Monthly Notices of the Royal Astronomical Society, Mar 14, 2022
The mechanism that produces fast radio burst (FRB) emission is poorly understood. Targeted monito... more The mechanism that produces fast radio burst (FRB) emission is poorly understood. Targeted monitoring of repeating FRB sources provides the opportunity to fully characterise the emission properties in a manner impossible with one-off bursts. Here we report observations of the source of FRB 20201124A, with the Australian Square Kilometre Array Pathfinder (ASKAP) and the Ultra-wideband Low (UWL) receiver at the Parkes 64-m radio telescope (Murriyang). The source entered a period of emitting bright bursts during early April 2021. We have detected 16 bursts from this source. One of the bursts detected with ASKAP is the brightest burst ever observed from a repeating FRB source with an inferred fluence of 640 ± 70 Jy ms. Of the five bursts detected with the UWL, none display any emission in the range 1.1-4 GHz. All UWL bursts are highly polarized, and we obtain an average Faraday rotation measure of -613 ± 2 rad m -2 for this source. In one of the UWL bursts, we see evidence of significant circularly-polarized emission with a fractional extent of 47%. Such a high degree of circular polarisation has never been seen before in bursts from repeating FRB sources. We also see evidence for variation in the polarization position angle in this UWL burst. Models for repeat burst emission will need to account for increasing diversity in the burst polarization properties.
The age-metallicity relation of the solar neighbourhood is studied using a sample of 552 late-typ... more The age-metallicity relation of the solar neighbourhood is studied using a sample of 552 late-type dwarfs. This sample was built from the intersection of photometric catalogues with chromospheric activity surveys of the Mount Wilson group. For these stars, metallicities were estimated from uvby data, and ages were calculated from their chromospheric emission levels using a new metallicity-dependent chromospheric activity-age relation developed by . A careful estimate of the errors in the chromospheric age is made. The errors in the chromospheric indices are shown to include partially the effects of the stellar magnetic cycles, although a detailed treatment of this error is still beyond our knowledge. It is shown that the results are not affected by the presence of unresolved binaries in the sample. We derive an age-metallicity relation which confirms the mean trend found by previous workers. The mean metallicity shows a slow, steady increase with time, amounting at least 0.56 dex in 15 Gyr. The initial metallicity of the disk is around -0.70 dex, in agreement with the G dwarf metallicity distribution . According to our data, the intrinsic cosmic dispersion in metal abundances is around 0.13 dex, a factor of two smaller than that found by . We show that chromospheric ages are compatible with isochrone ages, within the expected errors, so that the difference in the scatter cannot be caused by the accuracy of our ages and metallicities. This reinforces some suggestions that the Edvarsson et al.'s sample is not suitable to the determination of the age-metallicity relation.
Monthly Notices of the Royal Astronomical Society, Nov 25, 2021
Fast Radio Bursts (FRBs) are highly energetic transient events with duration of order of microsec... more Fast Radio Bursts (FRBs) are highly energetic transient events with duration of order of microseconds to milliseconds and of unknown origin. They are known to lie at cosmological distances, through localisation to host galaxies. Recently, an FRB-like event was seen from the Milky Way magnetar SGR 1935+2154 by the CHIME and STARE2 telescopes. This is the only magnetar that has produced FRB events in our galaxy. Finding similar events in the Milky Way is of great interest to understanding FRB progenitors. Such events will be strongly affected by the turbulent interstellar medium in the Milky Way, their intrinsic energy distribution and their spatial locations within the plane of the Milky Way. We examine these effects using models for the distribution of electrons in the ISM to estimate the dispersion measure and pulse scattering of mock events, and a range of models for the spatial distribution and luminosity functions, including models motivated by the spatial distribution of the Milky Way's magnetars. We evaluate the fraction of FRB events in the Milky Way that are detectable by STARE2 for a range of ISM models, spatial distributions and burst luminosity functions. In all the models examined, only a fraction of burst events are detectable, mainly due to the scattering effects of the ISM. We find that GReX, a proposed all-sky experiment, could increase the detection rate of Milky Way FRB events by an order of magnitude, depending on assumptions made about the luminosity function and scale-height of the FRBs.
Context. Fast radio bursts (FRBs) are extremely energetic pulses of millisecond duration and unkn... more Context. Fast radio bursts (FRBs) are extremely energetic pulses of millisecond duration and unknown origin. To understand the phenomenon that emits these pulses, targeted and un-targeted searches have been performed for multiwavelength counterparts, including the optical. Aims. The objective of this work is to search for optical transients at the positions of eight well-localized (<1 ) FRBs after the arrival of the burst on different timescales (typically at one day, several months, and one year after FRB detection). We then compare this with known optical light curves to constrain progenitor models. Methods. We used the Las Cumbres Observatory Global Telescope (LCOGT) network to promptly take images with its network of 23 telescopes working around the world. We used a template subtraction technique to analyze all the images collected at differing epochs. We have divided the difference images into two groups: In one group we use the image of the last epoch as a template, and in the other group we use the image of the first epoch as a template. We then searched for optical transients at the localizations of the FRBs in the template subtracted images. Results. We have found no optical transients and have therefore set limiting magnitudes to the optical counterparts. Typical limits in apparent and absolute magnitudes for our LCOGT data are ∼22 and -19 mag in the r band, respectively. We have compared our limiting magnitudes with light curves of super-luminous supernovae (SLSNe), Type Ia supernovae (SNe Ia), supernovae associated with gamma-ray bursts (GRB-SNe), a kilonova, and tidal disruption events (TDEs). Conclusions. Assuming that the FRB emission coincides with the time of explosion of these transients, we rule out associations with SLSNe (at the ∼99.9% confidence level) and the brightest subtypes of SNe Ia, GRB-SNe, and TDEs (at a similar confidence level). However, we cannot exclude scenarios where FRBs are directly associated with the faintest of these subtypes or with kilonovae.
Monthly Notices of the Royal Astronomical Society, May 27, 2015
A new sample of stars, representative of the solar neighbourhood luminosity function, is construc... more A new sample of stars, representative of the solar neighbourhood luminosity function, is constructed from the Hipparcos catalogue and the Fifth Catalogue of Nearby Stars. We have cross-matched to sources in the 2MASS catalogue so that for all stars individually determined Near Infrared photometry (NIR) is available on a homogeneous system (typically K s ). The spatial completeness of the sample has been carefully determined by statistical methods, and the NIR luminosity function of the stars has been derived by direct star counts. We find a local volume luminosity of 0.121 ± 0.004 L K pc -3 , corresponding to a volumetric mass-to-light ratio of M/L K = 0.31 ± 0.02 M /L K , where giants contribute 80 per cent to the light but less than 2 per cent to the stellar mass. We derive the surface brightness of the solar cylinder with the help of a vertical disc model. We find a surface brightness of 99 L K pc -2 with an uncertainty of approximately 10 per cent. This corresponds to a mass-to-light ratio for the solar cylinder of M/L K = 0.34 M /L K . The mass-tolight ratio for the solar cylinder is only 10 per cent larger than the local value despite the fact that the local population has a much larger contribution of young stars. It turns out that the effective scale heights of the lower main sequence carrying most of the mass is similar to that of the giants, which are dominating the NIR light. The corresponding colour for the solar cylinder is V -K = 2.89 mag compared to the local value of V -K = 2.46 mag. An extrapolation of the local surface brightness to the whole Milky Way yields a total luminosity of M K = -24.2 mag. The Milky Way falls in the range of K band Tully-Fisher (TF) relations from the literature.
Monthly Notices of the Royal Astronomical Society, Feb 27, 2019
The discovery of fast radio bursts (FRBs) about a decade ago opened up new possibilities for prob... more 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 occuured 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.
Monthly Notices of the Royal Astronomical Society, May 17, 2023
We use the latest parallaxes measurements from Gaia DR3 to obtain a geometric calibration of the ... more We use the latest parallaxes measurements from Gaia DR3 to obtain a geometric calibration of the tip of the red giant branch (TRGB) in Cousins I magnitudes as a standard candle for cosmology. We utilise the following surveys: SkyMapper DR3, APASS DR9, ATLAS Refcat2, and Gaia DR3 synthetic photometry to obtain multiple zero-point calibrations of the TRGB magnitude, M T RGB I . Our sample contains Milky Way halo stars at high galactic latitudes (|b| > 36) where the impact of metallicity, dust, and crowding are minimised. The magnitude of the TRGB is identified using Sobel edge detection, but this approach introduced a systematic offset. To address this issue, we utilised simulations with PARSEC isochrones and showed how to calibrate and remove this bias. Applying our method within the colour range where the slope of the TRGB is relatively flat for metal-poor halo stars (1.55 < (BP -RP ) < 2.25), we find a weighted average M T RGB I = -4.042 ± 0.041 (stat) ±0.031 (sys) mag. A geometric calibration of the Milky Way TRGB has the benefit of being independent of other distance indicators and will help probe systematics in the local distance ladder, leading to improved measurements of the Hubble constant.
Monthly Notices of the Royal Astronomical Society, Dec 23, 2015
In decomposing the HI rotation curves of disc galaxies, it is necessary to break a degeneracy bet... more In decomposing the HI rotation curves of disc galaxies, it is necessary to break a degeneracy between the gravitational fields of the disc and the dark halo by estimating the disc surface density. This is done by combining measurements of the vertical velocity dispersion of the disc with the disc scale height. The vertical velocity dispersion of the discs is measured from absorption lines (near the V-band) of near-face-on spiral galaxies, with the light coming from a mixed population of giants of all ages. However, the scale heights for these galaxies are estimated statistically from near-IR surface photometry of edge-on galaxies. The scale height estimate is therefore dominated by a population of older (> 2 Gyr) red giants. In this paper, we demonstrate the importance of measuring the velocity dispersion for the same older population of stars that is used to estimate the vertical scale height. We present an analysis of the vertical kinematics of K-giants in the solar vicinity. We find the vertical velocity distribution best fit by two components with dispersions of 9.6 ± 0.5 km s -1 and 18.6 ± 1.0 km s -1 , which we interpret as the dispersions of the young and old disc populations respectively. Combining the (single) measured velocity dispersion of the total young + old disc population (13.0 ± 0.1 km s -1 ) with the scale height estimated for the older population would underestimate the disc surface density by a factor of ∼ 2. Such a disc would have a peak rotational velocity that is only 70% of that for the maximal disc, thus making it appear submaximal.
Monthly Notices of the Royal Astronomical Society, Feb 11, 2019
We present Monte-Carlo simulations of a cosmological population of repeating fast radio burst (FR... more 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.
Fast Radio Bursts (FRBs) represent a novel tool for probing the properties of the universe at cos... more Fast Radio Bursts (FRBs) represent a novel tool for probing the properties of the universe at cosmological distances. The dispersion measures of FRBs, combined with the redshifts of their host galaxies, has very recently yielded a direct measurement of the baryon content of the universe, and has the potential to directly constrain the location of the "missing baryons". The first results are consistent with the expectations of ΛCDM for the cosmic density of baryons, and have provided the first constraints on the properties of the very diffuse intergalactic medium (IGM) and circumgalactic medium (CGM) around galaxies. FRBs are the only known extragalactic sources that are compact enough to exhibit diffractive scintillation in addition to showing exponential tails which are typical of scattering in turbulent media. This will allow us to probe the turbulent properties of the circumburst medium, the host galaxy ISM/halo, and intervening halos along the path, as well as the IGM. Measurement of the Hubble constant and the dark energy parameter w can be made with FRBs, but require very large samples of localised FRBs (>10 3 ) to be effective on their own-they are best combined with other independent surveys to improve the constraints. Ionisation events, such as for He II, leave a signature in the dispersion measure-redshift relation, and if FRBs exist prior to these times, they can be used to probe the reionisation era, although more than 10 3 localised FRBs are required.
Monthly Notices of the Royal Astronomical Society, Dec 16, 2022
The rich absorption spectra of Sun-like stars are enticing probes for variations in the fine-stru... more The rich absorption spectra of Sun-like stars are enticing probes for variations in the fine-structure constant, 𝛼, which gauges the strength of electromagnetism. While individual line wavelengths are sensitive to 𝛼, they are also sensitive to physical processes in the stellar atmospheres, which has precluded their use so far. Here we demonstrate a new, differential approach using solar twins: velocity separations between close pairs of transitions are compared across stars with very similar physical properties, strongly suppressing astrophysical and instrumental systematic errors. We utilise 423 archival exposures of 18 solar twins from the High-Accuracy Radial velocity Planetary Searcher (HARPS), in which calibration errors can be reduced to 3 m s -1 . For stars with ≈10 high signal-to-noise ratio spectra (≥200 per pixel), velocity separations between pairs are measured with ≈10 m s -1 statistical precision. A companion paper assesses a range of systematic error sources using 130 stars, with a greater range of stellar parameters, providing accurate corrections for astrophysical effects and a residual, intrinsic star-to-star scatter of 0-13 m s -1 . Within these uncertainties, we find no evidence for velocity separation differences in 17 transition pairs between solar twins. In a second companion paper, this is found to limit local ( 50 pc) variations in 𝛼 to ≈50 parts per billion, ∼2 orders of magnitude less than other Galactic constraints.
Publications of the Astronomical Society of Australia, 2021
Galactic electron density distribution models are crucial tools for estimating the impact of the ... more Galactic electron density distribution models are crucial tools for estimating the impact of the ionised interstellar medium on the impulsive signals from radio pulsars and fast radio bursts. The two prevailing Galactic electron density models are YMW16 ) and NE2001 (Cordes & Lazio, 2002). Here, we introduce a software package P y G E D M which provides a unified application programming interface (API) for these models and the YT20 (Yamasaki & Totani, 2020) model of the Galactic halo. We use P y G E D M to compute all-sky maps of Galactic dispersion measure (DM) for YMW16 and NE2001, and compare the large-scale differences between the two. In general, YMW16 predicts higher DM values toward the Galactic anticentre. YMW16 predicts higher DMs at low Galactic latitudes, but NE2001 predicts higher DMs in most other directions. We identify lines of sight for which the models are most discrepant, using pulsars with independent distance measurements. YMW16 performs better on average than NE2001, but both models show significant outliers. We suggest that future campaigns to determine pulsar distances should focus on targets where the models show large discrepancies, so future models can use those measurements to better estimate distances along those line of sight. We also suggest that the Galactic halo should be considered as a component in future GEDMs, to avoid overestimating the Galactic DM contribution for extragalactic sources such as FRBs.
Monthly Notices of the Royal Astronomical Society, May 3, 2018
We report a new Fast Radio Burst (FRB) discovered in real-time as part of the UTMOST project at t... more 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 ± 0.04 pc cm -3 , is the lowest of the FRB population. The Milky Way contribution along the line of sight is ∼ 40 pc cm -3 , leaving an excess DM of ∼ 145 pc cm -3 . The FRB has a fluence > 20 ± 7 Jy ms, and is narrow, with a width of ∼ 400 µs at 10% of its maximum amplitude. However, the burst shows three temporal components, the narrowest of which is ∼ 30 µs, and a scattering timescale of 4.1 ± 2.7 µs. 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.
Monthly Notices of the Royal Astronomical Society, Dec 12, 2014
Fast radio bursts (FRBs) are one of the most tantalizing mysteries of the radio sky; their progen... more Fast radio bursts (FRBs) are one of the most tantalizing mysteries of the radio sky; their progenitors and origins remain unknown and until now no rapid multiwavelength follow-up of an FRB has been possible. New instrumentation has decreased the time between observation and discovery from years to seconds, and enables polarimetry to be performed on FRBs for the first time. We have discovered an FRB (FRB 140514) in real-time on 2014 May 14 at 17:14:11.06 UTC at the Parkes radio telescope and triggered follow-up at other wavelengths within hours of the event. FRB 140514 was found with a dispersion measure (DM) of 562.7(6) cm -3 pc, giving an upper limit on source redshift of z 0.5. FRB 140514 was found to be 21 ± 7 per cent (3σ ) circularly polarized on the leading edge with a 1σ upper limit on linear polarization <10 per cent. We conclude that this polarization is intrinsic to the FRB. If there was any intrinsic linear polarization, as might be expected from coherent emission, then it may have been depolarized by Faraday rotation caused by passing through strong magnetic fields and/or high-density environments. FRB 140514 was discovered during a campaign to re-observe known FRB fields, and lies close to a previous discovery, FRB 110220; based on the difference in DMs of these bursts and time-on-sky arguments, we attribute the proximity to sampling bias and conclude that they are distinct objects. Follow-up conducted by 12 telescopes observing from X-ray to radio wavelengths was unable to identify a variable multiwavelength counterpart, allowing us to rule out models in which FRBs originate from nearby (z < 0.3) supernovae and long duration gamma-ray bursts.
In this paper, we investigate some chemokinematical properties of the Milky Way disk, by using a ... more In this paper, we investigate some chemokinematical properties of the Milky Way disk, by using a sample composed by 424 late-type dwarfs. We show that the velocity dispersion of a stellar group correlates with the age of this group, according to a law proportional to t 0.26 , where t is the age of the stellar group. The temporal evolution of the vertex deviation is considered in detail. It is shown that the vertex deviation does not seem to depend strongly on the age of the stellar group. Previous studies in the literature seem to not have found it due to the use of statistical ages for stellar groups, rather than individual ages. The possibility to use the orbital parameters of a star to derive information about its birthplace is investigated, and we show that the mean galactocentric radius is likely to be the most reliable stellar birthplace indicator. However, this information cannot be presently used to derive radial evolutionary constraints, due to an intrinsic bias present in all samples constructed from nearby stars. An extensive discussion of the secular and stochastic heating mechanisms commonly invoked to explain the age-velocity dispersion relation is presented. We suggest that the age-velocity dispersion relation could reflect the gradual decrease in the turbulent velocity dispersion from which disk stars form, a suggestion originally made by Tinsley & Larson (1978, ApJ, 221, 554) and supported by several more recent disk evolution calculations. A test to distinguish between the two types of models using high-redshift galaxies is proposed.
Monthly Notices of the Royal Astronomical Society, Mar 19, 2019
We have conducted a search for bright repeating Fast Radio Bursts in our nearby Universe with the... more We have conducted a search for bright repeating Fast Radio Bursts in our nearby Universe with the Australian Square Kilometer Array Pathfinder (ASKAP) in singledish mode. We used eight ASKAP 12-m dishes, each equipped with a Chequerboard Phased Array Feed forming 36 beams on the sky, to survey ∼30,000 deg 2 of the southern sky (-90 • < δ < +30 • ) in 158 antenna days. The fluence limit of the survey is 22 Jy ms. We report the detection of FRB 180515 in our survey. We found no repeating FRBs in a total mean observation of 3 hrs per pointing divided into one hour intervals, which were separated in time ranging between a day to a month. Using our non-detection, we exclude the presence of a repeating FRB similar to FRB 121102 closer than z = 0.004 in the survey area -a volume of at least 9.4 × 10 4 Mpc 3 -at 95% confidence.
Monthly Notices of the Royal Astronomical Society, Jul 4, 2019
We detail a new fast radio burst (FRB) survey with the Molonglo Radio Telescope, in which six FRB... more We detail a new fast radio burst (FRB) survey with the Molonglo Radio Telescope, in which six FRBs were detected between 2017 June and 2018 December. 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 μs) studies of the bursts. Five of the FRBs show temporal broadening consistent with interstellar and/or intergalactic scattering, with scattering time-scales ranging from 0.16 to 29.1 ms. One burst, FRB181017, shows remarkable temporal structure, with three 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 d -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 ∝ ν α ). Our results suggest that FRB spectra may turn over below 1 GHz. Optical, radio, and X-ray follow-up has been made for most of the reported bursts, with no associated transients found. No repeat bursts were found in the survey.
Chemical tagging of stellar debris from disrupted open clusters and associations underpins the sc... more Chemical tagging of stellar debris from disrupted open clusters and associations underpins the science cases for next-generation multi-object spectroscopic surveys. As part of the Galactic Archaeology project TraCD (Tracking Cluster Debris), a preliminary attempt at reconstructing the birth clouds of now phase-mixed thin disk debris is undertaken using a parametric minimum spanning tree (MST) approach. Empirically-motivated chemical abundance pattern uncertainties (for a 10-dimensional chemistry-space) are applied to NBODY6-realised stellar associations dissolved into a background sea of field stars, all evolving in a Milky Way potential. We demonstrate that significant population reconstruction degeneracies appear when the abundance uncertainties approach ∼0.1 dex and the parameterised MST approach is employed; more sophisticated methodologies will be required to ameliorate these degeneracies.
A chromospheric age distribution of 552 latetype dwarfs is transformed into a star formation hist... more A chromospheric age distribution of 552 latetype dwarfs is transformed into a star formation history by the application of scale height corrections, stellar evolutionary corrections and volume corrections. We show that the disk of our Galaxy has experienced enhanced episodes of star formation at 0-1 Gyr, 2-5 Gyr and 7-9 Gyr ago, although the reality of the latter burst is still uncertain. The star sample birthsites are distributed over a very large range of distances because of orbital diffusion, and so give an estimate of the global star formation rate. These results are compared with the metal-enrichment rate, given by the age-metallicity relation, with the expected epochs of close encounters between our Galaxy and the Magellanic Clouds, and with previous determinations of the star formation history. Simulations are used to examine the age-dependent smearing of the star formation history due to age uncertainties, and the broadening of the recovered features, as well as to measure the probability level that the history derived to be produced by statistical fluctuations of a constant star formation history. We show, with a significance level greater than 98%, that the Milky Way have not had a constant star formation history.
Monthly Notices of the Royal Astronomical Society, Mar 14, 2022
The mechanism that produces fast radio burst (FRB) emission is poorly understood. Targeted monito... more The mechanism that produces fast radio burst (FRB) emission is poorly understood. Targeted monitoring of repeating FRB sources provides the opportunity to fully characterise the emission properties in a manner impossible with one-off bursts. Here we report observations of the source of FRB 20201124A, with the Australian Square Kilometre Array Pathfinder (ASKAP) and the Ultra-wideband Low (UWL) receiver at the Parkes 64-m radio telescope (Murriyang). The source entered a period of emitting bright bursts during early April 2021. We have detected 16 bursts from this source. One of the bursts detected with ASKAP is the brightest burst ever observed from a repeating FRB source with an inferred fluence of 640 ± 70 Jy ms. Of the five bursts detected with the UWL, none display any emission in the range 1.1-4 GHz. All UWL bursts are highly polarized, and we obtain an average Faraday rotation measure of -613 ± 2 rad m -2 for this source. In one of the UWL bursts, we see evidence of significant circularly-polarized emission with a fractional extent of 47%. Such a high degree of circular polarisation has never been seen before in bursts from repeating FRB sources. We also see evidence for variation in the polarization position angle in this UWL burst. Models for repeat burst emission will need to account for increasing diversity in the burst polarization properties.
The age-metallicity relation of the solar neighbourhood is studied using a sample of 552 late-typ... more The age-metallicity relation of the solar neighbourhood is studied using a sample of 552 late-type dwarfs. This sample was built from the intersection of photometric catalogues with chromospheric activity surveys of the Mount Wilson group. For these stars, metallicities were estimated from uvby data, and ages were calculated from their chromospheric emission levels using a new metallicity-dependent chromospheric activity-age relation developed by . A careful estimate of the errors in the chromospheric age is made. The errors in the chromospheric indices are shown to include partially the effects of the stellar magnetic cycles, although a detailed treatment of this error is still beyond our knowledge. It is shown that the results are not affected by the presence of unresolved binaries in the sample. We derive an age-metallicity relation which confirms the mean trend found by previous workers. The mean metallicity shows a slow, steady increase with time, amounting at least 0.56 dex in 15 Gyr. The initial metallicity of the disk is around -0.70 dex, in agreement with the G dwarf metallicity distribution . According to our data, the intrinsic cosmic dispersion in metal abundances is around 0.13 dex, a factor of two smaller than that found by . We show that chromospheric ages are compatible with isochrone ages, within the expected errors, so that the difference in the scatter cannot be caused by the accuracy of our ages and metallicities. This reinforces some suggestions that the Edvarsson et al.'s sample is not suitable to the determination of the age-metallicity relation.
Monthly Notices of the Royal Astronomical Society, Nov 25, 2021
Fast Radio Bursts (FRBs) are highly energetic transient events with duration of order of microsec... more Fast Radio Bursts (FRBs) are highly energetic transient events with duration of order of microseconds to milliseconds and of unknown origin. They are known to lie at cosmological distances, through localisation to host galaxies. Recently, an FRB-like event was seen from the Milky Way magnetar SGR 1935+2154 by the CHIME and STARE2 telescopes. This is the only magnetar that has produced FRB events in our galaxy. Finding similar events in the Milky Way is of great interest to understanding FRB progenitors. Such events will be strongly affected by the turbulent interstellar medium in the Milky Way, their intrinsic energy distribution and their spatial locations within the plane of the Milky Way. We examine these effects using models for the distribution of electrons in the ISM to estimate the dispersion measure and pulse scattering of mock events, and a range of models for the spatial distribution and luminosity functions, including models motivated by the spatial distribution of the Milky Way's magnetars. We evaluate the fraction of FRB events in the Milky Way that are detectable by STARE2 for a range of ISM models, spatial distributions and burst luminosity functions. In all the models examined, only a fraction of burst events are detectable, mainly due to the scattering effects of the ISM. We find that GReX, a proposed all-sky experiment, could increase the detection rate of Milky Way FRB events by an order of magnitude, depending on assumptions made about the luminosity function and scale-height of the FRBs.
Context. Fast radio bursts (FRBs) are extremely energetic pulses of millisecond duration and unkn... more Context. Fast radio bursts (FRBs) are extremely energetic pulses of millisecond duration and unknown origin. To understand the phenomenon that emits these pulses, targeted and un-targeted searches have been performed for multiwavelength counterparts, including the optical. Aims. The objective of this work is to search for optical transients at the positions of eight well-localized (<1 ) FRBs after the arrival of the burst on different timescales (typically at one day, several months, and one year after FRB detection). We then compare this with known optical light curves to constrain progenitor models. Methods. We used the Las Cumbres Observatory Global Telescope (LCOGT) network to promptly take images with its network of 23 telescopes working around the world. We used a template subtraction technique to analyze all the images collected at differing epochs. We have divided the difference images into two groups: In one group we use the image of the last epoch as a template, and in the other group we use the image of the first epoch as a template. We then searched for optical transients at the localizations of the FRBs in the template subtracted images. Results. We have found no optical transients and have therefore set limiting magnitudes to the optical counterparts. Typical limits in apparent and absolute magnitudes for our LCOGT data are ∼22 and -19 mag in the r band, respectively. We have compared our limiting magnitudes with light curves of super-luminous supernovae (SLSNe), Type Ia supernovae (SNe Ia), supernovae associated with gamma-ray bursts (GRB-SNe), a kilonova, and tidal disruption events (TDEs). Conclusions. Assuming that the FRB emission coincides with the time of explosion of these transients, we rule out associations with SLSNe (at the ∼99.9% confidence level) and the brightest subtypes of SNe Ia, GRB-SNe, and TDEs (at a similar confidence level). However, we cannot exclude scenarios where FRBs are directly associated with the faintest of these subtypes or with kilonovae.
Monthly Notices of the Royal Astronomical Society, May 27, 2015
A new sample of stars, representative of the solar neighbourhood luminosity function, is construc... more A new sample of stars, representative of the solar neighbourhood luminosity function, is constructed from the Hipparcos catalogue and the Fifth Catalogue of Nearby Stars. We have cross-matched to sources in the 2MASS catalogue so that for all stars individually determined Near Infrared photometry (NIR) is available on a homogeneous system (typically K s ). The spatial completeness of the sample has been carefully determined by statistical methods, and the NIR luminosity function of the stars has been derived by direct star counts. We find a local volume luminosity of 0.121 ± 0.004 L K pc -3 , corresponding to a volumetric mass-to-light ratio of M/L K = 0.31 ± 0.02 M /L K , where giants contribute 80 per cent to the light but less than 2 per cent to the stellar mass. We derive the surface brightness of the solar cylinder with the help of a vertical disc model. We find a surface brightness of 99 L K pc -2 with an uncertainty of approximately 10 per cent. This corresponds to a mass-to-light ratio for the solar cylinder of M/L K = 0.34 M /L K . The mass-tolight ratio for the solar cylinder is only 10 per cent larger than the local value despite the fact that the local population has a much larger contribution of young stars. It turns out that the effective scale heights of the lower main sequence carrying most of the mass is similar to that of the giants, which are dominating the NIR light. The corresponding colour for the solar cylinder is V -K = 2.89 mag compared to the local value of V -K = 2.46 mag. An extrapolation of the local surface brightness to the whole Milky Way yields a total luminosity of M K = -24.2 mag. The Milky Way falls in the range of K band Tully-Fisher (TF) relations from the literature.
Monthly Notices of the Royal Astronomical Society, Feb 27, 2019
The discovery of fast radio bursts (FRBs) about a decade ago opened up new possibilities for prob... more 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 occuured 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.
Monthly Notices of the Royal Astronomical Society, May 17, 2023
We use the latest parallaxes measurements from Gaia DR3 to obtain a geometric calibration of the ... more We use the latest parallaxes measurements from Gaia DR3 to obtain a geometric calibration of the tip of the red giant branch (TRGB) in Cousins I magnitudes as a standard candle for cosmology. We utilise the following surveys: SkyMapper DR3, APASS DR9, ATLAS Refcat2, and Gaia DR3 synthetic photometry to obtain multiple zero-point calibrations of the TRGB magnitude, M T RGB I . Our sample contains Milky Way halo stars at high galactic latitudes (|b| > 36) where the impact of metallicity, dust, and crowding are minimised. The magnitude of the TRGB is identified using Sobel edge detection, but this approach introduced a systematic offset. To address this issue, we utilised simulations with PARSEC isochrones and showed how to calibrate and remove this bias. Applying our method within the colour range where the slope of the TRGB is relatively flat for metal-poor halo stars (1.55 < (BP -RP ) < 2.25), we find a weighted average M T RGB I = -4.042 ± 0.041 (stat) ±0.031 (sys) mag. A geometric calibration of the Milky Way TRGB has the benefit of being independent of other distance indicators and will help probe systematics in the local distance ladder, leading to improved measurements of the Hubble constant.
Monthly Notices of the Royal Astronomical Society, Dec 23, 2015
In decomposing the HI rotation curves of disc galaxies, it is necessary to break a degeneracy bet... more In decomposing the HI rotation curves of disc galaxies, it is necessary to break a degeneracy between the gravitational fields of the disc and the dark halo by estimating the disc surface density. This is done by combining measurements of the vertical velocity dispersion of the disc with the disc scale height. The vertical velocity dispersion of the discs is measured from absorption lines (near the V-band) of near-face-on spiral galaxies, with the light coming from a mixed population of giants of all ages. However, the scale heights for these galaxies are estimated statistically from near-IR surface photometry of edge-on galaxies. The scale height estimate is therefore dominated by a population of older (> 2 Gyr) red giants. In this paper, we demonstrate the importance of measuring the velocity dispersion for the same older population of stars that is used to estimate the vertical scale height. We present an analysis of the vertical kinematics of K-giants in the solar vicinity. We find the vertical velocity distribution best fit by two components with dispersions of 9.6 ± 0.5 km s -1 and 18.6 ± 1.0 km s -1 , which we interpret as the dispersions of the young and old disc populations respectively. Combining the (single) measured velocity dispersion of the total young + old disc population (13.0 ± 0.1 km s -1 ) with the scale height estimated for the older population would underestimate the disc surface density by a factor of ∼ 2. Such a disc would have a peak rotational velocity that is only 70% of that for the maximal disc, thus making it appear submaximal.
Monthly Notices of the Royal Astronomical Society, Feb 11, 2019
We present Monte-Carlo simulations of a cosmological population of repeating fast radio burst (FR... more 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.
Fast Radio Bursts (FRBs) represent a novel tool for probing the properties of the universe at cos... more Fast Radio Bursts (FRBs) represent a novel tool for probing the properties of the universe at cosmological distances. The dispersion measures of FRBs, combined with the redshifts of their host galaxies, has very recently yielded a direct measurement of the baryon content of the universe, and has the potential to directly constrain the location of the "missing baryons". The first results are consistent with the expectations of ΛCDM for the cosmic density of baryons, and have provided the first constraints on the properties of the very diffuse intergalactic medium (IGM) and circumgalactic medium (CGM) around galaxies. FRBs are the only known extragalactic sources that are compact enough to exhibit diffractive scintillation in addition to showing exponential tails which are typical of scattering in turbulent media. This will allow us to probe the turbulent properties of the circumburst medium, the host galaxy ISM/halo, and intervening halos along the path, as well as the IGM. Measurement of the Hubble constant and the dark energy parameter w can be made with FRBs, but require very large samples of localised FRBs (>10 3 ) to be effective on their own-they are best combined with other independent surveys to improve the constraints. Ionisation events, such as for He II, leave a signature in the dispersion measure-redshift relation, and if FRBs exist prior to these times, they can be used to probe the reionisation era, although more than 10 3 localised FRBs are required.
Monthly Notices of the Royal Astronomical Society, Dec 16, 2022
The rich absorption spectra of Sun-like stars are enticing probes for variations in the fine-stru... more The rich absorption spectra of Sun-like stars are enticing probes for variations in the fine-structure constant, 𝛼, which gauges the strength of electromagnetism. While individual line wavelengths are sensitive to 𝛼, they are also sensitive to physical processes in the stellar atmospheres, which has precluded their use so far. Here we demonstrate a new, differential approach using solar twins: velocity separations between close pairs of transitions are compared across stars with very similar physical properties, strongly suppressing astrophysical and instrumental systematic errors. We utilise 423 archival exposures of 18 solar twins from the High-Accuracy Radial velocity Planetary Searcher (HARPS), in which calibration errors can be reduced to 3 m s -1 . For stars with ≈10 high signal-to-noise ratio spectra (≥200 per pixel), velocity separations between pairs are measured with ≈10 m s -1 statistical precision. A companion paper assesses a range of systematic error sources using 130 stars, with a greater range of stellar parameters, providing accurate corrections for astrophysical effects and a residual, intrinsic star-to-star scatter of 0-13 m s -1 . Within these uncertainties, we find no evidence for velocity separation differences in 17 transition pairs between solar twins. In a second companion paper, this is found to limit local ( 50 pc) variations in 𝛼 to ≈50 parts per billion, ∼2 orders of magnitude less than other Galactic constraints.
Publications of the Astronomical Society of Australia, 2021
Galactic electron density distribution models are crucial tools for estimating the impact of the ... more Galactic electron density distribution models are crucial tools for estimating the impact of the ionised interstellar medium on the impulsive signals from radio pulsars and fast radio bursts. The two prevailing Galactic electron density models are YMW16 ) and NE2001 (Cordes & Lazio, 2002). Here, we introduce a software package P y G E D M which provides a unified application programming interface (API) for these models and the YT20 (Yamasaki & Totani, 2020) model of the Galactic halo. We use P y G E D M to compute all-sky maps of Galactic dispersion measure (DM) for YMW16 and NE2001, and compare the large-scale differences between the two. In general, YMW16 predicts higher DM values toward the Galactic anticentre. YMW16 predicts higher DMs at low Galactic latitudes, but NE2001 predicts higher DMs in most other directions. We identify lines of sight for which the models are most discrepant, using pulsars with independent distance measurements. YMW16 performs better on average than NE2001, but both models show significant outliers. We suggest that future campaigns to determine pulsar distances should focus on targets where the models show large discrepancies, so future models can use those measurements to better estimate distances along those line of sight. We also suggest that the Galactic halo should be considered as a component in future GEDMs, to avoid overestimating the Galactic DM contribution for extragalactic sources such as FRBs.
Monthly Notices of the Royal Astronomical Society, May 3, 2018
We report a new Fast Radio Burst (FRB) discovered in real-time as part of the UTMOST project at t... more 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 ± 0.04 pc cm -3 , is the lowest of the FRB population. The Milky Way contribution along the line of sight is ∼ 40 pc cm -3 , leaving an excess DM of ∼ 145 pc cm -3 . The FRB has a fluence > 20 ± 7 Jy ms, and is narrow, with a width of ∼ 400 µs at 10% of its maximum amplitude. However, the burst shows three temporal components, the narrowest of which is ∼ 30 µs, and a scattering timescale of 4.1 ± 2.7 µs. 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.
Monthly Notices of the Royal Astronomical Society, Dec 12, 2014
Fast radio bursts (FRBs) are one of the most tantalizing mysteries of the radio sky; their progen... more Fast radio bursts (FRBs) are one of the most tantalizing mysteries of the radio sky; their progenitors and origins remain unknown and until now no rapid multiwavelength follow-up of an FRB has been possible. New instrumentation has decreased the time between observation and discovery from years to seconds, and enables polarimetry to be performed on FRBs for the first time. We have discovered an FRB (FRB 140514) in real-time on 2014 May 14 at 17:14:11.06 UTC at the Parkes radio telescope and triggered follow-up at other wavelengths within hours of the event. FRB 140514 was found with a dispersion measure (DM) of 562.7(6) cm -3 pc, giving an upper limit on source redshift of z 0.5. FRB 140514 was found to be 21 ± 7 per cent (3σ ) circularly polarized on the leading edge with a 1σ upper limit on linear polarization <10 per cent. We conclude that this polarization is intrinsic to the FRB. If there was any intrinsic linear polarization, as might be expected from coherent emission, then it may have been depolarized by Faraday rotation caused by passing through strong magnetic fields and/or high-density environments. FRB 140514 was discovered during a campaign to re-observe known FRB fields, and lies close to a previous discovery, FRB 110220; based on the difference in DMs of these bursts and time-on-sky arguments, we attribute the proximity to sampling bias and conclude that they are distinct objects. Follow-up conducted by 12 telescopes observing from X-ray to radio wavelengths was unable to identify a variable multiwavelength counterpart, allowing us to rule out models in which FRBs originate from nearby (z < 0.3) supernovae and long duration gamma-ray bursts.
In this paper, we investigate some chemokinematical properties of the Milky Way disk, by using a ... more In this paper, we investigate some chemokinematical properties of the Milky Way disk, by using a sample composed by 424 late-type dwarfs. We show that the velocity dispersion of a stellar group correlates with the age of this group, according to a law proportional to t 0.26 , where t is the age of the stellar group. The temporal evolution of the vertex deviation is considered in detail. It is shown that the vertex deviation does not seem to depend strongly on the age of the stellar group. Previous studies in the literature seem to not have found it due to the use of statistical ages for stellar groups, rather than individual ages. The possibility to use the orbital parameters of a star to derive information about its birthplace is investigated, and we show that the mean galactocentric radius is likely to be the most reliable stellar birthplace indicator. However, this information cannot be presently used to derive radial evolutionary constraints, due to an intrinsic bias present in all samples constructed from nearby stars. An extensive discussion of the secular and stochastic heating mechanisms commonly invoked to explain the age-velocity dispersion relation is presented. We suggest that the age-velocity dispersion relation could reflect the gradual decrease in the turbulent velocity dispersion from which disk stars form, a suggestion originally made by Tinsley & Larson (1978, ApJ, 221, 554) and supported by several more recent disk evolution calculations. A test to distinguish between the two types of models using high-redshift galaxies is proposed.
Monthly Notices of the Royal Astronomical Society, Mar 19, 2019
We have conducted a search for bright repeating Fast Radio Bursts in our nearby Universe with the... more We have conducted a search for bright repeating Fast Radio Bursts in our nearby Universe with the Australian Square Kilometer Array Pathfinder (ASKAP) in singledish mode. We used eight ASKAP 12-m dishes, each equipped with a Chequerboard Phased Array Feed forming 36 beams on the sky, to survey ∼30,000 deg 2 of the southern sky (-90 • < δ < +30 • ) in 158 antenna days. The fluence limit of the survey is 22 Jy ms. We report the detection of FRB 180515 in our survey. We found no repeating FRBs in a total mean observation of 3 hrs per pointing divided into one hour intervals, which were separated in time ranging between a day to a month. Using our non-detection, we exclude the presence of a repeating FRB similar to FRB 121102 closer than z = 0.004 in the survey area -a volume of at least 9.4 × 10 4 Mpc 3 -at 95% confidence.
Monthly Notices of the Royal Astronomical Society, Jul 4, 2019
We detail a new fast radio burst (FRB) survey with the Molonglo Radio Telescope, in which six FRB... more We detail a new fast radio burst (FRB) survey with the Molonglo Radio Telescope, in which six FRBs were detected between 2017 June and 2018 December. 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 μs) studies of the bursts. Five of the FRBs show temporal broadening consistent with interstellar and/or intergalactic scattering, with scattering time-scales ranging from 0.16 to 29.1 ms. One burst, FRB181017, shows remarkable temporal structure, with three 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 d -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 ∝ ν α ). Our results suggest that FRB spectra may turn over below 1 GHz. Optical, radio, and X-ray follow-up has been made for most of the reported bursts, with no associated transients found. No repeat bursts were found in the survey.
Chemical tagging of stellar debris from disrupted open clusters and associations underpins the sc... more Chemical tagging of stellar debris from disrupted open clusters and associations underpins the science cases for next-generation multi-object spectroscopic surveys. As part of the Galactic Archaeology project TraCD (Tracking Cluster Debris), a preliminary attempt at reconstructing the birth clouds of now phase-mixed thin disk debris is undertaken using a parametric minimum spanning tree (MST) approach. Empirically-motivated chemical abundance pattern uncertainties (for a 10-dimensional chemistry-space) are applied to NBODY6-realised stellar associations dissolved into a background sea of field stars, all evolving in a Milky Way potential. We demonstrate that significant population reconstruction degeneracies appear when the abundance uncertainties approach ∼0.1 dex and the parameterised MST approach is employed; more sophisticated methodologies will be required to ameliorate these degeneracies.
A chromospheric age distribution of 552 latetype dwarfs is transformed into a star formation hist... more A chromospheric age distribution of 552 latetype dwarfs is transformed into a star formation history by the application of scale height corrections, stellar evolutionary corrections and volume corrections. We show that the disk of our Galaxy has experienced enhanced episodes of star formation at 0-1 Gyr, 2-5 Gyr and 7-9 Gyr ago, although the reality of the latter burst is still uncertain. The star sample birthsites are distributed over a very large range of distances because of orbital diffusion, and so give an estimate of the global star formation rate. These results are compared with the metal-enrichment rate, given by the age-metallicity relation, with the expected epochs of close encounters between our Galaxy and the Magellanic Clouds, and with previous determinations of the star formation history. Simulations are used to examine the age-dependent smearing of the star formation history due to age uncertainties, and the broadening of the recovered features, as well as to measure the probability level that the history derived to be produced by statistical fluctuations of a constant star formation history. We show, with a significance level greater than 98%, that the Milky Way have not had a constant star formation history.
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Papers by Chris Flynn