K. S. Long

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 X-ray polarization of the Cygnus sources

Multiple observations of Cygnus X-1, Cygnus X-2, and Cygnus X-3 were carried out with the Bragg crystal polarimeters aboard OSO 8. Of these sources, an unambiguous detection of the time-averaged polarization was obtained only for Cygnus X-1, which has a polarization of 2.4% plus or minus 1.1% at 2.6 keV and 5.3% plus or minus 2.5% at 5.2 keV. While statistically significant evidence for polarization of Cygnus X-2 of 4.9% at 2.6 keV was obtained in a 1975 observation, this result was not confirmed in later observations. For Cygnus X-3, modulation of the observed flux at twice the rotation frequency of the satellite, the signature of polarization, was also detected, but spurious modulation due to the presence of additional sources in the field of view cannot be ruled out.

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 linear X-ray polarization of Scorpius X-1

Scorpius X-1 was observed with the Bragg crystal polarimeters aboard OSO 8 in August 1977 and again in August 1978. An analysis of these observations reveals a time-averaged polarization of 0.39% plus or minus 0.20% at 2.6 keV and 1.31% plus or minus 0.40% at 5.2 keV. The corresponding position angles are 29 deg plus or minus 10 deg and 57 deg plus or minus 6 deg, respectively. Binary phase-dependent polarization was searched for but not observed.

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.

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 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)