Observational characteristics of the Very Low Mass X-ray Binary Pulsars

4U 0142+614

The properties of 4U 0142+614 (White et al. 1987) remained puzzling for a long time, owing to confusion problems with a nearby pulsating and transient Be/neutron star system (Motch et al. 1991; Mereghetti, Stella & De Nile 1993). Despite the small error box (5" radius), no optical counterpart has yet been identified, down to V;SPMgt;24 (Steinle et al. 1987), thus excluding the presence of a massive companion. Using data from the EXOSAT archive, Israel, Mereghetti & Stella (1994) discovered periodic pulsations at 8.7 s, which were later confirmed with ROSAT (Hellier 1994b). No delays in the pulse arrival times caused by orbital motion were found, with upper limits on of about  lt-s for orbital periods between 7 min and 12 hr (Israel, Mereghetti & Stella 1994). The EXOSAT and ROSAT period measurements, obtained in 1984 and 1993, provide a spin-down rate of  s yr . The 2-10 keV spectrum, a power law with photon index of , is extremely soft (White et al. 1987) and led to the initial classification of this source as a possible black hole candidate. The X-ray luminosity of 4U 0142+614 did not show large secular variations around an average value of  erg s (assuming a distance of 2 kpc).

1E 1048.1-5937

This source was serendipitously discovered with the Einstein Observatory in 1979, and found to pulsate at 6.44 s (Seward, Charles & Smale 1986). The brightest candidate counterparts in the small error box have V;SPMgt;20 (Mereghetti, Caraveo & Bignami 1992) indicating that also 1E 1048.1-5937 is a LMXB. 1E 1048.1-5937 was repeatedly observed with ROSAT in 1992 and 1993. While all the previous observations with EXOSAT and GINGA (Corbet & Day 1990) were consistent with a constant spin-down at a rate of  s yr , the ROSAT data (Mereghetti 1996) indicate a doubling of the spin-down rate (Fig. A.6) . FIG A.6: Spin period evolution of 1E 1048.1-5937 (from Mereghetti 1996). The power law photon index derived with EXOSAT ( =2.3, Seward, Charles & Smale 1986) implies also for this source a spectrum somewhat softer than the ``canonical" spectrum of X-ray pulsars. The high column density suggests that 1E 1048.1-5937 lies behind the Carina nebula, i.e. at more than 2.8 kpc. The luminosity corresponding to this distance is  erg s .

4U 1626-67

This LMXBs is optically identified with the V 18.5, blue star KZ TrA (McClintock et al. 1977). In addition to the X-ray periodicity at 7.7 s (Rappaport et al. 1977), a pulsation at a slightly lower frequency is present in the optical band (Middleditch et al. 1981). This is probably due to reprocessing of the X-ray pulses occurring near the companion star, and the difference of the two periodicities can be explained with an orbital period of 41.4 min. The high X-ray to optical flux ratio, as well as the very strong limits on the optical mass function ( ;SPMlt; 0.013 lt-s for 10 min   10 hr; Levine et al. 1988), clearly indicate that 4U 1626-67 is a LMXB. The period measurements obtained before 1990 were consistent with a constant spin-up rate of  s yr (Nagase 1989), but in 1991 the period derivative changed sign (Lutovinov et al. 1994; Bildsten et al. 1994). On the basis of the luminosity required to explain the spin-up torque ( - ) Levine et al. (1988) estimated a distance of 3 to 6 kpc. With the exception of quasi-periodic flares with a characteristic timescale of  s, little intensity variations are present in 4U 1626-67. The pulse averaged spectrum, a flat power law (photon index ) followed by an exponential cut-off at  keV (Pravdo et al. 1979), is similar to that of HMXB pulsars (White, Swank & Holt 1983). A recent ASCA observation revealed the presence of spectral features around 1 keV, interpreted as emission lines of hydrogen-like Neon (Angelini et al. 1995). This indicates an overabundance of Ne in 4U 1626-67, which might give interesting constraints on the nature and evolution of its companion star.

RX J1838.4-0301

This pulsar, recently discovered with ROSAT, is embedded in a region of diffuse X-ray and radio emission, which is interpreted as a supernova remnant years old at a distance of  kpc (Schwentker 1994). The brightest optical object in its 10" radius error box has V=14. The spectrum in the 0.1-2.4 keV band, and therefore the unabsorbed flux, are not well constrained by the data (Schwentker 1994), but there is evidence that also this source is quite soft (best fit power law photon index of ). Also RX J1838.4-0301 is likely a LMXB.

1E 2259+586

The source 1E 2259+586 was discovered with the Einstein Observatory at the center of the X-ray and radio supernova remnant G109.1-1.0 (Fahlman & Gregory 1981). Extensive searches for optical, IR and radio counterparts were carried out without success (Fahlman et al. 1982; Coe & Jones 1992; Coe, Jones & Letho 1996), but they definitely exclude the presence of a massive companion (Davies & Coe 1991). The spin period of 1E 2259+586 has been increasing at  s yr until 1992 (Koyama et al. 1989; Iwasawa, Koyama & Halpern 1992). Recent ROSAT data revealed the first spin-up episode for this source. A detailed analysis of all the period measurements over the last 15 years (Baykal & Swank 1996) showed that 1E 2259+586 undergoes random angular velocity variations similar to those observed in other accreting binary neutron stars (Fig. A.7).

FIG A.7: Spin period evolution of 1E 2259+586 (adapted from Baykal & Swank 1996). The upper limit on is of 0.08 lt-s for 10  s 10  s (Koyama et al. 1989). 1E 2259+586 has a very soft spectrum, as recently confirmed by BBXRT and ASCA observations (Corbet et al. 1995b). Also in this source, no long term variability greater than a factor 2 has been reported. A distance of 3.6 0.4 kpc for the supernova remnant G109.1-1.0 was estimated by using the classical relation between distance and radio surface brightness (Gregory & Fahlman 1980). Based on improved radio observations, Hughes et al. (1984) have subsequently derived a value of 5.6 kpc. For this distance the average X-ray luminosity of 1E 2259+586 is 2 erg s .

Note --Recently (Haberl et al. 1996) a new X-ray pulsator was discovered, namely RX J0720.4-3125, in the ROSAT All-Sky Survey with similar characteristics of this sample of pulsating source. A periodic modulation of the X-ray flux is detected with a period of 8.38s while the energy spectrum is well-described by a blackbody with kT=75-83 eV. No optical counterpart was detected to a limiting magnitude of V about 21.2 .