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Fast radio bursts are millisecond-duration, extragalactic radio flashes of unknown physical origin1,2,3. The only known repeating fast radio burst source4,5,6—FRB 121102—has been localized to a star-forming region in a dwarf galaxy7,8,9 at redshift 0.193 and is spatially coincident with a compact, persistent radio source7,10. The origin of the bursts, the nature of the persistent source and the properties of the local environment are still unclear. Here we report observations of FRB 121102 that show almost 100 per cent linearly polarized emission at a very high and variable Faraday rotation measure in the source frame (varying from +1.46 × 105 radians per square metre to +1.33 × 105 radians per square metre at epochs separated by seven months) and narrow (below 30 microseconds) temporal structure. The large and variable rotation measure demonstrates that FRB 121102 is in an extreme and dynamic magneto-ionic environment, and the short durations of the bursts suggest a neutron star origin. Such large rotation measures have hitherto been observed11,12 only in the vicinities of massive black holes (larger than about 10,000 solar masses). Indeed, the properties of the persistent radio source are compatible with those of a low-luminosity, accreting massive black hole10. The bursts may therefore come from a neutron star in such an environment or could be explained by other models, such as a highly magnetized wind nebula13 or supernova remnant14 surrounding a young neutron star.

Fast radio bursts (FRBs) are highly dispersed millisecond-duration radio flashes probably arriving from far outside the Milky Way1,2. This phenomenon was discovered at radio frequencies near 1.4 gigahertz and so far has been observed in one case3 at as high as 8 gigahertz, but not at below 700 megahertz in spite of substantial searches at low frequencies4,5,6,7. Here we report detections of 13 FRBs at radio frequencies as low as 400 megahertz, on the Canadian Hydrogen Intensity Mapping Experiment (CHIME) using the CHIME/FRB instrument8. They were detected during a telescope pre-commissioning phase, when the sensitivity and field of view were not yet at design specifications. Emission in multiple events is seen down to 400 megahertz, the lowest radio frequency to which the telescope is sensitive. The FRBs show various temporal scattering behaviours, with the majority detectably scattered, and some apparently unscattered to within measurement uncertainty even at our lowest frequencies. Of the 13 reported here, one event has the lowest dispersion measure yet reported, implying that it is among the closest yet known, and another has shown multiple repeat bursts, as described in a companion paper9. The overall scattering properties of our sample suggest that FRBs as a class are preferentially located in environments that scatter radio waves more strongly than in the diffuse interstellar medium in the Milky Way.

The discovery of a radioactively powered kilonova associated with the binary neutron-star merger GW170817 remains the only confirmed electromagnetic counterpart to a gravitational-wave event 1 , 2 . Observations of the late-time electromagnetic emission, however, do not agree with the expectations from standard neutron-star merger models. Although the large measured ejecta mass 3 , 4 could be explained by a progenitor system that is asymmetric in terms of the stellar component masses (that is, with a mass ratio q of 0.7 to 0.8) 5 , the known Galactic population of merging double neutron-star systems (that is, those that will coalesce within billions of years or less) has until now consisted only of nearly equal-mass ( q  > 0.9) binaries 6 . The pulsar PSR J1913+1102 is a double system in a five-hour, low-eccentricity (0.09) orbit, with an orbital separation of 1.8 solar radii 7 , and the two neutron stars are predicted to coalesce in 470 − 11 + 12 million years owing to gravitational-wave emission. Here we report that the masses of the pulsar and the companion neutron star, as measured by a dedicated pulsar timing campaign, are 1.62 ± 0.03 and 1.27 ± 0.03 solar masses, respectively. With a measured mass ratio of q  = 0.78 ± 0.03, this is the most asymmetric merging system reported so far. On the basis of this detection, our population synthesis analysis implies that such asymmetric binaries represent between 2 and 30 per cent (90 per cent confidence) of the total population of merging binaries. The coalescence of a member of this population offers a possible explanation for the anomalous properties of GW170817, including the observed kilonova emission from that event. Pulsar timing measurements show a mass ratio of about 0.8 for the double neutron-star system PSR J1913+1102, and population synthesis models indicate that such asymmetric systems represent 2–30% of merging binaries.

We report the detection of magnetar-like x-ray bursts from the young pulsar PSR J1846-0258, at the center of the supernova remnant Kes 75. This pulsar, long thought to be exclusively rotation-powered, has an inferred surface dipolar magnetic field of 4.9 x 10¹³ gauss, which is higher than those of the vast majority of rotation-powered pulsars, but lower than those of the approximately 12 previously identified magnetars. The bursts were accompanied by a sudden flux increase and an unprecedented change in timing behavior. These phenomena lower the magnetic and rotational thresholds associated with magnetar-like behavior and suggest that in neutron stars there exists a continuum of magnetic activity that increases with inferred magnetic field strength.

Observations of repeated fast radio bursts, having dispersion measures and sky positions consistent with those of FRB 121102, show that the signals do not originate in a single cataclysmic event and may come from a young, highly magnetized, extragalactic neutron star. FRB 121102's repeat performance Fast radio bursts (FRBs) are transient radio pulses that last a few milliseconds. They are thought to be extragalactic, and are of unknown physical origin. Many FRB models have proposed the cause to be one-time-only cataclysmic events. Follow-up monitoring of detected bursts did not reveal repeat bursts, consistent with such models. However, this paper reports ten additional bursts from the direction of FRB 121102, demonstrating that its source survived the energetic events that caused the bursts. Although there may be multiple physical origins for the burst, the repeating bursts seen from FRB 121102 support an origin in a young, highly magnetized, extragalactic neutron star. Fast radio bursts are millisecond-duration astronomical radio pulses of unknown physical origin that appear to come from extragalactic distances 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 . Previous follow-up observations have failed to find additional bursts at the same dispersion measure (that is, the integrated column density of free electrons between source and telescope) and sky position as the original detections 9 . The apparent non-repeating nature of these bursts has led to the suggestion that they originate in cataclysmic events 10 . Here we report observations of ten additional bursts from the direction of the fast radio burst FRB 121102. These bursts have dispersion measures and sky positions consistent with the original burst 4 . This unambiguously identifies FRB 121102 as repeating and demonstrates that its source survives the energetic events that cause the bursts. Additionally, the bursts from FRB 121102 show a wide range of spectral shapes that appear to be predominantly intrinsic to the source and which vary on timescales of minutes or less. Although there may be multiple physical origins for the population of fast radio bursts, these repeat bursts with high dispersion measure and variable spectra specifically seen from the direction of FRB 121102 support an origin in a young, highly magnetized, extragalactic neutron star 11 , 12 .

Anomalous X-ray pulsars (AXPs) are a class of rare X-ray emitting pulsars whose energy source has been perplexing for some 20 years 1 , 2 , 3 . Unlike other X-ray emitting pulsars, AXPs cannot be powered by rotational energy or by accretion of matter from a binary companion star, hence the designation ‘anomalous’. Many of the rotational and radiative properties of the AXPs are strikingly similar to those of another class of exotic objects, the soft-γ-ray repeaters (SGRs). But the defining property of the SGRs—their low-energy-γ-ray and X-ray bursts—has not hitherto been observed for AXPs. Soft-γ-ray repeaters are thought to be ‘magnetars’, which are young neutron stars whose emission is powered by the decay of an ultra-high magnetic field 4 , 5 ; the suggestion that AXPs might also be magnetars has been controversial 6 . Here we report two X-ray bursts, with properties similar to those of SGRs, from the direction of the anomalous X-ray pulsar 1E1048.1 - 5937. These events imply a close relationship (perhaps evolutionary) between AXPs and SGRs, with both being magnetars.

The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a novel transit radio telescope operating across the 400–800 MHz band. CHIME is composed of four 20 m × 100 m semicylindrical paraboloid reflectors, each of which has 256 dual-polarization feeds suspended along its axis, giving it a ≳200 deg² field of view. This, combined with wide bandwidth, high sensitivity, and a powerful correlator, makes CHIME an excellent instrument for the detection of fast radio bursts (FRBs). The CHIME Fast Radio Burst Project (CHIME/FRB) will search beam-formed, high time and frequency resolution data in real time for FRBs in the CHIME field of view. Here we describe the CHIME/FRB back end, including the real-time FRB search and detection software pipeline, as well as the planned offline analyses. We estimate a CHIME/FRB detection rate of 2–42 FRBs sky⁻¹ day⁻¹ normalizing to the rate estimated at 1.4 GHz by Vander Wiel et al. Likely science outcomes of CHIME/FRB are also discussed. CHIME/FRB is currently operational in a commissioning phase, with science operations expected to commence in the latter half of 2018.

Precision timing and multiwavelength observations of a millisecond pulsar in a triple system show that the gravitational interactions between the bodies are strong; this allows the mass of each body to be determined accurately and means that the triple system will provide precise tests of the strong equivalence principle of general relativity. A millisecond pulsar in a threesome Millisecond pulsars act as high-precision celestial clocks, and astronomers can use them to test aspects of basic physics and astrophysics. A triple system containing a radio pulsar could provide measurements of the interior structures of the bodies and a test of theories of gravity, but the only previously known system with a millisecond pulsar shows only weak interactions. Scott Ransom et al . report precision timing and multiwavelength observations of a unique object, the millisecond pulsar PSR J0337+1715, in orbit with two white dwarf companions. Strong gravitational interactions are apparent in this triple system, making it possible to estimate the masses of the pulsar and the two white dwarf companions, as well as the inclinations of the orbits. The surprisingly coplanar and nearly circular orbits indicate a complex and exotic evolutionary past that differs from known stellar systems. Gravitationally bound three-body systems have been studied for hundreds of years 1 , 2 and are common in our Galaxy 3 , 4 . They show complex orbital interactions, which can constrain the compositions, masses and interior structures of the bodies 5 and test theories of gravity 6 , if sufficiently precise measurements are available. A triple system containing a radio pulsar could provide such measurements, but the only previously known such system, PSR B1620-26 (refs 7 , 8 ; with a millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of several decades), shows only weak interactions. Here we report precision timing and multiwavelength observations of PSR J0337+1715, a millisecond pulsar in a hierarchical triple system with two other stars. Strong gravitational interactions are apparent and provide the masses of the pulsar (1.4378(13) , where is the solar mass and the parentheses contain the uncertainty in the final decimal places) and the two white dwarf companions (0.19751(15) and 0.4101(3) ), as well as the inclinations of the orbits (both about 39.2°). The unexpectedly coplanar and nearly circular orbits indicate a complex and exotic evolutionary past that differs from those of known stellar systems. The gravitational field of the outer white dwarf strongly accelerates the inner binary containing the neutron star, and the system will thus provide an ideal laboratory in which to test the strong equivalence principle of general relativity.

We have detected an x-ray nebula around the binary millisecond pulsar B1957+20. A narrow tail, corresponding to the shocked pulsar wind, is seen interior to the known Hα bow shock and proves the long-held assumption that the rotational energy of millisecond pulsars is dissipated through relativistic winds. Unresolved x-ray emission likely represents the shock where the winds of the pulsar and its companion collide. This emission indicates that the efficiency with which relativistic particles are accelerated in the postshock flow is similar to that for young pulsars, despite the shock proximity and much weaker surface magnetic field of this millisecond pulsar.