Star-Forming Regions near GRB 990123

We reduced the Hubble Space Telescope Space Telescope Imaging Spectrograph images of the gamma-ray burst GRB 990123 that were obtained on 8-9 February 1999 and find V_0 = 25.36 +/- 0.10, which corresponds to a flux of 0.258 +/- 0.023 micro-Jy for the optical transient 16.644 days after the burst's peak. The probable host galaxy has V_0 = 24.25 +/- 0.07 (= 0.716 +/- 0.046 micro-Jy) and the optical transient is located 0.65 arcseconds (= 5.5 kpc) south of the galaxy's nucleus. We fit and subtracted a scaled point-spread function to the optical transient and found evidence for three bright knots situated within 0.5 arcseconds (= 4.3 kpc) of the optical transient. Each knot has V_0 ~ 28.1 +/- 0.3, a rest-frame V-band luminosity of between approximately 5e8 L_Sun and 8e8 L_Sun, and a star-formation rate of at least 0.1-0.2 Solar masses per year. The knots are centrally concentrated with full-width at half-maximum of approximately 0.17 arcseconds (= 1.5 kpc). Their sizes and luminosities are consistent with their being star-forming regions. The optical transient is located 0.15 arcseconds (= 1.3 kpc) southeast of the centre of one of these knots.Comment: 4 pages, 2 figures, to appear in Astronomy & Astrophysics Letter

First Measurements from a New Broadband Vibrothermography Measurement System

We report on the construction and development of a broad‐spectrum vibrothermography (Sonic IR) measurement system at the Center for NDE. The new system uses a broadband actuator instead of an ultrasonic welder to generate vibration and induce heating of cracks. A high‐resolution infrared camera captures the IR signature of a crack, and a reconfigurable data acquisition software system acquires and processes the IR images and vibrometry waveforms in real time. We present and discuss results from initial experiments with this system, including the frequency dependence of vibrothermographic heating of flaws in a jet turbine stator vane and an analysis of the correlation of heating with vibration frequency in a cracked test specimen

Thermographic signal reconstruction for vibrothermography

Vibrothermography, also known as thermosonics or sonic infrared, is a method of nondestructive evaluation that finds cracks or delaminations from the heat given off in response to vibration. In vibrothermography, finding cracks requires identifying and localizing pulsed surface and subsurface heat sources from a time sequence of infrared images. Traditionally this identification involves manually stepping through and studying the images. Careful observation of the heating and subsequent cooling is needed to distinguish cracks from false indications. In this paper, we present an algorithm that reduces the entire time sequence to a single static plot. The plot uses only a few coefficients per pixel to reconstruct the original sequence; this is possible because the reduction is based on a physical model. As an added bonus, the algorithm reduces noise and improves sensitivity. A single false-color image summarizes all the information from the entire sequence, simplifying the task of identifying cracks

High‐Sensitivity Air‐Coupled Ultrasonic Imaging with the First‐Order Symmetric Lamb Mode at Zero Group Velocity

A new method for high‐sensitivity non‐contact, through‐transmission, air‐coupled imaging of small material property changes or discontinuities in plates is demonstrated. Our approach exploits the excitation of the first‐order symmetric Lamb wave mode at its minimum frequency point of zero group velocity. Because this Lamb wave resonance couples energy extremely efficiently with the air and does not propagate energy in the plane of the plate, it is the dominant mode of transmission of an airborne focussed‐beam broadband impulse through the plate. We take advantage of the sensitivity of this mode by performing C‐scans at the frequency of the group‐velocity zero to image spatial discontinuities and property changes. Our results show that images measured at this frequency are more sensitive and more consistent than those measured elsewhere in the plate‐wave spectrum

Air-coupled acoustic imaging with zero-group-velocity Lamb modes

A Lamb wave resonance has been found that allows unusually efficient transmission of airborne sound waves through plates. This occurs at the zero-group-velocity point at the frequency minimum of the first-order symmetric (S1 ) Lamb mode. At this frequency, plane waves with a range of incident angles can couple between the air and the Lamb mode in the solid plate, dominating the spectrum of transmitted focused sound beams by 10 dB or more. We use this frequency for C-scan imaging, and demonstrate the detection of both a 3.2-mm-diameter buried flaw and a subwavelength thickness changes of .005l ~1%!

Full-field Vibration Measurement for Vibrothermography

Vibrothermography is a nondestructive technique for finding defects through vibration‐induced heating imaged with an infrared camera. To model the crack heating process in Vibrothermography, it is essential first to understand the vibration that causes heat generation. We describe a method for calculating internal motions from surface vibrometry measurements. A reciprocity integral and Gauss\u27s law allow representation of internal motion by a surface integral of boundary motion times the Green\u27s Function. We present experimental results showing internal motions calculated from measured surface motions of a vibrating sample. This will ultimately allow estimation of the detectability of a hypothetical crack at an arbitrary location in a specimen

Automatic determination of acoustic plate source-detector separation from one waveform

We discuss an automated procedure for determining the separation between a transient acoustic source and a detector on a plate. We use a time-estimation algorithm based on the assumption that the detected signal is represented by a small, finite number of discrete band-limited impulses. This is carried out using the MUSIC (multiple signal classification) algorithm in time-estimation mode to automatically estimate both the first arrival time of the lowest order antisymmetric (A/sub 0/) mode and the arrival time of the Rayleigh wave. Using the material and geometric properties of the plate and these two arrival times, we calculated the distance to the source. This technique allows the automatic determination of source-receiver separation from a single transient waveform

Model-based Inversion for Flash Thermography

The thermal image sequences from thermography experiments are blurred by lateral diffusion and therefore hard to interpret. The widely used one-dimensional heat flow model provides a robust interpretation of thickness or delamination from “break time” where lateral diffusion is significant, but is less effective otherwise. As a result, it remains quite common to interpret defects by contrast from the surrounding “acreage” rather than by the intrinsic properties of the defect signal itself. In this paper, we present an approach for model-based inversion of flash thermography image sequences that attempts to approximately reconstruct the flow or back-surface geometry from the thermal image sequence. The reconstruction is based on representing reflectors as buried heatsources and interpretingthe spatial and temporal heat distribution on the surface as a linear combination of the Green’s functions of those sources through linear inversion. The result is a spatial map of reflector intensity at a series of layers. Resolution decreases with depth, representing the inherent blurring due to thermal diffusion. The reconstruction is not perfect; the representation of lateral diffusion is approximate and the reconstruction causes substantial noise gain. Defects behind or nearly behind other defects may not be represented correctly. But the reconstruction does provide a physical interpretation that includes lateral heat flows observed in a flash thermography experiment