T. V. Smirnova

Five Years of Pulsar Flux Density Monitoring: Refractive Scintillation and the Interstellar Medium

We have monitored the radio flux density of 21 pulsars on a daily basis for five years. The 610 MHz flux density time series for these pulsars range from nearly constant for the most distant and heavily scattered pulsars to rapidly varying, saturated time series for more nearby pulsars. The measured stability of the flux density from the most distant pulsars (variations less than 5%) implies that the average radio emission from pulsars, before it has been affected by propagation through the interstellar medium, is constant in strength on timescales of a few hours to several years. The modulation index of the flux density variations never exceeds 0.5, ruling out a density inhomogeneity spectrum with a steep power-law exponent (β > 4). The flux density variations for 15 of the pulsars are consistent with a Kolmogorov turbulence spectrum over a range of more than 5 orders of magnitude in scattering strength, with no detectable presence of an inner scale. For these lines of sight we constrain the inhomogeneity slope to be in the range 3.5 ≤ β ≤ 3.7, which brackets the Kolmogorov value of β = 3.67. The flux density variations are greater than predicted by this model for six pulsars—including the Crab and Vela—but this group is consistent with a Kolmogorov spectrum and an inner scale of ≈1010 cm. The lines of sight to three of the other pulsars in this group pass through H II regions around young, hot stars. For six pulsars we have found a change in the slope of the intensity structure function, which could be connected with a change in the slope of the inhomogeneity power spectrum at a scale of ≈1013 cm.

PSR B0329+54: Statistics of Substructure Discovered within the Scattering Disk on RadioAstron Baselines of up to 235,000 km

We discovered fine-scale structure within the scattering disk of PSR B0329+54 in observations with the RadioAstron ground-space radio interferometer. Here, we describe this phenomenon, characterize it with averages and correlation functions, and interpret it as the result of decorrelation of the impulse-response function of interstellar scattering between the widely-separated antennas. This instrument included the 10-m Space Radio Telescope, the 110-m Green Bank Telescope, the 14x25-m Westerbork Synthesis Radio Telescope, and the 64-m Kalyazin Radio Telescope. The observations were performed at 324 MHz, on baselines of up to 235,000 km in November 2012 and January 2014. In the delay domain, on long baselines the interferometric visibility consists of many discrete spikes within a limited range of delays. On short baselines it consists of a sharp spike surrounded by lower spikes. The average envelope of correlations of the visibility function show two exponential scales, with characteristic delays of

EFFECTS OF INTERMITTENT EMISSION: NOISE INVENTORY FOR THE SCINTILLATING PULSAR B0834+06

\tau_1=4.1\pm 0.3\ \mu{\rm s}

PSR B0329+54: substructure in the scatter-broadened image discovered with RadioAstron on baselines up to 330 000 km

and

Interstellar scintillations of PSR B1919+21: space–ground interferometry

\tau_2=23\pm 3\ \mu{\rm s}

Distribution of interstellar plasma in the direction of PSR B0525+21 from data obtained on a ground–space interferometer

, indicating the presence of two scales of scattering in the interstellar medium. These two scales are present in the pulse-broadening function. The longer scale contains 0.38 times the scattered power of the shorter one. We suggest that the longer tail arises from highly-scattered paths, possibly from anisotropic scattering or from substructure at large angles.

Revealing compact structures of interstellar plasma in the Galaxy with RadioAstron

We compare signal and noise for observations of the scintillating pulsar B0834+06, using very-long baseline interferometry and a single-dish spectrometer. Comparisons between instruments and with models suggest that amplitude variations of the pulsar strongly affect the amount and distribution of self-noise. We show that noise follows a quadratic polynomial with flux density, in spectral observations. Constant coefficients, indicative of background noise, agree well with expectation; whereas second-order coefficients, indicative of self-noise, are about 3 times values expected for a pulsar with constant on-pulse flux density. We show that variations in flux density during the 10-sec integration account for the discrepancy. In the secondary spectrum, about 97% of spectral power lies within the pulsars typical scintillation bandwidth and timescale; an extended scintillation arc contains about 3%. For a pulsar with constant on-pulse flux density, noise in the dynamic spectrum will appear as a uniformly-distributed background in the secondary spectrum. We find that this uniform noise background contains 95% of noise in the dynamic spectrum for interferometric observations; but only 35% of noise in the dynamic spectrum for single-dish observations. Receiver and sky dominate noise for our interferometric observations, whereas self-noise dominates for single-dish. We suggest that intermittent emission by the pulsar, on timescales < 300 microseconds, concentrates self-noise near the origin in the secondary spectrum, by correlating noise over the dynamic spectrum. We suggest that intermittency sets fundamental limits on pulsar astrometry or timing. Accounting of noise may provide means for detection of intermittent sources, when effects of propagation are unknown or impractical to invert.

Study of scattering material with RadioAstron-VLBI observations

We have resolved the scatter-broadened image of PSR B0329+54 and detected substructure within it. These results are not influenced by any extended structure of a source but instead are directly attributed to the interstellar medium. We obtained these results at 324 MHz with the ground-space interferometer RadioAstron which included the space radio telescope (SRT), ground-based Westerbork Synthesis Radio Telescope and 64-m Kalyazin Radio Telescope on baseline projections up to 330,000 km in 2013 November 22 and 2014 January 1 to 2. At short 15,000 to 35,000 km ground-space baseline projections the visibility amplitude decreases with baseline length providing a direct measurement of the size of the scattering disk of 4.8

Refractive interstellar scintillations of pulsars

\pm

Five years of pulsar flux monitoring: probing the inhomogeneity spectrum of the interstellar medium.

0.8 mas. At longer baselines no visibility detections from the scattering disk would be expected. However, significant detections were obtained with visibility amplitudes of 3 to 5% of the maximum scattered around a mean and approximately constant up to 330,000 km. These visibilities reflect substructure from scattering in the interstellar medium and offer a new probe of ionized interstellar material. The size of the diffraction spot near Earth is 17,000