2005ApJ...633..367B

The ROSAT source RX J0630.8-2834 was suggested by positional coincidence to be the X-ray counterpart of the pulsar PSR B0628-28. This association, however, was regarded to be unlikely on the basis of the computed energetics of the putative X-ray counterpart. In this paper we report on multiwavelength observations of PSR B0628-28 made with the ESO NTT observatory in La Silla, the Lovell telescope at Jodrell Bank, and XMM-Newton. Although the optical observations do not detect any counterpart of RX J0630.8-2834 down to a limiting magnitude of V=26.1 mag and B=26.3 mag, XMM-Newton observations finally confirm it to be the pulsar's X-ray counterpart by detecting X-ray pulses with the radio pulsar's spin period. The X-ray pulse profile is not sinusoidal but characterized by a two-component pulse profile, consisting of a broad peak with a second narrow pulse leading the main pulse by ∼144°. The fraction of pulsed photons is (39±6)% with no strong energy dependence in the XMM-Newton bandpass. The pulsar's X-ray spectrum is well described by a power law with photon index α=2.63^+0.23_–0.15. A composite Planckian-plus-power-law spectral model yields an interesting alternative that formally describes the observed energy spectrum equally well. Inferred from best fits are a blackbody temperature of ∼1.7x10⁶ K and a projected blackbody radius of ∼69⁺³⁰_–25m, yielding a thermal flux contribution of ∼20% within the 0.1-2.4 keV band. The pulsar's spin-down to X-ray energy conversion efficiency as obtained from the single-power-law spectral model is ∼16%, assuming the distance inferred from the radio dispersion measure. If confirmed, PSR B0628-28 would be the first X-ray-overluminous rotation-powered pulsar identified among all ∼1400 radio pulsars known today. The emission beam geometry of PSR B0628-28 is estimated from radio polarization data taken at 408 and 1400 MHz. A formal best fit of the 1400 MHz data yields α∼11° for the inclination of the magnetic axis to the rotation axis and β~-3° for the impact angle of the line of sight. A combination of results obtained from 408 and 1400 MHz data, however, makes a nearly orthogonal solution, with α∼70° and β~-12° the most likely one.