Engineering topics

Boldt, E. A.

Publications and source records attributed to Boldt, E. A..

At least 145 records · Page 8

NASA NTRS1976

All-sky monitor observations of flares from Aquila X-1

The All-Sky Monitor on Ariel 5 has observed the Aquila-Serpens region on a quasi-continuous basis since launch in 1974 October. During this time Aql X-1 exhibited major flares in 1975 June and 1976 June, with the measured X-ray intensities increasing more than an order of magnitude to approximately that measured from the Crab nebula, and remaining above the experiment threshold (approximately 0.1 x s crab) for almost two months in both instances. These outbursts resemble the X-ray light curves of transient sources such as A0620-00 and are interpreted in terms of episodic accretion in a dwarf nova-like binary. Combination of the epoch of phase minimum for the 1.3d period (1976) with the 1975 data yields a value of P = 1.28d + or - 0.02d with a corresponding modulation of approximately 3% (3-6 keV). Modulation at this period is not apparent in the 1976 data, with an upper limit of 2% during that time.

Kaluzienski, L. J.↗

NASA NTRS1976

Spectra of Her X-1 near a turn-on in the 35-day cycle

X-ray spectra for Her X-l are presented for times before, during, and after a turn-on in the 35 day on-off cycle, as well as during an anomalous dip in the X-ray intensity. All four spectra are well represented by a power law of number index approximately .9 with a high energy cutoff near 20 keV. However, the column density of cold matter along the line of sight as estimated by the low energy cutoff varies substantially among the four intervals. The low level flux present prior to turn-on does not pulse and shows very little low energy absorption, in contrast to the X-rays observed during the turn-on and the anomalous dip. It seems likely that the pre-turn-on flux is composed of X-rays scattered into the line of sight by material away from the accretion disk.

Becker, R. H.↗

NASA NTRS1976

X-radiation from clusters of galaxies: Spectral evidence for a hot evolved gas

OSO-8 observations of the X-ray flux in the range 2-60 keV from the Virgo, Perseus, and Coma Clusters provide strong evidence for the thermal origin of the radiation, including iron line emission. The data are adequately described by emission from an isothermal plasma with an iron abundance in near agreement with cosmic levels. A power law description is generally less acceptable and is ruled out in the case of Perseus. Implications on the origin of the cluster gas are discussed.

Serlemitsos, P. J.↗

NASA NTRS1976

Spectral evolution of a long X-ray burst

An X-ray burst-like event with a peak intensity 1 1/2 times that of the Crab and a decay time of approximately 100s was observed. Significant spectral changes occurred during the burst. The spectra were best fit by the black form with kT ranging from .87 keV to 2.3 keV. They suggest a source with smaller dimensions than a massive black hole. A weak source was observed after the burst with a 10 keV thermal spectrum and an indication of iron line emission.

Swank, J. H.↗

NASA NTRS1976

X-ray observations of the Vela pulsar - Statistics and spectrum

The Vela pulsar (3U 0833-45) was observed in the range 2-60 keV by a proportional-counter experiment on board OSO-8 with temporal resolution sufficient to make possible a sensitive search for pulsed X-rays at the radio pulsar period. A statistical analysis yielded 8 percent as the 3-sigma upper limit on the pulsed fraction. The energy spectrum is fitted well by a structureless power law with number index 2.2 (+ or - 0.2) and absorption by a hydrogen column density of 2.9(+ or - 2.0) by 10 to the 22nd power per sq cm.

Pravdo, S. H.↗

NASA NTRS1976

Evidence for a 16.6 day period from Circinus X-1

Analysis of All-Sky Monitor observations of Cir X-1 (3U 1516-56) over the period from October 1975 to April 1976 has revealed a well-defined modulation of the 3-6-keV flux at a period of 16.585(+ or - 0.01) days. The light-curve is characterized by an abrupt drop in emission occurring on a time scale of no more than about 0.07 day, with epoch JD 2,442,877.181(+ or - 0.07). No clear correspondingly sharp increase in emission is observed during the cycle, so a noneclipse origin for this effect cannot be ruled out.

Kaluzienski, L. J.↗

NASA NTRS1976

The 35-D X-ray profile of Her X-1

Substantial X-ray emission as reported by Ariel 5 from Her X-l midway through the extended low-state portion of its 35-day cycle is examined. A similar feature during another cycle measured from UHURU satellite is also examined. It is proposed that this feature should occur regularly, consistent with some models for the mass accretion disc which have been reported in the literature. The results of the analysis of 500 days of Ariel-5 All-Sky Monitor data are reported which support the view that the feature occurs regularly in the 35-day cycle.

Holt, S. S.↗

NASA NTRS1976

High resolution measurements of the low state of Cyg X-1

Cyg X-1 was observed on two occasions separated by a year by the same X-ray rocket payload. High resolution temporal and spectral data reveal that Cyg X-1 was essentially unchanged in these two observations a year apart, with bursts of millisecond duration observed in the earlier flight and also, observed in the second. Analysis of these bursts has failed to reveal any internal temporal structure, either luminous or spectral. The shot noise character of temporal fluctuations on timescales approximately 1 second can be explained by the presence of exponential pulses with a fraction of a second time constant and a rate near 8 sec/1. The possible connection of these pulses with the bursts is examined.

Rothschild, R. E.↗

NASA NTRS1976

Spectral variability in the X-ray pulsar GX 1 + 4

Observations of the galactic-center-region hard X-ray source GX 1 + 4 by the GSFC X-ray spectroscopy experiment on OSO-8 confirm that GX 1 + 4 is a slow X-ray pulsar. The amount of absorption by cold matter in the spectrum of GX 1 + 4 varies significantly within a 24-hour period, which is typical of many X-ray binary systems. The light curve for the pulsations from GX 1 + 4 appears to be energy-dependent.

Becker, R. H.↗

NASA NTRS1976

All-sky monitor observations of the decay of A0620-00 (Nova monocerotis 1975)

The All-Sky X-ray Monitor onboard Ariel 5 has observed the 3-6 keV decline of the bright transient X-ray source A0620-00 on a virtually continuous basis during the period September 1975 - March 1976. The source behavior on timescales 100 minutes is characterized by smooth, exponential decays interrupted by substantial increases in October and February. The latter increase was an order-of-magnitude rise above the extrapolated exponential fall-off, and was followed by a final rapid decline. Upper limits of 2.5% and 10% were found for any periodicities in the range 0d.2 - 10d during the early and later decay phases, respectively. A probable correlation between the optical and 3-6 keV emission from A0620-00 was noted, effectively ruling out models involving traditional optical novae in favor of Roche-lobe overflow in a binary system. The existing data on the transient X-ray sources is consistent with two distinct luminosity-lifetime classes of these objects.

Kaluzienski, L. J.↗

NASA NTRS1976

The Her X-1 spectrum - A measure of the field near the magnetic poles of a neutron star

The steep high energy cutoff observed in the spectrum for Her X-1 is analyzed in terms of the severely modified Thomson scattering that dominates the radiative transfer in a highly magnetized plasma near the surface of a neutron star. The data are shown to indicate a field of about 10 to the 13th G near the magnetic poles and the stopping of accreting matter by nuclear collisions in the neighboring plasma

Boldt, E. A.↗

NASA NTRS1976

X-ray emission from the supernova remnant G287.8-0.5

The GSFC Cosmic X-ray spectroscopy experiment on OSO-8 observed a weak galactic X-ray source near theta 2 at 288 deg, b2 at -1 deg. The spectrum for this source between 2-20 keV is well represented by a thermal spectrum of kT = 7.34(+3.6), sub -2.6 keV with an intense iron emission line centered at 6.5 + or - .2 keV. The error box of the Uhuru source 4U1043-59, the only known X-ray source in our field of view, contains the radio supernova remnant G287.8-0.5. The possible association of the X-ray source with this supernova remnant is discussed.

Becker, R. H.↗

NASA NTRS1976

Variations in the spectra and pulse period of GX301-2

Spectra from observations of GX301-2 (3U1223-62) by the Goddard X-ray spectroscopy experiment on OSO-8 show an absorption edge due to cool iron with column densities of a few x 10 to the 19th power/cm squared. A decrease in intensity was associated with increased absorption and a spectrum similar to those of binary X-ray sources in absorption dips. Observed variations of the pulse period are consistent with Doppler shifts for reasonable orbit parameters.

Swank, J. H.↗

NASA NTRS1976

A return to the pre-1971 intensity level and a 5.6-d modulation for Cyg X-1

A 5-day modulation in the 'high-state' X-ray intensity data obtained by the Ariel V all-sky monitor for November 1975-January 1976 is reported. The light curve decrement (the fraction of the total source emission over one 5-day cycle which is deficient at superior conjunction) is 0.019 plus or minus 0.003, compared with 0.029 plus or minus 0.004 for a similar 5.6-day modulation observed before the April 1975 flare. The results are most easily explained by scattering from ionized material between the two stellar components which extends outside the orbital plane.

Holt, S. S.↗

NASA NTRS1976

Iron line emission from a high-temperature plasma in Cassiopeia A

The letter reports a measurement of the X-ray spectrum of Cas A with a proportional counter on board OSO-8. The high-energy data (greater than 7 keV) are found to agree equally well with a power law having a number index of about 4.5 and with a thermal spectrum having kT of about 3.9 keV. A narrow iron line predicted by the thermal model is also observed; this line is centered at about 6.67 keV and has an equivalent width of approximately 1270 eV. The low-energy data (below 6 keV) are not fully accounted for by the above thermal spectrum, but are fitted well by an additional low-temperature thermal component with kT not exceeding 0.7 keV and an atomic hydrogen column density of approximately 10 to the 22nd power per sq cm. The iron abundance in the plasma at an electron temperature of 40 keV is estimated from the observed line-to-continuum ratio, and it is shown that a normal cosmic iron abundance is adequate to explain the observation.

Pravdo, S. H.↗

NASA NTRS1976

X-ray observations of the Vela pulsar: Statistics and spectrum

The Vela pulsar was observed in the range 2-60 keV by the GSFC proportional counter experiment onboard OSO-8 with temporal resolution sufficient to make possible a sensitive search for pulsed X-rays at the radio pulsar period. A statistical analysis yielded 8 per cent as the 3 sigma upper limit on the pulsed fraction. The energy spectrum is fit well by a structureless power law with number index 2.21 + or - 0.2 and absorption by a hydrogen column density of N sub H equals 2.9 + or - 2.0 times ten to the twenty-second power per sq.cm.

Pravdo, S. H.↗

NASA NTRS1976

Evidence for a 16d.6 period from Circinus X-1

Analysis of All-Sky Monitor observations of Cir X-1 (3U1516-56) over the period October 1975 - April 1976 revealed a well-defined modulation of the 3-6 keV flux at a period of 16.585 + or - 0.01. The light-curve is characterized by an abrupt drop in emission occurring on a timescale of 0.07, with epoch JD 2,442,877.181 + or - 0.07. No clear correspondingly sharp increase in emission is observed during the cycle, so that a noneclipse origin for this effect cannot be ruled out.

Kaluzienski, L. J.↗

NASA NTRS1976

Evidence for an 11.2 day periodicity from Cygnus X-2

Evidence is reported for a periodicity of 11.17 (+ or - 0.10) days in the emission from Cygnus X-2. The data were obtained quasi-continuously over a period of more than one year with the Ariel-5 All-Sky Monitor. It is noted that this period is more than an order of magnitude larger than the 0.787-day binary period of Sco X-1 and is not a simple alias of a similarly short period. It is suggested that the 11.17-day modulation might be precessionally induced; otherwise, it would be rather unusual since the Cyg X-2 candidate is assumed to have a low-mass primary and periods in excess of about 2 days are usually associated with high-mass primaries.

Holt, S. S.↗