Employing gadolinium micro-disks as temperature probes for magnetic resonance imaging
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Nobles, J. E.↗
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A Survey of Software Techniques for Using Non-Volatile Memories for Storage and Main Memory Systems
Non-volatile memory (NVM) devices, such as Flash, phase change RAM, spin transfer torque RAM, and resistive RAM, offer several advantages and challenges when compared to conventional memory technologies, such as DRAM and magnetic hard disk drives (HDDs). Here, we present a survey of software techniques that have been proposed to exploit the advantages and mitigate the disadvantages of NVMs when used for designing memory systems, and, in particular, secondary storage (e.g., solid state drive) and main memory. Furthermore, we classify these software techniques along several dimensions to highlight their similarities and differences. Given that NVMs are growing in popularity, we believe that this survey will motivate further research in the field of software technology for NVMs.
97 MATHEMATICS AND COMPUTING↗
EFFECTS OF SPIN ON HIGH-ENERGY RADIATION FROM ACCRETING BLACK HOLES
Observations of jets in X-ray binaries show a correlation between radio power and black hole spin. This correlation, if confirmed, points toward the idea that relativistic jets may be powered by the rotational energy of black holes. In order to examine this further, we perform general relativistic radiative transport calculations on magnetically arrested accretion flows, which are known to produce powerful jets via the Blandford–Znajek (BZ) mechanism. We find that the X-ray and γ -ray emission strongly depend on spin and inclination angle. Surprisingly, the high-energy power does not show the same dependence on spin as the BZ jet power, but instead can be understood as a redshift effect. In particular, photons observed perpendicular to the spin axis suffer little net redshift until originating from close to the horizon. Such observers see deeper into the hot, dense, highly magnetized inner disk region. This effect is largest for rapidly rotating black holes due to a combination of frame dragging and decreasing horizon radius. While the X-ray emission is dominated by the near horizon region, the near-infrared (NIR) radiation originates at larger radii. Therefore, the ratio of X-ray to NIR power is an observational signature of black hole spin.
79 ASTRONOMY AND ASTROPHYSICS↗
Detection of possible quasi-periodic oscillations in the long-term optical light curve of the BL Lac object OJ 287
The detection of periodicity in the broadband non-thermal emission of blazars has so far been proven to be elusive. However, there are a number of scenarios that could lead to quasi-periodic variations in blazar light curves. For example, an orbital or thermal/viscous period of accreting matter around central supermassive black holes could, in principle, be imprinted in the multi-wavelength emission of small-scale blazar jets, carrying such crucial information about plasma conditions within the jet launching regions. In this paper, we present the results of our time series analysis of the ∼9.2 yr long, and exceptionally well-sampled, optical light curve of the BL Lac object OJ 287. The study primarily used the data from our own observations performed at the Mt. Suhora and Kraków Observatories in Poland, and at the Athens Observatory in Greece. Additionally, SMARTS observations were used to fill some of the gaps in the data. The Lomb–Scargle periodogram and the weighted wavelet Z-transform methods were employed to search for possible quasi-periodic oscillations in the resulting optical light curve of the source. Both methods consistently yielded a possible quasi-periodic signal around the periods of ∼400 and ∼800 days, the former with a significance (over the underlying colored noise) of ⩾99%. A number of likely explanations for this are discussed, with preference given to a modulation of the jet production efficiency by highly magnetized accretion disks. This supports previous findings and the interpretation reported recently in the literature for OJ 287 and other blazar sources.
79 ASTRONOMY AND ASTROPHYSICS↗
Collapsar Gamma-Ray Bursts Grind Their Black Hole Spins to a Halt
Abstract The spin of a newly formed black hole (BH) at the center of a massive star evolves from its natal value due to two competing processes: accretion of gas angular momentum that increases the spin and extraction of BH angular momentum by outflows that decreases the spin. Ultimately, the final, equilibrium spin is set by a balance between both processes. In order for the BH to launch relativistic jets and power aγ-ray burst (GRB), the BH magnetic field needs to be dynamically important. Thus, we consider the case of a magnetically arrested disk (MAD) driving the spin evolution of the BH. By applying the semianalytic MAD BH spin evolution model of Lowell et al. to collapsars, we show that if the BH accretes ∼20% of its initial mass, its dimensionless spin inevitably reaches small values,a≲ 0.2. For such spins, and for mass accretion rates inferred from collapsar simulations, we show that our semianalytic model reproduces the energetics of typical GRB jets,L jet ∼ 10 50 erg s −1 . We show that our semianalytic model reproduces the nearly constant power of typical GRB jets. If the MAD onset is delayed, this allows powerful jets at the high end of the GRB luminosity distribution,L jet ∼ 10 52 erg s −1 , but the final spin remains low,a≲ 0.3. These results are consistent with the low spins inferred from gravitational wave detections of binary BH mergers. In a companion paper by Gottlieb et al., we use GRB observations to constrain the natal BH spin to bea≃ 0.2.
Astronomy & Astrophysics↗
Jets with a Twist: The Emergence of FR0 Jets in a 3D GRMHD Simulation of Zero-angular-momentum Black Hole Accretion
Abstract Spinning supermassive black holes (BHs) in active galactic nuclei magnetically launch relativistic collimated outflows, or jets. Without angular momentum supply, such jets are thought to perish within 3 orders of magnitude in distance from the BH, well before reaching kiloparsec scales. We study the survival of such jets at the largest scale separation to date, via 3D general relativistic magnetohydrodynamic simulations of rapidly spinning BHs immersed into uniform zero-angular-momentum gas threaded by a weak vertical magnetic field. We place the gas outside the BH sphere of influence, or the Bondi radius, chosen to be much larger than the BH gravitational radius, R B = 10 3 R g . The BH develops dynamically important large-scale magnetic fields, forms a magnetically arrested disk (MAD), and launches relativistic jets that propagate well outside R B and suppress BH accretion to 1.5% of the Bondi rate, M ̇ B . Thus, low-angular-momentum accretion in the MAD state can form large-scale jets in Fanaroff–Riley (FR) type I and II galaxies. Subsequently, the disk shrinks and exits the MAD state: barely a disk (BAD), it rapidly precesses, whips the jets around, globally destroys them, and lets 5%–10% of M ̇ B reach the BH. Thereafter, the disk starts rocking back and forth by angles 90°–180°: the rocking accretion disk (RAD) launches weak intermittent jets that spread their energy over a large area and suppress BH accretion to ≲2% M ̇ B . Because the BAD and RAD states tangle up the jets and destroy them well inside R B , they are promising candidates for the more abundant, but less luminous, class of FR0 galaxies.
Astronomy & Astrophysics↗
The QSO 1156+295 - A multifrequency study of recent activity
Photometric observations are presented for outbursts of the quasi-stellar object QSO 1156+295 in 1981 and 1982. High time resolution photometry is included showing variations on time scales of weeks to about half an hour. Data from early plate material show that the object may have been quite bright at the beginning of this century but was quite faint and probably inactive between about 1950 and the beginning of recent activity in 1977-1979. Some results of optical linear polarization are discussed. In examining spectrophotometric results, broadband spectra show very little change in spectral shape during changes in continuum brightness. High resolution spectra (0.1-A FWHM) show no evidence for the Mg II 2798-A absorption sometimes seen in optically violent variables and other QSOs. The radio flux density variations at several frequencies are documented and discussed and compared with the optical light curve. Models for 1156+295 are discussed including variability time scales, and the observed energy output is compared with a magnetic accretion disk model proposed by Shields and Wheeler (1976). Results are summarized and the different sizes of active and inactive regions of the object are compared.
Wills, B. J.↗
The magnetic field in the dust ring at the center of the Galaxy
Measurements of the polarization of the far-infrared thermal emission from six points in the dust ring at Sgr A are presented. The position angles are approximately perpendicular to the long axis of the ring as projected on the sky. The inferred magnetic field is therefore approximately in the plane of the ring. The pattern traced by the polarization vectors resembles that expected for a magnetic accretion disk. The measurements indicate a field in which the outward radial component is much greater than the axial component at the surface of the disk. The field thus appears to satisfy the condition proposed by Blandford and Payne (1982) for removing energy and angular momentum through centrifugal acceleration of surface material moving along the field lines.
Hildebrand, R. H.↗
On the apparent velocity of integrated sunlight. 2: 1983-1992 and comparisons with magnetograms
We report additional results in our program to monitor the wavelength stability of lines in the 2.3 micrometer spectrum of integrated sunlight. We use the McMath Fourier transform spectrometer (FTS) of the National Solar Observatory to monitor 16 delta V = 2 lines of (12)C(16)O, as well as five atomic lines. Wavenumber calibration is achieved using a low-pressure N2O absorption cell and checked against terrestrial atmospheric lines. Imperfect optical integration of the solar disk remains the principal source of error, but this error has been reduced by improved FTS/telescope collimation and observing procedures. The present results include data from an additional 13 quarterly observing runs since 1985. We continue to find that the apparent velocity of integrated sunlight is variable, in the sense of having a greater reshift at solar maximum. This is supported by the temporal dependence of the integrated light velocity, and by the presence of a correlation between velocity and the disk-averaged magnetic flux derived from Kitt Peak magnetograms. The indicated peak-to-peak apparent velocity amplitude over a solar cycle is approximately the same as the velocity amplitude of the Sun's motion about the solar system barycenter. This represents about half the amplitude which we inferred in Paper I (Deming et al. 1987), but the present result has a much greater statistical significance. Our results have implications for those investigations which search for the Doppler signatures of planetary-mass companions to solar-type stars. We contrast our results to the recent finding by McMillan et al. 1993 that solar absorption lines in the violet spectral region are wavelength-stable over the solar cycle.
Deming, Drake↗
Astrophysical Magnetic Fields and Topics in Galaxy Formation
The grant was used to support theoretical research on a variety of astro-physical topics falling broadly into those described by the proposal: galaxy formation, astrophysical magnetic fields, magnetized accretion disks in AGN, new physics, and other astrophysical problems. Work accomplished; references are to work authored by project personel.
Field, George B.↗
Applied Vertical Bloch Line Storage Technology
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storing data solid-state chips magnetic disks VBL ↗
Combination radial and thrust magnetic bearing
A combination radial and thrust magnetic bearing is disclosed that allows for both radial and thrust axes control of an associated shaft. The combination radial and thrust magnetic bearing comprises a rotor and a stator. The rotor comprises a shaft, and first and second rotor pairs each having respective rotor elements. The stator comprises first and second stator elements and a magnet-sensor disk. In one embodiment, each stator element has a plurality of split-poles and a corresponding plurality of radial force coils and, in another embodiment, each stator element does not require thrust force coils, and radial force coils are replaced by double the plurality of coils serving as an outer member of each split-pole half.
Blumenstock, Kenneth A.↗
Earth Observing System Data Gateway
The Earth Observing System Data Gateway (EDG) software provides a "one-stop-shopping" standard interface for exploring and ordering Earth-science data stored at geographically distributed sites. EDG enables a user to do the following: 1) Search for data according to high-level criteria (e.g., geographic location, time, or satellite that acquired the data); 2) Browse the results of a search, viewing thumbnail sketches of data that satisfy the user s criteria; and 3) Order selected data for delivery to a specified address on a chosen medium (e.g., compact disk or magnetic tape). EDG consists of (1) a component that implements a high-level client/server protocol, and (2) a collection of C-language libraries that implement the passing of protocol messages between an EDG client and one or more EDG servers. EDG servers are located at sites usually called "Distributed Active Archive Centers" (DAACs). Each DAAC may allow access to many individual data items, called "granules" (e.g., single Landsat images). Related granules are grouped into collections called "data sets." EDG enables a user to send a search query to multiple DAACs simultaneously, inspect the resulting information, select browseable granules, and then order selected data from the different sites in a seamless fashion.
Pfister, Robin↗
Accreting on the Edge: A Luminosity-Dependent Cyclotron Line in the Be/X-ray Binary 2S 1553-542 Accompanied by Accretion Regimes Transition
Accreting X-ray pulsars (XRPs) undergo luminous X-ray outbursts during which the luminosity-dependent spectral and timing features of the neutron star’s emission can be analyzed in detail, thus shedding light on the accretion regime at work. We took advantage of a monitoring campaign performed with NuSTAR, Swift/XRT, AstroSat and NICER, to follow the Be/X-ray Binary 2S 1553-542 along one of its rare outbursts and trace its spectral and timing evolution. We report the discovery of a luminosity-dependent cyclotron line energy for the first time in this source. The pulse profiles and pulsed fraction also show variability along the outburst, consistently with the interpretation that the source transitions from the sub-critical to the super-critical accretion regime, separated by a critical luminosity ofLcrit≈4×1037erg/s.
X-ray binary stars↗
Plasma physics in the IRCC-AFP collaboration: magnetic reconnection and stellar accretion disks
No abstract provided
Takasao, S.↗
Reconnection-driven oscillations in dwarf nova disks
A class of oscillations observed during eruption of dwarf novae has been interpreted as oscillations of the accretion disks in these systems.These oscillations are quasi-periodic with coherence times typically between three and 15 cycles. It is shown that magnetic field reconnection at high magnetic Reynolds number can drive disk oscillations. The expected stochastic geometry of disk magnetic fields could naturally produce the observed phase incoherency.
Tajima, T.↗