47,198 research outputs found
A Deep Targeted Search for Fast Radio Bursts from the Sites of Low-Redshift Short Gamma-Ray Bursts
Some short gamma-ray bursts (SGRBs) are thought to be caused by the mergers
of binary neutron stars which may sometimes produce massive neutron star
remnants capable of producing extragalactic fast radio bursts (FRBs). We
conducted a deep search for FRBs from the sites of six low-redshift SGRBs. We
collected high time- and frequency-resolution data from each of the sites for
10 hours using the 2 GHz receiver of the Green Bank Telescope. Two of the SGRB
sites we targeted were visible with the Arecibo Radio Telescope with which we
conducted an additional 10 hours of 1.4 GHz observations for each. We searched
our data for FRBs using the GPU-optimized dedispersion algorithm
and the machine-learning-based package
(Fast Extragalactic Transient Candidate Hunter). We did not discover any FRBs,
but would have detected any with peak flux densities in excess of 87 mJy at the
Green Bank Telescope or 21 mJy at Arecibo with a signal-to-noise ratio of at
least 10. The isotropic-equivalent energy of any FRBs emitted from these sites
in our bands during our observations must not have exceeded a few times
erg, comparable to some of the lowest energy bursts yet seen from the
first known repeating FRB 121102.Comment: 10 pages, 2 figures, submitted to A
Timing analysis techniques at large core distances for multi-TeV gamma ray astronomy
We present an analysis technique that uses the timing information of
Cherenkov images from extensive air showers (EAS). Our emphasis is on distant,
or large core distance gamma-ray induced showers at multi-TeV energies.
Specifically, combining pixel timing information with an improved direction
reconstruction algorithm, leads to improvements in angular and core resolution
as large as ~40% and ~30%, respectively, when compared with the same algorithm
without the use of timing. Above 10 TeV, this results in an angular resolution
approaching 0.05 degrees, together with a core resolution better than ~15 m.
The off-axis post-cut gamma-ray acceptance is energy dependent and its full
width at half maximum ranges from 4 degrees to 8 degrees. For shower directions
that are up to ~6 degrees off-axis, the angular resolution achieved by using
timing information is comparable, around 100 TeV, to the on-axis angular
resolution. The telescope specifications and layout we describe here are geared
towards energies above 10 TeV. However, the methods can in principle be applied
to other energies, given suitable telescope parameters. The 5-telescope cell
investigated in this study could initially pave the way for a larger array of
sparsely spaced telescopes in an effort to push the collection area to >10 km2.
These results highlight the potential of a `sparse array' approach in
effectively opening up the energy range above 10 TeV.Comment: Published in Astroparticle Physic
Swope Supernova Survey 2017a (SSS17a), the Optical Counterpart to a Gravitational Wave Source
On 2017 August 17, the Laser Interferometer Gravitational-wave Observatory
(LIGO) and the Virgo interferometer detected gravitational waves emanating from
a binary neutron star merger, GW170817. Nearly simultaneously, the Fermi and
INTEGRAL telescopes detected a gamma-ray transient, GRB 170817A. 10.9 hours
after the gravitational wave trigger, we discovered a transient and fading
optical source, Swope Supernova Survey 2017a (SSS17a), coincident with
GW170817. SSS17a is located in NGC 4993, an S0 galaxy at a distance of 40
megaparsecs. The precise location of GW170817 provides an opportunity to probe
the nature of these cataclysmic events by combining electromagnetic and
gravitational-wave observations.Comment: 25 pages, 10 figures, 2 tables, published today in Scienc
An optimal method for scheduling observations of large sky error regions for finding optical counterparts to transients
The discovery and subsequent study of optical counterparts to transient
sources is crucial for their complete astrophysical understanding. Various
gamma ray burst (GRB) detectors, and more notably the ground--based
gravitational wave detectors, typically have large uncertainties in the sky
positions of detected sources. Searching these large sky regions spanning
hundreds of square degrees is a formidable challenge for most ground--based
optical telescopes, which can usually image less than tens of square degrees of
the sky in a single night. We present algorithms for optimal scheduling of such
follow--up observations in order to maximize the probability of imaging the
optical counterpart, based on the all--sky probability distribution of the
source position. We incorporate realistic observing constraints like the
diurnal cycle, telescope pointing limitations, available observing time, and
the rising/setting of the target at the observatory location. We use
simulations to demonstrate that our proposed algorithms outperform the default
greedy observing schedule used by many observatories. Our algorithms are
applicable for follow--up of other transient sources with large positional
uncertainties, like Fermi--detected GRBs, and can easily be adapted for
scheduling radio or space--based X--ray followup.Comment: Submitted to ApJ. 18 pages, 15 figure
The UTMOST pulsar timing programme I: overview and first results
We present an overview and the first results from a large-scale pulsar timing
programme that is part of the UTMOST project at the refurbished Molonglo
Observatory Synthesis Radio Telescope (MOST) near Canberra, Australia. We
currently observe more than 400 mainly bright southern radio pulsars with up to
daily cadences. For 205 (8 in binaries, 4 millisecond pulsars) we publish
updated timing models, together with their flux densities, flux density
variability, and pulse widths at 843 MHz, derived from observations spanning
between 1.4 and 3 yr. In comparison with the ATNF pulsar catalogue, we improve
the precision of the rotational and astrometric parameters for 123 pulsars, for
47 by at least an order of magnitude. The time spans between our measurements
and those in the literature are up to 48 yr, which allows us to investigate
their long-term spin-down history and to estimate proper motions for 60
pulsars, of which 24 are newly determined and most are major improvements. The
results are consistent with interferometric measurements from the literature. A
model with two Gaussian components centred at 139 and fits the transverse velocity distribution best. The pulse duty
cycle distributions at 50 and 10 per cent maximum are best described by
log-normal distributions with medians of 2.3 and 4.4 per cent, respectively. We
discuss two pulsars that exhibit spin-down rate changes and drifting subpulses.
Finally, we describe the autonomous observing system and the dynamic scheduler
that has increased the observing efficiency by a factor of 2-3 in comparison
with static scheduling.Comment: 31 pages, 14 figures, 6 tables, accepted for publication in MNRA
Large amplitude tip/tilt estimation by geometric diversity for multiple-aperture telescopes
A novel method nicknamed ELASTIC is proposed for the alignment of
multiple-aperture telescopes, in particular segmented telescopes. It only needs
the acquisition of two diversity images of an unresolved source, and is based
on the computation of a modified, frequency-shifted, cross-spectrum. It
provides a polychromatic large range tip/tilt estimation with the existing
hardware and an inexpensive noniterative unsupervised algorithm. Its
performance is studied and optimized by means of simulations. They show that
with 5000 photo-electrons/sub-aperture/frame and 1024x1024 pixel images,
residues are within the capture range of interferometric phasing algorithms
such as phase diversity. The closed-loop alignment of a 6 sub-aperture mirror
provides an experimental demonstration of the effectiveness of the method.
Author accepted version. Final version is Copyright 2017 Optical Society of
America. One print or electronic copy may be made for personal use only.
Systematic reproduction and distribution, duplication of any material in this
paper for a fee or for commercial purposes, or modifications of the content of
this paper are prohibited.Comment: Final version:
https://www.osapublishing.org/josaa/abstract.cfm?uri=josaa-34-8-127
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