Journal of Astrophysics and Astronomy最新文献

Hard X-ray photons with energies in the range of hundreds of keV typically undergo Compton scattering when they are incident on a detector. In this process, an incident photon deposits a fraction of its energy at the point of incidence and continues onwards with a change in direction that depends on the amount of energy deposited. By using a pair of detectors to detect the point of incidence and the direction of the scattered photon, we can calculate the scattering direction and angle. The position of a source in the sky can be reconstructed using many Compton photon pairs from a source. We demonstrate this principle in the laboratory by using a pair of Cadmium Zinc Telluride (CZT) detectors sensitive in the energy range of 20–200 keV, similar to those used in AstroSat/CZT Imager (CZTI). The laboratory setup consists of two detectors placed perpendicular to each other in a lead-lined box. The detectors are read out by a custom-programmed Xilinx PYNQ-Z2 FPGA board, and data are then transferred to a personal computer (PC). There are two key updates from CZTI: the detectors are read concurrently rather than serially, and the time resolution has been improved from 20 to 7.5 (mu )s. We irradiated the detectors with a collimated (^{133} texttt {Ba}) source and identified Compton scattering events for the 356 keV line. We run a Compton reconstruction algorithm to correctly infer the location of the source in the detector frame, with a location-dependent angular response measure of 16(^circ )–30(^circ ). This comprises a successful technology demonstration for a Compton imaging camera in the hard X-ray regime. We present the details of our setup, the data acquisition process, and software algorithms, and showcase our results. We also quantify the limitations of this setup and discuss ways of improving the performance in future experiments.

In this study, we present our scientific approach to observationally infer possible correlations of the disruption age with the stellar surface density and two-dimensional radius for 200 open star clusters within the solar neighbourhood. A detailed statistical analysis of number density, core and tidal radius, mass, and log(age) has also been presented using the Gaia EDR3 database. The initial mass and disruption time of the individual clusters are calculated using the analytical relations available in the literature, considering the cluster mass loss mechanism due to stellar evolution and tidal interactions. Considering the mass loss models used in this study, we also observe the variation of the initial mass of the star clusters as a function of their present age. We carry out a linear fit for the variation of the cluster disruption time with its number density, which gives an approximate correlation of the form, (t_{textrm{dis}}propto rho _{o}^{0.230pm 0.052}). We also assess this linear relation using statistical correlations, which results in a moderate coefficient value of 0.3903. This statistical correlation and the power law relation illustrate that disruption time increases with the increasing stellar surface density of the cluster. We also observationally and statistically investigate the correlation between the radius and disruption time of the clusters, but no prominent dependence was found between them.

The thermodynamics of interplanetary coronal mass ejections (ICMEs) is often described using a polytropic process. Estimating the polytopic index ((gamma )) allows us to quantify the expansion or compression of the ICME plasma arising from changes in the plasma temperature. In this study, we estimate (gamma ) for protons inside the magnetic clouds (MCs), their associated sheaths, and ambient solar wind for a large sample of well-observed events observed by the Wind spacecraft at 1 AU. We find that (gamma ) shows a high (({approx }1.6)) – low (({approx } 1.05)) – high (({approx }1.2)) behavior inside the ambient solar wind, sheath, and MCs, respectively. We also find that the proton polytropic index is independent of small-scale density fluctuations. Furthermore, our results show that the stored energy inside MC plasma is not expended in expanding its cross-section at 1 AU. The sub-adiabatic nature of MC plasma implies external heating – possibly due to thermal conduction from the corona. We find that the heating gradient per unit mass from the corona to the protons of MC at 1 AU is ({approx } 0.21) erg cm(^{-1}) g(^{-1}), which is in agreement with the required proton heating budget.

The pulsation of white dwarfs provides crucial information on stellar parameters for understanding the atmosphere and interior structure of these stars. In this study, we present a comprehensive statistical analysis of known ZZ Ceti stars from historical literature. Our dataset includes stellar parameters and oscillation properties from 339 samples, with 194 of them having undergone asteroseismological analysis. We investigated the empirical instability strip of ZZ Ceti stars and confirmed the linear relationship between temperature and weighted mean pulsation periods (WMP). We found that the WMP distribution is well-described with two groups of stars with peak values at ({sim }254) and ({sim }719) s. Using seismic mass and trigonometrical radii derived from GAIA DR3 parallaxes, we tested the mass-radius relationship of white dwarfs through observational and seismic analysis of ZZ Cetis. They are generally larger than the theoretical values, with the discrepancy reaching up to ({sim }15)% for massive stars with a mass estimated by seismology.

Space weather poses significant risks to technical systems and the global economy, making it a critical area of research. Coronal mass ejections (CMEs) are the primary drivers of space weather and can cause intense geomagnetic disturbances. The solar wind (SW) governs CME propagation in the heliosphere and drives geomagnetic storm activities. Understanding the evolution of SW stream interaction regions (SIRs), CMEs, and their interactions in the inner heliosphere is essential for accurately predicting their arrival times and mitigating their impacts. This study presents a review of Space Weather Adaptive Simulation (SWASTi), an indigenous three-dimensional magnetohydrodynamic (MHD) modelling framework, with a focus on its SW and CME modules. Comparative analysis with in situ observations demonstrates the model’s robustness, revealing the significant role of ambient SW conditions in shaping the morphological and dynamical properties of CMEs. The geo-effective impact of CME-CME interactions are also explored with a particular focus on the recent Gannon storm. Furthermore, the study discusses how in situ measurements from the Aditya-L1 mission can synergise with the SWASTi framework. This integrated approach, leveraging Aditya-L1 data and SWASTi’s 3D MHD simulations, provides new insights into the complex behaviour of solar wind, SIRs, and CMEs, promising significant advancements in near-real-time space weather forecasting.

The Semnan University Radio Array (SURA) is a self-triggered radio array located on the roof of the Physics Faculty at Semnan University in Iran. It is designed to detect radio emissions from air showers generated by ultra-high energy (UHE) cosmic rays with energies exceeding (10^{17}) eV. The array consists of 4 Log-Periodic Dipole Antennas (LPDAs) operating in the 40–80 MHz range. In this study, we present a method for reconstructing the core location of extensive air showers (EAS) by comparing the signal intensities of simulated and experimental data. We employ a simulated dense array as a reference and determine the core location by matching the experimental signal intensity of each antenna with the corresponding reference antenna in the simulated dense array. The method is first validated using simulated events to estimate its accuracy. We then apply it to the cosmic ray candidates detected by the SURA. Our results show that the core location can be reconstructed with a minimum error of about 3 m. However, when the characteristics of the shower being reconstructed differ significantly from the reference array, the error increases. To enhance reconstruction precision and computational efficiency, we explore optimizations, including reducing the dense array size and accounting for variations in primary energy and arrival direction. Our findings demonstrate the potential of radio-based techniques for high-precision core location reconstruction, providing valuable insights for future large-scale cosmic ray observatories.

Geomagnetic storms are the result of interaction between Earth’s magnetic field and interplanetary magnetic field conducted by large-scale structures from the Sun, such as coronal mass ejections (CMEs) and stream interaction regions (SIRs). CMEs originate from closed magnetic regions on the Sun, such as active regions and quiescent filament regions. SIRs are formed in the interplanetary medium due to the fast solar wind originating from coronal holes—regions of open magnetic field lines and interacting with the slow wind ahead. Geomagnetic storms have significant space weather consequences, such as geomagnetically induced currents, atmospheric heating, ionospheric density changes and energization of Van Allen belt electrons to relativistic energies. In this paper, we describe a catalog of intense geomagnetic storms with the Dst index (le -)100 nT (https://cdaw.gsfc.nasa.gov/CME_list/dst100), which is obtained from the Dst data made available online by the World Data Center, Kyoto, Japan. The catalog contains detailed information on the solar source – whether a CME or a coronal hole, including the cases that are due to a combined action of the two. The catalog also provides detailed information on the CME sources and coronal holes. We also presented some statistical results derived from the catalog.

We aim to identify the cluster members, estimate cluster properties, study the dynamical state of the clusters as a function of mass, trace the existence of dynamical effects in massive stars, and check for spatial patterns of members in young clusters. We studied 14 young open clusters located within 1 kpc using the data from Gaia DR3 with the membership estimated using the GMM method. The cluster parameters, such as age, distance, metallicity, and extinction were estimated by fitting PARSEC isochrones to the CMDs. These clusters are found to have ages between 6 and 90 Myr, located between 334 and 910 pc, covering a mass range of 0.13–13.77 (hbox {M}_odot ). In five of these clusters, stars from F to M spectral type show increasing velocity dispersion, a signature for dynamical relaxation. We detect high proper motion for B and A-type stars, possible walkaway stars in the other five clusters, Alessi Teutsch 5, ASCC 16, ASCC 21, IC 2395, and NGC 6405. We demonstrate the existence of mass-dependent velocity dispersion in young clusters, suggestive of dynamical relaxation. The typical range of transverse velocity dispersion is found to be 0.40–0.70 km (hbox {s}^{-1}) for young clusters.

CubeSats present unique opportunities for observational astronomy in the modern era. They are useful in observing difficult-to-access wavelength regions and long-term monitoring of interesting astronomical sources. However, conventional telescope designs are not necessarily the best fit for the restricted envelope of a CubeSat. Additionally, fine-pointing stability on these platforms is difficult due to the low mass of the spacecraft, and special allocations within the optical design are needed to achieve stable pointing. We propose afocal telescope designs as the framework to realise imagers and low-resolution spectrographs on CubeSat platforms. These designs help reduce the number of components in the optical chain and aim to improve throughput and sensitivity compared to conventional designs. Additionally, they also provide a fine steering mechanism within a collimated beam section. Fine beam steering within the collimated beam section avoids issues of image degradation due to out-of-plane rotation of the image plane or offset in the rotation axis of the mirror. This permits using simple and mostly off-the-shelf tip-tilt mirrors for beam steering. The designs discussed here also allow for a standard telescope design to be used in many instrument types; thus reducing the complexity as well as the development time and cost. The optical design, performance, and SNR estimations of these designs, along with some interesting science cases, are discussed. Several practical aspects in implementation, such as guiding, tolerancing, choice of detectors, vibration analysis, and laboratory test setups, are also presented.

We investigate the phenomenon of clustering of galaxies in an expanding universe by applying the fluctuation theory. We evaluate the fluctuation moments for the number of particles as well as the correlated fluctuations for number and energy of particles (galaxies), clustering under their mutual gravitation. The correlated fluctuations (langle Delta NDelta Urangle ) show interesting results. The value of (langle Delta Nrangle ) can be both positive and negative, because it is the difference between N and the mean value of N. A negative (langle Delta Nrangle ) corresponds to regions of underdensity and a positive (langle Delta Nrangle ) corresponds to regions of overdensity, as described by the clustering parameter b. The present work is concerned with the region (bge 0), at which gravitational interaction has already started causing the galaxies to cluster. Thus, for this work, the value of (langle Delta Nrangle ) is positive. Similarly, the energy fluctuations (langle Delta Urangle ) can also be both positive and negative. For large correlations, the overdense regions typically have negative total energy, and the underdense regions have usually positive total energy. The critical value at which this switch occurs has been calculated analytically. The results obtained by fluctuation theory closely match those obtained earlier by specific heat analysis and Lee-Yang theory. The evaluation has been extended to multicomponent systems, having a variety of masses. It has been found that the gravitational clustering of galaxies is more sensitive to mass ratios and less sensitive to the number densities of galaxies. This means there is little effect of (nu ) (number density), but a significant effect of (mu ) (mass) of galaxies on the clustering phenomenon. The clustering of galaxies is quicker when the mass of individual galaxies increases. They become nuclei for condensation. As the mass of galaxies increases, the transition from positive to negative energy occurs at a higher stage of clustering than in a single-component system.