Evidence for a low-frequency gravitational-wave background using pulsar timing arrays has generated recent interest into its underlying contributing sources. However, multiple investigations have seen that the significance of the evidence does not change with choice of pulsar modeling techniques but the resulting parameters from the gravitational wave searches do. PSR J1455-3330 is one of the longest-observed pulsars in the array monitored by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) but showed no evidence for long-timescale red noise, either intrinsic or the common signal found among many pulsars in the array. In this work, we argue that NANOGrav's piecewise-constant function used to model variations in radio-frequency-dependent dispersive delay should not be used for this pulsar, and a much simpler physical model of a fixed solar wind density plus a linear trend in dispersion measure is preferred. When the original model is replaced, (i) the pulsar's timing parallax signal changes from an upper limit to a significant detection, (ii) red noise becomes significant, and (iii) the red noise is consistent with the common signal found for the other pulsars. Neither of these signals are radio-frequency dependent. While the same physical motivation will not apply to many of the pulsars currently used in pulsar timing arrays, we argue for careful physically-motivated timing and noise modeling of pulsars used in precision timing experiments.

Wideband timing of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) datasets was first done for the 12.5 yr dataset. This method, where a single time of arrival and a single dispersion measure (DM) are measured using the entire bandwidth of each observation, proved to be invaluable for characterizing the time-varying DM, improving handling of frequency-dependent profile variability, as well as data volume reduction. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) Telescope has been observing most NANOGrav millisecond pulsars (MSPs) at nearly daily cadence (compared to roughly monthly cadence for other NANOGrav observations) since 2019, with the objective of integration into future pulsar timing array datasets. In this paper, we show the results of integration of high-cadence, low-observing-frequency CHIME data with data from the NANOGrav experiment for an isolated MSP PSR J0645+5158 and three binary MSPs: PSR J1012+5307, PSR J2145-0750, and PSR J2302+4442. Using a wideband timing pipeline, which we also describe, we present updated timing results for all four sources, including improved relativistic post-Keplerian measurements for the three binary pulsars in this analysis. For PSR J2302+4442, we report an updated strong detection of Shapiro delay from which we measured a companion mass of 0.35-0.04+0.05M circle dot , a pulsar mass of 1.8-0.3+0.3M circle dot , and an orbital inclination of 80 degrees-2+1 . We also report updated constraints on the reflex motion for PSR J2145-0750 using a combination of Very Long Baseline Array astrometry and our updated measurement of the time derivative of the projected semimajor axis of the pulsar orbit as a prior.

We report the discovery and timing of two pulsars from a sample of four circularly polarized sources identified in radio continuum images taken as part of the Australian SKA Pathfinder Variables and Slow Transients survey. Observations with the Parkes (Murriyang) radio telescope confirmed both sources as normal pulsars with high dispersion measures (DMs). PSR J1646-4451 has a spin period of 217 ms and a DM of 928 cm-3 pc, while PSR J1837-0616 exhibits a spin period of 118 ms and a DM of 793 cm-3 pc. These pulsars show extreme pulse broadening due to scattering, with measured scattering timescales of 117 ms and 75 ms at observing frequencies of similar to 1.8 GHz, respectively. These measurements imply extrapolated scattering timescales at 1 GHz of similar to 1346 ms and 740 ms, placing them among the most heavily scattered known pulsars. Our findings underscore the potential of using circular polarization in radio continuum images as a tool for identifying highly scattered pulsars. Future wide-field radio continuum surveys are poised to uncover a broader population of extreme pulsars, particularly those that are heavily scattered at 1.4 GHz, intrinsically faint, or residing in binaries-offering valuable insights into both pulsar demographics and the complex structure of the ionized interstellar medium.

We test the impact of an evolving supermassive black hole mass scaling relation (M-BH-M-bulge) 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 M-BH-M-bulge relation. We additionally consider the influence of the choice of galaxy stellar mass function (GSMF) 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 M-BH-M-bulge amplitude (i.e., the MBH/Mbulge ratio was higher in the past). If we assume that the M-BH-M-bulge amplitude does not evolve, our models require a GSMF that implies an undetected population of massive galaxies (M-star >= 10(11)M circle dot at z > 1). When the M-BH-M-bulge amplitude is allowed to evolve, we can model the GWB spectrum with all fiducial values and an M-BH-M-bulge amplitude that evolves as alpha(z) = alpha 0(1 + z)(1.04 +/- 0.5).

We present the searches conducted with the Zwicky Transient Facility (ZTF) in response to S250206dm, a bona fide event with an online false alarm rate of one in 25 yr, detected by the International Gravitational Wave Network. Although the event is significant, the nature of the compact objects involved remains unclear, with at least one likely neutron star. ZTF covered 68% of the last refined Bilby localization region, though we did not identify any likely optical counterpart. We describe the ZTF strategy, potential candidates, and the observations that helped rule out candidates, including sources circulated by other collaborations. Similar to Ahumada et al., we perform a frequentist analysis, using simsurvey, as well as Bayesian analysis, using nimbus, to quantify the efficiency of our searches. We find that, given the nominal up-to-date distance to this event of 373 +/- 104 Mpc, our efficiencies are above 10% for KNe brighter than -17.5 absolute magnitude. Assuming the optical counterpart known as kilonova (KN) lies within the ZTF footprint, our limits constrain the brightest end of the KN parameter space. Through dedicated radiative transfer simulations of KNe from binary neutron star (BNS) and black hole-neutron star mergers, we exclude parts of the BNS KN parameter space. Up to 35% of the models with high wind ejecta mass (Mwind approximate to 0.13 M circle dot) are ruled out when viewed face-on ( cos theta obs=1.0 ). Finally, we present a joint analysis using the combined coverage from ZTF and the Gravitational Wave Multimessenger Dark Energy Camera Survey. The joint observations cover 73% of the Bilby localization region, and the combined efficiency has a stronger impact on rising and slowly fading models, allowing us to rule out 55% of the high-mass KN models viewed face-on.

Free-floating objects (FFOs) in interstellar space—rogue planets, brown dwarfs, and large asteroids that are not gravitationally bound to any star—are expected to be ubiquitous throughout the Milky Way. Recent microlensing surveys have discovered several free-floating planets that are not bound to any known stellar systems. Additionally, three interstellar objects, namely 1I/’Oumuamua, 2I/Borisov, and 3I/ATLAS, have been detected passing through our solar system on hyperbolic trajectories. In this work, we search for FFOs on hyperbolic orbits that pass near millisecond pulsars (MSPs), where their gravitational influence can induce detectable perturbations in pulse arrival times. Using the NANOGrav 15 yr narrow band dataset, which contains high-precision timing data for 68 MSPs, we conduct a search for such hyperbolic scattering events between FFOs and pulsars. Although no statistically significant events were detected, this nondetection enables us to place upper limits (ULs) on the number density of FFOs as a function of their mass within our local region of the Galaxy. For example, the UL on the number density for Jupiter-mass FFOs (∼10 ^−2.5 –10 ^−3.5 M _⊙ ) obtained from different pulsars ranges from 5.25 × 10 ^6 pc ^−3 to 5.37 × 10 ^9 pc ^−3 , while the UL calculated by combining results from all the pulsars is 6.03 × 10 ^5 pc ^−3 . These results represent the first constraints on FFO population derived from pulsar timing data.

We report on the follow-up observations of circularly polarized sources identified in the MeerKAT image-based survey of the Galactic bulge. Using the Parkes radio telescope, we observed 16 circularly polarized sources with the Ultra-Wideband Low receiver and detected nine pulsars, six of which are new discoveries. All pulsars are fast rotators with spin periods under 100 ms. Among the new discoveries, five are millisecond pulsars (MSPs), and one has a spin period of 53.6 ms. At least four new MSPs exhibit clear signs of binary motion in their discovery observations. The dispersion measures of these pulsars fall between 18 and 330 pc cm ^−3 , which are lower than expected for Galactic bulge members and indicate that these pulsars lie in the foreground along the line of sight rather than within the bulge itself. This is the first time such a large number of pulsars have been confirmed from candidates identified in an image-based survey. These discoveries underscore the exceptional efficacy of circular polarization selection in image-based pulsar surveys, demonstrate the powerful synergy between high-sensitivity imaging and targeted time-domain follow-up using wideband receivers, and strengthen prospects for future deep pulsation searches—e.g., with MeerKAT or the forthcoming SKA or DSA-2000—to uncover the true MSP population in the Galactic bulge.

Trojan asteroids are found in the equilateral triangle Lagrange points of the Sun–Jupiter system in a great number, although they also exist less prolifically in other parts of the solar system. Despite up to planetary mass Trojans being predicted in extrasolar systems (i.e., exotrojans), they remain unconfirmed, although strong candidate evidence has emerged recently. For the first time, we extend the search for exotrojans to radio pulsars with low-mass (∼0.01 M _⊙ ) companions using accurately measured pulse times of arrival. With techniques developed for detecting the reflex motion of a star due to a librating Trojan, we place ∼1 M _⊕ upper mass constraints on potential exotrojans around eight pulsars observed in the NANOGrav 15 yr dataset. We find weak evidence consistent with ∼2–4 M _J exotrojans in the PSR J0023+0923 and PSR J1705−1903 binary systems, although the signals likely have a different, unknown source. We also place a libration-independent upper mass constraint of ∼8 M _J on exotrojans in the PSR J1641+8049 system by looking for an inconsistency between the times of superior conjunction as measured by optical light curves and those predicted by radio timing. These results offer initial observational constraints on the existence of exotrojans around pulsars, while their possible formation mechanisms remain unexplored.

We present the discovery of the radio afterglow of the most distant ultralong gamma-ray burst (GRB) detected to date, GRB 220627A at redshift z = 3.084. Its prompt gamma-ray light curve shows a double-pulse profile, with the pulses separated by a period of quiescence lasting ∼15 minutes, leading to early speculation it could be a strongly gravitationally lensed GRB. However, our analysis of the Fermi Gamma-ray Burst Monitor spectra taken during the time intervals of both pulses show clear differences in their spectral energy distributions, disfavouring the lensing scenario. We observed the radio afterglow from 7 to 456 days postburst: an initial, steep decay ( F _ν ∝ t ^−2 ) is followed by a shallower decline ( F _ν ∝ t ^−1/2 ) after ∼20 days. There are three scenarios that could explain these radio properties: (i) energy injection from an additional, slower ejecta component catching up to the external shock; (ii) a stratified density profile going as n ∝ r ^−8/3 ; or alternatively, (iii) the presence of a slow, wide ejecta component in addition to a fast, narrow ejecta component. We also conducted an independent test of the lensing hypothesis via very long baseline interferometry (VLBI) observations at ∼12 days postburst by searching, for the first time, for multiple images of the candidate lensed GRB afterglow. Our experiment highlighted the growing need for developments in real-time correlation capabilities for time-critical VLBI experiments, particularly as we advance towards the SKA and ngVLA era of radio astronomy.

Predicting algal blooms in waterways receiving inflows from multiple sources is challenging since blooms and their drivers can originate from diverse sources. Models that mechanistically simulate the formation and transport of algal blooms are often computationally intensive, creating barriers to using them for daily decision- making. Given this challenge, we developed a statistical risk forecasting framework for the Caloosahatchee River and Estuary in southwest Florida, United States of America, which receives engineered water releases from the eutrophic Lake Okeechobee, as well as hydrologic inputs from the surrounding watershed. The forecasting approach considers two different hydrologic regimes (i.e., lake- versus runoff-dominated conditions) while maintaining structural simplicity such that water managers could readily apply the model for short-term decision-making. Using daily mean discharge at two United States Geological Survey stations and more than 14 years of discrete water quality sampling data with diverse temporal resolution from the South Florida Water Management District, two regression tree models were trained and tested for day-ahead bloom risk forecasting: one for conditions in which lake releases dominated, and one when watershed inputs dominated. For the model capturing lake-dominated conditions, the main bloom predictors were 30-day lagged total suspended solids at the lake and canal outlets averaged over transect residence time (R2 = 0.78 and RMSE = 6.10 mu g/L). For the model predicting watershed-dominated conditions, 30-day lagged dissolved phosphorus load, and chlorophyll-a averaged over transect residence time at the canal outlet were the most important predictors of next-day chlorophyll-a (R2 = 0.49 and RMSE = 14.50 mu g/L). Critically, algal blooms driven by Lake Okeechobee releases are potentially controllable, as they are related, at least in part, to the operation of water control structures. Thus, the strong performance of the model for lake-dominated conditions demonstrates its utility for informing release scheduling to mitigate downstream blooms.

We present results from a search for radio afterglows of compact object mergers conducted with the Australian SKA Pathfinder (ASKAP). We used data from four epochs of the Rapid ASKAP Continuum Survey to search compact binary merger localization regions observed during the LIGO/Virgo O2, and O3 observing runs. Our investigation focused on 11 events (published in the GWTC-1, GWTC-2, and GWTC-3 catalogues of gravitational-wave events) with 90 per cent posterior localizations smaller than 150deg(2) and >= 99 per cent probabilities of being of astrophysical origin, to identify potential radio afterglow-like transients up to less than or similar to 1500 d post-merger. We identified candidate afterglow-type variable sources in the 90 per cent localization for events GW190503, GW200202, and GW200208, which were ruled out as unlikely to be related to the corresponding GW event on further analysis. Since we find no likely candidate counterparts, we constrain the inclination angle and the circum-merger density at isotropic equivalent energies in the range 2x10(51)-1x10(54)erg. These constraints are based on the assumption that the electron energy distribution in the associated jets follows a power-law index of p=2.2, with 1 per cent of the shock energy in the magnetic field (& varepsilon;(B )= 0.01) and 10 per cent in the electrons (& varepsilon;(e )= 0.1). We discuss the detectability of late-time afterglows as a function of merger distance and inclination angles with millijansky surveys.

Evidence has emerged for a stochastic signal correlated among 67 pulsars within the 15 yr pulsar-timing data set compiled by the NANOGrav collaboration. Similar signals have been found in data from the European, Indian, Parkes, and Chinese pulsar timing arrays. This signal has been interpreted as indicative of the presence of a nanohertz stochastic gravitational-wave background (GWB). To explore the internal consistency of this result, we investigate how the recovered signal strength changes as we remove the pulsars one by one from the data set. We calculate the signal strength using the (noise-marginalized) optimal statistic, a frequentist metric designed to measure the correlated excess power in the residuals of the arrival times of the radio pulses. We identify several features emerging from this analysis that were initially unexpected. The significance of these features, however, can only be assessed by comparing the real data to synthetic data sets. After conducting identical analyses on simulated data sets, we do not find anything inconsistent with the presence of a stochastic GWB in the NANOGrav 15 yr data. The methodologies developed here can offer additional tools for application to future, more sensitive data sets. While this analysis provides an internal consistency check of the NANOGrav results, it does not eliminate the necessity for additional investigations that could identify potential systematics or uncover unmodeled physical phenomena in the data.

The cosmic merger history of supermassive black hole binaries (SMBHBs) is expected to produce a low-frequency gravitational wave background (GWB). Here we investigate how signs of the discrete nature of this GWB can manifest in pulsar timing arrays (PTAs) through excursions from, and breaks in, the expected fGW-2/3 power law of the GWB strain spectrum. To do this, we create a semianalytic SMBHB population model, fit to North American Nanohertz Observatory for Gravitational Waves (NANOGrav's) 15 yr GWB amplitude, and with 1000 realizations, we study the populations' characteristic strain and residual spectra. Comparing our models to the NANOGrav 15 yr spectrum, we find two interesting excursions from the power law. The first, at 2 nHz, is below our GWB realizations with a p-value significance p = 0.05-0.06 (approximate to 1.8 sigma-1.9 sigma). The second, at 16 nHz, is above our GWB realizations with p = 0.04-0.15 (approximate to 1.4 sigma-2.1 sigma). We explore the properties of a loud SMBHB that could cause such an excursion. Our simulations also show that the expected number of SMBHBs decreases by 3 orders of magnitude, from similar to 106 to similar to 103, between 2 and 20 nHz. This causes a break in the strain spectrum as the stochasticity of the background breaks down at 26-19+28nHz , consistent with predictions pre-dating GWB measurements. The diminished GWB signal from SMBHBs at frequencies above the 26 nHz break opens a window for PTAs to detect continuous GWs from individual SMBHBs or GWs from the early Universe.

Pulse profile stability is a central assumption of standard pulsar timing methods. Thus, it is important for pulsar timing array experiments such as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) to account for any pulse profile variability present in their data sets. We show that in the NANOGrav 15-yr data set, the integrated pulse profile of PSR J1022+1001 as seen by the Arecibo radio telescope at 430, 1380, and 2030 MHz varies considerably in its shape from observation to observation. We investigate the possibility that this is due to the "ideal feed assumption" (IFA), on which NANOGrav's routine polarization calibration procedure relies. PSR J1022+1001 is ∼ 90% polarized in one pulse profile component, and also has significant levels of circular polarization. Time-dependent deviations in the feed's polarimetric response (PR) could cause mixing between the intensity I and the other Stokes parameters, leading to the observed variability. We calibrate the PR using a mixture of Measurement Equation Modeling and Measurement Equation Template Matching techniques. The resulting profiles are no less variable than those calibrated using the IFA method, nor do they provide an improvement in the timing quality of this pulsar. We observe the pulse shape in 25-MHz bandwidths to vary consistently across the band, which cannot be explained by interstellar scintillation in combination with profile evolution with frequency. Instead, we favor phenomena intrinsic to the pulsar as the cause.

Phytoplankton composition and biomass were investigated in the C-43 Canal in southwest Florida during a period of shifting discharges from water control structures. The canal receives regulated discharges from eutrophic Lake Okeechobee via the S77 structure. During periods of high S77 discharge in spring and early summer, cyanobacteria biomass dominated the phytoplankton community, including blooms of the harmful algal bloom (HAB) species Raphidiopsis raciborskii, Limnothrix redekei and Microcystis aeruginosa. During periods of low discharges from the lake, in mid-summer and autumn, water inputs to the canal came primarily from tributaries in the watershed surrounding the C-43. Phytoplankton biomass decreased, but the relative importance of dinoflagellates increased, including a bloom in July. The dinoflagellate community included Ceratium, Durinskia baltica, Glochidinium penardiforme, Gymnodinium fuscum, Parvodinium goslaviense, Parvodinium umbonatum/inconspicuum complex, Peridiniopsis quadridens, Woloszynskia reticulata, and an unidentified thecate and athecate species. D. baltica and P. goslaviense were recorded for the first time in Florida. Data was also obtained on water temperature, conductivity, fluorescent dissolved organic matter, chlorophyll a, total nitrogen, dissolved inorganic nitrogen, total phosphorus, PO4, discharge rates from water control structures, and water residence times. Results show that temporal shifts in the sources and rates of water inputs to the C-43 influence the character of environmental conditions that define phytoplankton composition and biomass in the canal. This suggests that management of discharges can play a role in mitigating HABs in the canal and downstream coastal environments receiving water from the canal.

Changing-Look Active Galactic Nuclei (CLAGN) are a rare subset of AGN that show significant changes to the flux of broad Balmer emission lines. Recent studies of CLAGN, such as 1ES 1927+654 and Mrk 590, have revealed that changes in the optically observed accretion rate are accompanied by changes in radio activity. We present a time-domain population study of 474 spectroscopically confirmed CLAGN at radio wavelengths using the Australia SKA Pathfinder Variable and Slow Transients Survey and the Very Large Array Sky Survey. We compare the radio properties of this CLAGN sample to a control sample of AGN that have not had recent changing-look events, and to AGN that were found to have transitioned from radio-quiet to radio-loud over 10-year timescales in VLASS. For 20 newly studied CLAGN detected in ASKAP VAST, we do not detect Mrk 590 or 1ES 1927+654-like fading of the radio flux in the 10 years following changing-look events. For 6 CLAGN with a sufficiently low redshift and high enough mass, we rule out a Mrk 590-like flare. We find that at the population level, CLAGN have higher VAST/VLASS detection rates, lower fractions of radio loudness, and higher variability rates in the 1 GHz frequency compared to the control AGN. Through VLA observations of radio SEDs and Magellan spectroscopic observations, we do not find evidence of a link between CLAGN and AGN that transitioned from radio-loud to radio-quiet in VLASS. We discuss the implications of this study for the physical mechanisms that drive enhanced accretion episodes.

Accurate streamflow forecasts are critical for modeling and managing estuarine water quality, as freshwater fluxes significantly influence coastal dynamics. The National Water Model (NWM) provides high-resolution streamflow predictions, which are valuable for hydrodynamic modeling in poorly gauged coastal regions. However, inaccuracies in NWM forecasts can limit our ability to predict estuarine and nearshore water quality effectively. First, this study evaluates the accuracy of NWM predictions for 14 coastal reaches in southwest Florida's Charlotte Harbor and Caloosahatchee River estuaries from 2018 to 2024, where hydrologic management has impacted water quality. NWM forecasts showed varying bias and variance, with Nash-Sutcliffe efficiencies (NSE) ranging from -2.26 to 0.77. Next, hydrodynamic simulations for the flow-managed Caloosahatchee River Estuary (CRE) were performed using both NWM forecasts and observed streamflows, revealing that errors in NWM predictions during high-flow events caused significant deviations in the position of ecologically relevant isohalines, lasting weeks. Finally, to address these issues, a Long Short-Term Memory (LSTM) network was developed to bias-correct NWM forecasts, improving NSE from 0.41 to 0.53. However, the LSTM's inability to "learn" managed discharge schedules highlights the need for advanced data assimilation and simulation techniques in flow-managed coastal systems.

Pulsar timing array observations have found evidence for an isotropic gravitational-wave background with the Hellings–Downs angular correlations between pulsar pairs. This interpretation hinges on the measured shape of the angular correlations, which is predominantly quadrupolar under general relativity. Here we explore a more flexible parameterization: we expand the angular correlations into a sum of Legendre polynomials and use a Bayesian analysis to constrain their coefficients with the 15 yr pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). When including Legendre polynomials with multipoles ℓ ≥ 2, we only find a significant signal in the quadrupole with an amplitude consistent with general relativity and nonzero at the ∼95% confidence level and a Bayes factor of 200. When we include multipoles ℓ ≤ 1, the Bayes factor evidence for quadrupole correlations decreases by more than an order of magnitude due to evidence for a monopolar signal at approximately 4 nHz, which has also been noted in previous analyses of the NANOGrav 15 yr data. Further work needs to be done in order to better characterize the properties of this monopolar signal and its effect on the evidence for quadrupolar angular correlations.

This paper reports the discovery and follow-up of four candidate redback spider pulsars: GPM J1723 $-33$ , GPM J1734 $-28$ , GPM J1752 $-30$ , and GPM J1815 $-14$ , discovered with the Murchison Widefield Array (MWA) from an imaging survey of the Galactic Plane. These sources are considered to be redback candidates based on their eclipsing variability, steep negative spectral indices, and potential Fermi $\gamma$ -ray associations, with GPM J1723 $-33$ and GPM J1815 $-14$ lying within a Fermi 95 $\%$ error ellipse. Follow-up pulsation searches with MeerKAT confirmed pulsations from GPM J1723 $-33$ , while the non-detections of the other three are likely due to scattering by material ablated from their companion stars. We identify possible orbital periods by applying folding algorithms to the light curves and determine that all sources have short orbital periods ( $\lt$ 24 h), consistent with redback spider systems. Following up on the sources at multiple radio frequencies revealed that the sources exhibit frequency-dependent eclipses, with longer eclipses observed at lower frequencies. We place broad constraints on the eclipse medium, ruling out induced Compton scattering and cyclotron absorption. Three sources are spatially consistent with optical sources in the Dark Energy Camera Plane Survey imaging, which may contain the optical counterparts. Each field is affected by strong dust extinction, and follow-up with large telescopes is needed to identify the true counterparts. Identifying potential radio counterparts to four previously unassociated Fermi sources brings us closer to understanding the origin of the unexplained $\gamma$ -ray excess in the Galactic Centre.