Radio Frequency Interference (RFI) greatly reduces sensitivity of radio observations to astrophysical signals and creates false positive candidates in searches for radio transients. Real signals are missed while considerable computational and human resources are needed to remove RFI candidates. In the context of transient astrophysics, this makes effective RFI removal vital to effective searches for fast radio bursts and pulsars. Radio telescopes typically sample at rates that are high enough for there to be tens to hundreds of samples along the transient's pulse. Mitigation techniques should excise RFI on this timescale to account for a changing radio frequency environment. We evaluate the effectiveness of three filters, as well as a composite of the three, that excises RFI at the cadence that the data are recorded. Each of these filters operates in a different domain and thus excises as a different RFI morphology. We analyze the performance of these four filters in three different situations: (I) synthetic pulses in Gaussian noise; (II) synthetic pulses injected into real data; (III) four pulsar observations. From these tests, we gain insight into how the filters affect both the pulse and the noise level. This allows use to outline which and how the filters should be used based on the RFI present and the characteristics of the source signal. We show by flagging a small percentage of the spectrum we can substantially improve the quality of transit observations.

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 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).

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.

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.

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.

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 Large Area Telescope catalog (4FGL-DR4) to investigate the implications of proposed γ -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 γ -ray pulsars are beaming toward the Earth. We then explore the observable outcomes of seven different γ -ray luminosity models. Four of the models provide a good match to the observed number of PLUIDs, while three others significantly overpredict the number of PLUIDs. For these latter models, either the average beaming fraction of γ -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 γ -ray models we investigated could replicate the observed sample of 150 γ -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.

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.

>The Five-hundred-meter Aperture Spherical Telescope (FAST is living up to its promise as one of the world's premiere pulsa finding instruments.At the center of FAST's many significan pulsar discoveries (which in total number 1000 from all projects to date) is the announcement by Han et al.(2025) of a further 473pulsars.The new discoveries are the latest installment from th FAST Galactic Plane Snapshot Survey (GPPS),bringing its total to751 pulsars.Full details of the design and observational setup o the FAST GPPS can be found in Han et al.(2021).In brief,th survey is covering the northern Galactic plane in the FAST sky with latitudes|b|<10°with 5 minute pointings using the 19-beam1.4 GHz receiver on the FAST telescope,achieving a sensitivity to pulsars as faint as a fewμJy.To date,with just under 25%of th total survey area completed,the discoveries include 107 rotating radio transients (RRATs),177 millisecond pulsars (MSPs) and 157binary pulsars.With most of the observations close to the Galacti plane complete,the rate of future discoveries will likely fall of slightly,but GPPS is set to eclipse the Parkes Multibeam Pulsa Survey (Manchester et al.2001) as the most successful pulsa survey to date.

Free electrons in the interstellar medium refract and diffract radio waves along multiple paths, resulting in angular and temporal broadening of radio pulses that limits pulsar timing precision. We determine multifrequency, multiepoch scattering times for the large dispersion measure millisecond PSR J1903+0327 by developing a three-component model for the emitted pulse shape that is convolved with a best-fit pulse broadening function (PBF) identified from a family of thin-screen and extended-media PBFs. We show that the scattering time, τ , at a fiducial frequency of 1500 MHz changes by approximately 10% over a 5.5 yr span with a characteristic timescale of approximately 100 days. We also constrain the spectral index and inner scale of the wavenumber spectrum of electron density variations along this line of sight. We find that the scaling law for τ versus radio frequency is strongly affected by any mismatch between the true and assumed PBF or between the true and assumed intrinsic pulse shape. We show using simulations that refraction is a plausible cause of the epoch dependence of τ , manifesting as changes in the PBF shape and 1/ e timescale. Finally, we discuss the implications of our scattering results on pulsar timing including time of arrival delays and dispersion measure misestimation.

Based on the rate of change of its orbital period, PSR J2043+1711 has a substantial peculiar acceleration of 3.5 ± 0.8 mm s ^–1 yr ^–1 , which deviates from the acceleration predicted by equilibrium Milky Way (MW) models at a 4 σ level. The magnitude of the peculiar acceleration is too large to be explained by disequilibrium effects of the MW interacting with orbiting dwarf galaxies (∼1 mm s ^–1 yr ^–1 ), and too small to be caused by period variations due to the pulsar being a redback. We identify and examine two plausible causes for the anomalous acceleration: a stellar flyby, and a long-period orbital companion. We identify a main-sequence star in Gaia DR3 and Pan-STARRS DR2 with the correct mass, distance, and on-sky position to potentially explain the observed peculiar acceleration. However, the star and the pulsar system have substantially different proper motions, indicating that they are not gravitationally bound. However, it is possible that this is an unrelated star that just happens to be located near J2043+1711 along our line of sight (chance probability of 1.6%). Therefore, we also constrain possible orbital parameters for a circumbinary companion in a hierarchical triple system with J2043+1711; the changes in the spindown rate of the pulsar are consistent with an outer object that has an orbital period of 60 kyr, a companion mass of 0.3 M _⊙ (indicative of a white dwarf or low-mass star), and a semimajor axis of 1900 au. Continued timing and/or future faint optical observations of J2043+1711 may eventually allow us to differentiate between these scenarios.

Open clusters (OCs) are the primary contributors to the native stellar population of the Milky Way disk. Neutron stars (NSs) -- remnants of massive stars -- in the disk should also originate from OCs, along with the resulting pulsars. However, no NS associated with any OC has been validated, presumably due to high-velocity kicks, exceeding the OC's escape velocity, imparted by supernova explosions. We have achieved arcsec-level localization (~1/1000 of the OC's tidal radius) to place the newly found pulsar PSR J1921+3745 in the tidal tail as well as near the tidal radius edge of NGC 6791, one of the oldest and most massive Galactic OCs. Our N-body simulation shows that, for an OC like NGC 6791, a substantial fraction of NSs with low-velocity kicks, peaking around 3 km/s in our calculation, remain in the cluster's tidal tails. The radio PSR J1921+3745 in the cluster tidal tail thus represents a snapshot of an escaping NS from an OC to the Galactic disk field. Further identification and characterization of pulsars associated with OCs are crucial to probe the origin of Galactic disk neutron stars and their co-evolution with the clusters.

The NANOGrav 15 yr data provide compelling evidence for a stochastic gravitational-wave (GW) background at nanohertz frequencies. The simplest model-independent approach to characterizing the frequency spectrum of this signal consists of a simple power-law fit involving two parameters: an amplitude A and a spectral index gamma. In this Letter, we consider the next logical step beyond this minimal spectral model, allowing for a running (i.e., logarithmic frequency dependence) of the spectral index, gamma(run)(f) = gamma + beta ln (f/f(ref)). We fit this running-power-law (RPL) model to the NANOGrav 15 yr data and perform a Bayesian model comparison with the minimal constant-power-law (CPL) model, which results in a 95% credible interval for the parameter beta consistent with no running, beta is an element of[-0.80, 2.96], and an inconclusive Bayes factor, B(RPL versus CPL) = 0.69 +/- 0.01. We thus conclude that, at present, the minimal CPL model still suffices to adequately describe the NANOGrav signal; however, future data sets may well lead to a measurement of nonzero beta. Finally, we interpret the RPL model as a description of primordial GWs generated during cosmic inflation, which allows us to combine our results with upper limits from Big Bang nucleosynthesis, the cosmic microwave background, and LIGO-Virgo-KAGRA.

Accurate pulsar astrometric estimates play an essential role in almost all high-precision pulsar timing experiments. Traditional pulsar timing techniques refine these estimates by including them as free parameters when fitting a model to observed pulse time-of-arrival measurements. However, reliable sub-milliarcsecond astrometric estimations require years of observations and, even then, power from red noise can be inadvertently absorbed into astrometric parameter fits, biasing the resulting estimations and reducing our sensitivity to red noise processes, including gravitational waves (GWs). In this work, we seek to mitigate these shortcomings by using pulsar astrometric estimates derived from Very Long Baseline Interferometry (VLBI) as priors for the timing fit. First, we calibrated a frame tie to account for the offsets between the reference frames used in VLBI and timing. Then, we used the VLBI-informed priors and timing-based likelihoods of several astrometric solutions consistent with both techniques to obtain a maximum-posterior astrometric solution. We found offsets between our results and the timing-based astrometric solutions, which, if real, would lead to absorption of spectral power at frequencies of interest for single-source GW searches. However, we do not find significant power absorption due to astrometric fitting at the low-frequency domain of the GW background.

Pulsar timing array experiments have reported evidence for a stochastic background of nanohertz gravitational waves consistent with the signal expected from a population of supermassive black hole binaries. Their analyses assume power-law spectra for intrinsic pulsar noise and for the background, as well as a Hellings-Downs cross-correlation pattern among the gravitational-wave- induced residuals across pulsars. These assumptions may not be realized in actuality. We test them in the NANOGrav 15 yr dataset using Bayesian posterior predictive checks. After fitting our fiducial model to real data, we generate a population of simulated dataset replications. We use the replications to assess whether the optimal statistic significance, interpulsar correlations, and spectral coefficients are extreme. We recover Hellings-Downs correlations in simulated datasets at significance levels consistent with the correlations measured in the NANOGrav 15 yr dataset. A similar test on spectral coefficients shows that their values in real data are not extreme compared to their distributions across replications. We also evaluate the evidence for the stochastic background using posterior predictive versions of the frequentist optimal statistic and of Bayesian model comparison and find comparable significance (3.26 and 36 respectively) to what was previously reported for the standard statistics. We conclude with novel visualizations of the reconstructed gravitational waveforms that enter the residuals for each pulsar. Our analysis strengthens confidence in the identification and characterization of the gravitational-wave background.

Observations show that globular clusters (GCs) might be among the best places to find millisecond pulsars (MSPs). However, the GC Terzan 6 seems to be an exception without any pulsar discovered, although its high stellar encounter rate suggests that it harbors dozens of them. We report the discovery of the first radio pulsar, PSR J1750-3116A, likely associated with Terzan 6 in a search of C-band (4-8 GHz) data from the Robert C. Byrd Green Bank Telescope with a spin period of 5.33 ms and dispersion measure (DM) similar or equal to 383 pc cm-3. The mean flux density of this pulsar is approximately 3 mu Jy. The DM agrees well with predictions from the Galactic free electron density model, assuming a distance of 6.7 kpc for Terzan 6. PSR J1750-3116A is likely an isolated MSP, potentially formed through dynamical interactions, considering the core-collapsed classification and the exceptionally high stellar encounter rate of Terzan 6. This is the highest radio frequency observation that has led to the discovery of a pulsar in a GC to date. While L-band (1-2 GHz) observations of this cluster are unlikely to yield significant returns due to propagation effects, we predict that further pulsar discoveries in Terzan 6 will be made by existing radio telescopes at higher frequencies.

Pulsar timing array experiments have recently uncovered evidence for a nanohertz gravitational wave background by precisely timing an ensemble of millisecond pulsars. The next significant milestones for these experiments include characterizing the detected background with greater precision, identifying its source(s), and detecting continuous gravitational waves from individual supermassive black hole binaries. To achieve these objectives, generating accurate and precise times of arrival of pulses from pulsar observations is crucial. Incorrect polarization calibration of the observed pulsar profiles may introduce errors in the measured times of arrival. Further, previous studies (e.g., van Straten 2013; Manchester et al. 2013) have demonstrated that robust polarization calibration of pulsar profiles can reduce noise in the pulsar timing data and improve timing solutions. In this paper, we investigate and compare the impact of different polarization calibration methods on pulsar timing precision using three distinct calibration techniques: the Ideal Feed Assumption (IFA), Measurement Equation Modeling (MEM), and Measurement Equation Template Matching (METM). Three NANOGrav pulsars-PSRs J1643$-$1224, J1744$-$1134, and J1909$-$3744-observed with the 800 MHz and 1.5 GHz receivers at the Green Bank Telescope (GBT) are utilized for our analysis. Our findings reveal that all three calibration methods enhance timing precision compared to scenarios where no polarization calibration is performed. Additionally, among the three calibration methods, the IFA approach generally provides the best results for timing analysis of pulsars observed with the GBT receiver system. We attribute the comparatively poorer performance of the MEM and METM methods to potential instabilities in the reference noise diode coupled to the receiver and temporal variations in the profile of the reference pulsar, respectively.