The Astrophysical Journal · 1971 · 30 citations · 0 references
The X-ray pulsar in the Crab Nebula, integrated over the energy range 0.5-9 keV, has a primarypulse profile slightly narrower than that of its optical counterpart. The pulsed fraction of the total X-ray emission varies smoothly from about 2 percent near 1 keV to about 10 percent at 7 keV, corresponding to a pulsar spectral index of -0.2 + 0.1 for the energy flux. If this fraction is constant at 15 percent above 15 keV, then a break in the slope of the pulsar spectrum must occur near 10 keV. The apparent interstellar absorption of soft X-rays from the Crab Nebula implies 0.55 + 0.10 hydrogen atoms between us and the Crab. This is twice the density indicated by 21cm absorption measurements, but the difference may be due to X-ray absorption by interstellar grains.