F. A. Primini

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.

Observations of NGC 4151 at 2 keV to 2 MeV from HEAO 1

Broad-range X-ray and gamma-ray spectra of the Seyfert galaxy NGC 4151 obtained by the UCSD/MIT and GSFC/CIT detectors of HEAO 1 are presented. For two out of the three observational epochs the best-fit model is a power law with photon index 1.6 + or - 0.1 in the 2 keV-2 MeV spectral range. The third observation, in June, 1978, when the intensity was a factor of 2 higher, requires a change above 50 keV from a similar power law to a steeper power law or an exponential. Through comparisons with previous observations, a weak correlation is found between the high-energy spectral index and the flux level. Comparison of these data and other X-ray and gamma-ray observations with the spectra expected from several possible emission mechanisms supports a synchrotron-self-Compton model using infrared target photons or a Compton-scattering process involving X-rays of less than 10 keV energies.

The hard X-ray spectrum of X Persei

Results of observations of 3U 0352 + 30, the source associated with the X Persei system, in the energy range 12-180 keV are reported. The measurements were made on 1978 August 22 with the UCSD/MIT High-Energy X-ray and Low-Energy Gamma-ray Experiment on HEAO 1. Data taken simultaneously in the energy range 2-50 keV indicate that, in addition to a 7 keV thermal bremsstrahlung component, there is a second component of higher energy. It is found that this higher energy component fits a power law of photon index 1.3 (+0.4, -0.5) and is pulsed at the 13.95 minute period of the lower energy component.

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.

HEAO 1 high energy X-ray observations of the Virgo cluster and A2142

Observations are reported of the Virgo cluster and Abell 2142 in the energy range of 15-150 keV, detected by using hard X-ray and low energy gamma-ray instruments on board the HEAO-1 spacecraft, during 1977 and 1978. The Virgo cluster was detected at a mean flux of (1.3 + or - 0.3) x 10 to the 0.001 photons/sq cm-s in the 20-100 keV band, and A2142 was detected marginally at a flux of (2 + or - 0.8) x 10 to the 0.001 photons/sq cm-s in the 15-40 keV band. Inverse Compton emission was not observed in these clusters. Results are used to derive limits to the intracluster magnetic field of B approximately greater than 5 x 10 to the -7th gauss and B approximately 5 x 10 to the -8th gauss in the Virgo cluster and A1242, respectively. There is some evidence for variability in these sources, which implies that the hard X-ray emission originates in a relatively compact object or region rather than from the cluster as a whole.

HEAO 1 high-energy X-ray observations of Centaurus X-3

Pulsations of 4.8 sec were detected up to energies above 38 keV by the present High Energy X-ray and Low Energy Gamma-Ray HEAO 1 satellite experiment observations of Cen X-3, and an analysis of the X-ray spectrum as a function of pulse phase indicates that the spectrum hardens during an interval of about 1.2 sec which lags the pulse peak by about 0.6 sec. The results of correlated observations of pulse period and X-ray intensity include (1) the detection of a high intensity state during which the pulse period is on the average increasing, (2) the measurement of comparable high intensities during episodes of both period increase and decrease, (3) the detection of X-ray pulsations at a much reduced level during a period of low intensity, and (4) the detection of a transition between spin-down, and spin-up episodes that coincides with a rapid decrease in X-ray intensity.

Observations of 12-200 keV X-rays from GX 339-4

X-ray spectra of GX 339-4 measured on three occasions in 1977 and 1978 are presented. These are the first reported measurements above 10 keV. The spectra can be described as the superposition of a soft component, which is dominant below about 20 keV, and a hard component at higher energy. Simultaneous measurements at lower energy show that the soft component vanished during the observation in early 1978. The behavior of these two components is similar to that of the spectrum of Cygnus X-1; this reinforces the previously noted resemblance in rapid X-ray variability.