164,936 research outputs found
Frequency Limits on Naked-Eye Optical Transients Lasting from Minutes to Years
How often do bright optical transients occur on the sky but go unreported? To
constrain the bright end of the astronomical transient function, a systematic
search for transients that become bright enough to be noticed by the unaided
eye was conducted using the all-sky monitors of the Night Sky Live network. Two
fisheye continuous cameras (CONCAMs) operating over three years created a data
base that was searched for transients that appeared in time-contiguous CCD
frames. Although a single candidate transient was found (Nemiroff and Shamir
2006), the lack of more transients is used here to deduce upper limits to the
general frequency of bright transients. To be detected, a transient must have
increased by over three visual magnitudes to become brighter than visual
magnitude 5.5 on the time scale of minutes to years. It is concluded that, on
the average, fewer than 0.0040 ( seconds) transients with
duration between minutes and hours, occur anywhere on the sky at any
one time. For transients on the order of months to years, fewer than 160
( year) occur, while for transients on the order of years to
millennia, fewer than 50 ( year) occur.Comment: Accepted for publication in A
SubmilliJansky Transients in Archival Radio Observations
[ABRIDGED] We report the results of a 944-epoch survey for transient sources
with archival data from the Very Large Array spanning 22 years with a typical
epoch separation of 7 days. Observations were obtained at 5 or 8.4 GHz for a
single field of view with a full-width at half-maximum of 8.6' and 5.1',
respectively, and achieved a typical point-source detection threshold at the
beam center of ~300 microJy per epoch. Ten transient sources were detected with
a significance threshold such that only one false positive would be expected.
Of these transients, eight were detected in only a single epoch. Two transients
were too faint to be detected in individual epochs but were detected in
two-month averages. None of the ten transients was detected in longer-term
averages or associated with persistent emission in the deep image produced from
the combination of all epochs. The cumulative rate for the short timescale
radio transients above 370 microJy at 5 and 8.4 GHz is 0.07 < R < 40 deg^-2
yr^-1, where the uncertainty is due to the unknown duration of the transients,
20 min < t_char < 7 days. A two-epoch survey for transients will detect 1.5 +/-
0.4 transient per square degrees above a flux density of 370 microJy. Two
transients are associated with galaxies at z=0.040 and z=0.249. These may be
similar to the peculiar Type Ib/c radio supernova SN 1998bw associated with GRB
980428. Six transients have no counterparts in the optical or infrared (R=27,
Ks=18). The hosts and progenitors of these transients are unknown.Comment: Accepted for ApJ; full quality figures available at
http://astro.berkeley.edu/~gbower/ps/rt.pd
Classification methods for noise transients in advanced gravitational-wave detectors
Noise of non-astrophysical origin will contaminate science data taken by the
Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO) and
Advanced Virgo gravitational-wave detectors. Prompt characterization of
instrumental and environmental noise transients will be critical for improving
the sensitivity of the advanced detectors in the upcoming science runs. During
the science runs of the initial gravitational-wave detectors, noise transients
were manually classified by visually examining the time-frequency scan of each
event. Here, we present three new algorithms designed for the automatic
classification of noise transients in advanced detectors. Two of these
algorithms are based on Principal Component Analysis. They are Principal
Component Analysis for Transients (PCAT), and an adaptation of LALInference
Burst (LIB). The third algorithm is a combination of an event generator called
Wavelet Detection Filter (WDF) and machine learning techniques for
classification. We test these algorithms on simulated data sets, and we show
their ability to automatically classify transients by frequency, SNR and
waveform morphology
Reconstructing solar wind inhomogeneous structures from stereoscopic observations in white-light: Small transients along the Sun-Earth line
The Heliospheric Imagers (HI) on board the two spacecraft of the Solar
Terrestrial Relations Observatory (STEREO) provided white-light images of
transients in the solar wind from dual perspectives from 2007 to 2014. In this
paper, we develop a new method to identify and locate the transients
automatically from simultaneous images from the two inner telescopes, known as
HI-1, based on a correlation analysis. Correlation coefficient (cc) maps along
the Sun-Earth line are constructed for the period from 1 Jan 2010 to 28 Feb
2011. From the maps, transients propagating along the Sun-Earth line are
identified, and a 27-day periodic pattern is revealed, especially for
small-scale transients. Such a periodicity in the transient pattern is
consistent with the rotation of the Sun's global magnetic structure and the
periodic crossing of the streamer structures and slow solar wind across the
Sun-Earth line, and this substantiates the reliability of our method and the
high degree of association between the small-scale transients of the slow solar
wind and the coronal streamers. Besides, it is suggested by the cc map that
small-scale transients along the Sun-Earth line are more frequent than
large-scale transients by a factor of at least 2, and that they quickly
diffused into background solar wind within about 40 Rs in terms of the
signal-to-noise ratio of white-light emissions. The method provides a new tool
to reconstruct inhomogeneous structures in the heliosphere from multiple
perspectives.Comment: 24 pages, 9 figures, to be published on Journal of Geophysical
Research - Space Physic
Upconversion Luminescence Transients
Inhomogeneous active-ion distributions in laser materials lead to strong deviations of upconversion versus direct luminescence transients from the quadratic law of energy-transfer upconversion. Measured luminescence decay curves in and confirm experimentally the predicted deviations. Differences in energy migration within the metastable level of are indentified
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