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Apparao, K. M. V.

Publications and source records attributed to Apparao, K. M. V..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1994

Spectral variations in the low-mass X-ray binary 4U 0614+09

We report the X-ray spectra of a binary 4U 0614+09 observed with EXOSAT on five different occasions in 1984 and 1986. In four observations in 1984, the source intensity was 'low' compared to its intensity in the 1986 observation. The source was steady on timescales of 10 to 1000 s in each of the 1984 observations but highly variable during the 1986 variations. The best-fit spectral models for the continuum are a blackbody spectrum plus an emission from a Comptonized region. In addition, a component due to reflection from a cold disk around the source was needed as an absorbiton edge due to cold or ionized iron was seen near 7 keV with good significance in most of the spectra, particularly in the 'low' state. The relation of the hardness ratios with intensity follow the behavior seen in the 'Atoll' sources. We have studied the behavior of the hardness ratios with the variations in the parameters of the best-fit spectral models. Most of the variability is associated with the blackbody component thought to arise from the boundary layer on the surface of the neutron star. During the 'high' state in 1986, the blackbody temperature is higher, its size smaller, and its contribution significantly greater than in the 'low' state. The other notable difference between the two states is the lower opacity in the Comptonized region, and a significantly higher absorption in the 'high' state. A weak line emission (equivalent width = 10 to 40 eV) is detected during the 'high' state. The line energy lies between 6 and 7 keV wh ich implies its origin from ionized iron.

Singh, K. P.↗
NASA NTRS1992

The soft X-ray diffuse background observed with the HEAO 1 low-energy detectors

Results of a study of the diffuse soft-X-ray background as observed by the low-energy detectors of the A-2 experiment aboard the HEAO 1 satellite are reported. The observed sky intensities are presented as maps of the diffuse X-ray background sky in several energy bands covering the energy range 0.15-2.8 keV. It is found that the soft X-ray diffuse background (SXDB) between 1.5 and 2.8 keV, assuming a power law form with photon number index 1.4, has a normalization constant of 10.5 +/- 1.0 photons/sq cm s sr keV. Below 1.5 keV the spectrum of the SXDB exceeds the extrapolation of this power law. The low-energy excess for the NEP can be fitted with emission from a two-temperature equilibrium plasma model with the temperatures given by log I1 = 6.16 and log T2 = 6.33. It is found that this model is able to account for the spectrum below 1 keV, but fails to yield the observed Galactic latitude variation.

Garmire, G. P.↗
NASA NTRS1992

X-ray observations of planetary nebulae with binary nuclei

Einstein and EXOSAT satellite observations of X-ray emission from the planetary nebulae A63 and LoTr 5 are reviewed. Both of these systems contain binary central stars. No flux was detected from A63 (central star UU Sge). LoTr 5 (central star IN Com) is a previously unreported X-ray emitter; it showed no statistically significant X-ray variability. Three models for the source of the X-ray emission in such systems are considered in the light of these and previous results.

Apparao, K. M. V.↗
NASA NTRS1991

White dwarfs in Be star binary systems

An evaluation is made of possible reasons for the persistent inability to identify white dwarf stars in the Be binary systems. It is noted that many Be stars exhibiting large optical enhancements may be Be + WD and Be + He systems, and that observations of pulsations in the H-alpha emission, as well as observation of time delays between enhancements of optical line and continuum, can identify such systems.

Apparao, K. M. V.↗
NASA NTRS1985

Abundance of low energy (50-150 MeV) antiprotons in cosmic rays

The progress is presented of the nuclear emulsion experiment to determine abundance of low energy antiprotons in cosmic rays. No antiprotons have been detected so far at upper limit of p/p less than or similar to 4 x .0001 in the energy range 50 MeV to 15 MeV.

Apparao, K. M. V.↗
NASA NTRS1985

X-ray emission from Be star binaries

A model in which a neutron star in an eccentric inclined orbit around a Be star passes through a ring of matter around the Be star has been used to explain the recurrent flares of X-ray emission from these systems. The optical emission during X-ray flares is due to partial absorption and reprocessing of the X-rays in the ring.

Apparao, K. M. V.↗
NASA NTRS1984

Self-absorption of high-energy Gamma-rays in Cygnus X-3

Absorption of high-energy gamma-rays from Cyg X-3 in its own photon field by the reaction gamma + gamma yields e(+) + e(-) is considered. It is found that gamma rays with an energy around 10 to the 12th eV are attenuated, and the observed flux has to be corrected. Gamma rays with energy greater than 10 to the 15th eV which are observed are not affected by the photon field.

Apparao, K. M. V.↗
NASA NTRS1984

Cyclotron emission near stellar mass black holes

Cyclotron emission in the inner regions of an accretion disk around a matter accreting black hole can be appreciable. In the case of the X-ray source Cyg X-1, cyclotron emission may provide the soft photons needed for 'Comptonization' to produce high energy X-rays. The inverse correlation between the fluxes of high energy and low energy X-rays during the 'high' and 'low' states of Cyg X-1, may be understood as a result of the variation of the rate of accretion and the Compton scattering of the cyclotron photons. In the case of the X-ray source GX 339-4, the observed optical flux during the high states does not seem to be due to cyclotron emission, but probably due to reprocessing of high energy X-rays by the outer regions of the disk.

Apparao, K. M. V.↗
NASA NTRS1981

Discovery of X-ray pulsations from 2S 1417-624

X-ray pulsations having a period of 17.64 sec are detected from the hard, variable galactic X-ray source 2S 1417-624. It is noted that the pulsations were found through a Fourier analysis of 40 satellite orbits of data taken with the rotation modulation collimators aboard SAS 3 in July 1978. The pulse profile shows detailed interpulse structure that is similar in character to the pulse profiles of 4U 0900-40 and A0535+26. Significant changes in pulse period are detected over the eight-day observing interval. It is suggested that these changes may be the result of a large intrinsic spin-up rate for the neutron star or from binary orbital motion, with orbital periods longer than approximately 15 days.

Kelley, R. L.↗
NASA NTRS1979

Soft X-ray observation of the Rho Ophiuchus dark cloud region

Soft X-rays (0.1-0.8 keV) from the region including the Rho Oph dark cloud were observed with the SAS-3 low-energy X-ray telescope. No X-ray absorption by the cloud was observed. This indicates that the diffuse component of soft X-rays in this region is mostly from the foreground of the Rho Oph cloud which is located at a distance of 160-200 pc.

Apparao, K. M. V.↗
NASA NTRS1978

4U0241+61 - A luminous low-redshift QSO

The faint (1.8 micro Jy, 2-11 keV) X-ray QSO 4U0241+61 is considered in terms of its celestial position (galactic latitude = +2.4 plus or minus 30 sec), luminosity (M sub v = -25.8 plus or minus 1.8), redshift (z = 0.0438), and radio emission (0.2-0.5 Jy). Counting rates in two energy channels are presented, and found comparable to the Type 1 Seyfert 3C120. Spectrophotometric calibration to an approximately 8 A resolution was also achieved. Attention is given to the close relation between X-ray emitting QSOs and Seyferts.

Apparao, K. M. V.↗
NASA NTRS1978

Positions of galactic X-ray sources - At longitudes /l-II/ of 55-320 deg

Rotating modulation collimeter detectors on the SAS-3 X-ray observatory were used to supply precise (20-40 sec) positional data on 10 galactic X-ray sources in the l super II region between 55 and 320 deg. It is noted that the position for 2S0114+650 has led to the discovery of an optical counterpart. Attention is given to the temperature dependence of the experiments, and it is found that position measurements made at low temperatures are consistent with those obtained at normal (0 C) temperatures. Positions, error radii, and intensities (2-11 keV) are listed for each of the ten sources.

Dower, R. G.↗
NASA NTRS1978

Possible X-ray counterparts of gamma-ray sources

The results are presented of a survey regarding the X-ray source positions which fall within the error boxes of 10 unidentified gamma-ray sources observed with the aid of the COS-B satellite. In three cases, including CG 135-1, CG 312-1, and CG 327-0, an X-ray source was found within the gamma-ray error box. However, because of the large uncertainty regarding the gamma-ray source positions, the positional coincidence is not necessarily conclusive. It is, therefore, necessary to take into account additional information on the spectral or temporal characteristics of the X-ray sources. It is found that the X-ray source 4U 02416 plus 1 is a possible candidate as the X-ray-counterpart of CG 135 plus 1 in connection with both spectral hardness characteristics and positional coincidence.

Maraschi, L.↗
NASA NTRS1978

Positions of galactic X-ray sources with l/II/ between -20 deg and +6 deg

The precise positions of nine X-ray sources in the vicinity of the galactic center are reported. The data were obtained as part of the comprehensive survey of the galactic plane performed with the rotating modulation collimator detectors on the SAS-3 X-ray observatory. The sources include the binary X-ray source 4U 1700-37 which has a well established optical counterpart that lies 7 sec from the reported position. The other sources GX 349+2, 4U 1702-42, 4U 1705-44, MX 1716-31, A 1742-294, 4U 1755-33, GX 5-1, and 2S 1803-245 lack established counterparts in other wavelengths. The obtained position for GX 5-1 adds confidence to the radio counterpart proposed by Braes et al. (1972). The reported position for 4U 1755-33 excludes the optical counterpart proposed by Jones et al. (1974).

Jernigan, J. G.↗
NASA NTRS1978

Positions of galactic X-ray sources - l/II/ between 320 and 340 deg

The precise (20-40 sec) positions of seven X-ray sources in the celestial region corresponding to the galactic longitude range from 320 to 340 deg are reported. The sources include a recurrent transient X-ray source (MX1608-52) and a source 2S1553-542 coincident with a gamma-ray source within the given errors of position. The positions reported reduce the previously reported areas of the error region for six of the sources by factors ranging from 10 to 100. The presented results add confidence to the proposed radio candidates for 4U1624-49 and 4U1642-45 considered by Sanduleak and Dolan (1974) and Seaquist (1977).

Apparao, K. M. V.↗
NASA NTRS1977

Positions of galactic X-ray sources - Galactic latitudes between zero and 20 deg

Precise (to 20 or 25 arcsec) positions of six X-ray sources located in the galactic bulge are reported which were obtained as part of a survey of the galactic plane performed with the SAS-3 rotating modulation collimators. The sources include GX1+4, GX9+9, GX3+1, GX9+1, GX13+1, and GX17+2. The error radii and X-ray intensities (2 to 11 keV) determined for the sources are also presented along with finding charts and proposed optical and radio counterparts. The results are compared with those of previous satellite and sounding-rocket experiments. The status of each source with respect to optical or radio identifications is briefly discussed.

Doxsey, R. E.↗
NASA NTRS1977

Positions of three X-ray burst sources

Precise (20-30 arcsec) positions are reported for three steady X-ray sources that have been identified recently as X-ray burst sources: 4U (MXB) 1636-53, 4U 1728-33 (MXB 1728-34), and 4U (MXB) 1735-44. The positions were derived from data obtained with the SAS 3 rotating modulation collimators during a survey of the galactic plane. Possible optical counterparts are noted.

Jernigan, J. G.↗