Journal of Astrophysics and Astronomy最新文献

We have performed a detailed analysis on the Teutsch 76 (T76) open cluster using the deep near-infrared (NIR) observations taken with the TANSPEC instrument mounted on the 3.6m Devasthal optical telescope along with the recently available high quality proper motion data from the Gaia data release 3 and deep photometric data from Pan-STARRS1 survey. We have found that the T76 cluster is having a central density concentration with circular morphology, probably due to the star-formation processes. The radius of the T76 cluster is found to be 45(^{prime prime }) (1.24 pc) and 28 stars within this radius were marked as highly probable cluster members. We have found that the cluster is located at a distance of (5.7pm 1.0) kpc and is having an age of (50pm 10) Myr. The mass function slope ((Gamma )) in the cluster region in the mass range of (sim ) (0.75<M/M_odot <5.8) is estimated as (-1.3pm 0.2), which is similar to the value of −1.35 given by Salpeter (1955). The cluster is not showing any signatures of mass-segregation and is currently undergoing dynamical relaxation.

The low-frequency radio telescope of the Square Kilometre Array (SKA) is being built by the international radio astronomical community to (i) have orders of magnitude higher sensitivity and (ii) be able to map the sky several hundred times faster, than any other existing facilities over the frequency range of 50–350 MHz. The sensitivity of a radio telescope array is in general, dependent upon the number of electromagnetic sensors used to receive the sky signal. The total number of them is further constrained by the effects of mutual coupling between the sensor elements, allowable grating lobes in their radiation patterns, etc. The operating frequency band is governed by the desired spatial and spectral responses, acceptable sidelobe and backlobe levels, radiation efficiency, polarization purity and calibratability of sensors’ response. This paper presents a brief review of several broadband antennas considered as potential candidates by various engineering groups across the globe, for the low-frequency radio telescope of SKA covering the frequency range of 50–350 MHz, on the basis of their suitability for conducting primary scientific objectives.

Actively accreting young stellar objects show H(alpha ) emission line in their spectra. We present the results of survey for H(alpha ) emission-line stars in the star-forming region IC 5070 taken with 2-m Himalyan Chandra Telescope. Based on the H(alpha ) slitless spectroscopy data, we identified 131 emission-line stars in (sim )0.29 square degrees area of the IC 5070 region. Using Gaia early data release 3, we estimated the mean proper motion and parallax of the emission-line stars. We also estimated the mean distance and reddening toward the region using the emission-line stars, which are (sim )833 pc and (sim )2 mag, respectively. By examining the locations of these stars in the color–magnitude diagrams constructed using Gaia and PanSTARRS1 data, we found that a majority of the H(alpha ) emitters are young low-mass (({<}1.5, M_odot )) stars. We also compared our catalog of emission-line stars with the available young stellar catalogs and found that most of them are class ii/flat spectrum sources with the spectral type ranging from K to M. Based on the proper motion/parallax values and locations on the color–magnitude diagrams, about 20 emission-line stars are flagged as non-members. The relative proper motion of the emission-line stars with respect to the ionizing source suggest the possibility of the ‘rocket effect’ scenario in the remnant cloud (BRC 31).

Serpens molecular cloud is one of the most active sites of ongoing star formation at a distance of about 300 pc, and hence, is very well-suited for studies of young low-mass stars and sub-stellar objects. In this paper, for the Serpens star-forming region, we found potential members of the Young Stellar Objects (YSOs) population from the Gaia DR3 data and study their kinematics and distribution. We compiled a catalog of 656 YSOs from available catalogs ranging from X-ray to the infrared. We use this as a reference set and cross-match it to find 87 Gaia DR3 member stars to produce a control sample with revised parameters. We queried the DR3 catalog with these parameters and found 1196 stars. We then applied three different density-based machine learning algorithms (DBSCAN, OPTICS and HDBSCAN) to this sample and found potential YSOs. The three clustering algorithms identified a common set of 822 YSO members from Gaia DR3 in this region. We also classified these objects using 2MASS and WISE data to study their distribution and the progress of star-formation in Serpens.

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.