497 research outputs found
An Early Warning System for Asteroid Impact
Earth is bombarded by meteors, occasionally by one large enough to cause a
significant explosion and possible loss of life. Although the odds of a deadly
asteroid strike in the next century are low, the most likely impact is by a
relatively small asteroid, and we suggest that the best mitigation strategy in
the near term is simply to move people out of the way. We describe an "early
warning" system that could provide a week's notice of most sizable asteroids or
comets on track to hit the Earth. This system, dubbed "Asteroid
Terrestrial-impact Last Alert System" (ATLAS), comprises two observatories
separated by about 100km that simultaneously scan the visible sky twice a
night, and can be implemented immediately for relatively low cost. The
sensitivity of ATLAS permits detection of 140m asteroids (100 Mton impact
energy) three weeks before impact, and 50m asteroids a week before arrival. An
ATLAS alarm, augmented by other observations, should result in a determination
of impact location and time that is accurate to a few kilometers and a few
seconds. In addition to detecting and warning of approaching asteroids, ATLAS
will continuously monitor the changing universe around us: most of the variable
stars in our galaxy, many micro-lensing events from stellar alignments,
luminous stars and novae in nearby galaxies, thousands of supernovae, nearly a
million quasars and active galactic nuclei, tens of millions of galaxies, and a
billion stars. With two views per day ATLAS will make the variable universe as
familiar to us as the sunrise and sunset.Comment: 33 pages, 7 figures, accepted for publication in PASP, Jan 201
Determining the Type, Redshift, and Phase of a Supernova Spectrum
We present an algorithm to identify the types of supernova spectra, and
determine their redshift and phase. This algorithm, based on the correlation
techniques of Tonry & Davis, is implemented in the SuperNova IDentification
code (SNID). It is used by members of the ESSENCE project to determine whether
a noisy spectrum of a high-redshift supernova is indeed of type Ia, as opposed
to, e.g., type Ib/c. Furthermore, by comparing the correlation redshifts
obtained using SNID with those determined from narrow lines in the supernova
host galaxy spectrum, we show that accurate redshifts (with a typical error <
0.01) can be determined for SNe Ia for which a spectrum of the host galaxy is
unavailable. Last, the phase of an input spectrum is determined with a typical
accuracy of ~3 days.Comment: 10 pages, 7 figures. To appear in "The Multicoloured Landscape of
Compact Objects and their Explosive Progenitors: Theory vs Observations"
(Cefalu, Sicily, June 2006). Eds. L. Burderi et al. (New York: AIP
Fuzzy Supernova Templates II: Parameter Estimation
Wide field surveys will soon be discovering Type Ia supernovae (SNe) at rates
of several thousand per year. Spectroscopic follow-up can only scratch the
surface for such enormous samples, so these extensive data sets will only be
useful to the extent that they can be characterized by the survey photometry
alone. In a companion paper (Rodney and Tonry, 2009) we introduced the SOFT
method for analyzing SNe using direct comparison to template light curves, and
demonstrated its application for photometric SN classification. In this work we
extend the SOFT method to derive estimates of redshift and luminosity distance
for Type Ia SNe, using light curves from the SDSS and SNLS surveys as a
validation set. Redshifts determined by SOFT using light curves alone are
consistent with spectroscopic redshifts, showing a root-mean-square scatter in
the residuals of RMS_z=0.051. SOFT can also derive simultaneous redshift and
distance estimates, yielding results that are consistent with the currently
favored Lambda-CDM cosmological model. When SOFT is given spectroscopic
information for SN classification and redshift priors, the RMS scatter in
Hubble diagram residuals is 0.18 mags for the SDSS data and 0.28 mags for the
SNLS objects. Without access to any spectroscopic information, and even without
any redshift priors from host galaxy photometry, SOFT can still measure
reliable redshifts and distances, with an increase in the Hubble residuals to
0.37 mags for the combined SDSS and SNLS data set. Using Monte Carlo
simulations we predict that SOFT will be able to improve constraints on
time-variable dark energy models by a factor of 2-3 with each new generation of
large-scale SN surveys.Comment: 20 pages, 7 figures, accepted to ApJ; paper 1 is arXiv:0910.370
Reconciliation of the Surface Brightness Fluctuations and Type Ia Supernovae Distance Scales
We present Hubble Space Telescope measurements of surface brightness
fluctuations (SBF) distances to early-type galaxies that have hosted Type Ia
supernovae (SNIa). The agreement in the relative SBF and SNIa multicolor light
curve shape and delta-m_15 distances is excellent. There is no systematic scale
error with distance, and previous work has shown that SBF and SNIa give
consistent ties to the Hubble flow. However, we confirm a systematic offset of
about 0.25 mag in the distance zero points of the two methods, and we trace
this offset to their respective Cepheid calibrations. SBF has in the past been
calibrated with Cepheid distances from the H_0 Key Project team, while SNIa
have been calibrated with Cepheid distances from the team composed of Sandage,
Saha, and collaborators. When the two methods are calibrated in a consistent
way, their distances are in superb agreement. Until the conflict over the
``long'' and ``short'' extragalactic Cepheid distances among many galaxies is
resolved, we cannot definitively constrain the Hubble constant to better than
about 10%, even leaving aside the additional uncertainty in the distance to the
Large Magellanic Cloud, common to both Cepheid scales. However, recent
theoretical SBF predictions from stellar population models favor the Key
Project Cepheid scale, while the theoretical SNIa calibration lies between the
long and short scales. In addition, while the current SBF distance to M31/M32
is in good agreement with the RR Lyrae and red giant branch distances,
calibrating SBF with the longer Cepheid scale would introduce a 0.3 mag offset
with respect to the RR Lyrae scale.Comment: 13 pages, 3 PostScript figures, LaTeX with AASTeX 5.02 and natbib.sty
v7.0 (included). Accepted for publication in The Astrophysical Journa
Transit Analysis Package (TAP and autoKep): IDL Graphical User Interfaces for Extrasolar Planet Transit Photometry
We present an IDL graphical user interface-driven software package designed
for the analysis of extrasolar planet transit light curves. The Transit
Analysis Package (TAP) software uses Markov Chain Monte Carlo (MCMC) techniques
to fit light curves using the analytic model of Mandel and Agol (2002). The
package incorporates a wavelet based likelihood function developed by Carter
and Winn (2009) which allows the MCMC to assess parameter uncertainties more
robustly than classic chi-squared methods by parameterizing uncorrelated
"white" and correlated "red" noise. The software is able to simultaneously
analyze multiple transits observed in different conditions (instrument, filter,
weather, etc). The graphical interface allows for the simple execution and
interpretation of Bayesian MCMC analysis tailored to a user's specific data set
and has been thoroughly tested on ground-based and Kepler photometry. AutoKep
provides a similar GUI for the preparation of Kepler MAST archive data for
analysis by TAP or any other analysis software. This paper describes the
software release and provides instructions for its use.Comment: 8 pages, 2 figure
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