255 research outputs found
Flux density measurements of GPS candidate pulsars at 610 MHz using interferometric imaging technique
We conducted radio interferometric observations of six pulsars at 610 MHz
using the Giant Metrewave Radio Telescope (GMRT). All these objects were
claimed or suspected to be the gigahertz-peaked spectra (GPS) pulsars. For a
half of the sources in our sample the interferometric imaging provides the only
means to estimate their flux at 610 MHz due to a strong pulse
scatter-broadening. In our case, these pulsars have very high dispersion
measure values and we present their spectra containing for the first time
low-frequency measurements. The remaining three pulsars were observed at low
frequencies using the conventional pulsar flux measurement method. The
interferometric imaging technique allowed us to re-examine their fluxes at 610
MHz. We were able to confirm the GPS feature in the PSR B182313 spectrum and
select a GPS candidate pulsar. These results clearly demonstrate that the
interferometric imaging technique can be successfully applied to estimate flux
density of pulsars even in the presence of strong scattering.Comment: 7 pages, 3 tables, 5 figures, accepted for publication in MNRA
Gigahertz-peaked spectra pulsars and thermal absorption model
We present the results of our radio interferometric observations of pulsars
at 325 MHz and 610 MHz using the Giant Metrewave Radio Telescope (GMRT). We
used the imaging method to estimate the flux densities of several pulsars at
these radio frequencies. The analysis of the shapes of the pulsar spectra
allowed us to identify five new gigahertz-peaked spectra (GPS) pulsars. Using
the hypothesis that the spectral turnovers are caused by thermal free-free
absorption in the interstellar medium, we modeled the spectra of all known
objects of this kind. Using the model, we were able to put some observational
constrains on the physical parameters of the absorbing matter, which allows us
to distinguish between the possible sources of absorption. We also discuss the
possible effects of the existence of GPS pulsars on future search surveys,
showing that the optimal frequency range for finding such objects would be from
a few GHz (for regular GPS sources) to possibly 10 GHz for pulsars and
radio-magnetars exhibiting very strong absorption.Comment: Accepted for publication in ApJ, 28 pages, 8 figure
Modelling of the radio spectrum evolution in the binary pulsar B125963
In this paper we give the first attempt to model the evolution of the
spectrum of PSR B125963 radio emission while the pulsar orbits the companion
Be star. As suggested by Kijak et al. (Mon. Not. R. Astron. Soc. 418:L114,
2011a) this binary system can be useful in understanding the origin of the
gigahertz-peaked spectrum of pulsars. The model explains, at least
qualitatively, the observed alterations of the spectral shape depending on the
orbital phases of this pulsar. Thus, our results support the hypothesis that
the external factors have a significant impact on the observed radio emission
of a pulsar. The model can also contribute to our understanding of the origin
of some non-typical spectral shapes(e.g. flat or broken spectra).Comment: 9 pages, 2 tables, 3 figure
Flux-density spectral analysis for several pulsars and two newly-identified gigahertz-peaked spectra
In this paper we present results from flux density measurements for 21
pulsars over a wide frequency range, using the Giant Metrewave Radio Telescope
(GMRT) and the Effelsberg telescope. Our sample was a set of mostly newly
discovered pulsars from the selection of candidates for gigahertz-peaked
spectra (GPS) pulsars. Using the results of our observations along with
previously published data, we identify two new GPS pulsars. One of them, PSR
J1740+1000, with dispersion measure of 24 pc cm, is the first GPS pulsar
with such a low DM value.We also selected several strong candidates for objects
with high frequency turnover in their spectra which require further
investigation.We also revisit our source selection criteria for future searches
for GPS pulsars.Comment: 10 pages, 2 tables, 9 figures, accepted for publication in MNRA
Multi-frequency observations and spectral analysis of two gigahertz-peaked spectra pulsars
We report the multi-frequency observations of two pulsars: J1740+1000 and
B1800-21, using the Giant Metrewave Radio Telescope and the Green Bank
Telescope. The main aim of these observations was to estimate the flux density
spectrum of these pulsars, as both of them were previously reported to exhibit
gigahertz-peaked spectra. J1740+1000 is a young pulsar far from the Galactic
plane and the interpretation of its spectrum was inconclusive in the light of
the recent flux density measurements. Our result supports the gigahertz-peaked
interpretation of the PSR J1740+1000 spectrum. B1800-21 is a Vela-like pulsar
near the W30 complex, whose spectrum exhibit a significant change between 2012
and 2014 year. Our analysis shows that the current shape of the spectrum is
similar to that observed before 2009 and confirms that the observed spectral
change happen in a time-scale of a few years.Comment: 9 pages, 7 figure
Toward An Empirical Theory of Pulsar Emission. VII. On the Spectral Behavior of Conal Beam Radii and Emission Heights
In this paper we return to the old problem of conal component-pair widths and
profile dimensions. Observationally, we consider a set of 10 pulsars with
prominent conal component pairs, for which well measured profiles exist over
the largest frequency range now possible. Apart from some tendency to narrow at
high frequency, the conal components exhibit almost constant widths. We use all
three profile measures, the component separation as well as the outside
half-power and 10% widths, to determine conal beam radii, which are the focus
of our subsequent analysis. These radii at different frequencies are well
fitted by a relationship introduced by Thorsett (1991), but the resulting
parameters are highly correlated. Three different types of behavior are found:
one group of stars exhibits a continuous variation of beam radius which can be
extrapolated down to the stellar surface along the ``last open field lines''; a
second group exhibits beam radii which asymptotically approach a minimum high
frequency value that is 3--5 times larger; and a third set shows almost no
spectral change in beam radius at all. The first two behaviors are associated
with outer-cone component pairs; whereas the constant separation appears to
reflect inner-cone emission.Comment: 21 pages, 11 figures, accepted for publication in Astrophysical
Journal, uses aaste
Unraveling the drift behaviour of the remarkable pulsar PSR B0826-34
We present new results from high sensitivity GMRT observations of PSR
B0826-34. We provide a model to explain the observed subpulse drift properties
of this pulsar, including the apparent reversals of the drift direction. In
this model, PSR B0826-34 is close to being an aligned rotator. We solve for the
emission geometry of this pulsar and show that the angle between the rotation
and the magnetic axes is less than 5 deg. We see evidence for as many as 6 to 7
drifting bands in the main pulse at 318 MHz, which are part of a circulating
system of about 15 spark-associated subpulse emission beams. We provide
quantitative treatments of the aliasing problem and various effects of
geometry. The observed drift rate is an aliased version of the true drift rate,
such that a subpulse drifts to the location of the adjacent subpulse (or a
multiple thereof) in about one pulsar period. We show that small variations, of
the order of 3-8%, in the mean drift rate are then enough to explain the
apparent reversals of drift direction. We find the mean circulation time of the
drift pattern to be significantly longer than the predictions of the original
RS75 model and propose an explanation for this, based on modified models with
temperature regulated partial ion flow in the polar vacuum gap. From the
variation of the mean subpulse separation across the main pulse window, we show
that the spark pattern is not centred around the dipole axis, but around a
point much closer (within a degree or so) to the rotation axis -- we discuss
the implication of this.Comment: 23 pages (including 9 figure). Submitted to Astronomy and
Astrophysics on November 11, 200
Spectrum evolution in binary pulsar B1259-63/LS 2883 Be star and gigahertz-peaked spectra
We study the radio spectrum of PSR B1259-63 orbiting around the Be star LS
2883 and show that the shape of the spectrum depends on the orbital phase. At
frequencies below 3 GHz PSR B1259-63 flux densities are lower when measured
near the periastron passage than those measured far from periastron. We suggest
that an interaction of the radio waves with the Be star environment accounts
for this effect. While it is quite natural to explain the pulsar eclipse by the
presence of an equatorial disk around LS 2883, this disk alone cannot be
responsible for the observed spectral evolution of PSR B1259-63 and we,
therefore, propose a qualitative model which explains this evolution. We
consider two mechanisms that might influence the observed radio emission:
free-free absorption and cyclotron resonance. We believe that this binary
system can hold the clue to the understanding of gigahertz-peaked spectra of
pulsars.Comment: 5 pages, 4 figures, MNRAS Letters, 418, L114-L118 (2011
On Pair Production in the Crab Pulsar
We consider the widespread assumption that coherent pulsar radio emission is
based on extended pair production leading to plasma densities highly exceeding
the Goldreich-Julian density. We show as an example that the observed low
frequency (160 MHz) emission of the Crab pulsar is incompatible to the model of
extended pair production. Our results rule out significant pair production if a
plasma process is responsible for coherence and the radio emission originates
from inside the light cylinder.Comment: accepted for publication in ApJ Letters; 4 pages, no figure
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