Engineering topics
Publications and source records attributed to Serlemitsos, P. J..
The line energy, equivalent width, binary phase dependence, and intrinsic width of the iron line emission feature observed in the low state sepctrum of Hercules X-1 are examined. Deductions are made concerning secondary X-ray emission from this binary system.
The HEAO-1 A-2 experiment, designed to study the large scale structure of the galaxy and the universe at X-ray energies is described. The instrument consists of six gas proportional counters of three types nominally covering the energy ranges of 0.15-3 keV, 1.2-20 keV, and 2.5-60 keV. The two low energy detectors have about 400 sq cm open area each while the four others have about 800 sq cm each. Dual field of view collimators allow the unambiguous determination of instrument internal background and diffuse X-ray brightness. Instrument characteristics and early performance are discussed.
The 2-60 keV flux from NGC4151 changed by a factor of two on a timescale of 1.5 days. No fluctuations were detected in excess of a factor of three on timescales less than four hours. During a total observation of approximately 11 days there were no statistically significant changes in spectral shape. The spectrum was fitted by a power law with photon index alpha approximately 1.42 + or - 0.06 and column density N sub H approximately 7.5 + or - 0.5 x 10 to the 22d power at/cu cm. A 2 sigma residual to this fit implies fluorescent Fe line emission with E. W. approximately 240 eV. Both synchrotron self-Compton and thermal Compton models are consistent with the X-ray data.
The Goddard Space Flight Center (GSFC) Cosmic X-Ray experiment aboard OSO 8 viewed the X-ray binary pulsar, Vela X-1, on three occasions from late 1975 through late 1976. The X-ray spectrum is well-represented by a power law modified by photoelectric absorption, a high-energy cutoff, and a line feature at about 6.8 keV. When combined with other observations, the reported measurements show that the pulse period is not decreasing monotonically. The three eclipses observed all indicate a significant eclipse flux.
OSO-8 X-ray observations of Cen A in 1975 and 1976 are reported. The source spectrum can be well fitted in both years by a power law of number index 1.66 and absorption due to 1.3 by 10 to the 23rd power atoms/sq cm. The total flux varied by a factor of 2 between 1975 and 1976. In 1976 there were flux variations of approximately 40% on a time scale of days. Measurements of the 6.4-keV Fe fluorescent line and the 7.1-keV absorption edge imply Fe/H of approximately 0.000016. Simultaneous radio measurements show variation in phase with X-ray variability. Consideration of radio, millimeter, infrared, and X-ray data shows that all the data can be accounted for by a model in which the X-rays are due to a synchrotron self-Compton source embedded in a cold H2 cloud.
OSO-8 X-ray spectra from 2 to 20 keV were analyzed for 26 clusters of galaxies. Temperature, emission integrals, iron abundances, and low energy absorption measurements are given. Eight clusters have positive iron emission line detections at the 90% confidence level, and all twenty cluster spectra are consistent with Fe/H=0.000014 by number with the possible exception of Virgo. Physical correlations between X-ray spectral parameters and other cluster properties are examined. It is found that: (1) the X-ray temperature is approximately proportional to the square of the velocity dispersion of the galaxies; (2) the emission integral and therefore the bolometric X-ray luminosity is a strong function of the X-ray temperature; (3) the X-ray temperature and emission integral are better correlated with cluster central galaxy density than with richness; (4) temperature and emission integral are separately correlated with Rood-Sastry type; and (5) the fraction of galaxies which are spirals is correlated with the observed ram pressure in the cluster core.
Pulse phase-dependent spectral changes in the high energy (less than 20 keV) continuum of Hercules X-1 were observed. Cyclotron absorption of underlying continua can reproduce the observed angular dependence in the high energy cutoff. Implications of this model, which include the possibility of determining the angular separation between the line of sight and the neutron star magnetic field if the absorbing electron spectrum is known are discussed.
A single flare event with a rise time of about 7 s and a decay time of approximately 40 s was observed by the Goddard cosmic X-ray spectroscopy experiment on OSO-8 during observations of 3U 0614+09 in September 1975. The possibility is discussed that the source was 3U 0614+09, which has been compared with the low-mass binary Sco X-1. The spectra during the event could be fitted with blackbody (kT = 0.8-1.1 keV) or thermal bremsstrahlung (kT = 1.1-2.5 keV) models with absorption by cool material. In the blackbody model the source was of the order of 1 million cm in size.
The BL Lac objects MK501 and MK421 have been observed by the HEAO-1 experiment A-2 in the 2-60-keV band. Their spectra are similar with best-fitting power laws having an energy index between -0.2 and 0.2. There was no detectable X-ray absorption with implied column densities of less than 1.5 x 10 to the 22nd H atoms/sq cm. MK421 was a factor 6 weaker in November 1977 then in May 1977. A new identification of PKS0548-322 with a source near 4U 0543-31 is suggested.
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The Goddard Space Flight Center Cosmic X-Ray Experiment aboard OSO-8 viewed the X-ray binary pulsar, Vela X-1, on three occasions from late 1975 through late 1976. The X-ray spectrum is well represented by a power law modified by photoelectric absorption, a high energy cutoff, and a line feature at approximately 6.8 keV. When combined with other observations, our measurements show that the pulse period is not decreasing monotonically. The three eclipses observed all indicate a significant eclipse flux.
An X-ray burst source has been discovered near the X-ray source 3U 1915-05. The continuum spectra of both the burst source and the quiescent source 3U 1915-05 are hard, with kT for thermal bremsstrahlung models above 20 keV. The spectrum of 3U 1915-05 has a feature at 9.1 keV, which, if attributed to absorption by hydrogen and heliumlike iron, suggests the presence of a highly ionized cloud surrounding a central X-ray source.
An eclipsing X-ray binary pulsator consistent with the location of A1540-53 has been observed. The source pulse period was 528.93 + or - 0.10 s. The binary nature is confirmed by a Doppler curve for the pulsation period. The eclipse angle of 30.5 + or - 3 deg and the 4-hour transition to and from eclipse suggest an early-type giant or supergiant primary star.
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The X-ray spectrum of Her X-1 was observed in the energy range from 2 to 24 keV from August 29 to September 3, 1975. Emission features are observed near the K-alpha iron-line energy which exhibit both broadening and a double line structure. The total luminosity in these features is about 4 by 10 to the 35th power ergs/s. Iron line fluorescence from an opaque cool (not exceeding 1 million K) shell of material at the Alfven surface provides the necessary luminosity in this feature. The double line structure and the line energy width can be due to Doppler shifts if the shell is forced to corotate with the pulsar at a radius of at least 800 million cm. Implications of this model regarding physical conditions near Her X-1 are discussed.
X-ray spectra for Her X-1 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 0.9 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 was not seen to pulse and showed 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. An accurate determination of the pulsation period for Her X-1 during 1975 August finds the period has decreased to 1.2378065 plus or minus 0.0000001 s.
Data on X-ray bursts recorded by the OSO-8 and SAS-3 satellites and criteria (rise time, duration, recurrence) for X-ray bursts are examined in a study of possible correlation between the galactic longitude distribution of X-ray burst sources and globular clusters. Correlations are weak, with only two X-ray bursts sources (NGC6624 and the rapid burster) coinciding with globular clusters, another seven possibly coinciding, most not correlated. Ad hoc explanations (globular clusters obscured by dust clouds, bursts associated with remnants of disrupted globular clusters evolved into supermassive black holes) are considered briefly.
An eclipsing X-ray binary pulsator consistent with the location of A1540-53 was observed. The source pulse period was 528.93 plus or minus 0.10 seconds. The binary nature is confirmed by a Doppler curve for the pulsation period. The eclipse angle of 30.5 deg plus or minus 3 deg and the 4 h transition to and from eclipse suggest an early type, giant or supergiant, primary star.