Journal of Astrophysics and Astronomy最新文献

A key science project for SKA-Low is the interferometer imaging of spatio-temporal fluctuations in the hydrogen ionization fraction and spin temperature as a diagnostic of cosmic dawn (CD) and epoch of reionization (EoR) in the redshift range (6 lesssim z lesssim 28). However, detection of the global CD/EoR signal, which provides the differential HI 21-cm brightness temperature with respect to the background radiation temperature, is outside the current purview of SKA-Low observing in interferometer mode. This global CD/EoR signal can provide the critical zero-spacing measurement vital for establishing the base level of fluctuations measured by SKA-Low interferometers. We present here, a concept to detect the global CD/EoR signal as a supplement to the CD/EoR key science of SKA-Low. This would be enabled by adding a compact array of outrigger antennas interspersed between the core stations of SKA-Low. The autocorrelations of the outrigger antennas would form the measurement set for the global CD/EoR. The visibilities measured between outriggers and SKA stations would be ‘blind’ to the global CD/EoR signal and would provide a Global Sky Model and calibration of the bandpass and mode-coupling in the primary beam of the outriggers. The global signal measurement capability will strengthen the SKA-Low CD/EoR program.

The cosmological principle states that the Universe is statistically homogeneous and isotropic at large distance scales. Currently, there exist many observations which indicate a departure from this principle. It has been shown that many of these observations can be explained by invoking superhorizon cosmological perturbations and may be consistent with the Big Bang paradigm. Remarkably, these modes simultaneously explain the observed Hubble tension, i.e., the discrepancy between the direct and indirect measurements of the Hubble parameter. We propose several tests of the cosmological principle using SKA. In particular, we can reliably extract the signal of dipole anisotropy in the distribution of radio galaxies. The superhorizon perturbations also predict a significant redshift dependence of the dipole signal, which can be well tested by the study of signals of reionization and the dark ages using SKA. We also propose to study the alignment of radio galaxy axes as well as their integrated polarization vectors over distance scales ranging from a few Mpc to Gpc. We discuss data analysis techniques that can reliably extract these signals from data.

To investigate the star-formation process, we present a multi-wavelength study of a massive star-forming site RAFGL 5085, which has been associated with the molecular outflow, Hii region and near-infrared cluster. The continuum images at 12, 250, 350 and 500 (mu )m show a central region (having (M_textrm{clump}sim 225) (M_{odot })) surrounded by five parsec-scale filaments, revealing a hub-filament system (HFS). In the Herschel column density ((N(textrm{H}_{2}))) map, filaments are identified with higher aspect ratios (length/diameter) and lower (N(textrm{H}_{2})) values ((sim )0.1–(2.4 times 10^{21}) cm(^{-2})), while the central hub is found with a lower aspect ratio and higher (N(textrm{H}_{2})) values ((sim )3.5–(7.0 times 10^{21}) cm(^{-2})). The central hub displays a temperature range of [19, 22.5] K in the Herschel temperature map, and is observed with signatures of star formation (including radio continuum emission). The JCMT (^{13})CO(J (=) 3–2) line data confirm the presence of HFS and its hub is traced with supersonic and non-thermal motions having higher Mach number and lower thermal to non-thermal pressure ratio. In the (^{13})CO position–velocity diagrams, velocity gradients along the filaments towards the HFS appear to be observed, suggesting the gas flow in the RAFGL 5085 HFS and the applicability of the clump-fed scenario.

India has been a significant contributor to the SKA project from 2009, with a special emphasis on the Telescope Manager, the central control system for the SKA Observatory. This paper describes the details of the design and early prototyping phases of the Telescope Manager work package, including the accompanying life cycle activities. It also traces the history of India’s involvement in the work on the Telescope Manager in various phases of the project, describing the contribution starting with the concept design to the currently ongoing construction phase, and looks at the road ahead. The details of the design and the outcomes of the prototyping and development contributed by India in each phase, are also included.

Coronal mass ejections (CMEs), as they can inject a large amounts of mass and magnetic flux into the interplanetary space, are the primary source of space weather phenomena on the Earth. The present review first briefly introduces the solar surface signatures of the origins of CMEs and then focuses on the attempts to understand the kinematic evolution of CMEs from the Sun to the Earth. CMEs have been observed in the solar corona in white-light from a series of space missions over the last five decades. In particular, LASCO/SOHO has provided almost continuous coverage of CMEs for more than two solar cycles until today. However, the observations from LASCO suffered from projection effects and limited field-of-view (within 30 (R_odot ) from the Sun). In 2006, the launch of the twin STEREO spacecraft has made possible multiple viewpoints imaging observations, which enabled us to assess the projection effects on CMEs. Moreover, heliospheric imagers (HIs) onboard STEREO continuously observed the large and unexplored distance gap between the Sun and the Earth. Finally, the Earth-directed CMEs that earlier have been routinely identified only near the Earth at 1 AU in in situ observations from ACE and WIND, could also be identified at longitudes away from the Sun–Earth line using the in situ instruments onboard STEREO. We describe the key signatures for the identification of CMEs using in situ observations. Our review presents the frequently used methods for estimation of the kinematics of CMEs and their arrival time at 1 AU using primarily SOHO and STEREO observations. We emphasize the need of deriving the three-dimensional (3D) properties of Earth-directed CMEs from the locations away from the Sun–Earth line. The results improving the CME arrival time prediction at Earth and the open issues holding back progress are also discussed. Finally, we summarize the importance of heliospheric imaging and discuss the path forward to achieve improved space weather forecasting.

Detection of the redshifted 21-cm signal of neutral hydrogen from the cosmic dawn and the epoch of reionization (EoR) is one of the final frontiers of modern observational cosmology. The inherently faint signal makes it susceptible to contamination by several sources like astrophysical foregrounds and instrumental systematics. Nevertheless, developments achieved in the recent developments will combine to make signal detection possible with the upcoming Square Kilometer Array (SKA), both statistically and via tomography. This review describes an indigenously developed end-to-end pipeline that simulates sensitive interferometric observations. It mainly focuses on the requirements for Hi detection in interferometers. In its present form, it can mimic the effects of realistic point source foregrounds and systematics—calibration error and position error on 21-cm observations. The performance of the pipeline is demonstrated for test cases with 0.01% calibration error and position error. Its performance is consistent across telescope, foreground and signal models. The focus of the simulation pipeline during the initial stages was for EoR science. But since this is a general interferometric simulation pipeline, it will be helpful to the entire SKA user community, irrespective of the science goals.

The unique morphology of head–tail (HT) radio galaxies suggests that radio jets and their intra-cluster medium interact strongly. We conducted a systematic search for HT radio galaxies using the LOFAR two-meter sky survey first data release (LoTSS DR1) at 144 MHz. In this paper, a catalog of 55 new HT radio galaxies is presented, 10 of which are narrow-angle tailed sources (NATs) and 45 are wide-angle-tailed sources (WATs). NATs are characterized by tails that are bent like a narrow ‘V’ shape with a <90(^{circ }) opening angle. The opening angle between jets in WAT radio galaxies is >90(^{circ }), exhibiting wide ‘C’-like morphologies. We found that 30 out of 55 HT radio galaxies reported in this paper are associated with known galaxy clusters. Most of the sources presented in the current paper have redshifts <0.5. Various physical properties and statistical studies of these HT radio galaxies are presented.

This paper describes the polarisation study of a Lynds cloud, LDN 1340, (alpha = 2)h32m and (delta = 73^{circ } 00^prime ) corresponding to galactic coordinates of (ell = 130^{circ }.07) and (b=) 11(^{circ }.6), with emphasis on the RNO 8 area. The cloud has been observed using the 1.2 m telescope at Mt. Abu Infrared Observatory, in the infrared wavelength band using the Near-Infrared Camera, Spectrograph and Polarimeter instrument. The polarimetric observations were used to map the magnetic field geometry around the region. We combined our measurements with archival data from the 2MASS and WISE surveys. The Gaia EDR3 and DR3 data for the same region were used for distance, proper motion, and other astrophysical information. The analysis of the data reveals areas with ordered polarisation vectors in the region of RNO 8. The position angle measurements reveal polarisation due to dichroic extinction which is consistent with the Galactic magnetic field. The magnetic field strength was calculated for the RNO 8 region using the Chandrashekhar–Fermi method and the value estimated is (sim )42 (mu )G.

Possibilities to detect hostless supernova remnants (hl-SNR) in intergalactic medium using the 1 GHz band of the Square Kilometre Array (SKA) are discussed. The optical detection rate of the hl-SN constrained from the Sloan Digital Sky Survey is used to predict a number of detectable hl-SNRs in the radio band. With an anticipated detection sensitivity (({sim },1, ) (upmu )Jy) and angular resolution (({<}1'')) with the SKA, a significant number of hl-SNR are expected to get detected in the nearby galaxy groups, such as local group, Virgo, Fornax and Eridanus. A few very luminous hl-SNR may also be detected in the Coma cluster and other rich galaxy clusters up to about 100 Mpc distance. The identification of individual SNRs against other background radio sources will require sub-arcsec angular resolution at 1 GHz to resolve the characteristic circular morphology of the radio SNRs in nearby groups. At larger distances, where individual SNR cannot be resolved, a population of hl-SNR may also be constrained statistically in a small region, by estimating excess unresolved radio sources without a known optical host in a group or cluster, compared to the number density of the background radio sources at (upmu )Jy flux level.

In this paper, we have studied the relationship between the monthly variations of solar wind plasma velocity, interplanetary magnetic field (IMF) and geomagnetic activity index (Ap) during solar cycles 22–24 (1986–2020). The modulation parameter (ζ ( = ) V * B) is proportional to the product of solar wind plasma velocity, V and strength of the interplanetary magnetic field, B. We have investigated the periodicities and their evolution during solar cycles 22–24 using Fast Fourier Transform (FFT), RobPer periodogram and Continuous Wavelet Transformation methods. The significant periods present in the modulation parameter include the Rieger type, semi-annual period, annual period and quasi-biennial period (mid-term). In this study, we have found that the rotation rate at the base of the convection zone is ~1.30 years. Quasi-biennial oscillations (QBOs) of solar wind plasma velocity, IMF, modulation parameters and Ap have been compared. The modulation parameter appears to be a better representative of the geomagnetic changes than the other two.