[en] We investigate rotational spin noise (referred to as timing noise) in non-accreting pulsars: millisecond pulsars, canonical pulsars, and magnetars. Particular attention is placed on quantifying the strength and non-stationarity of timing noise in millisecond pulsars because the long-term stability of these objects is required to detect nanohertz gravitational radiation. We show that a single scaling law is sufficient to characterize timing noise in millisecond and canonical pulsars while the same scaling law underestimates the levels of timing noise in magnetars. The scaling law, along with a detailed study of the millisecond pulsar B1937+21, leads us to conclude that timing noise is latent in most millisecond pulsars and will be measurable in many objects when better arrival time estimates are obtained over long data spans. The sensitivity of a pulsar timing array to gravitational radiation is strongly affected by any timing noise. We conclude that detection of proposed gravitational wave backgrounds will require the analysis of more objects than previously suggested over data spans that depend on the spectra of both the gravitational wave background and of the timing noise. It is imperative to find additional millisecond pulsars in current and future surveys in order to reduce the effects of timing noise.

[en] Most millisecond pulsars, like essentially all other radio pulsars, show timing errors well in excess of what is expected from additive radiometer noise alone. We show that changes in amplitude, shape, and pulse phase for the millisecond pulsar J1713+0747 cause this excess error. These changes appear to be uncorrelated from one pulse period to the next. The resulting time of arrival (TOA) variations are correlated across a wide frequency range and is observed with different backend processors on different days, confirming that they are intrinsic in origin and not an instrumental effect or caused by strongly frequency-dependent interstellar scattering. Centroids of single pulses show an rms phase variation ≈40 μs, which dominates the timing error and is the same phase jitter phenomenon long known in slower spinning, canonical pulsars. We show that the amplitude modulations of single pulses are modestly correlated with their arrival time fluctuations. We also demonstrate that single-pulse variations are completely consistent with arrival time variations of pulse profiles obtained by integrating N pulses such that the arrival-time error decreases proportional to 1/√N. We investigate methods for correcting TOAs for these pulse-shape changes, including multi-component TOA fitting and principal component analysis. These techniques are not found to improve the timing precision of the observations. We conclude that when pulse-shape changes dominate timing errors, the timing precision of PSR J1713+0747 can be only improved by averaging over a larger number of pulses.

[en] The soft-gamma repeater Swift J1818.0−1607 is only the fifth magnetar found to exhibit pulsed radio emission. Using the Ultra-Wideband Low receiver system of the Parkes radio telescope, we conducted a 3 hr observation of Swift J1818.0−1607. Folding the data at a rotation period of P = 1.363 s, we obtained wideband polarization profiles and flux density measurements covering radio frequencies between 704 and 4032 MHz. After measuring, and then correcting for the pulsar’s rotation measure of 1442.0 ± 0.2 rad m⁻², we find the radio profile is between 80% and 100% linearly polarized across the wide observing band, with a small amount of depolarization at low frequencies that we ascribe to scatter broadening. We also measure a steep spectral index of $\mathrm{\alpha <!--\; ??\; -->}=-<!--\; ???\; -->{2.26}_{-<!--\; ???\; -->0.03}^{+0.02}$ across our large frequency range, a significant deviation from the flat or inverted spectra often associated with radio-loud magnetars. The steep spectrum and temporal rise in flux density bears some resemblance to the behavior of the magnetar-like, rotation-powered pulsar PSR J1119−6127. This leads us to speculate that Swift J1818.0−1607 may represent an additional link between rotation-powered pulsars and magnetars.

[en] The Five-hundred-meter Aperture Spherical radio Telescope (FAST) will become one of the world-leading telescopes for pulsar timing array (PTA) research. The primary goals for PTAs are to detect (and subsequently study) ultra-low-frequency gravitational waves, to develop a pulsar-based time standard and to improve solar system planetary ephemerides. FAST will have the sensitivity to observe known pulsars with significantly improved signal-to-noise ratios and will discover a large number of currently unknown pulsars. We describe how FAST will contribute to PTA research and show that jitter- and timing-noise will be the limiting noise processes for FAST data sets. Jitter noise will limit the timing precision achievable over data spans of a few years while timing noise will limit the precision achievable over many years. (paper)

[en] PSR J1012+5307, a millisecond pulsar in orbit with a helium white dwarf (WD), has been timed with high precision for about 25 yr. One of the main objectives of this long-term timing is to use the large asymmetry in gravitational binding energy between the neutron star and the WD to test gravitational theories. Such tests, however, will be eventually limited by the accuracy of the distance to the pulsar. Here, we present very long baseline interferometry (VLBI) astrometry results spanning approximately 2.5 yr for PSR J1012+5307, obtained with the Very Long Baseline Array as part of the $\mathrm{M}\mathrm{S}\mathrm{P}\mathrm{S}\mathrm{R}\mathrm{\pi <!--\; ??\; -->}$ project. These provide the first proper motion and absolute position for PSR J1012+5307 measured in a quasi-inertial reference frame. From the VLBI results, we measure a distance of ${0.83}_{-<!--\; ???\; -->0.02}^{+0.06}$ kpc (all the estimates presented in the abstract are at 68% confidence) for PSR J1012+5307, which is the most precise obtained to date. Using the new distance, we improve the uncertainty of measurements of the unmodeled contributions to orbital period decay, which, combined with three other pulsars, places new constraints on the coupling constant for dipole gravitational radiation ${\mathrm{\kappa <!--\; ??\; -->}}_D=(-<!--\; ???\; -->1.7\pm <!--\; ??\; -->1.7)\times <!--\; ??\; -->{10}^{-<!--\; ???\; -->4}$ and the fractional time derivative of Newton’s gravitational constant $\stackrel{\dot{}<!--\; ??\; -->}{G}/G=-<!--\; ???\; -->{1.8}_{-<!--\; ???\; -->4.7}^{+5.6}\times <!--\; ??\; -->{10}^{-<!--\; ???\; -->13}{\mathrm{y}\mathrm{r}}^{-<!--\; ???\; -->1}$ in the local universe. As the uncertainties of the observed decays of orbital period for the four leading pulsar-WD systems become negligible in ≈10 yr, the uncertainties for $\stackrel{\dot{}<!--\; ??\; -->}{G}/G$ and κ _D will be improved to ≤1.5 × 10⁻¹³ yr⁻¹ and ≤1.0 × 10⁻⁴, respectively, predominantly limited by the distance uncertainties.

[en] We have developed a new coherent dedispersion mode to study the emission of fast radio bursts (FRBs) 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 $15\pm <!--\; ??\; -->2\mathrm{r}\mathrm{a}\mathrm{d}{\mathrm{m}}^{-<!--\; ???\; -->2}$ and apparent dispersion measures that vary by $0.041\pm <!--\; ??\; -->0.004\mathrm{p}\mathrm{c}{\mathrm{c}\mathrm{m}}^{-<!--\; ???\; -->3}$. 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.

[en] Fast radio burst (FRB) 190608 was detected by the Australian Square Kilometre Array Pathfinder (ASKAP) and localized to a spiral galaxy at $z_{\mathrm{h}\mathrm{o}\mathrm{s}\mathrm{t}}=0.11778$ in the Sloan Digital Sky Survey (SDSS) footprint. The burst has a large dispersion measure (${\mathrm{D}\mathrm{M}}_{\mathrm{F}\mathrm{R}\mathrm{B}}=339.8\mathrm{p}\mathrm{c}{\mathrm{c}\mathrm{m}}^{-<!--\; ???\; -->3}$) compared to the expected cosmic average at its redshift. It also has a large rotation measure (${\mathrm{R}\mathrm{M}}_{\mathrm{F}\mathrm{R}\mathrm{B}}=353\mathrm{r}\mathrm{a}\mathrm{d}{\mathrm{m}}^{-<!--\; ???\; -->2}$) and scattering timescale (τ = 3.3 ms at 1.28 GHz). Chittidi et al. perform a detailed analysis of the ultraviolet and optical emission of the host galaxy and estimate the host DM contribution to be $110\pm <!--\; ??\; -->37\mathrm{p}\mathrm{c}{\mathrm{c}\mathrm{m}}^{-<!--\; ???\; -->3}$. This work complements theirs and reports the analysis of the optical data of galaxies in the foreground of FRB 190608 in order to explore their contributions to the FRB signal. Together, the two studies delineate an observationally driven, end-to-end study of matter distribution along an FRB sightline, the first study of its kind. Combining our Keck Cosmic Web Imager (KCWI) observations and public SDSS data, we estimate the expected cosmic dispersion measure ${\mathrm{D}\mathrm{M}}_{\mathrm{c}\mathrm{o}\mathrm{s}\mathrm{m}\mathrm{i}\mathrm{c}}$ along the sightline to FRB 190608. We first estimate the contribution of hot, ionized gas in intervening virialized halos (${\mathrm{D}\mathrm{M}}_{\mathrm{h}\mathrm{a}\mathrm{l}\mathrm{o}\mathrm{s}}\approx <!--\; ???\; -->7\text{--}28\mathrm{p}\mathrm{c}{\mathrm{c}\mathrm{m}}^{-<!--\; ???\; -->3}$). Then, using the Monte Carlo Physarum Machine methodology, we produce a 3D map of ionized gas in cosmic web filaments and compute the DM contribution from matter outside halos (${\mathrm{D}\mathrm{M}}_{\mathrm{I}\mathrm{G}\mathrm{M}}\approx <!--\; ???\; -->91\text{--}126\mathrm{p}\mathrm{c}{\mathrm{c}\mathrm{m}}^{-<!--\; ???\; -->3}$). This implies that a greater fraction of ionized gas along this sightline is extant outside virialized halos. We also investigate whether the intervening halos can account for the large FRB rotation measure and pulse width and conclude that it is implausible. Both the pulse broadening and the large Faraday rotation likely arise from the progenitor environment or the host galaxy.

[en] The Australian SKA Pathfinder (ASKAP) telescope has started to localize fast radio bursts (FRBs) to arcsecond accuracy from the detection of a single pulse, allowing their host galaxies to be reliably identified. We discuss the global properties of the host galaxies of the first four FRBs localized by ASKAP, which lie in the redshift range 0.11 < z < 0.48. All four are massive galaxies (log(M _*/M _⊙) ∼ 9.4–10.4) with modest star formation rates of up to 2 M _⊙ yr⁻¹—very different to the host galaxy of the first repeating FRB 121102, which is a dwarf galaxy with a high specific star formation rate. The FRBs localized by ASKAP typically lie in the outskirts of their host galaxies, which appears to rule out FRB progenitor models that invoke active galactic nuclei or free-floating cosmic strings. The stellar population seen in these host galaxies also disfavors models in which all FRBs arise from young magnetars produced by superluminous supernovae, as proposed for the progenitor of FRB 121102. A range of other progenitor models (including compact-object mergers and magnetars arising from normal core-collapse supernovae) remain plausible.

[en] The emission from PSR J1107−5907 is erratic. Sometimes the radio pulse is undetectable, at other times the pulsed emission is weak, and for short durations the emission can be very bright. In order to improve our understanding of these state changes, we have identified archival data sets from the Parkes radio telescope in which the bright emission is present, and find that the emission never switches from the bright state to the weak state, but instead always transitions to the “off” state. Previous work had suggested the identification of the “off” state as an extreme manifestation of the weak state. However, the connection between the “off” and bright emission reported here suggests that the emission can be interpreted as undergoing only two emission states: a “bursting” state consisting of both bright pulses and nulls, and the weak emission state.

[en] We present a new fast radio burst (FRB) 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 subarcsecond localization 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 15 μJy. 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 hr 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 that 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.