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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Magnetic activity in pre-main-sequence stars

The extensive new evidence for extremely high levels of magnetic activity on the surface of premain-sequence stars is reviewed and analyzed in the context of solar evolution. Most of these observations concern 'weak' T Tauri stars, in which the surface properties of the young stars are not dominated by circumstellar matter as in 'classical' T Tauri stars. Optical photometry shows that weak T Tauria stars are moderately rapid rotaters with cool starspots covering 5 to 40 percent of the surface. Optical and ultraviolet spectroscopy indicates that their chromospheres are enhanced about 50 times over contemporary solar levels. X-ray and radio intensities are of order 1000 to 100,000 times greater than peak solar levels, and they vary on time scales of days and (in a few cases) hours or minutes. The variability can only be explained in terms of some extremely powerful magnetic-flare model. The large numbers of weak T Tauri stars, probably larger than the population of classical T Tauri stars, strongly suggests that the sun was a weak T Tauri star for at least part of its premain-sequence evolutionary phase. These findings are summarized in a suggested portrait of the sun as a 1 Myr old weak T Tauri star.

Feigelson, Eric D.↗
DOE OSTI2014

THE PECULIAR PULSAR POPULATION OF THE CENTRAL PARSEC

Pulsars orbiting the Galactic center black hole, Sgr A*, would be potential probes of its mass, distance, and spin, and may even be used to test general relativity. Despite predictions of large populations of both ordinary and millisecond pulsars in the Galactic center, none have been detected within 25 pc by deep radio surveys. One explanation has been that hyperstrong temporal scattering prevents pulsar detections, but the recent discovery of radio pulsations from a highly magnetized neutron star (magnetar) within 0.1 pc shows that the temporal scattering is much weaker than predicted. We argue that an intrinsic deficit in the ordinary pulsar population is the most likely reason for the lack of detections to date: a ''missing pulsar problem'' in the Galactic center. In contrast, we show that the discovery of a single magnetar implies efficient magnetar formation in the region. If the massive stars in the central parsec form magnetars rather than ordinary pulsars, their short lifetimes could explain the missing pulsars. Efficient magnetar formation could be caused by strongly magnetized progenitors, or could be further evidence of a top-heavy initial mass function. Furthermore, current high-frequency surveys should already be able to detect bright millisecond pulsars, given the measured degree of temporal scattering.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1979

The ultraviolet flux distribution of Alpha-2 Canum Venaticorum

Intermediate- and narrow-band UV spectrophotometry from Copernicus, OAO 2, the S2-68 experiment on TD 1, and a sounding-rocket experiment are combined with ground-based observations to define the absolute flux distribution of the bright magnetic Ap star Alpha-2 CVn over the wavelength range from 1030 to 7580 A. Two flux distributions are presented which coincide more or less with the rare-earth maximum and minimum in the star's cycle. The results are compared with those for two normal stars, the UV variability of Alpha-2 CVn is characterized as a function of wavelength, and non-LTE effects on the UV continua of C I and Si I are analyzed. Some physical properties of Alpha-2 CVn are estimated, and the evolutionary age of the star is estimated to be between 220 million and 300 million years. It is concluded that Alpha-2 CVn is old enough to have been magnetically braked to its current rotational velocity by either the centrifugal-wind or the accretion mechanism and to have developed its chemical peculiarities by either mechanism.

Leckrone, D. S.↗
NASA NTRS1991

Basic research in solar physics

This grant, dating back more than 20 years has supported a variety of investigations of the chromospheres and coronae of the Sun and related cool stars by the Principal Investigator, his postdocs and graduate students, and colleagues at other institutions. This work involved studies of radiative transfer and spectral line formation theory, and the application of these techniques to the analysis of spectra obtained from space and ground-based observatories in the optical, ultraviolet, x-ray and radio portions of the spectrum. Space observations have included the analysis of spectra from OSO-7, Skylab, SMM, and the HRTS rocket experiments. Recent work has concentrated on the interaction of magnetic fields, plasma and radiation in the outer atmospheres of the Sun and other magnetically active stars with different fundamental parameters. Our study of phenomena common to the Sun and stars, the 'solar-stellar connection', can elucidate the fundamental physics, because spatially-resolved observations of the Sun provide us with the 'groundtruth,' while interpretation of stellar data permit us to isolate those parameters critical to stellar activity. Recently, we have studied the differences in physical properties between solar regions of high magnetic flux density and the surrounding plasma. High-resolution CN and CO spectroheliograms have been used to model the thermal inhomogeneities driven by unstable CO cooling, and we have analyzed spatially resolved UV spectra from HRTS to model the thermal structure and energy balance of small-scale structures. The study of nonlinear relations between atmospheric radiative losses and the photospheric magnetic flux density has been continued. We have also proposed a new model for the decay of plages by random walk diffusion of magnetic flux. Our analysis of phenomena common to the Sun and stars included the application of available spectroscopic diagnostics, establishing evidence that the atmospheres of the least active stars are heated at a 'basal' rate that is also found in the centers of solar supergranules, and using the Doppler-imaging technique to measure the position, size, and brightness of stellar active regions. We are computing multi-component models for solar and stellar atmospheres, and models for coronal loops and for the transition-region down flows. The study of solar and stellar flares permits us to assess the role of turbulent energy transport, to pinpoint the mechanism behind Type I radio bursts, to determine whether plasma radiation or cyclotron maser is responsible for microwave flares on M dwarfs, and to extend our knowledge of the basic physics pertinent to cyclotron-maser processes operating on the Sun.

Linsky, Jeffrey L.↗
DOE OSTI2021

Broadband X-ray burst spectroscopy of the fast-radio-burst-emitting Galactic magnetar

Magnetars are young, magnetically powered neutron stars that possess the strongest magnetic fields in the Universe. Fast radio bursts (FRBs) are extremely intense millisecond-long radio pulses of primarily extragalactic origin, and a leading attribution for their genesis focuses on magnetars. A hallmark signature of magnetars is their emission of bright, hard X-ray bursts of sub-second duration. On 27 April 2020, the Galactic magnetar SGR J1935+2154 emitted hundreds of X-ray bursts within a few hours. One of these temporally coincided with an FRB, the first known detection of an FRB from the Milky Way. Here, we present spectral and temporal analyses of 24 X-ray bursts emitted 13 hours prior to the FRB and seen simultaneously with the Neutron Star Interior Composition Explorer (NICER) mission of the National Aeronautics and Space Administration and with the Fermi Gamma-ray Burst Monitor (GBM) mission in their combined energy range of 0.2 keV to 30 MeV. These broadband spectra permit direct comparison with the spectrum of the FRB-associated X-ray burst (FRB-X). We demonstrate that all 24 NICER and GBM bursts are very similar temporally to the FRB-X, but strikingly different spectrally. The singularity of the FRB-X burst is perhaps indicative of an uncommon locale for its origin. We suggest that this event originated in quasi-polar open or closed magnetic field lines that extend to high altitudes.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2021

Broadband X-ray burst spectroscopy of the fast-radio-burst-emitting Galactic magnetar

Magnetars are young, magnetically powered neutron stars that possess the strongest magnetic fields in the Universe. Fast radio bursts (FRBs) are extremely intense millisecond-long radio pulses of primarily extragalactic origin, and a leading attribution for their genesis focuses on magnetars. A hallmark signature of magnetars is their emission of bright, hard X-ray bursts of sub-second duration. On 27 April 2020, the Galactic magnetar SGR J1935+2154 emitted hundreds of X-ray bursts within a few hours. One of these temporally coincided with an FRB, the first known detection of an FRB from the Milky Way. Here, we present spectral and temporal analyses of 24 X-ray bursts emitted 13 hours prior to the FRB and seen simultaneously with the Neutron Star Interior Composition Explorer (NICER) mission of the National Aeronautics and Space Administration and with the Fermi Gamma-ray Burst Monitor (GBM) mission in their combined energy range of 0.2 keV to 30 MeV. These broadband spectra permit direct comparison with the spectrum of the FRB-associated X-ray burst (FRB-X). We demonstrate that all 24 NICER and GBM bursts are very similar temporally to the FRB-X, but strikingly different spectrally. The singularity of the FRB-X burst is perhaps indicative of an uncommon locale for its origin. We suggest that this event originated in quasi-polar open or closed magnetic field lines that extend to high altitudes.

G. Younes↗
DOE OSTI2014

ANTI-GLITCH INDUCED BY COLLISION OF A SOLID BODY WITH THE MAGNETAR 1E 2259+586

Glitches have been frequently observed in neutron stars. Previously, these glitches have unexceptionally manifested as sudden spin-ups that can be explained as being due to impulsive transfer of angular momentum from the interior superfluid component to the outer solid crust. Alternatively, they may also be due to large-scale crust-cracking events. However, an unprecedented anti-glitch was recently reported for the magnetar 1E 2259+586, which clearly exhibited a sudden spin-down, strongly challenging previous glitch theories. Here we show that the anti-glitch can be well explained by the collision of a small solid body with the magnetar. The intruder has a mass of about 1.1 × 10{sup 21} g. Its orbital angular momentum is assumed to be antiparallel to that of the spinning magnetar, so that the sudden spin-down can be naturally accounted for. The observed hard X-ray burst and decaying softer X-ray emission associated with the anti-glitch can also be reasonably explained. Our study indicates that a completely different type of glitch due to collisions between small bodies and neutron stars should exist and may have already been observed previously. It also hints at a new way of studying capture events by neutron stars: through accurate timing observations of pulsars.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1984

Gravitational radiation from accreting neutron stars

Attention is given to the evolution of neutron stars whose magnetic fields are so weak that (1) the effects of the magnetosphere on the accretion disk are negligible, allowing it to extend to the surface of the star; and (2) the loss of energy and angular momentum by the pulsar mechanism is negligible compared with that provided by the accretion disk. The small nonaxisymmetric distortion of the neutron star should produce some modulation of the X-ray emission at the gravitational wave frequency, and it is hypothesized that this frequency will lie in the 200-800 Hz range. This is significantly below the rotational frequency of the neutron star.

Wagoner, R. V.↗
DOE OSTI2016

TIMING OBSERVATIONS OF PSR J1023+0038 DURING A LOW-MASS X-RAY BINARY STATE

Transitional millisecond pulsars (tMSPs) switch, on roughly multi-year timescales, between rotation-powered radio millisecond pulsar (RMSP) and accretion-powered low-mass X-ray binary (LMXB) states. The tMSPs have raised several questions related to the nature of accretion flow in their LMXB state and the mechanism that causes the state switch. The discovery of coherent X-ray pulsations from PSR J1023+0038 (while in the LMXB state) provides us with the first opportunity to perform timing observations and to compare the neutron star’s spin variation during this state to the measured spin-down in the RMSP state. Whereas the X-ray pulsations in the LMXB state likely indicate that some material is accreting onto the neutron star’s magnetic polar caps, radio continuum observations indicate the presence of an outflow. The fraction of the inflowing material being ejected is not clear, but it may be much larger than that reaching the neutron star’s surface. Timing observations can measure the total torque on the neutron star. We have phase-connected nine XMM-Newton observations of PSR J1023+0038 over the last 2.5 years of the LMXB state to establish a precise measurement of spin evolution. We find that the average spin-down rate as an LMXB is 26.8 ± 0.4% faster than the rate (−2.39 × 10{sup −15} Hz s{sup −1}) determined during the RMSP state. This shows that negative angular momentum contributions (dipolar magnetic braking, and outflow) exceed positive ones (accreted material), and suggests that the pulsar wind continues to operate at a largely unmodified level. We discuss implications of this tight observational constraint in the context of possible accretion models.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1990

Long and short timescale variability of magnetic activity on the BY Dra star BD+26 deg 730

BD+26 degree 730 is a very active K5V flare star with a 60 year starspot cycle. Since it is also nearly pole-on, it presents an unusual opportunity to study purely temporal evolution of magnetic activity. Contemporaneous International Ultraviolet Explorer (IUE), magnetic flux, polarimetric, and H alpha observations of the star taken over several rotational periods in 1988 are presented. When combined with older photometry, optical and IUE archive spectra, the data show a surprising lack of any activity variability on either short (few days) or long (years) timescales. The lack of variability is suggested to be due to a nearly saturated level of magnetic activity on the star, as indicated by its large magnetic filling factor.

Saar, S. H.↗
DOE OSTI2013

POPULATION SYNTHESIS OF YOUNG ISOLATED NEUTRON STARS: THE EFFECT OF FALLBACK DISK ACCRETION AND MAGNETIC FIELD EVOLUTION

The spin evolution of isolated neutron stars (NSs) is dominated by their magnetic fields. The measured braking indices of young NSs show that the spin-down mechanism due to magnetic dipole radiation with constant magnetic fields is inadequate. Assuming that the NS magnetic field is buried by supernova fallback matter and re-emerges after accretion stops, we carry out a Monte Carlo simulation of the evolution of young NSs, and show that most of the pulsars have braking indices ranging from –1 to 3. The results are compatible with the observational data of NSs associated with supernova remnants. They also suggest that the initial spin periods of NSs might occupy a relatively wide range.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1984

Self-consistent models for Coulomb-heated X-ray pulsar atmospheres

Calculations of accreting magnetized neutron star atmospheres heated by the gradual deceleration of Protons via Coulomb collisions are presented. Self consistent determinations of the temperature and density structure for different accretion rates are made by assuming hydrostatic equilibrium and energy balance, coupled with radiative transfer. The full radiative transfer in two polarizations, using magnetic cross sections but with cyclotron resonance effects treated approximately, is carried out in the inhomogeneous atmospheres. Previously announced in STAR as N84-12012

Harding, A. K.↗
DOE OSTI2017

A Measurement of Radius Inflation in the Pleiades and Its Relation to Rotation and Lithium Depletion

Precise measurements of eclipsing binary parameters and statistical studies of young clusters have suggested that some magnetically active low-mass dwarfs possess radii inflated by ∼5%–15% relative to theoretical expectations. If true, this effect should be pronounced in young open clusters, due to the rapid rotation and strong magnetic activity of their most extreme members. We explore this possibility by determining empirical radii for 83 members of the nearby Pleiades open cluster, using spectral energy distribution fitting to establish F{sub bol} with a typical accuracy of ≈3% together with color and spectro-photometric indices to determine T{sub eff}. We find several Pleiades members with radii inflated above radius-T{sub eff} models from state-of-the-art calculations, and apparent dispersions in radii for the K-dwarfs of the cluster. Moreover, we demonstrate that this putative radius inflation correlates strongly with rotation rate, consistent with inflation of young stars by magnetic activity and/or starspots. We argue that this signal is not a consequence of starspot-induced color anomalies, binarity, or depth effects in the cluster, employing Gaia DR1 distances as a check. Finally, we consider the lithium abundances of these stars, demonstrating a triple correlation between rotation rate, radius inflation, and enhanced lithium abundance. Our result—already significant to ∼99.99% confidence—provides strong support for a magnetic origin of the inflated radii and lithium dispersion observed in young, low-mass stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

SPECTROPOLARIMETRY OF THE CLASSICAL T TAURI STAR BP TAU

We implement a least-squares deconvolution (LSD) code to study magnetic fields on cool stars. We first apply our code to high-resolution optical echelle spectra of 53 Cam (a magnetic Ap star) and three well-studied cool stars (Arcturus, 61 Cyg A, and ξ Boo A) as well as the Sun (by observing the asteroid Vesta) as tests of the code and the instrumentation. Our analysis is based on several hundred photospheric lines spanning the wavelength range 5000 Å to 9000 Å. We then apply our LSD code to six nights of data on the Classical T Tauri Star BP Tau. A maximum longitudinal field of 370 ± 80 G is detected from the photospheric lines on BP Tau. A 1.8 kG dipole tilted at 129° with respect to the rotation axis and a 1.4 kG octupole tilted at 104° with respect to the rotation axis, both with a filling factor of 0.25, best fit our LSD Stokes V profiles. Measurements of several emission lines (He I 5876 Å, Ca II 8498 Å, and 8542 Å) show the presence of strong magnetic fields in the line formation regions of these lines, which are believed to be the base of the accretion footpoints. The field strength measured from these lines shows night-to-night variability consistent with rotation of the star.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1990

Determining the properties of accretion-gap neutron stars

If neutron stars have radii as small as has been argued by some, observations of accretion-powered X-rays could verify the existence of innermost stable circular orbits (predicted by general relativity) around weakly magnetized neutron stars. This may be done by detecting X-ray emission from clumps of matter before and after they cross the gap (where matter cannot be supported by rotation) between the inner accretion disk and the stellar surface. Assuming the validity of general relativity, it would then be possible to determine the masses of such neutron stars independently of any knowledge of binary orbital parameters. If an accurate mass determination were already available through any of the methods conventionally used, the new mass determination method proposed here could then be used to quantitatively test strong field effects of gravitational theory.

Kluzniak, Wlodzimierz↗
NASA NTRS1989

Timing analysis of binary X ray pulsars observed by HEAO 1

Timing analysis of the pointed observations of 5 selected binary X ray pulsars, in two categories of disk-fed and wind-fed sources, by HEAO 1 A-2 is reported. The power spectral analysis was performed on the data in the frequency range from approx. 1 MHz to 6.25 Hz. The coherent signal of the pulsation, the continuum of the power spectrum, varies in time and differs among sources. Quasi-Periodic Oscillation (QPO) is probably related to a fast spinning but weakly magnetized neutron star in the low mass X ray binaries. QPO was searched for in this frequency range to see if scaling laws exist among these two systems which may have possessed different order of magnitude of magnetic field strengths and the inner disk radii. One possible QPO is found centering at 0.062 Hz in 4U0115+63 during a flare.

Soong, Y.↗
DOE OSTI2015

COMMON PATTERNS IN THE EVOLUTION BETWEEN THE LUMINOUS NEUTRON STAR LOW-MASS X-RAY BINARY SUBCLASSES

The X-ray transient XTE J1701–462 was the first source observed to evolve through all known subclasses of low-magnetic-field neutron star low-mass X-ray binaries (NS-LMXBs), as a result of large changes in its mass accretion rate. To investigate to what extent similar evolution is seen in other NS-LMXBs we have performed a detailed study of the color–color and hardness–intensity diagrams (CDs and HIDs) of Cyg X-2, Cir X-1, and GX 13+1—three luminous X-ray binaries, containing weakly magnetized neutron stars, known to exhibit strong secular changes in their CD/HID tracks. Using the full set of Rossi X-ray Timing Explorer Proportional Counter Array data collected for the sources over the 16 year duration of the mission, we show that Cyg X-2 and Cir X-1 display CD/HID evolution with close similarities to XTE J1701–462. Although GX 13+1 shows behavior that is in some ways unique, it also exhibits similarities to XTE J1701–462, and we conclude that its overall CD/HID properties strongly indicate that it should be classified as a Z source, rather than as an atoll source. We conjecture that the secular evolution of Cyg X-2, Cir X-1, and GX 13+1—illustrated by sequences of CD/HID tracks we construct—arises from changes in the mass accretion rate. Our results strengthen previous suggestions that within single sources Cyg-like Z source behavior takes place at higher luminosities and mass accretion rates than Sco-like Z behavior, and lend support to the notion that the mass accretion rate is the primary physical parameter distinguishing the various NS-LMXB subclasses.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Preface for frontiers of magnetic reconnection research in heliophysical, astrophysical, and laboratory plasmas

Magnetic reconnection—the topological rearrangement of magnetic field—underlies many explosive phenomena across a wide range of natural and laboratory plasmas.3 It plays a pivotal role in electron and ion heating, particle acceleration to high energies, energy transport, and self-organization. Reconnection can have a complex relationship with turbulence at both large and small scales, leading to various effects that are only beginning to be understood. In heliophysics, magnetic reconnection plays a key role in solar flares, coronal mass ejections, coronal heating, solar wind dissipation, the interaction of interplanetary plasma with magnetospheres, dynamics of planetary magnetospheres such as magnetic substorms, and the heliospheric boundary with the interstellar medium. Here, the magnetic reconnection is integral to the solar and planetary dynamo processes.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗