Chandra and RXTE studies of the X-ray/gamma-ray millisecond pulsar PSR J0218+4232

We report on high-resolution spatial and timing observations of the millisecond pulsar PSR J0218+4232 performed with the Chandra X-ray Observatory (CXO) and the Rossi X-ray Timing Explorer (RXTE). With these observations we were able to study: (a) the possible spatial extent at X-ray energies of the DC source coincident with PSR J0218+4232 in detail (CXO); (b) the relative phasing between the X-ray, radio and gamma-ray profiles (CXO and RXTE); and (c) the spectral properties at energies beyond 10 keV (RXTE). We found no indications for extended emission at X-ray energies down to ~1'' scales and confirmed the presence of a point-like DC-component. The two non-thermal pulses in the X-ray profile are found to be aligned with two of the three pulses visible at radio-frequencies and more importantly with the two gamma-ray pulses seen in the EGRET 100-1000 MeV pulse profile. The latter reduces now the random occurrence probability for the detected gamma-ray signal to ~10^-6, which corresponds to a 4.9sigma detection significance. This strengthens the credibility of our earlier claimed detection of pulsed high-energy gamma-ray emission from this millisecond pulsar. The pulsed RXTE spectrum appears to extend to ~20 keV and can be described by a power-law with a photon index of 1.14, slightly softer than measured by BeppoSAX and XMM for energies below 10 keV. We compare the high-energy characteristics of PSR J0218+4232 with those of two other X-ray emitting millisecond pulsars PSR B1821-24 and PSR B1937+21.