11,517 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
Pulsar Scintillation and the Local Bubble
We present here the results from an extensive scintillation study of twenty
pulsars in the dispersion measure (DM) range 3 - 35 pc cm^-3 caried out using
the Ooty Radio Telescope (ORT) at 327 MHz, to investigate the distribution of
ionized material in the local interstellar medium. Observations were made
during the period January 1993 to August 1995, in which the dynamic
scintillation spectra of these pulsars were regularly monitored over 10 - 90
epochs spanning 100 days. Reliable and accurate estimates of strengths of
scattering have been deduced from the scintillation parameters averaged out for
their long-term fluctuations arising from refractive scintillation (RISS)
effects. Our analysis reveals several anomalies in the scattering strength,
which suggest tht the distribution of scattering material in the Solar
neighborhood is not uniform.
We have modelled these anomalous scattering effects in terms of
inhomogeneities in the distribution of electron dnsity fluctuations in the
local interstellar medium (LISM). Our model suggests the presence of a low
density bubble surrounded by a shell of much higher density fluctuations. We
are able to put constraints on geometrical and scattering properties of such a
structure, and find it to be morphologically similar to the Local Bubble known
from other studies.Comment: 35 pages, 12 figure
Long-Term Scintillation Studies of Pulsars: III. Testing Theoretical Models of Refractive Scintillation
Refractive interstellar scintillation (RISS) is thought to be the cause
behind a variety of phenomena seen at radio wavelengths in pulsars and compact
radio sources. Though there is substantial observational data to support
several consequences of it, the quantitative predictions from theories have not
been thoroughly tested. In this paper, data from our long-term scintillation
study of 18 pulsars are used to test the predictions. The fluctuations of
decorrelation bandwidth (), scintillation time scale () and flux
density (F) are examined for their cross-correlations and compared with the
predictions. The theory predicts a strong correlation between and
, and strong anti-correlations between and F, and and
F. For 5 pulsars, we see a reasonable agreement. There is considerable
difficulty in reconciling the results for the rest of the pulsars. Our analysis
shows the underlying noise sources can sometimes reduce the correlation, but
cannot cause an absence of correlation. It is also unlikely that the poor flux
correlations arise from a hitherto unrecognized intrinsic flux variations. For
PSR B0834+06, which shows anomalous behaviour of persistent drift slopes,
positive correlation is found between and the drift-corrected .
Many pulsars show an anti-correlation between and the drift slope, and
this is in accordance with the simple models of RISS. The detections of
correlated variations of observables and a reasonable agreement between the
predicted and measured correlations for some pulsars confirm RISS as the
primary cause of the observed fluctuations. However, the complexity seen with
the detailed results suggests the necessity of more comprehensive theoretical
treatments for describing refractive fluctuations and their correlations.Comment: 27 pages, 6 Figures, 6 Tables. Accepted for publication in The
Astrophysical Journa
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 Îźm
A monolithic InP-based grating spectrometer for use in wavelength-division multiplexed systems at 1.5 Îźm 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
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