262 research outputs found
TCF periodogram's high sensitivity: A method for optimizing detection of small transiting planets
We conduct a methodological study for statistically comparing the
sensitivities of two periodograms for weak signal planet detection in transit
surveys: the widely used Box-Least Squares (BLS) algorithm following light
curve detrending and the Transit Comb Filter (TCF) algorithm following
autoregressive ARIMA modeling. Small depth transits are injected into light
curves with different simulated noise characteristics. Two measures of spectral
peak significance are examined: the periodogram signal-to-noise ratio (SNR) and
a False Alarm Probability (FAP) based on the generalized extreme value
distribution. The relative performance of the BLS and TCF algorithms for small
planet detection is examined for a range of light curve characteristics,
including orbital period, transit duration, depth, number of transits, and type
of noise. The TCF periodogram applied to ARIMA fit residuals with the SNR
detection metric is preferred when short-memory autocorrelation is present in
the detrended light curve and even when the light curve noise had white
Gaussian noise. BLS is more sensitive to small planets only under limited
circumstances with the FAP metric. BLS periodogram characteristics are inferior
when autocorrelated noise is present. Application of these methods to TESS
light curves with small exoplanets confirms our simulation results. The study
ends with a decision tree that advises transit survey scientists on procedures
to detect small planets most efficiently. The use of ARIMA detrending and TCF
periodograms can significantly improve the sensitivity of any transit survey
with regularly spaced cadence.Comment: 30 pages, 13 figures, submitted to AAS Journal
The Remarkably Featureless High Resolution X-ray Spectrum of Mrk 478
An observation of Mrk 478 using the Chandra Low Energy Transmission Grating
Spectrometer is presented. The source exhibited 30-40% flux variations on
timescales of order 10000 s together with a slow decline in the spectral
softness over the full 80 ks observation. The 0.15--3.0 keV spectrum is well
fitted by a single power law with photon index of Gamma = 2.91 +/- 0.03.
Combined with high energy data from BeppoSAX, the spectrum from 0.15 to 10 keV
is well fit as the sum of two power laws with Gamma = 3.03 +/- 0.04, which
dominates below 2 keV and 1.4 +/- 0.2, which dominates above 2 keV (quoting 90%
confidence uncertainties). No significant emission or absorption features are
detected in the high resolution spectrum, supporting our previous findings
using the Extreme Ultraviolet Explorer but contradicting the claims of emission
lines by Hwang & Bowyer (1997). There is no evidence of a warm absorber, as
found in the high resolution spectra of many Sy 1 galaxies including others
classified as narrow line Sy 1 galaxies such as Mrk 478. We suggest that the
X-ray continuum may result from Comptonization of disk thermal emission in a
hot corona through a range of optical depths.Comment: 21 pages, 7 figures; accepted for publication in the Astronomical
Journa
Neutral description and exchange of design computational workflows
Proposed in this paper is a neutral representation of design computational workflows which allows
their exchange and sharing between different project partners and across design stages. This is
achieved by the de-coupling of configuration and execution logic. Thus, the same underlying
workflow can be executed with different (fidelity) models and different software tools as long as the
inputs and outputs of the constituent process are kept the same. To this purpose, an object model is
proposed to define different simulation objects, their scope, and hierarchy in the simulation process.
An XML based computer readable representation of workflows based on the proposed object model, is
also suggested. The application of the proposed representation is demonstrated via a case study
involving the exchange of workflows between two design partners. The case study also demonstrates
how the same workflow can be executed using different execution tools and involving different
fidelity models
The Low CO Content of the Extremely Metal Poor Galaxy I Zw 18
We present sensitive molecular line observations of the metal-poor blue
compact dwarf I Zw 18 obtained with the IRAM Plateau de Bure interferometer.
These data constrain the CO J=1-0 luminosity within our 300 pc (FWHM) beam to
be L_CO < 1 \times 10^5 K km s^-1 pc^2 (I_CO < 1 K km s^-1), an order of
magnitude lower than previous limits. Although I Zw 18 is starbursting, it has
a CO luminosity similar to or less than nearby low-mass irregulars (e.g. NGC
1569, the SMC, and NGC 6822). There is less CO in I Zw 18 relative to its
B-band luminosity, HI mass, or star formation rate than in spiral or dwarf
starburst galaxies (including the nearby dwarf starburst IC 10). Comparing the
star formation rate to our CO upper limit reveals that unless molecular gas
forms stars much more efficiently in I Zw 18 than in our own galaxy, it must
have a very low CO-to-H_2 ratio, \sim 10^-2 times the Galactic value. We detect
3mm continuum emission, presumably due to thermal dust and free-free emission,
towards the radio peak.Comment: 5 pages in emulateapj style, accepted by the Astrophysical Journa
The Case for Optically-Thick High Velocity Broad Line Region Gas in Active Galactic Nuclei
A combined analysis of the profiles of the main broad quasar emission lines
in both Hubble Space Telescope and optical spectra shows that while the
profiles of the strong UV lines are quite similar, there is frequently a strong
increase in the Ly-alpha/H-alpha ratio in the high-velocity gas. We show that
the suggestion that the high velocity gas is optically-thin presents many
problems. We show that the relative strengths of the high velocity wings arise
naturally in an optically-thick BLR component. An optically-thick model
successfully explains the equivalent widths of the lines, the Ly-alpha/H-alpha
ratios and flatter Balmer decrements in the line wings, the strengths of CIII]
and the lambda 1400 blend, and the strong variability of high-velocity,
high-ionization lines (especially HeII and HeI).Comment: 34 pages in AASTeX, including 10 pages of figures. Submitted to
Astrophysical Journa
Time Dependence of the Ultraviolet Radiation Field in the Local Interstellar Medium
Far Ultraviolet radiation has been recognized as the main source of heating
of the neutral interstellar gas, and, as a consequence, it determines whether
the thermal balance of the neutral gas results in cold ()
clouds (CNM), warm () clouds (WNM), or a combination of the two.
High FUV fields convert the neutral gas to WNM, while low fields result in CNM.
The sources of FUV radiation are the short-lived massive stars that generally
originate in associations that form in Giant Molecular Clouds present in the
galactic disk. Using McKee & Williams' (1997) distribution of birthrates for OB
associations in the Galaxy, we determine the expected behavior of the
time-dependent FUV field for random positions in the local ISM. The FUV field
is calculated in two bands (912-1100 \AA and 912-2070 \AA) and at the
wavelength 1400 \AA. Our median value for the [912-2070 \AA] band is
times Habing's (1968) value for the radiation field at the solar circle in this
band, and quite close to Draine's (1976) value, . Due to attenuation
by dust, only associations within about 500 pc contribute significantly to the
energy density at a given point. The FUV field is asymmetric at a given point,
and the asymmetry grows for higher fields. The FUV field fluctuates with a
variety of amplitudes, the larger ones being less frequent. The mean field is
about twice the median field because of these fluctuations, or spikes, in the
radiation field. These spikes, which last Myr, are caused by the
infrequent birth of nearby associations. We also model shorter duration spikes
caused by runaway OB stars. The presence of a fluctuating heating rate created
by the fluctuating FUV field converts CNM to WNM and vice versa.Comment: 43 pages, 23 figures, submitted to Ap
Broadening of the Iron emission line in MCG-6-30-15 by Comptonization
We show that the Iron K emission line from MCG-6-30-15 could be broadened due
to Comptonization by a surrounding highly ionized cloud with radius cms. We calculate the temperature of the cloud to be \sim0.21\kev,
provided a reasonable estimate of the UV flux is made. The X-ray/-ray
emission observed from the source is compatible with this model. Such a cloud
should be highly ionized and strong absorption edges are not expected from the
source (Fabian et al 1995).
For a \onlyten{6}\msol black hole the size of the could corresponds to
about 300 Schwarzschild radius. The intrinsic line could then be emitted far
from the black hole and gravitational red-shift and Doppler effects would be
negligible. If the black hole mass is much larger than \onlyten{6}\msol,
gravitational/Doppler red-shifts would also contribute significantly to the
broadening.
We argue that the broad red wing observed in the source does not by itself
imply emission from regions close () to the black hole. However,
Comptonization cannot produce a double peak. The presence of such a feature is
a clear sign of inner disk emission influenced by gravitational and Doppler
effects, perhaps broadened by the Comptonization. We note that simultaneous
broad band (2-100 keV) study of this source can also reveal (or rule out) the
presence of such a Comptonizing cloud.Comment: 2 figures. uses aasms4.sty, accepted by ApJ, email:
[email protected]
Forces on Dust Grains Exposed to Anisotropic Interstellar Radiation Fields
Grains exposed to anisotropic radiation fields are subjected to forces due to
the asymmetric photon-stimulated ejection of particles. These forces act in
addition to the ``radiation pressure'' due to absorption and scattering. Here
we model the forces due to photoelectron emission and the photodesorption of
adatoms. The ``photoelectric'' force depends on the ambient conditions relevant
to grain charging. We find that it is comparable to the radiation pressure when
the grain potential is relatively low and the radiation spectrum is relatively
hard. The calculation of the ``photodesorption'' force is highly uncertain,
since the surface physics and chemsitry of grain materials are poorly
understood at present. For our simple yet plausible model, the photodesorption
force dominates the radiation pressure for grains with size >~0.1 micron
exposed to starlight from OB stars. We find that the anisotropy of the
interstellar radiation field is ~10% in the visible and ultraviolet. We
estimate size-dependent drift speeds for grains in the cold and warm neutral
media and find that micron-sized grains could potentially be moved across a
diffuse cloud during its lifetime.Comment: LaTeX(41 pages, 19 figures), submitted to Ap
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