10,746 research outputs found
Searches for radio transients
Exploration of the transient Universe is an exciting and fast-emerging area
within radio astronomy. Known transient phenomena range in time scales from
sub-nanoseconds to years or longer, thus spanning a huge range in time domain
and hinting a rich diversity in their underlying physical processes. Transient
phenomena are likely locations of explosive or dynamic events and they offer
tremendous potential to uncover new physics and astrophysics. A number of
upcoming next-generation radio facilities and recent advances in computing and
instrumentation have provided a much needed impetus for this field which has
remained a relatively uncharted territory for the past several decades. In this
paper we focus mainly on the class of phenomena that occur on very short time
scales (i.e. from milliseconds to nanoseconds), known as {\it
fast transients}, the detections of which involve considerable signal
processing and data management challenges, given the high time and frequency
resolutions required in their explorations, the role of propagation effects to
be considered and a multitude of deleterious effects due to radio frequency
interference. We will describe the techniques, strategies and challenges
involved in their detections and review the world-wide efforts currently under
way, both through scientific discoveries enabled by the ongoing large-scale
surveys at Parkes and Arecibo, as well as technical developments involving the
exploratory use of multi-element array instruments such as VLBA and GMRT. Such
developments will undoubtedly provide valuable inputs as next-generation arrays
such as LOFAR and ASKAP are designed and commissioned. With their wider fields
of view and higher sensitivities, these instruments, and eventually the SKA,
hold great potential to revolutionise this relatively nascent field, thereby
opening up exciting new science avenues in astrophysics.Comment: To appear in the special issue of the Bulletin of the Astronomical
Society of India on Transients at different wavelengths, eds D.J. Saikia and
D.A. Green. 21 pages, 5 figures. http://www.ncra.tifr.res.in/~bas
A CLEAN-based Method for Deconvolving Interstellar Pulse Broadening from Radio Pulses
Multipath propagation in the interstellar medium distorts radio pulses, an
effect predominant for distant pulsars observed at low frequencies. Typically,
broadened pulses are analyzed to determine the amount of propagation-induced
pulse broadening, but with little interest in determining the undistorted pulse
shapes. In this paper we develop and apply a method that recovers both the
intrinsic pulse shape and the pulse broadening function that describes the
scattering of an impulse. The method resembles the CLEAN algorithm used in
synthesis imaging applications, although we search for the best pulse
broadening function, and perform a true deconvolution to recover intrinsic
pulse structre. As figures of merit to optimize the deconvolution, we use the
positivity and symmetry of the deconvolved result along with the mean square
residual and the number of points below a given threshold. Our method makes no
prior assumptions about the intrinsic pulse shape and can be used for a range
of scattering functions for the interstellar medium. It can therefore be
applied to a wider variety of measured pulse shapes and degrees of scattering
than the previous approaches. We apply the technique to both simulated data and
data from Arecibo observations.Comment: 9 pages, 6 figures, Accepted for publication in the Astrophysical
Journa
Monolithic InP-Based Grating Spectrometer for Wavelength-Division Multiplexed Systems at 1.5 ÎŒm
A monolithic InP-based grating spectrometer for use in wavelength-division multiplexed systems at 1.5 ÎŒm is reported.
The spectrometer uses a single etched reflective focusing diffraction grating and resolves >50 channels at 1 nm spacing with a ~0.3nm channel width and at least 19dB channel isolation. Operation is essentially of the state of the input polarisation
The High Time Resolution Universe Pulsar Survey - X. Discovery Of Four Millisecond Pulsars And Updated Timing Solutions Of A Further 12
Here, we report on the discovery of four millisecond pulsars (MSPs) in the High Time Resolution Universe (HTRU) pulsar survey being conducted at the Parkes 64 m radio telescope. All four MSPs are in binary systems and are likely to have white dwarf companions. Additionally, we present updated timing solutions for 12 previously published HTRU MSPs, revealing new observational parameters such as five proper motion measurements and significant temporal dispersion measure variations in PSR J1017-7156. We discuss the case of PSR J1801-3210, which shows no significant period derivative after four years of timing data. Our best-fitting solution shows a of the order of 10-23, an extremely small number compared to that of a typical MSP. But, it is likely that the pulsar lies beyond the Galactic Centre, and an unremarkable intrinsic is reduced to close to zero by the Galactic potential acceleration. Furthermore, we highlight the potential to employ PSR J1801-3210 in the strong equivalence principle test due to its wide and circular orbit. In a broader comparison with the known MSP population, we suggest a correlation between higher mass functions and the presence of eclipses in âvery low mass binary pulsarsâ, implying that eclipses are observed in systems with high orbital inclinations. We also suggest that the distribution of the total mass of binary systems is inversely related to the Galactic height distribution. Finally, we report on the first detection of PSRs J1543-5149 and J1811-2404 as gamma-ray pulsars
RFI Identification and Mitigation Using Simultaneous Dual Station Observations
RFI mitigation is a critically important issue in radio astronomy using
existing instruments as well as in the development of next-generation radio
telescopes, such as the Square Kilometer Array (SKA). Most designs for the SKA
involve multiple stations with spacings of up to a few thousands of kilometers
and thus can exploit the drastically different RFI environments at different
stations. As demonstrator observations and analysis for SKA-like instruments,
and to develop RFI mitigation schemes that will be useful in the near term, we
recently conducted simultaneous observations with Arecibo Observatory and the
Green Bank Telescope (GBT). The observations were aimed at diagnosing RFI and
using the mostly uncorrelated RFI between the two sites to excise RFI from
several generic kinds of measurements such as giant pulses from Crab-like
pulsars and weak HI emission from galaxies in bands heavily contaminated by
RFI. This paper presents observations, analysis, and RFI identification and
excision procedures that are effective for both time series and spectroscopy
applications using multi-station data.Comment: 12 pages, 9 figures (4 in ps and 5 in jpg formats), Accepted for
publication in Radio Scienc
Quasars: a supermassive rotating toroidal black hole interpretation
A supermassive rotating toroidal black hole (TBH) is proposed as the
fundamental structure of quasars and other jet-producing active galactic
nuclei. Rotating protogalaxies gather matter from the central gaseous region
leading to the birth of massive toroidal stars whose internal nuclear reactions
proceed very rapidly. Once the nuclear fuel is spent, gravitational collapse
produces a slender ring-shaped TBH remnant. These events are typically the
first supernovae of the host galaxies. Given time the TBH mass increases
through continued accretion by several orders of magnitude, the event horizon
swells whilst the central aperture shrinks. The difference in angular
velocities between the accreting matter and the TBH induces a magnetic field
that is strongest in the region of the central aperture and innermost
ergoregion. Due to the presence of negative energy states when such a
gravitational vortex is immersed in an electromagnetic field, circumstances are
near ideal for energy extraction via non-thermal radiation including the
Penrose process and superradiant scattering. This establishes a self-sustaining
mechanism whereby the transport of angular momentum away from the quasar by
relativistic bi-directional jets reinforces both the modulating magnetic field
and the TBH/accretion disk angular velocity differential. Quasar behaviour is
extinguished once the BH topology becomes spheroidal. Similar mechanisms may be
operating in microquasars, SNe and GRBs when neutron density or BH tori arise.
In certain circumstances, long-term TBH stability can be maintained by a
negative cosmological constant, otherwise the classical topology theorems must
somehow be circumvented. Preliminary evidence is presented that Planck-scale
quantum effects may be responsible.Comment: 26 pages, 14 figs, various corrections and enhancements, final
versio
Relativistic spin precession in the binary PSR J11416545
PSR J11416545 is a precessing binary pulsar that has the rare potential to
reveal the two-dimensional structure of a non-recycled pulsar emission cone. It
has undergone of relativistic spin precession in the
years since its discovery. In this paper, we present a detailed Bayesian
analysis of the precessional evolution of the width of the total intensity
profile, to understand the changes to the line-of-sight impact angle ()
of the pulsar using four different physically motivated prior distribution
models. Although we cannot statistically differentiate between the models with
confidence, the temporal evolution of the linear and circular polarisations
strongly argue that our line-of-sight crossed the magnetic pole around MJD
54000 and that only two models remain viable. For both these models, it appears
likely that the pulsar will precess out of our line-of-sight in the next
years, assuming a simple beam geometry. Marginalising over suggests
that the pulsar is a near-orthogonal rotator and provides the first
polarization-independent estimate of the scale factor () that
relates the pulsar beam opening angle () to its rotational period ()
as : we find it to be at 1.4
GHz with 99\% confidence. If all pulsars emit from opposite poles of a dipolar
magnetic field with comparable brightness, we might expect to see evidence of
an interpulse arising in PSR J11416545, unless the emission is patchy.Comment: Accepted for publication in Astrophysical Journal Letter
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