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At least 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

The influence of quantum vacuum friction on pulsars

We first revisit the energy loss mechanism known as quantum vacuum friction (QVF), clarifying some of its subtleties. Then we investigate the observables that could easily differentiate QVF from the classical magnetic dipole radiation for pulsars with accurately measured braking indices (n). We show that this is particularly the case for the time evolution of a pulsar’s magnetic dipole direction ( ϕ-dot ) and surface magnetic field ( B-dot {sub 0}). As is well known in the context of the classic magnetic dipole radiation, n < 3 would only be possible for positive ( B-dot {sub 0}/B{sub 0}+ ϕ-dot /tanϕ), which, for instance, leads to B-dot {sub 0}>0 ( ϕ-dot >0) when ϕ (B {sub 0}) is constant. On the other hand, we show that QVF can result in very different predictions with respect to those above. Finally, even if B-dot {sub 0} has the same sign in both of the aforementioned models for a pulsar, then, for a given ϕ, we show that they give rise to different associated timescales, which could be another way to falsify QVF.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

RELATIONSHIPS BETWEEN FLUID VORTICITY, KINETIC HELICITY, AND MAGNETIC FIELD ON SMALL-SCALES (QUIET-NETWORK) ON THE SUN

We derive horizontal fluid motions on the solar surface over large areas covering the quiet-Sun magnetic network from local correlation tracking of convective granules imaged in continuum intensity and Doppler velocity by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory . From these we calculate the horizontal divergence, the vertical component of vorticity, and the kinetic helicity of fluid motions. We study the correlations between fluid divergence and vorticity, and between vorticity (kinetic helicity) and the magnetic field. We find that the vorticity (kinetic helicity) around small-scale fields exhibits a hemispherical pattern (in sign) similar to that followed by the magnetic helicity of large-scale active regions (containing sunspots). We identify this pattern to be a result of the Coriolis force acting on supergranular-scale flows (both the outflows and inflows), consistent with earlier studies using local helioseismology. Furthermore, we show that the magnetic fields cause transfer of vorticity from supergranular inflow regions to outflow regions, and that they tend to suppress the vortical motions around them when magnetic flux densities exceed about 300 G (from HMI). We also show that such an action of the magnetic fields leads to marked changes in the correlations between fluid divergence and vorticity. These results are speculated to be of importance to local dynamo action (if present) and to the dynamical evolution of magnetic helicity at the small-scale.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

TESTING MODELS FOR THE SHALLOW DECAY PHASE OF GAMMA-RAY BURST AFTERGLOWS WITH POLARIZATION OBSERVATIONS

The X-ray afterglows of almost one-half of gamma-ray bursts have been discovered by the Swift satellite to have a shallow decay phase of which the origin remains mysterious. Two main models have been proposed to explain this phase: relativistic wind bubbles (RWBs) and structured ejecta, which could originate from millisecond magnetars and rapidly rotating black holes, respectively. Based on these models, we investigate polarization evolution in the shallow decay phase of X-ray and optical afterglows. We find that in the RWB model, a significant bump of the polarization degree evolution curve appears during the shallow decay phase of both optical and X-ray afterglows, while the polarization position angle abruptly changes its direction by 90°. In the structured ejecta model, however, the polarization degree does not evolve significantly during the shallow decay phase of afterglows whether the magnetic field configuration in the ejecta is random or globally large-scale. Therefore, we conclude that these two models for the shallow decay phase and relevant central engines would be testable with future polarization observations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

TEMPORAL AND SPATIAL RELATIONSHIP OF FLARE SIGNATURES AND THE FORCE-FREE CORONAL MAGNETIC FIELD

We investigate the plasma and magnetic environment of active region NOAA 11261 on 2011 August 2 around a GOES M1.4 flare/CME (SOL2011-08-02T06:19). We compare coronal emission at the (extreme) ultraviolet and X-ray wavelengths, using SDO AIA and RHESSI images, in order to identify the relative timing and locations of reconnection-related sources. We trace flare ribbon signatures at ultraviolet wavelengths in order to pin down the intersection of previously reconnected flaring loops in the lower solar atmosphere. These locations are used to calculate field lines from three-dimensional (3D) nonlinear force-free magnetic field models, established on the basis of SDO HMI photospheric vector magnetic field maps. Using this procedure, we analyze the quasi-static time evolution of the coronal model magnetic field previously involved in magnetic reconnection. This allows us, for the first time, to estimate the elevation speed of the current sheet’s lower tip during an on-disk observed flare as a few kilometers per second. A comparison to post-flare loops observed later above the limb in STEREO EUVI images supports this velocity estimate. Furthermore, we provide evidence for an implosion of parts of the flaring coronal model magnetic field, and identify the corresponding coronal sub-volumes associated with the loss of magnetic energy. Finally, we spatially relate the build up of magnetic energy in the 3D models to highly sheared fields, established due to the dynamic relative motions of polarity patches within the active region.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Gravitational waves from pulsars and their braking indices: the role of a time dependent magnetic ellipticity

We study the role of time dependent magnetic ellipticities (ϵ{sub B}) on the calculation of the braking index of pulsars. Moreover, we study the consequences of such a ϵ{sub B} on the amplitude of gravitational waves (GWs) generated by pulsars with measured braking indices. We show that, since the ellipticity generated by the magnetic dipole is extremely small, the corresponding amplitude of GWs is much smaller than the amplitude obtained via the spindown limit.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

The effect of an offset polar cap dipolar magnetic field on the modeling of the vela pulsar's γ-ray light curves

We performed geometric pulsar light curve modeling using static, retarded vacuum, and offset polar cap (PC) dipole B-fields (the latter is characterized by a parameter ϵ), in conjunction with standard two-pole caustic (TPC) and outer gap (OG) emission geometries. The offset-PC dipole B-field mimics deviations from the static dipole (which corresponds to ϵ=0). In addition to constant-emissivity geometric models, we also considered a slot gap (SG) E-field associated with the offset-PC dipole B-field and found that its inclusion leads to qualitatively different light curves. Solving the particle transport equation shows that the particle energy only becomes large enough to yield significant curvature radiation at large altitudes above the stellar surface, given this relatively low E-field. Therefore, particles do not always attain the radiation-reaction limit. Our overall optimal light curve fit is for the retarded vacuum dipole field and OG model, at an inclination angle α=78{sub −1}{sup +1{sup {sup ∘}}} and observer angle ζ=69{sub −1}{sup +2{sup {sup ∘}}}. For this B-field, the TPC model is statistically disfavored compared to the OG model. For the static dipole field, neither model is significantly preferred. We found that smaller values of ϵ are favored for the offset-PC dipole field when assuming constant emissivity, and larger ϵ values favored for variable emissivity, but not significantly so. When multiplying the SG E-field by a factor of 100, we found improved light curve fits, with α and ζ being closer to best fits from independent studies, as well as curvature radiation reaction at lower altitudes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

NUMERICAL AND ANALYTICAL SOLUTIONS OF NEUTRINO-DOMINATED ACCRETION FLOWS WITH A NON-ZERO TORQUE BOUNDARY CONDITION AND ITS APPLICATIONS IN GAMMA-RAY BURSTS

A stellar-mass black hole (BH) surrounded by a neutrino-dominated accretion flow (NDAF) has been discussed in a number of works as the central engine of gamma-ray bursts (GRBs). It is widely believed that NDAF cannot liberate enough energy for bright GRBs. However, these works have been based on the assumption of a “no torque” boundary condition, which is invalid when the disk is magnetized. In this paper, we present both numerical and analytical solutions for NDAFs with non-zero boundary stresses and reexamine their properties. We find that an NDAF with such a boundary torque can be powerful enough to account for those bright short GRBs, energetic long GRBs, and ultra-long GRBs. The disk becomes viscously unstable, which makes it possible to interpret the variability of GRB prompt emission and the steep decay phase in the early X-ray afterglow. Finally, we study the gravitational waves radiated from a processing BH-NDAF. We find that the effects of the boundary torque on the strength of the gravitational waves can be ignored.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

DIFFERENCES BETWEEN RADIO-LOUD AND RADIO-QUIET γ -RAY PULSARS AS REVEALED BY FERMI

By comparing the properties of non-recycled radio-loud γ -ray pulsars and radio-quiet γ -ray pulsars, we have searched for the differences between these two populations. We found that the γ -ray spectral curvature of radio-quiet pulsars can be larger than that of radio-loud pulsars. Based on the full sample of non-recycled γ -ray pulsars, their distributions of the magnetic field strength at the light cylinder are also found to be different. We note that this might result from an observational bias. By reexamining the previously reported difference of γ -ray-to-X-ray flux ratios, we found that the significance can be hampered by their statistical uncertainties. In the context of the outer gap model, we discuss the expected properties of these two populations and compare with the possible differences that are identified in our analysis.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

AR Sco: A Precessing White Dwarf Synchronar?

The emission of the white dwarf–M dwarf binary AR Sco is driven by the rapid synchronization of its white dwarf, rather than by accretion. Synchronization requires a magnetic field ∼100 Gauss at the M dwarf and ∼10{sup 8} Gauss at the white dwarf, larger than the fields of most intermediate polars but within the range of fields of known magnetic white dwarfs. The spindown power is dissipated in the atmosphere of the M dwarf, within the near zone of the rotating white dwarf’s field, by magnetic reconnection, accelerating particles that produce the observed synchrotron radiation. The displacement of the optical maximum from conjunction may be explained either by dissipation in a bow wave as the white dwarf’s magnetic field sweeps past the M dwarf or by a misaligned white dwarf rotation axis and oblique magnetic moment. In the latter case the rotation axis precesses with a period of decades, predicting a drift in the orbital phase of the optical maximum. Binaries whose emission is powered by synchronization may be termed synchronars, in analogy to magnetars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Properties of Magnetars Mimicking {sup 56}Ni-Powered Light Curves in Type IC Superluminous Supernovae

Many Type Ic superluminous supernovae have light-curve decline rates after their luminosity peak, which are close to the nuclear decay rate of {sup 56}Co, consistent with the interpretation that they are powered by {sup 56}Ni and possibly pair-instability supernovae. However, their rise times are typically shorter than those expected from pair-instability supernovae, and Type Ic superluminous supernovae are often suggested to be powered by magnetar spin-down. If magnetar spin-down is actually a major mechanism to power Type Ic superluminous supernovae, it should be able to produce decline rates similar to the {sup 56}Co decay rate rather easily. In this study, we investigate the conditions for magnetars under which their spin-down energy input can behave like the {sup 56}Ni nuclear decay energy input. We find that an initial magnetic field strength within a certain range is sufficient to keep the magnetar energy deposition within a factor of a few of the {sup 56}Co decay energy for several hundreds of days. Magnetar spin-down needs to be by almost pure dipole radiation with the braking index close to three to mimic {sup 56}Ni in a wide parameter range. Not only late-phase {sup 56}Co-decay-like light curves, but also rise time and peak luminosity of most {sup 56}Ni-powered light curves can be reproduced by magnetars. Bolometric light curves for more than 700 days are required to distinguish the two energy sources solely by them. We expect that more slowly declining superluminous supernovae with short rise times should be found if they are mainly powered by magnetar spin-down.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

XMM-Newton Observation of the Nearby Pulsar B1133+16

We constrain the X-ray properties of the nearby (360 pc), old (5 Myr) pulsar B1133+16 with ∼100 ks effective exposure time by XMM-Newton. The observed pulsar flux in the 0.2–3 keV energy range is ∼10{sup −14} erg cm{sup −2} s{sup −1}, which results in the recording of ∼600 source counts with the EPIC pn and MOS detectors. The X-ray radiation is dominated by nonthermal radiation and is well described by both a single power-law model (PL) and a sum of blackbody and power-law emission (BB+PL). The BB+PL model results in a spectral photon index Γ=2.4{sub −0.3}{sup +0.4} and a nonthermal flux in the 0.2–3 keV energy range of (7±2)×10{sup −15} erg cm{sup −2} s{sup −1}. The thermal emission is consistent with the blackbody emission from a small hot spot with a radius of R{sub pc}≈14{sub −5}{sup +7} m and a temperature of T{sub s}=2.9{sub −0.4}{sup +0.6} MK. Assuming that the hot spot corresponds to the polar cap of the pulsar, we can use the magnetic flux conservation law to estimate the magnetic field at the surface B{sub s}≈3.9×10{sup 14} G. The observations are in good agreement with the predictions of the partially screened gap model, which assumes the existence of small-scale surface magnetic field structures in the polar cap region.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Magnetar Central Engine and Possible Gravitational Wave Emission of Nearby Short GRB 160821B

GRB 160821B is a short gamma-ray burst (SGRB) at redshift z = 0.16, with a duration less than 1 s and without any “extended emission” detected up to more than 100 s in both Swift /BAT and Fermi /GBM bands. An X-ray plateau with a sharp drop 180 s after the BAT trigger was observed with Swift /XRT. No supernova or kilo-nova signature was detected. Assuming the central engine of this SGRB is a recently born supra-massive magnetar, we can explain the SGRB as jet radiation and its X-ray plateau as the internal energy dissipation of the pulsar wind as it spins down. We constrain its surface magnetic field to B {sub p} < 3.12 × 10{sup 16} G and initial spin period to P{sub 0} < 8.5 × 10{sup −3} s. Its equation of state is consistent with the GM1 model with M{sub TOV} ∼ 2.37 M {sub ⊙} and ellipticity ϵ < 0.07. Its gravitational wave (GW) radiation may be detectable with the future Einstein Telescope, but is much weaker than the current detectability limit of Advanced LIGO. The GW radiation of such an event would be detectable by Advanced LIGO if it occurred at a distance of 100 Mpc ( z = 0.023).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Magnetic Fields in the Massive Dense Cores of the DR21 Filament: Weakly Magnetized Cores in a Strongly Magnetized Filament

We present Submillimeter Array 880 μ m dust polarization observations of six massive dense cores in the DR21 filament. The dust polarization shows complex magnetic field structures in the massive dense cores with sizes of 0.1 pc, in contrast to the ordered magnetic fields of the parsec-scale filament. The major axes of the massive dense cores appear to be aligned either parallel or perpendicular to the magnetic fields of the filament, indicating that the parsec-scale magnetic fields play an important role in the formation of the massive dense cores. However, the correlation between the major axes of the cores and the magnetic fields of the cores is less significant, suggesting that during the core formation, the magnetic fields below 0.1 pc scales become less important than the magnetic fields above 0.1 pc scales in supporting a core against gravity. Our analysis of the angular dispersion functions of the observed polarization segments yields a plane-of-sky magnetic field strength of 0.4–1.7 mG for the massive dense cores. We estimate the kinematic, magnetic, and gravitational virial parameters of the filament and the cores. The virial parameters show that the gravitational energy in the filament dominates magnetic and kinematic energies, while the kinematic energy dominates in the cores. Our work suggests that although magnetic fields may play an important role in a collapsing filament, the kinematics arising from gravitational collapse must become more important than magnetic fields during the evolution from filaments to massive dense cores.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

What Can We Learn about GRB from the Variability Timescale Related Correlations?

Recently, two empirical correlations related to the minimum variability timescale (MTS) of the light curves are discovered in gamma-ray bursts (GRBs). One is the anti-correlation between MTS and Lorentz factor Γ, and the other is the anti-correlation between the MTS and gamma-ray luminosity L {sub γ}. Both of the two correlations might be used to explore the activity of the central engine of GRBs. In this paper, we try to understand these empirical correlations by combining two popular black hole central engine models (namely, the Blandford and Znajek mechanism (BZ) and the neutrino-dominated accretion flow (NDAF)). By taking the MTS as the timescale of viscous instability of the NDAF, we find that these correlations favor the scenario in which the jet is driven by the BZ mechanism.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Magnetohydrodynamic Turbulence Mediated by Reconnection

Magnetic field fluctuations in magnetohydrodynamic turbulence can be viewed as current sheets that are progressively more anisotropic at smaller scales. As suggested by Loureiro and Boldyrev and Mallet et al., below a certain critical thickness, λ{sub c}, such current sheets become tearing-unstable. We propose that the tearing instability changes the effective alignment of the magnetic field lines in such a way as to balance the eddy turnover rate at all scales smaller than λ{sub c}. As a result, turbulent fluctuations become progressively less anisotropic at smaller scales, with the alignment angle increasing as θ∼(λ/λ{sub ∗}){sup −4/5+β}, where λ{sub ∗}∼L{sub 0}S{sub 0}{sup −3/4} is the resistive dissipation scale. Here L {sub 0} is the outer scale of the turbulence, S {sub 0} is the corresponding Lundquist number, and 0⩽β<4/5 is a parameter. The resulting Fourier energy spectrum is E(k{sub ⊥})∝k{sub ⊥}{sup −11/5+2β/3}, where k{sub ⊥} is the wavenumber normal to the local mean magnetic field, and the critical scale is λ{sub c}∼S{sub L}{sup −(4−5β)/(7−20β/3)}. The simplest model corresponds to β = 0, in which case the predicted scaling formally agrees with one of the solutions obtained in Mallet et al. from a discrete hierarchical model of abruptly collapsing current sheets, an approach different from and complementary to ours. We also show that the reconnection-mediated interval is non-universal with respect to the dissipation mechanism. Hyper-resistivity of the form η-tilde k{sup 2+2s} leads (in the simplest case of β = 0) to the different transition scale λ{sub c}∼L{sub 0} S-tilde {sub 0}{sup −4/(7+9s)} and the energy spectrum E(k{sub ⊥})∝k{sub ⊥}{sup −(11+9s)/(5+3s)}, where S-tilde {sub 0} is the corresponding hyper-resistive Lundquist number.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Constraining Parameters in Pulsar Models of Repeating FRB 121102 with High-energy Follow-up Observations

Recently, a precise (sub-arcsecond) localization of the repeating fast radio burst (FRB) 121102 led to the discovery of persistent radio and optical counterparts, the identification of a host dwarf galaxy at a redshift of z = 0.193, and several campaigns of searches for higher-frequency counterparts, which gave only upper limits on the emission flux. Although the origin of FRBs remains unknown, most of the existing theoretical models are associated with pulsars, or more specifically, magnetars. In this paper, we explore persistent high-energy emission from a rapidly rotating highly magnetized pulsar associated with FRB 121102 if internal gradual magnetic dissipation occurs in the pulsar wind. We find that the efficiency of converting the spin-down luminosity to the high-energy (e.g., X-ray) luminosity is generally much smaller than unity, even for a millisecond magnetar. This provides an explanation for the non-detection of high-energy counterparts to FRB 121102. We further constrain the spin period and surface magnetic field strength of the pulsar with the current high-energy observations. In addition, we compare our results with the constraints given by the other methods in previous works and expect to apply our new method to some other open issues in the future.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Generation Mechanisms of Quasi-parallel and Quasi-circular Flare Ribbons in a Confined Flare

We analyze a confined multiple-ribbon M2.1 flare (SOL2015-01-29T11:42) that originated from a fan-spine coronal magnetic field configuration, within active region NOAA 12268. The observed ribbons form in two steps. First, two primary ribbons form at the main flare site, followed by the formation of secondary ribbons at remote locations. We observe a number of plasma flows at extreme-ultraviolet temperatures during the early phase of the flare (as early as 15 minutes before the onset) propagating toward the formation site of the secondary ribbons. The secondary ribbon formation is co-temporal with the arrival of the pre-flare generated plasma flows. The primary ribbons are co-spatial with Ramaty High Energy Spectroscopic Imager ( RHESSI ) hard X-ray sources, whereas no enhanced X-ray emission is detected at the secondary ribbon sites. The (E)UV emission, associated with the secondary ribbons, peaks ∼1 minute after the last RHESSI hard X-ray enhancement. A nonlinear force-free model of the coronal magnetic field reveals that the secondary flare ribbons are not directly connected to the primary ribbons, but to regions nearby. Detailed analysis suggests that the secondary brightenings are produced due to dissipation of kinetic energy of the plasma flows (heating due to compression), and not due to non-thermal particles accelerated by magnetic reconnection, as is the case for the primary ribbons.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Ultrahigh-energy cosmic rays and black hole mergers

The recent detection of the gravitational-wave source GW150914 by the LIGO collaboration motivates a speculative source for the origin of ultrahigh-energy cosmic rays as a possible byproduct of the immense energies achieved in black hole (BH) mergers, provided that the BHs have spin, as seems inevitable, and there are relic magnetic fields and disk debris remaining from the formation of the BHs or from their accretion history. We argue that given the modest efficiency <0.01 required per event per unit of gravitational-wave energy release, merging BHs potentially provide an environment for accelerating cosmic rays to ultrahigh energies. The presence of tidally disrupted planetary or asteroidal debris could lead to associated fast radio bursts.

79 ASTRONOMY AND ASTROPHYSICS↗