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A. Melatos

Publications and source records attributed to A. Melatos.

At least 19 recordsLinked to original sources
arXiv2026

The G347.3-0.5 outlier from O3: a follow-up case study for continuous gravitational-wave candidates

We report a multi-pipeline follow-up study of the continuous gravitational-wave (CW) outlier identified by the Ming et al. Einstein@Home directed search for the young supernova remnant G347.3-0.5 (containing the central compact object RXJ1713.7-3946). The outlier was initially identified in LIGO data from the O3a observing run and followed up with the addition of O3b and O4a data by the Einstein@Home pipeline, with O4a producing weaker evidence. Here, we show extended data quality checks and results from three pipelines that have independently investigated this outlier, including the newly released O4b data set. We can robustly recover the outlier in O3, while we find no evidence for a related standard CW signal in O4a (consistent with Ming et al.) and O4b data. This work serves as a useful test case for future multi-pipeline follow-ups of CW candidates that aim to investigate their astrophysical or noise nature, and it is therefore an important step in preparation for the first CW detection.

gr-qc↗
arXiv2025

First results from the UTMOST-NS pulsar timing programme

The UTMOST-NS pulsar timing programme operated at the Molonglo Observatory Synthesis Telescope from April 2021 to June 2023, observing 173 pulsars with an average cadence of 50 pulsars per day. An overview of the programme is presented, detailing the hardware, software, and observing strategy. Pulsar timing results are discussed, focusing on timing noise and glitches. It is shown that the scaling of residuals due to timing noise with pulsar parameters and observing timespan is consistent with earlier studies, and that the recovered timing noise parameters remain consistent as the observing timespan is increased. Second frequency derivatives are investigated, and it is shown that the uncertainty on $\ddotν$ is sensitive to the frequency cutoff in the timing noise model, varying by three-fold approximately depending on whether Fourier modes with frequency lower than the reciprocal of the observing timespan are included. We measure 39 non-zero values of $\ddotν$ when considering both models with and without low-frequency modes. An analytic scaling relating anomalous braking indices to timing noise amplitude is also validated. Glitches in the sample are discussed, including three detected by an ``online'' glitch detection pipeline using a hidden Markov model (HMM). In total 17 glitches are discussed, one of which, in PSR J1902+0615, has not been reported elsewhere. An ``offline'' glitch search pipeline using the HMM framework is used to search for previously undetected glitches. Systematic upper limits are set on the size of undetected glitches. The mean upper limit is $Δν^{90\%}/ν= 6.3 \times 10^{-9}$ at 90\% confidence.

astro-ph.HE↗
arXiv2025

Flux tube clustering from magnetic coupling of adjacent type-I and -II superconductors in a neutron star: persistent gravitational radiation

Adjacent type-I and -II proton superconductors in a rotation-powered pulsar are predicted to exist in a metastable state containing macroscopic and quantized flux tubes, respectively. Previous studies show that the type-I and -II regions are coupled magnetically, when macroscopic flux tubes divide dendritically into quantized flux tubes near the type-I-II interface, through a process known as flux branching. The studies assume that the normal-superconducting boundary is sharp, and the quantized flux tubes do not repel mutually. Here the sharp-interface approximation is refined by accounting for magnetic repulsion. It is found that flux tubes in the same flux tree cluster with a minimum-energy separation two to seven times less than that of isolated flux tubes. Neutron vortices pin and cluster about flux trees. We find that the maximum characteristic wave strain $h_0$ of the current quadrupole gravitational radiation emitted by a rectilinear array of clustered vortices exceeds by $(1+N_{\rm v,t})^{1/2}$ the strain $h_0 \sim 10^{-32}(f/30 {\rm Hz})^{5/2} (D/1 {\rm kpc})^{-1}$ emitted by uniformly distributed vortices, where $N_{\rm v,t}$ is the mean number of pinned vortices per flux tree, $f$ is the star's spin frequency, and $D$ is the star's distance from Earth. The factor $(1 + N_{\rm v,t})^{1/2}$ brings $h_0$ close to the sensitivity limit of the current generation of interferometric gravitational wave detectors under certain circumstances, specifically when flux branching forms relatively few (and hence relatively large) flux trees.

astro-ph.HE↗
arXiv2025

Search for continuous gravitational waves from neutron stars in five globular clusters with a phase-tracking hidden Markov model in the third LIGO observing run

A search is performed for continuous gravitational waves emitted by unknown neutron stars in five nearby globular clusters using data from the third Laser Interferometer Gravitational-Wave Observatory (LIGO) observing run, over the frequency range $100$--$800\,\mathrm{Hz}$. The search uses a hidden Markov model to track both the frequency and phase of the continuous wave signal from one coherent segment to the next. It represents the first time that a phase-tracking hidden Markov model has been used in a LIGO search. After applying vetoes to reject candidates consistent with non-Gaussian artifacts, no significant candidates are detected. Estimates of the strain sensitivity at 95\% confidence $h_{0,\mathrm{eff}}^{95\%}$ and corresponding neutron star ellipticity $ε^{95\%}$ are presented. The best strain sensitivity, $h_{0,\mathrm{eff}}^{95\%} = 2.7 \times 10^{-26}$ at $211\,\mathrm{Hz}$, is achieved for the cluster NGC6544.

gr-qc↗
arXiv2024

Newtonian restricted three-body gravitational problem with positive and negative point masses

The Newtonian restricted three-body problem involving a positive primary point mass, $m_+$, and a negative secondary point mass, $m_-$, in a circular orbit, and a positive or negative tertiary point mass, $m_3$, with $m_+ > |m_-| \gg |m_3|$, is solved. Five Lagrange points are found for $m_3$, three of which are coplanar with $m_+$ and $m_-$, and two of which are not, a subtle consequence of the gravitational repulsion from $m_-$. All Lagrange points are linearly unstable, except for one point in the regime $m_+ \gtrsim 8.4 |m_-|$, which is linearly stable and collinear with $m_+$ and $m_-$.

physics.class-ph↗
arXiv2024

Magnetic coupling through flux branching of adjacent type-I and -II superconductors in a neutron star

The inner and outer cores of neutron stars are believed to contain type-I and -II proton superconductors, respectively. The type-I superconductor exists in an intermediate state, comprising macroscopic flux-free and flux-containing regions, while the type-II superconductor is flux-free, except for microscopic, quantized flux tubes. Here, we show that the inner and outer cores are coupled magnetically, when the macroscopic flux tubes subdivide dendritically into quantized flux tubes, a phenomenon called flux branching. An important implication is that up to $\sim 10^{12} (r_1/10^6 \, {\rm cm}) \, {\rm erg}$ of energy are required to separate a quantized flux tube from its progenitor macroscopic flux tube, where $r_1$ is the length of the macroscopic flux tube. Approximating the normal-superconducting boundary as sharp, we calculate the magnetic coupling energy between a quantized and macroscopic flux tube due to flux branching as a function of, $f_1$, the radius of the type-I inner core divided by the radius of the type-II outer core. Strong coupling delays magnetic field decay in the type-II superconductor. For an idealised inner core containing only a type-I proton superconductor and poloidal flux, and in the absence of ambipolar diffusion and diamagnetic screening, the low magnetic moments ($\lesssim 10^{27} \, {\rm G \, cm^3}$) of recycled pulsars imply $f_1 \lesssim 10^{-1.5}$.

astro-ph.HE↗
arXiv2024

Measuring the magnetic dipole moment and magnetospheric fluctuations of SXP 18.3 with a Kalman filter

The magnetic dipole moment $μ$ of an accretion-powered pulsar in magnetocentrifugal equilibrium cannot be inferred uniquely from time-averaged pulse period and aperiodic X-ray flux data, because the radiative efficiency $η_0$ of the accretion is unknown, as are the mass, radius, and distance of the star. The degeneracy associated with the radiative efficiency is circumvented, if fluctuations of the pulse period and aperiodic X-ray flux are tracked with a Kalman filter, whereupon $μ$ can be measured uniquely up to the uncertainties in the mass, radius, and distance. Here the Kalman filter analysis is demonstrated successfully in practice for the first time on Rossi X-ray Timing Explorer observations of the X-ray transient SXP 18.3 in the Small Magellanic Cloud, which is monitored regularly. The analysis yields $μ= 8.0^{+1.3}_{-1.2} \, \times \, 10^{30} \, {\rm G \, cm^3}$ and $η_0 = 0.04^{+0.02}_{-0.01}$, compared to $μ= 5.0^{+1.0}_{-1.0} \times 10^{30} \, {\rm G \, cm^3}$ as inferred traditionally from time-averaged data assuming $η_0=1$. The analysis also yields time-resolved estimates of two hidden state variables, the mass accretion rate and the Maxwell stress at the disk-magnetosphere boundary. The success of the demonstration confirms that the Kalman filter analysis can be applied in the future to study the magnetic moments and disk-magnetosphere physics of accretion-powered pulsar populations in the Small Magellanic Cloud and elsewhere.

astro-ph.HE↗
arXiv2023

Stability of interlinked neutron vortex and proton flux-tube arrays in a neutron star -- III. Proton feedback

The coupled, time-dependent Gross-Pitaevskii and Ginzburg-Landau equations are solved simultaneously in three dimensions to investigate the equilibrium state and far-from-equilibrium, spin-down dynamics of an interpenetrating neutron superfluid and proton type-II superconductor, as an idealized description of the outer core of a neutron star. The simulations generalize previous calculations without the time-dependent Ginzburg-Landau equation, where proton feedback is absent. If the angle $θ$ between the rotation and magnetic axes does not equal zero, the equilibrium state consists of geometrically complicated neutron vortex and proton flux-tube tangles, as the topological defects pin to one another locally but align with different axes globally. During spin-down, new types of motion are observed. For $θ= 0$, entire vortices pair rectilinearly with flux tubes and move together while pinned. For $θ\neq 0$, vortex segments pair with segments from one or more flux tubes, and the paired segments move together while pinned. The degree to which proton feedback impedes the deceleration of the crust is evaluated as a function of $θ$ and the pinning strength, $η$. Key geometric properties of vortex-flux-tube tangles, such as filament length, mean curvature, and polarity are analysed. It is found that proton feedback smooths the deceleration of the crust, reduces the rotational glitch sizes, and stabilizes the vortex tangle dynamics. The dimensionless control parameters in the simulations are mutually ordered to match what is expected in a real neutron star, but their central values and dynamics ranges differ from reality by many orders of magnitude due to computational limitations.

astro-ph.HE↗
arXiv2023

A new small glitch in Vela discovered with a hidden Markov model

A striking feature of the Vela pulsar (PSR J0835$-$4510) is that it undergoes sudden increases in its spin frequency, known as glitches, with a fractional amplitude on the order of $10^{-6}$ approximately every 900 days. Glitches of smaller magnitudes are also known to occur in Vela. Their distribution in both time and amplitude is less well constrained but equally important for understanding the physical process underpinning these events. In order to better understand these small glitches in Vela, an analysis of high-cadence observations from the Mount Pleasant Observatory is presented. A hidden Markov model (HMM) is used to search for small, previously undetected glitches across 24 years of observations covering MJD 44929 to MJD 53647. One previously unknown glitch is detected around MJD 48636 (Jan 15 1992), with fractional frequency jump $Δf/f = (8.19 \pm 0.04) \times 10^{-10}$ and frequency derivative jump $Δ\dot{f}/\dot{f} = (2.98 \pm 0.01) \times 10^{-4}$. Two previously reported small glitches are also confidently re-detected, and independent estimates of their parameters are reported. Excluding these events, 90% confidence frequentist upper limits on the sizes of missed glitches are also set, with a median upper limit of $Δf^{90\%}/f = 1.35 \times 10^{-9}$. Upper limits of this kind are enabled by the semi-automated and computationally efficient nature of the HMM, and are crucial to informing studies which are sensitive to the lower end of the glitch size distribution.

astro-ph.HE↗
arXiv2023

Measuring the vortex-nucleus pinning force from pulsar glitch rates

Superfluid vortex avalanches are one plausible cause of pulsar glitch activity. If they occur according to a state-dependent Poisson process, the measured long-term glitch rate is determined by the spin-down rate of the stellar crust, $\dotΩ_{\rm c}$, and two phenomenological parameters quantifying the vortex-nucleus pinning force: a crust-superfluid angular velocity lag threshold, $X_{\rm cr}$, and a reference unpinning rate, $λ_0$. A Bayesian analysis of 541 glitches in 177 pulsars, with $N_{\rm g} \geq 1$ events per pulsar, yields $X_{\rm cr} = 0.15^{+0.09}_{-0.04} \, {\rm rad \, s^{-1}}$, $λ_{\rm ref} = 7.6^{+3.7}_{-2.6} \times 10^{-8} \, {\rm s^{-1}}$, and $a = -0.27^{+0.04}_{-0.03}$ assuming the phenomenological rate law $λ_0 = λ_{\rm ref} [τ/(1 \, {\rm yr})]^a$, where $τ$ denotes the characteristic spin-down age. The results are broadly similar, whether one includes or excludes quasiperiodic glitch activity, giant glitches, or pulsars with $N_{\rm g}=0$, up to uncertainties about the completeness of the sample and the total observation time per pulsar. The $X_{\rm cr}$ and $λ_0$ estimates are consistent with first-principles calculations based on nuclear theory, e.g. in the semiclassical local density approximation.

astro-ph.HE↗
arXiv2022

Tracking hidden magnetospheric fluctuations in accretion-powered pulsars with a Kalman filter

X-ray flux and pulse period fluctuations in an accretion-powered pulsar convey important information about the disk-magnetosphere interaction. It is shown that simultaneous flux and period measurements can be analysed with a Kalman filter based on the standard magnetocentrifugal accretion torque to generate accurate time-dependent estimates of three hidden state variables, which fluctuate stochastically and cannot be measured directly: the mass accretion rate, the Maxwell stress at the disk-magnetosphere boundary, and the radiative efficiency of accretion onto the stellar surface. The inferred fluctuation statistics carry implications for the physics of hydromagnetic instabilities at the disk-magnetosphere boundary and searches for continuous gravitational radiation from low-mass X-ray binaries.

astro-ph.HE↗
arXiv2022

Estimating and detecting random processes on the unit circle

The problem of detecting a sinusoidal signal with randomly varying frequency has a long history. It is one of the core problems in signal processing, arising in many applications including, for example, underwater acoustic frequency line tracking, demodulation of FM radio communications, laser phase drift in optical communications and, recently, continuous gravitational wave astronomy. In this paper we describe a Markov Chain Monte Carlo based procedure to compute a specific detection posterior density. We demonstrate via simulation that our approach results in an up to $25$ percent higher detection rate than Hidden Markov Model based solutions, which are generally considered to be the leading techniques for these problems.

eess.SP↗
arXiv2022

Bayesian Detection of a Sinusoidal Signal with Randomly Varying Frequency

The problem of detecting a sinusoidal signal with randomly varying frequency has a long history. It is one of the core problems in signal processing, arising in many applications including, for example, underwater acoustic frequency line tracking, demodulation of FM radio communications, laser phase drift in optical communications and, recently, continuous gravitational wave astronomy. In this paper we describe a Markov Chain Monte Carlo based procedure to compute a specific detection posterior density. We demonstrate via simulation that our approach results in an up to $25$ percent higher detection rate than Hidden Markov Model based solutions, which are generally considered to be the leading techniques for these problems.

eess.SP↗
arXiv2022

Testing for a Random Walk Structure in the Frequency Evolution of a Tone in Noise

Inference and hypothesis testing are typically constructed on the basis that a specific model holds for the data. To determine the veracity of conclusions drawn from such data analyses, one must be able to identify the presence of the assumed structure within the data. In this paper, a model verification test is developed for the presence of a random walk-like structure in the variations in the frequency of complex-valued sinusoidal signals measured in additive Gaussian noise. This test evaluates the joint inference of the random walk hypothesis tests found in economics literature that seek random walk behaviours in time series data, with an additional test to account for how the random walk behaves in frequency space.

eess.SP↗
arXiv2022

Mountain formation by repeated, inhomogeneous crustal failure in a neutron star

The elastic crust of a neutron star fractures repeatedly as it spins down electromagnetically. An idealised, macroscopic model of inhomogeneous crustal failure is presented based on a cellular automaton with nearest-neighbour tectonic interactions involving strain redistribution and thermal dissipation. Predictions are made of the size and waiting-time distributions of failure events, as well as the rate of failure as the star spins down. The last failure event typically occurs when the star spins down to approximately 1% of its birth frequency with implications for rotational glitch activity. Neutron stars are commonly suggested as sources of continuous gravitational waves. The output of the automaton is converted into predictions of the star's mass ellipticity and gravitational wave strain as functions of its age, with implications for future observations with instruments such as the Laser Interferometer Gravitational Wave Observatory (LIGO), the Virgo interferometer, or the Kamioka Gravitational Wave Detector (KAGRA).

astro-ph.HE↗
arXiv2022

Thermal luminosity degeneracy of magnetized neutron stars with and without hyperon cores

The dissipation of intense crustal electric currents produces high Joule heating rates in cooling neutron stars. Here it is shown that Joule heating can counterbalance fast cooling, making it difficult to infer the presence of hyperons (which accelerate cooling) from measurements of the observed thermal luminosity $L_γ$. Models with and without hyperon cores match $L_γ$ of young magnetars (with poloidal-dipolar field $B_{\textrm{dip}} \gtrsim 10^{14}$ G at the polar surface and $L_γ \gtrsim 10^{34}$ erg s$^{-1}$ at $t \lesssim 10^5$ yr) as well as mature, moderately magnetized stars (with $B_{\textrm{dip}} \lesssim 10^{14}$ G and $10^{31} \ \textrm{erg s}^{-1} \lesssim L_γ \lesssim 10^{32}$ erg s$^{-1}$ at $t \gtrsim 10^5$ yr). In magnetars, the crustal temperature is almost independent of hyperon direct Urca cooling in the core, regardless of whether the latter is suppressed or not by hyperon superfluidity. The thermal luminosities of light magnetars without hyperons and heavy magnetars with hyperons have $L_γ$ in the same range and are almost indistinguishable. Likewise, $L_γ$ data of neutron stars with $B_{\textrm{dip}} \lesssim 10^{14}$ G but with strong internal fields are not suitable to extract information about the equation of state as long as hyperons are superfluid, with maximum amplitude of the energy gaps of the order $\approx 1$ MeV.

astro-ph.HE↗
arXiv2022

Anti-glitches in accreting pulsars from superfluid vortex avalanches

Three sudden spin-down events, termed `anti-glitches', were recently discovered in the accreting pulsar NGC 300 ULX-1 by the \textit{Neutron Star Interior Composition Explorer} (NICER) mission. Unlike previous anti-glitches detected in decelerating magnetars, these are the first anti-glitches recorded in an accelerating pulsar. One standard theory is that pulsar spin-up glitches are caused by avalanches of collectively unpinning vortices that transfer angular momentum from the superfluid interior to the crust of a neutron star. Here we test whether vortex avalanches are also consistent with the anti-glitches in NGC 300 ULX-1, with the angular momentum transfer reversed. We perform $N$-body simulations of up to $5 \times 10^{3}$ pinned vortices in two dimensions in secularly accelerating and decelerating containers. Vortex avalanches routinely occur in both scenarios, propagating inwards and outwards respectively. The implications for observables, such as size and waiting time statistics, are considered briefly.

astro-ph.HE↗
arXiv2022

Systematic upper limits on the size of missing pulsar glitches in the first UTMOST open data release

A systematic, semi-automated search for pulsar glitches in the first UTMOST public data release is presented. The search is carried out using a hidden Markov model which incorporates both glitches and timing noise into the model of the assumed phase evolution of the pulsar. Glitches are detected through Bayesian model selection between models with and without glitches present with minimal human intervention. Nine glitches are detected among seven objects, all of which have been previously reported. No new glitches were detected. Injection studies are used to place 90\% frequentist upper limits on the size of undetected glitches in each of the 282 objects searched. The mean upper limit obtained is $Δf^{90\%}/f = 1.9 \times 10^{-8}$, with a range of $4.1 \times 10^{-11} \leq Δf^{90\%}/f \leq 2.7 \times 10^{-7}$, assuming step events with no post-glitch recoveries. It is demonstrated that including glitch recovery has a mild effect, in most cases increasing the upper limit by a factor of $\lesssim 5$ conservatively assuming complete recovery on a timescale of $100\,\mathrm{d}$.

astro-ph.HE↗