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Relativistic jets are collimated plasma outflows with relativistic speeds. Astrophysical objects involving relativistic jets are a system comprising a compact object such as a black hole, surrounded by rotating accretion flows, with the relativistic jets produced near the central compact object. The most accepted models explaining the origin of relativistic jets involve magnetohydrodynamic (MHD) processes. Over the past few decades, many general relativistic MHD (GRMHD) codes have been developed and applied to model relativistic jet formation in various conditions. This short review provides an overview of the recent progress of GRMHD simulations in generating relativistic jets and their modeling for observations.

Intestinal stem cells (ISCs) of the fruit fly, Drosophila melanogaster , offer an excellent genetic model to explore homeostatic roles of ISCs in animal physiology. Among available genetic tools, the escargot ( esg ) -GAL4 driver, expressing the yeast transcription factor gene, GAL4 , under control of the esg gene promoter, has contributed significantly to ISC studies. This driver facilitates activation of genes of interest in proximity to a GAL4-binding element, Upstream Activating Sequence, in ISCs and progenitor enteroblasts (EBs). While esg-GAL4 has been considered an ISC/EB-specific driver, recent studies have shown that esg-GAL4 is also active in other tissues, such as neurons and ovaries. Therefore, the ISC/EB specificity of esg-GAL4 is questionable. In this study, we reveal esg-GAL4 expression in the corpus allatum (CA), responsible for juvenile hormone (JH) production. When driving the oncogenic gene, Ras V12 , esg-GAL4 induces overgrowth in ISCs/EBs as reported, but also increases CA cell number and size. Consistent with this observation, animals alter expression of JH-response genes. Our data show that esg-GAL4 -driven gene manipulation can systemically influence JH-mediated animal physiology, arguing for cautious use of esg-GAL4 as a “specific” ISC/EB driver to examine ISC/EB-mediated animal physiology.

Our Galactic Centre, Sagittarius A*, is believed to harbour a supermassive black hole, as suggested by observations tracking individual orbiting stars 1 , 2 . Upcoming submillimetre very-long baseline interferometry images of Sagittarius A* carried out by the Event Horizon Telescope collaboration (EHTC) 3 , 4 are expected to provide critical evidence for the existence of this supermassive black hole 5 , 6 . We assess our present ability to use EHTC images to determine whether they correspond to a Kerr black hole as predicted by Einstein’s theory of general relativity or to a black hole in alternative theories of gravity. To this end, we perform general-relativistic magnetohydrodynamical simulations and use general-relativistic radiative-transfer calculations to generate synthetic shadow images of a magnetized accretion flow onto a Kerr black hole. In addition, we perform these simulations and calculations for a dilaton black hole, which we take as a representative solution of an alternative theory of gravity. Adopting the very-long baseline interferometry configuration from the 2017 EHTC campaign, we find that it could be extremely difficult to distinguish between black holes from different theories of gravity, thus highlighting that great caution is needed when interpreting black hole images as tests of general relativity. The authors predict the ability of the Event Horizon Telescope (in its 2017 campaign) to distinguish between different theories of gravity based on images of Sagittarius A*; they suggest that it will not be possible.

The authors present a streamlined setup for Brillouin scattering observation, which negates the need for an independent reference path by incorporating a variable reflectivity mirror at the terminus of the sensing fibre. The authors’ study uncovers an optimal reflectivity point, specifically at −9 dB, where the signal‐to‐noise ratio (SNR) of the observed Brillouin gain spectrum (BGS) reaches its zenith, approximately doubling the SNR of a conventional simplified setup that utilises Fresnel reflection. Furthermore, the authors elucidate that this setup inherently mitigates the polarisation‐dependent fluctuations of the BGS. A simplified Brillouin observation setup is developed by removing an independent reference path and incorporating a variable reflectivity mirror at the fibre end. This setup inherently mitigates the polarisation‐dependent fluctuations of the Brillouin gain spectrum.

In Brillouin optical correlation-domain reflectometry (BOCDR), sinusoidal modulation is applied to the output frequency of a light source, with spatial resolution inversely related to the modulation amplitude. We have developed an effective method to estimate the modulation amplitude using the width of the noise spectrum caused by Rayleigh scattering, eliminating the need for an optical spectrum analyzer or modifications to existing equipment. However, the Rayleigh noise spectrum often displays a three-layered structure, complicating the identification of the appropriate spectral components for estimating the modulation amplitude. In this work, we investigate the origins of this three-layered Rayleigh noise spectrum and identify the directivity of an optical circulator as the source of the third noise component. As replacing the circulator with alternative optical components is not easy, it remains an essential part of the system. Our analysis shows that the third noise component exhibits significantly small variation in spectral width with changes in modulation frequency compared to the first and second components. This characteristic allows for the effective separation and identification of the third noise component, thereby enhancing the accuracy and convenience of modulation amplitude estimation in BOCDR.

Optical fibre sensors based on Brillouin scattering have been vigorously studied in the context of structural health monitoring on account of their capacity for distributed strain and temperature measurements. However, real-time distributed strain measurement has been achieved only for two-end-access systems; such systems reduce the degree of freedom in embedding the sensors into structures, and furthermore render the measurement no longer feasible when extremely high loss or breakage occurs at a point along the sensing fibre. Here, we demonstrate real-time distributed measurement with an intrinsically one-end-access reflectometry configuration by using a correlation-domain technique. In this method, the Brillouin gain spectrum is obtained at high speed using a voltage-controlled oscillator, and the Brillouin frequency shift is converted into a phase delay of a synchronous sinusoidal waveform; the phase delay is subsequently converted into a voltage, which can be directly measured. When a single-point measurement is performed at an arbitrary position, a strain sampling rate of up to 100 kHz is experimentally verified by detecting locally applied dynamic strain at 1 kHz. When distributed measurements are performed at 100 points with 10 times averaging, a repetition rate of 100 Hz is verified by tracking a mechanical wave propagating along the fibre. Some drawbacks of this ultrahigh-speed configuration, including the reduced measurement accuracy, lowered spatial resolution and limited strain dynamic range, are also discussed. Fibre sensors: ultrafast sampling An optical fibre sensing scheme that measures strain with a high spatial resolution and a very high sampling rate has been developed. Optical fibre sensors based on Brillouin scattering are promising for monitoring structural health. The system built by Yosuke Mizuno of Tokyo Institute of Technology and colleagues measures the frequency shift induced in the fibre’s Brillouin gain spectrum on stretching the fibre. This frequency shift is converted into a phase delay of a sinusoidal waveform, which enables the direct detection of the frequency shift. The approach allows single-point strain measurements to be performed at a rate of up to 100 kilohertz at any point along the fibre. Distributed measurements at multiple points along the fibre are also possible, although at lower repetition rates. Importantly, the scheme only requires access from one end of the fibre.

M87 has been the target of numerous astronomical observations across the electromagnetic spectrum, and very long baseline interferometry has resolved an edge-brightened jet 1 – 4 . However, the origin and formation of its jets remain unclear. In our current understanding, black holes (BH) are the driving engine of jet formation 5 , and indeed the recent Event Horizon Telescope observations revealed a ring-like structure in agreement with theoretical models of accretion onto a rotating Kerr BH 6 . In addition to the spin of the BH being a potential source of energy for the launching mechanism, magnetic fields are believed to play a key role in the formation of relativistic jets 7 , 8 . A priori, the spin, a ⋆ , of the BH in M87 ⋆ is unknown; however, when accounting for the estimates of the X-ray luminosity and jet power, values of a ⋆ ≳ 0.5 appear favoured 6 . Besides the properties of the accretion flow and the BH spin, the radiation microphysics including the particle distribution (thermal 6 and non-thermal 9 , 10 ) as well as the particle acceleration mechanism 11 play a crucial role. We show that general relativistic magnetohydrodynamic simulations and general relativistic radiative transfer calculations can reproduce the broadband spectrum from the radio to the near-infrared regime and simultaneously match the observed collimation profile of M87, thus allowing us to set rough constraints on the dimensionless spin of M87* to be 0.5 ≲  a ⋆  ≲ 1.0, with higher spins being possibly favoured. General relativistic magnetohydrodynamic simulations with general relativistic radiative transfer reproduce remarkably well the broadband spectrum and morphology of the innermost jet of M87, allowing some first rough constraints on the spin of M87*.

Although it is generally accepted that cellular differentiation requires changes to transcriptional networks, dynamic regulation of promoters and enhancers at specific sets of genes has not been previously studied en masse. Exploiting the fact that active promoters and enhancers are transcribed, we simultaneously measured their activity in 19 human and 14 mouse time courses covering a wide range of cell types and biological stimuli. Enhancer RNAs, then messenger RNAs encoding transcription factors, dominated the earliest responses. Binding sites for key lineage transcription factors were simultaneously overrepresented in enhancers and promoters active in each cellular system. Our data support a highly generalizable model in which enhancer transcription is the earliest event in successive waves of transcriptional change during cellular differentiation or activation.

Fibre‐optic distributed Brillouin sensing that utilises correlation‐domain techniques often requires an optical delay fibre in the reference path to control the correlation peak order in the sensing fibre. Nonetheless, a suboptimal delay fibre length can result in decreased performance. In this work, as a first step towards clarifying the optimal delay fibre length for Brillouin correlation‐domain sensing, the authors evaluate the effect of changing the delay fibre length on the Brillouin gain spectra in a full‐length measurement scenario. The authors investigate the impact of delay fibre length on Brillouin gain spectra. Increasing delay fibre length reduces the power of the lower‐frequency peak from a sensing fibre and increases the power of the higher‐frequency peak from the delay fibre. This result indicates the existence of an optimal delay fibre length.

In Brillouin optical correlation-domain reflectometry (BOCDR), spatial resolution relies on the modulation amplitude of the light. We propose a Rayleigh-based method that utilizes the spectral width of Rayleigh-induced noise to measure this amplitude without altering the setup or requiring an optical spectrum analyzer. With high frequency resolution and ease of implementation, our approach enhances the convenience and accuracy of spatial resolution evaluation in BOCDR.