188 research outputs found
The stationary phase point method for transitional scattering: diffractive radio scintillation for pulsar
The stationary phase point (SPP) method in one-dimensional case is introduced
to treat the diffractive scintillation. From weak scattering, where the SPP
number N=1, to strong scattering (N1), via transitional scattering regime
(N2,3), we find that the modulation index of intensity experiences the
monotonically increasing from 0 to 1 with the scattering strength,
characterized by the ratio of Fresnel scale \rf to diffractive scale
\rdiff.Comment: Hanas Meeting paper, appear in ChJAA, 2006, 6, Su
First Results from MASIV: The Micro-Arcsecond Scintillation-Induced Variability Survey
We are undertaking a large-scale, Micro-Arcsecond Scintillation-Induced
Variability (MASIV) survey of the northern sky, Dec > 0 deg, at 4.9 GHz with
the VLA. Our objective is to construct a sample of 100 to 150 scintillating
extragalactic sources with which to examine both the microarcsecond structure
and the parent populations of these sources, and to probe the turbulent
interstellar medium responsible for the scintillation. We report on our first
epoch of observations which revealed variability on timescales ranging from
hours to days in 85 of 710 compact flat-spectrum sources. The number of highly
variable sources, those with RMS flux density variations greater than 4% of the
mean, increases with decreasing source flux density but rapid, large amplitude
variables such as J1819+3845 are very rare. When compared with a model for the
scintillation due to irregularities in a 500 pc thick electron layer, our
preliminary results indicate maximum brightness temperatures ~10E+12 K, similar
to those obtained from VLBI surveys even though interstellar scintillation is
not subject to the same angular resolution limit.Comment: 18 pages, 5 figures. To appear in the Astronomical Journa
The Size of the Radio-Emitting Region in Low-luminosity Active Galactic Nuclei
We have used the VLA to study radio variability among a sample of 18 low
luminosity active galactic nuclei (LLAGNs), on time scales of a few hours to 10
days. The goal was to measure or limit the sizes of the LLAGN radio-emitting
regions, in order to use the size measurements as input to models of the radio
emission mechanisms in LLAGNs. We detect variability on typical time scales of
a few days, at a confidence level of 99%, in half of the target galaxies.
Either variability that is intrinsic to the radio emitting regions, or that is
caused by scintillation in the Galactic interstellar medium, is consistent with
the data. For either interpretation, the brightness temperature of the emission
is below the inverse-Compton limit for all of our LLAGNs, and has a mean value
of about 1E10 K. The variability measurements plus VLBI upper limits imply that
the typical angular size of the LLAGN radio cores at 8.5 GHz is 0.2
milliarcseconds, plus or minus a factor of two. The ~ 1E10 K brightness
temperature strongly suggests that a population of high-energy nonthermal
electrons must be present, in addition to a hypothesized thermal population in
an accretion flow, in order to produce the observed radio emission.Comment: 61 pages, 17 figures, 10 tables. Accepted for publication in the
Astrophysical Journa
Intra-day variability observations of S5 0716+714 over 4.5 years at 4.8 GHz
We aim to search for evidence of annual modulation in the time scales of the
BL Lac object S5 0716+714. The intra-day variability (IDV) observations were
carried out monthly from 2005 to 2009, with the Urumqi 25m radio telescope at
4.8 GHz. The source has shown prominent IDV as well as long-term flux
variations. The IDV time scale does show evidence in favor of an annual
modulation, suggesting that the IDV of 0716+714 is dominated by interstellar
scintillation. The source underwent a strong outburst phase between mid-2008
and mid-2009; a second intense flare was observed in late 2009, but no
correlation between the total flux density and the IDV time scale is found,
implying that the flaring state of the source does not have serious
implications for the general characteristics of its intra-day variability.
However, we find that the inner-jet position angle is changing throughout the
years, which could result in an annual modulation noise in the anisotropic ISS
model fit. There is also an indication that the lowest IDV amplitudes (rms flux
density) correspond to the slowest time scales of IDV, which would be
consistent with an ISS origin of the IDV of 0716+714.Comment: 6 pages, 7 figures, accepted for publication in A&A; corrected typos
in Table
Probing the Eclipse of J0737-3039A with Scintillation
We have examined the interstellar scintillations of the pulsars in the double
pulsar binary system. Near the time of the eclipse of pulsar A by the
magnetosphere of B, the scintillations from both pulsars should be highly
correlated because the radiation is passing through the same interstellar
plasma. We report confirmation of this effect using 820 and 1400 MHz
observations made with the Green Bank Telescope. The correlation allows us to
constrain the projected relative position of the two pulsars at closest
approach to be 4000 +/- 2000 km, corresponding to an inclination that is only
0.29 +/-0.14 degrees away from 90 degrees. It also produces a two-dimensional
map of the spatial correlation of the interstellar scintillation. This shows
that the interstellar medium in the direction of the pulsars is significantly
anisotropic. When this anisotropy is included in the orbital fitting, the
transverse velocity of the center of mass is reduced from the previously
published value of 141 +/- 8.5 km/s to 66 +/- 15 km/s.Comment: 7 pages, 7 figures, accepted by Ap
Profile instabilities of the millisecond pulsar PSR J1022+1001
We present evidence that the integrated profiles of some millisecond pulsars
exhibit severe changes that are inconsistent with the moding phenomenon as
known from slowly rotating pulsars. We study these profile instabilities in
particular for PSR J1022+1001 and show that they occur smoothly, exhibiting
longer time constants than those associated with moding. In addition, the
profile changes of this pulsar seem to be associated with a relatively
narrow-band variation of the pulse shape. Only parts of the integrated profile
participate in this process which suggests that the origin of this phenomenon
is intrinsic to the pulsar magnetosphere and unrelated to the interstellar
medium. A polarization study rules out profile changes due to geometrical
effects produced by any sort of precession. However, changes are observed in
the circularly polarized radiation component. In total we identify four
recycled pulsars which also exhibit instabilities in the total power or
polarization profiles due to an unknown phenomenon (PSRs J1022+1001,
J1730-2304, B1821-24, J2145-0750).
The consequences for high precision pulsar timing are discussed in view of
the standard assumption that the integrated profiles of millisecond pulsars are
stable. As a result we present a new method to determine pulse times-of-arrival
that involves an adjustment of relative component amplitudes of the template
profile. Applying this method to PSR J1022+1001, we obtain an improved timing
solution with a proper motion measurement of -17 \pm 2 mas/yr in ecliptic
longitude. Assuming a distance to the pulsar as inferred from the dispersion
measure this corresponds to an one-dimensional space velocity of 50 km/s.Comment: 29 pages, 12 figures, accepted for publication in Ap
The Asymmetry Coefficient for Interstellar Scintillation of Extragalactic Radio Sources
Comparing the asymmetry coefficients and scintillation indices for observed
time variations of the intensity of the radiation of extragalactic sources and
the predictions of theoretical models is a good test of the nature of the
observed variations. Such comparisons can be used to determine whether
flux-density variations are due to scintillation in the interstellar medium or
are intrinsic to the source. In the former case, they can be used to estimate
the fraction of the total flux contributed by the compact component (core)
whose flux-density variations are brought about by inhomogeneities in the
interstellar plasma. Results for the radio sources PKS 0405-385, B0917+624, PKS
1257-336, and J1819+3845 demonstrate that the scintillating component in these
objects makes up from 50% to 100% of the total flux, and that the intrinsic
angular sizes of the sources at 5 GHz is 10-40 microarcseconds. The
characteristics of the medium giving rise to the scintillations are presented
Interstellar scintillation as the origin of rapid radio variability in the quasar J1819+3845
Quasars shine brightly due to the liberation of gravitational energy as matter falls onto a supermassive black hole in the centre of a galaxy. Variations in the radiation received from active galactic nuclei (AGN) are studied at all wavelengths, revealing the tiny dimensions of the region and the processes of fuelling the black hole. Some AGN are variable at optical and shorter wavelengths, and display radio outbursts over years and decades. These AGN often also show faster variations at radio wavelengths (intraday variability, IDV) which have been the subject of much debate. The simplest explanation, supported by a correlation in some sources between the optical (intrinsic) and faster radio variations, is that the rapid radio variations are intrinsic. However, this explanation implies physically difficult brightness temperatures, suggesting that the variations may be due to scattering of the incident radiation in the interstellar medium of our Galaxy. Here we present results which show unambiguously that the variations in one extreme case are due to interstellar scintillation. We also measure the transverse velocity of the scattering material, revealing a surprising high velocity plasma close to the Solar System
Circular Polarization Induced by Scintillation in a Magnetized Medium
A new theory is presented for the development of circular polarization as
radio waves propagate through the turbulent, birefringent interstellar medium.
The fourth order moments of the wavefield are calculated and it is shown that
unpolarized incident radiation develops a nonzero variance in circular
polarization. A magnetized turbulent medium causes the Stokes parameters to
scintillate in a non-identical manner. A specific model for this effect is
developed for the case of density fluctuations in a uniform magnetic field.Comment: 16 pages, 1 figure, Phys. Rev. E, accepte
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