Journal of Astrophysics and Astronomy最新文献

Our study focuses on analysing the coronal, transition and chromospheric activity of four rapidly rotating stars located within 50 pc in the solar neighborhood. We have used the multi-wavelength capabilities of AstroSat to investigate the outer atmospheres of AB Dor, BO Mic, DG CVn and GJ 3331. These stars, classified as M and K type active stars, are known for their short rotation periods, leading to increased surface magnetic activity. Our soft X-ray observations provide the coronal properties, such as emission measures, temperatures and elemental coronal abundances. We reported the detection of X-ray flares from AB Dor, BO Mic and DG CVn, while UV light curves reveal flares in both BO Mic and DG CVn.

We present the spectral properties of a selection of comets from both the Jupiter family and Oort cloud in the ultraviolet (UV) and mid-infrared (IR) wavelength regions. Spectroscopic measurements are retrieved from public archival data of the International Ultraviolet Explorer (IUE) and Hubble Space Observatory (HST) in the near-UV (NUV) and far-UV (FUV) wavelengths and reveal emissions from Polycyclic Aromatic Hydrocarbon (PAH) molecules, such as pentacene (C(_{22})H(_{14})) at 1902, 2795 Å and toluene (C(_{7})H(_{8})) at 2681 Å. UV spectra of the comets also show emissions due to CS, Fe II, CO Cameron bands, C I, S I and O I. Mid-IR spectroscopic observations made using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope for a few comets reveal the presence of PAH bands at 6.2, 7.7, 8.6, 11.2 (mu )m. Mid-IR spectra of the comets also reveal the presence of silicate at 9.7 (mu )m. The similarity of PAH signatures observed in the proto-planetary disks of young stellar objects, meteorites, interplanetary dust particles and comets with those observed in the interstellar medium (ISM) suggests a possible scenario for the incorporation of PAH from ISM into the primordial solar nebula.

Pulsation frequencies offer a unique opportunity to probe the interior of hot B subdwarf (sdB) stars. In this study, we present the results by analysing the pulsation properties of the sdB star TIC 293165262 using the TESS 21-sector photometry in Cycles 1, 3 and 5. Fifteen significant frequencies were detected within the frequency range of <500 (mu )Hz, all of which were identified as low-frequency g-mode oscillations. By analysing the two resolved incomplete multiplets with spacings of (sim ) 0.056 and (sim ) 0.099 (mu )Hz, we derived a rotational period of (99.8pm 1.8) days, indicating that TIC 293165262 is a relatively slow rotating single sdB star. The period spacings of (ell =1) and (ell =2) sequences were determined as 264 and 154 s, respectively, which resulted in a mode discriminant of seven (ell =1) modes, three (ell =2) modes and two satisfied the both. During the five observational segments, the frequency and amplitude variations of six significant pulsations were clearly observed, revealing nonlinear weak mode interactions. These linear and nonlinear properties of pulsations in TIC 293165262 highlighted the continuous provision of high-quality photometry for pulsating sdB stars by the TESS mission.

Understanding the spectral properties of sources is crucial for the characterization of the radio source population. In this work, we have extensively studied the ELAIS N1 field using various low-frequency radio observations. For the first time, we presented the 1250 MHz observations of the field using the upgraded Giant Meterwave Radio Telescope (uGMRT) that reach a central off-source RMS noise of (sim )12 (upmu )Jy beam(^{-1}). A source catalog of 1086 sources is compiled at (5sigma ) threshold (>60 (upmu )Jy) to derive the normalized differential source counts at this frequency, which is consistent with existing observations and simulations. We presented the spectral indices derived in two ways: two-point spectral indices and by fitting a power-law. The latter yielded a median (alpha = -0.57pm 0.14), and we identified nine ultra-steep spectrum sources using these spectral indices. Further, using a radio color diagram, we identified the three mega-hertz peaked spectrum (MPS) sources, while three other MPS sources are identified from the visual inspection of the spectra, the properties of which are discussed. In our study of the classified sources in the ELAIS N1 field, we presented the relationship between (alpha ) and z. We found no evidence of an inverse correlation between these two quantities and suggested that the nature of the radio spectrum remains independent of the large-scale properties of the galaxies that vary with redshifts.

X-ray polarimetry is a powerful tool to probe the geometry of the hot X-ray corona in active galactic nuclei (AGN). Here, we present our results on the characterization of the X-ray polarization of the radio-quiet Seyfert-type AGN IC 4329A at a redshift of (z = 0.016). This is based on observations carried out by the Imaging X-ray Polarimeter (IXPE). IXPE observed IC 4329A during 5–15 January 2023, for a total observing time of 458 ks. From the model-independent analysis, we found a polarization degree ((Pi _{X})) of (3.7pm 1.5)% and a polarization position angle ((Psi _{X})) of 61(^{circ }pm 12^{circ }) in the 2–8 keV energy range (at 68% confidence). This is also in agreement with the values of (Pi _{X}) and (Psi _{X}) of (4.7pm 2.2)% and (71^{circ } pm 14^{circ }), respectively, obtained from spectro-polarimetric analysis of the I, Q and U Stokes spectra in the 2–8 keV energy band (at 90% confidence). The value of (Pi _X) in the 2–8 keV band obtained from the model-independent analysis is lower than the minimum detectable polarization (MDP) value of 4.5%. However, (Pi _X) obtained from spectro-polarimetric analysis in the 2–8 keV band is larger than the MDP value. In the 3–5 keV band, we found (Pi _X) of (6.5 pm 1.8), which is larger than the MDP value of 5.5%. The observed moderate value of (Pi _{X}) obtained from the analysis of the IXPE data in the 3–5 keV band argues against a spherical lamp-post geometry for the X-ray corona in IC 4329A; however, considering simulations, the observed polarization measurements tend to favor a conical shape geometry for the corona. This is the first time measurement of X-ray polarization in IC 4329A. Measurements of the X-ray polarization in many such radio-quiet AGN will help in constraining the geometry of the X-ray corona in AGN.

We study detectability of the deviation in interacting and non-interacting cubic Galileon models from the (Lambda )CDM model through the 21-cm power spectrum. We show that the interferometric observations like the upcoming SKA1-mid can detect the deviations in both interacting and non-interacting cubic Galileon model from the (Lambda )CDM model depending on the parameter values.

Incomplete sky analysis of cosmic microwave background (CMB) polarization spectra poses a major problem of leakage between E- and B-modes. We present a machine learning approach to remove this E-to-B leakage using a convolutional neural network (CNN) in presence of detector noise. The CNN predicts the full sky E- and B-modes spectra for multipoles (2 le ell le 384) from the partial sky spectra for (N_textrm{side} = 256). We use tensor-to-scalar ratio (r=0.001) to simulate the CMB polarization maps. We train our CNN using (10^5) full sky target spectra and an equal number of noise contaminated partial sky spectra obtained from the simulated maps. The CNN works well for two masks covering the sky area of (sim )80% and (sim )10%, respectively after training separately for each mask. For the assumed theoretical E- and B-modes spectra, predicted full sky E- and B-modes spectra agree well with the corresponding target spectra and their means agree with theoretical spectra. The CNN preserves the cosmic variances at each multipole, effectively removes correlations of the partial sky E- and B-modes spectra, and retains the entire statistical properties of the targets avoiding the problem of so-called E-to-B leakage for the chosen theoretical model.

With the third data release of the Gaia mission, Gaia DR3 with its precise photometry and astrometry, it is now possible to study the behavior of stars at a scale never seen before. In this paper, we developed new criteria to identify T-Tauri stars (TTS) candidates using UV and optical color-magnitude diagrams (CMDs) by combining the GALEX and Gaia surveys. We found 19 TTS candidates and five of them are newly identified TTS in the Taurus molecular cloud (TMC), not cataloged before as TMC members. For some of the TTS candidates, we also obtained optical spectra from several Indian telescopes. We also present the analysis of distance and proper motion of young stars in the Taurus using data from Gaia DR3. We found that the stars in Taurus show a bimodal distribution with distance, having peaks at (130.17_{-1.24}^{1.31}) pc and (156.25_{-5.00}^{1.86}) pc. The reason for this bimodality, we think, is due to the fact that different clouds in the TMC region are at different distances. We further showed that the two populations have similar ages and proper motion distribution. Using the Gaia DR3 CMD, we showed that the age of Taurus is consistent with 1 Myr.

We have modeled the diffuse background at the Galactic Poles in the far-ultraviolet (FUV: 1536 Å) and the near-ultraviolet (NUV: 2316 Å). The background is well-fit using a single-scattering dust model with an offset representing the extragalactic light plus any other contribution to the diffuse background. We have found a dust albedo of 0.35–0.40 (FUV) and 0.11–0.19 in the NGP ((b > 70^{circ })) and 0.46–0.56 (FUV) and 0.31–0.33 (NUV) in the SGP ((b < 70^{circ })). The differences in the albedo may reflect changes in the dust-to-gas ratio over the sky or in the dust distribution. We find offsets at zero-reddening of 273–286 and 553–581 photons (hbox {cm}^{-2}) (hbox {s}^{-1}) (hbox {sr}^{-1}) Å(^{-1}) in the FUV and NUV, respectively, in the NGP with similar values in the SGP.

In general, the initial designs of periodic orbits around the Lagrangian points and transfers to them from the Earth are generated under the Circular Restricted Three Body Problem (CRTBP) framework and used as reference designs to generate realistic designs under Ephemeris models. The realistic ephemeris designs are quasi-periodic multi-revolution orbits. So, the use of periodic orbits, which are multi-revolution in nature can be explored as reference designs as an alternative. Although, the periodic orbits generated in the Elliptical Restricted Three Body Problem (ERTBP) framework are multi-revolution in nature, in the Sun–Earth system, these orbits have large in-plane and out-of-plane amplitudes and hence, these are not suitable for scientific missions. For viable smaller amplitudes, only quasi-halo orbits do exist around the Sun–Earth Lagrangian point L₁ in the ERTBP framework. Hence, as an alternative to the periodic orbits under the CRTBP framework, the quasi-halo orbits under the ERTBP framework are designed and used as reference designs to generate ephemeris designs. These quasi-halo orbits are generated using a methodology, proposed in this paper, which involves differential evolution, an evolutionary optimization technique. Using a similar approach based on the differential evolution technique, the quasi-halo orbits are designed in the Ephemeris model also. The methodology could generate quasi-halo orbits that do not require any theoretical velocity corrections for about five years (10 revolutions). The pros and cons of reference designs in generating ephemeris design are analyzed. It is found that the design in the Ephemeris model is close to both the initial designs in CRTBP and ERTBP frameworks. However, the use of ERTBP design as the reference design does not result in a reduction in computational time. Furthermore, optimal two-impulse transfers to the quasi-halo orbit from an Earth parking orbit, are generated under the ERTBP framework and the Ephemeris model. The location of insertion and the components of orbit insertion velocity are treated as unknowns and determined using differential evolution. The transfer cost in the Ephemeris model is found to be less compared to transfers in CRTBP and ERTBP frameworks.