1,225 research outputs found
A Search for Single Radio Pulses and Bursts from Southern AXPs
We observed four southern AXPs in 1999 near 1400 MHz with the Parkes 64-m
radio telescope to search for periodic radio emission. No Fourier candidates
were discovered in the initial analysis, but the recent radio activity observed
for the AXP XTE J1810-197 has prompted us to revisit these data to search for
single radio pulses and bursts. The data were searched for both persistent and
bursting radio emission at a wide range of dispersion measures, but no
detections of either kind were made. These results further weaken the proposed
link between rotating radio transient sources and magnetars. However, continued
radio searches of these and other AXPs at different epochs are warranted given
the transient nature of the radio emission seen from XTE J1810-197, which until
very recently was the only known radio-emitting AXP.Comment: 3 pages, including 1 table. To appear in the proceedings of "40 Years
of Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17, 2007,
McGill University, Montreal, Canad
An axisymmetric hydrodynamical model for the torus wind in AGN. III: Spectra from 3D radiation transfer calculations
We calculate a series of synthetic X-ray spectra from outflows originating
from the obscuring torus in active galactic nuclei (AGN). Such modeling
includes 2.5D hydrodynamical simulations of an X-ray excited torus wind,
including the effects of X-ray heating, ionization, and radiation pressure. 3D
radiation transfer calculations are performed in the 3D Sobolev approximation.
Synthetic X-ray line spectra and individual profiles of several strong lines
are shown at different inclination angles, observing times, and for different
characteristics of the torus.
Our calculations show that rich synthetic warm absorber spectra from 3D
modeling are typically observed at a larger range of inclinations than was
previously inferred from simple analysis of the transmitted spectra. In
general, our results are supportive of warm absorber models based on the
hypothesis of an "X-ray excited funnel flow" and are consistent with
characteristics of such flows inferred from observations of warm absorbers from
Seyfert 1 galaxies.Comment: 31 pages, 10 figure
Remarkable Spectral Variability of PDS 456
We report on the highest to date signal-to-noise-ratio X-ray spectrum of the
luminous quasar PDS 456, as obtained during two XMM-Newton orbits in September
2007. The present spectrum is considerably different from several previous
X-ray spectra recorded for PDS 456 since 1998. The ultra-high-velocity outflow
seen as recently as February 2007 is not detected in absorption. Conversely, a
significant reflection component is detected. The reflection model suggests the
reflecting medium may be outflowing at a velocity v/c = -0.06 +/- 0.02. The
present spectrum is analyzed in the context of the previous ones in an attempt
to understand all spectra within the framework of a single model. We examine
whether an outflow with variable partial covering of the X-ray source along the
line of sight that also reflects the source from other lines of sight can
explain the dramatic variations in the broad-band spectral curvature of PDS
456. It is established that absorption plays a major role in shaping the
spectrum of other epochs, while the 2007 XMM-Newton spectrum is dominated by
reflection, and the coverage of the source by the putative outflow is small (<
20%).Comment: submitted to Ap
Radio Pulse Properties of the Millisecond Pulsar PSR J0437-4715. I. Observations at 20cm
We present a total of 48 minutes of observations of the nearby, bright
millisecond pulsar PSR J0437-4715 taken at the Parkes radio observatory in
Australia. The data were obtained at a central radio frequency of 1380 MHz
using a high-speed tape recorder that permitted coherent Nyquist sampling of 50
MHz of bandwidth in each of two polarizations. Using the high time resolution
available from this voltage recording technique, we have studied a variety of
single-pulse properties, most for the first time in a millisecond pulsar. We
find no evidence for "diffractive" quantization effects in the individual pulse
arrival times or amplitudes as have been reported for this pulsar at lower
radio frequency using coarser time resolution (Ables et al. 1997). Overall, we
find that the single pulse properties of PSR J0437-4715 are similar to those of
the common slow-rotating pulsars, even though this pulsar's magnetosphere and
surface magnetic field are several orders of magnitude smaller than those of
the general population. The pulsar radio emission mechanism must therefore be
insensitive to these fundamental neutron star properties.Comment: 24 Postscript pages, 11 eps figures. Accepted for publication in the
Astrophysical Journal. Abbreviated abstract follow
What makes the Crab pulsar shine?
Our high time resolution observations of individual pulses from the Crab
pulsar show that the main pulse and interpulse differ in temporal behavior,
spectral behavior, polarization and dispersion. The main pulse properties are
consistent with one current model of pulsar radio emission, namely, soliton
collapse in strong plasma turbulence. The high-frequency interpulse is quite
another story. Its dynamic spectrum cannot easily be explained by any current
emission model; its excess dispersion must come from propagation through the
star's magnetosphere. We suspect the high-frequency interpulse does not follow
the ``standard model'', but rather comes from some unexpected region within the
star's magnetosphere. Similar observations of other pulsars will reveal whether
the radio emission mechanisms operating in the Crab pulsar are unique to that
star, or can be identified in the general population.Comment: 5 pages, 2 figures, to appear in proceedings of meeting "Forty Years
of Pulsars: Millisecond Pulsars, Magnetars and More", Montreal, August 200
The GBT350 Survey of the Northern Galactic Plane for Radio Pulsars and Transients
Using the Green Bank Telescope (GBT) and Pulsar Spigot at 350MHz, we have
surveyed the Northern Galactic Plane for pulsars and radio transients. This
survey covers roughly 1000 square degrees of sky within 75 deg < l < 165 deg
and |b| < 5.5 deg, a region of the Galactic Plane inaccessible to both the
Parkes and Arecibo multibeam surveys. The large gain of the GBT along with the
high time and frequency resolution provided by the Spigot make this survey more
sensitive by factors of about 4 to slow pulsars and more than 10 to millisecond
pulsars (MSPs), compared with previous surveys of this area. In a preliminary,
reduced-resolution search of all the survey data, we have discovered 33 new
pulsars, almost doubling the number of known pulsars in this part of the
Galaxy. While most of these sources were discovered by normal periodicity
searches, 5 of these sources were first identified through single, dispersed
bursts. We discuss the interesting properties of some of these new sources.
Data processing using the data's full-resolution is ongoing, with the goal of
uncovering MSPs missed by our first, coarse round of processing.Comment: To appear in the proceedings of "Forty Years of Pulsars: Millisecond
Pulsars, Magnetars and More" held in Montreal, Canada, August 12-17, 2007. 3
pages, 2 figure
The Subpulse Modulation Properties of Pulsars and its Frequency Dependence
A large sample of about two hundred pulsars have been observed to study their
subpulse modulation at an observing wavelength of (when achievable) both 21 and
92 cm using the Westerbork Synthesis Radio Telescope. For 57 pulsars drifting
subpulses are discovered for the first time and are confirmed for many others.
This leads to the conclusion that it could well be that the drifting subpulse
mechanism is an intrinsic property of the emission mechanism itself, although
for some pulsars it is difficult or impossible to detect. It appears that the
youngest pulsars have the most disordered subpulses and the subpulses become
more and more organized into drifting subpulses as the pulsar ages. Drifting
subpulses are in general found at both frequencies and the measured values of
P3 at the two frequencies are highly correlated, showing the broadband nature
of this phenomenon. Also the modulation indices measured at the two frequencies
are clearly correlated, although at 92 cm they are on average possibly higher.
The correlations with the modulation indices are argued to be consistent with
the picture in which the radio emission is composed out of a drifting subpulse
signal plus a quasi-steady signal which becomes, on average, stronger at high
observing frequencies. There is no obvious correlation found between P3 and the
pulsar age (or any other pulsar parameter) contrary to reports in the past.Comment: Proceedings of the 40 Years of Pulsars: Millisecond Pulsars,
Magnetars and More conference in Montrea
Modeling Variable Emission Lines in AGNs: Method and Application to NGC 5548
We present a new scheme for modeling the broad line region in active galactic
nuclei (AGNs). It involves photoionization calculations of a large number of
clouds, in several pre-determined geometries, and a comparison of the
calculated line intensities with observed emission line light curves. Fitting
several observed light curves simultaneously provides strong constraints on
model parameters such as the run of density and column density across the
nucleus, the shape of the ionizing continuum, and the radial distribution of
the emission line clouds. When applying the model to the Seyfert 1 galaxy NGC
5548, we were able to reconstruct the light curves of four ultraviolet
emission-lines, in time and in absolute flux. This has not been achieved by any
previous work. We argue that the Balmer lines light curves, and possibly also
the MgII2798 light curve, cannot be tested in this scheme because of the
limitations of present-day photoionization codes. Our fit procedure can be used
to rule out models where the particle density scales as r^{-2}, where r is the
distance from the central source. The best models are those where the density
scales as r^{-1} or r^{-1.5}. We can place a lower limit on the column density
at a distance of 1 ld, of N_{col}(r=1) >~ 10^{23} cm^{-2} and limit the
particle density to be in the range of 10^{12.5}>N(r=1)>10^{11} cm^{-3}. We
have also tested the idea that the spectral energy distribution (SED) of the
ionizing continuum is changing with continuum luminosity. None of the
variable-shape SED tried resulted in real improvement over a constant SED case
although models with harder continuum during phases of higher luminosity seem
to fit better the observed spectrum. Reddening and/or different composition
seem to play a minor role, at least to the extent tested in this work.Comment: 12 pages, including 9 embedded EPS figures, accepted for publication
in Ap
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