We consider the X-ray properties of the redback class of eclipsing millisecond pulsars .
These are transitional systems between accreting low-mass X-ray binaries and binary millisecond pulsars orbiting white dwarfs , and hence their companions are non-degenerate and nearly Roche-lobe filling .
The X-ray luminosity seems to scale with the fraction of the pulsar sky subtended by the companion , suggesting the shock region is not much larger than the companion , which is supported by modeling of the orbital light curves .
The typical X-ray photon spectral index is \sim 1 and the typical 0.3-8 keV X-ray efficiency , assuming a shock size on the order of the companion ’ s Roche lobe cross-section , is on the order of 10 % .
We present an overview of previous investigations , and present new observations of two redbacks , a Chandra observation of PSR J1628 - 3205 and a XMM-Newton observation of PSR J2129 - 0429 .
The latter shows a clearly double peaked orbital light curve with variation of the non-thermal flux by a factor of \sim 11 , with peaks around orbital phases 0.6 and 0.9 .
We suggest the magnetic field of the companion plays a significant role in the X-ray emission from intrabinary shocks in redbacks .