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 (N$\gg$1), via transitional scattering regime (N$\sim$2,3), 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