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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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At least 505 records · Page 28
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DOE OSTI2014

JVLA observations of IC 348 SW: Compact radio sources and their nature

We present sensitive 2.1 and 3.3 cm Jansky Very Large Array radio continuum observations of the region IC 348 SW. We detect a total of 10 compact radio sources in the region, 7 of which are first reported here. One of the sources is associated with the remarkable periodic time-variable infrared source LRLL 54361, opening the possibility of monitoring this object at radio wavelengths. Four of the sources appear to be powering outflows in the region, including HH 211 and HH 797. In the case of the rotating outflow HH 797, we detect a double radio source at its center, separated by ∼3''. Two of the sources are associated with infrared stars that possibly have gyrosynchrotron emission produced in active magnetospheres. Finally, three of the sources are interpreted as background objects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Observation and modeling of geocoronal charge exchange X-ray emission during solar wind gusts

Solar wind charge exchange (SWCX) X-rays are emitted when highly charged solar wind ions such as O{sup 7{sup +}} collide with neutral gas, including the Earth's tenuous outer atmosphere (exosphere or geocorona) and hydrogen and helium from the local interstellar medium drifting through the heliosphere. This geocoronal and heliospheric emission comprises a significant and varying fraction of the soft X-ray background (SXRB) and is seen in every X-ray observation, with the intensity dependent on solar wind conditions and observation geometry. Under the right conditions, geocoronal emission can increase the apparent SXRB by roughly an order of magnitude for an hour or more. In this work, we study a dozen occasions when the near-Earth solar wind flux was exceptionally high. These gusts of wind lead to abrupt changes in SWCX X-ray emission around Earth, which may or may not be seen by X-ray observatories depending on their line of sight. Using detailed three-dimensional magnetohydrodynamical simulations of the solar wind's interaction with the Earth's magnetosphere, and element abundances and ionization states measured by ACE, we model the time-dependent brightness of major geocoronal SWCX emission lines during those gusts and compare with changes in the X-ray background measured by the Chandra X-ray Observatory. We find reasonably good agreement between model and observation, with measured geocoronal line brightnesses averaged over 1 hr of up to 136 photons s{sup –1} cm{sup –2} sr{sup –1} in the O VII Kα triplet around 564 eV.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Frequency independent quenching of pulsed emission

Simultaneous observations at four different frequencies, 313, 607, 1380, and 4850 MHz, for three pulsars, PSRs B0031–07, B0809+74, and B2319+60, are reported in this paper. Identified null and burst pulses are highly concurrent across more than a decade of frequency. A small fraction of non-concurrent pulses (≤3%) is observed, most of which occur at the transition instances. We report, with very high significance for the first time, the full broadband nature of the nulling phenomenon in these three pulsars. These results suggest that nulling invokes changes on the global magnetospheric scale.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ACCRETING CIRCUMPLANETARY DISKS: OBSERVATIONAL SIGNATURES

I calculate the spectral energy distributions of accreting circumplanetary disks using atmospheric radiative transfer models. Circumplanetary disks only accreting at 10{sup –10} M {sub ☉} yr{sup –1} around a 1 M{sub J} planet can be brighter than the planet itself. A moderately accreting circumplanetary disk ( M-dot ∼10{sup −8} M{sub ⊙} yr{sup −1}; enough to form a 10 M{sub J} planet within 1 Myr) around a 1 M{sub J} planet has a maximum temperature of ∼2000 K, and at near-infrared wavelengths (J, H, K bands), this disk is as bright as a late-M-type brown dwarf or a 10 M{sub J} planet with a ''hot start''. To use direct imaging to find the accretion disks around low-mass planets (e.g., 1 M{sub J} ) and distinguish them from brown dwarfs or hot high-mass planets, it is crucial to obtain photometry at mid-infrared bands (L', M, N bands) because the emission from circumplanetary disks falls off more slowly toward longer wavelengths than those of brown dwarfs or planets. If young planets have strong magnetic fields (≳100 G), fields may truncate slowly accreting circumplanetary disks ( M-dot ≲10{sup −9} M{sub ⊙} yr{sup −1}) and lead to magnetospheric accretion, which can provide additional accretion signatures, such as UV/optical excess from the accretion shock and line emission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

CIRCULAR POLARIZATION OF PULSAR WIND NEBULAE AND THE COSMIC-RAY POSITRON EXCESS

Recent observations by the PAMELA and AMS-02 telescopes have uncovered an anomalous rise in the positron fraction at energies above 10 GeV. One possible explanation for this excess is the production of primary electron/positron pairs through electromagnetic cascades in pulsar magnetospheres. This process results in a high multiplicity of electron/positron pairs within the wind-termination shock of pulsar wind nebulae (PWNe). A consequence of this scenario is that no circular polarization should be observed within PWNe, since the contributions from electrons and positrons exactly cancel. Here we note that current radio instruments are capable of setting meaningful limits on the circular polarization of synchrotron radiation in PWNe, which observationally test the model for pulsar production of the local positron excess. The observation of a PWN with detectable circular polarization would cast strong doubt on pulsar interpretations of the positron excess, while observations setting strong limits on the circular polarization of PWNe would lend credence to these models. Finally, we indicate which PWNe are likely to provide the best targets for observational tests of the AMS-02 excess.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ON THE ORIGIN OF RADIO EMISSION FROM MAGNETARS

Magnetars are the most magnetized objects in the known universe. Powered by the magnetic energy, and not by the rotational energy as in the case of radio pulsars, they have long been regarded as a completely different class of neutron stars. The discovery of pulsed radio emission from a few magnetars weakened the idea of a clean separation between magnetars and normal pulsars. We use the partially screened gap (PSG) model to explain radio emission of magnetars. The PSG model requires that the temperature of the polar cap is equal to the so-called critical value, i.e., the temperature at which the thermal ions outflowing from the stellar surface screen the acceleration gap. We show that a magnetar has to fulfill the temperature, power, and visibility conditions in order to emit radio waves. First, in order to form PSG, the residual temperature of the surface has to be lower than the critical value. Second, since the radio emission is powered by the rotational energy, it has to be high enough to enable heating of the polar cap by backstreaming particles to the critical temperature. Finally, the structure of the magnetic field has to be altered by magnetospheric currents in order to widen a radio beam and increase the probability of detection. Our approach allows us to predict whether a magnetar can emit radio waves using only its rotational period, period derivative, and surface temperature in the quiescent mode.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE CRAB PULSAR AT CENTIMETER WAVELENGTHS. I. ENSEMBLE CHARACTERISTICS

We have observed the pulsar in the Crab Nebula at high radio frequencies and high time resolution. We present continuously sampled data at 640 ns time resolution and individual bright pulses recorded at down to 0.25 ns time resolution. Combining our new data with previous data from our group and from the literature shows the dramatic changes in the pulsar’s radio emission between low and high radio frequencies. Below about 5 GHz the mean profile is dominated by the bright Main Pulse and Low-Frequency Interpulse. Everything changes, however, above about 5 GHz; the Main Pulse disappears and the mean profile of the Crab pulsar is dominated by the High-Frequency Interpulse (which is quite different from its low-frequency counterpart) and the two High-Frequency Components. We present detailed observational characteristics of these different components which future models of the pulsar’s magnetosphere must explain.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A BROADBAND RADIO STUDY OF THE AVERAGE PROFILE AND GIANT PULSES FROM PSR B1821-24A

We present the results of a wideband (720–2400 MHz) study of PSR B1821–24A (J1824–2452A, M28A), an energetic millisecond pulsar (MSP) visible in radio, X-rays and γ-rays. In radio, the pulsar has a complex average profile that spans ≳85% of the spin period and exhibits strong evolution with observing frequency. For the first time we measure phase-resolved polarization properties and spectral indices of radio emission throughout almost all of the on-pulse window. We synthesize our findings with high-energy information to compare M28A to other known γ-ray MSPs and to speculate that M28A’s radio emission originates in multiple regions within its magnetosphere (i.e., both in the slot or outer gaps near the light cylinder and at lower altitudes above the polar cap). M28A is one of a handful of pulsars that are known to emit giant radio pulses (GRPs)—short, bright radio pulses of unknown nature. We report a drop in the linear polarization of the average profile in both windows of GRP generation and also a “W”-shaped absorption feature (resembling a double notch), partly overlapping with one of the GRP windows. The GRPs themselves have broadband spectra consisting of multiple patches with Δν/ν ∼ 0.07. Although our time resolution was not sufficient to resolve the GRP structure on the μs scale, we argue that GRPs from this pulsar most closely resemble the GRPs from the main pulse of the Crab pulsar, which consist of a series of narrowband nanoshots.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

SIMULTANEOUS NuSTAR/CHANDRA OBSERVATIONS OF THE BURSTING PULSAR GRO J1744-28 DURING ITS THIRD REACTIVATION

We report on a 10 ks simultaneous Chandra/High Energy Transmission Grating (HETG)–Nuclear Spectroscopic Telescope Array (NuSTAR) observation of the Bursting Pulsar, GRO J1744-28, during its third detected outburst since discovery and after nearly 18 yr of quiescence. The source is detected up to 60 keV with an Eddington persistent flux level. Seven bursts, followed by dips, are seen with Chandra, three of which are also detected with NuSTAR. Timing analysis reveals a slight increase in the persistent emission pulsed fraction with energy (from 10% to 15%) up to 10 keV, above which it remains constant. The 0.5–70 keV spectra of the persistent and dip emission are the same within errors and well described by a blackbody (BB), a power-law (PL) with an exponential rolloff, a 10 keV feature, and a 6.7 keV emission feature, all modified by neutral absorption. Assuming that the BB emission originates in an accretion disk, we estimate its inner (magnetospheric) radius to be about 4 × 10{sup 7} cm, which translates to a surface dipole field B ≈ 9 × 10{sup 10} G. The Chandra/HETG spectrum resolves the 6.7 keV feature into (quasi-)neutral and highly ionized Fe xxv and Fe xxvi emission lines. XSTAR modeling shows these lines to also emanate from a truncated accretion disk. The burst spectra, with a peak flux more than an order of magnitude higher than Eddington, are well fit with a PL with an exponential rolloff and a 10 keV feature, with similar fit values compared to the persistent and dip spectra. The burst spectra lack a thermal component and any Fe features. Anisotropic (beamed) burst emission would explain both the lack of the BB and any Fe components.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ENERGY DISSIPATION IN MAGNETIC NULL POINTS AT KINETIC SCALES

We use kinetic particle-in-cell and MHD simulations supported by an observational data set to investigate magnetic reconnection in clusters of null points in space plasma. The magnetic configuration under investigation is driven by fast adiabatic flux rope compression that dissipates almost half of the initial magnetic field energy. In this phase powerful currents are excited producing secondary instabilities, and the system is brought into a state of “intermittent turbulence” within a few ion gyro-periods. Reconnection events are distributed all over the simulation domain and energy dissipation is rather volume-filling. Numerous spiral null points interconnected via their spines form null lines embedded into magnetic flux ropes; null point pairs demonstrate the signatures of torsional spine reconnection. However, energy dissipation mainly happens in the shear layers formed by adjacent flux ropes with oppositely directed currents. In these regions radial null pairs are spontaneously emerging and vanishing, associated with electron streams and small-scale current sheets. The number of spiral nulls in the simulation outweighs the number of radial nulls by a factor of 5–10, in accordance with Cluster observations in the Earth's magnetosheath. Twisted magnetic fields with embedded spiral null points might indicate the regions of major energy dissipation for future space missions such as the Magnetospheric Multiscale Mission.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A STATISTICAL SURVEY OF DYNAMIC PRESSURE PULSES IN THE SOLAR WIND BASED ON WIND OBSERVATIONS

Solar wind dynamic pressure pulse (DPP) structures, across which the dynamic pressure changes abruptly over timescales from a few seconds to several minutes, are often observed in the near-Earth space environment. The space weather effects of DPPs on the magnetosphere–ionosphere coupling system have been widely investigated in the last two decades. In this study, we perform a statistical survey on the properties of DPPs near 1 AU based on nearly 20 years of observations from the WIND spacecraft. It is found that only a tiny fraction of DPPs (around 4.2%) can be regarded as interplanetary shocks. For most DPPs, the total pressure (the sum of the thermal pressure and magnetic pressure) remains in equilibrium, but there also exists a small fraction of DPPs that are not pressure-balanced. The overwhelming majority of DPPs are associated with solar wind disturbances, including coronal mass ejection-related flows, corotating interaction regions, as well as complex ejecta. The annual variations of the averaged occurrence rate of DPPs are roughly in phase with the solar activity during solar cycle 23, and during the rising phase of solar cycle 24.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

OPTICAL HYDROGEN ABSORPTION CONSISTENT WITH A THIN BOW SHOCK LEADING THE HOT JUPITER HD 189733B

Bow shocks are ubiquitous astrophysical phenomena resulting from the supersonic passage of an object through a gas. Recently, pre-transit absorption in UV metal transitions of the hot Jupiter (HJ) exoplanets HD 189733b and WASP12-b have been interpreted as being caused by material compressed in a planetary bow shock. Here we present a robust detection of a time-resolved pre-transit, as well as in-transit absorption signature around the HJ exoplanet HD 189733b using high spectral resolution observations of several hydrogen Balmer lines. The line shape of the pre-transit feature and the shape of the timeseries absorption provide the strongest constraints on the morphology and physical characteristics of extended structures around an exoplanet. The in-transit measurements confirm the previous exospheric Hα detection, although the absorption depth measured here is ∼50% lower. The pre-transit absorption feature occurs 125 minutes before the predicted optical transit, a projected linear distance from the planet to the stellar disk of 7.2 R{sub p}. The absorption strength observed in the Balmer lines indicates an optically thick, but physically small, geometry. We model this signal as the early ingress of a planetary bow shock. If the bow shock is mediated by a planetary magnetosphere, the large standoff distance derived from the model suggests a large planetary magnetic field strength of B{sub eq} = 28 G. Better knowledge of exoplanet magnetic field strengths is crucial to understanding the role these fields play in planetary evolution and the potential development of life on planets in the habitable zone.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

HIGH-TEMPERATURE IONIZATION IN PROTOPLANETARY DISKS

We calculate the abundances of electrons and ions in the hot (≳500 K), dusty parts of protoplanetary disks, treating for the first time the effects of thermionic and ion emission from the dust grains. High-temperature ionization modeling has involved simply assuming that alkali elements such as potassium occur as gas-phase atoms and are collisionally ionized following the Saha equation. We show that the Saha equation often does not hold, because free charges are produced by thermionic and ion emission and destroyed when they stick to grain surfaces. This means the ionization state depends not on the first ionization potential of the alkali atoms, but rather on the grains’ work functions. The charged species’ abundances typically rise abruptly above about 800 K, with little qualitative dependence on the work function, gas density, or dust-to-gas mass ratio. Applying our results, we find that protoplanetary disks’ dead zone, where high diffusivities stifle magnetorotational turbulence, has its inner edge located where the temperature exceeds a threshold value ≈1000 K. The threshold is set by ambipolar diffusion except at the highest densities, where it is set by Ohmic resistivity. We find that the disk gas can be diffusively loaded onto the stellar magnetosphere at temperatures below a similar threshold. We investigate whether the “short-circuit” instability of current sheets can operate in disks and find that it cannot, or works only in a narrow range of conditions; it appears not to be the chondrule formation mechanism. We also suggest that thermionic emission is important for determining the rate of Ohmic heating in hot Jupiters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

STRONG SOLAR WIND DYNAMIC PRESSURE PULSES: INTERPLANETARY SOURCES AND THEIR IMPACTS ON GEOSYNCHRONOUS MAGNETIC FIELDS

In this investigation, we first present a statistical result of the interplanetary sources of very strong solar wind dynamic pressure pulses (DPPs) detected by WIND during solar cycle 23. It is found that the vast majority of strong DPPs reside within solar wind disturbances. Although the variabilities of geosynchronous magnetic fields (GMFs) due to the impact of positive DPPs have been well established, there appears to be no systematic investigations on the response of GMFs to negative DPPs. Here, we study both the decompression effects of very strong negative DPPs and the compression from strong positive DPPs on GMFs at different magnetic local time sectors. In response to the decompression of strong negative DPPs, GMFs on the dayside near dawn and near dusk on the nightside, are generally depressed. But near the midnight region, the responses of GMF are very diverse, being either positive or negative. For part of the events when GOES is located at the midnight sector, the GMF is found to abnormally increase as the result of magnetospheric decompression caused by negative DPPs. It is known that under certain conditions magnetic depression of nightside GMFs can be caused by the impact of positive DPPs. Here, we find that a stronger pressure enhancement may have a higher probability of producing the exceptional depression of GMF at the midnight region. Statistically, both the decompression effect of strong negative DPPs and the compression effect of strong positive DPPs depend on the magnetic local time, which are stronger at the noon sector.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

OPTICAL I-BAND LINEAR POLARIMETRY OF THE MAGNETAR 4U 0142+61 WITH SUBARU

Magnetars are known to have optical and/or infrared (IR) emission, but the origin of the emission is not well understood. In order to fully study their emission properties, we have carried out for the first time optical linear polarimetry of the magnetar 4U 0142+61, which has been determined from different observations to have a complicated broadband spectrum over optical and IR wavelengths. From our I-band imaging polarimetric observation, conducted with the 8.2-m Subaru telescope, we determine the degree of linear polarization to be P = 1.0 ± 3.4%, or P ≤ 5.6% (90% confidence level). Considering models that were suggested for optical emission from magnetars, we discuss the implications of our result. The upper limit measurement indicates that, differing from radio pulsars, magnetars probably would not have strongly polarized optical emission if the emission arises from their magnetosphere as suggested.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

V899 MON: AN OUTBURSTING PROTOSTAR WITH A PECULIAR LIGHT CURVE, AND ITS TRANSITION PHASES

We present a detailed study of V899 Mon (a new member in the FUors/EXors family of young low-mass stars undergoing outburst), based on our long-term monitoring of the source starting from 2009 November to 2015 April. Our optical and near-infrared photometric and spectroscopic monitoring recorded the source transitioning from its first outburst to a short-duration quiescence phase (<1 yr), and then returning to a second outburst. We report here the evolution of the outflows from the inner region of the disk as the accretion rate evolved in various epochs. Our high-resolution (R ∼ 37,000) optical spectrum could resolve interesting clumpy structures in the outflow traced by various lines. Change in far-infrared flux was also detected between two outburst epochs. Based on our observations, we constrained various stellar and envelope parameters of V899 Mon, as well as the kinematics of its accretion and outflow. The photometric and spectroscopic properties of this source fall between classical FUors and EXors. Our investigation of V899 Mon hints at instability associated with magnetospheric accretion being the physical cause of the sudden short-duration pause of the outburst in 2011. It is also a good candidate to explain similar short-duration pauses in outbursts of some other FUors/EXors sources.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

DISCOVERY OF LOW DM FAST RADIO TRANSIENTS: GEMINGA PULSAR CAUGHT IN THE ACT

We report the discovery of several energetic radio bursts at 34 MHz, using the Gauribidanur radio telescope. The radio bursts exhibit two important properties associated with the propagation of astronomical signals through the interstellar medium: (i) frequency dependent dispersive delays across the observing bandwidth and (ii) Faraday rotation of the plane of linear polarization. These bursts sample a range of dispersion measures (DM; 1.4–3.6 pc cm{sup −3}) and show DM-variation at timescales of the order of a minute. Using groups of bursts having a consistent DM, we show that the bursts have originated from the radio-quiet gamma-ray pulsar Geminga. Detection of these bursts supports the existence of occasional radio emission from Geminga. The rare occurrence of these bursts, and the short timescale variation in their DM (if really caused by the intervening medium or the pulsar magnetosphere), might provide clues as to why the pulsar has not been detected in earlier sensitive searches. We present details of the observations and search procedure used to discover these bursts, a detailed analysis of their properties, and evidences of these bursts being associated with Geminga pulsar, and briefly discuss the possible emission mechanism of these bursts.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

THE SECOND FERMI LARGE AREA TELESCOPE CATALOG OF GAMMA-RAY PULSARS

This catalog summarizes 117 high-confidence ≥0.1 GeV gamma-ray pulsar detections using three years of data acquired by the Large Area Telescope (LAT) on the Fermi satellite. Half are neutron stars discovered using LAT data through periodicity searches in gamma-ray and radio data around LAT unassociated source positions. The 117 pulsars are evenly divided into three groups: millisecond pulsars, young radio-loud pulsars, and young radio-quiet pulsars. We characterize the pulse profiles and energy spectra and derive luminosities when distance information exists. Spectral analysis of the off-peak phase intervals indicates probable pulsar wind nebula emission for four pulsars, and off-peak magnetospheric emission for several young and millisecond pulsars. We compare the gamma-ray properties with those in the radio, optical, and X-ray bands. We provide flux limits for pulsars with no observed gamma-ray emission, highlighting a small number of gamma-faint, radio-loud pulsars. The large, varied gamma-ray pulsar sample constrains emission models. Fermi's selection biases complement those of radio surveys, enhancing comparisons with predicted population distributions.

79 ASTRONOMY AND ASTROPHYSICS↗