A multiwavelength study of PSR B0628-28: The first overluminous rotation-powered pulsar?

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 ESONTT 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 similar to 144 degrees. 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 alpha = 2.63 +/-(+0.23)(-0.15) A composite Planckianplus -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 similar to 1.7 x 10(6) K and a projected blackbody radius of similar to 69-(+30)(25)m, yielding a thermal flux contribution of similar to 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 similar to 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 similar to 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 alpha similar to 11 degrees for the inclination of the magnetic axis to the rotation axis and beta similar to -3 degrees 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 alpha similar to 70 degrees and beta similar to -12 degrees the most likely one.