Journal of Astrophysics and Astronomy最新文献

This research focuses on parametrization of deceleration parameter within the structure of modified symmetric teleparallel gravity or (fleft(Qright)) gravity, where (Q) represents the nonmetricity scalar. To explore evolutionary timeline of the Universe, we considered the logarithmic form: (fleft(Qright)=m+n text,{rm{{ln}}}(Q)), where (m) and (n) are constants. In this context, we utilize a particular form of deceleration parameter given by (qleft(zright)=frac{1}{2}+frac{{q}_{1}z+{q}_{2}}{{(1+z)}^{2}},) where ({q}_{1}), ({q}_{2}) and redshift, (z) are the parameters. This form allows a transition from a decelerating phase to an accelerating phase. Solution for the Hubble parameter is derived using the given parametric form of (q), which is then applied to the Friedmann equations. Following this, we estimated the model parameters’ best-fit values by using 115 supernovae Ia data points and Planck Collaboration (2018). We also focus on testing energy conditions in the context of cosmological acceleration. Moreover, we analysed the evolution of density, pressure, equation of state (EoS) parameter and Om(z) diagnostics to understand accelerated expansion phase of the Universe.

Non-specular meteor trail echoes are radar reflections from plasma instabilities, which are caused by field-aligned irregularities (FAI). Here, we reported on characteristics of non-specular trail echoes frequently detected with 53 MHz Gadanki ((13.5^circ )N, (79.2^circ )E) MST radar. These echoes are characteristically different in size and nature, having non-specular reflections over several range-bins with several seconds of duration. We presented few representative examples of such echoes detected with Gadanki MST radar. These examples were analysed and discussed on case by case to understand their evolutionary mechanism. Based on the current understanding of meteor trail theories, we also discussed the most possible factors responsible for the evolution of such echoes.

In this paper, we presented the relationship between prominence eruptions (PEs) and coronal mass ejections (CMEs) from May 2010 to December 2019 covering most of the solar cycle 24. We used data from the atmospheric imaging assembly (AIA) for PEs and the large angle and spectrometric coronagraph (LASCO) for CMEs. We identified 1225 PEs, with 67% being radial, 32% transverse and 1% failed PEs. The radial and transverse PEs, and the combined set have average speeds of (approx )53, 9 and 38 (text {km s}^{-1}), respectively. PE association with CMEs is examined by assigning a confidence level (CL) from 0 (no association) to 3 (clear association). Out of 1225 PEs, 662 (54%) are found to be associated with CMEs including CLs 1, 2 and 3. Our study reveals that the spatial and temporal relationships between PEs and CMEs vary over the solar cycle. During solar minima, CMEs tend to deflect towards the equator, possibly due to a stronger polar field. Temporal offsets are larger during solar maxima and smaller during the minima. This implies that the PEs appear earlier in LASCO C2 FOV during the minima than the maxima. Among the 662 CMEs associated with PEs, 78% show clear bright core structures. Investigation of morphological and temporal behaviour’s of these CMEs indicate that the prominences evolve into CME cores at higher altitudes suggesting that PEs and CME cores are the same structure. Average speeds of the PEs, CME core and CME leading edge are 62, 390 and 525 (text {km s}^{-1}), respectively. The speed of CME cores is faster than the PEs because the former was observed at larger heights, where they have accelerated to higher speeds.

We are exploring the possibility of carrying out radio interferometric observations of the solar chromosphere at ({approx }) 11.2 GHz (({lambda }=2.68) cm), in both total intensity (Stokes-I) and circularly polarized intensity (Stokes-V), using low-cost commercial dish TV antennas. Here, we present our initial results on the magnetic field strength (B) estimated using data obtained with a prototype set-up, and compare them with similar observations.

Astrophysical compact objects, viz., white dwarfs, neutron stars and black holes, are the remnants of stellar deaths at the end of their life cycles. They are ideal testbeds for various fundamental physical processes under extreme conditions that are unique in nature. Observational radio astronomy with uGMRT and OORT facilities has led to several important breakthroughs in studies of different kinds of pulsars and their emission mechanisms. On the other hand, accretion processes around compact objects are at the core of Indian astronomy research. In this context, AstroSat mission revolutionized spectro-temporal observations and measurements of accretion phenomena, quasi-periodic oscillations, and jet behaviour in binary systems hosting compact objects. Moreover, recently launched XPoSat mission is set to provide an impetus to these high-energy phenomena around compact objects by enabling us to conduct polarization measurements in the X-ray band. Further, during the past decade, numerous gravitational wave signals have been observed from coalescing black holes and neutron stars in binary systems. Recent simultaneous observation of GW170817 event in both gravitational waves and electromagnetic channels has ushered in the era of multi-messenger astronomy. In the future, synergistic efforts among several world-class observational facilities, e.g., LIGO-India, SKA, TMT, etc., within the Indian astrophysics community will provide a significant boost to achieve several key science goals that have been delineated here. In general, this paper plans to highlight scientific projects being pursued across Indian institutions in this field, the scientific challenges that this community would be focusing, and the opportunities available in the coming decade. Finally, we have also mentioned the required resources, both in the form of infrastructural and human resources.

Discoveries in cosmology over the last few decades, using multi-band electromagnetic (EM) observations from radio to gamma rays, have shaped our understanding of the Universe and opened a plethora of open questions. The open questions span from the early stages of the Universe, focused on uncovering the physical processes that governed its formation and rapid expansion, to the later evolutionary phases characterized by a transition from dark matter domination to the current epoch dominated by dark energy components that collectively account for (sim )95% of the Universe’s total energy budget. Though their existence is indicated by multiple independent observations, the law of physics, which governs them remains unknown. In the coming years along with multi-band EM observations from telescopes with better sensitivity, an independent cosmological messenger gravitational waves (GW) spanning over nearly 20 decades in frequencies will be able to probe and bring insights to these open questions from the early phase of the Universe till the current stage, and possibly will unveil cosmic mysteries which are currently unknown. These observations will open discovery space in the early epoch of cosmic acceleration known as cosmic inflation, the nature of dark matter, the cosmic evolution of dark energy, the total mass of neutrinos and beyond standard model particle physics. It will also shed light on the cosmic evolution of galaxies, and black holes, and how their interplay has shaped the observable Universe. Furthermore, the area of multi-messenger cosmology by exploring the synergy between GW, EM and neutrino observations will bring to light several uncharted territories in cosmology and fundamental physics. This document provides a summary of the current progress in cosmology and outlines future directions and prospects in the field.

Fast radio bursts (FRBs) are short-duration, highly-energetic radio transients with unclear origins and emission mechanisms, typically found at cosmological distances. Despite extensive searches, no credible prompt electromagnetic counterparts have been found for extragalactic FRBs. We presented the results from a comprehensive search for prompt X-ray counterparts using AstroSat-CZTI , which regularly detects other high-energy fast transients like gamma-ray bursts (GRBs). Our systematic search in CZTI data for hard X-ray transients temporally and spatially coincident with 572 FRBs yielded no credible counterparts. We estimated flux upper limits for these events and converted them to upper limits on X-ray-to-radio fluence ratios and found them to be distributed between (10^{7}) and (10^{13}) for all the three search timescales – 0.01, 0.1 and 1 s. Using redshifts derived from dispersion measures, we placed ((L_textrm{iso})), upper limits ranging from (10^{49}) to (10^{55}) (mathrm {ergs~s^{-1}})  on isotropic equivalent luminosities. We compared them with the isotropic luminosities of GRBs, to examine potential similarities between these transient classes. Finally, we explored the prospects for X-ray counterpart detections using other current and upcoming X-ray monitors, including Fermi-GBM, Swift-BAT, SVOM-ECLAIRs and Daksha, with next-generation FRB detection facilities, such as DSA-2000, CHORD and BURSTT. Our results highlight that highly sensitive X-ray monitors with large sky coverage, like Daksha, will provide the best opportunities to detect X-ray counterparts of bright FRBs.

This study comprehensively analyses three open star clusters: SAI 16, SAI 81 and SAI 86 using Gaia DR3 data. Based on the ASteCA code, we determined the most probable star candidates ((Pge 50)%) and estimated the number of star members of each cluster as 125, 158 and 138, respectively. We estimated the internal structural parameters by fitting the King model to the observed radial density profiles, including the core, limited and tidal radii. Isochrone fitting to the colour–magnitude diagram provided (log ) age of (9.13 pm 0.04), (8.10 pm 0.04) and (8.65 pm 0.04) and distances (d) of (3790 pm 94), (3900 pm 200) and (3120 pm 30) pc for SAI 16, SAI 81 and SAI 86, respectively. Moreover, we have calculated their projected distances from the galactic plane ((X_{odot }), ( Y_{odot })) as well as their projected distance from the galactic plane ((Z_{odot })), distance from the galactic centre ((R_{gc})) and total mass ((M_{C})) in solar units are about (142pm 12), (302pm 17) and (192pm 14) for SAI 16, SAI 81 and SAI 86, respectively. Examining the dynamical relaxation state indicates that all three clusters are dynamically relaxed. By undertaking a kinematic analysis of the cluster data, space velocity was determined. We calculated the coordinates of the apex point ((A_o,D_o)) using the AD diagram method along with the derivation of solar elements ((S_{odot }), (l_A), (b_A)). Through our detailed dynamic orbit analysis, we determined that the three SAI clusters belong to the young stellar disc, confirming their membership within this component of the galactic structure.

Indian scientists have made significant contributions to the study of supermassive black holes (SMBH) and active galactic nuclei (AGN) through observational efforts, advanced data analysis, and theoretical modelling and simulations. Observational studies have been carried out using India’s own facilities’, such as AstroSat (India’s first dedicated multi-wavelength space observatory in UV and X-ray bands), the Giant Metrewave Radio Telescope (GMRT), and optical/IR ground-based observatories as well as leading international observatories. This paper provides an overview of India’s contributions and outlines a vision for advancing future research in SMBH and AGN. It is an expanded and detailed version of the chapter on SMBH and AGN featured in the recently released Vision Document of the Astronomical Society of India.

X-ray afterglow of the gamma-ray burst (GRB), GRB 170714A, may exhibit an intriguing characteristic: It comprises two plateaus, each followed by a precipitous decay segment. We posit that this unusual feature can be rationalized within the framework of GRB magnetar scenario. Specifically, initial break in the X-ray afterglow is attributed to the decay of magnetic field of the central magnetar, whereas the subsequent break corresponds to the collapse of this magnetar. We argue that the decay of magnetar’s magnetic field is caused by the fall-back accretion. Given this current understanding, we deduce that rare hypercritical fall-back accretion is layered, with density profile of the fallback matter along the radial direction being notably discontinuous. Our work potentially shows the prospect of employing GRBs as a tool to investigate intricacies of highly uncertain fall-back accretion processes.