F. K. Li

The discovery of rapidly repetitive X-ray bursts from a new source in Scorpius

Rapidly repetitive X-ray bursts have been observed from a new X-ray source in Scorpius. More than 2000 bursts were observed during the approx.4 day continual SAS-3 observations of this source which we designated MXB 1730--335. The time interval between bursts varied from a minimum of approx. 6 s to a maximum of approx.5 minutes. The energy in a given burst is approximately linearly proportional to the time interval to the next burst. The largest bursts observed last for approx.60 s and represent an energy release of approx.10/sup 40/ ergs for an assumed distance to the source of 10 kpc. The smallest bursts observed last only for a few seconds. We suggest that the bursts are caused by sporadic precipitations of plasma from a reservoir in the magnetosphere of a neutron star. The reservoir is replenished at a nearly constant rate by mass transferred from a binary companion. (AIP)

Discovery of a 7.68 second X-ray periodicity in 3U 1626-67

SAS-3 observations of the X-ray source 3U 1626-67 have revealed the presence of a stable 7.68-sec pulse period. This source was selected for study because of its hard X-ray spectrum. The compilation of source spectra used in the selection process is also presented. Pulse arrival times are analyzed for effects of possible binary orbital motion. Upper limits to the projected orbital radius are obtained which tend to exclude orbital periods in the range from about 0.5 to 35 days. Binary systems with either a very long orbit (at least 175 days) or a very short orbit (no more than about 0.3 day) are most probable

Observations of galactic X-ray sources by OSO-7

We present the MIT data from the OSO-7 satellite for observations of the galactic plane between 1971 and 1974. A number of sources discovered in the MIT all-sky survey are described in detail: MX 0049 + 59, MX 0836 - 42, MX 1353 - 64, MX 1406 - 61, MX 1418 - 61, MX 1709 - 40, and MX 1608 - 52 (the persistent source suggested to be associated with the X-ray burst source XB 1608 - 52). Upper limits to the X-ray emission from a number of interesting objects are also derived. General results describing all of our observations of galactic sources are presented. Specifically, we display the number-intensity diagrams, luminosity functions, and color-color diagrams for all of the sources we detected. The data are divided between disk and bulge populations, and the characteristics of the two groups are contrasted. Finally, the concept of X-ray source populations and the relationship of globular cluster sources and burst sources to the disk and bulge populations are discussed.

Discovery of a 272 second periodic variation in the X-ray source GX 304-1

Observations of GX 304-1 (3U 1258-61) performed with the SAS-3 satellite revealed regular X-ray pulsations with a period of 272 s. Average pulse profiles, derived from 7 days of observation, are presented in four energy intervals covering the range 1-19 keV. The pulses are basically characterized by a single broad peak, and the amount of modulation decreases rapidly with increasing energy. The pulse arrival times are analyzed for effects of possible orbital motion of the X-ray star, and significant constraints are placed on the orbital parameters. For example, if it is established that the companion is an early-type star of at least 5 solar masses, then the orbital period is likely to be greater than about 15 days. Three 100-s flares were observed during which the source intensity rose by a factor of 4 in approximately 10 s.

Recurrent brief X-ray bursts from the globular cluster NGC 6624

Ten brief bursts of X-rays, recurrent at nearly equal intervals, have been detected in data obtained during a two-day observation of the X-ray source 3U 1820--30 in the globular cluster NGC 6624 by the SAS-3 X-ray observatory during 1975 May. The bursts occurred at times near those in the sequence i/sub k/=JD 2,442,549.4121+0.1822k with k=1,2,3,5,6,7,8,9,10,11. The rms deviation between the actual occurrence times and the corresponding values of i/sub k/ is 621 s, which is equivalent to an rms phase jitter of 3.9 percent. The co-added light curve of the five best observed bursts rises to peak intensity in about 1 s, decays exponentially with a time constant of approx.3.5 s, and has a total energy flux of approximately 3 x 10/sup -7/ ergs cm/sup -2/ per burst in the 1--10 keV energy range. The spectrum changes from very soft to very hard during the first 4 s, and then remains constant. The observed flux implies a total energy release per burst of approx.10/sup 39/ ergs if the source is at a distance of 5 kpc and radiates isotropically. (AIP)

Orbital elements and masses for the SMC X-1/Sanduleak 160 binary system

The binary x-ray source SMC X-1 was observed with the SAS-3 satellite for approximately four days (1976 February 26--29). Sufficient timing data were obtained on the 0.71 s x-ray pulsations to measure the Doppler velocity curve of the x-ray source and thereby derive orbital and stellar parameters for the system. The projected velocity of the x-ray star is 301.5 +- 2.0 (1 sigma) km s/sup -1/, and the corresponding mass function is 11.05 +- 0.22 (1 sigma) M/sub sun/. Combining the x-ray timing data with constraints imposed by the x-ray eclipse duration and the available optical data on Sk 160, we obtain a range of allowable values of 1.1 M/sub sun/approximately-less-thanM/sub x/approximately-less-than4.0 M/sub sun/ for the mass of the x-ray star. (AIP)

Observations of Hercules X-1 with SAS-3 during 1975 July

X-ray pulsations from Her X-1 with energies between 0.1 and 30 keV were observed for four days with the SAS-3 satellite, during the 1975 July-August ON state of the source. The existence of a strong flux between 0.1 and 0.4 keV, with pulsations that are out of phase with those above 1 keV, is confirmed. A pulsed flux in the 19-30 keV band was discovered. The average fractional rate of change in pulse period between 1972 and 1975 was about 3 x 10 to the -6th/yr, and the absolute value of the average fractional rate of change in orbital period during the same interval was not greater than 5 x 10 to the -7th/yr