Accurate, rapid identification of dislocation lines in coherent diffractive imaging via a min-max optimization formulation

Defects such as dislocations impact materials properties and their response during external stimuli. Defect engineering has emerged as a possible route to improving the performance of materials over a wide range of applications, including batteries, solar cells, and semiconductors. Imaging these defects in their native operating conditions to establish the structure-function relationship and, ultimately, to improve performance has remained a considerable challenge for both electron-based and x-ray-based imaging techniques. However, the advent of Bragg coherent x-ray diffractive imaging (BCDI) has made possible the 3D imaging of multiple dislocations in nanoparticles ranging in size from 100 nm to1000 nm. While the imaging process succeeds in many cases, nuances in identifying the dislocations has left manual identification as the preferred method. Derivative-based methods are also used, but they can be inaccurate and are computationally inefficient. Here we demonstrate a derivative-free method that is both more accurate and more computationally efficient than either derivative- or human-based methods for identifying 3D dislocation lines in nanocrystal images produced by BCDI. We formulate the problem as a min-max optimization problem and show exceptional accuracy for experimental images. We demonstrate a 260x speedup for a typical experimental dataset with higher accuracy over current methods. We discuss the possibility of using this algorithm as part of a sparsity-based phase retrieval process. We also provide the MATLAB code for use by other researchers

Radio structures of the nuclei of nearby Seyfert galaxies and the nature of the missing diffuse emission

We present archival high spatial resolution VLA and VLBA data of the nuclei of seven of the nearest and brightest Seyfert galaxies in the Southern Hemisphere. At VLA resolution (~0.1 arcsec), the nucleus of the Seyfert galaxies is unresolved, with the exception of MCG-5-23-16 and NGC 7469 showing a core-jet structure. Three Seyfert nuclei are surrounded by diffuse radio emission related to star-forming regions. VLBA observations with parsec-scale resolution pointed out that in MRK 1239 the nucleus is clearly resolved into two components separated by ~30 pc, while the nucleus of NGC 3783 is unresolved. Further comparison between VLA and VLBA data of these two sources shows that the flux density at parsec scales is only 20% of that measured by the VLA. This suggests that the radio emission is not concentrated in a single central component, as in elliptical radio galaxies, and an additional low-surface brightness component must be present. A comparison of Seyfert nuclei with different radio spectra points out that the ``presence'' of undetected flux on milli-arcsecond scale is common in steep-spectrum objects, while in flat-spectrum objects essentially all the radio emission is recovered. In the steep-spectrum objects, the nature of this ``missing'' flux is likely due to non-thermal AGN-related radiation, perhaps from a jet that gets disrupted in Seyfert galaxies because of the denser environment of their spiral hosts.Comment: 13 pages, 9 figures; paper accepted for publication in MNRA

Radio Emission from the Intermediate-mass Black Hole in the Globular Cluster G1

We have used the Very Large Array (VLA) to search for radio emission from the globular cluster G1 (Mayall-II) in M31. G1 has been reported by Gebhardt et al. to contain an intermediate-mass black hole (IMBH) with a mass of ~2 x 10^4 solar masses. Radio emission was detected within an arcsecond of the cluster center with an 8.4 GHz power of 2 x 10^{15} W/Hz. The radio/X-ray ratio of G1 is a few hundred times higher than that expected for a high-mass X-ray binary in the cluster center, but is consistent with the expected value for accretion onto an IMBH with the reported mass. A pulsar wind nebula is also a possible candidate for the radio and X-ray emission from G1; future high-sensitivity VLBI observations might distinguish between this possibility and an IMBH. If the radio source is an IMBH, and similar accretion and outflow processes occur for hypothesized ~ 1000-solar-mass black holes in Milky Way globular clusters, they are within reach of the current VLA and should be detectable easily by the Expanded VLA when it comes on line in 2010.Comment: ApJ Letters, accepted, 11 pages, 1 figur

X-band system performance of the very large array

The Very Large Array (VLA) is being equipped to receive telemetry from Voyager 2 during the Neptune encounter in 1989. Cryogenically cooled amplifiers are being installed on each of the 27 antennas. These amplifiers are currently a mix of field effect transistors (FETs) and high electron mobility transistors (HEMTs) and exhibit zenith system temperatures that range from 30 to 52 K. The system temperatures and aperture efficiencies determined during the past year are summarized. The nominal values of the noise diode calibration are compared with derived values made under the assumption of a uniform atmosphere over the array. Gain values are determined from observations of unresolved radio sources whose flux densities are well known. The tests suggest that the completed VLA will have a ratio of gain to system temperature that is approximately 4.4 dB above that of a single 64 m antenna of the Deep Space Network

Steep-Spectrum Radio Emission from the Low-Mass Active Galactic Nucleus GH 10

GH 10 is a broad-lined active galactic nucleus (AGN) energized by a black hole of mass 800,000 Solar masses. It was the only object detected by Greene et al. in their Very Large Array (VLA) survey of 19 low-mass AGNs discovered by Greene & Ho. New VLA imaging at 1.4, 4.9, and 8.5 GHz reveals that GH 10's emission has an extent of less than 320 pc, has an optically-thin synchrotron spectrum with a spectral index -0.76+/-0.05, is less than 11 percent linearly polarized, and is steady - although poorly sampled - on timescales of weeks and years. Circumnuclear star formation cannot dominate the radio emission, because the high inferred star formation rate, 18 Solar masses per year, is inconsistent with the rate of less than 2 Solar masses per year derived from narrow Halpha and [OII] 3727 emission. Instead, the radio emission must be mainly energized by the low-mass black hole. GH 10's radio properties match those of the steep-spectrum cores of Palomar Seyfert galaxies, suggesting that, like those Seyferts, the emission is outflow-driven. Because GH 10 is radiating close to its Eddington limit, it may be a local analog of the starting conditions, or seeds, for supermassive black holes. Future imaging of GH 10 at higher resolution thus offers an opportunity to study the relative roles of radiative versus kinetic feedback during black-hole growth.Comment: 7 pages; 2 figures; emulateapj; to appear in Ap

Blazar Counterparts for 3EG Sources at -40 < decl. < 0: Pushing South Through the Bulge

Supplementing existing survey data with VLA observations, we have extended $\gamma-$ray counterpart identifications down to decl. = -40$^\circ$ using our Figure of Merit approach. We find blazar counterparts for $\sim$ 70% of EGRET sources above decl. = -40$^\circ$ away from the Galaxy. Spectroscopic confirmation is in progress, and spectra for $\sim$ two dozen sources are presented here. We find evidence that increased exposure in the bulge region allowed EGRET to detect relatively faint blazars; a clear excess of non-blazar objects in this region however argues for an additional (new) source class.Comment: ApJ accepte

Gigahertz-Peaked Spectrum Radio Sources in Nearby Galaxies

There is now strong evidence that many low-luminosity AGNs (LLAGNs) contain accreting massive black holes and that the nuclear radio emission is dominated by parsec-scale jets launched by these black holes. Here, we present preliminary results on the 1.4 GHz to 667 GHz spectral shape of a well-defined sample of 16 LLAGNs. The LLAGNs have a falling spectrum at high GHz frequencies. Several also show a low-frequency turnover with a peak in the 1-20 GHz range. The results provide further support for jet dominance of the core radio emission. The LLAGNs show intriguing similarities with gigahertz-peaked spectrum (GPS) sources.Comment: 6 pages, to appear in ASP Conference series, 2002, Vol. 25