Daniel L. Smythe

Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre

The cores of most galaxies are thought to harbour supermassive black holes, which power galactic nuclei by converting the gravitational energy of accreting matter into radiation. Sagittarius A* (Sgr A*), the compact source of radio, infrared and X-ray emission at the centre of the Milky Way, is the closest example of this phenomenon, with an estimated black hole mass that is 4,000,000 times that of the Sun. A long-standing astronomical goal is to resolve structures in the innermost accretion flow surrounding Sgr A*, where strong gravitational fields will distort the appearance of radiation emitted near the black hole. Radio observations at wavelengths of 3.5 mm and 7 mm have detected intrinsic structure in Sgr A*, but the spatial resolution of observations at these wavelengths is limited by interstellar scattering. Here we report observations at a wavelength of 1.3 mm that set a size of microarcseconds on the intrinsic diameter of Sgr A*. This is less than the expected apparent size of the event horizon of the presumed black hole, suggesting that the bulk of Sgr A* emission may not be centred on the black hole, but arises in the surrounding accretion flow.

Jet-launching structure resolved near the supermassive black hole in m87

Black Hole Close-Up M87 is a giant elliptical galaxy about 55 million light-years away. Accretion of matter onto its central massive black hole is thought to power its relativistic jet. To probe structures on scales similar to that of the black holes event horizon, Doeleman et al. (p. 355, published online 27 September) observed the relativistic jet in M87 at a wavelength of 1.3 mm using the Event Horizon Telescope, a special purpose, very-long-baseline interferometry array consisting of four radio telescopes located in Arizona, California, and Hawaii. The analysis suggests that the accretion disk that powers the jet orbits in the same direction as the spin of the black hole. High-resolution observations of the jet in the galaxy M87 probe structures very close to the galaxy’s central black hole. Approximately 10% of active galactic nuclei exhibit relativistic jets, which are powered by the accretion of matter onto supermassive black holes. Although the measured width profiles of such jets on large scales agree with theories of magnetic collimation, the predicted structure on accretion disk scales at the jet launch point has not been detected. We report radio interferometry observations, at a wavelength of 1.3 millimeters, of the elliptical galaxy M87 that spatially resolve the base of the jet in this source. The derived size of 5.5 ± 0.4 Schwarzschild radii is significantly smaller than the innermost edge of a retrograde accretion disk, suggesting that the M87 jet is powered by an accretion disk in a prograde orbit around a spinning black hole.

1.3 mm WAVELENGTH VLBI OF SAGITTARIUS A*: DETECTION OF TIME-VARIABLE EMISSION ON EVENT HORIZON SCALES

Sagittarius A*, the ~4 × 10^6 M_⊙ black hole candidate at the Galactic center, can be studied on Schwarzschild radius scales with (sub)millimeter wavelength very long baseline interferometry (VLBI). We report on 1.3 mm wavelength observations of Sgr A* using a VLBI array consisting of the JCMT on Mauna Kea, the Arizona Radio Observatory’s Submillimeter Telescope on Mt. Graham in Arizona, and two telescopes of the CARMA array at Cedar Flat in California. Both Sgr A* and the quasar calibrator 1924−292 were observed over three consecutive nights, and both sources were clearly detected on all baselines. For the first time, we are able to extract 1.3mmVLBI interferometer phase information on Sgr A* through measurement of closure phase on the triangle of baselines. On the third night of observing, the correlated flux density of Sgr A* on all VLBI baselines increased relative to the first two nights, providing strong evidence for time-variable change on scales of a few Schwarzschild radii. These results suggest that future VLBI observations with greater sensitivity and additional baselines will play a valuable role in determining the structure of emission near the event horizon of Sgr A*.

PERSISTENT ASYMMETRIC STRUCTURE OF SAGITTARIUS A* ON EVENT HORIZON SCALES

The Galactic Center black hole Sagittarius A* (Sgr A*) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Potentially much richer source information is contained in the phases. We report on 1.3 mm phase information on Sgr A* obtained with the EHT on a total of 13 observing nights over 4 years. Closure phases, the sum of visibility phases along a closed triangle of interferometer baselines, are used because they are robust against phase corruptions introduced by instrumentation and the rapidly variable atmosphere. The median closure phase on a triangle including telescopes in California, Hawaii, and Arizona is nonzero. This result conclusively demonstrates that the millimeter emission is asymmetric on scales of a few Schwarzschild radii and can be used to break 180-degree rotational ambiguities inherent from amplitude data alone. The stability of the sign of the closure phase over most observing nights indicates persistent asymmetry in the image of Sgr A* that is not obscured by refraction due to interstellar electrons along the line of sight.

SAW/FET Programmable Transversal Filter with 100-MHz Bandwidth and Enhanced Programmability

The authors previously reported on a SAW/FET filter that uses air-gap coupling of a LiNbO/sub 3/ SAW delay line to a Si integrated circuit, thus implementing a programmable tapped delay line. The device incorporates edge-bonded transducers for wide bandwidth and short (15 ns) pedestal of delay. Modifications to the Si circuit are reported here, which both increase the programmable RF bandwidth to 100 MHz and provide means of increasing the programmable tap on/off ratio to 30 dB. This filter has 350 metal-oxide-silicon (MOS) analog sampling fingers spanning 1/5 microsec. of delay. The varactor inherent in each sampling finger provides programmable tap weighting; a digital shift register controls programming FETs that connect each MOS varactor tap to a common programming voltage line. The 100-MHz bandwidth results from doubling the number of taps by the addition of a 2-to-1 analog multiplexer to the FET array.

Interference temperature measurements from 70 to 1500 MHz in suburban and rural environments of the Northeast

We report sensitive spectrum measurements from 70 to 1500 MHz of the radio interference (RFI) environment at locations in Westford, Massachusetts and Hancock, New Hampshire. These measurements show that below 400 MHz the interference in bands allocated to passive services is often limited by unintentional signals from power lines and part 15 devices

SAW/FET programmable filter with varistor taps for improved performance

The SAW (surface acoustic wave)/FET (field-effect transistor) programmable transversal filter utilizes a LiNbO/sub 3/ SAW delay line that is coupled to a silicon integrated circuit across a 300 nm air gap. A SAW/FET with an integrated circuit that incorporates a varistor-controlled tap-weighting circuit has been built. The varistor is a MOSFET (metal-oxide-silicon FET) whose resistance determines the tap weight and is controlled by the voltage stored on the capacitance of the MOSFET gate. The integrated circuit contains the taps, tap weighting and RF summing circuitry to accomplish the transversal filter function. A digital shift register operates as a commutator that allows serial programming of the individual analog tap weights. There are 350 analog taps that yield a 1.5- mu s delay with a programmable bandwidth of 100 MHz at a center frequency of 175 MHz. The programmable tap on/off is greater than 30 dB with a +or-1.0-dB uniformity. When programmed as a narrowband filter, the out-of-band rejection is greater than 35 dB across the entire 100-MHz programmable bandwidth.<<ETX>>

A SAW/CCD programmable matched filter

A charge‐coupled device (CCD) has been integrated with a surface‐acoustic‐wave (SAW) piezoelectric delay line in a hybrid gap‐coupled structure to produce an electronically programmable analog matched filter in which the filter function is controlled by the charge pattern clocked into the CCD. This device allows analog‐analog correlation of SAW input signals of ≲40‐MHz bandwidth and ≲3.5‐μs duration. Programming time is 6 ms at a 100‐kHz clock rate, and the reference holding time is 1 s for a 3‐dB degradation of the correlation peak. Operation as a matched filter for a 3.25‐μs‐long 13‐bit Barker‐encoded signal yields a peak correlation level of −41 dB m.

Adaptive and Matched Filtering with a SAW/FET Programmable Transversal Filter

We describe the operation of a SAW/FET programmable transversal filter as both a matched filter and an adaptive filter. The SAW/FET device incorporates surface-acoustic-wave (SAW) signal propagation in conjunction with a field-effecttransistor(FET)-controlled sampling array. The SAW/FET device has a 1.5-us impulse response, 350 taps, and 100-MHz programmable bandwidth centered at 175 MHz. The on/off ratio of each individual tap is 30 dB. As a matched filter, the SAW/FET device is demonstrated using both 31- and 63-chip m-sequences, showing nearly ideal sldelobe performance. The adaptive processor uses a digital computer to calculate the tap-weight updates according to the least-mean-squares (LMS) algorithm and achieves up to 25 dB of CW cancellation.

A SAW/CCD accumulating correlator

A new analog‐signal‐processing device is reported in which a charge‐coupled device (CCD) is coupled to a piezoelectric surface acoustic wave (SAW) delay line across a 300‐nm gap. The CCD samples multiply, accumulate, and read out the cross‐correlation function of two wide‐band SAW input signals which counterpropagate on the delay line. A signal‐processing gain of 30 dB at a bandwidth of 20 MHz has been observed by correlating biphase modulated pseudonoise waveforms of 100‐μs duration in the presence of Gaussian noise. This compact hybrid structure provides 300 discrete samples of a 7‐μs segment of the correlation function.