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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS2014

Gamma-Ray Emission in Dissipative Pulsar Magnetospheres: from Theory to Fermi Observations

We compute the patterns of gamma-ray emission due to curvature radiation in dissipative pulsar magnetospheres. Our ultimate goal is to construct macrophysical models that are able to reproduce the observed gamma-ray light curve phenomenology recently published in the Second Fermi Pulsar Catalog. We apply specific forms of Ohm's law on the open field lines using a broad range for the macroscopic conductivity values that result in solutions ranging, from near-vacuum to near-force-free. Using these solutions, we generate model gamma-ray light curves by calculating realistic trajectories and Lorentz factors of radiating particles under the influence of both the accelerating electric fields and curvature radiation reaction. We further constrain our models using the observed dependence of the phase lags between the radio and gamma-ray emission on the gamma-ray peak separation. We perform a statistical comparison of our model radio-lag versus peak-separation diagram and the one obtained for the Fermi standard pulsars. We find that for models of uniform conductivity over the entire open magnetic field line region, agreement with observations favors higher values of this parameter. We find, however, significant improvement in fitting the data with models that employ a hybrid form of conductivity, specifically, infinite conductivity interior to the light cylinder and high but finite conductivity on the outside. In these models the gamma-ray emission is produced in regions near the equatorial current sheet but modulated by the local physical properties. These models have radio lags near the observed values and statistically best reproduce the observed light curve phenomenology. Additionally, they also produce GeV photon cut-off energies.

stars↗
NASA NTRS2016

A New, Low Braking Index for the LMC Pulsar B0540-69

We report the results of a 16-month monitoring campaign using the Swift satellite of PSR 0540, a young pulsar in the Large Magellanic Cloud. Phase connection was maintained throughout the campaign so that a reliable ephemeris could be determined, and the length of the campaign is adequate to accurately determine the spin frequency and its first and second derivatives. The braking index n is 0.031 +/- 0.013 (90% confidence), a value much lower than previously reported for 0 540 and almost all other young pulsars. We use data from the extensive monitoring campaign with RXTE to showt hat timing noise is unlikely to significantly affect the measurement. This is the first measurement of the braking index in the pulsars recently discovered high spin-down state. We discuss possible mechanisms for producing the low braking index.

pulsars individual (PSR B0540−69)↗
NASA NTRS2017

Where are the r-modes? Chandra Observations of Millisecond Pulsars

We present the results of Chandra observations of two non-accreting millisecond pulsars, PSRs J1640+2224(J1640) and J1709+2313 (J1709), with low inferred magnetic fields and spin-down rates in order to constrain their surface temperatures, obtain limits on the amplitude of unstable r-modes in them, and make comparisons with similar limits obtained for a sample of accreting low-mass X-ray binary (LMXB) neutron stars. We detect both pulsars in the X-ray band for the first time. They are faint, with inferred soft X-ray fluxes(0.3-3 keV) of approx. 6 x 10(exp -15) and 3 x 10( exp -15) erg/sq cm for J1640 and J1709, respectively. Spectral analysis assuming hydrogen atmosphere emission gives global effective temperature upper limits (90% confidence) of 3.3-4.3 x 10(exp 5) K for J1640 and 3.6-4.7 x 10(exp 5) K for J1709, where the low end of the range corresponds to canonical neutron stars (M = 1.4 Stellar Mass), and the upper end corresponds to higher-mass stars (M = 2.21 Stellar Mass). Under the assumption that r-mode heating provides the thermal support, we obtain dimensionless r-mode amplitude upper limits of 3.2-4.8 x 10(exp -8) and 1.8-2.8 x 10(exp -7) for J1640 and J1709, respectively, where again the low end of the range corresponds to lower-mass, canonical neutron stars (M =1.4 Stellar Mass). These limits are about an order of magnitude lower than those we derived previously for a sample of LMXBs, except for the accreting millisecond X-ray pulsar SAX J1808.43658, which has a comparable amplitude limit to J1640 and J1709.

oscillations↗
DOE OSTI2022

Discovery, Timing, and Multiwavelength Observations of the Black Widow Millisecond Pulsar PSR J1555–2908

Abstract We report the discovery of PSR J1555−2908, a 1.79 ms radio and gamma-ray pulsar in a 5.6 hr binary system with a minimum companion mass of 0.052 M ⊙ . This fast and energetic ( E ̇ = 3 × 10 35 erg s −1 ) millisecond pulsar was first detected as a gamma-ray point source in Fermi Large Area Telescope (LAT) sky survey observations. Guided by a steep-spectrum radio point source in the Fermi error region, we performed a search at 820 MHz with the Green Bank Telescope that first discovered the pulsations. The initial radio pulse timing observations provided enough information to seed a search for gamma-ray pulsations in the LAT data, from which we derive a timing solution valid for the full Fermi mission. In addition to the discovery and timing of radio and gamma-ray pulsations, we searched for X-ray pulsations using NICER but no significant pulsations were detected. We also obtained time-series r -band photometry that indicates strong heating of the companion star by the pulsar wind. Material blown off the heated companion eclipses the 820 MHz radio pulse during inferior conjunction of the companion for ≈10% of the orbit, which is twice the angle subtended by its Roche lobe in an edge-on system.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Multiwavelength Observations of a New Redback Millisecond Pulsar 4FGL J1910.7-5320

We present the study of multiwavelength observations of an unidentified Fermi Large Area Telescope (LAT) source, 4FGL J1910.7-5320, a new candidate redback millisecond pulsar binary. In the 4FGL 95% error region of 4FGL J1910.7-5320, we find a possible binary with a 8.36 hr orbital period from the Catalina Real-Time Transient Survey, confirmed by optical spectroscopy using the SOAR telescope. This optical source was recently independently discovered as a redback pulsar by the TRAPUM project, confirming our prediction. We fit the optical spectral energy distributions of 4FGL J1910.7-5320 with a blackbody model, inferring a maximum distance of 4.1 kpc by assuming that the companion fills its Roche lobe with a radius of R = 0.7 R ⊙ . Using a 12.6 ks Chandra X-ray observation, we identified an X-ray counterpart for 4FGL J1910.7-5320, with a spectrum that can be described by an absorbed power law with a photon index of 1.0 ± 0.4. The spectrally hard X-ray emission shows tentative evidence for orbital variability. Using more than 12 yr of Fermi-LAT data, we refined the position of the γ-ray source, and the optical candidate still lies within the 68% positional error circle. In addition to 4FGL J1910.7-5320, we find a variable optical source with a periodic signal of 4.28 hr inside the 4FGL catalog 95% error region of another unidentified Fermi source, 4FGL J2029.5-4237. However, the γ-ray source does not have a significant X-ray counterpart in an 11.7 ks Chandra observation, with a 3σ flux upper limit of 2.4 × 10 -14 erg cm -2 s -1 (0.3–7 keV). Moreover, the optical source is outside our updated Fermi-LAT 95% error circle. These observational facts all suggest that this new redback millisecond pulsar powers the gamma-ray source 4FGL J1910.7-5320 while 4FGL J2029.5-4237 is unlikely the γ-ray counterpart to the 4.28 hr variable.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

A Multiwavelength Investigation of PSR J2229+6114 and its Pulsar Wind Nebula in the Radio, X-Ray, and Gamma-Ray Bands

Abstract G106.3+2.7, commonly considered to be a composite supernova remnant (SNR), is characterized by a boomerang-shaped pulsar wind nebula (PWN) and two distinct (“head” and “tail”) regions in the radio band. A discovery of very-high-energy gamma-ray emission (E γ > 100 GeV) followed by the recent detection of ultrahigh-energy gamma-ray emission (E γ > 100 TeV) from the tail region suggests that G106.3+2.7 is a PeVatron candidate. We present a comprehensive multiwavelength study of the Boomerang PWN (100″ around PSR J2229+6114) using archival radio and Chandra data obtained two decades ago, a new NuSTAR X-ray observation from 2020, and upper limits on gamma-ray fluxes obtained by Fermi-LAT and VERITAS observatories. The NuSTAR observation allowed us to detect a 51.67 ms spin period from the pulsar PSR J2229+6114 and the PWN emission characterized by a power-law model with Γ = 1.52 ± 0.06 up to 20 keV. Contrary to the previous radio study by Kothes et al., we prefer a much lower PWNB-field (B∼ 3μG) and larger distance (d∼ 8 kpc) based on (1) the nonvarying X-ray flux over the last two decades, (2) the energy-dependent X-ray size of the PWN resulting from synchrotron burn-off, and (3) the multiwavelength spectral energy distribution (SED) data. Our SED model suggests that the PWN is currently re-expanding after being compressed by the SNR reverse shock ∼1000 yr ago. In this case, the head region should be formed by GeV–TeV electrons injected earlier by the pulsar propagating into the low-density environment.

Astronomy & Astrophysics↗
DOE OSTI2024

A Model for Pair Production Limit Cycles in Pulsar Magnetospheres

Abstract It was recently proposed that the electric field oscillation as a result of self-consistent e ± pair production may be the source of coherent radio emission from pulsars. Direct particle-in-cell simulations of this process have shown that the screening of the parallel electric field by this pair cascade manifests as a limit cycle, as the parallel electric field is recurrently induced when pairs produced in the cascade escape from the gap region. In this work, we develop a simplified time-dependent kinetic model of e ± pair cascades in pulsar magnetospheres that can reproduce the limit-cycle behavior of pair production and electric field screening. This model includes the effects of a magnetospheric current, the escape of e ± , as well as the dynamic dependence of pair production rate on the plasma density and energy. Using this simple theoretical model, we show that the power spectrum of electric field oscillations averaged over many limit cycles is compatible with the observed pulsar radio spectrum.

Astronomy & Astrophysics↗
NASA NTRS1971

Second decrease in the period of the Vela Pulsar.

Preliminary analysis of the period discontinuities of the Vela Pulsar observed in 1969 and 1971. The fractional change in period for the 1971 discontinuity is -0.000002, slightly less than the magnitude of the previous change. The time span between the discontinuities is 2.50 plus or minus 0.03 yr; the uncertainty is determined by the time span between observations. It is pointed out that the Vela Pulsar could very well be as old as the supernova remnant Vela X (Shklovsky, 1970), which apparently surrounds the pulsar (Brandt et al., 1971).

Reichley, P. E.↗
NASA NTRS1972

The measurement of pulsar periods.

The method used for determining the pulsar period is discussed. In connection with the computation of the time of arrival of a pulse the right ascension and declination of the earth relative to the barycenter, the center of mass of the solar system, are calculated. The data obtained together with the position of the pulsar are used to calculate what the time of arrival would have been had the antenna been at the barycenter. A matrix approach is used for series observations. Pulsar positions obtained are listed in a table. A graph is presented showing the rate of change of the period as a function of the period for several sources.

Downs, G. S.↗
NASA NTRS1972

Pulsar observations and neutron star models

Information about the physical parameters of neutron stars is obtained from pulsar observations. The energy balance of the Crab nebula and the Vela X remnant allows derivation of limits for the masses of the Crab and Vela pulsars. Glitch observations provide further clues on the masses of these two pulsars. The degree of confidence in the derived numbers is pointed out. The possibility of observing neutron stars in binary systems as pulsating X-ray sources is discussed. The importance of observing redshifted gamma ray lines from the surface of neutron stars, and thus directly measuring either individual or statistical properties of these objects is pointed out.

Boerner, G.↗
NASA NTRS1973

The magnetospheric structure of pulsars

A model of pulsar magnetospheres is described which has evolved inductively from the work of Sturrock, where the radiation is produced near the surface of a neutron star. Some of the theoretical ideas of others, particularly those of Sturrock, are discussed. The braking index n and period-pulse-width distribution of pulsars are first reinvestigated by relaxing the conventional assumption that R sub Y = R sub L, where R sub Y is the radius of the neutral points marking the transition from closed to open magnetic field lines, and R sub L is the radius of the light cylinder. This is replaced by the parameterization R sub Y = R sub * (1- eta )power R sub L (eta), where R sub * is the neutron star radius. If the ratio frequency radiation is created near the surface and beamed along open field lines, it is found that a good fit to the period-pulse-width distribution can be obtained for eta in the range 0.5 = or eta = or 0.7. The relation n = 1 + 2 eta then gives n = 2.2 + or - 0.2, which is in good agreement with the values measured for the Crab pulsar.

Roberts, D. H.↗
NASA NTRS1972

Stellar evolution and pulsars.

It has been found that pulsars are rotating magnetic neutron stars, which are created during catastrophic collapses of old stars whose nuclear fuel has long since been used up. The maximum size of pulsars, based on the fastest rotation period of 33 msec, cannot exceed 100 km. The densest star the theory predicts is the neutron star. Its diameter is only 10 km. The processes producing radiation from pulsars are discussed, giving attention to a process similar to that by which a klystron operates and to a process based on a maser mechanism.

Chiu, H.-Y.↗
NASA NTRS1973

VLBI observations of the Crab Nebula pulsar.

Observations of the Crab Nebula pulsar at meter wavelengths using very long baseline interferometry techniques have been made. At 196.5 MHz, no resolution of the pulsar is observed, all the pulse shapes observed with the interferometers are similar to single-dish profiles, and all the power pulsates. At 111.5 MHz, in addition to the pulsing power, there is always a steady component, presumably due to multipath propagation effects. The pulsar is slightly resolved at 111.5 MHz with an apparent angular diameter of 0.07 plus or minus 0.01 sec. Fifty per cent linear polarization of the time-averaged power is observed at 196.5 MHz; at 111.5 MHz, 20% of the total time-averaged power is polarized, and 35% of the pulsing power is polarized. The steady component is unpolarized. The total flux of the steady plus pulsating component appears to remain constant, while the distribution of power between these components varies.

Vandenberg, N. R.↗
NASA NTRS1974

Electromagnetic processes in the atmosphere of pulsars

The work consists of two parts. The first deals with the fine structure of radio pulses. Based on kinetic theory, processes occurring in the plasma shell of a pulsar when external electromagnetic radiation is present are investigated. It is shown that electromagnetic waves cause electrons to drift relative to ions, and initiate longitudinal oscillations. A dispersion equation describing the longitudinal oscillations in magnetized plasma is derived. Conditions for excitation of oscillations are found. Correlation functions of electron density are calculated, along with the coefficients of electromagnetic wave scattering. It is shown that variations in the amplitude of pulsar pulses are associated with scintillations caused by fluctuations in the plasma electron density. The second part of the study presents a mechanism for the radio emission of pulsars. The model of a rotating and a pulsating star, a neutron star with dipolar or more complex magnetic field, is examined.

Yukhimuk, A. K.↗
NASA NTRS1974

Pulsar flux-density spectra

Time-averaged flux-density spectra of ten pulsars have been obtained on the basis of simultaneous observations at six frequencies between 250 and 8085 MHz. The effects of interstellar scintillation on the accuracy of the measurements are discussed. All of the pulsars exhibited a power-law spectrum at high frequencies with spectral indices between negative 1.6 and negative 3.0. Several of the pulsars show a more positive spectral index at the low-frequency end of the data.

Backer, D. C.↗
NASA NTRS1974

An upper limit on soft X-ray pulsations from the pulsar PSR 0833-45

The Vela pulsar PSR 0833-45, located in the Vela X nebula, has the second shortest period of any of the known radio pulsars. Soft X-ray observations are described which place an upper limit of 0.3% on the pulsed fraction of the Vela pulsar relative to the flux from the entire Vela X Nebula, in the 0.5- to 1.0-keV energy band. The present results are about a factor of 7 below the value of 2% measured by Harnden and Gorenstein (1973) for the pulsed fraction relative to the flux from the Vela X Nebula.

Moore, W. E.↗
NASA NTRS1974

Pulsar observations and neutron star models

Information about the physical parameters of neutron stars is obtained from pulsar observations. The energy balance of the Crab Nebula and the Vela X remnant allows one to derive limits for the masses of the Crab and Vela pulsars. Glitch observations provide further clues on the masses of these two pulsars. The degree of confidence with which one should believe the derived numbers is pointed out. The possibility of observing neutron stars in binary systems as pulsating X-ray sources is discussed. Finally, the importance of observing redshifted gamma ray lines from the surface of neutron stars, and thus directly measuring either individual or statistical properties of these objects, is pointed out.

Boerner, G.↗
NASA NTRS1975

SAS-2 High energy gamma-ray observations of the Vela pulsar

The second Small Astronomy Satellite high-energy (35 MeV) gamma-ray telescope detected pulsed gamma-ray emission at the radio period from PSR 0833-45, the Vela pulsar, as well as an unpulsed flux from the Vela region. The pulsed emission consists of two peaks, one following the radio peak by about 13 msec, and the other 0.4 period after the first. The luminosity of the pulsed emission above 100 MeV from Vela is about 0.1 that of the pulsar NP0532 in the Crab nebula, whereas the pulsed emission from Vela at optical wavelengths is less than 0.0004 that from the Crab. The relatively high intensity of the pulsed gamma-ray emission and the double peak structure, compared to the single pulse in the radio emission, suggests that the high energy gamma-ray pulsar emission may be produced under different conditions from those found at lower energies.

Thompson, D. J.↗