Overall properties

The main properties of the sources considered above are summarised in Table A.1. Their most important similarities are the following:

 
Table: Properties of the LMXB pulsars

(a)

When compared to the other X-ray pulsars, the most striking property of these objects is their narrow spin period distribution (Fig. A.5). A Kolmogorov-Smirnov test yields a probability of 4 10 that the period distribution of these five sources and that of the HMXBs pulsars are drawn from the same parent distribution.

(b)

With the exception of 4U 1626-67, their spectra are much softer than those of the other X-ray pulsars (White, Swank & Holt 1983). Figure A.8 shows a comparison of the EXOSAT spectra of Vela X-1 and 4U 0142+614. FIG A.8: A comparison of the EXOSAT ME spectra of Vela X-1 and 4U 0142+614 (adapted from Stella, Mereghetti & Israel 1996). The steep power law spectra of 4U 0142+614 and 1E 2259+586 are even softer than those of most LMXBs (White, Stella & Parmar 1988). This is also apparent from the position occupied by these source in the X-ray colour-colour diagram of Figure A.9. FIG A.9: X-ray colour-colour diagram for a sample of X-ray binaries (adapted from Stella, Mereghetti & Israel 1996).

(c)

These sources are probably less luminous than most persistent LMXBs. Indeed, for 1E 2259+586, 1E 1048.1-5937 and 4U 0142+614 firm upper limits of a few 10 erg s can be derived by requiring that they lie within the Galaxy. The best estimates for the distances of 4U 1626-67 and RX J1838.4-0301 also imply luminosities of this order of magnitude or smaller.

(d)

Their flux appears to be relatively constant on timescales from months to years. This is unlike most of the other X-ray pulsars (including Her X-1 and GX 1+4) for which large flux variations (encompassing transient activity) have been observed.

(e)

Two of these sources are likely associated to supernova remnants. Another source, 1E 1048.1-5937, lies in the direction of the Carina Nebula, a complex region of radio, optical and X-ray diffuse emission, clearly associated with recent star formation activity.

Based on these similarities, Mereghetti & Stella (1995) proposed that these pulsars belong to a homogeneous subclass of LMXBs characterised by lower luminosities (  erg s ) and higher magnetic fields (  G) than classical, non-pulsating LMXBs.