Pub Date : 2015-01-01DOI: 10.1007/978-3-319-09894-4_4
S. Sengupta
{"title":"Brown Dwarfs: The Missing Link Between Stars and Planets","authors":"S. Sengupta","doi":"10.1007/978-3-319-09894-4_4","DOIUrl":"https://doi.org/10.1007/978-3-319-09894-4_4","url":null,"abstract":"","PeriodicalId":50722,"journal":{"name":"Bulletin of the Astronomical Society of India","volume":"27 1","pages":"59-65"},"PeriodicalIF":0.0,"publicationDate":"2015-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51002859","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Compact extragalactic sources can display intensity fluctuations which are caused by fluctuations in the electron density of the interstellar medium. The 1/e time scale of these ‘‘refractive scintillations’’ can be many years. The intensity structure function D1(τ)=<[I—t)−I(T+τ)]2≳ has been computed for various distributions of the density fluctuations in the interstellar medium. The results show that the intensity fluctuations observed for periods much small than the 1/e time scale are dominated by the region near the earth. Thus most observations actually sample only the local region.
{"title":"Refractive Scintillation of Extragalactic Radio Sources","authors":"W. Coles","doi":"10.1063/1.37585","DOIUrl":"https://doi.org/10.1063/1.37585","url":null,"abstract":"Compact extragalactic sources can display intensity fluctuations which are caused by fluctuations in the electron density of the interstellar medium. The 1/e time scale of these ‘‘refractive scintillations’’ can be many years. The intensity structure function D1(τ)=<[I—t)−I(T+τ)]2≳ has been computed for various distributions of the density fluctuations in the interstellar medium. The results show that the intensity fluctuations observed for periods much small than the 1/e time scale are dominated by the region near the earth. Thus most observations actually sample only the local region.","PeriodicalId":50722,"journal":{"name":"Bulletin of the Astronomical Society of India","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2008-06-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1063/1.37585","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"59298245","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2008-02-04DOI: 10.5194/ANGEO-26-47-2008
S. Chakraborty, R. Hajra
Long-term (1978-1990) total electron content (TEC) data have been analyzed to show the dependence of ambient ionization on EUV radiation from the Sun. TEC observations were made at Calcutta (22.58 N, 88.38 E ge- ographic, dip: 32 N), situated virtually below the north- ern crest of the equatorial ionization anomaly. Day-to-day changes in TEC at different local times do not show any sig- nificant correlation with F10.7 solar flux. A good correlation is, however, observed between the F10.7 solar flux and the monthly mean TEC when both are considered on a long-term basis, i.e. either in the ascending (1986-1990) or in the de- scending (1979-1985) phase. In the early morning hours the correlation coefficient maximizes around the 08:00-10:00 h IST interval. The flux independent nature of diurnal TEC is evident around the noon time hours of only a few months in the descending phase for F10.7 values greater than 150 unit. Variation of TEC for the whole time period (1979-1990) also exhibits a prominent hysteresis effect. The remarkable feature of the hysteresis effect is its local time dependence, leading to a temporal flip-over. Solar flux-normalized TEC values show a clear seasonal dependence with asymmetri- cal variations in the two equinoxes. The amplitudes of the equinoctial peaks reveal a prominent local time dependence. A further normalization leads to a typical local time varia- tion of TEC. Based on solar flux, seasonal and local time dependent features of TEC, an empirical formula has been developed to represent the TEC variation in the early morn- ing hours. It yields a quantitative estimate of the solar flux dependent nature of the TEC variation. The formula has been validated using the available TEC data and data from the neu- ral network.
{"title":"Solar control of ambient ionization of the ionosphere near the crest of the equatorial anomaly in the Indian zone","authors":"S. Chakraborty, R. Hajra","doi":"10.5194/ANGEO-26-47-2008","DOIUrl":"https://doi.org/10.5194/ANGEO-26-47-2008","url":null,"abstract":"Long-term (1978-1990) total electron content (TEC) data have been analyzed to show the dependence of ambient ionization on EUV radiation from the Sun. TEC observations were made at Calcutta (22.58 N, 88.38 E ge- ographic, dip: 32 N), situated virtually below the north- ern crest of the equatorial ionization anomaly. Day-to-day changes in TEC at different local times do not show any sig- nificant correlation with F10.7 solar flux. A good correlation is, however, observed between the F10.7 solar flux and the monthly mean TEC when both are considered on a long-term basis, i.e. either in the ascending (1986-1990) or in the de- scending (1979-1985) phase. In the early morning hours the correlation coefficient maximizes around the 08:00-10:00 h IST interval. The flux independent nature of diurnal TEC is evident around the noon time hours of only a few months in the descending phase for F10.7 values greater than 150 unit. Variation of TEC for the whole time period (1979-1990) also exhibits a prominent hysteresis effect. The remarkable feature of the hysteresis effect is its local time dependence, leading to a temporal flip-over. Solar flux-normalized TEC values show a clear seasonal dependence with asymmetri- cal variations in the two equinoxes. The amplitudes of the equinoctial peaks reveal a prominent local time dependence. A further normalization leads to a typical local time varia- tion of TEC. Based on solar flux, seasonal and local time dependent features of TEC, an empirical formula has been developed to represent the TEC variation in the early morn- ing hours. It yields a quantitative estimate of the solar flux dependent nature of the TEC variation. The formula has been validated using the available TEC data and data from the neu- ral network.","PeriodicalId":50722,"journal":{"name":"Bulletin of the Astronomical Society of India","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2008-02-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.5194/ANGEO-26-47-2008","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"70921236","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2006-01-01DOI: 10.1051/0004-6361:20052755
S. Pal, S. Chakrabarti, A. Kraus, S. Mandal
We obtained a broadband radio spectrum of the galactic compact object SS433 through the multi-wavelength campaign using the Giant Meter Radio Telescope (GMRT) and the Effelsberg 100-m Radio Telescope. The observations took place from 2005 January 03 to 2005 February 08 using eight different frequencies. We observed a flare which appeared to have started prior to January 18 and lasted till February 08. The light curves show a progressively larger time-delay as the radio frequency goes down. The peak of the 6 cm lightcurve is delayed by ∼ 3 days with respect to the peak of the 9mm lightcurve. There is an indication of the flattening of the spectrum and possibly a turnover at ∼ 1.5GHz. If we interpret this to be due to synchrotron self-absorption, the required magnetic field would become too large, however, free-free absorption by hot thermal surrounding medium formed due to stellar winds is still a possibility. All sky monitor (ASM) aboard Rossi X-ray Timing Experiment (RXTE) showed very high X-ray count on 2005 January 25 when the flare was well underway. This may be due to slamming of the radio ‘bullets’ with previously ejected, relatively slowly moving material.
{"title":"Broadband radio spectrum of SS433","authors":"S. Pal, S. Chakrabarti, A. Kraus, S. Mandal","doi":"10.1051/0004-6361:20052755","DOIUrl":"https://doi.org/10.1051/0004-6361:20052755","url":null,"abstract":"We obtained a broadband radio spectrum of the galactic compact object SS433 through the multi-wavelength campaign using the Giant Meter Radio Telescope (GMRT) and the Effelsberg 100-m Radio Telescope. The observations took place from 2005 January 03 to 2005 February 08 using eight different frequencies. We observed a flare which appeared to have started prior to January 18 and lasted till February 08. The light curves show a progressively larger time-delay as the radio frequency goes down. The peak of the 6 cm lightcurve is delayed by ∼ 3 days with respect to the peak of the 9mm lightcurve. There is an indication of the flattening of the spectrum and possibly a turnover at ∼ 1.5GHz. If we interpret this to be due to synchrotron self-absorption, the required magnetic field would become too large, however, free-free absorption by hot thermal surrounding medium formed due to stellar winds is still a possibility. All sky monitor (ASM) aboard Rossi X-ray Timing Experiment (RXTE) showed very high X-ray count on 2005 January 25 when the flare was well underway. This may be due to slamming of the radio ‘bullets’ with previously ejected, relatively slowly moving material.","PeriodicalId":50722,"journal":{"name":"Bulletin of the Astronomical Society of India","volume":"34 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2006-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1051/0004-6361:20052755","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"57813509","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2005-03-01DOI: 10.1111/J.1365-2966.2005.08721.X
A. Ahuja, Y. Gupta, D. Mitra, A. Kembhavi
In this paper, we describe a novel experiment for the accurate estimation of pulsar dispersion measures (DMs) using the Giant Metrewave Radio Telescope. This experiment was carried out for a sample of 12 pulsars, over a period of more than one year (2001 January to 2002 May) with observations about once every fortnight. At each epoch, the pulsar DMs were obtained from simultaneous dual-frequency observations, without requiring any absolute timing information. The DM estimates were obtained from both the single-pulse data streams and from the average profiles. The accuracy of the DM estimates at each epoch is ∼1 part in 104 or better, making the data set useful for many different kinds of studies. The time-series of DMs shows significant variations on time-scales of weeks to months for most of the pulsars. An analysis of the mean DM values from these data shows significant deviations from catalogue values (as well as from other estimates in the literature) for some of the pulsars, with PSR B1642−03 showing the most notable differences. From our analysis results it appears that the constancy of pulsar DMs (at the level of 1 in 103 or better) cannot be taken for granted. For PSR B2217+47, we see evidence of a large-scale DM gradient over a 1-yr period, which is modelled as being due to a blob of enhanced electron density sampled by the line of sight. For some pulsars, including pulsars with fairly simple profiles such as PSR B1642−03, we find evidence for small changes in DM values for different frequency pairs of measurement, a result that needs to be investigated in detail. Another interesting result is that we find significant differences in DM values obtained from average profiles and single-pulse data.
{"title":"Tracking pulsar dispersion measures using the Giant Metrewave Radio Telescope","authors":"A. Ahuja, Y. Gupta, D. Mitra, A. Kembhavi","doi":"10.1111/J.1365-2966.2005.08721.X","DOIUrl":"https://doi.org/10.1111/J.1365-2966.2005.08721.X","url":null,"abstract":"In this paper, we describe a novel experiment for the accurate estimation of pulsar dispersion measures (DMs) using the Giant Metrewave Radio Telescope. This experiment was carried out for a sample of 12 pulsars, over a period of more than one year (2001 January to 2002 May) with observations about once every fortnight. At each epoch, the pulsar DMs were obtained from simultaneous dual-frequency observations, without requiring any absolute timing information. The DM estimates were obtained from both the single-pulse data streams and from the average profiles. The accuracy of the DM estimates at each epoch is ∼1 part in 104 or better, making the data set useful for many different kinds of studies. \u0000 \u0000 \u0000 \u0000The time-series of DMs shows significant variations on time-scales of weeks to months for most of the pulsars. An analysis of the mean DM values from these data shows significant deviations from catalogue values (as well as from other estimates in the literature) for some of the pulsars, with PSR B1642−03 showing the most notable differences. From our analysis results it appears that the constancy of pulsar DMs (at the level of 1 in 103 or better) cannot be taken for granted. For PSR B2217+47, we see evidence of a large-scale DM gradient over a 1-yr period, which is modelled as being due to a blob of enhanced electron density sampled by the line of sight. For some pulsars, including pulsars with fairly simple profiles such as PSR B1642−03, we find evidence for small changes in DM values for different frequency pairs of measurement, a result that needs to be investigated in detail. Another interesting result is that we find significant differences in DM values obtained from average profiles and single-pulse data.","PeriodicalId":50722,"journal":{"name":"Bulletin of the Astronomical Society of India","volume":"357 1","pages":"1013-1021"},"PeriodicalIF":0.0,"publicationDate":"2005-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1111/J.1365-2966.2005.08721.X","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"62853661","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2005-01-01DOI: 10.1007/978-3-642-02859-5_90
K. R. Rao, B. E. Reddy
{"title":"FIP effect in RV Tauri stars.","authors":"K. R. Rao, B. E. Reddy","doi":"10.1007/978-3-642-02859-5_90","DOIUrl":"https://doi.org/10.1007/978-3-642-02859-5_90","url":null,"abstract":"","PeriodicalId":50722,"journal":{"name":"Bulletin of the Astronomical Society of India","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2005-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"51069083","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2002-01-01DOI: 10.1088/0264-9381/19/13/307
B P Schmidt
Over the past five years two teams have used type Ia supernovae to trace the expansion of the universe to a look-back time more than 70% of the age of the universe. These observations show an accelerating universe which is best explained by a cosmological constant, or other form of dark energy with an equation of state near w = p/ρ = −1. There are many possible lurking systematic effects. However, while difficult to completely eliminate, none of these appears large enough to challenge current results. However, as future experiments attempt to better characterize the equation of state of the matter leading to the observed acceleration, these systematic effects will ultimately limit progress.
{"title":"Measuring global curvature and cosmic acceleration with supernovae","authors":"B P Schmidt","doi":"10.1088/0264-9381/19/13/307","DOIUrl":"https://doi.org/10.1088/0264-9381/19/13/307","url":null,"abstract":"Over the past five years two teams have used type Ia supernovae to trace the expansion of the universe to a look-back time more than 70% of the age of the universe. These observations show an accelerating universe which is best explained by a cosmological constant, or other form of dark energy with an equation of state near w = p/ρ = −1. There are many possible lurking systematic effects. However, while difficult to completely eliminate, none of these appears large enough to challenge current results. However, as future experiments attempt to better characterize the equation of state of the matter leading to the observed acceleration, these systematic effects will ultimately limit progress.","PeriodicalId":50722,"journal":{"name":"Bulletin of the Astronomical Society of India","volume":"19 1","pages":"3487 - 3503"},"PeriodicalIF":0.0,"publicationDate":"2002-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1088/0264-9381/19/13/307","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"60536825","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2000-03-01DOI: 10.1017/S007418090006109X
Sarbani Basu
Helioseismic techniques allow us to probe the interior of the Sun with very high precision and in the process test the physical inputs to stellar models. The picture of the Sun that has been built in this manner may be termed “The Seismic Sun”. After a brief discussion of some of the inversion techniques used in the process, our current view of the seismic Sun shall be reviewed. What we know so far suggests that the internal structure of the Sun can be represented by a standard model, however, one which has a smoother sound-speed and abundance variation than the solar models with the usual treatment of diffusion.
{"title":"The seismic sun","authors":"Sarbani Basu","doi":"10.1017/S007418090006109X","DOIUrl":"https://doi.org/10.1017/S007418090006109X","url":null,"abstract":"Helioseismic techniques allow us to probe the interior of the Sun with very high precision and in the process test the physical inputs to stellar models. The picture of the Sun that has been built in this manner may be termed “The Seismic Sun”. After a brief discussion of some of the inversion techniques used in the process, our current view of the seismic Sun shall be reviewed. What we know so far suggests that the internal structure of the Sun can be represented by a standard model, however, one which has a smoother sound-speed and abundance variation than the solar models with the usual treatment of diffusion.","PeriodicalId":50722,"journal":{"name":"Bulletin of the Astronomical Society of India","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2000-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1017/S007418090006109X","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"57406552","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1999-11-10DOI: 10.1142/S0217732399002443
C. Sivaram
An impressive variety of recent observations which include luminosity evolutions of high redshift supernovae strongly suggest that the cosmological constant (∧) is not zero. Even though the ∧-term may dominate cosmic dynamics at the present epoch, such a value for the vacuum energy is actually unnaturally small. The difficulties in finding a suitable explanation (based on fundamental physics) for such a small residual value for the cosmological term has led several authors to resort to an anthropic explanation for its existence. Here we present a few examples which invoke phase transitions in the early universe involving strong or electroweak interactions to show how the cosmical term of the correct observed magnitude can arise from fundamental physics involving gravity.
{"title":"A non-anthropic origin for a small cosmological constant","authors":"C. Sivaram","doi":"10.1142/S0217732399002443","DOIUrl":"https://doi.org/10.1142/S0217732399002443","url":null,"abstract":"An impressive variety of recent observations which include luminosity evolutions of high redshift supernovae strongly suggest that the cosmological constant (∧) is not zero. Even though the ∧-term may dominate cosmic dynamics at the present epoch, such a value for the vacuum energy is actually unnaturally small. The difficulties in finding a suitable explanation (based on fundamental physics) for such a small residual value for the cosmological term has led several authors to resort to an anthropic explanation for its existence. Here we present a few examples which invoke phase transitions in the early universe involving strong or electroweak interactions to show how the cosmical term of the correct observed magnitude can arise from fundamental physics involving gravity.","PeriodicalId":50722,"journal":{"name":"Bulletin of the Astronomical Society of India","volume":"47 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"1999-11-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1142/S0217732399002443","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"64256828","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 1997-01-01DOI: 10.1016/S0273-1177(97)00789-8
H. Chandra, G. D. Vyas, S. Sharma
{"title":"Ionospheric effects of the total solar eclipse of 24 October, 1995 over Ahmedabad","authors":"H. Chandra, G. D. Vyas, S. Sharma","doi":"10.1016/S0273-1177(97)00789-8","DOIUrl":"https://doi.org/10.1016/S0273-1177(97)00789-8","url":null,"abstract":"","PeriodicalId":50722,"journal":{"name":"Bulletin of the Astronomical Society of India","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"1997-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.1016/S0273-1177(97)00789-8","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"56032226","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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