W. A. Baity

Discovery of a 30. 5 day periodicity in LMC X-4

Observations covering a 500/sup d/ time span made with High-Energy X-ray and Low-Energy Gamma-Ray Instrument on HEAO 1 reveal a 30./sup d/48 +- 0./sup d/06 peroidic variability in the approx.13 to approx.80 keV flux from LMC X-4. The source intensity is modulated by at least a factor of 5 through the 30./sup d/5 cycle and is low for approx.40% of the cycle. A value of the orbital period P = 1./sup d/40839 +- 0./sup d/00010 is determined from the observations. Possible origins of the 30./sup d/5 period are discussed.

Centaurus A /NGC 5128/ at 2 keV-2.3 MeV - HEAO 1 observations and implications

The nearby active-nucleus galaxy Centaurus A (NGC 5128) has been studied at 2 keV--2.3 MeV using data from both the UCSD/MIT hard X-ray and low energy ..gamma..-ray instrument and from the GSFC/CIT cosmic X-ray experiment on HEAO 1. We find that an E/sup -1.60plus-or-minus0.03/ power law spectrum breaking to E/sup -2.0plus-or-minus0.2/ at 140 keV best describes the 1978 January and July data. The average intensity was 50% higher during the January observations. We have searched our data for faster variations and set limits in several energy ranges over times from fractions of a day to several days. Upper limits to unresolved lines at 511 keV and 1.6 MeV are 6.5 x 10/sup -4/ photons cm/sup -2/ s/sup -1/ and 2.2 x 10/sup -4/ photons cm/sup -2/ s/sup -1/, respectively, at the 90% confidence level. Continuation of the observed power law to higher energies is used to constrain various models of energy generation in the nucleus of NGC 5128.

Hercules X-1 hard X-ray pulsations observed from HEAO 1

The 1.24 s pulsations of Her X-1 in the energy range 13--75 keV has been analyzed in data obtained from the UCSD/MIT experiment on HEAO 1 during observations of the source on three dates in 1978 February and three in 1978 August. Observational results are (1) the main pulse broadens somewhat with increasing energy; (2) the pulsation light curve undergoes pronounced changes at the leading edge of the main pulse from day to day; (3) spectral hardening within the main pulse is confirmed; (4) a 40--60 keV spectral feature in the spectrum is confirmed; (5) this feature is resolved, ..delta..E/E>20%, and its its centroid varies with pulsation phase; and (6) the 13--75 keV spectrum does not noticeably vary from day to day, except for an overall intensity factor. Some implications of these results for the prevailing models of Her X-1 and the HZ Her-Her X-1 system are briefly discussed.

HEAO 1 observations of the Perseus cluster above 10 keV

We present the results of HEAO 1 observations of the Perseus cluster from 10 to 150 keV in 1977 August and 1978 February and August. The spectrum exhibits a previously unknown hard (>25 keV) component in addition to the previously known thermal bremsstrahlung emission. The data presented show no significant evidence of variability from 10.5 keV to 93.5 keV, and a comparison of our results with earlier results indicates no strong evidence for variability above 25 keV over a time scale of approx. 4 yr. If the hard-component excess is due to NGC 1275, the data imply a 2--6 keV X-ray luminosity of approx.1 x 10/sup 44/ ergs s /sup -1/ for the galaxy, or approx.15% of the total cluster emission from 2 to 6 keV and a 25--40 luminosity of approx.8 x 10/sup 43/ ergs s/sup -1/. In the context of a model of hard-X-ray emission by inverse Compton scattering of relativistic electrons on the 2.7 K background, the X-ray spectrum, in conjuction with previously reported radio data, implies a magnetic field of B>10/sup -7/ gauss.

HEAO 1 observations of the long-term variability of Hercules X-1

Observations are reported of Hercules X-1 in the energy range 13-180 keV which covered two complete 35d cycles of high and low states of the X-ray intensity during 1978. Three high ON states and two low ON states were observed. Features resembling absorption dips were observed in the two high ON states and one low ON state in which good quality data were available. The results are interpreted in the context of precessing tilted accretion disk-periodic mass transfer models. Since the line of sight to Her X-1 lies nearer the plane of the disk rim during low ON states than during high ON states, the observed X-ray intensity during low ON states may be more susceptible to changes in the disk structure.

Long-term observations of Cygnus X-2 from OSO-7

Results of observations of Cygnus X-2 are reported that were made with an X-ray telescope on board the OSO-7 satellite during the three intervals of January 9-16 and 21-26, 1972, and July 8-11, 1972. Random factor-of-two variations were found in the data. No statistically significant periodic variations were found in the range from 10 hours to 2 days.