Artem V. Tuntsov

The magnetic field and turbulence of the cosmic web measured using a brilliant fast radio burst

Probing the intergalactic magnetic field Fast radio bursts (FRBs) are powerful flashes of astronomical radio waves that last just milliseconds and whose origins are a matter of debate. Ravi et al. discovered a FRB that is exceptionally bright (see the Perspective by Kaspi). This allowed them to measure how the polarization of the signal varies with wavelength (Faraday rotation). Combining this with the time delay of the flash at different wavelengths revealed the mean magnetic field along the line of sight. Assuming that the FRB originates from a colocated galaxy, the results constrain the intergalactic magnetic field and will inform models of galaxy formation and cosmology. Science, this issue p. 1249; see also p. 1230 A fast radio burst allows astronomers to constrain the intergalactic magnetic field. Fast radio bursts (FRBs) are millisecond-duration events thought to originate beyond the Milky Way galaxy. Uncertainty surrounding the burst sources, and their propagation through intervening plasma, has limited their use as cosmological probes. We report on a mildly dispersed (dispersion measure 266.5 ± 0.1 parsecs per cubic centimeter), exceptionally intense (120 ± 30 janskys), linearly polarized, scintillating burst (FRB 150807) that we directly localize to 9 square arc minutes. On the basis of a low Faraday rotation (12.0 ± 0.7 radians per square meter), we infer negligible magnetization in the circum-burst plasma and constrain the net magnetization of the cosmic web along this sightline to <21 nanogauss, parallel to the line-of-sight. The burst scintillation suggests weak turbulence in the ionized intergalactic medium.

The AAT/WFI survey of the Monoceros Ring and Canis Major dwarf galaxy – I. From l= (193–276)°

We present the results of an Anglo-Australian Telescope (AAT) wide field camera survey of the stars in the Monoceros Ring (MRi) and purported Canis Major (CMa) overdensity in the Galactic longitudes of l = (193‐276) ◦ . Current numerical simulations suggest that both of these structures are the result of a single on-going accretion event, although an alternative solution is that the warped and flared disc of the Galaxy can explain the origin of both of these structures. Our results show that, with regards the MRi, the warped and flared disc is unable to reproduce the locations and strengths of the detections observed around the Galaxy. This supports a non-Galactic origin for this structure. We report eight new detections and two tentative detections of the MRi in this survey. The exact nature of the CMa overdensity is still unresolved, although this survey provides evidence that invoking the Galactic warp is not a sufficient solution when compared with observation. Several fields in this survey are highly inconsistent with the current Galactic disc models that include a warp and flare, to such an extent that explaining their origins with these structures is problematic. We also report that the blue plume stars previously invoked to support the dwarf galaxy hypothesis are unfounded, and associating them with an outer spiral arm is equally problematic. Standard Galactic models are unable to accommodate all the observations of these new structures, leading away from a warped/flared disc explanation for their origins and more toward a non-Galactic source. Additionally, evidence is presented in favour of a detection of the CMa dwarf stream away from the CMa region. As the outer reaches of the Galactic disc continue to be probed, the fascinating structures that are the MRi and CMa overdensity will no doubt continue to inform us of the unique structure and formation of the Milky Way.

Real-time detection of an extreme scattering event: Constraints on Galactic plasma lenses

Identifying live radio scattering events Radio emissions from distant quasars are occasionally modified for a few weeks by foreground interstellar plasma, in an extreme scattering event (ESE). Understanding this process has been difficult, because existing techniques do not allow events to be identified fast enough for follow-up before they finish. Bannister et al. developed a radio survey technique that allows ESEs to be identified in real time. After finding their first live ESE, they followed it up with additional radio and optical telescopes. The results constrain the size and density of the plasma and rule out one popular model of ESEs. Science, this issue p. 354 Interstellar extreme scattering events are identified in real time with radio observations. Extreme scattering events (ESEs) are distinctive fluctuations in the brightness of astronomical radio sources caused by occulting plasma lenses in the interstellar medium. The inferred plasma pressures of the lenses are ~103 times the ambient pressure, challenging our understanding of gas conditions in the Milky Way. Using a new survey technique, we discovered an ESE while it was in progress. Here we report radio and optical follow-up observations. Modeling of the radio data demonstrates that the lensing structure is a density enhancement and the lens is diverging, ruling out one of two competing physical models. Our technique will uncover many more ESEs, addressing a long-standing mystery of the small-scale gas structure of our Galaxy.

Dense plasma dispersion of fast radio bursts

Stellar coronae have been invoked to explain the apparently extragalactic dispersion measures observed in fast radio bursts. This paper demonstrates that the suggested plasma densities would lead to deviations from the standard dispersion curve that are inconsistent with the data. The problem is then turned around and higher-order dispersion terms are connected to the moments of the density distribution along the line of sight. The deviations quantified in three observed bursts are analysed and a lower limit on the maximum electron density is obtained in one case, although with considerable uncertainty. Selection effects are then discussed and shown to be non-restrictive in relation to plasma density, except at the lowest frequencies and highest temperatures.

Extreme Radio-wave Scattering Associated with Hot Stars

We use data on extreme radio scintillation to demonstrate that this phenomenon is associated with hot stars in the solar neighbourhood. The ionized gas responsible for the scattering is found at distances up to 1.75pc from the host star, and on average must comprise 1.E5 distinct structures per star. We detect azimuthal velocities of the plasma, relative to the host star, up to 9.7 km/s, consistent with warm gas expanding at the sound speed. The circumstellar plasma structures that we infer are similar in several respects to the cometary knots seen in the Helix, and in other planetary nebulae. There the ionized gas appears as a skin around tiny molecular clumps. Our analysis suggests that molecular clumps are ubiquitous circumstellar features, unrelated to the evolutionary state of the star. The total mass in such clumps is comparable to the stellar mass.

Detecting compact dark matter in galaxy clusters via gravitational microlensing: A2218 and A370

After decades of searching, the true nature of dark matter still eludes us. One potential probe of the form of dark matter in galaxy clusters is to search for microlensing variability in the giant arcs and arclets. In this paper, a simple method is introduced to characterize pixel variability in the limit of high optical depth to microlensing. Expanding on earlier work, the expected microlensing signal for two massive clusters, A2218 & A370 is calculated. It is found that the microlensing signal depends sensitively upon the mix of smooth and compact dark matter in the cluster. Comparison of two deep exposures taken with James Webb Space Telescope or two hour long exposures taken with a 30-metre class telescope in two epochs separated by a few years will possibly detect about a few dozen pixels which show strong variability due to microlensing at five sigma level, revealing wealth of information on the microlensing population.

On a systematic bias in surface brightness fluctuations based distances due to gravitational microlensing

The effect of gravitational microlensing on the determination of extragalactic distances using the surface brightness fluctuations (SBF) technique is considered and a method to calculate SBF amplitudes in the presence of microlensing is presented. With a simple approximation for the magnification power spectrum at low optical depth, the correction to the SBF-based luminosity distance is calculated. The results suggest the effect can be safely neglected at present but may become important for SBF-based Hubble diagrams at luminosity distances of about 1 Gpc and beyond.

Measuring transverse velocities in gravitationally lensed extragalactic systems using an annual parallax effect

A parallax method to determine transverse velocity in a gravitationally lensed system is described. Using the annual motion of the Earth around the Sun allows us to probe the local structure of the magnification map that, under certain assumptions, can be used to infer the effective transverse velocity. The method is applied to OGLE data for QSO 2237+0305, and the velocity value is estimated to be about 15 ± 10 km s −1 if attributed to the lensing galaxy or about 420 ± 300 km s −1 if attributed to the quasar. We find this estimate unreasonably small and conclude that we have not measured a parallax effect. We give a short list of properties that a system should possess to allow a successful implementation of this method.

Scintillation kinks, bumps and wiggles in the radio spectrum of the quasar PMN J1106−3647

We report radio observations of the quasar PMN J1106-3647. Our data, taken with the Australia Telescope Compact Array, show large variations in the amplitude and shape of its spectrum, on a short time-scale. A great variety of spectral features is evident, including: sharp kinks; broad spectral peaks; and wiggles. No two spectra are alike. We interpret the variations as interstellar scintillation of a radio source that is compact, but not point-like. Under this interpretation, complex spectral structure can arise purely refractively, under high magnification conditions, or from interference between waves that have been scattered by small-scale density fluctuations (diffractive scintillation). Both effects may be playing a role in J1106-3647, and we tentatively identify kinks with the former, and wiggles with the latter. Diffractive scintillation of AGN is uncommon, as the fringe visibility is low for all but the most compact radio sources. Refractive interpretation of the kink implies that the source has a sharp, concave boundary. Our data are consistent with a mildly boosted synchrotron source, provided the scattering material is at a distance ~50 pc from us.

Microlensing in phase space - II. Correlations analysis

Applications of the phase-space approach to the calculation of the microlensing autocorrelation function are presented. The continuous propagation equation for a random star field with a Gaussian velocity distribution is solved in the leading non-trivial approximation using the perturbation technique. It is shown that microlensing modulations can be important in the interpretation of optical and shorter-wavelength light curves of pulsars, power spectra of active galactic nuclei and coherence estimates for quasi-periodic oscillations of dwarf novae and low-mass X-ray binaries. Extra scatter in the brightness of Type Ia supernovae due to gravitational microlensing is shown to be of order up to 0.2 mag depending on the extent of the light curves.