set of features that are consistently predictive for both in vivo and in vitro enzyme turnover rates, revealing novel protein structural correlates of catalytic turnover. We use our predictions to parameterize two mechanistic genome-scale modelling frameworks for proteome-limited metabolism, leading to significantly higher accuracy in the prediction of quantitative proteome data than previous approaches. The presented machine learning models thus provide a valuable tool for understanding metabolism and the proteome at the genome scale, and elucidate structural, biochemical, and network properties that underlie enzyme kinetics
Schur Partial Derivative Operators
A lattice diagram is a finite list L=((p_1,q_1),...,(p_n,q_n) of lattice
cells. The corresponding lattice diagram determinant is \Delta_L(X;Y)=\det \|
x_i^{p_j}y_i^{q_j} \|. These lattice diagram determinants are crucial in the
study of the so-called ``n! conjecture'' of A. Garsia and M. Haiman. The space
M_L is the space spanned by all partial derivatives of \Delta_L(X;Y). The
``shift operators'', which are particular partial symmetric derivative
operators are very useful in the comprehension of the structure of the M_L
spaces. We describe here how a Schur function partial derivative operator acts
on lattice diagrams with distinct cells in the positive quadrant.Comment: 8 pages, LaTe
Primordial magnetic fields and the HI signal from the epoch of reionization
The implication of primordial magnetic-field-induced structure formation for
the HI signal from the epoch of reionization is studied. Using semi-analytic
models, we compute both the density and ionization inhomogeneities in this
scenario. We show that: (a) The global HI signal can only be seen in emission,
unlike in the standard CDM models, (b) the density perturbations
induced by primordial fields, leave distinctive signatures of the magnetic
field Jeans' length on the HI two-point correlation function, (c) the length
scale of ionization inhomogeneities is \la 1 \rm Mpc. We find that the peak
expected signal (two-point correlation function) is in
the range of scales for magnetic field strength in the
range . We also discuss the
detectability of the HI signal. The angular resolution of the on-going and
planned radio interferometers allows one to probe only the largest magnetic
field strengths that we consider. They have the sensitivity to detect the
magnetic field-induced features. We show that thefuture SKA has both the
angular resolution and the sensitivity to detect the magnetic field-induced
signal in the entire range of magnetic field values we consider, in an
integration time of one week.Comment: 19 pages, 5 figures, to appear in JCA
Can Virialization Shocks be Detected Around Galaxy Clusters Through the Sunyaev-Zel'dovich Effect?
In cosmological structure formation models, massive non-linear objects in the
process of formation, such as galaxy clusters, are surrounded by large-scale
shocks at or around the expected virial radius. Direct observational evidence
for such virial shocks is currently lacking, but we show here that their
presence can be inferred from future, high resolution, high-sensitivity
observations of the Sunyaev-Zel'dovich (SZ) effect in galaxy clusters. We study
the detectability of virial shocks in mock SZ maps, using simple models of
cluster structure (gas density and temperature distributions) and noise
(background and foreground galaxy clusters projected along the line of sight,
as well as the cosmic microwave background anisotropies). We find that at an
angular resolution of 2'' and sensitivity of 10 micro K, expected to be reached
at ~ 100 GHz frequencies in a ~ 20 hr integration with the forthcoming ALMA
instrument, virial shocks associated with massive M ~ 10^15 M_Sun clusters will
stand out from the noise, and can be detected at high significance. More
generally, our results imply that the projected SZ surface brightness profile
in future, high-resolution experiments will provide sensitive constraints on
the density profile of cluster gas.Comment: 15 pages, submitted to Ap
Constraints on the small-scale power spectrum of density fluctuations from high-redshift gamma-ray bursts
Cosmological models that include suppression of the power spectrum of density
fluctuations on small scales exhibit an exponential reduction of high-redshift,
non-linear structures, including a reduction in the rate of gamma ray bursts
(GRBs). Here we quantify the constraints that the detection of distant GRBs
would place on structure formation models with reduced small-scale power. We
compute the number of GRBs that could be detectable by the Swift satellite at
high redshifts (z > 6), assuming that the GRBs trace the cosmic star formation
history, which itself traces the formation of non-linear structures. We
calibrate simple models of the intrinsic luminosity function of the bursts to
the number and flux distribution of GRBs observed by the Burst And Transient
Source Experiment (BATSE). We find that a discovery of high-z GRBs would imply
strong constraints on models with reduced small-scale power. For example, a
single GRB at z > 10, or 10 GRBs at z > 5, discovered by Swift during its
scheduled two-year mission, would rule out an exponential suppression of the
power spectrum on scales below R_c=0.09 Mpc (exemplified by warm dark matter
models with a particle mass of m_x=2 keV). Models with a less sharp suppression
of small-scale power, such as those with a red tilt or a running scalar index,
n_s, are more difficult to constrain, because they are more degenerate with an
increase in the power spectrum normalization, sigma_8, and with models in which
star-formation is allowed in low-mass minihalos. We find that a tilt of \delta
n_s ~ 0.1 is difficult to detect; however, an observed rate of 1 GRB/yr at z >
12 would yield an upper limit on the running of the spectral index, alpha =
d(n_s)/d(ln k) > -0.05.Comment: 10 pages, 6 figures; Minor changes to match version published in Ap
Probing the Reionization History of the Universe using the Cosmic Microwave Background Polarization
The recent discovery of a Gunn--Peterson (GP) trough in the spectrum of the
redshift 6.28 SDSS quasar has raised the tantalizing possibility that we have
detected the reionization of the universe. However, a neutral fraction (of
hydrogen) as small as 0.1% is sufficient to cause the GP trough, hence its
detection alone cannot rule out reionization at a much earlier epoch. The
Cosmic Microwave Background (CMB) polarization anisotropy offers an alternative
way to explore the dark age of the universe. We show that for most models
constrained by the current CMB data and by the discovery of a GP trough
(showing that reionization occurred at z > 6.3), MAP can detect the
reionization signature in the polarization power spectrum. The expected 1-sigma
error on the measurement of the electron optical depth is around 0.03 with a
weak dependence on the value of that optical depth. Such a constraint on the
optical depth will allow MAP to achieve a 1-sigma error on the amplitude of the
primordial power spectrum of 6%. MAP with two years (Planck with one year) of
observation can distinguish a model with 50% (6%) partial ionization between
redshifts of 6.3 and 20 from a model in which hydrogen was completely neutral
at redshifts greater than 6.3. Planck will be able to distinguish between
different reionization histories even when they imply the same optical depth to
electron scattering for the CMB photons.Comment: ApJ version. Added Figure 2 and reference
A Population of Short-Period Variable Quasars from PTF as Supermassive Black Hole Binary Candidates
Supermassive black hole binaries (SMBHBs) at sub-parsec separations should be
common in galactic nuclei, as a result of frequent galaxy mergers.
Hydrodynamical simulations of circumbinary discs predict strong periodic
modulation of the mass accretion rate on time-scales comparable to the orbital
period of the binary. As a result, SMBHBs may be recognized by the periodic
modulation of their brightness. We conducted a statistical search for periodic
variability in a sample of 35,383 spectroscopically confirmed quasars in the
photometric database of the Palomar Transient Factory (PTF). We analysed
Lomb-Scargle periodograms and assessed the significance of our findings by
modeling each individual quasar's variability as a damped random walk (DRW). We
identified 50 quasars with significant periodicity beyond the DRW model,
typically with short periods of a few hundred days. We find 33 of these to
remain significant after a re-analysis of their periodograms including
additional optical data from the intermediate-PTF and the Catalina Real-Time
Transient Survey (CRTS). Assuming that the observed periods correspond to the
redshifted orbital periods of SMBHBs, we conclude that our findings are
consistent with a population of unequal-mass SMBHBs, with a typical mass ratio
as low as q = M2/M1 ~ 0.01.Comment: MNRAS (accepted), new section 4.
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