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On 11 September 2021, two small thunderstorms developed over the Telescope Array Surface Detector (TASD) that produced an unprecedented number of six downward terrestrial gamma ray flashes (TGFs) within one‐hour timeframe. The TGFs occurred during the initial stage of negative cloud‐to‐ground flashes whose return strokes had increasingly large peak currents up to −${-}$223 kA, 147 GeV energy deposit in up to 25 1.2 km‐spaced surface detectors, and intermittent bursts of gamma‐rays with total durations up to 717 μ${\upmu }$s. The analyses are based on observations recorded by the TASD network, complemented by data from a 3D lightning mapping array, broadband VHF interferometer, fast electric field change sensor, high‐speed video camera, and the National Lightning Detection Network. The TGFs of the final two flashes had gamma fluences of ≃2×1014$\simeq 2\times 1{0}^{14}$and 8×1014${\times} 1{0}^{14}$, logarithmically bridging the gap between previous TASD and satellite‐based detections. The observations further emphasize the similarity between upward and downward TGF varieties, suggesting a common mechanism for their production. Terrestrial Gamma‐Ray Flashes (TGFs) are high‐energy lightning‐produced radiation events that have primarily been detected by satellites, but are increasingly detected by ground‐based instrumentation. While the upward satellite‐detected events exhibit extremely high numbers of gamma photons, the downward ground‐based versions observed by the Telescope Array Surface Detector have, until now, displayed much weaker numbers by orders of magnitude. In this paper, we report observations of downward TGFs obtained at the large‐area cosmic ray detector Telescope Array facility in west‐central Utah, showing strengths midway between previously detected downward TGFs and upward satellite‐detected events. The results indicate that TGFs span a wide range of fluences, a finding further supported by recent airborne observations over storm tops. Downward‐directed TGFs were observed whose fluences bridge the gap between previous downward and satellite‐detected TGFsThe observations support the idea that satellites detect only the very strongest TGFs of a continuumFurther multi‐instrument observations are expected to provide additional insights into the causes of both short‐ and long‐duration TGFs Downward‐directed TGFs were observed whose fluences bridge the gap between previous downward and satellite‐detected TGFs The observations support the idea that satellites detect only the very strongest TGFs of a continuum Further multi‐instrument observations are expected to provide additional insights into the causes of both short‐ and long‐duration TGFs

Optical emissions associated with Terrestrial Gamma ray Flashes (TGFs) have recently become important subjects in space‐based and ground‐based observations as they can help us understand how TGFs are produced during thunderstorms. In this paper, we present the first time‐resolved leader spectra of the optical component associated with a downward TGF. The TGF was observed by the Telescope Array Surface Detector (TASD) simultaneously with other lightning detectors, including a Lightning Mapping Array (LMA), an INTerFerometer (INTF), a Fast Antenna (FA), and a spectroscopic system. The spectroscopic system recorded leader spectra at 29,900 frames per second (33.44 μ${\upmu }$s time resolution), covering a spectral range from 400 to 900 nm, with 2.1 nm per pixel. The recordings of the leader spectra began 11.7 ms before the −18${-}18$kA return stroke and at a height of 2.37 km above the ground. These spectra reveal that optical emissions of singly ionized nitrogen and oxygen occur between 167 μ${\upmu }$s before and 267 μ${\upmu }$s after the TGF detection, while optical emissions of neutrals (H I, 656 nm; N I, 744 nm, and O I, 777 nm) occur right at the moment of the detection. The time‐dependent spectra reveal differences in the optical emissions of lightning leaders with and without downward TGFs. Spectroscopy is a powerful tool in lightning research, as it enables the identification of chemical components within the lightning channel. In this study, we present time‐resolved leader spectra of the optical components associated with a downward TGF detection. The chemical makeup of these optical emissions includes nitrogen, oxygen, and hydrogen, along with measurements of their intensities and lifetimes. This marks the first time that spectroscopic optical components of lightning leaders associated with downward TGFs have been recorded. Optical emissions of ions appear before and after the detection of the TGF while optical emissions of neutrals appear at the moment of the TGF detection. The first time‐resolved leader spectra (400–900 nm) linked with downward Terrestrial Gamma ray Flash (TGF) were measuredIon optical emissions start 167 μs before detecting the downward TGF and end 267 μs after itNeutral optical emissions occur right at the moment of the downward TGF detection The first time‐resolved leader spectra (400–900 nm) linked with downward Terrestrial Gamma ray Flash (TGF) were measured Ion optical emissions start 167 μs before detecting the downward TGF and end 267 μs after it Neutral optical emissions occur right at the moment of the downward TGF detection

This study was aimed to investigate associations between birth weight and multiple adiposity indicators in youth, and to examine potential mediating effects by biological maturation. This was a school-based study involving 981 Brazilian adolescents aged between 10 and 17 years. Birth weight was reported retrospectively by mothers. Maturation was estimated by age of peak height velocity. Adiposity indicators included body mass index (BMI), waist circumference and percent body fat estimated from triceps and subscapular skinfolds. Multilevel mediation analyses were performed using the Sobel test, adjusted for chronological age, gestational age, cardiorespiratory fitness and socio-economic status. Except for body fat in girls, biological maturation partly or fully mediated (P<0.05) positive relationships between birth weight with all other obesity indicators in both sexes with their respective values of indirect effects with 95% confidence intervals: BMI [boys: 0.44 (0.06–0.82); girls: 0.38 (0.13–0.64)], waist circumference [boys: 1.14 (0.22–2.05); girls: 0.87 (0.26–1.48)] and body fat [boys: 0.60 (0.13–1.07)]. To conclude, birth weight is associated with elevated obesity risk in adolescence and biological maturation seems to at least partly mediate this relationship.

In this paper, we present the first high‐speed video observation of a cloud‐to‐ground lightning flash and its associated downward‐directed Terrestrial Gamma‐ray Flash (TGF). The optical emission of the event was observed by a high‐speed video camera running at 40,000 frames per second in conjunction with the Telescope Array Surface Detector, Lightning Mapping Array, interferometer, electric‐field fast antenna, and the National Lightning Detection Network. The cloud‐to‐ground flash associated with the observed TGF was formed by a fast downward leader followed by a very intense return stroke peak current of −154 kA. The TGF occurred while the downward leader was below cloud base, and even when it was halfway in its propagation to ground. The suite of gamma‐ray and lightning instruments, timing resolution, and source proximity offer us detailed information and therefore a unique look at the TGF phenomena. This study provides the very first simultaneous observations of a downward‐directed terrestrial gamma‐ray flash (TGF) together with its associated cloud‐to‐ground lightning flash using a high‐speed camera in addition to gamma‐ray and radio measurements. The camera, running at 40,000 frames per second, allowed us to check the characteristics of the downward leader, the development stage of the lightning flash, and the luminosity variations in coincidence with TGF production. Simultaneous recordings of a downward‐directed terrestrial gamma‐ray flash (TGF), high‐speed video images, and radio emissionsTGF events occurred while the leader was already branching below cloud base and even when it was halfway in its propagation to groundEnergetic downward‐directed TGFs were associated with fast downward leaders that produced high return stroke peak currents Simultaneous recordings of a downward‐directed terrestrial gamma‐ray flash (TGF), high‐speed video images, and radio emissions TGF events occurred while the leader was already branching below cloud base and even when it was halfway in its propagation to ground Energetic downward‐directed TGFs were associated with fast downward leaders that produced high return stroke peak currents

A sample of carbon-rich stars (C-stars) in the Small Magellanic Cloud (SMC) was selected from the combined 2MASS and DENIS catalogues on the basis of their $J-K_{\rm s}$colour. This sample was extended to include confirmed C-stars from the Rebeirot et al. (1993) spectroscopic atlas. In this combined sample (N= 1149), a smaller number (N= 1079) were found to have MACHO observations. For this sub-sample, light curves were determined and 919 stars were found to have high quality light-curves with amplitudes of at least 0.05 mag. Of these stars, only 4% have a well-defined single period – most of these have multiple well-defined periods, while 15% have highly irregular light-curves. The distribution of the logarithm of the period versus magnitude, colour, period ratio (if applicable), and amplitude was analyzed and compared with previous works. Variable C-stars are distributed in three sequences: B, C, and Dfrom Wood et al. (1999), and do not populate sequences with periods shorter than $\log P \sim 1.5$. Stellar ages and masses were estimated using stellar evolutionary models.

Long Period Variable stars are ubiquitous among the bright red giant branch stars in NGC 5128. Mostly they are found to be brighter than the tip of the first ascent giant branch with magnitudes ranging from about $K=19$to $K=21.5$. They have periods between 155 and 1000 days and K-band amplitudes between 0.1 and 2 mag, characteristic of semi-regular and Mira variables. We compare the colors, periods and amplitudes of these variables with those found in old stellar populations like Galactic globular clusters and Galactic bulge as well as with intermediate-age Magellanic Cloud long period variables. The population of stars above the tip of the red giant branch (RGB) amounts to 2176 stars in the outer halo field (Field 1) and 6072 stars in the inner halo field (Field 2). The comparison of the luminosity functions of the Galactic bulge, M 31 bulge and NGC 5128 halo fields shows an excess of bright AGB stars extending to $M_K\simeq -8.65$. The large majority of these sources belong to the asymptotic giant branch (AGB) population in NGC 5128. Subtracting the foreground Galactic stars and probable blends, at least 26% and 70% of AGB stars are variable in Fields 1 and 2, respectively. The average period of NGC 5128 LPVs is 395 days and the average amplitude 0.77 mag. Many more short period Miras are present in Field 2 than in Field 1 indicating a difference in the stellar populations between the two fields. Period and amplitude distributions and near-IR colors of the majority of LPVs in NGC 5128 are similar to the Galactic bulge variables. However, some ~10% of LPVs have periods longer than 500 days and thus probably more massive, hence younger, progenitor stars. A few carbon star candidates are identified based on their red $J-H$and $H-K$colors.

The first variable star catalogue in a giant elliptical galaxy NGC 5128 (Centaurus A) is presented. Using multi-epoch observations with ISAAC at the VLT we have detected 1504 red variables in two halo fields covering 10.46 arcmin square. For the variables with at least 10 good measurements, periods and amplitudes were determined using Fourier analysis and non-linear sine-curve fitting algorithms. The final catalogue contains 1146 long period variables with well established light curve parameters. Periods, amplitudes, $J_{\rm s}HK_{\rm s}$photometry as well as individual Ks-band magnitudes are provided for all the variables. The distribution of amplitudes ranges from 0.3 to a few magnitudes in the Ks-band, with a median value around 0.7 mag. The amplitudes, mean magnitudes and periods indicate that the majority of variables belong to the class of long period variables with semiregular and Mira variables. Exhaustive simulations were performed in order to assess the completeness of our catalogue and the accuracy of the derived periods.

We resolve stars of the nearest giant elliptical galaxy NGC 5128 using VLT with FORS1 and ISAAC. We construct deep U, Vand Kscolor-magnitude and color-color diagrams in two different halo fields (in the halo and in the north-eastern diffuse shell). In the outer, shell field, at ~14 kpc from the center of the galaxy, there is a significant recent star formation with stars as young as 10 Myr, approximately aligned with the prominent radio and X-ray jet from the nucleus of the host AGN. Ionized gas filaments are evident in ultraviolet images near the area where neutral Hiand CO molecular gas was previously observed. The underlying stellar population of the halo of the giant elliptical is predominantly old with a very broad metallicity distribution. The presence of an extended giant branch reaching $M_{\rm bol}=-5$mag suggests the existence of a significant intermediate-age AGB population in the halo of this galaxy.

We have observed K-band Surface Brightness Fluctuations (SBF) in NGC 1427 and NGC 720. We combine our measurements with distance measurements based on I-band SBF distance moduli and distance moduli derived by other distance indicators. When using distances from I-band SBF, we obtain $\overline M_K = -6.55 \pm 0.27$for NGC 1427 and $\overline M_K = -6.43 \pm 0.18$for NGC 720. When using an average of direct distance measurements, we obtain $\overline M_K = -6.17 \pm 0.23$for NGC 1427 and $\overline M_K = -6.14 \pm 0.24$for NGC 720. For both galaxies, $\overline M_K$lies well above the average value used as a calibration for distance indication ($\overline M_K = -5.61 \pm 0.12$Jensen et al. [CITE]). In NGC 720 the high K-band SBF are most probably due to an intermediate-age population. However, there is no hint from current data for any anomaly in the stellar population of NGC 1427. These large variations of $\overline M_K$in several galaxies underline once more the need for a better understanding of $\overline M_K$as a function of stellar population properties (Mei et al. [CITE]; Liu et al. [CITE]), especially with respect to the potential presence of intermediate-age populations.

We evaluate the method of optical and infrared Surface Brightness Fluctuations (SBF) as a distance indicator and its application on 8-m class telescopes, such as the Very Large Telescope (VLT). The novelty of our approach resides in the development of Monte Carlo simulations of SBF observations incorporating realistic elliptical galaxy stellar population models, the effects induced by globular clusters and background galaxies, instrumental noise, sky background and PSF blurring. We discuss, for each band and in different observational conditions, the errors on distance measurements arising from stellar population effects, data treatment and observational constraints. With 8-m class telescopes, one can extend I-band SBF measurements out to 6000-10 000 km s-1. Integration times in the K-band are too expensive from the ground, due to the high infrared background for large-scale distance determination projects. Nevertheless ground-based K-band measurements are necessary to understand stellar population effects on the SBF calibration, and to prepare future space-based observations, where this band is more efficient.

Anomalously high K-band surface brightness fluctuations (SBF) have been reported in NGC 4489 by Pahre & Mould ([CITE]), Jensen et al. ([CITE]) and Jensen et al. ([CITE]). However, these conclusions were uncertain because of relatively low signal-to-noise data. New high signal-to-noise data for NGC 4489 have been obtained at the NOAO/KPNO 2.1 m and the ESO/La Silla 3.5 m NTT telescopes. Adopting the I-band SBF distance modulus determined by Tonry et al. ([CITE]) and the $\overline M_I$versus ($V-I$) calibration of Tonry et al. ([CITE]), a value of $\overline M_K =-6.29\pm0.10 $mag was derived. Relative to the average empirical $\overline M_K$derived for giant ellipticals by Jensen et al. ([CITE]) ($\overline M_K=-5.61\pm0.12$mag), the detection of an anomalous K-band SBF in NGC 4489 is confirmed at the two sigma level. Such anomalous fluctuations could be caused by an extended giant branch, consisting of either intermediate-age AGB stars above the tip of the first-ascent giant branch or high-metallicity first-ascent giants, or by lack of a full understanding of the K-band SBF calibration. This result raises questions about the accuracy of K-band SBF distance measurements for elliptical galaxies with unknown stellar composition and underscores the need for $\overline M_K$measurements over a larger range of color and luminosity.

The three X-ray selected mid-K stars with the highest values of (fx/fv) in the Einstein Medium Sensitivity Survey have been studied spectroscopically at moderate and high spectral resolution. One of these objects (1E1548.7+1125) is presented as a binary-star system with characteristics similar to the RS CVn class of binaries. A second of these stars (1E1751.0+7046) is a rapid rotator, while the third star (1E1339.9+6030) has not been shown to be either a binary system or a rapid rotator. The presence of both Ca II H and K emission core and high X-ray flux is attributed to chromospheric and coronal activity, but no single underlying cause is indicated.

Current stellar population models have arguably the largest uncertainties in the near-IR wavelength range, partly due to a lack of large and well calibrated empirical spectral libraries. In this paper we present a project whose aim it is to provide the first library of luminosity weighted integrated near-IR spectra of globular clusters to be used to test the current stellar population models and serve as calibrators for future ones. Our pilot study presents spatially integrated K-band spectra of three old (≥10 Gyr) and metal poor ([Fe/H] ~ -1.4), and three intermediate age (1–2 Gyr) and more metal rich ([Fe/H] ~ - 0.4) globular clusters in the LMC. We measured the line strengths of the $\ion{Na}{i}$, $\ion{Ca}{i}$and 12CO (2–0) absorption features. The $\ion{Na}{i}$index decreases with increasing age and decreasing metallicity of the clusters. The $D_{\rm CO}$index, used to measure the 12CO (2–0) line strength, is significantly reduced by the presence of carbon-rich TP-AGB stars in the globular clusters with age ~1 Gyr. This is in contradiction to the predictions of the stellar population models of Maraston (2005, MNRAS, 362, 799). We find that this disagreement is due to the different CO absorption strength of carbon-rich Milky Way TP-AGB stars used in the models and the LMC carbon stars in our sample. For globular clusters with age ≥2 Gyr we find $D_{\rm CO}$index measurements consistent with the model predictions.

Current stellar population models have arguably the largest uncertainties in the near-IR wavelength range, partly due to a lack of large and well calibrated empirical spectral libraries. In this paper we present a project whose aim it is to provide the first library of luminosity weighted integrated near-IR spectra of globular clusters to be used to test the current stellar population models and serve as calibrators for future ones. Our pilot study presents spatially integrated K-band spectra of three old (=10 Gyr) and metal poor ([Fe/H] ~ -1.4), and three intermediate age (1–2 Gyr) and more metal rich ([Fe/H] ~ - 0.4) globular clusters in the LMC. We measured the line strengths of the $\ion{Na}{i}$, $\ion{Ca}{i}$and 12CO?(2–0) absorption features. The $\ion{Na}{i}$index decreases with increasing age and decreasing metallicity of the clusters. The DCOindex, used to measure the 12CO?(2–0) line strength, is significantly reduced by the presence of carbon-rich TP-AGB stars in the globular clusters with age ~1 Gyr. This is in contradiction to the predictions of the stellar population models of Maraston (2005, MNRAS, 362, 799). We find that this disagreement is due to the different CO absorption strength of carbon-rich Milky Way TP-AGB stars used in the models and the LMC carbon stars in our sample. For globular clusters with age = 2 Gyr we find DCOindex measurements consistent with the model predictions.