11,162 research outputs found
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
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
The High Time Resolution Universe Survey - V: Single-pulse energetics and modulation properties of 315 pulsars
We report on the pulse-to-pulse energy distributions and phase-resolved
modulation properties for catalogued pulsars in the southern High Time
Resolution Universe intermediate-latitude survey. We selected the 315 pulsars
detected in a single-pulse search of this survey, allowing a large sample
unbiased regarding any rotational parameters of neutron stars. We found that
the energy distribution of many pulsars is well-described by a log-normal
distribution, with few deviating from a small range in log-normal scale and
location parameters. Some pulsars exhibited multiple energy states
corresponding to mode changes, and implying that some observed "nulling" may
actually be a mode-change effect. PSRJ1900-2600 was found to emit weakly in its
previously-identified "null" state. We found evidence for another state-change
effect in two pulsars, which show bimodality in their nulling time scales; that
is, they switch between a continuous-emission state and a single-pulse-emitting
state. Large modulation occurs in many pulsars across the full integrated
profile, with increased sporadic bursts at leading and trailing sub-beam edges.
Some of these high-energy outbursts may indicate the presence of "giant pulse"
phenomena. We found no correlation with modulation and pulsar period, age, or
other parameters. Finally, the deviation of integrated pulse energy from its
average value was generally quite small, despite the significant phase-resolved
modulation in some pulsars; we interpret this as tenuous evidence of energy
regulation between distinct pulsar sub-beams.Comment: Before full MNRAS publication, supplementary material is available
temporarily at http://dl.dropbox.com/u/22076931/supplementary_material.pd
Evidence for a Photospheric Component in the Prompt Emission of the Short GRB120323A and its Effects on the GRB Hardness-Luminosity Relation
The short GRB 120323A had the highest flux ever detected with the Fermi/GBM.
Here we study its remarkable spectral properties and their evolution using two
spectral models: (i) a single emission component scenario, where the spectrum
is modeled by the empirical Band function, and (ii) a two component scenario,
where thermal (Planck-like) emission is observed simultaneously with a
non-thermal component (a Band function). We find that the latter model fits the
integrated burst spectrum significantly better than the former, and that their
respective spectral parameters are dramatically different: when fit with a Band
function only, the Epeak of the event is unusually soft for a short GRB, while
adding a thermal component leads to more typical short GRB values. Our
time-resolved spectral analysis produces similar results. We argue here that
the two-component model is the preferred interpretation for GRB 120323A, based
on: (i) the values and evolution of the Band function parameters of the two
component scenario, which are more typical for a short GRB, and (ii) the
appearance in the data of a significant hardness-intensity correlation,
commonly found in GRBs, when we employee two-component model fits; the
correlation is non-existent in the Band-only fits. GRB 110721A, a long burst
with an intense photospheric emission, exhibits the exact same behavior. We
conclude that GRB 120323A has a strong photospheric emission contribution,
first time observed in a short GRB. Magnetic dissipation models are difficult
to reconcile with these results, which instead favor photospheric thermal
emission and fast cooling synchrotron radiation from internal shocks. Finally,
we derive a possibly universal hardness-luminosity relation in the source frame
using a larger set of GRBs L,i=(1.59+/-0.84).10^50 (Epeak,i)^(1.33+/-0.07)
erg/s), which could be used as a possible redshift estimator for cosmology.Comment: 27 pages, 13 figures, Accepted by ApJ (April, 7th 2013
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
pulsar_spectra: A pulsar flux density catalogue and spectrum fitting repository
We present the pulsar_spectra software repository, an open-source pulsar flux
density catalogue and automated spectral fitting software that finds the best
spectral model and produces publication-quality plots. The Python-based
software includes features that enable users in the astronomical community to
add newly published spectral measurements to the catalogue as they become
available. The spectral fitting software is an implementation of the method
described in Jankowski et al. (2018) which uses robust statistical methods to
decide on the best-fitting model for individual pulsar spectra. pulsar_spectra
is motivated by the need for a centralised repository for pulsar flux density
measurements to make published measurements more accessible to the astronomical
community and provide a suite of tools for measuring spectra
Cosmic ray produced Mg<SUP>28</SUP>, Si<SUP>31</SUP>, S<SUP>38</SUP>, C<SUP>l38</SUP>, Cl<SUP>34m</SUP> and other short-lived radioisotopes in wet precipitation
The concentrations of seven radioisotopes, expected to be produced in the troposphere by interactions of secondary cosmic rays with atmospheric nuclei, have been measured in "fresh" rain collections. The half-lives of these isotopes range from about half an hour to a day. The procedures developed for rapid, specific and sensitive analyses of these nuclides are discussed. Detection of two of the isotopes studied, Cl39 (half-life: 55 mins.) and Na24 (15 hrs.), has been reported earlier by Winsberg and Rodel respectively. Amongst the remaining nuclides, two: S38 (2.9 hrs.) and Cl38 (37.3 mins.) were independently and almost simultaneously detected by us and Perkins and his collaborators. Three other isotopes, Cl34m (32 mins.), Si31 (2.6 hrs.) and Mg28 (21.2 hrs.), detected in the present work have not yet been reported elsewhere. The nature of cosmic ray secondary particles responsible for the production of these short-lived radionuclides in the troposphere is discussed. Isotope production is found to vary strongly with altitude in the troposphere; it increases by a factor of two every 1.5-2 km depending on the radioisotope under question. This fact combined with the availability of several isotopes of half-lives ranging from about half an hour to a day leads to the possibility of using them as tracers for studying short-term tropospheric processes, e.g. those occurring prior to and during condensation in a precipitating cloud. The implications of the present measurements are discussed
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