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 tobe the X-ray counterpart of the old field pulsar PSR B0628-28. Thisassociation, however, was regarded to be unlikely based on the computedenergetics of the putative X-ray counterpart. In this paper we report onmultiwavelength observations of PSR B0628-28 made with the ESO/NTT observatoryin La Silla, the Jodrell Bank radio observatory and XMM-Newton. Although theoptical observations do not detect any counterpart of RX J0630.8-2834 down to alimiting magnitude of V=26.1 mag and B=26.3 mag, XMM-Newton observationsfinally confirmed it to be the pulsar's X-ray counterpart by detecting X-raypulses with the radio pulsar's spin-period. The X-ray pulse profile ischaracterized by a single broad peak with a second smaller peak leading themain pulse component by ~144 degree. The fraction of pulsed photons is (38 +-7)% with no strong energy dependence in the XMM-Newton bandpass. The pulsar'sX-ray spectrum is well described by a single component power law with photonindex 2.63^{+0.23}_{-0.15}, indicating that the pulsar's X radiation isdominated by non-thermal emission processes. A low level contribution ofthermal emission from residual cooling or from heated polar caps, cannot beexcluded. The pulsar's spin-down to X-ray energy conversion efficiency isobtained to be ~16% for the radio dispersion measure inferred pulsar distance.If confirmed, PSR B0628-28 would be the first X-ray overluminousrotation-powered pulsar identified among all ~1400 radio pulsars known today.Comment: Accepted for publication in ApJ. Find a paper copy with higher resolution images at ftp://ftp.xray.mpe.mpg.de/people/web/astro-ph-0505488_rev2.pd