The lack of an optical counterpart down to limits of and (Steinle et al. 1987; White et al. 1987) implies an X-ray/optical flux ratio . The only known classes of galactic sources which can yield such a high value are low mass X-ray binaries (LMXBs) and isolated neutron stars.
A neutron star accreting from a low mass companion is the most likely explanation for 4U 0142+614. Coherent pulsations are rarely seen in LMXBs: the only known examples among optically identified systems, 4U 1626-67, Her X-1 and GX 1+4, have very different X-ray properties, companion stars and evolutionary origins (see, e.g., White, Nagase & Parmar 1995). The spin period of 4U 0142+614 is very similar to that of 4U 1626-67 (7.7 s, Rappaport et al. 1977), and it is interesting to note that two other optically unidentified pulsars, which are likely accreting from low mass companions, 1E 2259+586 and 1E 1048.1-5937 (Coe & Jones 1992; Mereghetti, Caraveo & Bignami 1992), have periods of the same order, 6.98 and 6.44 s respectively (Davies et al. 1990; Corbet & Day 1990).
In order to derive some information from the available optical limits, some considerations on the likely distance and interstellar reddening of 4U 0142+614 are required. The position of 4U 0142+614 is close (;SPMlt;0.5 ) to that of two open clusters with well determined distances and reddening: NGC 654 (2.5 kpc and A =2.67), and NGC 663 (2.1 kpc and A =2.43) (see Leisawitz, Bash & Thaddeus 1989, and references therein). The column density N cm derived from the power law spectral fits of 4U 0142+614 (White et al. 1987) corresponds to a higher absorption, A 7 (Gorenstein 1975), hinting to a greater distance. However, 4U 0142+614 is not necessarily much further than these clusters, since a part of its absorption could be intrinsic to the source or due to a local (d;SPMlt;1 kpc) molecular cloud which is present in this region, as clearly visible on the POSS prints. 4U 0142+614 lies near to the edge of this cloud, which does not significantly affect NGC 654 and NGC 663 (Leisawitz, Bash, & Thaddeus 1989). A distance of 4 kpc would yield a 1-10 keV luminosity of erg s , similar to 4U 1626-67. At this distance and reddening the faint optical counterpart of the latter source would be fainter than the present limits for 4U 0142+614. On the other hand, an evolved companion similar to that of GX 1+4 or Cyg X-2 (M -1, van Paradijs 1991) would have been detected even at 10 kpc (which for this direction is well outside the Galaxy). A companion star similar to that of 4U 1626-67, i.e. either a main sequence star with M or a white dwarf of 0.02 (Verbunt, Wijers & Burm 1990), is also compatible with the limits on derived, which however also allow for more massive companions. For instance, a hydrogen main sequence star of 0.3 , would fill the Roche lobe for an orbital period of hr, requiring i .
4U 0142+614 is similar to the unidentified source 1E 2259+586, whose spectrum can be described by a power law with energy index 3, plus some possible cyclotron features suggesting a magnetic field B (Iwasawa, Koyama & Halpern 1992).
The energy spectrum of the Aug. 1984 EXOSAT ME observation was modelled by a two power law model with photon index =3.2 and =1 for the soft component (1-3 keV) and the hard component (4-11 keV), respectively. The intrinsic absorption was worked out to be and for the soft and hard component, respectively. In the Nov. 1985 observation only the soft part of the spectrum was present and fit with a single power law model ( =3.2; see Fig. A.4).
The possibility that 4U 0142+614 is an isolated neutron star is suggested by its very high , its ultrasoft spectrum, and the absence of significant variability on long timescales (of course this possibility requires that the evidence for secular spin-up is the result of chance detections in the 1985 and 1991 data). In principle the X-ray emission could be due to non-thermal magnetospheric processes powered by the rotational energy, to thermal emission from the neutron star surface, or to accretion from the interstellar medium. While examples of the first two mechanisms are well known (see, e.g., Mereghetti, Caraveo & Bignami 1994), no compelling evidence for a compact object accreting from the interstellar medium has yet been found, despite several studies show that such sources could be relatively common (Treves & Colpi 1991; Blaes & Madau 1993).
Very recently new results were obtained by investigating the properties of the 4U0142+614 using new data obtained with the ASCA observatory and archival data from the Einstein and ROSAT observatories (White et al. 1996; see §A.2 for more details).