Spectral measurements of Cyg X-3: A thermal source embedded in hot plasma within a cold shell     

R. K. Manchanda 《Journal of Astrophysics and Astronomy》2002,23(3-4):197-212

The attempts at unified model fitting to explain the spectral variations in Cyg X-3 suggest equally probable fits with a combination of an absorbed blackbody and a separately absorbed power law with an exponential cut-off or a composite of absorbed free-free emission with a power law hard X-ray component apart from the iron emission line. These seemingly ordinary but ad hoc mixtures of simple X-ray emission mechanisms have a profound implication about the geometry of the X-ray source. While the first set suggests a black-hole nature of the compact object, the second combination is consistent with a neutron star binary picture. The spectral variability at hard X-ray energies above 30 keV can provide crucial input for the unified picture. In this paper, we present spectral observations of Cyg X-3, made in our on-going survey of galactic and extragalactic X-ray sources in the 20–200 keV energy region, using Large Area Scintillation counter Experiment. The data show a clear power-law photon spectrum of the form dN/dE ∼ E^−2.8 in the 20 to 130 keV energy range. A comparison with earlier data suggests that the total number of X-ray photons in the entire 2–500 keV energy band is conserved at all time for a given luminosity level irrespective of the state. We propose that this behaviour can be explained by a simple geometry in which a thermal X-ray source is embedded in a hot plasma formed by winds from the accretion disk within a cold shell. The high/soft and low/hard X-ray states of the source are simply the manifestation of the extent of the surrounding scattering medium in which the seed photons are Comptonized and hot plasma can be maintained by either the X-ray driven winds or the magneto-centrifugal winds.  相似文献   

Energy spectrum and time variations of hard X-rays from Cyg X-1     

P. C. Agrawal  G. S. Gokhale  V. S. Iyengar  P. K. Kunte  R. K. Manchanda  B. V. Sreekantan 《Astrophysics and Space Science》1972,18(2):408-424

Experimental results on the intensity, energy spectrum and time variations in hard X-ray emission from Cyg X-1 based on a balloon observation made on 1971, April 6 from Hyderabad (India) are described. The average energy spectrum of Cyg X-1 in the 22–154 keV interval on 1971 April 6 is best represented by a power law dN/dE=(5.41±1.53)E ^{–(1.92±0.10)} photons cm^–2s^–1 keV^–1 which is in very good agreement with the spectrum of Cyg X-1 derived from an earlier observation made by us on 1969 April 16 in the 25–151 keV band and given by dN/dE=(3.54±2.44)E ^{–(1.89±0.22)} photons cm^–2s^–1 keV^–1. A thermal bremsstrahlung spectrum fails to give a good fit over the entire energy range for both the observations. Comparison with the observations of other investigators shows that almost all balloon experiments consistently give a spectrum of E ^–2, while below 20 keV the spectrum varies fromE ^–1.7 toE ^–5. There is some indication of a break in the Cyg X-1 spectrum around 20 keV. Spectral analysis of data in different time intervals for the 1971 April 6 flight demonstrates that while the source intensity varies over time scales of a few minutes, there is no appreciable variation in the spectral slope. Analysis of various hard X-ray observations for long term variations shows that over a period of about a week the intensity of Cyg X-1 varies upto a factor of four. The binary model proposed by Dolan is examined and the difficulties in explaining the observed features of Cyg X-1 by this model are pointed out.  相似文献   

X-ray measurements of black hole X-ray binary source GRS 1915+105 and the evolution of hard X-ray spectrum     

R. K. Manchanda 《Journal of Astrophysics and Astronomy》2000,21(1-2):39-52

We report the spectral measurement of GRS 1915+105 in the hard X-ray energy band of 20–140keV. The observations were made on March 30th, 1997 during a quiescent phase of the source. We discuss the mechanism of emission of hard X-ray photons and the evolution of the spectrum by comparing the data with earlier measurements and an axiomatic model for the X-ray source.  相似文献   

Utility of Hyperspectral Data for Potato Late Blight Disease Detection   总被引：1，自引：0，他引：1  

Shibendu Shankar Ray  Namrata Jain  R. K. Arora  S. Chavan  Sushma Panigrahy 《Journal of the Indian Society of Remote Sensing》2011,39(2):161-169

The study was carried out to investigate the utility of hyperspectral reflectance data for potato late blight disease detection. The hyperspectral data was collected for potato crop at different level of disease infestation using hand-held spectroradiometer over the spectral range of 325–1075 nm. The data was averaged into 10-nm wide wavebands, resulting in 75 narrowbands. The reflectance curve was partitioned into five regions, viz. 400–500 nm, 520–590 nm, 620–680 nm, 770–860 nm and 920–1050 nm. The notable differences in healthy and diseased potato plants were noticed in 770–860 nm and 920–1050 nm range. Vegetation indices, namely NDVI, SR, SAVI and red edge were calculated using reflectance values. The differences between the vegetation indices for plants at different levels of disease infestation were found highly significant. The optimal hyperspectral wavebands to discriminate the healthy plants from disease infested plants were 540, 610, 620, 700, 710, 730, 780 and 1040 nm whereas upto 25% infestation could be discriminated using reflectance at 710, 720 and 750 nm.  相似文献   

Greenhouse gas emissions from Indian livestock     

Abha Chhabra  K. R. Manjunath  Sushma Panigrahy  J. S. Parihar 《Climatic change》2013,117(1-2):329-344

Livestock constitutes an integral component of Indian agriculture sector and also a major source of GHGs emissions. The study presents a detailed inventory of GHG emissions at district/state level from different age-groups, indigenous and exotic breed of different Indian livestock categories estimated using the recent census 2003 and country-specific emission coefficients based on IPCC guidelines. The total methane emission including enteric fermentation and manure management of livestock was estimated at 11.75 Tg/year for the year 2003. Enteric fermentation constitutes ~91 % of the total methane emissions from Indian livestock. Dairy buffalo and indigenous dairy cattle together contribute 60 % of the methane emissions. The total nitrous oxide emission from Indian livestock for the year 2003 is estimated at 1.42 Gg/year, with 86.1 % contribution from poultry. The total GHGs emission from Indian livestock is estimated at 247.2 Mt in terms of CO₂ equivalent emissions. Although the Indian livestock contributes substantially to the methane budget, the per capita emission is only 24.23 kgCH₄/animal/year. Using the remote sensing derived potential feed/fodder area available for livestock, the average methane flux was calculated as 74.4 kg/ha. The spatial patterns derived in GIS environment indicated the regions with high GHGs emissions that need to be focused subsequently for mitigation measures. The projected estimates indicate a likely increase of 40 % in methane emissions from buffalo population.  相似文献   

Observation of ultra low hard X-ray flux from Cyg X-1 —A possible partial occulation of the X-ray source     

R. K. Manchanda 《Journal of Astrophysics and Astronomy》2001,22(1):9-20

The black hole candidate Cyg X-1 was observed in ultra low state on march 30, 1997 using Large Area Scintillation counter Experiment (LASE) in the hard X-ray energy region of 20–180 keV. During the 30 minute exposure a combined signal of 68 sigma was obtained, however, the measured flux at 50 keV was lower by a factor of 2 than the minimum flux reported so far. Using the recent orbital ephemeris of the source, our snap-shot observations were made at ϕ_5.6 = 0.915, which corresponds to the binary minimum revealed by the ASM light curves. The daily average data from the BATSE detectors give the source intensity level to be higher by a factor of 5. Very low flux values measured in the present experiment suggest that the hard X-ray source may have been partially occulted by the primary companion during its transit near the X-ray minimum.  相似文献   

Spectral variability in hard X-rays and the evidence for a 13.5 years period in the bright quasar 3C273     

R. K. Manchanda 《Journal of Astrophysics and Astronomy》2002,23(3-4):243-258

We report the observation of nearest quasar 3C273 made with LASE instrument on November 20th, 1998 as a part of our continuing programme of balloon borne hard X-ray observations in the 20–200 keV band using high sensitivity Large Area Scintillation counter Experiment. Our data clearly show a steep spectrum in the 20–200 keV with power law spectral indexα = 2.26 ± 0.07. This is in complete contrast to the reported data from OSSE and BeppoSAX which suggest the value of 1.3 to 1.6 for the power law index in the X-ray energy band, but is quite consistent with the value derived for the high energy gamma ray data. A single power law fit in the X-ray and gamma ray energy bands points to a common origin of these photons and the absence of spectral break around 1 MeV as suggested in literature. We have reanalyzed the available data to study the temporal variability of the spectrum in the hard X-ray band. Our analysis reveals that 50 keV flux from the source, shows a strong modulation with a period of about 13.5 years. The analysis of the optical light curve of the source also supports the 5000 day period. We discuss the emission mechanism and the possible sites for X-ray photons along with the implications of the long term periodicity with respect to source geometry.  相似文献   

Classification of road damage due to earthquakes     

Panjamani Anbazhagan  Sushma Srinivas  Deepu Chandran 《Natural Hazards》2012,60(2):425-460

Earthquakes cause massive road damage which in turn causes adverse effects on the society. Previous studies have quantified the damage caused to residential and commercial buildings; however, not many studies have been conducted to quantify road damage caused by earthquakes. In this study, an attempt has been made to propose a new scale to classify and quantify the road damage due to earthquakes based on the data collected from major earthquakes in the past. The proposed classification for road damage due to earthquake is called as road damage scale (RDS). Earthquake details such as magnitude, distance of road damage from the epicenter, focal depth, and photographs of damaged roads have been collected from various sources with reported modified Mercalli intensity (MMI). The widely used MMI scale is found to be inadequate to clearly define the road damage. The proposed RDS is applied to various reported road damage and reclassified as per RDS. The correlation between RDS and earthquake parameters of magnitude, epicenter distance, hypocenter distance, and combination of magnitude with epicenter and hypocenter distance has been studied using available data. It is observed that the proposed RDS correlates well with the available earthquake data when compared with the MMI scale. Among several correlations, correlation between RDS and combination of magnitude and epicenter distance is appropriate. Summary of these correlations, their limitations, and the applicability of the proposed scale to forecast road damages and to carry out vulnerability analysis in urban areas is presented in the paper.  相似文献   

Influence of Atmospheric Water Vapour on IRS NIR Measurements for Detecting Vegetation Signal. Part I: A Simulation Study Using MODTRAN     

Mehul R. Pandya  Raghavendra Pratap Singh  Sushma Panigrahy  Jai Singh Parihar 《Journal of the Indian Society of Remote Sensing》2012,40(2):215-223

Quantitative remote sensing involving accurate estimation of vegetation properties relies greatly on the measurements of the near infrared (NIR) channel because of unique interaction property between light and leaf. It is generally assumed that the NIR measurements are made in the absence of atmospheric absorption. However, relatively weak water vapour absorption features still persist in the NIR channel, which has bearing on the quantitative estimates of the vegetation properties and long-term data series. This paper reports the results of a study that was carried out to infer the possible influence of the atmospheric water vapour (WV) on the NIR measurements (0.77–0.86 μm) of Indian Remote Sensing (IRS) satellite sensors through radiative transfer simulations using MODTRAN model. The study also suggests and evaluates the alternate band-positions for the NIR channel to improve the IRS NIR measurements. It was observed that the water absorption features present around 0.810 μm reduces the WV transmission of IRS NIR channel from 1 to 0.91 when atmospheric WV content increased from 0 to 6 g/cm² and thus hampered the NIR reflectance by 14% as compared to reference signal. A significant improvement of the order of 6.5 to 12% in the NIR reflectance and 4.2 to 7% in NDVI was observed, when IRS NIR channel was split into NIR1 (0.775–0.805 μm) and NIR2 (0.845–0.875 μm) channels by avoiding the WV absorption features. The companion paper in this issue (Pandya et al. 2011) will support results of this simulation study through the EO1-Hyperion data analysis.  相似文献   

The flattening of the X-ray spectrum of sco X-1 at energies beyond 40 keV     

P. C. Agrawal  S. Biswas  G. S. Gokhale  V. S. Iyengar  P. K. Kunte  R. K. Manchanda  B. V. Sreekantan 《Astrophysics and Space Science》1971,10(3):500-507

The measurements of the hard X-ray spectrum of Sco X-1 in the energy interval 20–150 keV in three balloon flights from Hyderabad, India are reported. These results show conclusively that the spectrum of Sco X-1 is very flat in the energy interval 40–150 keV and the measured fluxes beyond 60 keV are several orders of magnitude higher than those expected on the basis of an extrapolation of bremsstrahlung spectrum from a thin hot plasma at a temperature corresponding tokT=5 keV, which is applicable for Sco X-1 for energies <40 keV. The results are compared with those of other investigators of hard X-rays from Sco X-1, and the implication of the results is briefly discussed.  相似文献