The X-ray Luminosity of rotation-powered Neutron Stars

Joachim Trümper, Werner Becker and Rodrigo Supper

The nearly 750 radio pulsars known so far are interpreted as rapidly spinning and strongly magnetized neutron stars which are radiating at the expense of rotational energy. With ROSAT and ASCA, 27 of these pulsars have been detected in the soft X-ray domain, representing a wide range of ages ( tex2html_wrap_inline40 yrs), magnetic field strength ( tex2html_wrap_inline42 G) and spin periods (1.6 - 530 ms). Despite these dispersions in parameters Becker & Trümper (1997) found that the X-ray luminosity of the detected pulsars is closely correlated with the pulsar's rotational energy loss tex2html_wrap_inline46.

Figure 1a depicts the X-ray luminosities of all 26 pulsars measured in the ROSAT band as a function of their rotational energy loss. For the cooling neutron stars we have plotted the luminosities derived for their non-thermal component. Fitting a tex2html_wrap_inline48 distribution to the data we find tex2html_wrap_inline50 and a correlation coefficient of r = 0.946. Adopting b=1 the efficiency of conversion is tex2html_wrap_inline56, indicated by the dashed-line.

Another way to look to the sample of the X-ray detected rotation-powered pulsars is shown in Figure 1b where the X-ray luminosity is plotted as a function of the pulsar's characteristic age. Assuming the standard picture of magnetic braking we find tex2html_wrap_inline58. The corresponding tex2html_wrap_inline60 curves are indicated. All pulsars lie close to the braking curves calculated for their respective polar magnetic fields.

The strong correlation suggests that the prime energy source of the X-ray emission is the pulsar's rotational energy. In particular the millisecond pulsars, which are separate from the majority of ordinary field pulsars, obey the same linear relationship with the same X-ray efficiency as the Crab type pulsars, indicating that their emission is mainly due to magnetospheric processes. The result is in line with the occurrence of power-law spectra and the similarity between the radio and X-ray pulse profiles (Becker & Trümper et al. 1997 in prep.).

Becker,W., Trümper, J., A&A, 1997, 326, 682-691


Author: Werner Becker (e-mail: )