| 1. |
- Alfvén, Hannes
(författare)
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Cosmology : Myth or science
- 1984
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Ingår i: Journal of astrophysics and astronomy. - : Indian Academy of Science. - 0250-6335 .- 0973-7758. ; 5:1, s. 79-98
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Tidskriftsartikel (refereegranskat)
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| 2. |
- Chattopadhyay, Tanmoy, et al.
(författare)
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Sub-MeV spectroscopy with AstroSat-CZT imager for gamma ray bursts
- 2021
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Ingår i: Journal of astrophysics and astronomy. - : Springer Nature. - 0250-6335 .- 0973-7758. ; 42:2
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Tidskriftsartikel (refereegranskat)abstract
- Cadmium-Zinc-Telluride Imager (CZTI) onboard AstroSat has been a prolific Gamma-Ray Burst (GRB) monitor. While the 2-pixel Compton scattered events (100-300 keV) are used to extract sensitive spectroscopic information, the inclusion of the low-gain pixels (similar to 20% of the detector plane) after careful calibration extends the energy range of Compton energy spectra to 600 keV. The new feature also allows single-pixel spectroscopy of the GRBs to the sub-MeV range which is otherwise limited to 150 keV. We also introduced a new noise rejection algorithm in the analysis ('Compton noise'). These new additions not only enhances the spectroscopic sensitivity of CZTI, but the sub-MeV spectroscopy will also allow proper characterization of the GRBs not detected by Fermi. This article describes the methodology of single, Compton event and veto spectroscopy in 100-900 keV combined for the GRBs detected in the first year of operation. CZTI in last five years has detected similar to 20 bright GRBs. The new methodologies, when applied on the spectral analysis for this large sample of GRBs, has the potential to improve the results significantly and help in better understanding the prompt emission mechanism.
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| 3. |
- Heald, G., et al.
(författare)
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LOFAR: Recent Imaging Results and Future Prospects
- 2011
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Ingår i: Journal of Astrophysics and Astronomy. - : Springer Science and Business Media LLC. - 0250-6335 .- 0973-7758. ; 32:4, s. 589-598
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Tidskriftsartikel (refereegranskat)abstract
- The Low-Frequency Array (LOFAR) is under construction in the Netherlands and in several surrounding European countries. In this contribution, we describe the layout and design of the telescope, with particular emphasis on the imaging characteristics of the array when used in its 'standard imaging' mode. After briefly reviewing the calibration and imaging software used for LOFAR image processing, we show some recent results from the ongoing imaging commissioning efforts. We conclude by summarizing future prospects for the use of LOFAR in observing the little-explored low-frequency Universe.
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| 4. |
- Rottgering, H., et al.
(författare)
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LOFAR and APERTIF Surveys of the Radio Sky: Probing Shocks and Magnetic Fields in Galaxy Clusters
- 2011
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Ingår i: Journal of Astrophysics and Astronomy. - : Springer Science and Business Media LLC. - 0250-6335 .- 0973-7758. ; 32:4, s. 557-566
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Tidskriftsartikel (refereegranskat)abstract
- At very low frequencies, the new pan-European radio telescope LOFAR is opening the last unexplored window of the electromagnetic spectrum for astrophysical studies. The revolutionary APERTIF-phased arrays that are about to be installed on the Westerbork radio telescope (WSRT) will dramatically increase the survey speed for the WSRT. Combined surveys with these two facilities will deeply chart the northern sky over almost two decades in radio frequency from similar to 15 up to 1400 MHz. Here we briefly describe some of the capabilities of these new facilities and what radio surveys are planned to study fun-damental issues related to the formation and evolution of galaxies and clusters of galaxies. In the second part we briefly review some recent observational results directly showing that diffuse radio emission in clusters traces shocks due to cluster mergers. As these diffuse radio sources are relatively bright at low frequencies, LOFAR should be able to detect thousands of such sources up to the epoch of cluster formation. This will allow addressing many question about the origin and evolution of shocks and magnetic fields in clusters. At the end we briefly review some of the first and very preliminary LOFAR results on clusters.
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| 5. |
- Ryde, Nils
(författare)
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On the cosmic origin of fluorine
- 2020
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Ingår i: Journal of Astrophysics and Astronomy. - : Springer Science and Business Media LLC. - 0250-6335 .- 0973-7758. ; 41:1
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Tidskriftsartikel (refereegranskat)abstract
- The cosmic origin of fluorine, the ninth element of the periodic table, is still under debate. The reason for this fact is the large difficulties in observing stellar diagnostic lines, which can be used for the determination of the fluorine abundance in stars. Here we discuss some recent work on the chemical evolution of fluorine in the Milky Way and discuss the main contributors to the cosmic budget of fluorine.
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| 6. |
- Shaw, Abinash Kumar, et al.
(författare)
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Detecting galaxies in a large H I spectral cube
- 2022
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Ingår i: Journal of astrophysics and astronomy. - : Springer Science and Business Media LLC. - 0250-6335 .- 0973-7758. ; 43:2
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Tidskriftsartikel (refereegranskat)abstract
- The upcoming square kilometer array (SKA) is expected to produce humongous amount of data for undertaking H I science. We have developed an MPI-based PYTHON pipeline to deal with the large data efficiently with the present computational resources. Our pipeline divides such large H I 21-cm spectral cubes into several small cubelets, and then processes them in parallel using publicly available H I source finder SOFIA-2. The pipeline also takes care of sources at the boundaries of the cubelets and also filters out false and redundant detections. By comapring with the true source catalog, we find that the detection efficiency depends on the SOFIA-2 parameters, such as the smoothing kernel size, linking length and threshold values. We find the optimal kernel size for all flux bins to be between 3–5 and 7–15 pixels, respectively, in the spatial and frequency directions. Comparing the recovered source parameters with the original values, we find that the output of SOFIA-2 is highly dependent on kernel sizes and a single choice of kernel is not sufficient for all types of H I galaxies. We also propose the use of alternative methods to SOFIA-2, which can be used in our pipeline to find sources more robustly.
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| 7. |
- Sreehari, H., et al.
(författare)
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Observational aspects of outbursting black hole sources : Evolution of spectro-temporal features and X-ray variability
- 2018
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Ingår i: Journal of astrophysics and astronomy. - : Indian Academy of Sciences. - 0250-6335 .- 0973-7758. ; 39:1
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Tidskriftsartikel (refereegranskat)abstract
- We report on our attempt to understand the outbursting profile of Galactic Black Hole sources, keeping in mind the evolution of temporal and spectral features during the outburst. We present results of evolution of quasi-periodic oscillations, spectral states and possible connection with jet ejections during the outburst phase. Further, we attempt to connect the observedX-ray variabilities (i.e., 'class'/'structured' variabilities, similar to GRS 1915+105) with spectral states of black hole sources. Towards these studies, we consider three black hole sources that have undergone single (XTE J1859+226), a few (IGR J17091-3624) and many (GX 339-4) outbursts since the start of RXTE era. Finally, we model the broadband energy spectra (3-150 keV) of different spectral states using RXTE and NuSTAR observations. Results are discussed in the context of two-component advective flow model, while constraining the mass of the three black hole sources.
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