Journal of Astrophysics and Astronomy最新文献

The Square Kilometre Array Observatory (SKAO) is perhaps the most ambitious radio telescope envisaged yet. It will enable unprecedented studies of the Sun, corona and heliosphere and help to answer many of the outstanding questions in these areas. Its ability to make a vast previously unexplored phase space accessible, also promises a large discovery potential. The Indian solar and heliospheric physics community has been preparing for this science opportunity. A significant part of this effort has been towards playing a leading role in pursuing science with SKAO precursor instruments. This paper briefly summarises the current status of the various aspects of work done as a part of this enterprise and our future goals.

We present an extended study of the Be/X-ray pulsar 2S 1553-542 during its type II outbursts. We have incorporated NICER, Swift-XRT, RXTE-PCA, NuSTAR and FERMI observations to carry out the detailed phase and time resolved spectral analysis of the source. We have summarized the evidence of variability of the cyclotron feature observed in the X-ray continuum of the source with respect to the pulse phases of the pulsar by using the recent NuSTAR observation of 2021 outburst of the source. The time-resolved spectral analysis has been performed by considering RXTE observations of the 2008 outburst of the pulsar. The hardness intensity diagram (HID) has been obtained using 2008 observations in which the intensity follows distinct branches with respect to hardness ratio. Diagonal branch is observed in the high intensity state, whereas the horizontal branch corresponds to the low intensity state. The transition from the diagonal to horizontal branch occurs at the luminosity of ((4.88pm 0.24)times 10^{37}) erg (hbox {s}^{-1}). The photon-index exhibits a weak positive correlation with flux along the diagonal branch and negative correlation along the horizontal branch. The existence of two different diagonal and horizontal branches further reflects the possibility of two different accretion states separated by the critical luminosity. The spin-up rate during the outburst phase is found to depend on the flux and is found to increase with an increase in the flux.

Diffuse radio emission has been detected in a considerable number of galaxy clusters and groups, revealing the presence of pervasive cosmic magnetic fields, and of relativistic particles in the large scale structure of the Universe. Since the radio emission in galaxy systems is faint and its spectrum is steep, its observations are largely limited by the instrument sensitivity and frequency of observation, leading to a dearth of information, more so for lower-mass systems. The recent commissioning or upgrade of several large radio telescope arrays, particularly at the low frequency bands (<GHz) is, therefore, a significant step forward. The unprecedented sensitivity of these new instruments, aided by the development of advanced calibration and imaging techniques, have helped in achieving unparalleled image quality and revolutionised the study of cluster-scale radio emission. At the same time, the development of state-of-the-art numerical simulations and the availability of supercomputing facilities have paved the way for high-resolution numerical modelling of radio emission, and the structure of the cosmic magnetic fields, associated with large-scale structures in the Universe, leading to predictions matching the capabilities of observational facilities. In view of these rapidly-evolving developments in modeling and observations, in this review, we summarise the role of new telescope arrays and the development of advanced imaging techniques and discuss the range of detections of various kinds of cluster radio sources, both in dedicated surveys as well as in numerous individual studies. We pay specific attention to the kinds of diffuse radio structures that have been able to reveal the underlying physics in recent observations. In particular, we discuss observations of large-scale sections of the cosmic web in the form of supercluster filaments, and studies of emission in low-mass systems, such as poor clusters and groups of galaxies, and of ultra-steep spectrum sources, the last two being notably aided by low-frequency observations and high sensitivity of the instruments being developed. We also discuss and review the current theoretical understanding of various diffuse radio sources in clusters and the associated magnetic field and polarisation in view of the current observations and simulations. As the statistics of detections improve along with our theoretical understanding, we update the source classification schemes based on the intrinsic properties of these sources. We conclude by summarising the role of the upgraded GMRT (uGMRT) and our expectations from the upcoming Square Kilometre Array (SKA) observatories.

Radio Frequency Interference (RFI) of impulsive nature is created by sources like sparking on high-power transmission lines due to gap or corona discharge and automobile sparking, and it affects the entire observing frequency bands of low-frequency radio telescopes. Such RFI is a significant problem at the Upgraded Giant Metrewave Radio Telescope (uGMRT). A real-time RFI filtering scheme has been developed and implemented to mitigate the effect on astronomical observations. The scheme works in real-time on pre-correlation data from each antenna and allows the detection of RFI based on median absolute deviation statistics. The samples are identified as RFI-based on user-defined thresholds and are replaced by digital noise, a constant or zeros. We review the testing and implementation of this system at the uGMRT. We illustrate the effectiveness of the filtering for continuum, spectral line and time-domain data. The real-time filter is released for regular observations in the bands falling in 250–1450 MHz, and recent observing cycles show growing usage. Further, we explain the relevance of the released system to the Square Kilometer Array (SKA) receiver chain and possible ways of implementation to meet the computational requirements.

The SKA pulsar search pipeline will be used for real time detection of pulsars. Modern radio telescopes, such as SKA will be generating petabytes of data in their full scale of operation. Hence, experience-based and data-driven algorithms are being investigated for applications, such as candidate detection. Here, we describe our findings from testing a state of the art object detection algorithm called Mask R-CNN to detect candidate signatures in the SKA pulsar search pipeline. We have trained the Mask R-CNN model to detect candidate images. A custom semi-auto annotation tool was developed and investigated to rapidly mark the regions of interest in large datasets. We have used a simulation dataset to train and build the candidate detection algorithm. A more detailed analysis is planned. This paper presents details of this initial investigation highlighting the future prospects.

Understanding the explosion mechanism of type Ia supernova is among the most challenging issues in astrophysics. Accretion of matter on a carbon–oxygen (CO) white dwarf (WD) from a companion star is one of the most important keys in this regard. Our aim is to study the effects of WD composition on various parameters during the accretion of helium-rich matter at a slow rate. We have used the computer simulation code Modules for Experiments in Stellar Astrophysics (MESA) to understand the variations in the properties, such as specific heat ((C_P)) and degeneracy parameter ((eta )). The profile of specific heat shows a discontinuity and that of the degeneracy parameter shows a dip near the ignition region. As expected, the size of WD decreases and g increases during the accretion. However, a red-giant-like expansion is observed after the rapid ignition towards the end. Our study explains the reason behind the delay in onset of helium ignition due to the difference in carbon abundance in a CO-WD. We found that WDs of the lower abundance of carbon, accrete slightly longer before the onset of helium ignition.

We present a study of the molecular cloud in Sh2-112 massive star forming region using the 3-2 transition of CO isotopologues: CO, (^{13})CO and C(^{18})O; supplemented in part by CGPS H i line emission and MSX data. Sh2-112 is an optically visible region powered by an O8V type massive star BD(+)45 3216, and hosts two Red MSX Survey sources: G083.7962(+)03.3058 and G083.7071(+)03.2817, classified as H ii region and young stellar object, respectively. Reduced spectral data products from the James Clerk Maxwell Telescope archive, centered on the two RMS objects with (sim ) (7'times 7') field-of-view each, were utilized for the purpose. The (^{13})CO(3-2) channel map of the region shows the molecular cloud to have filamentary extensions directed away from the massive star, which also seems to be at the edge of a cavity like structure. Multiple molecular cloud protrusions into this cavity structure, host local peaks of emission. The integrated emission map of the region constructed from only those emission clumps, detected above 5(sigma ) level in the position–position–velocity space affirms the same. MSX sources were found distributed along the cavity boundary, where the gas has been compressed. Spectral extraction at these positions yielded high Mach numbers and low ratios of thermal to non-thermal pressures, suggesting a dominance of supersonic and non-thermal motion in the cloud.

The 100 m Green Bank Telescope detected ketenimine (CH(_2)CNH) in absorption towards the star-forming region Sagittarius B2(N) by means of three rotational transitions: 7(_{16})–8(_{08}) at 41.5 GHz, 8(_{19})–9(_{09}) at 23.2 GHz and 9(_{18})–10(_{0,10}) at 4.9 GHz. This information was recently brought to light by Atacama Large Millimeter/submillimeter Array (ALMA). Below 50 GHz, the rotational spectrum of ketenimine is sparse. In this context, we present the 1-pyrroline rotational spectra for the same frequency range. For spectroscopic parameter calculations, we used quantum chemistry. The PGOPHER program has been used to replicate the species’ pure rotational spectrum. This molecule’s rotating spectrum makes it a viable candidate for upcoming astronomical detections because the radio lines can be estimated with a high degree of precision in mm/sub-mm wave region.

With the credible detection of C(_{60}), C(_{70}) and C(_{70}^+) in the interstellar medium (ISM), new prospects have opened up for the search of other fullerenes and their derivatives. Since fullerenes show high proton affinities, their protonated forms should predominate in the ISM, which can easily go through deuterium enrichment. Here, we present the infrared (IR) spectra and standard enthalpy of formation of C(_{60})H(^{+}), C(_{60})D(^{+}), C(_{60})H(_{18}^{+}) and C(_{60})D(_{18}^{+}) using Density Functional Theory (DFT) in singly ionized forms. The obtained computed IR spectra are compared with the observations. The results show that the four mid-infrared bands of neutral C(_{60}) are still visible in C(_{60})H(^{+}) and C(_{60})D(^{+}), but their strength diminishes in C(_{60})H(_{18}^{+}) and C(_{60})D(_{18}^{+}). As a conclusion, it is possible that the IR bands ascribed to C(_{60}) are a mixture of pure and slightly protonated and deuteronated fullerenes. In this way, the observed scattering of the C(_{60}) band ratios could be explained.

Interstellar detection of the straight-chain (n-propyl cyanide, n-C(_{3})H(_{7})CN) and branched-chain (i-propyl cyanide, i-C(_{3})H(_{7})CN) molecules toward the star-forming region, Sagittarius B2(N2) (Sgr B2(N2)) has attracted attention to study the formation mechanism and chemical evolution of branched carbon-chain molecules. These molecules are the precursors of biologically relevant prebiotic molecules, i.e., amino acids. In this light, we consider n-butyl cyanide and higher-order branched chain molecule, t-butyl cyanide from the C(_{5})H(_{9})N isomeric group. Quantum chemical calculations, such as rotational constants, dipole moments and other spectroscopic information will assist to study the chemical evolution and examine the possibility of detecting higher-order branched-chain molecules in high-mass star-forming regions.