8,384 research outputs found
The Non-homogeneous Poisson Process for Fast Radio Burst Rates
This paper presents the non-homogeneous Poisson process (NHPP) for modeling
the rate of fast radio bursts (FRBs) and other infrequently observed
astronomical events. The NHPP, well-known in statistics, can model changes in
the rate as a function of both astronomical features and the details of an
observing campaign. This is particularly helpful for rare events like FRBs
because the NHPP can combine information across surveys, making the most of all
available information. The goal of the paper is two-fold. First, it is intended
to be a tutorial on the use of the NHPP. Second, we build an NHPP model that
incorporates beam patterns and a power law flux distribution for the rate of
FRBs. Using information from 12 surveys including 15 detections, we find an
all-sky FRB rate of 586.88 events per sky per day above a flux of 1 Jy (95\%
CI: 271.86, 923.72) and a flux power-law index of 0.91 (95\% CI: 0.57, 1.25).
Our rate is lower than other published rates, but consistent with the rate
given in Champion et al. 2016.Comment: 19 pages, 2 figure
Eroded Soils Need Phosphorus
When the top soil has washed away or blown away—crop yields drop. Most farmers know this. But just why won’t the subsoil produce as well as the original top soil? Is there any particular element that we can add to eroded soils which will step up crop yields? These are questions we have been looking into at the Iowa Station
The First Detailed X-ray Observations of High-Redshift, Optically-Selected Clusters: XMM-Newton Results for Cl 1324+3011 at z = 0.76 and Cl 1604+4304 at z = 0.90
We present the first detailed X-ray observations of optically-selected
clusters at high redshift. Two clusters, Cl 1324+3011 at z = 0.76 and Cl
1604+4304 at z = 0.90, were observed with XMM-Newton. The optical center of
each cluster is coincident with an extended X-ray source whose emission is
detected out to a radius of 0.5 Mpc. The emission from each cluster appears
reasonably circular, with some indication of asymmetries and more complex
morphologies. Similarly to other optically-selected clusters at redshifts of z
> 0.4, both clusters are modest X-ray emitters with bolometric luminosities of
only Lx = 1.4 - 2.0 x 10^(44) erg/s. We measure gas temperatures of T = 2.88
(+0.71/-0.49) keV for Cl 1324+3011 and 2.51 (+1.05/-0.69) keV for Cl 1604+4304.
The X-ray properties of both clusters are consistent with the high-redshift
Lx-T relation measured from X-ray-selected samples at z > 0.5. However, based
on the local relations, their X-ray luminosities and temperatures are low for
their measured velocity dispersions (sigma). The clusters are cooler by a
factor of 2 - 9 compared to the local sigma-T relation. We briefly discuss the
possible explanations for these results.Comment: 14 pages, 4 figures; accepted for publication in Astrophysical
Journal Letters; version with full resolution figures available at
http://bubba.ucdavis.edu/~lubin/xmm.pd
Colors, magnitudes and velocity dispersions in early-type galaxies: Implications for galaxy ages and metallicities
We present an analysis of the color-magnitude-velocity dispersion relation
for a sample of 39320 early-type galaxies within the Sloan Digital Sky Survey.
We demonstrate that the color-magnitude relation is entirely a consequence of
the fact that both the luminosities and colors of these galaxies are correlated
with stellar velocity dispersions. Previous studies of the color-magnitude
relation over a range of redshifts suggest that the luminosity of an early-type
galaxy is an indicator of its metallicity, whereas residuals in color from the
relation are indicators of the luminosity-weighted age of its stars. We show
that this, when combined with our finding that velocity dispersion plays a
crucial role, has a number of interesting implications. First, galaxies with
large velocity dispersions tend to be older (i.e., they scatter redward of the
color-magnitude relation). Similarly, galaxies with large dynamical mass
estimates also tend to be older. In addition, at fixed luminosity, galaxies
which are smaller, or have larger velocity dispersions, or are more massive,
tend to be older. Second, models in which galaxies with the largest velocity
dispersions are also the most metal poor are difficult to reconcile with our
data. However, at fixed velocity dispersion, galaxies have a range of ages and
metallicities: the older galaxies have smaller metallicities, and vice-versa.
Finally, a plot of velocity dispersion versus luminosity can be used as an age
indicator: lines of constant age run parallel to the correlation between
velocity dispersion and luminosity.Comment: 12 pages, 9 figures. Accepted by A
The Spectrum and Variability of Circular Polarization in Sagittarius A* from 1.4 to 15 GHz
We report here multi-epoch, multi-frequency observations of the circular
polarization in Sagittarius A*, the compact radio source in the Galactic
Center. Data taken from the VLA archive indicate that the fractional circular
polarization at 4.8 GHz was -0.31% with an rms scatter of 0.13% from 1981 to
1998, in spite of a factor of 2 change in the total intensity. The sign
remained negative over the entire time range, indicating a stable magnetic
field polarity. In the Summer of 1999 we obtained 13 epochs of VLA A-array
observations at 1.4, 4.8, 8.4 and 15 GHz. In May, September and October of 1999
we obtained 11 epochs of Australia Telescope Compact Array observations at 4.8
and 8.5 GHz. In all three of the data sets, we find no evidence for linear
polarization greater than 0.1% in spite of strong circular polarization
detections. Both VLA and ATCA data sets support three conclusions regarding the
fractional circular polarization: the average spectrum is inverted with a
spectral index ~0.5 +/- 0.2; the degree of variability is roughly constant on
timescales of days to years; and, the degree of variability increases with
frequency. We also observed that the largest increase in fractional circular
polarization was coincident with the brightest flare in total intensity.
Significant variability in the total intensity and fractional circular
polarization on a timescale of 1 hour was observed during this flare,
indicating an upper limit to the size of 70 AU at 15 GHz. The fractional
circular polarization at 15 GHz reached -1.1% and the spectral index is
strongly inverted during this flare. We conclude that the spectrum has two
components that match the high and low frequency total intensity components.
(abridged)Comment: Accepted for publication in ApJ, 40 pages, 18 figure
The Planetary Nebula System and Dynamics in the Outer Halo of NGC 5128
The halos of elliptical galaxies are faint and difficult to explore, but they
contain vital clues to both structure and formation. We present the results of
an imaging and spectroscopic survey for planetary nebulae (PNe) in the nearby
elliptical NGC 5128. We extend the work of Hui et al.(1995) well into the halo
of the galaxy--out to distances of 100 and 50 kpc along the major and minor
axes. We now know of 1141 PNe in NGC 5128, 780 of which are confirmed. Of these
780 PNe, 349 are new from this survey, and 148 are at radii beyond 20 kpc. PNe
exist at distances up to 80 kpc (~15 r_e), showing that the stellar halo
extends to the limit of our data. This study represents by far the largest
kinematic study of an elliptical galaxy to date, both in the number of velocity
tracers and in radial extent. We confirm the large rotation of the PNe along
the major axis, and show that it extends in a disk-like feature into the halo.
The rotation curve of the stars flattens at ~100 km/s with V/sigma between 1
and 1.5, and with the velocity dispersion of the PNe falling gradually at
larger radii. The two-dimensional velocity field exhibits a zero-velocity
contour with a pronounced twist, showing that the galaxy potential is likely
triaxial in shape, tending toward prolate. The total dynamical mass of the
galaxy within 80 kpc is ~5 x 10^{11} M_sun, with M/L_B ~ 13. This mass-to-light
ratio is much lower than what is typically expected for elliptical galaxies.Comment: 21 pages, 13 figures (figures 3-8 best viewed in color), accepted for
publication in the Astrophysical Journa
Sunyaev - Zel'dovich fluctuations from spatial correlations between clusters of galaxies
We present angular power spectra of the cosmic microwave background radiation
anisotropy due to fluctuations of the Sunyaev-Zel'dovich (SZ) effect through
clusters of galaxies. A contribution from the correlation among clusters is
especially focused on, which has been neglected in the previous analyses.
Employing the evolving linear bias factor based on the Press-Schechter
formalism, we find that the clustering contribution amounts to 20-30% of the
Poissonian one at degree angular scales. If we exclude clusters in the local
universe, it even exceeds the Poissonian noise, and makes dominant contribution
to the angular power spectrum. As a concrete example, we demonstrate the
subtraction of the ROSAT X-ray flux-limited cluster samples. It indicates that
we should include the clustering effect in the analysis of the SZ fluctuations.
We further find that the degree scale spectra essentially depend upon the
normalization of the density fluctuations, i.e., \sigma_8, and the gas mass
fraction of the cluster, rather than the density parameter of the universe and
details of cluster evolution models. Our results show that the SZ fluctuations
at the degree scale will provide a possible measure of \sigma_8, while the
arc-minute spectra a probe of the cluster evolution. In addition, the
clustering spectrum will give us valuable information on the bias at high
redshift, if we can detect it by removing X-ray luminous clusters.Comment: 11 pages, 4 figures, submitted to Astrophysical Journa
Towards a Holistic View of the Heating and Cooling of the Intracluster Medium
(Abridged) X-ray clusters are conventionally divided into two classes: "cool
core" (CC) clusters and "non-cool core" (NCC) clusters. Yet relatively little
attention has been given to the origins of this dichotomy and, in particular,
to the energetics and thermal histories of the two classes. We develop a model
for the entropy profiles of clusters starting from the configuration
established by gravitational shock heating and radiative cooling. At large
radii, gravitational heating accounts for the observed profiles and their
scalings well. However, at small and intermediate radii, radiative cooling and
gravitational heating cannot be combined to explain the observed profiles of
either type of cluster. The inferred entropy profiles of NCC clusters require
that material is preheated prior to cluster collapse in order to explain the
absence of low entropy (cool) material in these systems. We show that a similar
modification is also required in CC clusters in order to match their properties
at intermediate radii. In CC clusters, this modification is unstable, and an
additional process is required to prevent cooling below a temperature of a few
keV. We show that this can be achieved by adding a self-consistent AGN feedback
loop in which the lowest-entropy, most rapidly cooling material is heated so
that it rises buoyantly to mix with material at larger radii. The resulting
model does not require fine tuning and is in excellent agreement with a wide
variety of observational data. Some of the other implications of this model are
briefly discussed.Comment: 27 pages, 13 figures, MNRAS accepted. Discussion of cluster heating
energetics extended, results unchange
Is there a Supermassive Black Hole at the Center of the Milky Way?
This review outlines the observations that now provide an overwhelming
scientific case that the center of our Milky Way Galaxy harbors a supermassive
black hole. Observations at infrared wavelength trace stars that orbit about a
common focal position and require a central mass (M) of 4 million solar masses
within a radius of 100 Astronomical Units. Orbital speeds have been observed to
exceed 5,000 km/s. At the focal position there is an extremely compact radio
source (Sgr A*), whose apparent size is near the Schwarzschild radius
(2GM/c^2). This radio source is motionless at the ~1 km/s level at the
dynamical center of the Galaxy. The mass density required by these observations
is now approaching the ultimate limit of a supermassive black hole within the
last stable orbit for matter near the event horizon.Comment: Invited review submitted to International Journal of Modern Physics
D; 23 pages; 10 figure
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