H. L. Kestenbaum

A precision measurement of the X-ray polarization of the Crab Nebula without pulsar contamination

The linear X-ray polarization of the Crab Nebula has been precisely measured at 2.6 keV and 5.2 keV with the OSO 8 graphite crystal polarimeters. The 1.4 ms time reolution of these instruments permitted the removal of any contribution to the polarization from the pulsar. The nebular polarization is 19.2% +- 1.0% at a position angle of 156/sup 0/.4 +- 1/sup 0/.4 at 2.6 keV. At 5.2 keV the corresponding results are 19.5% +- 2.8% at 152/sup 0/.6 +- 4/sup 0/.0.

Measurement of the X-ray polarization of the Crab Nebula

The graphite crystal X-ray polarimeters aboard the OSO-8 satellite were used to observe the Crab Nebula for six days from March 11 through March 17, 1976 (UT). Analysis of 15 orbits of quick-look data shows that the polarization and position angles at 2.6 and 5.2 keV are 15.7%(+ or - 1.5%) at 161.1(+ or - 2.8) deg and 18.3%(+ or - 4.2%) at 155.5(+ or - 6.6) deg, respectively. These results confirm the previous measurement and the hypothesis of synchrotron X-ray emission.

The first search for X-ray polarization in the Centaurus X-3 and Hercules X-1 pulsars

The first search for X-ray polarization in the Cen X-3 and Her X-1 pulsars was performed by the OSO 8 polarimeters in 1975 July and 1975 August, respectively. Three-sigma upper limits to the polarization in Cen X-3 of 13.5% and 19% at 2.6 keV and 5.2 keV, respectively, were obtained when the data were averaged over the pulse and binary periods. The upper limit for Her X-1 at 2.6 keV is 60%. A search for pulse-phase dependent X-ray polarization from both objects was also performed. At the 91% confidence level, emission from Cen X-3 exhibits evidence for X-ray polarization at 2.6 keV that varies with pulse phase. Upper limits to polarization are presented for the leading and trailing edges and peak of the Her X-1 pulse at 2.6 keV.

Search for X-ray polarization in the Crab pulsar

Results of a search for X-ray polarization in the Crab pulsar are presented which were obtained by observing the pulsar at 2.6 and 5.2 keV with identical X-ray polarimeters aboard OSO 8. The polarization in different portions of the X-ray light curve is analyzed, the polarization contribution due to the nebula is removed, and the results of the analysis are given for isolated portions of both the primary pulse and the interpulse at 2.6 and 5.2 keV. No evidence for X-ray polarization is found at the 99% confidence level; 3-sigma upper limits to the polarization at 2.6 keV are given along with the largest polarization allowed at 5.2 keV by the 99% confidence counter. It is noted that the trailing edge of the interpulse at 2.6 keV is 26% polarized at the 92% confidence level, while the 5.2-keV polarization measurement of 40% in the leading edge of the primary pulse is different from zero at 96% confidence. X-ray and optical polarization measurements are compared.

The graphite crystal X-ray spectrometer on OSO-8

The large-area graphite crystal X-ray spectrometer aboard the OSO-8 satellite is described. The instrument can be used to measure continuum profiles over the energy band from 2 to 8 keV with a resolution of several tens of electron volts and to detect narrow line emission from strong X-ray sources with a limiting sensitivity (3 sigmas) of the order of 10-eV, equivalent width for a three-day observing period.

Energy dependence of the size of the X-ray source in the Crab Nebula

The size of the x-ray emitting region in the Crab Nebula observed during the 1974 December 28 (UT) lunar occultation at p.a.=300degree is found to decrease with increasing photon energy. A power law fitted to the source size versus energy with an exponent ..gamma..= -0.148 +- 0.012 agrees with the optical and x-ray data but does not predict the observed size and energy dependence in the radio region. Consideration of the effects of interstellar grains indicates that scattering does not contribute appreciably to the observed nebular size variation. The results are used to interpret theoretical models based on diffusion and bulk transport processes for electrons in the nebula. Energy spectral parameters for different regions of the nebula are also derived from the x-ray data. (AIP)

An upper limit to the linear X-ray polarization of Scorpius X-1

A measurement of Sco X-1 with the graphite-crystal polarimeters on the OSO 8 satellite yields upper limits (3 sigmas) to the linear polarization of 2.7% and 4.9% at 2.6 keV and 5.2 keV, respectively. The results are discussed in terms of an accretion-disk model.

Search for X-ray polarization in Cygnus X-1

Cygnus X-1 was observed at 2.6 and 5.2 keV during a three-day period in November 1975 by the X-ray polarimeters on the OSO-8 satellite. At the 99% confidence level, the results are consistent with the source being unpolarized. Positive evidence for polarization at 2.6 keV was obtained at the 96% confidence level.

Search for continuous X-ray emission from NP 0532

A search for continuous X-ray emission from the pulsar NP 0532 has been performed using lunar occultations and high temporal resolution. No increase in the nonpulsed phase of the flux from the Crab Nebula was observed when NP 0532 was exposed. The result is used to establish an upper limit of 4.7 million K (99% confidence) on the surface temperature of the neutron star.

Search for X-ray line emission from A0620--00

Measurements of the X-ray spectrum (1.85--7 keV) of A0620--00 obtained on 1975 October 17 and 1976 January 1--9 with the graphite crystal spectrometer on OSO-8 show a smooth continuum with an absence of emission lines. Upper limits are given for line emission from Si and S ions and are used to establish that the source is not optically thin. The results support a dense plasma model. (AIP)