Present-day galaxies are surrounded by cool and enriched halo gas extending for hundreds of kiloparsecs. This halo gas is thought to be the dominant reservoir of material available to fuel future star formation, but direct constraints on its mass and physical properties have been difficult to obtain. We report the detection of a fast radio burst (FRB 181112), localized with arcsecond precision, that passes through the halo of a foreground galaxy. Analysis of the burst shows that the halo gas has low net magnetization and turbulence. Our results imply predominantly diffuse gas in massive galactic halos, even those hosting active supermassive black holes, contrary to some previous results.

We have developed a new coherent dedispersion mode to study the emission of Fast Radio Bursts that trigger the voltage capture capability of the Australian SKA Pathfinder (ASKAP) interferometer. In principle the mode can probe emission timescales down to 3 ns with full polarimetric information preserved. Enabled by the new capability, here we present a spectropolarimetric analysis of FRB 181112 detected by ASKAP, localized to a galaxy at redshift 0.47. At microsecond time resolution the burst is resolved into four narrow pulses with a rise time of just 15 µs for the brightest. The pulses have a diversity of morphology, but do not show evidence for temporal broadening by turbulent plasma along the line of sight, nor is there any evidence for periodicity in their arrival times. The pulses are highly polarized (up to 95%), with the polarization position angle varying both between and within pulses. The pulses have apparent rotation measures that vary by 15 ± 2 rad m −2 and apparent dispersion measures that vary by 0.041 ± 0.004 pc cm −3 . Conversion between linear and circular polarization is observed across the brightest pulse. We conclude that the FRB 181112 pulses are most consistent with being a direct manifestation of the emission process or the result of propagation through a relativistic plasma close to the source. This demonstrates that our method, which facilitates high-time-resolution polarimetric observations of FRBs, can be used to study not only burst emission processes, but also a diversity of propagation effects present on the gigaparsec paths they traverse.

We present a new fast radio burst at 920 MHz discovered during commensal observations conducted with the Australian Square Kilometre Array Pathfinder (ASKAP) as part of the Commensal Real-time ASKAP Fast Transients (CRAFT) survey. FRB 191001 was detected at a dispersion measure (DM) of 506.92(4) pc cm −3 and its measured fluence of 143(15) Jy ms is the highest of the bursts localized to host galaxies by ASKAP to date. The sub-arcsecond localisation of the FRB provided by ASKAP reveals that the burst originated in the outskirts of a highly star-forming spiral in a galaxy pair at redshift z = 0.2340(1). Radio observations show no evidence for a compact persistent radio source associated with the FRB 191001 above a flux density of 15µJy. However, we detect diffuse synchrotron radio emission from the disk of the host galaxy that we ascribe to ongoing star formation. FRB 191001 was also detected as an image-plane transient in a single 10-s snapshot with a flux density of 19.3 mJy in the low-time-resolution visibilities obtained simultaneously with CRAFT data. The commensal observation facilitated a search for repeating and slowly varying radio emissions 8 hrs before and 1 hr after the burst. We found no variable radio emission on timescales ranging from 1 ms to 1.4 hr. We report our upper limits and briefly review FRB progenitor theories in the literature which predict radio afterglows. Our data are still only weakly constraining of any afterglows at the redshift of the FRB. Future commensal observations of more nearby and bright FRBs will potentially provide stronger constraints.

Despite existing constraints, it remains possible that up to 35% of all dark matter is comprised of compact objects, such as the black holes in the 10–100 M ⊙ range whose existence has been confirmed by LIGO. The strong gravitational lensing of transients such as fast radio bursts (FRBs) and gamma-ray bursts has been suggested as a more sensitive probe for compact dark matter than intensity fluctuations observed in microlensing experiments. Recently the Australian Square Kilometre Array Pathfinder has reported burst substructure down to 15 μs timescales in FRBs in the redshift range 0.3–0.5. We investigate here the implications of this for the detectability of compact dark matter by FRBs. We find that a sample size of ∼130 FRBs would be required to constrain compact dark matter to less than the existing 35% limit with 95% confidence, if it were distributed along ≳1 Gpc-long FRB sightlines through the cosmic web. Conversely, existing constraints on the fraction of compact dark matter permit as many as 1 in ≈40 of all z ≲ 0.4 FRBs to exhibit microlensed burst structure. Approximately 170 FRBs intercepting halos within ∼50 kpc would be required to exclude the fraction of compact dark matter in each intercepted halo to a similar level. Furthermore, we consider the cumulative effects of lensing of the FRB signal by a macroscopic dark matter distribution. We conclude that lensing from a uniform distribution of compact objects is likely not observable, but suggest that FRBs may set meaningful limits on power-law distributions of dark matter.

State transitions in black hole X-ray binaries are likely caused by gas evaporation from a thin accretion disk into a hot corona. We present a height-integrated version of this process, which is suitable for analytical and numerical studies. With radius r scaled to Schwarzschild units and coronal mass accretion rate m ̇ c to Eddington units, the results of the model are independent of black hole mass. State transitions should thus be similar in X-ray binaries and an active galactic nucleus. The corona solution consists of two power-law segments separated at a break radius r b ∼ 103(α/0.3)−2, where α is the viscosity parameter. Gas evaporates from the disk to the corona for r > r b , and condenses back for r < r b . At r b , m ̇ c reaches its maximum, m ̇ c , max ≈ 0.02 ( α / 0.3 ) 3 . If at r ≫ r b the thin disk accretes with m ̇ d < m ̇ c , max , then the disk evaporates fully before reaching r b , giving the hard state. Otherwise, the disk survives at all radii, giving the thermal state. While the basic model considers only bremsstrahlung cooling and viscous heating, we also discuss a more realistic model that includes Compton cooling and direct coronal heating by energy transport from the disk. Solutions are again independent of black hole mass, and r b remains unchanged. This model predicts strong coronal winds for r > r b , and a T ∼ 5 × 108 K Compton-cooled corona for r < r b . Two-temperature effects are ignored, but may be important at small radii.

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