Further X-ray observations are of paramount importance to clarify the nature of this sample of X-ray pulsators. 4U 0142+614 (see §A.1.3), 1E 1048.1-5937 and 1E 2259+586 are included in the SAX Core Program and will be observed soon. Moreover 1E 1048.1-5937 and 1E 2259+586 were also observed by the XTE satellite in Aug. 1996 and data will soon be available. As in the case of 4U 0142+614 (see §A.1.3) the main objectives are to search for periodic delays of the pulse arrival times caused by the orbital motion and orbital modulation, and to obtain a new measurement of the spin period and X-ray flux.
\2box16.611 Summary -- The most likely interpretation is that the X-ray pulsars 4U 0142+614, 1E 1048.1-5937, 4U 1626-67, RX J1838.4-0301 and 1E 2259+586 are rotating close to their equilibrium spin periods and accreting from similar low mass companions. Their magnetic fields are likely of the order of a few 10 G, i.e. lower than in HMXB pulsars, but still much higher than those of non-pulsating LMXBs (see Fig. A.10).
Though the orbital period is known only for 4U 1626-67, the faintness of the optical counterpart and the absence of Doppler modulations in the X-ray pulses indicate that these systems have small orbital periods and/or companions of very low mass. The main contribution to the optical emission is thus expected to come from the X-ray heated accretion disc. An empirical relation between absolute magnitude, X-ray luminosity and orbital period for LMXBs has been derived by van Paradijs & McClintock (1994). For 10 erg s it predicts hr). Therefore, we would expect to detect the optical counterparts only for the closest and least absorbed of our LMXB pulsars, as indeed is the case (the optical absorption of 4U 1626-67 is only A ; van Paradijs et al. 1986). The relatively low luminosities imply that only the closest members of this class of LMXBs have so far been discovered. It is also possible that some of the already known, but poorly studied, LMXBs contain a pulsar with a spin period in the 5-10 s range.
Future studies of this new class of X-ray pulsars should concentrate on: (a) more accurate X-ray pulse timing studies that could reveal unambiguously the presence of a companion star; (b) deeper searches for the optical counterparts; (c) periodicity searches in low luminosity X-ray sources close to the galactic plane.