2002ApJ...570..245S

We study the conditions under which a main-sequence binary companion to the central ionizing star of a planetary nebula (PN) might become magnetically active and thereby display strong X-ray luminosity, L_X≳5x10²⁹ ergs .s^–1. Since most PNs are older than few billion years, any main-sequence companion will rotate too slowly to have magnetic activity and hence bright X-ray emission unless it is spun up. We demonstrate that if the orbital separation during the asymptotic giant branch (AGB) phase of the PN progenitor is a≲30-60 AU, main-sequence companions in the spectral type range F7-M4 (mass range 0.3M_☉≲M₂≲1.3M_☉) will accrete enough angular momentum from the AGB wind to rotate rapidly, become magnetically active, and exhibit X-ray luminosities L_X≳5x10²⁹ ergs .s^–1. Lower mass M stars and brown dwarfs can also become magnetically active, but they should have small orbital separations and hence are less likely to survive the AGB phase of the progenitor. For orbital separation of a≲0.3 AU, i.e., for a binary systems that went through a common envelope phase, the fast wind from the central white dwarf (WD) star will interact with (and potentially disrupt) the companion's corona on the side facing the central star, while for a≲6 R_☉, i.e., an orbital period of P_orb≲30 hr, the WD's fast wind will compress a dense small region near the surface of the companion. This region may thermally emit X-rays with nonnegligible luminosity. We estimate that 20%-30% of elliptical PNs and 30%-50% of bipolar PNs are likely to have magnetically active companions that will reveal themselves in X-ray observations. Reanalysis of Chandra spectroscopy of the compact central source of NGC 7293 indicates that the emitting region of this object possesses abundance anomalies similar to those of coronally active main-sequence stars. High-resolution X-ray spectroscopy of this and other compact sources in PNs are necessary to confirm a coronal origin for the X-ray emission.