The BYU 0.9-meter Reflector

We study the broadband emission of Mrk 501 using multiwavelength observations from 2017 to 2020 performed with a multitude of instruments, involving, among others, MAGIC, Fermi's Large Area Telescope (LAT), NuSTAR, Swift, GASP-WEBT, and the Owens Valley Radio Observatory. Mrk 501 showed an extremely low broadband activity, which may help to unravel its baseline emission. Nonetheless, significant flux variations are detected at all wave bands, with the highest occurring at X-rays and very-high-energy (VHE) γ-rays. A significant correlation (>3σ) between X-rays and VHE γ-rays is measured, supporting leptonic scenarios to explain the variable parts of the emission, also during low activity. This is further supported when we extend our data from 2008 to 2020, and identify, for the first time, significant correlations between the Swift X-Ray Telescope and Fermi-LAT. We additionally find correlations between high-energy γ-rays and radio, with the radio lagging by more than 100 days, placing the γ-ray emission zone upstream of the radio-bright regions in the jet. Furthermore, Mrk 501 showed a historically low activity in X-rays and VHE γ-rays from mid-2017 to mid-2019 with a stable VHE flux (>0.2 TeV) of 5% the emission of the Crab Nebula. The broadband spectral energy distribution (SED) of this 2 yr long low state, the potential baseline emission of Mrk 501, can be characterized with one-zone leptonic models, and with (lepto)-hadronic models fulfilling neutrino flux constraints from IceCube. We explore the time evolution of the SED toward the low state, revealing that the stable baseline emission may be ascribed to a standing shock, and the variable emission to an additional expanding or traveling shock.

Understanding the acoustic source characteristics of supersonic jets is vital to accurate noise field modeling and jet noise reduction strategies. This paper uses advanced, coherence- based partial field decomposition methods to characterize the acoustic sources in an installed, supersonic GE F404 engine. Partial field decomposition is accomplished using an equivalent source reconstruction via acoustical holography. Bandwidth is extended through the application of an array phase-unwrapping and interpolating scheme. The optimized-location virtual references method is used, and variations on this method are discussed. Apparent source distributions and source-related partial fields are shown as a function of frequency. Local maxima are observed in holography reconstructions at the nozzle lipline, distinct in frequency and space. It is hypothesized that the first local maximum may relate to noise generated by large-scale turbulence structures around and downstream of the supersonic core tip. Other local maxima are correlated primarily with Mach wave radiation originating from throughout the shear layer and into the region downstream of the potential core tip but before the end of the supersonic core. Source-elucidating decompositions show that the order and behavior of the decomposition lends to the local maxima being related to distinct source mechanisms, while between the local maxima, there is a combination of mechanisms active, which is likely the cause of spatiospectral lobes observed previously with other full-scale, supersonic jets.

Spatiospectral lobes are features identified in the noise fields radiated from full-scale tactical aircraft that are unseen in lab-scale experiments. Prior studies have explored lobe frequency-domain characteristics, but a joint time-frequency domain (JTFD) analysis has the potential to further explore these phenomena and connect them to source-related events. This paper applies an event-based beamforming technique to acoustical data collected at a 120-microphone array near a T-7A-installed F404 engine. The algorithm correlates time domain events between pairs of adjacent microphones to find an event propagation direction and then ray traces to the jet centerline to identify an apparent source location. A wavelet transform is used to identify frequency triggers tied to the spatiospectral lobes to gain insights. It is shown that the source responsible for the spatiospectral lobes is composed of multiple, overlapping sources each with a different peak radiation angle. Events are also observed in the time-frequency domain, and it is found that there is an underlying temporal structure reminiscent of mode switching seen with lab-scale jet screech. Using a Markov-style analysis, this temporal structure is characterized. It is found that spectral peaks related to the spatiospectral lobes are composed of discrete but randomly distributed temporospectral events, each with a frequency-dependent directivity.

This paper describes investigations into convective Mach number and its relationship to maximum radiation angle for an installed afterburner-capable military jet engine. The convective Mach number describes the velocity of coherent structures in the turbulent mixing layer of a jet. For supersonic jets, this parameter should be useful in predicting the maximum noise radiation angle. However, of the several definitions of the convective Mach number, none have been successful in predicting the peak radiation angle of all jets. In this paper, physicsbased and empirically derived convective Mach numbers are calculated from data collected from a T-7A-installed GE F404 engine and are compared against measured maximum noise directivity angles. Of the physics-based definitions, the T-7A data how the convective Mach number associated with Oertel’s first family of instability waves predicts the maximum radiation angle within 6° over a range of engine conditions. Additionally, the so-called “Oertel convective Mach number,” which has successfully predicted peak directivity angles in rocket noise studies, is a relatively poor predictor of the T-7A maximum directivity angle. An empirical formulation of the convective Mach number suggests that, for the T-7A, the “convective velocity” of coherent structures in the shear layer is about 60% of the fullyexpanded centerline velocity for supersonic engine conditions. Evaluating this empirical definition of the convective Mach number using data from other jet noise studies shows that the acoustic Mach number appears to be the best predictor of the convective velocity. Finally, a frequency-dependent study of the convective Mach number at afterburner shows the peak directivity angle is roughly constant at low and high frequencies, while the frequencies associated with the transition from the potential core to the supersonic core show the greatest change in directivity.

In September 2021, an Atlas V rocket without solid rocket boosters was launched from Vandenberg Space Force Base, California, carrying the NASA/USGS Landsat 9 satellite. In this launch configuration, the plumes from the RD-180 engine’s two nozzles are unobstructed, providing the opportunity to analyze the sound generated by a liquid-fuel rocket engine with an azimuthally asymmetric nozzle geometry. Acoustical data were collected at various locations surrounding the launch pad, ranging from a few hundred meters to several kilometers. This paper discusses an overview of the measurement logistics, maximum overall sound pressure levels at the measurement stations, and an initial analysis of the azimuthal variability of the overall sound pressure level and spectra along two radials. Within the constraints of this measurement, the Atlas V radiation does not appear to be azimuthally asymmetric.

In 2021 BL Lacertae underwent an extraordinary activity phase, which was intensively followed by the Whole Earth Blazar Telescope (WEBT) Collaboration. We present the WEBT optical data in the BVRI bands acquired at 36 observatories around the world. In mid-2021 the source showed its historical maximum, with R = 11.14. The light curves display many episodes of intraday variability, whose amplitude increases with source brightness, in agreement with a geometrical interpretation of the long-term flux behaviour. This is also supported by the long-term spectral variability, with an almost achromatic trend with brightness. In contrast, short-term variations are found to be strongly chromatic and are ascribed to energetic processes in the jet. We also analyse the optical polarimetric behaviour, finding evidence of a strong correlation between the intrinsic fast variations in flux density and those in polarization degree, with a time delay of about 13 h. This suggests a common physical origin. The overall behaviour of the source can be interpreted as the result of two mechanisms: variability on time-scales greater than several days is likely produced by orientation effects, while either shock waves propagating in the jet, or magnetic reconnection, possibly induced by kink instabilities in the jet, can explain variability on shorter time-scales. The latter scenario could also account for the appearance of quasi-periodic oscillations, with periods from a few days to a few hours, during outbursts, when the jet is more closely aligned with our line of sight and the time-scales are shortened by relativistic effects.