D. Yentis

Discovery of X-ray pulsations in SMC X-1

Observations of SMC X-1 from an Aerobee rocket and an Apollo spacecraft have detected X-ray pulsations with a period of 0.716 s. The pulsed fraction in the 1.6--10 keV energy range is 25--35 percent. Evidence for significant pulse shape and pulsed fraction changes in the 0.6--1.6 keV range is also presented. The spectrum during both observations is fitted by a photon power law, 0.040 E/sup -0/./sup 8/. (AIP)

Observations of outbursts from the recurrent X-ray transient A0538-66 and LMC X-4

Nine X-ray outbursts from the LMC have been observed with the HEAO 1 Large-Area Sky Survey Instrument. Some are shown to originate in the recurrent transient A0538-66, confirming the proposed 16 day periodicity and showing that the duration of the events can be as long as 14 days or as short as a few hours. Deviations from precise periodicity can be attributed to phase jitter or to a change in period occurring around the time of an exceptionally long outburst. Other outbursts which are irregular and consistently shorter originate in LMC X-4. A long-term light curve indicates that the LMC X-4 outbursts occur only when the source is in a high state, but are not strongly correlated with the binary phase.

Millisecond variability of Cygnus X-1

The May 7, 1978 HEAO 1 spacecraft observations of Cygnus X-1 with a 10 microsec time resolution have revealed 3 ms variability, as well as a 300 ms component and features which extend to 6 s. Emission on time scales shorter than 1 s can be approximately modeled as a superposition of uncorrelated 3 ms and 300 ms shot contributions, both with peak luminosities near 10 to the 37th ergs/s, and with shot rates of about 10/s and 0.2/s, respectively. The 3 ms variability is consistent with the time scales expected for variability near the inner edge of a viscous accretion disk that surrounds a 10-solar mass black hole.

Evidence for coherent emission with a 12 millisecond period during a burst from MXB 1728--34

Four bursts of MXB 1728--34 have been observed with the HEAO A-1 instrument using 5 ms timing resolution. The rapid initial rise has been resolved in all four bursts. Fourier analysis indicates the presence of a periodic component with a 12.2 ms period during one of the bursts. Epoch-folding analysis confirms this period and further indicates that the period decreases with time, with dP/dt = 10/sup -6/. The signal has a probability of 2.5 x 10/sup -9/ to occur at random, and an expectation of 3 x 10/sup -3/ to occur in the number of epoch folding performed.

Identification of a faint X-ray source with the W Ursae Majoris star VW Cephei

The NRL instrument aboard the HEAO 1 satellite has detected a faint X-ray source, which has been identified tentatively with the contact binary star (W UMa variable) VW Cephei. Its luminosity is between 3 x 10/sup 30/ and 4 x 10/sup 31/ ergs s/sup -1/ (0.1--10 keV), and the results suggest some variation of the X-ray flux with phase.

An 80 day X-ray light curve of 3C 371

The Large Area Sky Survey (LASS) experiment on HEAO 1 that monitored the N galaxy/BL Lacertae object 3C 371 from August 19 to November 7, 1977 is discussed, noting that statistically significant threefold or larger intensity variations were observed in the flux range 0.5-20 keV. It is found that the degree of variability depends on the time of observation, with the source sometimes appearing constant and sometimes changing on time scales of 1 to 2 weeks. Also observed was an X-ray flare that lasted approximately 25 days. The total 80-day X-ray light curve is seen as suggesting either a superposition of X-ray flares or a variable injection rate that changes on weekly time scales. The observed X-ray flux is found to be consistent with that expected by extrapolating the nonthermal optical spectrum, suggesting that the optical to X-ray flux could be synchrotron radiation, provided that it is possible to account for the patterns of variability seen at optical and X-ray wavelengths and to dispose of inverse Compton difficulties.

A survey of rich clusters of galaxies with HEAO 1. II

Results are presented of an X-ray survey of approximately 1900 Abell clusters of galaxies performed with the NRL large-area X-ray detectors on HEAO 1. The cluster luminosity function derived from the observations made at ecliptic longitudes between 80 and 180 deg and 260 and 360 deg is fit with both power law and exponential relations, and used to estimate a total contribution of Abell clusters to the volume emissivity of the diffuse X-ray background of between 3 and 10%. No strong correlation is found between X-ray luminosity and Bautz-Morgan class, richness class or 26-MHz radio power, although B-M type I and Abell richness class 3 appear more likely to be strong X-ray sources. A correlation is suggested between optical radius and X-ray luminosity.

An X-ray survey of distant rich clusters of galaxies with HEAO 1

We present a 1--10keV survey of distance class 4, 5 and 6 Abell clusters using the HEAO 1 NRL large area survey experiment. The survey is a little less than 1/30 of the Abell catalog. Eleven clusters are identified with X-ray sources, and X-ray upper limits are provided for 60 others.

H0547-14: X-ray flux from a weak gamma-ray burst

Data concerning a fast X ray transient observed by the HEAO 1 on March 17, 1978 are discussed. The object, labeled H0547-14, was located in Lepus, with galactic coordinates 219.37 deg longitude, b = -20.12 deg in the anticenter direction. A 10 sec scan of the transient was obtained with proportional counter detectors. An X ray luminosity of more than 5.1 x 10 to the 31st ergs/sec was measured as a lower limit, with the decay taking 17 min. A lower limit to the flare was set at 30 sec. The flux intensity resided in the range 4.4/1 billion-4/100 million erg/sq cm per sec, with a rise time of a factor of 10 in about 5 sec. Possible characterizations of the object, as either a gamma ray burst or as an M dwarf star, are discussed.

The X-ray light curve of 4U 2129+47

We present the light curve of 4U 2129+47 and show that it almost certainly has a period of approx.0.22 days. This is the same period as the optical candidate proposed by Thorstensen et al. Binary X-ray source models, consisting of an M dwarf plus either a neutron star or a white dwarf, are briefly discussed.