X-ray Census of Millisecond Pulsars in the Galactic field
We have conducted a systematic survey for the X-ray properties of millisecond pulsars (MSPs). Currently, there are 47 MSPs with confirmed X-ray detections. We have also placed the upper limits for the X-ray emission from the other 36 MSPs by using the archival data. We have normalized their X-ray luminosities $L_{x}$ and their effective photon indices $\Gamma$ into a homogeneous data set, which enable us to carry out a detailed statistical analysis. Based on our censored sample, we report a relation of $L_{x}\simeq10^{31.05}\left(\dot{E}/10^{35}\right)^{1.31}$ erg/s (2-10 keV) for the MSPs. The inferred X-ray conversion efficiency is found to be lower than previously reported estimate that could be affected by selection bias. $L_{x}$ also correlates/anti-correlates with the magnetic field strength at the light cylinder $B_{LC}$/characteristic age $\tau$. On the other hand, there is no correlation between $L_{x}$ and their surface magnetic field strength $B_{s}$. We have further divided the sample into four classes: (i) black-widows, (ii) redbacks, (iii) isolated MSPs and (iv) other MSP binaries, and compare the properties among them. We noted that while the rotational parameters and the orbital periods of redbacks and black-widow are similar, $L_{x}$ of redbacks are significantly higher than those of black-widows in the 2-10 keV band. Also the $\Gamma$ of redbacks are apparently smaller than those of black-widows, which indicates the X-ray emission of redbacks are harder than that of black-widows. This can be explained by the different contribution of intrabinary shocks in the X-ray emission of these two classes.