arXiv subjects

Duncan Lorimer

Publications and source records attributed to Duncan Lorimer.

At least 19 recordsLinked to original sources

arXiv2025

Inferring Mbh-Mbulge Evolution from the Gravitational Wave Background

We test the impact of an evolving supermassive black hole (SMBH) mass scaling relation (Mbh-Mbulge) on the predictions for the gravitational wave background (GWB). The observed GWB amplitude is 2-3 times higher than predicted by astrophysically informed models which suggests the need to revise the assumptions in those models. We compare a semi-analytic model's ability to reproduce the observed GWB spectrum with a static versus evolving-amplitude Mbh-Mbulge relation. We additionally consider the influence of the choice of galaxy stellar mass function on the modeled GWB spectra. Our models are able to reproduce the GWB amplitude with either a large number density of massive galaxies or a positively evolving Mbh-Mbulge amplitude (i.e., the Mbh / Mbulge ratio was higher in the past). If we assume that the Mbh-Mbulge amplitude does not evolve, our models require a galaxy stellar mass function that implies an undetected population of massive galaxies (Mstellar > 10^11 Msun at z > 1). When the Mbh-Mbulge amplitude is allowed to evolve, we can model the GWB spectrum with all fiducial values and an Mbh-Mbulge amplitude that evolves as alpha(z) = alpha_0 (1 + z)^(1.04 +/- 0.5).

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arXiv2025

Monte Carlo evaluations of gamma-ray and radio pulsar populations

Based on well-grounded Galactic neutron star populations formed from radio pulsar population syntheses of canonical pulsars (CPs) and millisecond pulsars (MSPs), we use the latest Fermi-LAT catalog (4FGL-DR4) to investigate the implications of proposed $\gamma-$ray luminosity models. Using Monte Carlo techniques, we calculate the number of CPs and MSPs that would comprise the sample of pulsar-like unidentified sources (PLUIDs) in 4FGL-DR4. While radio beaming fractions were used to scale the sizes of the populations, when forming the mock 4FGL-DR4 samples, we make the simplifying assumption that all $\gamma-$ray pulsars are beaming towards the Earth. We then explore the observable outcomes of seven different $\gamma-$ray luminosity models. Four of the models provide a good match to the observed number of PLUIDs, while three others significantly over-predict the number of PLUIDs. For these latter models, either the average beaming fraction of $\gamma-$ray pulsars is more like 25--50\%, or a revision in the luminosity scaling is required. Most of the radio detectable MSPs that our models predict as part of the PLUIDs within 4FGL-DR4 are, unsurprisingly, fainter than the currently observed sample and at larger dispersion measures. For CPs, in spite of an excellent match to the observed radio population, none of the $\gamma-$ray models we investigated could replicate the observed sample of 150 $\gamma-$ray CPs. Further work is required to understand this discrepancy. For both MSPs and CPs, we provide encouraging forecasts for targeted radio searches of PLUIDs from 4FGL-DR4 to elucidate the issues raised in this study.

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arXiv2023

The Petabyte Project

Transient radio sources, such as fast radio bursts, intermittent pulsars, and rotating radio transients, can offer a wealth of information regarding extreme emission physics as well as the intervening interstellar and/or intergalactic medium. Vital steps towards understanding these objects include characterizing their source populations and estimating their event rates across observing frequencies. However, previous efforts have been undertaken mostly by individual survey teams at disparate observing frequencies and telescopes, and with non-uniform algorithms for searching and characterization. The Petabyte Project (TPP) aims to address these issues by uniformly reprocessing data from several petabytes of radio transient surveys covering two decades of observing frequency (300 MHz-20 GHz). The TPP will provide robust event rate analyses, in-depth assessment of survey and pipeline completeness, as well as revealing discoveries from archival and ongoing radio surveys. We present an overview of TPP's processing pipeline, scope, and our potential to make new discoveries.

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arXiv2021

Insights into the Milky Way pulsar--black hole population using radio and gravitational wave observations

The detection of two NS--BH mergers by LIGO-Virgo provided the first direct confirmation of the existence of this type of system in the Universe. These detections also imply the existence of pulsar--black hole (PSR--BH) systems. In this analysis, we use the non-detection of any PSR--BH systems in current radio surveys to estimate a 95\% upper limit of $\sim$150 PSR--BH binary systems that are beaming towards the Earth in the Milky Way. This corresponds to a 95\% upper limit of $\mathcal{R}_{\rm LIGO} = 7.6$~yr$^{-1}$ on the merger detection rate for the LIGO-Virgo network scaled to a range distance of 100~Mpc, which is consistent with the rates derived by LIGO-Virgo. In addition, for the first time, we use the merger detection rates estimate by LIGO-Virgo to predict the number of detectable PSR--BH systems in the Milky Way. We find there to be $\left< N_{\rm obs, NSBH, e} \right> = 2^{+5}_{-1}$ and $\left< N_{\rm obs, NSBH, p} \right> = 6^{+7}_{-4}$ detectable PSR--BH systems in the Milky Way corresponding to the event-based and population-based merger detection rates estimated by LIGO-Virgo respectively. We estimate the probability of detecting these PSR--BH systems with current radio pulsar surveys, showing that the Arecibo PALFA survey has the highest probability of detecting a PSR--BH system, while surveys with recently commissioned and planned telescopes are almost guaranteed to detect one of these systems. Finally, we discuss the hurdles in detecting PSR--BH systems and how these can be overcome in the future.

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arXiv2021

FAST Globular Cluster Pulsar Survey: Twenty-Four Pulsars Discovered in Fifteen Globular Clusters

We present the discovery of 24 pulsars in 15 Globular Clusters (GCs) using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). These include the first pulsar discoveries in M2, M10, and M14. Most of the new systems are either confirmed or likely members of binary systems. M53C, NGC6517H and I are the only three pulsars confirmed to be isolated. M14A is a black widow pulsar with an orbital period of 5.5 hours and a minimum companion mass of 0.016 \Ms. M14E is an eclipsing binary pulsar with an orbital period of 20.3 hours. With the other 8 discoveries that have been reported elsewhere, in total 32 GC pulsars have been discovered by FAST so far. In addition, We detected M3A twice. This was enough to determine that it is a black widow pulsar with an orbital period of 3.3 hours and a minimum companion mass of 0.0125 \Ms.

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arXiv2021

The NANOGrav 12.5-Year Data Set: Polarimetry and Faraday Rotation Measures from Observations of Millisecond Pulsars with the Green Bank Telescope

In this work, we present polarization profiles for 23 millisecond pulsars observed at 820 MHz and 1500 MHz with the Green Bank Telescope as part of the NANOGrav pulsar timing array. We calibrate the data using Mueller matrix solutions calculated from observations of PSRs B1929+10 and J1022+1001. We discuss the polarization profiles, which can be used to constrain pulsar emission geometry, and present both the first published radio polarization profiles for nine pulsars and the discovery of very low intensity average profile components ("microcomponents") in four pulsars. Using the Faraday rotation measures, we measure for each pulsar and use it to calculate the Galactic magnetic field parallel to the line of sight for different lines of sight through the interstellar medium. We fit for linear and sinusoidal trends in time in the dispersion measure and Galactic magnetic field and detect magnetic field variations with a period of one year in some pulsars, but overall find that the variations in these parameters are more consistent with a stochastic origin.

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arXiv2020

A Fast Radio Burst discovered in FAST drift scan survey

We report the discovery of a highly dispersed fast radio burst, FRB~181123, from an analysis of $\sim$1500~hr of drift-scan survey data taken using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The pulse has three distinct emission components, which vary with frequency across our 1.0--1.5~GHz observing band. We measure the peak flux density to be $>0.065$~Jy and the corresponding fluence $>0.2$~Jy~ms. Based on the observed dispersion measure of 1812~cm$^{-3}$~pc, we infer a redshift of $\sim 1.9$. From this, we estimate the peak luminosity and isotropic energy to be $\lesssim 2\times10^{43}$~erg~s$^{-1}$ and $\lesssim 2\times10^{40}$~erg, respectively. With only one FRB from the survey detected so far, our constraints on the event rate are limited. We derive a 95\% confidence lower limit for the event rate of 900 FRBs per day for FRBs with fluences $>0.025$~Jy~ms. We performed follow-up observations of the source with FAST for four hours and have not found a repeated burst. We discuss the implications of this discovery for our understanding of the physical mechanisms of FRBs.

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arXiv2020

An Updated Galactic Double Neutron Star Merger Rate Based on Radio Pulsar Populations

Following the procedure described in Pol et al., we update the Galactic double neutron star (DNS) merger rate by including the new, highly eccentric DNS system J0509+3801 (Lynch et al.). This leads to a new Galactic DNS merger rate of $\mathcal{R}_{\rm MW} = 37^{+24}_{-11}$ Myr$^{-1}$, where the errors represent 90% confidence intervals. The corresponding DNS merger detection rate for Advanced LIGO is $\mathcal{R} = 1.9^{+1.2}_{-0.6} \times \left( D_{\rm r} / 100 \ \rm Mpc \right)^3 \rm yr^{-1},$ where $D_{\rm r}$ is the range distance. Using the LIGO O3 range distance of 130 Mpc (Abbott et al.), we predict that LIGO will detect anywhere between three and seven DNS mergers per year of observing at O3 sensitivity.

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arXiv2018

Verifying and Reporting Fast Radio Bursts

Fast Radio Bursts (FRBs) are a class of short-duration transients at radio wavelengths with inferred astrophysical origin. The prototypical FRB is a broadband signal that occurs over the extent of the receiver frequency range, is narrow in time, and is highly dispersed, following a $\nu^{-2}$ relation. However, some FRBs appear band-limited, and show apparent scintillation, complex frequency-dependent structure, or multi-component pulse shapes. While there is sufficient evidence that FRBs are indeed astrophysical, their one-off nature necessitates extra scrutiny when reporting a detection as bona fide and not a false positive. Currently, there is no formal validation framework for FRBs, rather a set of community practices. In this article, we discuss potential sources of false positives, and suggest a framework in which FRB-like events can be evaluated as real or otherwise. We present examples of false-positive events in data from the Arecibo, LOFAR, and Nanshan telescopes, which while FRB-like, are found to be due to instrumental variations, noise, and radio-frequency interference. Differentiating these false-positive detections from astrophysical events requires knowledge and tests beyond thresholded single-pulse detection. We discuss post-detection analyses, verification tests, and datasets which should be provided when reporting an FRB detection.

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arXiv2018

The Pulsar Search Collaboratory: Expanding Nationwide

The Pulsar Search Collaboratory (PSC) engages high school students and teachers in analyzing real data from the Robert C. Byrd Green Bank Telescope for the purpose of discovering exotic stars called pulsars. These cosmic clocks can be used as a galactic-scale detector of gravitational waves, ripples in space-time that have recently been directly detected from the mergers of stellar-mass black holes. Through immersing students in an authentic, positive learning environment to build a sense of belonging and competency, the goal of the PSC is to promote students' long-term interests in science and science careers. PSC students have discovered 7 pulsars since the start of the PSC in 2008. Originally targeted at teachers and students in West Virginia, over time the program has grown to 18 states. In a new effort to scale the PSC nationally, the PSC has developed an integrated online training program with both self-guided lectures and homework and real-time interactions with pulsar astronomers. Now, any high school student can join in the exciting search for pulsars and the discovery of a new type of gravitational waves.

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arXiv2018

The Galactic halo pulsar population

Most population studies of pulsars have hitherto focused on the disc of the Galaxy, the Galactic centre, globular clusters, and nearby galaxies. It is expected that pulsars, by virtue of their natal kicks, are also to be found in the Galactic halo. We investigate the possible population of canonical (i.e. non-recycled) radio pulsars in the halo, estimating the number of such pulsars, and the fraction that is detectable via single pulse and periodicity searches. Additionally, we explore the distributions of flux densities and dispersion measures of this population. We also consider the effects of different velocity models and the evolution of inclination angle and magnetic field on our results. We show that $\sim$33 % of all pulsars beaming towards the Earth are in the halo but the fraction reduces to $\sim$1.5 % if we let the inclination angle and the magnetic field evolve as a falling exponential. Moreover, the fraction that is detectable is significantly limited by the sensitivity of surveys. This population would be most effectively probed by surveys using time-domain periodicity search algorithms. The current non-detections of pulsars in the halo can be explained if we assume that the inclination angle and magnetic field of pulsars evolve with time. We also highlight a possible confusion between bright pulses from halo pulsars and Fast Radio Bursts with low dispersion measures where further follow-up is warranted.

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arXiv2017

Initial Results from the ALFABURST Survey

Here, we present initial results from the ALFABURST radio transient survey, which is currently running in a commensal mode with the ALFA receiver at the Arecibo telescope. We observed for a total of 1400 hours and have detected single pulses from known pulsars but did not detect any FRBs. The non-detection of FRBs is consistent with the current FRB sky rates.

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arXiv2017

VOEvent Standard for Fast Radio Bursts

Fast radio bursts are a new class of transient radio phenomena currently detected as millisecond radio pulses with very high dispersion measures. As new radio surveys begin searching for FRBs a large population is expected to be detected in real-time, triggering a range of multi-wavelength and multi-messenger telescopes to search for repeating bursts and/or associated emission. Here we propose a method for disseminating FRB triggers using Virtual Observatory Events (VOEvents). This format was developed and is used successfully for transient alerts across the electromagnetic spectrum and for multi-messenger signals such as gravitational waves. In this paper we outline a proposed VOEvent standard for FRBs that includes the essential parameters of the event and where these parameters should be specified within the structure of the event. An additional advantage to the use of VOEvents for FRBs is that the events can automatically be ingested into the FRB Catalogue (FRBCAT) enabling real-time updates for public use. We welcome feedback from the community on the proposed standard outlined below and encourage those interested to join the nascent working group forming around this topic.

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arXiv2016

Detecting pulsars in the Galactic centre

Although high-sensitivity surveys have revealed a number of highly dispersed pulsars in the inner Galaxy, none have so far been found in the Galactic centre (GC) region, which we define to be within a projected distance of 1~pc from Sgr~A*. This null result is surprising given that several independent lines of evidence predict a sizable population of neutron stars in the region. Here, we present a detailed analysis of both the canonical and millisecond pulsar populations in the GC and consider free-free absorption and multi-path scattering to be the two main sources of flux density mitigation. We demonstrate that the sensitivity limits of previous surveys are not sufficient to detect GC pulsar population, and investigate the optimum observing frequency for future surveys. Depending on the degree of scattering and free-free absorption in the GC, current surveys constrain the size of the potentially observable population (i.e.~those beaming towards us) to be up to 52 canonical pulsars and 10000 millisecond pulsars. We find that the optimum frequency for future surveys is in the range of 9--13~GHz. We also predict that future deeper surveys with the Square Kilometer array will probe a significant portion of the existing radio pulsar population in the GC.

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arXiv2016

Detecting fast radio bursts at decametric wavelengths

Fast radio bursts (FRBs) are highly dispersed, sporadic radio pulses that are likely extragalactic in nature. Here we investigate the constraints on the source population from surveys carried out at frequencies $<1$~GHz. All but one FRB has so far been discovered in the 1--2~GHz band, but new and emerging instruments look set to become valuable probes of the FRB population at sub-GHz frequencies in the near future. In this paper, we consider the impacts of free-free absorption and multi-path scattering in our analysis via a number of different assumptions about the intervening medium. We consider previous low frequency surveys alongwith an ongoing survey with the University of Technology digital backend for the Molonglo Observatory Synthesis Telescope (UTMOST) as well as future observations with the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Hydrogen Intensity and Real-Time Analysis Experiment (HIRAX). We predict that CHIME and HIRAX will be able to observe $\sim$ 30 or more FRBs per day, even in the most extreme scenarios where free-free absorption and scattering can significantly impact the fluxes below 1~GHz. We also show that UTMOST will detect 1--2 FRBs per month of observations. For CHIME and HIRAX, the detection rates also depend greatly on the assumed FRB distance scale. Some of the models we investigated predict an increase in the FRB flux as a function of redshift at low frequencies. If FRBs are truly cosmological sources, this effect may impact future surveys in this band, particularly if the FRB population traces the cosmic star formation rate.

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arXiv2016

Commissioning of ALFABURST: initial tests and results

Fast Radio Bursts (FRBs) are apparently one-time, relatively bright radio pulses that have been observed in recent years. The origin of FRBs is currently unknown and many instruments are being built to detect more of these bursts to better characterize their physical properties and identify the source population. ALFABURST is one such instrument. ALFABURST takes advantage of the 7-beam Arecibo L-band Feed Array (ALFA) receiver on the 305-m Arecibo Radio Telescope in Puerto Rico, to detect FRBs in real-time at L-band (1.4 GHz). We present the results of recent on-sky tests and observations undertaken during the commissioning phase of the instrument. ALFABURST is now available for commensal observations with other ALFA projects.

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arXiv2016

Ordinary X-rays from Three Extraordinary Millisecond Pulsars: XMM-Newton Observations of PSRs J0337+1715, J0636+5129, and J0645+5158

We present the first X-ray observations of three recently discovered millisecond pulsars (MSPs) with interesting characteristics: PSR J0337+1715, PSR J0636+5129, and PSR J0645+5158. PSR J0337+1715 is a fast-spinning, bright, and so-far unique MSP in a hierarchical triple system with two white dwarf (WD) companions. PSR J0636+5129 is a MSP in a very tight 96-min orbit with a low-mass, 8 $M_J$ companion. PSR J0645+5158 is a nearby, isolated MSP with a very small duty cycle (1-2%), which has led to its inclusion in high-precision pulsar timing programs. Using data from XMM-Newton, we have analyzed X-ray spectroscopy for these three objects, as well as optical/ultraviolet photometry for PSR J0337+1715. The X-ray data for each are largely consistent with expectations for most MSPs with regards to the ratios of thermal and non-thermal emission. We discuss the implications of these data on the pulsar population, and prospects for future observations of these pulsars.

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arXiv2015

ALFABURST: A realtime fast radio burst monitor for the Arecibo telescope

Fast radio bursts (FRBs) constitute an emerging class of fast radio transient whose origin continues to be a mystery. Realizing the importance of increasing coverage of the search parameter space, we have designed, built, and deployed a realtime monitor for FRBs at the 305-m Arecibo radio telescope. Named 'ALFABURST', it is a commensal instrument that is triggered whenever the 1.4 GHz seven-beam Arecibo $L$-Band Feed Array (ALFA) receiver commences operation. The ongoing commensal survey we are conducting using ALFABURST has an instantaneous field of view of 0.02 sq. deg. within the FWHM of the beams, with the realtime software configurable to use up to 300 MHz of bandwidth. We search for FRBs with dispersion measure up to 2560 cm$^{-3}$ pc and pulse widths ranging from 0.128 ms to 16.384 ms. Commissioning observations performed over the past few months have demonstrated the capability of the instrument in detecting single pulses from known pulsars. In this paper, I describe the instrument and the associated survey.

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