Engineering Papers⌕ Search

Engineering topics

Pravdo, S. H.

Publications and source records attributed to Pravdo, S. H..

At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

New hard X-ray sources observed with HEAO A-2

A search for new hard X-ray sources using data from the first complete view of the sky with the HEAO A-2 experiment has discovered 47 new sources, detected seven sources recently discovered with other experiments, and significantly reduced the size of the error boxes for six previously known sources. Intensities and error boxes are given for each of these sources; identifications are suggested when an error box contains an object similar to known X-ray sources. The new identifications consist of seven type 1 Seyfert galaxies, including two whose Seyfert characteristics were discovered as a result of their location in an X-ray error box; one intermediate Seyfert galaxy; three Abell clusters; five N galaxies; two bursting radio sources; and an additional three nearby galaxies with bright nuclei and narrow emission lines.

Marshall, F. E.↗
NASA NTRS1979

A search for X-ray emission from rich clusters, extended halos around clusters, and superclusters

The all-sky data base acquired with the HEAO A-2 experiment was searched for X-ray emission on a variety of metagalactic size scales which were either predicted or previously detected. Results in the 0.2-60 keV energy range are presented. The optically richest clusters, including those from which a microwave decrement were observed, appear to be relatively underluminous in X-rays. Observations of Abell 576 show its luminosity to be less than earlier estimates, and moreover less than the luminosity predicted from its microwave decrement, unless the intracluster gas is a factor of approximately 10 hotter than in typical clusters. Near SC0627 there are two X-ray sources, and the identification of the dominant source with SCO627 is probably incorrect. New spectral observations of Abell 401 and 2147, possible superclusters, reveal that they have typical cluster spectra with iron line emission.

Pravdo, S. H.↗
NASA NTRS1979

Pulse phase spectroscopy of Hercules X-1

The 6-18 keV and the 40-60 keV X-ray spectrum of Hercules X-1 were simultaneously observed with the A-2 experiment on HEAO 1. By combining these measurements with the results of an earlier observation of this X-ray pulsar, evidence is found for a component of emission above 40 keV which is above an extrapolation from lower energies, by a factor which is pulse phase dependent. These data are compared to previous hard X-ray observations and possible models are discussed.

Pravdo, S. H.↗
NASA NTRS1979

HEAO 1 observations of the X-ray pulsar 4U1626-67

Results of an observation of the 7-s pulsar 4U1626-67 with the A2 experiments on HEAO 1 are reported. The phase-averaged X-ray spectra which change radically as a function of pulse phase. Included in this spectral change is the sudden appearance and subsequent decay of a continuum or emission feature with a mean energy of 19 keV which contains about 1/2 the power in this spectral range. Pulse timing results include a new determination of the pulse period and a factor 8 reduction in the upper limit for the light travel time for orbital periods between 1 and 7 hours. The findings for this system are discussed and compared with the general nature of pulsar spectra.

Pravdo, S. H.↗
NASA NTRS1979

Observations of the transient X-ray source 4U0115+63

Results of HEAO-2 pointed observations and Ariel 5 All Sky Monitor observations of the X-ray transient 4U0115+63 are presented. The transient source pulses with a period of 3.6136 s + or - .0004 s, has a hard spectrum typical of an X-ray binary pulsar, and has a broad iron line emission feature. A discussion of the transient behavior is given and inferences are made concerning the nature of the X-ray source based on the pointed data.

Rose, L. A.↗
NASA NTRS1979

X-ray spectra of X Persei

The GSFC cosmic X-ray spectroscopy experiment on OSO 8 observed X Per for 20 days during two observations in 1976 February and 1977 February. The spectrum of X Per varies in phase with its 13.9 minute period, hardening significantly at X-ray minimum. Unlike other X-ray binary pulsar spectra, those of X Per do not exhibit iron line emission or strong absorption features. Our data show no evidence for a 22 hour periodicity in the X-ray intensity of X Per. These results indicate that the X-ray emission from X Per may be originating from a neutron star in a low-density region far from the optically identified Be star.

Becker, R. H.↗
NASA NTRS1979

Iron lines from X-ray binaries

A summary of the observational evidence of iron lines from satellite X-ray detectors and binary X-ray pulsars is presented. The current theories consider that the continua X-rays from X-ray pulsars originate at or near the surface of neutron stars, and they can then heat or fluoresce iron ions in surrounding physical regions to create the observed emission. Measurements of the line energy, intensity, and width, and the phase dependence of these quantities can help determine the relative importance of contributions of relative sources. The evidence is presented that the iron emission from some sources contains more than one component, and the spectra of other X-ray binaries including transients and cataclysmic variables also contain iron lines. Some implications of the observed line emission are discussed.

Pravdo, S. H.↗
NASA NTRS1979

Recent observations of Hercules X-1 with HEAO-1 and OSO-8

HEAO 1 X-ray observations of Her X-1 near an onset of the high state are discussed. An X-ray light curve is determined which indicates that for about 0.5 day before the X-ray intensity turn-on there was 1.2-sec pulsed emission from the source at a level intermediate between the high- and low-state intensities. These results are taken as demonstrating the stability in the 35-day cycle when compared with previous observations. An inconclusive search for 58-keV line emission from Her X-1 is also reported.

Pravdo, S. H.↗
NASA NTRS1979

X-ray pulsar spectroscopy - Observational prospects

Three experimental concepts have been suggested to advance the study of X-ray pulsar spectroscopy. They are: (1) a higher bit rate for pulse phase spectroscopy; (2) an emphasis on the 10 keV 'pulsar surface' emission, and (3) an improved resolution to understand the nature of broadened iron lines in pulsars. It is shown that for conventional X-ray spectroscopy at least 5 KB/s are needed to substantially improve pulse phase spectroscopy; the broadened iron line emission in the pulsar spectra will require improved spectral resolution at 6.7 keV to discriminate among the models for this emission. It is concluded that this work will elucidate the physics of neutron star surface and circumstellar X-ray emission.

Pravdo, S. H.↗
NASA NTRS1978

New hard X-ray sources observed with HEAO-A2

A search for new hard X-ray sources using data from the first complete view of the sky with the HEAO-A2 experiment discovered 47 new sources, detected 7 sources recently discovered with other experiments, and significantly reduced the size of the error boxes for 6 previously known sources. Intensities and error boxes are given for each of these sources; identifications are suggested when an error contains an object similar to known X-ray sources. The new identifications consist of seven Type 1 Seyfert galaxies, including two whose Seyfert characteristics were discovered due to their location in an X-ray error box; one intermediate Seyfert galaxy; three Abell clusters; five N-galaxies; two bursting radio sources; and an additional three nearby galaxies with bright nuclei and narrow emission lines.

Marshall, F. E.↗
NASA NTRS1978

X-ray observations of BL Lacertae objects

Results are reported for OSO 8 and HEAO 1 X-ray observations of the BL Lac objects Mrk 421, Mrk 501, and PKS 0548-322 over the energy range from 2 to 60 keV. It is found that the X-ray spectra of Mrk 421 and 501 are similar, exhibit statistically significant soft X-ray excesses, lack low-energy absorption, and are best fitted by power laws over the 2-30-keV range with respective photon indices of approximately 0.91 and 1.2 and respective hydrogen column densities of less than 8 x 10 to the 21st per sq cm and no more than 1.8 x 10 to the 22nd per sq cm. It is noted that the spectral index for both sources is identical, within errors, to the HF radio spectral index and that Mrk 421 experienced a factor of 6 decrease in the 2-10-keV band over a six-month period. A tentative identification of PKS 0548-322 with the source H0548-32 is suggested.

Mushotzky, R. F.↗
NASA NTRS1978

Discovery of iron line emission in the Hercules X-1 low-state spectrum with HEAO 1

Results are reported for HEAO 1 observations of Her X-1 which confirm the existence of the iron line-emission feature discovered with OSO 8 in the high-state spectrum and also reveal an iron emission feature in the low-state spectrum. The iron line-emission feature in the low-state spectrum was detected at a significance level greater than 5 sigmas and is characterized by a line energy of 6.4 keV, an equivalent width larger than that of the high-state line, a binary phase dependence of the line intensity, and a line width that is intrinsically narrow in contrast to the broad high-state line. The observed line energy and narrow line width are shown to be consistent with an interpretation in terms of fluorescence emission due to X-ray heating. It is suggested that HZ Her contributes no more than about 10% of the predicted line flux, that this emission is reflected from a highly ionized plasma, and that a hot coronal gas is an important source of the low-state X-rays.

Pravdo, S. H.↗
NASA NTRS1978

A 4.8 hour periodicity in the spectra of Cygnus X-3

The X-ray binary Cyg X-3 has been observed on three occasions during 1975 and 1976 by the GSFC Cosmic X-Ray Spectroscopy Experiment on OSO 8. The X-ray spectra from all three observations can be represented by power-law continua with strong iron line emission. Comparisons of spectra taken within the same observation at various phases of the 4.8 hour period reveal a relative excess of low-energy X-ray emission near zero phase (i.e., the minimum) of the 4.8 hour modulation. In addition, the centroid of the line emission is observed to vary in phase with the 4.8 hour cycle. The possibility of persistent thermal X-ray emission from material surrounding the binary system is introduced in an effort to account for the observed effects.

Becker, R. H.↗
NASA NTRS1978

X ray spectra of X Per

The cosmic X-ray spectroscopy experiment on OSO-8 observed X Per for twenty days during two observations in Feb. 1976 and Feb. 1977. The spectrum of X Per varies in phase with its 13.9 min period, hardening significantly at X-ray minimum. Unlike other X-ray binary pulsar spectra, X Per's spectra do not exhibit iron line emission or strong absorption features. The data show no evidence for a 22 hour periodicity in the X-ray intensity of X Per. These results indicate that the X-ray emission from X Per may be originating from a neutron star in a low density region far from the optically identified Be star.

Becker, R. H.↗
NASA NTRS1978

Galactic center iron line emission

Data from the Goddard Space Flight Center experiment on HEAO 1 were examined for galactic center iron line emission. Evidence for a 6.8 keV line from the summed flux within 10 deg longitude and 10 deg latitude of the galactic center was found; however, firm limits cannot be put on the intrinsic line width. If the line emission is diffuse and cosmic-ray-induced, the calculations of Bussard et al (1978) yield an upper limit to c-12 4.44 MeV line emission, which is not inconsistent with the results of Haymes et al (1975). Significant contributions to the iron line from discrete sources can not be ruled out.

Worrall, D. M.↗
NASA NTRS1978

X-ray observations of BL Lac objects

The BL Lac objects MK501 and MK421 were observed with OSO-8 and HEAO-1 X-ray detectors in the 2-60 keV band. Their spectra are similar with best fitting power laws having energy index -.4 alpha 4. A soft X-ray excess is indicated in their spectra. There was no detectable X-ray absorption with implied column densities N sub H 1.5 x 10 to the 22d power at sq cm. MK421 was a factor 6 weaker in November 1977 than in May 1977. An identification of PKS0548-322 with a new source H0548,-32 is suggested.

Mushotzky, R. F.↗
NASA NTRS1978

Observations of low-luminosity X-ray sources in Vela-Puppis

Results are presented for a study of the X-ray emission from a small portion of the galactic plane near galactic longitude 260 deg. This region contains at least six low-luminosity X-ray sources within about 10 deg of PSR 0833-45, which is near the center of the Gum nebula. The X-ray source 4U 0833-45, associated with the Vela pulsar, is observed at twice its 4U catalog intensity. The lack of X-ray pulsations at the pulsar period (greater than 99% nonpulsed), the nonthermal power-law spectrum, and models of the X-ray source distribution in this region suggest that a large fraction of the X-rays come from an extended source about 1 deg of arc in radius. The observation of a high-temperature (effective temperature at least 100 million K) spectrum in a field of view containing only Puppis A among known sources has led to the discovery of new OSO 8 source, OS 0752-39. Other spectra from this region are discussed.

Pravdo, S. H.↗