Diffractive Interstellar Scintillation Research Articles

Jet-cocoon geometry in the optically dark, very high energy gamma-ray burst 201216C

ABSTRACT We present the results of a radio observing campaign on GRB 201216C, combined with publicly available optical and X-ray data. The detection of very high energy (VHE, >100 GeV) emission by MAGIC makes this the fifth VHE GRB at the time of publication. Comparison between the optical and X-ray light curves show that GRB 201216C is a dark GRB, i.e. the optical emission is significantly absorbed and is fainter than expected from the X-ray detections. Our e-MERLIN data also shows evidence of diffractive interstellar scintillation. We can study the column density along the line of sight to the GRB in both the host galaxy, from the damped optical light curve, and the Milky Way, via scintillation studies. We find that the afterglow is best modelled using a jet-cocoon geometry within a stellar wind environment. Fitting the data with a multicomponent model, we estimate that the optical, X-ray, and higher frequency radio data before ∼25 d originates from an ultrarelativistic jet with an isotropic equivalent kinetic energy of (0.6–10) × 1052 erg and an opening angle of ∼1–9°. The lower frequency radio emission detected by MeerKAT, from day 28 onwards, is produced by the cocoon with a kinetic energy that is between two and seven orders of magnitude lower (0.02–50) × 1048 erg. The energies of the two components are comparable to those derived in simulations of such scenarios.

Detection of Bursts from FRB 121102 with the Effelsberg 100 m Radio Telescope at 5 GHz and the Role of Scintillation

Abstract FRB 121102, the only repeating fast radio burst (FRB) known to date, was discovered at 1.4 GHz and shortly after the discovery of its repeating nature, detected up to 2.4 GHz. Here, we present three bursts detected with the 100 m Effelsberg radio telescope at 4.85 GHz. All three bursts exhibited frequency structure on broad and narrow frequency scales. Using an autocorrelation function analysis, we measured a characteristic bandwidth of the small-scale structure of 6.4 ± 1.6 MHz, which is consistent with the diffractive scintillation bandwidth for this line of sight through the Galactic interstellar medium (ISM) predicted by the NE2001 model. These were the only detections in a campaign totaling 22 hr in 10 observing epochs spanning five months. The observed burst detection rate within this observation was inconsistent with a Poisson process with a constant average occurrence rate; three bursts arrived in the final 0.3 hr of a 2 hr observation on 2016 August 20. We therefore observed a change in the rate of detectable bursts during this observation, and we argue that boosting by diffractive interstellar scintillations may have played a role in the detectability. Understanding whether changes in the detection rate of bursts from FRB 121102 observed at other radio frequencies and epochs are also a product of propagation effects, such as scintillation boosting by the Galactic ISM or plasma lensing in the host galaxy, or an intrinsic property of the burst emission will require further observations.

Highest Frequency Detection of FRB 121102 at 4–8 GHz Using the Breakthrough Listen Digital Backend at the Green Bank Telescope

Abstract We report the first detections of the repeating fast radio burst source FRB 121102 above 5.2 GHz. Observations were performed using the 4–8 GHz receiver of the Robert C. Byrd Green Bank Telescope with the Breakthrough Listen digital backend. We present the spectral, temporal, and polarization properties of 21 bursts detected within the first 60 minutes of a total of 6 hr of observations. These observations comprise the highest burst density yet reported in the literature, with 18 bursts being detected in the first 30 minutes. A few bursts clearly show temporal sub-structure with distinct spectral properties. These sub-structures superimpose to provide an enhanced peak signal-to-noise ratio at higher trial dispersion measures. Broad features occur in ∼1 GHz wide subbands that typically differ in peak frequency between bursts within the band. Finer-scale structures (∼10–50 MHz) within these bursts are consistent with the structure expected from Galactic diffractive interstellar scintillation. The bursts exhibit nearly 100% linear polarization, and a large average rotation measure of 9.359 ± 0.012 × 104 rad m−2 (in the observer’s frame). No circular polarization was found for any burst. We measure an approximately constant polarization position angle in the 13 brightest bursts. The peak flux densities of the reported bursts have average values (0.2 ± 0.1 Jy) similar to those seen at lower frequencies (<3 GHz), while the average burst widths (0.64 ± 0.46 ms) are relatively narrower.

A VLA Study of High-redshift GRBs. II. The Complex Radio Afterglow of GRB 140304A: Shell Collisions and Two Reverse Shocks

Abstract We present detailed multifrequency, multiepoch radio observations of GRB 140304A at z = 5.283 from 1 to 86 GHz and from 0.45 to 89 days. The radio and millimeter data exhibit unusual multiple spectral components, which cannot be simply explained by standard forward and reverse shock scenarios. Through detailed multiwavelength analysis spanning radio to X-rays, we constrain the forward shock parameters to E k,iso ≈ 4.9 × 1054 erg, A * ≈ 2.6 × 10−2, ϵ e ≈ 2.5 × 10−2, ϵ B ≈ 5.9 × 10−2, p ≈ 2.6, and θ jet ≈ 1.°1, yielding a beaming-corrected γ-ray and kinetic energy, E γ ≈ 2.3 × 1049 erg and E K ≈ 9.5 × 1050 erg, respectively. We model the excess radio emission as due to a combination of a late-time reverse shock (RS) launched by a shell collision, which also produces a rebrightening in the X-rays at ≈0.26 days, and either a standard RS or diffractive interstellar scintillation (ISS). Under the standard RS interpretation, we invoke consistency arguments between the forward and reverse shocks to derive a deceleration time, t dec ≈ 100 s, the ejecta Lorentz factor, Γ(t dec) ≈ 300, and a low RS magnetization, R B ≈ 0.6. Our observations highlight both the power of radio observations in capturing RS emission and thus constraining the properties of GRB ejecta and central engines and the challenge presented by ISS in conclusively identifying RS emission in GRB radio afterglows.

Detecting pulsars with interstellar scintillation in variance images

Pulsars are the only cosmic radio sources known to be sufficiently compact to show diffractive interstellar scintillations. Images of the variance of radio signals in both time and frequency can be used to detect pulsars in large-scale continuum surveys using the next generation of synthesis radio telescopes. This technique allows a search over the full field of view while avoiding the need for expensive pixel-by-pixel high time resolution searches. We investigate the sensitivity of detecting pulsars in variance images. We show that variance images are most sensitive to pulsars whose scintillation time-scales and bandwidths are close to the subintegration time and channel bandwidth. Therefore, in order to maximise the detection of pulsars for a given radio continuum survey, it is essential to retain a high time and frequency resolution, allowing us to make variance images sensitive to pulsars with different scintillation properties. We demonstrate the technique with Murchision Widefield Array data and show that variance images can indeed lead to the detection of pulsars by distinguishing them from other radio sources.

Time-domain and spectral properties of pulsars at 154 MHz

We present 154 MHz Murchison Widefield Array imaging observations and variability information for a sample of pulsars. Over the declination range $-80^{\circ} < {\delta} < 10^{\circ}$ we detect 17 known pulsars with mean flux density greater than 0.3 Jy. We explore the variability properties of this sample on timescales of minutes to years. For three of these pulsars, PSR J0953+0755, PSR J0437-4715 and PSR J0630-2834 we observe interstellar scintillation and variability on timescales of greater than 2 minutes. One further pulsar, PSR J0034-0721, showed significant variability, the physical origins of which are difficult to determine. The dynamic spectra for PSR J0953+0755 and PSR J0437-4715 show discrete time and frequency structure consistent with diffractive interstellar scintillation and we present the scintillation bandwidth and timescales from these observations. The remaining pulsars within our sample were statistically non-variable. We also explore the spectral properties of this sample and find spectral curvature in pulsars PSR J0835-4510, PSR J1752-2806 and PSR J0437-4715.

On the paucity of fast radio bursts at low Galactic latitudes

We examine the effect of Galactic diffractive interstellar scintillation as a means of explaining the reported deficit of fast radio burst (FRB) detections at low Galactic latitude. We model the unknown underlying FRBflux density distribution as a power lawwith a rate scaling as S−5/2+δ ν and account for the fact that the FRBs are detected at unknown positions within the telescope beam. We find that the event rate of FRBs located off the Galactic plane may be enhanced by a factor of ∼30–300 per cent relative to objects near the Galactic plane without necessarily affecting the slope of the distribution. For FRBs whose flux densities are subject to relatively weak diffractive scintillation, as is typical for events detected at high Galactic latitudes, we demonstrate that an effect associated with Eddington bias is responsible for the enhancement. The magnitude of the enhancement increases with the steepness of the underlying flux density distribution, so that existing limits on the disparity in event rates between high and lowGalactic latitudes suggest that the FRB population has a steep differential flux density distribution, scaling as S−3.5 ν or steeper. Existing estimates of the event rate in the flux density range probed by the High Time Resolution Universe survey overestimate the true rate by a factor of ∼3.

Short timescale intensity fluctuations of PSR B1133+16 and PSR B1237+25 due to interstellar scintillation at 1.54 GHz

We report on the short timescale intensity fluctuations of PSR B1133+16 and PSR B1237+25 due to scintillation at 1.54 GHz. The structure functions of intensity fluctuations are constructed and the linear fittings are applied to the structure regime in log–log plots to get the slope values. The slope of the SFs for PSR B1237+25 are less than that of PSR B1133+16, and both of them are much less than 2. For PSR B1133+16, the slope values agree very well with a Kolmogorov spectrum predicted value 5/3, whereas PSR B1237+25 not. We investigate the dynamic spectrum and obtain its auto-correlation function (ACF). Scintillation parameters are obtained by fitting to the autocorrelation function of the dynamic spectrum. The observed diffractive interstellar scintillation (DISS) timescales agree well with the observations and are consistent with some expected values, but the observed de-correlation frequency bandwidths are much less than predicted ones. The expected refractive interstellar scintillation (RISS) timescales are also estimated by using our derived diffractive scintillation parameters. They are consistent with the timescales of slow pulse intensity fluctuations. We proposed that the short timescale of pulse intensity variations caused by the DISS are modulated by the slow variations due to the effects of RISS.

Pulsar diffractive scintillation at 1.4 and 4.8 GHz

Abstract The diffractive interstellar scintillation of 11 pulsars at 1410 MHz and of five pulsars at 4800 MHz was analysed to estimate the spectrum of the electron density fluctuations as well as the ACF, CCF and the structure function. We present dynamic spectra for a few pulsars at high frequencies in the strong and weak regime and show that a lensing effect probably exists for pulsar PSR B2020+28 at 1410 MHz. The mean value of the indexes of the power law spectra for an ensemble of pulsars is in good agreement with a Kolmogorov spectrum.

Magnetic field limits and spectral variability in the Circinus galaxy H2O megamasers

We have used the Australia Telescope Compact Array (ATCA) to search for polarized emission from the Circinus galaxy H2O megamasers. No linear or circular polarized emission was detected with 3σ upper limits of 24 and 39 mJy, respectively, corresponding to fractional polarizations of 0.6 and 0.9 per cent for the strongest line. These results allow us to place upper limits on the strength of the nuclear magnetic field of 150 mG near the outer edge of the masing disc and 360 mG at the inner edge. Rapid variability, thought to be due to interstellar scintillation, is a well-known feature of the Circinus H2O megamasers. Our Stokes-I data show evidence for changes in the shape of the maser line spectral profiles, as well as their intensity. In addition to the rapid variations, the maser light curves also exhibit variations with a time-scale of the order of days (although our observations are too short to characterize this well). These behaviours support the hypothesis that the Circinus H2O megamasers undergo diffractive interstellar scintillation, which has previously only been observed in pulsars and in one active galactic nucleus.

Diffractive interstellar scintillation of the quasar J1819+3845 at ${\vec{\lambda}}$21 cm

We report the discovery of fast, frequency-dependent intensity variations from the scintillating intra-day variable quasar J1819+3845 at cm which resemble diffractive interstellar scintillations observed in pulsars. The observations were taken with the Westerbork Synthesis Radio Telescope on a dozen occasions in the period between Aug. 2002 and Jan. 2005. The data were sampled at both high temporal and high frequency resolution and have an overall simultaneous frequency span of up to 600 MHz. In constructing the light curves and dynamic spectra the confusion from background sources has been eliminated. The timescale (down to 20 min) and the bandwidth (frequency decorrelation bandwidth of 160 MHz) of the observed variations jointly imply that the component of the source exhibiting this scintillation must possess a brightness temperature well in excess of the inverse Compton limit. A specific model in which both the source and scintillation pattern are isotropic implies a brightness temperature K, where previous estimates place the distance to the scattering medium in the range pc, yielding a minimum brightness temperature >20 times the inverse Compton limit. An independent estimate of the screen distance using the 21 cm scintillation properties alone indicates a minimum screen distance of pc and a brightness temperature above K. There is no evidence for anisotropy in the scattering medium or source from the scintillation characteristics, but these estimates may be reduced by a factor comparable to the axial ratio if the source is indeed elongated. The observed scintillation properties of J1819+3845 at 21 cm are compared with those at 6 cm, where a significantly larger source size has been deduced for the bulk of the emission by Dennett-Thorpe & de Bruyn (2003). However, opacity effects within the source and the different angular scales probed in the regimes of weak and strong scattering complicate this comparison.

Long-term scintillation observations of five pulsars at 1540 MHz

From 2001 January to 2002 June, we monitored PSRs B0329+54, B0823+26, B1929+10, B2020+28 and B2021+51 using the Nanshan 25-m radio telescope of Urumqi Observatory to study their diffractive interstellar scintillation (DISS). The average interval between observations was about 9 days and the observation duration ranged between 2 and 6 hours depending on the pulsar. Wide variations in the DISS parameters were observed over the 18-month data span. Despite this, the average scintillation velocities are in excellent agreement with the proper motion velocities. The average two-dimensional autocorrelation function for PSR B0329+54 is well described by a thin-screen Kolmogorov model, at least along the time and frequency axes. Observed modulation indices for the DISS time scale and bandwidth and the pulsar flux density are greater than values predicted for a Kolmogorov spectrum of electron density fluctuations. Correlated variations over times long compared to the nominal refractive scintillation time are observed, suggesting that larger-scale density fluctuations are important. For these pulsars, the scintillation bandwidth as a function of frequency has a power-law index (~3.6) much less than expected for Kolmogorov turbulence (~4.4). Sloping fringes are commonly observed in the dynamic spectra, especially for PSR B0329+54. The detected range of fringe slopes are limited by our observing resolution. Our observations are sensitive to larger-scale fringes and hence smaller refractive angles, corresponding to the central part of the scattering disk.

Diffractive Interstellar Scintillation Timescales and Velocities

We derive general relationships between the observed timescale of diffractive interstellar scintillations and the physical velocities of the observer, the source, and the scattering medium. Our treatment applies exclusively to saturated scintillations of point sources in the strong scattering regime. We show how scintillation observations may be combined with other observables (proper motion and dispersion measure) to yield (1) improvements in galactic models for the free-electron density and (2) estimates of the distance and transverse velocity of individual pulsars. We explicitly consider cases of current astrophysical interest, including hypervelocity pulsars too far above the Galactic plane to allow distance estimates from dispersion measures alone. We also briefly consider scintillations of extragalactic sources, including gamma-ray burst sources at great distances from the Galaxy.

Interstellar Fringes from PSR B0834+06

The spectra of radio flux from pulsars show deep modulations, over a typical time scale Δtdand frequency scale Δfd. These random diffractive interstellar scintillations are caused by inhomogeneities in the interstellar plasma on scales of 105m to 106m; on occasion there are also refractive modulations caused by very much larger scales. We report here an episode of periodic modulations of the dynamic spectrum of PSR B0834+06 seen in the sequence of observations centered at 408 MHz reported by Gupta et al. (1994 - GRL). There were 12 observations of about 2 hours covering 5 MHz spaced over 14 months. An episode of very fine periodic fringes was detected, though the period 45 kHz in frequency was barely resolved by the 19.5 kHz channel bandwiths; the period in time was about 5 minutes and was fully resolved. The normal Δfdwas about 4 MHz (GRL) - ≈90 times the fringe period.

Long-term intensity variations of 20 pulsars

Intensity fluctuations of 20 pulsars at 0.43 GHz are analyzed over a 4 yr time span. The objects were selected from a long-term timing study of pulsars performed at the Arecibo Observatory covering a range of dispersion measures between 8 and 403 pc/cc. A structure function analysis was used to determine the characteristic time scales and modulation indices of long-term (greater than or equal to few days) intensity variations. Variability generally agrees with a model where refractive scintillations superpose with diffractive interstellar scintillations, with the latter quenched to varying degrees from the finite bandwidth. The refractive scintillations, or limits, are consistent with amplitudes expected from a Kolmogorov spectrum for the electron density.

Refractive interstellar scintillation in pulsar dynamic spectra

The evolution of the dynamic spectra of eight pulsars over 16 months is reported. Observations were made at Jodrell Bank at 408 MHz from 1984 September to 1986 January. The changing form of the spectra is interpreted as refractive modulation of diffractive interstellar scintillation. The correlation function versus time and frequency was computed for each observation and estimates were made of the apparent decorrelation widths in frequency and time, and the frequency drift rate of scintillation features. A model is developed for how these three parameters vary with epoch under changing angles of refraction. The model successfully describes the variations for four of the pulsars, for which the drift rate shows some sign reversals between epochs. Two pulsars kept a constant sign of drift rate, which we interpret as due to persistent refracting structures in their lines of sight, which are consistent with an intervening H II region. Studies were made of the variability of the diffractive bandwidth, the visibility of the drifting features in the dynamic spectra, the refractive shifts and the speed estimated for the diffraction pattern. These quantities mostly agree with expectations from the model. The pulsar PSRB2016+28, which has an unusually low proper motion, shows clear modulation of the pattern speed by the Earth's annual motion. In the 11 epochs observed for PSRB2016+28, two showed unmistakable periodic modulation in the dynamic spectra. Several other pulsars exhibited occasional periodic patterns with low modulation. The phenomenon varies substantially between the pulsars, being more common for lower strengths of scattering.