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W. Becker (MPE), M.C. Weisskopf, A.F. Tennant (MSFC), A. Jessner (MPIfR), J. Dyks, A.K. Harding (GSFC), S.N. Zhang (UAH)
We have completed part of a program to study the X-ray emission properties of old rotation-powered pulsars with XMM-Newton in order to probe and identify the origin of their X-radiation. The X-ray emission from these old pulsars is largely dominated by non-thermal processes. None of the observed spectra required adding a thermal component consisting of either a hot polar cap or surface cooling emission to model the data. The X-ray spectrum of PSR B0950+08 is best described by a single power law of photon-index 1.93 (+0.14,-0.12). Taking optical data from the VLT into account a broken power law modelis found to describe the pulsar's broadband spectrum from the optical to the X-ray band. Temperature upper limits for possible contributions from a heated polar cap or the whole neutron star surface are < 0.87 MK and < 0.48 MK, respectively. We also find that the X-ray emission from PSR B0950+08 is pulsed with two peaks per rotation period. The phase separation between the two X-ray peaks is ~144 degree (maximum to maximum) which is similar to the pulse peak separation observed in the radio band at 1.4 GHz. The main radio peak and the trailing X-ray peak are almost phase aligned. The fraction of X-ray pulsed photons is ~30%. A phase-resolved spectral analysis confirms the non-thermal nature of the pulsed emission. Detailed pulse profile simulations constrain the pulsar's emission geometry to be that of an almost orthogonal rotator. The spectral emission properties observed for PSR B0823+26 are similar to those of PSR B0950+08. Its energy spectrum is very well described by a single power law with photon-index 2.5 (+0.9,-0.45). Temperature upper limits for thermal contributions from a hot polar cap or from the entire neutron star surface are <1.17 MK and < 0.5 MK, respectively. There is evidence for pulsed X-ray emission at the ~97% confidence level with a pulsed fraction of 49 +- 22%. For PSR J2043+2740 we report the first detection of X-ray emission. A power law spectrum, or a combination of a thermal and a power law spectrum all yield acceptable descriptions of its X-ray spectrum.
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.