1,062 research outputs found
Alien Registration- Fillmore, Perley C. (Baldwin, Cumberland County)
https://digitalmaine.com/alien_docs/32923/thumbnail.jp
All Transients, All the Time: Real-Time Radio Transient Detection with Interferometric Closure Quantities
We demonstrate a new technique for detecting radio transients based on
interferometric closure quantities. The technique uses the bispectrum, the
product of visibilities around a closed-loop of baselines of an interferometer.
The bispectrum is calibration independent, resistant to interference, and
computationally efficient, so it can be built into correlators for real-time
transient detection. Our technique could find celestial transients anywhere in
the field of view and localize them to arcsecond precision. At the Karl G.
Jansky Very Large Array (VLA), such a system would have a high survey speed and
a 5-sigma sensitivity of 38 mJy on 10 ms timescales with 1 GHz of bandwidth.
The ability to localize dispersed millisecond pulses to arcsecond precision in
large volumes of interferometer data has several unique science applications.
Localizing individual pulses from Galactic pulsars will help find X-ray
counterparts that define their physical properties, while finding host galaxies
of extragalactic transients will measure the electron density of the
intergalactic medium with a single dispersed pulse. Exoplanets and active stars
have distinct millisecond variability that can be used to identify them and
probe their magnetospheres. We use millisecond time scale visibilities from the
Allen Telescope Array (ATA) and VLA to show that the bispectrum can detect
dispersed pulses and reject local interference. The computational and data
efficiency of the bispectrum will help find transients on a range of time
scales with next-generation radio interferometers.Comment: Accepted to ApJ. 8 pages, 5 figures, 2 tables. Revised to include
discussion of non-Gaussian statistics of techniqu
Detection of Signals from Cosmic Reionization using Radio Interferometric Signal Processing
Observations of the HI 21cm transition line promises to be an important probe
into the cosmic dark ages and epoch of reionization. One of the challenges for
the detection of this signal is the accuracy of the foreground source removal.
This paper investigates the extragalactic point source contamination and how
accurately the bright sources ( ~Jy) should be removed in order to
reach the desired RMS noise and be able to detect the 21cm transition line.
Here, we consider position and flux errors in the global sky-model for these
bright sources as well as the frequency independent residual calibration
errors. The synthesized beam is the only frequency dependent term included
here. This work determines the level of accuracy for the calibration and source
removal schemes and puts forward constraints for the design of the cosmic
reionization data reduction scheme for the upcoming low frequency arrays like
MWA,PAPER, etc. We show that in order to detect the reionization signal the
bright sources need to be removed from the data-sets with a positional accuracy
of arc-second. Our results also demonstrate that the efficient
foreground source removal strategies can only tolerate a frequency independent
antenna based mean residual calibration error of in amplitude
or degree in phase, if they are constant over each days of
observations (6 hours). In future papers we will extend this analysis to the
power spectral domain and also include the frequency dependent calibration
errors and direction dependent errors (ionosphere, primary beam, etc).Comment: accepted by ApJ; 12 pages, 10 figure
The Radio Luminosity Function of the NEP Distant Cluster Radio Galaxies
A complete sample of 18 X-ray selected clusters of galaxies belonging to the
ROSAT North Ecliptic Pole (NEP) survey has been observed with the Very Large
Array at 1.4 GHz. These are the most distant clusters in the X-ray survey with
redshift in the range 0.3 < z < 0.8.Seventy-nine radio sources are detected
within half an Abell radius with an observed peak brightness >=0.17 mJy/beam,
except for three sources, belonging to the same cluster, which have a higher
peak brightness limit of 0.26 mJy/beam. The NEP field source counts are in good
agreement with the source counts of a comparison survey, the VLA-VIRMOS deep
field survey, indicating that the NEP sample is statistically complete.
Thirty-two out of the 79 sources are within 0.2 Abell radii, twenty-two of them
are considered cluster members based on spectroscopic redshifts or their
optical magnitude and morphological classification. The cluster radio galaxies
are used to construct the Radio Luminosity Function (RLF) of distant X-ray
selected clusters. A comparison with two nearby cluster RLFs shows that the NEP
RLF lies above the local ones, has a steeper slope at low radio powers (<=
10^(24) W/Hz) and shows no evidence for a break at about 6 X 10^(24) W/Hz which
is observed in the nearby cluster RLFs. We discuss briefly the origin and
possible explanations of the differences observed in the radio properties of
nearby and distant clusters of galaxies. The main result of this study is that
the RLF of the distant X-ray clusters is very different from that of the local
rich Abell clusters.Comment: 32 pages, 14 figures, Latex file with use of bib.tex. To appear in
Astronomy and Astrophysics, Main Journal. To appear in Astronomy and
Astrophysics Main Journal. To appear in Astronomy & Astrophysics, Main
Journa
The Expanded Very Large Array -- a New Telescope for New Science
Since its commissioning in 1980, the Very Large Array (VLA) has consistently
demonstrated its scientific productivity. However, its fundamental capabilities
have changed little since 1980, particularly in the key areas of sensitivity,
frequency coverage, and velocity resolution. These limitations have been
addressed by a major upgrade of the array, which began in 2001 and will be
completed at the end of 2012. When completed, the Expanded VLA -- the EVLA --
will provide complete frequency coverage from 1 to 50 GHz, a continuum
sensitivity of typically 1 microJy/beam (in 9 hours with full bandwidth), and a
modern correlator with vastly greater capabilities and flexibility than the
VLA's. In this paper we describe the goals of the EVLA project, its current
status, and the anticipated expansion of capabilities over the next few years.
User access to the array through the OSRO and RSRO programs is described. The
following papers in this special issue, derived from observations in its early
science period, demonstrate the astonishing breadth of this most flexible and
powerful general-purpose telescope.Comment: 6 pages; 2 figures; emulateapj.cls; to appear in the ApJL EVLA
special issu
Full-Stokes polarimetry with circularly polarized feeds - Sources with stable linear and circular polarization in the GHz regime
We present a pipeline that allows recovering reliable information for all
four Stokes parameters with high accuracy. Its novelty relies on the treatment
of the instrumental effects already prior to the computation of the Stokes
parameters contrary to conventional methods, such as the M\"uller matrix one.
The instrumental linear polarization is corrected across the whole telescope
beam and significant Stokes and can be recovered even when the recorded
signals are severely corrupted. The accuracy we reach in terms of polarization
degree is of the order of 0.1-0.2 %. The polarization angles are determined
with an accuracy of almost 1. The presented methodology was applied
to recover the linear and circular polarization of around 150 Active Galactic
Nuclei. The sources were monitored from July 2010 to April 2016 with the
Effelsberg 100-m telescope at 4.85 GHz and 8.35 GHz with a cadence of around
1.2 months. The polarized emission of the Moon was used to calibrate the
polarization angle. Our analysis showed a small system-induced rotation of
about 1 at both observing frequencies. Finally, we identify five
sources with significant and stable linear polarization; three sources remain
constantly linearly unpolarized over the period we examined; a total of 11
sources have stable circular polarization degree and four of
them with non-zero . We also identify eight sources that maintain
a stable polarization angle over the examined period. All this is provided to
the community for polarization observations reference. We finally show that our
analysis method is conceptually different from the traditionally used ones and
performs better than the M\"uller matrix method. Although it was developed for
a system equipped with circularly polarized feeds it can easily be modified for
systems with linearly polarized feeds as well.Comment: 19 pages, 17 figures, accepted for publication in Astronomy &
Astrophysics on May 30, 201
Radio Lobes of Pictor A: an X-ray spatially resolved Study
A new XMM observation has made possible a detailed study of both lobes of the
radio galaxy Pictor A. Their X-ray emission is of non thermal origin and due to
Inverse Compton scattering of the microwave background photons by relativistic
electrons in the lobes, as previously found. In both lobes, the equipartition
magnetic field (Beq) is bigger than the Inverse Compton value (Bic), calculated
from the radio and X-ray flux ratio. The Beq/Bic ratio never gets below 2, in
spite of the large number of reasonable assumptions tested to calculate Beq,
suggesting a lobe energetic dominated by particles. The X-ray data quality is
good enough to allow a spatially resolved analysis. Our study shows that Bic
varies through the lobes. It appears to increase behind the hot spots. On the
contrary, a rather uniform distribution of the particles is observed. As a
consequence, the radio flux density variation along the lobes appears to be
mainly driven by magnetic field changes.Comment: 15 pages, 3 figures, ApJ accepte
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
GRB 050408: An Atypical Gamma-Ray Burst as a Probe of an Atypical Galactic Environment
The bright GRB 050408 was localized by HETE-II near local midnight, enabling
an impressive ground-based followup effort as well as space-based followup from
Swift. The Swift data from the X-Ray Telescope (XRT) and our own optical
photometry and spectrum of the afterglow provide the cornerstone for our
analysis. Under the traditional assumption that the visible waveband was above
the peak synchrotron frequency and below the cooling frequency, the optical
photometry from 0.03 to 5.03 days show an afterglow decay corresponding to an
electron energy index of p_lc = 2.05 +/- 0.04, without a jet break as suggested
by others. A break is seen in the X-ray data at early times (at ~12600 sec
after the GRB). The spectral slope of the optical spectrum is consistent with
p_lc assuming a host-galaxy extinction of A_V = 1.18 mag. The optical-NIR
broadband spectrum is also consistent with p = 2.05, but prefers A_V = 0.57
mag. The X-ray afterglow shows a break at 1.26 x 10^4 sec, which may be the
result of a refreshed shock. This burst stands out in that the optical and
X-ray data suggest a large H I column density of N_HI ~ 10^22 cm^-2; it is very
likely a damped Lyman alpha system and so the faintness of the host galaxy (M_V
> -18 mag) is noteworthy. Moreover, we detect extraordinarily strong Ti II
absorption lines with a column density through the GRB host that exceeds the
largest values observed for the Milky Way by an order of magnitude.
Furthermore, the Ti II equivalent width is in the top 1% of Mg II
absorption-selected QSOs. This suggests that the large-scale environment of GRB
050408 has significantly lower Ti depletion than the Milky Way and a large
velocity width (delta v > 200 km/s).Comment: ApJ submitte
The RRAT Trap: Interferometric Localization of Radio Pulses from J0628+0909
We present the first blind interferometric detection and imaging of a
millisecond radio transient with an observation of transient pulsar J0628+0909.
We developed a special observing mode of the Karl G. Jansky Very Large Array
(VLA) to produce correlated data products (i.e., visibilities and images) on a
time scale of 10 ms. Correlated data effectively produce thousands of beams on
the sky that can localize sources anywhere over a wide field of view. We used
this new observing mode to find and image pulses from the rotating radio
transient (RRAT) J0628+0909, improving its localization by two orders of
magnitude. Since the location of the RRAT was only approximately known when
first observed, we searched for transients using a wide-field detection
algorithm based on the bispectrum, an interferometric closure quantity. Over 16
minutes of observing, this algorithm detected one transient offset roughly 1'
from its nominal location; this allowed us to image the RRAT to localize it
with an accuracy of 1.6". With a priori knowledge of the RRAT location, a
traditional beamforming search of the same data found two, lower significance
pulses. The refined RRAT position excludes all potential multiwavelength
counterparts, limiting its optical luminosity to L_i'<1.1x10^31 erg/s and
excluding its association with a young, luminous neutron star.Comment: Submitted to ApJ, 7 pages, 5 figure
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