arXiv subjects

C. Flynn

Publications and source records attributed to C. Flynn.

At least 37 records · Page 2Linked to original sources

arXiv2019

The UTMOST Survey for Magnetars, Intermittent pulsars, RRATs and FRBs I: System description and overview

We describe the ongoing `Survey for Magnetars, Intermittent pulsars, Rotating radio transients and Fast radio bursts' (SMIRF), performed using the newly refurbished UTMOST telescope. SMIRF repeatedly sweeps the southern Galactic plane performing real-time periodicity and single-pulse searches, and is the first survey of its kind carried out with an interferometer. SMIRF is facilitated by a robotic scheduler which is capable of fully autonomous commensal operations. We report on the SMIRF observational parameters, the data analysis methods, the survey's sensitivities to pulsars, techniques to mitigate radio frequency interference and present some early survey results. UTMOST's wide field of view permits a full sweep of the Galactic plane to be performed every fortnight, two orders of magnitude faster than previous surveys. In the six months of operations from January to June 2018, we have performed $\sim 10$ sweeps of the Galactic plane with SMIRF. Notable blind re-detections include the magnetar PSR J1622$-$4950, the RRAT PSR J0941$-$3942 and the eclipsing pulsar PSR J1748$-$2446A. We also report the discovery of a new pulsar, PSR J1705$-$54. Our follow-up of this pulsar with the UTMOST and Parkes telescopes at an average flux limit of $\leq 20$ mJy and $\leq 0.16$ mJy respectively, categorizes this as an intermittent pulsar with a high nulling fraction of $< 0.002$

astro-ph.IM↗

arXiv2019

Five new real-time detections of Fast Radio Bursts with UTMOST

We detail a new fast radio burst (FRB) survey with the Molonglo Radio Telescope, in which six FRBs were detected between June 2017 and December 2018. By using a real-time FRB detection system, we captured raw voltages for five of the six events, which allowed for coherent dedispersion and very high time resolution (10.24 $μ$s) studies of the bursts. Five of the FRBs show temporal broadening consistent with interstellar and/or intergalactic scattering, with scattering timescales ranging from 0.16 to 29.1 ms. One burst, FRB181017, shows remarkable temporal structure, with 3 peaks each separated by 1 ms. We searched for phase-coherence between the leading and trailing peaks and found none, ruling out lensing scenarios. Based on this survey, we calculate an all-sky rate at 843 MHz of $98^{+59}_{-39}$ events sky$^{-1}$ day$^{-1}$ to a fluence limit of 8 Jy-ms: a factor of 7 below the rates estimated from the Parkes and ASKAP telescopes at 1.4 GHz assuming the ASKAP-derived spectral index $α=-1.6$ ($F_ν\proptoν^α$). Our results suggest that FRB spectra may turn over below 1 GHz. Optical, radio and X-ray followup has been made for most of the reported bursts, with no associated transients found. No repeat bursts were found in the survey.

astro-ph.HE↗

arXiv2019

Relativistic spin precession in the binary PSR J1141$-$6545

PSR J1141$-$6545 is a precessing binary pulsar that has the rare potential to reveal the two-dimensional structure of a non-recycled pulsar emission cone. It has undergone $\sim 25 °$ of relativistic spin precession in the $\sim18$ years since its discovery. In this paper, we present a detailed Bayesian analysis of the precessional evolution of the width of the total intensity profile, to understand the changes to the line-of-sight impact angle ($β$) of the pulsar using four different physically motivated prior distribution models. Although we cannot statistically differentiate between the models with confidence, the temporal evolution of the linear and circular polarisations strongly argue that our line-of-sight crossed the magnetic pole around MJD 54000 and that only two models remain viable. For both these models, it appears likely that the pulsar will precess out of our line-of-sight in the next $3-5$ years, assuming a simple beam geometry. Marginalising over $β$ suggests that the pulsar is a near-orthogonal rotator and provides the first polarization-independent estimate of the scale factor ($\mathbb{A}$) that relates the pulsar beam opening angle ($ρ$) to its rotational period ($P$) as $ρ= \mathbb{A}P^{-0.5}$ : we find it to be $> 6 \rm~deg~s^{0.5}$ at 1.4 GHz with 99\% confidence. If all pulsars emit from opposite poles of a dipolar magnetic field with comparable brightness, we might expect to see evidence of an interpulse arising in PSR J1141$-$6545, unless the emission is patchy.

astro-ph.HE↗

arXiv2019

Constraining the era of helium reionization using fast radio bursts

The discovery of fast radio bursts (FRBs) about a decade ago opened up new possibilities for probing the ionization history of the Intergalactic Medium (IGM). In this paper we study the use of FRBs for tracing the epoch of HeII reionization, using simulations of their dispersion measures. We model dispersion measure contributions from the Milky Way, the IGM (homogeneous and inhomogeneous) and a possible host galaxy as a function of redshift and star formation rate. We estimate the number of FRBs required to distinguish between a model of the Universe in which helium reionization occurred at z = 3 from a model in which it occurred at z = 6 using a 2-sample Kolmogorov-Smirnoff test. We find that if the IGM is homogeneous >1100 FRBs are needed and that an inhomogeneous model in which traversal of the FRB pulse through galaxy halos increases the number of FRBs modestly, to >1600. We also find that to distinguish between a reionization that occurred at z = 3 or z = 3.5 requires ~5700 FRBs in the range 3 < z < 5.

astro-ph.HE↗

arXiv2019

Are all fast radio bursts repeating sources?

We present Monte-Carlo simulations of a cosmological population of repeating fast radio burst (FRB) sources whose comoving density follows the cosmic star formation rate history. We assume a power-law model for the intrinsic energy distribution for each repeating FRB source located at a randomly chosen position in the sky and simulate their dispersion measures (DMs) and propagation effects along the chosen lines-of-sight to various telescopes. In one scenario, an exponential distribution for the intrinsic wait times between pulses is chosen, and in a second scenario we model the observed pulse arrival times to follow a Weibull distribution. For both models we determine whether the FRB source would be deemed a repeater based on the telescope sensitivity and time spent on follow-up observations. We are unable to rule out the existence of a single FRB population based on comparisons of our simulations with the longest FRB follow-up observations performed. We however rule out the possibility of FRBs 171020 and 010724 repeating with the same rate statistics as FRB 121102 and also constrain the slope of a power-law fit to the FRB energy distribution to be $-2.0 < γ<-1.0$. All-sky simulations of repeating FRB sources imply that the detection of singular events correspond to the bright tail-end of the adopted energy distribution due to the combination of the increase in volume probed with distance, and the position of the burst in the telescope beam.

astro-ph.HE↗

arXiv2018

Hunting for radio emission from the intermittent pulsar J1107-5907 at low frequencies

The rare intermittent pulsars pose some of the most challenging questions surrounding the pulsar emission mechanism, but typically have relatively minimal low-frequency ($\lesssim$ 300 MHz) coverage. We present the first low-frequency detection of the intermittent pulsar J1107-5907 with the Murchison Widefield Array (MWA) at 154 MHz and the simultaneous detection from the recently upgraded Molonglo Observatory Synthesis Telescope (UTMOST) at 835 MHz, as part of an on-going observing campaign. During a 30-minute simultaneous observation, we detected the pulsar in its bright emission state for approximately 15 minutes, where 86 and 283 pulses were detected above a signal-to-noise threshold of 6 with the MWA and UTMOST, respectively. Of the detected pulses, 51 had counterparts at both frequencies and exhibited steep spectral indices for both the bright main pulse component and the precursor component. We find that the bright state pulse energy distribution is best parameterised by a log-normal distribution at both frequencies, contrary to previous results which suggested a power law distribution. Further low-frequency observations are required in order to explore in detail aspects such as pulse-to-pulse variability, intensity modulations and to better constrain the signal propagation effects due to the interstellar medium and intermittency characteristics at these frequencies. The spectral index, extended profile emission covering a large fraction of pulse longitude, and the broadband intermittency of PSR J1107-5907 suggests that future low-frequency pulsar searches, for instance those planned with SKA-Low, will be in an excellent position to find and investigate new pulsars of this type.

astro-ph.HE↗

arXiv2018

The UTMOST pulsar timing programme I: overview and first results

We present an overview and the first results from a large-scale pulsar timing programme that is part of the UTMOST project at the refurbished Molonglo Observatory Synthesis Radio Telescope (MOST) near Canberra, Australia. We currently observe more than 400 mainly bright southern radio pulsars with up to daily cadences. For 205 (8 in binaries, 4 millisecond pulsars) we publish updated timing models, together with their flux densities, flux density variability, and pulse widths at 843 MHz, derived from observations spanning between 1.4 and 3 yr. In comparison with the ATNF pulsar catalogue, we improve the precision of the rotational and astrometric parameters for 123 pulsars, for 47 by at least an order of magnitude. The time spans between our measurements and those in the literature are up to 48 yr, which allows us to investigate their long-term spin-down history and to estimate proper motions for 60 pulsars, of which 24 are newly determined and most are major improvements. The results are consistent with interferometric measurements from the literature. A model with two Gaussian components centred at 139 and $463~\text{km} \: \text{s}^{-1}$ fits the transverse velocity distribution best. The pulse duty cycle distributions at 50 and 10 per cent maximum are best described by log-normal distributions with medians of 2.3 and 4.4 per cent, respectively. We discuss two pulsars that exhibit spin-down rate changes and drifting subpulses. Finally, we describe the autonomous observing system and the dynamic scheduler that has increased the observing efficiency by a factor of 2-3 in comparison with static scheduling.

astro-ph.HE↗

arXiv2018

FRB microstructure revealed by the real-time detection of FRB170827

We report a new Fast Radio Burst (FRB) discovered in real-time as part of the UTMOST project at the Molonglo Observatory Synthesis Radio Telescope (MOST). FRB170827 is the first detected with our low-latency ($< 24$ s), machine-learning-based FRB detection system. The FRB discovery was accompanied by the capture of voltage data at the native time and frequency resolution of the observing system, enabling coherent dedispersion and detailed off-line analysis, which have unveiled fine temporal and frequency structure. The dispersion measure (DM) of 176.80 $\pm$ 0.04 pc cm$^{-3}$, is the lowest of the FRB population. The Milky Way contribution along the line of sight is $\sim$ 40 pc cm$^{-3}$, leaving an excess DM of $\sim$ 145 pc cm$^{-3}$. The FRB has a fluence $>$ 20 $\pm$ 7 Jy ms, and is narrow, with a width of $\sim$ 400 $μ$s at 10$\%$ of its maximum amplitude. However, the burst shows three temporal components, the narrowest of which is $\sim$ 30 $μ$s, and a scattering timescale of $4.1 \pm 2.7$ $μ$s. The FRB shows spectral modulations on frequency scales of 1.5 MHz and 0.1 MHz. Both are prominent in the dynamic spectrum, which shows a very bright region of emission between 841 and 843 MHz, and weaker, patchy emission across the entire band. We show the fine spectral structure could arise in the FRB host galaxy, or its immediate vicinity.

astro-ph.HE↗

arXiv2017

Imaging atomic vacancies in commercially available black phosphorus

Black phosphorus (BP) is receiving significant attention because of its direct 0.4-1.5 eV layer-dependent band gap and high mobility. Because BP devices rely on exfoliation from bulk crystals, there is a need to understand native impurities and defects in the source material. In particular, samples are typically p-doped, but the source of the doping is not well understood. Here, we use scanning tunneling microscopy and spectroscopy to compare atomic defects of BP samples from two commercial sources. Even though the sources produced crystals with an order of magnitude difference in impurity atoms, we observed a similar defect density and level of p-doping. We attribute these defects to phosphorus vacancies and provide evidence that they are the source of the p-doping. We also compare these native defects to those induced by air exposure and show they are distinct and likely more important for control of electronic structure. These results indicate that impurities in BP play a minor role compared to vacancies, which are prevalent in commercially-available materials, and call for better control of vacancy defects.

cond-mat.mes-hall↗

arXiv2017

Strong field tests of gravity with PSR J1141-6545

The initial results from timing observations of PSR J1141-6545, a relativistic pulsar white-dwarf binary system, are presented. Predictions from the timing baseline hint at the most stringent test of gravity by an asymmetric binary yet. The timing precision has been hindered by the dramatic variations of the pulse profile due to geodetic precession, a pulsar glitch and red timing noise. Methods to overcome such timing irregularities are briefly presented along with preliminary results from the test of the General Theory of Relativity (GR) from this pulsar

gr-qc↗

arXiv2017

The SUrvey for Pulsars and Extragalactic Radio Bursts II: New FRB discoveries and their follow-up

We report the discovery of four Fast Radio Bursts (FRBs) in the ongoing SUrvey for Pulsars and Extragalactic Radio Bursts (SUPERB) at the Parkes Radio Telescope: FRBs 150610, 151206, 151230 and 160102. Our real-time discoveries have enabled us to conduct extensive, rapid multi-messenger follow-up at 12 major facilities sensitive to radio, optical, X-ray, gamma-ray photons and neutrinos on time scales ranging from an hour to a few months post-burst. No counterparts to the FRBs were found and we provide upper limits on afterglow luminosities. None of the FRBs were seen to repeat. Formal fits to all FRBs show hints of scattering while their intrinsic widths are unresolved in time. FRB 151206 is at low Galactic latitude, FRB 151230 shows a sharp spectral cutoff, and FRB 160102 has the highest dispersion measure (DM = $2596.1\pm0.3$ pc cm$^{-3}$) detected to date. Three of the FRBs have high dispersion measures (DM >$1500$ pc cm$^{-3}$), favouring a scenario where the DM is dominated by contributions from the Intergalactic Medium. The slope of the Parkes FRB source counts distribution with fluences $>2$ Jyms is $α=-2.2^{+0.6}_{-1.2}$ and still consistent with a Euclidean distribution ($α=-3/2$). We also find that the all-sky rate is $1.7^{+1.5}_{-0.9}\times10^3$FRBs/($4π$ sr)/day above $\sim2$ Jyms and there is currently no strong evidence for a latitude-dependent FRB sky-rate.

astro-ph.HE↗

arXiv2017

The UTMOST: A hybrid digital signal processor transforms the MOST

The Molonglo Observatory Synthesis Telescope (MOST) is an 18,000 square meter radio telescope situated some 40 km from the city of Canberra, Australia. Its operating band (820-850 MHz) is now partly allocated to mobile phone communications, making radio astronomy challenging. We describe how the deployment of new digital receivers (RX boxes), Field Programmable Gate Array (FPGA) based filterbanks and server-class computers equipped with 43 GPUs (Graphics Processing Units) has transformed MOST into a versatile new instrument (the UTMOST) for studying the dynamic radio sky on millisecond timescales, ideal for work on pulsars and Fast Radio Bursts (FRBs). The filterbanks, servers and their high-speed, low-latency network form part of a hybrid solution to the observatory's signal processing requirements. The emphasis on software and commodity off-the-shelf hardware has enabled rapid deployment through the re-use of proven 'software backends' for its signal processing. The new receivers have ten times the bandwidth of the original MOST and double the sampling of the line feed, which doubles the field of view. The UTMOST can simultaneously excise interference, make maps, coherently dedisperse pulsars, and perform real-time searches of coherent fan beams for dispersed single pulses. Although system performance is still sub-optimal, a pulsar timing and FRB search programme has commenced and the first UTMOST maps have been made. The telescope operates as a robotic facility, deciding how to efficiently target pulsars and how long to stay on source, via feedback from real-time pulsar folding. The regular timing of over 300 pulsars has resulted in the discovery of 7 pulsar glitches and 3 FRBs. The UTMOST demonstrates that if sufficient signal processing can be applied to the voltage streams it is possible to perform innovative radio science in hostile radio frequency environments.

astro-ph.IM↗

arXiv2017

The SUrvey for Pulsars and Extragalactic Radio Bursts I: Survey Description and Overview

We describe the Survey for Pulsars and Extragalactic Radio Bursts (SUPERB), an ongoing pulsar and fast transient survey using the Parkes radio telescope. SUPERB involves real-time acceleration searches for pulsars and single-pulse searches for pulsars and fast radio bursts. We report on the observational setup, data analysis, multi-wavelength/messenger connections, survey sensitivities to pulsars and fast radio bursts and the impact of radio frequency interference. We further report on the first 10 pulsars discovered in the project. Among these is PSR~J1306$-$40, a millisecond pulsar in a binary system where it appears to be eclipsed for a large fraction of the orbit. PSR~J1421$-$4407 is another binary millisecond pulsar; its orbital period is $30.7$ days. This orbital period is in a range where only highly eccentric binaries are known, and expected by theory; despite this its orbit has an eccentricity of $10^{-5}$.

astro-ph.IM↗

arXiv2017

The first interferometric detections of Fast Radio Bursts

We present the first interferometric detections of Fast Radio Bursts (FRBs), an enigmatic new class of astrophysical transient. In a 180-day survey of the Southern sky we discovered 3 FRBs at 843 MHz with the UTMOST array, as part of commissioning science during a major ongoing upgrade. The wide field of view of UTMOST ($\approx 9$ deg$^{2}$) is well suited to FRB searches. The primary beam is covered by 352 partially overlapping fan-beams, each of which is searched for FRBs in real time with pulse widths in the range 0.655 to 42 ms, and dispersion measures $\leq$2000 pc cm$^{-3}$. Detections of FRBs with the UTMOST array places a lower limit on their distances of $\approx 10^4$ km (limit of the telescope near-field) supporting the case for an astronomical origin. Repeating FRBs at UTMOST or an FRB detected simultaneously with the Parkes radio telescope and UTMOST, would allow a few arcsec localisation, thereby providing an excellent means of identifying FRB host galaxies, if present. Up to 100 hours of follow-up for each FRB has been carried out with the UTMOST, with no repeating bursts seen. From the detected position, we present 3$σ$ error ellipses of 15 arcsec x 8.4 deg on the sky for the point of origin for the FRBs. We estimate an all-sky FRB rate at 843 MHz above a fluence $\cal F_\mathrm{lim}$ of 11 Jy ms of $\sim 78$ events sky$^{-1}$ d$^{-1}$ at the 95 percent confidence level. The measured rate of FRBs at 843 MHz is of order two times higher than we had expected, scaling from the FRB rate at the Parkes radio telescope, assuming that FRBs have a flat spectral index and a uniform distribution in Euclidean space. We examine how this can be explained by FRBs having a steeper spectral index and/or a flatter log$N$-log$\mathcal{F}$ distribution than expected for a Euclidean Universe.

astro-ph.HE↗

arXiv2016

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

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 pc cm^-3), exceptionally intense (120+-30 Jy), linearly polarized, scintillating burst (FRB 150807) that we directly localize to 9 arcmin^2. Based on a low Faraday rotation (12.0+-0.7 rad m^-2), we infer negligible magnetization in the circum-burst plasma and constrain the net magnetization of the cosmic web along this sightline to <21 nG, parallel to the line-of-sight. The burst scintillation suggests weak turbulence in the ionized intergalactic medium.

astro-ph.HE↗

arXiv2016

A Fast Radio Burst Host Galaxy

In recent years, millisecond duration radio signals originating from distant galaxies appear to have been discovered in the so-called Fast Radio Bursts. These signals are dispersed according to a precise physical law and this dispersion is a key observable quantity which, in tandem with a redshift measurement, can be used for fundamental physical investigations. While every fast radio burst has a dispersion measurement, none before now have had a redshift measurement, due to the difficulty in pinpointing their celestial coordinates. Here we present the discovery of a fast radio burst and the identification of a fading radio transient lasting $\sim 6$ days after the event, which we use to identify the host galaxy; we measure the galaxy's redshift to be $z=0.492\pm0.008$. The dispersion measure and redshift, in combination, provide a direct measurement of the cosmic density of ionised baryons in the intergalactic medium of $Ω_{\mathrm{IGM}}=4.9 \pm 1.3\%$, in agreement with the expectation from WMAP, and including all of the so-called "missing baryons". The $\sim6$-day transient is largely consistent with a short gamma-ray burst radio afterglow, and its existence and timescale do not support progenitor models such as giant pulses from pulsars, and supernovae. This contrasts with the interpretation of another recently discovered fast radio burst, suggesting there are at least two classes of bursts.

astro-ph.HE↗

arXiv2016

Fast Radio Transient searches with UTMOST at 843 MHz

We report the first radio interferometric search at 843 MHz for fast transients, particularly Fast Radio Bursts (FRBs). The recently recommissioned Swinburne University of Technology's digital backend for the Molonglo Observatory Synthesis Telescope array (the UTMOST) with its large collecting area (18,000 $\mathrm{m^2}$) and wide instantaneous field of view (7.80 $\mathrm{deg^2}$) is expected to be an efficient tool to detect FRBs. As an interferometer it will be capable of discerning whether the FRBs are truly a celestial population. We show that UTMOST at full design sensitivity can detect an event approximately every few days. We report on 2 preliminary FRB surveys at about 7% and 14% respectively of the array's final sensitivity. Several pulsars have been detected via single pulses and no FRBs were discovered with pulse widths ($W$), in the range 655.36 $μ$s $< W < 41.9$ ms and dispersion measures (DMs) in the range $100 < $DM$< 2000$ $\mathrm{pc\,cm^{-3}}$. This non-detection sets a 2$σ$ upper limit of the sky rate of not more than 1000 events $\mathrm{sky^{-1}}$ $\mathrm{day^{-1}}$ at 843 MHz down to a flux limit of 11 Jy for 1 ms FRBs. We show that this limit is consistent with previous survey limits at 1.4 GHz and 145 MHz and set a lower limit on the mean spectral index of FRBs of $α> -3.2$.

astro-ph.HE↗

arXiv2016

Are the distributions of Fast Radio Burst properties consistent with a cosmological population?

High time resolution radio surveys over the last few years have discovered a population of millisecond-duration transient bursts called Fast Radio Bursts (FRBs), which remain of unknown origin. FRBs exhibit dispersion consistent with propagation through a cold plasma and dispersion measures indicative of an origin at cosmological distances. In this paper we perform Monte Carlo simulations of a cosmological population of FRBs, based on assumptions consistent with observations of their energy distribution, their spatial density as a function of redshift and the properties of the interstellar and intergalactic media. We examine whether the dispersion measures, fluences, inferred redshifts, signal-to-noises and effective widths of known FRBs are consistent with a cosmological population. Statistical analyses indicate that at least 50 events at Parkes are required to distinguish between a constant co-moving FRB density, and a FRB density that evolves with redshift like the cosmological star formation rate density.

astro-ph.HE↗