Chandra pulsar survey (ChaPS)

Abstract

Taking advantage of the high sensitivity of the Chandra Advanced CCD Imaging Spectrometer, we have conducted a snap-shot survey of pulsars previously undetected in X-rays. We detected 12 pulsars and established deep flux limits for 11 pulsars. Using these new results, we revisit the relationship between the X-ray luminosity, L_psr_x, and spin-down power, Edot. We find that the obtained limits further increase the extremely large spread in the non-thermal X-ray efficiencies, eta_psr_x=L_psr_x/Edot, with some of them being now below 1e-5. Such a spread cannot be explained by poorly known distances or by beaming of pulsar radiation. We also find evidence of a break in the dependence of L_psr_x on Edot, such that pulsars become more X-ray efficient at Edot<~ 1e34-1e35 erg/s. We examine the relationship between the gamma-ray luminosity, L_psr_g, and Edot, which exhibits a smaller scatter compared to that in X-rays. This confirms that the very large spread in the X-ray efficiencies cannot be explained just by beaming because the gamma-ray emission is generally expected to be beamed stronger than the X-ray emission. Intriguingly, there is also an indication of a break in the L_psr_g(Edot) dependence at Edot~1e35 erg/s, with lower-Edot pulsars becoming less gamma-ray efficient. We also examine the distance-independent L_psr_f/L_psr_x ratio as a function of Edot for a sample of gamma-ray pulsars observed by Chandra and find that it peaks at Edot~1e35 erg/s, showing that the breaks cannot originate from poorly measured distances. We discuss the implications of our findings for existing models of magnetospheric emission and venues for further exploration.