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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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All sourcesNASA NTRSDOE OSTI
NASA NTRS2020

The NANOGrav 11 yr data set: Evolution of Gravitational-wave Background Statistics

An ensemble of inspiraling supermassive black hole binaries should produce a stochastic background of very low frequency gravitational waves. This stochastic background is predicted to be a power law, with a gravitational-wave strain spectral index of −2/3, and it should be detectable by a network of precisely timed millisecond pulsars, widely distributed on the sky. This paper reports a new “time slicing” analysis of the 11 yr data release from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) using 34 millisecond pulsars. Methods to flag potential “false-positive” signatures are developed, including techniques to identify responsible pulsars. Mitigation strategies are then presented. We demonstrate how an incorrect noise model can lead to spurious signals, and we show how independently modeling noise across 30 Fourier components, spanning NANOGrav’s frequency range, effectively diagnoses and absorbs the excess power in gravitational-wave searches. This results in a nominal, and expected, progression of our gravitational-wave statistics. Additionally, we show that the first interstellar medium event in PSR J1713+0747 pollutes the common red-noise process with low spectral index noise, and we use a tailored noise model to remove these effects.

J S Hazboun↗
NASA NTRS1995

Binary-binary interactions and the formation of the PSR B1620-26 triple system in M4

The hierarchical triple system containing the millisecond pulsar PSR B1620-26 in M4 is the first star system ever detected in a globular cluster. Such systems should form in globular clusters as a result of dynamical interactions between binaries. We propose that the triple system containing PSR B1620-26 formed through an exchange interaction between a wide primordial binary and a pre-existing binary millisecond pulsar. This scenario would have the advantage of reconciling the approximately 10(exp 9) yr timing age of the pulsar with the much shorter lifetime of the triple system in the core of M4.

Rasio, Frederic A.↗
NASA NTRS2005

Hotspot or Heatwave? Getting to Grips with Neutron Star Burst Oscillations

Many accreting neutron stars, including two of the millisecond pulsars, exhibit high frequency oscillations during Type I X-ray bursts. The properties of the burst oscillations reflect the nature of the thermal asymmetry on the stellar surface. The mechanism that gives rise to the aspzetry, however , remains unclear: possibilities include a hotspot due to uneven fuel distribution, modes of oscillation in the surface layers of the neutron star, or vortices driven by the Coriolis force. I will review some of the latest theory and observations, and present the results of a recent study of variability in the burst oscillations of the millisecond pulsar 51814-338.

Watts, A.↗
DOE OSTI2022

Return of the templates: Revisiting the Galactic Center excess with multimessenger observations

The Galactic center excess (GCE) remains one of the most intriguing discoveries from the Fermi Large Area Telescope (LAT) observations. We revisit the characteristics of the GCE by first producing a new set of high-resolution galactic diffuse gamma-ray emission templates. This diffuse emission, which accounts for the bulk of the observed gamma rays, is ultimately due to cosmic-ray interactions with the interstellar medium. Using recent high-precision cosmic-ray observations, in addition to the continuing Fermi-LAT observations and observations from lower energy photons, we constrain the properties of the galactic diffuse emission. We describe a large set of diffuse gamma-ray emission templates which account for a very wide range of initial assumptions on the physical conditions in the inner galaxy. The broad properties of the GCE that we find in this work are qualitatively unchanged despite the introduction of this new set of templates, though its quantitative features appear mildly different than those obtained in previous analyses. In particular, we find a high-energy tail at higher significance than previously reported. This tail is very prominent in the northern hemisphere, and less so in the southern hemisphere. This strongly affects one prominent interpretation of the excess: known millisecond pulsars are incapable of producing this high-energy emission, even in the relatively softer southern hemisphere, and are therefore disfavored as the sole explanation of the GCE. The annihilation of dark matter particles of mass 40 - 7 + 10 GeV (95% CL) to b quarks with a cross-section of ⟨ σ A v ⟩ = 1. 4 - 0.3 + 0.6 × 10 - 26 cm 3 s − 1 provides a good fit to the excess especially in the relatively cleaner southern sky. Dark matter of the same mass range annihilating to b quarks or heavier dark matter particles annihilating to heavier Standard Model bosons can combine with millisecond pulsars to provide a good fit to the southern hemisphere emission as well, as can a broken power-law spectrum which would be related to recent cosmic-ray burst activity. As part of this paper, we make publicly available all of our templates and the data covariance matrix we have generated to account for systematic uncertainties.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1994

Noise and drift analysis of non-equally spaced timing data

Generally, it is possible to obtain equally spaced timing data from oscillators. The measurement of the drifts and noises affecting oscillators is then performed by using a variance (Allan variance, modified Allan variance, or time variance) or a system of several variances (multivariance method). However, in some cases, several samples, or even several sets of samples, are missing. In the case of millisecond pulsar timing data, for instance, observations are quite irregularly spaced in time. Nevertheless, since some observations are very close together (one minute) and since the timing data sequence is very long (more than ten years), information on both short-term and long-term stability is available. Unfortunately, a direct variance analysis is not possible without interpolating missing data. Different interpolation algorithms (linear interpolation, cubic spline) are used to calculate variances in order to verify that they neither lose information nor add erroneous information. A comparison of the results of the different algorithms is given. Finally, the multivariance method was adapted to the measurement sequence of the millisecond pulsar timing data: the responses of each variance of the system are calculated for each type of noise and drift, with the same missing samples as in the pulsar timing sequence. An estimation of precision, dynamics, and separability of this method is given.

Vernotte, F.↗
DOE OSTI2024

Robustness of the Galactic Center excess morphology against masking

The Galactic Center excess (GCE) remains an enduring mystery, with leading explanations being annihilating dark matter or an unresolved population of millisecond pulsars. Analyzing the morphology of the GCE provides critical clues to identify its exact origin. We investigate the robustness of the inferred GCE morphology against the effects of masking, an important step in the analysis where the gamma-ray emission from point sources and the galactic disk are excluded. Using different masks constructed from Fermi point source catalogs and a wavelet method, we find that the GCE morphology, particularly its ellipticity and cuspiness, is relatively independent of the choice of mask for energies above 2–3 GeV. The GCE morphology systematically favors an approximately spherical shape, as expected for dark matter annihilation. Compared to various stellar bulge profiles, a spherical dark matter annihilation profile better fits the data across different masks and galactic diffuse emission backgrounds, except for the stellar bulge profile which provides a similar fit to the data. Modeling the GCE with two components, one from dark matter annihilation and one tracing the Coleman bulge, we find this two-component model outperforms any single component or combinations of dark matter annihilation and other stellar bulge profiles. Uncertainty remains about the exact fraction contributed by each component across different background models and masks. Furthermore, when the Coleman bulge dominates, its corresponding spectrum lacks characteristics typically associated with millisecond pulsars, suggesting that it mostly models the emission from other sources instead of the GCE that is still present and spherically symmetric.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS2007

Erratum: "Discovery of a Second Millisecond Accreting Pulsar: XTE J1751-305"

The original Table 1 ("Timing Parameters of XTE J1751-305") contains one error. The epoch of pulsar mean longitude 90deg is incorrect due to a numerical conversion error in the preparation of the original table text. A corrected version of Table 1 is shown. For reference, the epoch of the ascending node is also included. The correct value was used in all of the analysis leading up to the paper. As T(sub 90) is a purely fiducial reference time, the scientific conclusions of the paper are unchanged.

Markwardt, Craig↗
NASA NTRS1993

On the verification of the planetary system around PSR 1257 + 12

The magnitude and nature of residuals expected in the times of pulsar arrival (TOA) of the millisecond pulsar PSR 1257 + 12 as a function of the observational time span is estimated so that an observing progam may be planned to optimize the detection of a perturbation signature of a possible planetary system. The numerical and analytic procedures for evaluating the residuals are described. Calculations results are given in which a recognizable signature is shown to be a modulation of the amplitude of the TOA residual differences as a function of both epoch and observational interval. A supplementary approach involving intensive closely spaced observations over selected time intervals to find the time of a particular zero crossing of the TOA residuals is also discussed.

Peale, S. J.↗
NASA NTRS2005

Pulsar Wind Nebulae, Space Velocities and Supernova Remnant

The original proposal for this LTSA grant was for X-ray studies of pulsars, and especially pulsar wind nebulae and what they could tell us about pulsar properties, especially their space velocities. By any metric, this program has been very successful. No fewer than 14 papers on directly related topics (and several dozen more on related topics) have been published in refereed journals with the PI as lead or co-author, all observational results that have had significant impact on the field. These include the first X-ray detection of the "Duck" pulsar, a clear demonstration that estimated pulsar ages can be off by over an order of magnitude (via observations of the young supernova remnant G11.2-0.3) and the detection of the first pulsar wind nebula around a millisecond pulsar. These publications have also resulted in 4 press releases. Moreover, they also represent the thesis work of two PhD students at MIT (Froney Crawford and Mike Pivovaroff) and one postdoctoral fellow, Bryan Gaensler, now Assistant Professor at Harvard.

Source record↗
NASA NTRS2014

Gravitational Waves from Known Pulsars: Results from the Initial Detector Era

We present the results of searches for gravitational waves from a large selection of pulsars using data from the most recent science runs (S6, VSR2 and VSR4) of the initial generation of interferometric gravitational wave detectors LIGO (Laser Interferometric Gravitational-wave Observatory) and Virgo. We do not see evidence for gravitational wave emission from any of the targeted sources but produce upper limits on the emission amplitude. We highlight the results from seven young pulsars with large spin-down luminosities. We reach within a factor of five of the canonical spin-down limit for all seven of these, whilst for the Crab and Vela pulsars we further surpass their spin-down limits. We present new or updated limits for 172 other pulsars (including both young and millisecond pulsars). Now that the detectors are undergoing major upgrades, and, for completeness, we bring together all of the most up-to-date results from all pulsars searched for during the operations of the first-generation LIGO, Virgo and GEO600 detectors. This gives a total of 195 pulsars including the most recent results described in this paper.

waves↗
DOE OSTI2023

The Third Fermi Large Area Telescope Catalog of Gamma-Ray Pulsars

We present 294 pulsars found in GeV data from the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope. Another 33 millisecond pulsars (MSPs) discovered in deep radio searches of LAT sources will likely reveal pulsations once phase-connected rotation ephemerides are achieved. A further dozen optical and/or X-ray binary systems colocated with LAT sources also likely harbor gamma-ray MSPs. This catalog thus reports roughly 340 gamma-ray pulsars and candidates, 10% of all known pulsars, compared to ≤11 known before Fermi. Half of the gamma-ray pulsars are young. Of these, the half that are undetected in radio have a broader Galactic latitude distribution than the young radio-loud pulsars. The others are MSPs, with six undetected in radio. Overall, ≥236 are bright enough above 50 MeV to fit the pulse profile, the energy spectrum, or both. For the common two-peaked profiles, the gamma-ray peak closest to the magnetic pole crossing generally has a softer spectrum. The spectral energy distributions tend to narrow as the spindown power $\dot{E}$ decreases to its observed minimum near 10 33 erg s -1 , approaching the shape for synchrotron radiation from monoenergetic electrons. We calculate gamma-ray luminosities when distances are available. Our all-sky gamma-ray sensitivity map is useful for population syntheses. The electronic catalog version provides gamma-ray pulsar ephemerides, properties, and fit results to guide and be compared with modeling results.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2018

SEXTANT X-Ray Pulsar Navigation Demonstration: Initial On-Orbit Results

Millisecond pulsars (MSPs) are rapidly rotating neutron stars that appear to pulsate across the electromagnetic spectrum. Some MSPs have long-term timing stability that rivals that of atomic clocks. Pulse arrival phase can be predicted with great accuracy at any reference point in the Solar System through use of a pulsar timing model on a spacecraft. Comparing observed phase to predictions gives information that may be used in a navigation process. Why X-rays? Some stable MSPs have conveniently detectable X-ray emissions. X-rays are immune to interstellar dispersion effects thought to limit radio pulsar timing models. Highly directional compact detectors possible.

Navigatio↗
NASA NTRS2021

A NICER View of the Massive Pulsar PSR J0740+6620 Informed by Radio Timing and XMM-Newton Spectroscopy

We report on Bayesian estimation of the radius, mass, and hot surface regions of the massive millisecond pulsar PSR J0740+6620, conditional on pulse-profile modeling of Neutron Star Interior Composition Explorer X-ray Timing Instrument event data. We condition on informative pulsar mass, distance, and orbital inclination priors derived from the joint North American Nanohertz Observatory for Gravitational Waves and Canadian Hydrogen Intensity Mapping Experiment/Pulsar wideband radio timing measurements of Fonseca et al. We use XMM-Newton European Photon Imaging Camera spectroscopic event data to inform our X-ray likelihood function. The prior support of the pulsar radius is truncated at 16 km to ensure coverage of current dense matter models. We assume conservative priors on instrument calibration uncertainty. We constrain the equatorial radius and mass of PSR J0740+6620 to be-+12.390.981.30km and-+2.0720.0660.067Me respectively, each reported as the posterior credible interval bounded by the 16% and 84% quantiles, conditional on surface hot regions that are non-overlapping spherical caps of fully ionized hydrogen atmosphere with uniform effective temperature; a posteriori, the temperature is=-+TlogK5.99100.060.05([])for each hot region. All software for the X-ray modeling framework is open-source and all data, model, and sample information is publicly available, including analysis notebooks and model modules in the Python language. Our marginal likelihood function of mass and equatorial radius is proportional to the marginal joint posterior density of those parameters(within the prior support)and can thus be computed from the posterior samples.

Millisecond pulsars↗
NASA NTRS2010

Pair Cascades and Deathlines in Offset Magnetic Dipole Fields

We investigate electron-positron pair cascades in a dipole magnetic field whose axis is offset from the neutron star center. In such a field geometry, the polar cap is displaced from the neutron star symmetry axis and the field line radius of curvature is modified. Using the modified parallel electric field near the polar cap of an offset dipole, we simulate pair cascades to determine the pair deathlines and pair multiplicities as a function of the offset parameter. We find that the pair multiplicity can change dramatically with a modest offset, with a significant increase on one side of the polar cap. Lower pair deathlines allow a larger fraction of the pulsar population, that include old and millisecond pulsars, to produce cascades with high multiplicity.

Harding, Alice↗
NASA NTRS2012

Pair Cascades and Deathlines in Magnetic Fields with Offset Polar Caps

We present results of electron-positron pair cascade simulations in a dipole magnetic field whose polar cap is offset from the dipole axis. In such a field geometry, the polar cap is displaced a small fraction of the neutron star radius from the star symmetry axis and the field line radius of curvature is modified. Using the modified parallel electric field near the offset polar cap, we simulate pair cascades to determine the pair deathlines and pair multiplicities as a function of the offset. We find that the pair multiplicity can change dr;unatically with a modest offset, with a significant increase on one side of the polar cap. Lower pair deathlines allow a larger fraction of the pulsar population, that include old and millisecond pulsars, to produce cascades with high multiplicity. The results have some important implications for pulsar particle production, high-energy emission and cosmic-ray contribution.

Harding, Alice K.↗
NASA NTRS2019

Microarcsecond VLBI Pulsar Astrometry with PSRπ II. Parallax Distances for 57 Pulsars

We present the results of PSRπ, a large astrometric project targeting radio pulsars using the Very Long Baseline Array (VLBA). From our astrometric database of 60 pulsars, we have obtained parallax-based distance measurements for all but 3, with a parallax precision that is typically ∼45 μas and approaches 10 μas in the best cases. Our full sample doubles the number of radio pulsars with a reliable (≳5σ) model-independent distance constraint. Importantly, many of the newly measured pulsars are well outside the solar neighborhood, and so PSRπ brings a near-tenfold increase in the number of pulsars with a reliable model-independent distance at d > 2 kpc. Our results show that both widely used Galactic electron density distribution models contain significant shortcomings, particularly at high Galactic latitudes. When comparing our results to pulsar timing, two of the four millisecond pulsars in our sample exhibit significant discrepancies in their proper motion estimates. With additional VLBI observations that extend our sample and improve the absolute positional accuracy of our reference sources, we will be able to additionally compare pulsar absolute reference positions between VLBI and timing, which will provide a much more sensitive test of the correctness of the solar system ephemerides used for pulsar timing. Finally, we use our large sample to estimate the typical accuracy attainable for differential VLBA astrometry of pulsars, showing that for sufficiently bright targets observed eight times over 18 months, a parallax uncertainty of 4 μas per arcminute of separation between the pulsar and calibrator can be expected.

A. T. Deller↗
DOE OSTI2022

Incremental Fermi Large Area Telescope Fourth Source Catalog

We present an incremental version (4FGL-DR3, for Data Release 3) of the fourth Fermi Large Area Telescope (LAT) catalog of γ-ray sources. Based on the first 12 years of science data in the energy range from 50 MeV to 1 TeV, it contains 6658 sources. The analysis improves on that used for the 4FGL catalog over eight years of data: more sources are fit with curved spectra, we introduce a more robust spectral parameterization for pulsars, and we extend the spectral points to 1 TeV. The spectral parameters, spectral energy distributions, and associations are updated for all sources. Light curves are rebuilt for all sources with 1 yr intervals (not 2 month intervals). Among the 5064 original 4FGL sources, 16 were deleted, 112 are formally below the detection threshold over 12 yr (but are kept in the list), while 74 are newly associated, 10 have an improved association, and seven associations were withdrawn. Pulsars are split explicitly between young and millisecond pulsars. Pulsars and binaries newly detected in LAT sources, as well as more than 100 newly classified blazars, are reported. We add three extended sources and 1607 new point sources, mostly just above the detection threshold, among which eight are considered identified, and 699 have a plausible counterpart at other wavelengths. We discuss the degree-scale residuals to the global sky model and clusters of soft unassociated point sources close to the Galactic plane, which are possibly related to limitations of the interstellar emission model and missing extended sources.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1993

Planets around pulsars - Implications for planetary formation

Data on planets around pulsars are summarized, and different models intended to explain the formation mechanism are described. Both theoretical and observational evidence suggest that very special circumstances are required for the formation of planetary systems around pulsars, namely, the prior presence of a millisecond pulsar with a close binary companion, probably a low mass main-sequence star. It is concluded that the discovery of two planets around PSR 1257+12 is important for better understanding the problems of dynamics and stellar evolution. The process of planetary formation should be learned through intensive studies of the properties of disks near young objects and application of techniques for detection of planets around main-sequence solar-type stars.

Bodenheimer, Peter↗