Abstract Detecting the 21 cm power spectrum from the Cosmic Dawn requires extremely accurate calibration and substantial suppression of foreground emission. In this study, we analyze drift-scan observations in the Murchison Widefield Array ultra-low-frequency band (73.6–104.3 MHz) and demonstrate from real data that the gain autocorrelation functions have zero-crossing timescales of ∼2–3 h, while the Allan-deviation minima indicate characteristic stability timescales of ∼45 min for the gain amplitude and 1.3 h for the gain phase. Using simulations, we further evaluate how representative catalog errors—positional errors, flux-scale errors, and catalog incompleteness—induce frequency-dependent structures in the calibration gains and generate systematic power within the Epoch of Reionization window. The results show that the accuracy required for the calibration sky catalog depends on the assumed amplitude of the 21 cm signal. For strong absorption scenarios such as those suggested by EDGES, the signal remains detectable even with $\sim$0.1% flux-scale errors and arcsecond-level positional accuracy. However, for standard scenarios, we find that $\sim$0.01% flux-scale accuracy and ${\sim }0{_{.}^{\prime\prime}} 5$ positional accuracy are sufficient to achieve the required dynamic range. We also find that source completeness down to $1\, \mathrm{mJy}$ remains inadequate for achieving the necessary dynamic range for either the strong or weak standard 21 cm signal scenario, although it is sufficient for the EDGES-inspired exotic scenario. Motivated by the gain-stability timescale indicated by the Allan-deviation analysis, we apply a gain moving average in time (${\sim } 50$ min), which reduces the standard deviation of the gain by 30%–60% and partially relaxes the requirements on the calibration catalog. These results demonstrate that the accuracy of the calibration catalog is a key limiting factor for low-frequency 21 cm cosmology and indicate that future Cosmic Dawn observations will require next-generation catalogs with improved accuracy and depth.
Astrometric observations provide a powerful means of determining the distances, motions, and properties of stars, particularly in binary systems where orbital elements enable direct mass measurements. We investigate how binary orbital motion affects astrometric precision through degeneracies with proper motion and annual parallax. Using an analytic approach based on the variance-covariance matrix, we examine the dependence of measurement precision on the observational time span, sampling pattern, and orbital parameters. We first analyze circular orbits with one-dimensional observations. For uniformly sampled observations, position precision remains robust regardless of the binary period, while proper motion and annual parallax exhibit significant degeneracies with binary motion at periods of >2.2 and 0.8-1.5 yr for 3-yr observations, and >4.2 and 0.9-1.2 yr for 5-yr observations, respectively. We then extend the analysis to non-uniform sampling, showing that additional degeneracies arise at shorter periods (0.3-0.7 yr), although their impact is generally limited. Furthermore, we investigate binary systems with elliptical orbits observed from various viewing directions and analyze the precision of the orbital parameters. We find that positional precision deteriorates by a factor of a few as eccentricity increases. The precisions of the semi-major axis and orbital inclination degrade similarly, remaining within a factor of a few for moderate eccentricities (e 0.8). In contrast, the precision of the eccentricity itself shows little variation over the parameter space. The reference epoch, argument of periastron, and longitude of the ascending node are generally determined with poor precision. For e 0.8, the precisions of the position coordinates, semi-major axis, eccentricity, and orbital inclination remain within a factor of approximately 5 of the fundamental astrometric precision scale, sigma/root N. These results clarify the conditions under which astrometric measurements of binary systems are reliable and highlight the importance of observational strategy and orbital characteristics in determining parameter precision
Abstract Striae are fine structures occasionally observed in the dust tails of comets, typically near perihelion, shortly after a comet begins receding from the Sun. In this study, we investigate the observational conditions under which striae appear by examining eleven comets in which striae were observed during their past apparitions. Publicly available images are primarily used, and the geometric configurations for ground-based observations are compared. Apparent total magnitudes are also considered. We find that, with the exception of the unusual case of C/1995 O1 (Hale–Bopp), striae are most likely to be observed from the ground when the heliocentric distance $r \lt 0.7 \, \mathrm{au}$ and the perihelion distance $q \lt {0.6} \, \mathrm{au}$, within approximately 22 days after perihelion passage. Favorable conditions further include a phase angle $40^\circ \lt \alpha \lt 120^\circ$, a line-of-sight angle to the comet’s orbital plane more than $20^\circ$ from edge-on, and an apparent total magnitude brighter than roughly 2–3. Although apparent total magnitude depends not only on geocentric distance but also on intrinsic physical properties such as nucleus size, dust-to-gas ratio, and dust production rate, these properties show complex dependencies on heliocentric distance. Therefore, we focus on geometric parameters and apparent total magnitude as practical indicators for the detection of striae in future apparitions. Based on these criteria, we present a simple flowchart for evaluating the likelihood of detecting striae in ground-based observations.
In large-scale astronomical data classification, the accurate classification of galaxies, stars, and quasars typically relies on spectral labels. However, spectral data account for only a small fraction of all astronomical observations, making the automatic fine classification of massive photometric data a persistent challenge. To tackle this challenge, we propose a photometric celestial source detection and classification algorithm based on YOLOv11 and a Hybrid Vision Transformer (HybridViT) embedded with the Astrostem convolutional module. We first utilize YOLOv11 to detect and localize photometric celestial sources, then feed the detected sources into the HybridViT network for fine-grained classification. By introducing a task-adaptive convolutional stem between the raw input and the Patch Embedding layer, HybridViT enhances the extraction of local morphological features for both point sources and extended sources. Experimental results demonstrate that YOLOv11 achieves an mAP@0.5 of 0.979 and an mAP@0.5:0.95 of 0.897. On the SDSS DR16 dataset, HybridViT achieves three-class classification of stars, quasars, and galaxies with an accuracy of 0.892. On the Galaxy Zoo 2 (GZ2) dataset, it further achieves fine-grained morphological classification of six galaxy types-barred spiral galaxies, cigar galaxies, elliptical galaxies, intermediate galaxies, spiral galaxies, and irregular galaxies-with an accuracy of 0.940. The proposed model outperforms mainstream CNN baselines across all metrics. The HybridViT model incorporating Astrostem can accurately extract morphological features from photometric sources, effectively improving the recognition accuracy for various celestial types, and is well suited for intelligent classification of large-scale sky survey photometric data.
Abstract Wave equations describing acoustic waves in transonic accretion flows have singular behaviors at the transonic point, because any acoustic informations cannot transport themselves upstream through the sonic radius. Under a simplified geometrically thin (relativistic) advection-dominated disk, the conditions that the singularity at the sonic radius is not a real singularity but a regular (apparent) one are derived for the inertial-acoustic oscillations (p-mode oscillations). A purpose of this derivation is to use the results as the inner boundary conditions in studying numerically the behaviors of the global trapped p-mode oscillations in transonic accretion disks. The p-mode oscillation, which is degenerated to a single mode in the disk with no accretion flow, is split into two ones by the effects of accretion flows. For each of these two split modes, the conditions of regular singularity at the sonic radius are derived. The derived conditions should be used as inner boundary conditions of oscillations when we examine trapping of global p-mode oscillations in transonic advection-dominated accretion disks.
