Engineering topics

White, N. E.

Publications and source records attributed to White, N. E..

At least 37 records · Page 2

NASA NTRS1993

The luminosity dependence of the X-ray spectrum of the transient 42 second pulsar EXO 2030 + 375

We report on the evolution in the X-ray spectrum of the transient X-ray pulsar EXO 2030 + 375 during part of an outburst in 1985 May-August. The overall continuum spectral shape is similar to that of other accreting pulsars and can be represented by a power-law spectrum modified at low energies by significant absorption and at high energies either by an exponential cutoff or by the effects of cyclotron scattering. As the luminosity decreased by a factor of about 100, the X-ray spectrum became harder with the photon index decreasing from 1.83 +/- 0.01 to 1.29 +/- 0.01. In addition, the high-energy cutoff energy decreased from 20 to 10 keV during the same interval. If the cutoff is interpreted in terms of cyclotron resonance scattering, then this variation implies a magnetic field strength that decreased from 2.6 x 10 exp 12 to 1.3 x 10 exp 12. This variation implies that the cutoff energy does not provide a reliable measure of the surface magnetic field strength in this system.

Reynolds, A. P.↗

NASA NTRS1991

A study of the flaring and quiescent X-ray and UV emission from II Pegasi

An investigation has been conducted of the rotational modulation of the transition-region UV and coronal X-ray emission for the RS CVn system II Pegasi. The X-ray light curve is dominated by a strong flare detected at orbital phase, where the minimum of the photometric wave occurred. The flare parameters derived show that the flare originates with a height greater than half the stellar radius. The characteristics of the flare are similar to those of a solar two-ribbon flare; a comparison of the midtransition region density with that in the coronal region shows a very steep pressure gradient.

Tagliaferri, G.↗

NASA NTRS1990

The X-ray orbital lightcurve of TY Pyx - A 3.2-day eclipsing RS CVn system

An Exosat observation of one orbital cycle of the eclipsing 3.2-d RS CVn system TY Pyx is reported. This system contains two almost identical GV stars, which is unusual because most RS CVns contain F or G main-sequence star and a K subgiant. The X-ray light curves of the primary and secondary eclipses are different above and below 1 keV. At lower energies a sharp eclipse is seen, but at higher energies it is absent. This confirms earlier observations that the 1.5-4.0 x 10 to the 7th K emission from the RS CVn systems is not eclipsed and comes from an extended region, whereas the 7 x 10 to the 6th K emission comes from more compact solarlike active regions. Modeling the X-ray eclipse detected at low energies requires two emission regions, one on each star, covering less than 20 percent of the stellar surface. To avoid a strong orbital modulation caused by both regions being either simultaneously visible, or not visible, they must be located 180 deg apart in longitude. The derived longitudes are 0 and 180 deg; i.e., they point at each other on the inward faces of the two stars.

Culhane, J. L.↗

NASA NTRS1990

An Exosat observation of spectral variability from the RS CVn binary AR Lac

The eclipsing RS CVn system AR Lac (G2IV + K0IV) has been observed continuously for one 2-day binary cycle with the Exosat observatory. Below 1 keV, a factor of two intensity modulation is seen with a flat bottomed minimum around the time of primary eclipse and a shallow dip preceeding primary eclipse. Above 1 keV, where only emission with temperatures greater than 10 exp 7 K would be detected, no orbital modulation or eclipse is seen. This suggests that the greater than 20 million degree emission comes from a large region, comparable in size to the binary separation. The modulation in the less than 1 keV lightcurve has been modeled by chi-squared fitting to X-ray bright spots and by using maximum entropy deconvolution. The lower temperature emission is found to originate in compact regions with a pressure and temperature similar to that of the flaring sun.

White, N. E.↗

NASA NTRS1989

X-ray spectroscopy of the ultrasoft transient 4U 1543 - 47

The X-ray transient 4U 1543 - 47 was observed in August 1983 by the Exosat observatory near the maximum of an outburst. The X-ray spectrum was measured using a gas scintillation proportional counter (GSPC) and a transmission grating spectrometer (TGS). A broad (FWHM about 2.7 keV) line at 5.9 keV is detected in the GSPC, which is interpreted as a redshifted and broadened iron K-alpha line. The line broadening and redshift may arise from either Compton scattering in a cool plasma with small optical depth and/or from Droppler and relativistic effects in the vicinity of compact object. The spectrum below 2 keV, obtained with the TGS, shows evidence for a broad emission feature at 0.74 keV, which may be an iron L-transition complex. However, such an emission feature could be an artifact caused by an anomalously low interstellar absorption by neutral oxygen. The contimuum emission is extremely soft and is well described by an unsaturated Comptonized spectrum from very cool plasma (kT = 0.84 keV) with large scattering depth.

Van Der Woerd, H.↗

NASA NTRS1989

An Exosat X-ray observation of one orbital cycle of 4U 1700-37/HD 153919

Exosat observations of close to one complete binary cycle of 4U 1700-37/HD 153919 are presented, and X-ray absorption variations are determined as a function of orbital phase in order to study the properties of the stellar wind of the companion HD 153919. The decrease noted in absorption between eclipse egress and phase 0.5 may be due to X-rays from a neutron star being absorbed by an inner wind with a radial velocity law close to that predicted by Castor et al. (1975). Under the assumption that the X-ray source is powered by material captured from the steller wind, the absorption variations with orbital phase and the time-average luminosity have been modeled to investigate the suppression of the radiative acceleration of the wind via the ionization of the wind by the X-ray source.

Haberl, F.↗

NASA NTRS1989

Iron K lines from low-mass X-ray binaries

Models are presented for the 6-7 keV iron line emission from low-mass X-ray binaries. A simplified model for an accretion disk corona is used to examine the dependence of the observable line properties, line width and mean energy, on the radial distance of the emission region from the X-ray source, and on the fraction of the X-rays from the source which reach the disk surface. The effects of blending of multiple line components and of Comptonization of the line profile are included in numerical calculations of the emitted profile shape. The results of these calculations, when compared with the line properties observed from several low-mass X-ray binaries, suggest that the broadening is dominated either by rotation or by Compton scattering through a greater optical depth than is expected from an accretion disk corona.

Kallman, T.↗

NASA NTRS1988

Simultaneous X-ray and radio observations of GX 13 + 1

The results of simultaneous X-ray and radio observations of GX 13 + 1, intrinsically one of the brightest Galactic bulge X-ray sources, are presented and possible models are discussed. No evidence is found for correlated X-ray and radio flux density variations. The variation of the 6-20 cm wavelength spectral index from +0.5 to -1.0 indicates that the radio flux is probably produced by a nonthermal emission mechanism. Application of an equipartition synchrotron model to the data suggests that the radio emission originates in a region with a magnetic field of about 1 gauss. If the radio flux density variations are due to injection of relativistic electrons, the equivalent nonthermal particle energy is 10 to the 36th - 10 to the 41st ergs/s.

Garcia, M. R.↗

NASA NTRS1986

X-rays from the magnetic white dwarf PG 1658 + 441

X-ray emission has been detected from PG 1658 + 441, a hot, isolated, magnetic white dwarf. The source was first detected in an Einstein IPC observation and was later identified in an Exosat channel multiplier array observation. Both imaging observations were motivated by an attempt to find the optical counterpart to the HEAO 1 soft X-ray source H1659 + 44. Spectral analysis, however, indicates that H1659 + 44 and PG 1658 + 441 are unrelated. The broad-band spectrum of this DA white dwarf can be explained as emission from a homogeneous, high-gravity, pure hydrogen atmosphere with a temperature near 28,000 K. The observations of PG 1658 + 441 support the suggestion that a correlation exists between temperature and helium abundance in white dwarf atmospheres.

Pravdo, S. H.↗

NASA NTRS1985

The identification of H2311 + 77 with HD220140, a probable RS CVn star

H2311 + 77, one of the bright soft X-ray sources in the sky originally detected with HEAO 1, has been identified in an Exosat imaging observation with a 7.7-mag late-type star, HD220140. The X-ray luminosity and the optical characteristics of this star indicate that it is probably an RS CVn binary system.

Pravdo, S. H.↗

NASA NTRS1985

The discovery of a 25 min regular modulation in the X-ray flux from 2S0142 + 61

A 13 hr observation of 2S0142 + 61 on August 27, 1984 by EXOSAT shows the X-ray flux of 2S0142 + 61 to be modulated with a period of 1456+/-6 s. The 1-10 keV spectrum is two component with an approximately 0.7 keV thermal and 0.0 energy index power law, with 30 percent of the total luminosity in the thermal component. The spectrum is absorbed by about 1 x 10 to the 22nd H per sq cm. Only the hard component is pulsed with a 3 to 10 keV peak to mean amplitude of 35 percent. Below 2 keV the modulation is less than a few percent. The total 1-10 keV luminosity is 3.5 x 10 to the 32nd erg/s for a distance of 100 pc. Possible optical counterparts are discussed.

White, N. E.↗

NASA NTRS1985

The mass transfer rate in X1916-053 - It is driven by gravitational radiation?

A 50-minute period for a binary system harboring an X-ray burster would allow several alternatives for the mass-giving secondary, including an H-shell burning-plus-He degenerate core composite model. The burst properties of X1916-053 are presently used to argue against the He degenerate as well as the He main sequence solutions and to estimate whether, for any of the other solutions, the mass transfer rate could be consistent with that expected from gravitational radiation (GR). Within an uncertainty of a factor of 2, the transfer rate for the composite model solution is consistent with gravitational radiation, but enhancement by other mechanisms should be investigated.

Swank, J. H.↗

NASA NTRS1984

The 41.5 day binary X-ray pulsar 4U 1223 - 62 (GX 301 - 2)

Ariel 5 and SAS 3 X-ray pulse timing data for the 700 s X-ray pulsar 4U 1223 - 62 (GX 301 - 2) are reexamined in order to deduce the orbital elements of this system. The Ariel 5/SAS 3 arrival times are first reanalyzed to obtain the best solution for an orbital period close to 41.5 days and to estimate the uncertainties in it caused by unknown variations in the underlying pulse period. Spectral measurements by OSO 8 and HEAO 1 at a variety of orbital phases are presented and the origin of an unabsorbed, unpulsed soft component is discussed. OSO 8 observations of occasions when the amplitude and shape of the pulse profile changed dramatically are discussed. The global X-ray properties of the system are explained in terms of a simple stellar wind accretion model, and the interaction of the inflowing material with the neutron star magnetosphere is considered.

White, N. E.↗

NASA NTRS1984

New evidence on the nature of the neutron star and accretion flow in Vela X-1 from pulse timing observations

It is believed that variations in the intrinsic pulse frequencies of neutron star X-ray sources reflect changes in the rotation rate of the stellar crust produced by torque originating outside and inside the star. The external torque depends on the flow pattern of the accreting plasma, while the internal torque depends on the state of the interior and its coupling to the crust. Information about the accretion flow and the star itself can, therefore, be obtained on the basis of a study of intrinsic frequency variations. The present investigation is concerned with the results of a detailed study of the 283 s pulsing X-ray star Vela X-1 using pulse timing techniques. It is found that the frequency variations in this source are consistent with the occurrence of a succession of temporally unresolved events which can be characterized mathematically as second-order red noise in the pulse phase.

Boynton, P. E.↗

NASA NTRS1984

The unusually soft X-ray spectrum of LMC X-3

Spectral studies of LMC X-3 using the HEAO 1 A-2 experiment and the Einstein Solid State Spectrometer show that the X-ray spectrum is extremely soft relative to the vast majority of galactic accretion-driven X-ray sources, but similar to that of black hole candidates in their 'high state'. Rapid approximately 1 s flickering comparable to that seen from Cyg X-1 in its low state was not detected. In the 1977-1979 interval LMC X-3 was for most (if not all) of its time in a 'high state'. The fact that several other steady and transient X-ray sources also have similarly soft X-ray spectra and should be regarded as potential black hole candidates is noted. The spectra of these sources are not well described by any one simple model.

White, N. E.↗

NASA NTRS1984

VV Puppis - The soft X-ray machine

The detection of soft and hard X rays from VV Pup is reported, and an orbital light curve is presented for both. The light curves are found to be in good agreement with the orbital light curve observed in the optical (Warner and Nather, 1972), suggesting that both X rays and optical light arise from the vicinity of a bright spot near one magnetic pole of the white dwarf. The radius of the soft X-ray-emitting bright spot is in the range 20-120 km, and its temperature is in the range (2-6) x 10 to the 5th K. The observed ratio of soft X-ray to hard X-ray fluxes is approximately 15, the highest of all the AM Her stars.

Patterson, J.↗

NASA NTRS1984

X-ray spectral signatures of accreting black holes

The X-ray properties of the binary black hole candidates include distinctive bimodal spectral behavior where the X-ray spectra switch between being very soft (alpha greater than 3) and hard (alpha about 0.7). It is pointed out that this spectral transition occurs at about 1 percent L(edd) (for accretion onto a 10 solar masses black hole), and that this corresponds to the luminosity where an optically thick electron-positron pair plasma will form. Active galactic nuclei (AGN) are often considered to be accreting black holes. An X-ray spectral similarity between the above binary sources and both the Seyfert I and BL Lac classes of AGN is demonstrated. As many of these objects radiate most of their luminosity as X-rays, such a similarity may be expected if the underlying mechanisms are similar.

White, N. E.↗

NASA NTRS1984

X-ray observations of X Persei

The HEAO 2 Einstein Observatory was used to confirm the identification of 4U 0352 + 30 with X Per. The HEAO 2 data were analyzed in order to determine an additional point in the pulse period history of the X-ray source and to search for short binary periods. In addition, a majority of the historical X-ray observations was reanalyzed and more refined values for the pulse period as a function of time were obtained. The period history possess significant scatter, but no evidence for a measurable long-term spin-up trend is found, and hence a lower limit to the spin-up time scale is set. This lower limit is consistent with the presence of a neutron star, but it does not rule out a degenerate dwarf. The period history was also searched for evidence of binary motion, and upper limits to a(x) sin i were obtained.

Weisskopf, M. C.↗