126 research outputs found
Detection of polarized quasi-periodic microstructure emission in millisecond pulsars
Microstructure emission, involving short time scale, often quasi-periodic,
intensity fluctuations in subpulse emission, is well known in normal period
pulsars. In this letter, we present the first detections of quasi-periodic
microstructure emission from millisecond pulsars (MSPs), from Giant Metrewave
Radio Telescope (GMRT) observations of two MSPs at 325 and 610 MHz. Similar to
the characteristics of microstructure observed in normal period pulsars, we
find that these features are often highly polarized, and exhibit quasi-periodic
behavior on top of broader subpulse emission, with periods of the order of a
few s. By measuring their widths and periodicities from single pulse
intensity profiles and their autocorrelation functions, we extend the
microstructure timescale - rotation period relationship by more than an order
of magnitude down to rotation periods 5 ms, and find it to be consistent
with the relationship derived earlier for normal pulsars. The similarity of
behavior is remarkable, given the significantly different physical properties
of MSPs and normal period pulsars, and rules out several previous speculations
about the possible different characteristics of microstructure in MSP radio
emission. We discuss the possible reasons for the non-detection of these
features in previous high time resolution MSP studies along with the physical
implications of our results, both in terms of a geometric beam sweeping model
and temporal modulation model for micropulse production.Comment: 6 pages, 4 figures, 1 table. Accepted for publication in ApJ Letter
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
Single pulse polarization study of pulsars B0950+08 and B1642-03: micropulse properties and mixing of orthogonal modes
We present the results of a high-time resolution polarization study of single
pulses from pulsars B0950+08 and B1642-03. Single pulses from pulsar B0950+08
sometimes show isolated micropulses without any significant associated subpulse
emission. Assuming that the properties of such micropulses represent the
intrinsic nature of micropulse emission, we characterize the width and
polarization properties of these `intrinsic' microstructures. Most of the
`intrinsic' micropulses (~90%) follow common characteristic polarization
properties, while the average width of these micropulses is consistent with the
general micropulse population from this pulsar. Single pulses from these
pulsars show a diverse range of polarization properties, including
depolarization and mixing of two orthogonal modes resulting in polarization
position angle jumps. We present a superposition model of the two orthogonal
modes which can explain depolarization, the observed position angle jumps, and
associated changes in other polarization parameters.Comment: 12 pages, 18 figures, Accepted for publication in the Monthly Notices
of the Royal Astronomical Society (MNRAS
Implementing and Characterizing Real-time Broadband RFI Excision for the GMRT Wideband Backend
The Giant Metrewave Radio Telescope (GMRT) is being upgraded to increase the
receiver sensitivity. This makes the receiver more susceptible to man-made
Radio Frequency Interference (RFI). To improve the receiver performance in
presence of RFI, real-time RFI excision (filtering) is incorporated in the GMRT
wideband backend (GWB). The RFI filtering system is implemented on FPGA and
CPU-GPU platforms to detect and remove broadband and narrowband RFI. The RFI is
detected using a threshold-based technique where the threshold is computed
using Median Absolute Deviation (MAD) estimator. The filtering is carried out
by replacing the RFI samples by either noise samples or constant value or
threshold. This paper describes the status of the real-time broadband RFI
excision system in the wideband receiver chain of the upgraded GMRT (uGMRT).
The test methodology for carrying out various tests to demonstrate the
performance of broadband RFI excision at the system level and on radio
astronomical imaging experiments are also described.Comment: 7 pages, 7 figure
Absolute Broadband Polarization Behaviour of PSR B0329+54: A Glimpse of the Core Emission Process
In this paper we report multifrequency single pulse polarization observations
of the PSR B0329+54 normal mode using the Giant Meterwave Radio Telescope at
325 and 610 MHz and the Effelsberg Observatory at 2695 MHz. Our observations
show that towards the central part of the polarization position-angle traverse
there is an unusual ``arc''-like structure, which is comprised of a broadband
``kink'' and a frequency-dependent ``spot.'' The features are intimately
connected with the intensity dependence of the core component: the stronger
emission arrives earlier and its linear polarization is displaced farther along
the ``kink''. Moreover, at high intensities, the circular polarization is --/+
antisymmetric; the nearly complete positive circular is characteristic of the
weaker, later core subpulses. We find that the ``kink'' emission is associated
with the extraordinary (X) propagation mode, and hence propagation effects do
not appear capable of producing the core component's broadband,
intensity-dependent emission. Rather, the overall evidence points to a largely
geometric interpretation in which the ``kink'' provides a rare glimpse of the
accelerating cascade or height-dependent amplifier responsible for the core
radiation.Comment: Accepted for publication in MNRAS, replaced with a newer versio
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