169 research outputs found
Using R-based VOStat as a low resolution spectrum analysis tool
We describe here an online software suite VOStat written mainly for the Virtual Observatory, a novel structure in which astronomers share terabyte scale data. Written mostly in the public-domain statistical computing language and environment R, it can do a variety of statistical analysis on multidimensional, multi-epoch data with errors.
Included are techniques which allow astronomers to start with multi-color data in the form of low-resolution spectra and select special kinds of sources in a variety of ways including color outliers. Here we describe the tool and demonstrate it with an example from Palomar-QUEST, a synoptic sky survey
Towards an Automated Classification of Transient Events in Synoptic Sky Surveys
We describe the development of a system for an automated, iterative,
real-time classification of transient events discovered in synoptic sky
surveys. The system under development incorporates a number of Machine Learning
techniques, mostly using Bayesian approaches, due to the sparse nature,
heterogeneity, and variable incompleteness of the available data. The
classifications are improved iteratively as the new measurements are obtained.
One novel feature is the development of an automated follow-up recommendation
engine, that suggest those measurements that would be the most advantageous in
terms of resolving classification ambiguities and/or characterization of the
astrophysically most interesting objects, given a set of available follow-up
assets and their cost functions. This illustrates the symbiotic relationship of
astronomy and applied computer science through the emerging discipline of
AstroInformatics.Comment: Invited paper, 15 pages, to appear in Statistical Analysis and Data
Mining (ASA journal), ref. proc. CIDU 2011 conf., eds. A. Srivasatva & N.
Chawla, in press (2011
Effective Radii and Color Gradients in Radio Galaxies
We present de Vaucouleurs' effective radii in B and R bands for a sample of
Molonglo Reference Catalogue radio galaxies and a control sample of normal
galaxies. We use the ratio of the scale lengths in the two bands as an
indicator to show that the radio galaxies tend to have excess of blue color in
their inner region much more frequently than the control galaxies. We show that
the scale length ratio is a useful indicator of radial color variation even
when the conventional color gradient is too noisy to serve the purpose.Comment: 11 pages, 4 figures, (LaTeX: aaspp4, epsfig), to appear in ApJL 199
The Next Decade of Astroinformatics and Astrostatistics
Over the past century, major advances in astronomy and astrophysics have been largely driven by improvements in instrumentation and data collection. With the amassing of high quality data from new telescopes, and especially with the advent of deep and large astronomical surveys, it is becoming clear that future advances will also rely heavily on how those data are analyzed and interpreted. New methodologies derived from advances in statistics, computer science, and machine learning are beginning to be employed in sophisticated investigations that are not only bringing forth new discoveries, but are placing them on a solid footing. Progress in wide-field sky surveys, interferometric imaging, precision cosmology, exoplanet detection and characterization, and many subfields of stellar, Galactic and extragalactic astronomy, has resulted in complex data analysis challenges that must be solved to perform scientific inference. Research in astrostatistics and astroinformatics will be necessary to develop the state-of-the-art methodology needed in astronomy. Overcoming these challenges requires dedicated, interdisciplinary research. We recommend: (1) increasing funding for interdisciplinary projects in astrostatistics and astroinformatics; (2) dedicating space and time at conferences for interdisciplinary research and promotion; (3) developing sustainable funding for long-term astrostatisics appointments; and (4) funding infrastructure development for data archives and archive support, state-of-the-art algorithms, and efficient computing
Understanding extreme quasar optical variability with CRTS: I. Major AGN flares
There is a large degree of variety in the optical variability of quasars and
it is unclear whether this is all attributable to a single (set of) physical
mechanism(s). We present the results of a systematic search for major flares in
AGN in the Catalina Real-time Transient Survey as part of a broader study into
extreme quasar variability. Such flares are defined in a quantitative manner as
being atop of the normal, stochastic variability of quasars. We have identified
51 events from over 900,000 known quasars and high probability quasar
candidates, typically lasting 900 days and with a median peak amplitude of
mag. Characterizing the flare profile with a Weibull
distribution, we find that nine of the sources are well described by a
single-point single-lens model. This supports the proposal by Lawrence et al.
(2016) that microlensing is a plausible physical mechanism for extreme
variability. However, we attribute the majority of our events to explosive
stellar-related activity in the accretion disk: superluminous supernovae, tidal
disruption events, and mergers of stellar mass black holes.Comment: 25 pages, 18 figures, accepted for publication by MNRA
A systematic search for close supermassive black hole binaries in the Catalina Real-Time Transient Survey
Hierarchical assembly models predict a population of supermassive black hole
(SMBH) binaries. These are not resolvable by direct imaging but may be
detectable via periodic variability (or nanohertz frequency gravitational
waves). Following our detection of a 5.2 year periodic signal in the quasar PG
1302-102 (Graham et al. 2015), we present a novel analysis of the optical
variability of 243,500 known spectroscopically confirmed quasars using data
from the Catalina Real-time Transient Survey (CRTS) to look for close (< 0.1
pc) SMBH systems. Looking for a strong Keplerian periodic signal with at least
1.5 cycles over a baseline of nine years, we find a sample of 111 candidate
objects. This is in conservative agreement with theoretical predictions from
models of binary SMBH populations. Simulated data sets, assuming stochastic
variability, also produce no equivalent candidates implying a low likelihood of
spurious detections. The periodicity seen is likely attributable to either jet
precession, warped accretion disks or periodic accretion associated with a
close SMBH binary system. We also consider how other SMBH binary candidates in
the literature appear in CRTS data and show that none of these are equivalent
to the identified objects. Finally, the distribution of objects found is
consistent with that expected from a gravitational wave-driven population. This
implies that circumbinary gas is present at small orbital radii and is being
perturbed by the black holes. None of the sources is expected to merge within
at least the next century. This study opens a new unique window to study a
population of close SMBH binaries that must exist according to our current
understanding of galaxy and SMBH evolution.Comment: 29 pages, 10 figures, accepted for publication in MNRAS - this
version contains extended table and figur
Connecting the time domain community with the Virtual Astronomical Observatory
The time domain has been identified as one of the most important areas of
astronomical research for the next decade. The Virtual Observatory is in the
vanguard with dedicated tools and services that enable and facilitate the
discovery, dissemination and analysis of time domain data. These range in scope
from rapid notifications of time-critical astronomical transients to annotating
long-term variables with the latest modeling results. In this paper, we will
review the prior art in these areas and focus on the capabilities that the VAO
is bringing to bear in support of time domain science. In particular, we will
focus on the issues involved with the heterogeneous collections of (ancillary)
data associated with astronomical transients, and the time series
characterization and classification tools required by the next generation of
sky surveys, such as LSST and SKA.Comment: Submitted to Proceedings of SPIE Observatory Operations: Strategies,
Processes and Systems IV, Amsterdam, 2012 July 2-
A possible close supermassive black-hole binary in a quasar with optical periodicity
Quasars have long been known to be variable sources at all wavelengths. Their
optical variability is stochastic, can be due to a variety of physical
mechanisms, and is well-described statistically in terms of a damped random
walk model. The recent availability of large collections of astronomical time
series of flux measurements (light curves) offers new data sets for a
systematic exploration of quasar variability. Here we report on the detection
of a strong, smooth periodic signal in the optical variability of the quasar PG
1302-102 with a mean observed period of 1,884 88 days. It was identified
in a search for periodic variability in a data set of light curves for 247,000
known, spectroscopically confirmed quasars with a temporal baseline of
years. While the interpretation of this phenomenon is still uncertain, the most
plausible mechanisms involve a binary system of two supermassive black holes
with a subparsec separation. Such systems are an expected consequence of galaxy
mergers and can provide important constraints on models of galaxy formation and
evolution.Comment: 19 pages, 6 figures. Published online by Nature on 7 January 201
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