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Gravitational waves are expected to be radiated by supermassive black hole binaries formed during galaxy mergers. A stochastic superposition of gravitational waves from all such binary systems would modulate the arrival times of pulses from radio pulsars. Using observations of millisecond pulsars obtained with the Parkes radio telescope, we constrained the characteristic amplitude of this background, Ac,yr, to be <1.0 × 10–15 with 95% confidence. This limit excludes predicted ranges for Ac,yr from current models with 91 to 99.7% probability. We conclude that binary evolution is either stalled or dramatically accelerated by galactic-center environments and that higher-cadence and shorter-wavelength observations would be more sensitive to gravitational waves.

Abstract Casestudy Gastroblastoma is a rare tumor with biphasic components showing epithelial and mesenchymal differentiation. To date, <15 cases have been reported, with molecular confirmation of the recently identified MALAT1-GLI1 translocations only in a subset. Aspiration cytologic and small biopsy findings have not yet been reported. We present a case of gastroblastoma, arising in a 22-year-old female. Results A CT scan was performed, showing a 7 cm heterogeneous mass in the distal stomach and pancreas, clinically suspected to represent at gastrointestinal stromal tumor (GIST). She underwent two preoperative samples, including endoscopic ultrasound guided-fine needle aspiration and core biopsy, followed by a distal gastrectomy. Diff- Quik stained touch preparations performed on the core needle biopsy during rapid on-site evaluation showed a hypercellular neoplasm composed of large, three-dimensional aggregates of neoplastic cells in a background of numerous isolated single cells and bare nuclei. The neoplastic cells were bland with spindled to epithelioid nuclei, occasional nuclear grooves, and small nucleoli. Immunostains were only helpful in excluding GIST (CD117 and DOG1 negative). Distal gastrectomy showed a nodular/plexiform tumor with variably epithelioid to spindle cell cytology and solid to focally myxoid/microcystic architecture. Pancytokeratins CAM5.2 (patchy) and AE1/AE3 (very focal) were positive, with negative S100, SMA, Desmin, Melan-A, Inhibin, Calretinin, and Synaptophysin. Based on the age, location, histology and immunophenotype, gastroblastoma was suspected, and multiplex NGS-based fusion sequencing identified a MALAT1-GLI1 fusion. Staging studies were negative for metastasis at presentation. Conclusion Based on this experience, we recommend consideration of gastroblastoma for a gastric tumor in a young patient, especially if encountering a cytologic sample showing non-pleomorphic epithelioid and spindle cell cytology. Lack of expression of GIST, smooth muscle, neuroendocrine, and neural sheath-associated markers should particularly raise consideration of this rare neoplasm. While in this case molecular studies clinched the diagnosis upon resection, increasingly used GLI1 immunostain may be of use prospectively for diagnosis of limited samples.

We present a flux density study of 89 millisecond pulsars (MSPs) regularly monitored as part of the MeerKAT Pulsar Timing Array (MPTA) using the L-Band receiver with an approximately two week cadence between 2019-2022. For each pulsar, we have determined the mean flux densities at each epoch in eight \\(\\sim\\)97 MHz sub-bands ranging from 944 to 1625 MHz. From these we have derived their modulation indices, their average and peak-to-median flux densities in each sub-band, as well as their mean spectral indices across the entire frequency range. We find that the vast majority of the MSPs have spectra that are well described by a simple power law, with a mean spectral index of -1.86(6). Using the temporal variation of the flux densities we measured the structure functions and determined the refractive scintillation timescale for seven. The structure functions provide strong evidence that the intrinsic radio luminosities of MSPs are stable. As a population, the average modulation index at 20 cm wavelengths peaks near unity at dispersion measures (DMs) of \\(\\sim\\)20 pc cm\\(^{-3}\\) and by a DM of 100 pc cm\\(^{-3}\\) are closer to 0.2, due to refractive scintillation. We find that timing arrays can improve their observing efficiency by reacting to scintillation maxima, and that 20 cm FRB surveys should prioritise highly scintillating mid-latitude regions of the Galactic sky where they will find \\(\\sim\\)30% more events and bursts at greater distances.

The double pulsar system, PSR J0737\\(-\\)3039A/B, consists of two neutron stars bound together in a highly relativistic orbit that is viewed nearly edge-on from the Earth. This alignment results in brief radio eclipses of the fast-rotating pulsar A when it passes behind the toroidal magnetosphere of the slow-rotating pulsar B. The morphology of these eclipses is strongly dependent on the geometric orientation and rotation phase of pulsar B, and their time-evolution can be used to constrain the geodetic precession rate of the pulsar. We demonstrate a Bayesian inference framework for modelling eclipse light-curves obtained with MeerKAT between 2019-2023. Using a hierarchical inference approach, we obtained a precession rate of \\(\\Omega_{\\rm SO}^{\\rm B} = {5.16^{\\circ}}^{+0.32^{\\circ}}_{-0.34^{\\circ}}\\) yr\\(^{-1}\\) for pulsar B, consistent with predictions from General Relativity to a relative uncertainty of 6.5%. This updated measurement provides a 6.1% test of relativistic spin-orbit coupling in the strong-field regime. We show that a simultaneous fit to all of our observed eclipses can in principle return a \\(\\sim\\)1.5% test of spin-orbit coupling. However, systematic effects introduced by the current geometric orientation of pulsar B along with inconsistencies between the observed and predicted eclipse light curves result in difficult to quantify uncertainties. Assuming the validity of General Relativity, we definitively show that the spin-axis of pulsar B is misaligned from the total angular momentum vector by \\(40.6^{\\circ} \\pm 0.1^{\\circ}\\) and that the orbit of the system is inclined by approximately \\(90.5^{\\circ}\\) from the direction of our line of sight. Our measured geometry for pulsar B suggests the largely empty emission cone contains an elongated horseshoe shaped beam centered on the magnetic axis, and that it may not be re-detected as a radio pulsar until early-2035.

PSR J1933\\(-\\)6211 is a 3.5-ms pulsar in a 12.8-d orbit with a white dwarf (WD). Its high proper motion and low dispersion measure result in such significant interstellar scintillation that high signal-to-noise detections require long observing durations or fortuitous timing. We turn to the sensitive MeerKAT telescope and, combined with historic Parkes data, leverage PSR J1933\\(-\\)6211's kinematic and relativistic effects to constrain its 3D orbital geometry and the component masses. We obtain precise proper motion and parallax estimates, and measure their effects as secular changes in the Keplerian orbital parameters: a variation in orbital period of \\(7(1) \\times 10^{-13}\\) s s\\(^{-1}\\) and a change in projected semi-major axis of \\(1.60(5) \\times 10^{-14}\\) s s\\(^{-1}\\). A self-consistent analysis of all kinematic and relativistic effects yields a distance of \\(1.6^{+0.2}_{-0.3}\\) kpc, an orbital inclination, \\(i = 55(1)\\) deg and a longitude of the ascending node, \\(\\Omega = 255^{+8}_{-14}\\) deg. The probability densities for \\(\\Omega\\) and \\(i\\) and their symmetric counterparts, (\\(180-i\\), \\(360-\\Omega\\)), are seen to depend on the fiducial orbit used to measure the time of periastron passage. We investigate this unexpected dependence and rule out software-related causes using simulations. Nevertheless, we constrain the pulsar and WD masses to \\(1.4^{+0.3}_{-0.2}\\) M\\(_\\odot\\) and \\(0.43(5)\\) M\\(_\\odot\\) respectively. These strongly disfavour a helium-dominated WD. The orbital similarities between PSRs J1933\\(-\\)6211 and J1614\\(-\\)2230 suggest they underwent Case A Roche lobe overflow, an extended evolution while the companion star is still on the Main Sequence. However, with a mass of \\(\\sim 1.4\\) M\\(_\\odot\\), PSR J1933\\(-\\)6211 has not accreted significant matter. This highlights the low accretion efficiency of the spin-up process and suggests that observed neutron star masses are mostly a result of supernova physics.

We present the detection of 107 pulsars with interstellar scintillation arcs at 856--1712\\,MHz, observed with the MeerKAT Thousand Pulsar Array Programme. Scintillation arcs appear to be ubiquitous in clean, high S/N observations, their detection mainly limited by short observing durations and coarse frequency channel resolution. This led the survey to be sensitive to nearby, lightly scattered pulsars with high effective velocity -- from a large proper motion, a screen nearby the pulsar, or a screen near the Earth. We measure the arc curvatures in all of our sources, which can be used to give an estimate of screen distances in pulsars with known proper motion, or an estimate of the proper motion. The short scintillation timescale in J1731\\(-\\)4744 implies a scattering screen within 12\\,pc of the source, strongly suggesting the association between this pulsar and the supernova remnant RCW 114. We measure multiple parabolic arcs of 5 pulsars, all of which are weakly scintillating with high proper motion. Additionally, several sources show hints of inverted arclets suggesting scattering from anisotropic screens. Building on this work, further targeted MeerKAT observations of many of these pulsars will improve understanding of our local scattering environment and the origins of scintillation; annual scintillation curves would lead to robust screen distance measurements, and the evolution of arclets in time and frequency can constrain models of scintillation.

MeerTime is a five-year Large Survey Project to time pulsars with MeerKAT, the 64-dish South African precursor to the Square Kilometre Array. The science goals for the programme include timing millisecond pulsars (MSPs) to high precision (< 1 \\(\\mu\\)s) to study the Galactic MSP population and to contribute to global efforts to detect nanohertz gravitational waves with the International Pulsar Timing Array (IPTA). In order to plan for the remainder of the programme and to use the allocated time most efficiently, we have conducted an initial census with the MeerKAT \"L-band\" receiver of 189 MSPs visible to MeerKAT and here present their dispersion measures, polarization profiles, polarization fractions, rotation measures, flux density measurements, spectral indices, and timing potential. As all of these observations are taken with the same instrument (which uses coherent dedispersion, interferometric polarization calibration techniques, and a uniform flux scale), they present an excellent resource for population studies. We used wideband pulse portraits as timing standards for each MSP and demonstrated that the MeerTime Pulsar Timing Array (MPTA) can already contribute significantly to the IPTA as it currently achieves better than 1 \\(\\mu\\)s timing accuracy on 89 MSPs (observed with fortnightly cadence). By the conclusion of the initial five-year MeerTime programme in July 2024, the MPTA will be extremely significant in global efforts to detect the gravitational wave background with a contribution to the detection statistic comparable to other long-standing timing programmes.

We present pulse width measurements for a sample of radio pulsars observed with the MeerKAT telescope as part of the Thousand-Pulsar-Array (TPA) programme in the MeerTime project. For a centre frequency of 1284 MHz, we obtain 762 \\(W_{10}\\) measurements across the total bandwidth of 775 MHz, where \\(W_{10}\\) is the width at the 10% level of the pulse peak. We also measure about 400 \\(W_{10}\\) values in each of the four or eight frequency sub-bands. Assuming, the width is a function of the rotation period P, this relationship can be described with a power law with power law index \\(\\mu=-0.29\\pm 0.03\\). However, using orthogonal distance regression, we determine a steeper power law with \\(\\mu=-0.63\\pm 0.06\\). A density plot of the period-width data reveals such a fit to align well with the contours of highest density. Building on a previous population synthesis model, we obtain population-based estimates of the obliquity of the magnetic axis with respect to the rotation axis for our pulsars. Investigating the width changes over frequency, we unambiguously identify a group of pulsars that have width broadening at higher frequencies. The measured width changes show a monotonic behaviour with frequency for the whole TPA pulsar population, whether the pulses are becoming narrower or broader with increasing frequency. We exclude a sensitivity bias, scattering and noticeable differences in the pulse component numbers as explanations for these width changes, and attempt an explanation using a qualitative model of five contributing Gaussian pulse components with flux density spectra that depend on their rotational phase.

The main goal of pulsar timing array experiments is to detect correlated signals such as nanohertz-frequency gravitational waves. Pulsar timing data collected in dense monitoring campaigns can also be used to study the stars themselves, their binary companions, and the intervening ionised interstellar medium. Timing observations are extraordinarily sensitive to changes in path length between the pulsar and the Earth, enabling precise measurements of the pulsar positions, distances and velocities, and the shapes of their orbits. Here we present a timing analysis of 25 pulsars observed as part of the Parkes Pulsar Timing Array (PPTA) project over time spans of up to 24 yr. The data are from the second data release of the PPTA, which we have extended by including legacy data. We make the first detection of Shapiro delay in four Southern pulsars (PSRs J1017\\(-\\)7156, J1125\\(-\\)6014, J1545\\(-\\)4550, and J1732\\(-\\)5049), and of parallax in six pulsars. The prominent Shapiro delay of PSR J1125\\(-\\)6014 implies a neutron star mass of \\(M_p = 1.5 \\pm 0.2 M_\\odot\\) (68% credibility interval). Measurements of both Shapiro delay and relativistic periastron advance in PSR J1600\\(-\\)3053 yield a large but uncertain pulsar mass of \\(M_p = 2.06^{+0.44}_{-0.41}\\) M\\(_\\odot\\) (68% credibility interval). We measure the distance to PSR J1909\\(-\\)3744 to a precision of 10 lyr, indicating that for gravitational wave periods over a decade, the pulsar provides a coherent baseline for pulsar timing array experiments.

PSR J0955\\(-\\)6150 is a member of a class of eccentric MSP+He WD systems (eMSPs), whose binary evolution is poorly understood and believed to be different to that of traditional MSP+He WD systems. Measuring the masses of the stars in this system is important for testing hypotheses for the formation of eMSPs. We have carried out observations of this pulsar with the Parkes and MeerKAT radio telescopes. Our observations reveal a strong frequency evolution of this pulsar's intensity, with a spectral index (\\(\\alpha\\)) of \\(-3.13(2)\\). The sensitivity of MeerKAT has resulted in a \\(>10\\)-fold improvement in the timing precision compared to older Parkes observations. Combined with the 8-year timing baseline, it has allowed precise measurements of a proper motion and three orbital \"post-Keplerian\" parameters: the rate of advance of periastron, \\(\\dot{\\omega} = 0.00152(1) \\, {\\rm deg} \\, yr^{-1}\\) and the orthometric Shapiro delay parameters, \\(h_3 = 0.89(7) \\, \\mu\\)s and \\(\\varsigma = 0.88(2)\\). Assuming general relativity, we obtain \\(M_{p} = 1.71(2) \\, M_{\\odot}\\) for the mass of the pulsar and \\(M_{c} = 0.254(2) \\, M_{\\odot}\\) for the mass of the companion; the orbital inclination is 83.2(4) degrees. We find that the spin axis has a misalignment relative to the orbital angular momentum of \\(> 4.8\\) degrees at 99% CI. While the value of \\(M_{\\rm p}\\) falls within the wide range observed in eMSPs, \\(M_{\\rm c}\\) is significantly smaller than expected, allowing several formation hypotheses being ruled out. \\(M_{\\rm c}\\) is also significantly different from the expected value for an ideal low mass X-ray binary evolution scenario. The putative misalignment between the spin axis of the pulsar and the orbital angular momentum suggests that the unknown process that created the orbital eccentricity of the binary was also capable of changing its orbital orientation, an important evidence for understanding the origin of eMSPs.