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Astron. Astrophys. 326, 662-668 (1997)

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3. Discussion

We have discovered the soft X-ray source RX J0720.4-3125 in the ROSAT all-sky survey. From optical observations we found no counterpart down to a limiting B magnitude of 21.0. The source neither appears in the ROSAT WFC 2RE catalogue (Pye et al. 1995), as EUVE source (Malina et al. 1994), in radio catalogues, as IRAS nor as EGRET source. RX J0720.4-3125 is probably identical with the Einstein IPC slew source 1ES0718-313, only 11" away from the best ROSAT position. The IPC count rate was 0.39 [FORMULA] 0.16 counts s-1, compatible with the ROSAT fluxes, although with a large error and from a different energy band. The lower limit for f [FORMULA] /f [FORMULA] of 500 would still be compatible with a low mass X-ray binary nature as these systems have typical ratios of 100-1000 (White et al. 1993). The EXOSAT LE fluxes together with the ROSAT measurements indicate no large changes of the X-ray intensity ( [FORMULA] 5%) on time scales of years, in contrast to the highly variable X-ray binaries. Also the derived X-ray luminosity using distance estimates (see below) is orders of magnitudes lower than for typical low mass X-ray binaries.

The 8.391 s pulse period, if interpreted as spin period, makes it unlikely to come from a white dwarf. A massive white dwarf with this rotation period is still stable (e.g. Chanmugam et al. 1987) but would be expected in a binary system, spun up by accretion, and optically visible as cataclysmic variable. An isolated white dwarf which has fully accreted or evaporated its low-mass companion and is accreting from the interstellar medium would need to be about 30 times closer than a corresponding neutron star (see discussion below), to account for the observed X-ray flux. Also in this case it should be seen in the optical.

The similarities in the X-ray properties of RX J0720.4-3125 compared to RX J1856.5-3754 and the high limit of the f [FORMULA] /f [FORMULA] ratio suggest it as very likely candidate for an isolated neutron star. If the bolometric luminosity of RX J0720.4-3125 and RX J1856.5-3754 is comparable then the distance to RX J0720.4-3125 can not be far in excess of 100 pc. In this case the derived emission area is only a fraction of the neutron star surface, compatible with the observation of pulsed 8.391 s modulation of the soft X-ray flux from RX J0720.4-3125. In fact to keep the emission area smaller than the surface of a neutron star, the distance must be less than about 440 pc for a standard neutron star with 10 km radius.

The low value of photo-electric absorption derived from the PSPC spectrum (1.3  10 [FORMULA] cm-2 compared to the integrated galactic absorption in this direction of 1.89  10 [FORMULA] cm-2, Dickey & Lockman 1990) also excludes an extragalactic origin. The absorption may further provide an estimate for the distance of RX J0720.4-3125. Studies of the local interstellar medium conclude that in the direction to RX J0720.4-3125 the density is very low (Paresce 1984). No absorbing interstellar clouds are visible in CO-maps (Dame et al. 1987) and dark cloud maps (Feitzinger & Stüwe 1986). Welsh (1991) finds a `tunnel' with n [FORMULA] 0.1 cm-3, 300 pc long, about 5o away from RX J0720.4-3125. The nearby (1o) open cluster Collinder 140 shows an E(B-V) of 0.05, corresponding to an N [FORMULA] of 2.8  10 [FORMULA] cm-2, and is located at a distance of 400pc. Hence the distance may well be up to around 300 pc, limiting the X-ray luminosity to about 2.3  10 [FORMULA] erg s-1.

An isolated neutron star may be visible in X-rays for a relatively short period as young cooling object or when it is accreting matter from the interstellar medium. For the latter case, assuming blackbody-like emission, Blaes & Madau (1993) predict neutron star surface temperatures of kT = 20 (M_10 /f) [FORMULA] eV, where M_10 is the mass accretion rate in units of 1010 g s-1 and f is the fraction of the neutron star surface covered by accretion. Using the observed kT of 80 eV one derives M_10 = 250f. For the bolometric luminosity Blaes & Madau (1993) give Lbol = 2 1030 M_10 erg s-1 for a standard 1.4 M [FORMULA], 10 km radius neutron star, i.e. for RX J0720.4-3125 a luminosity Lbol of 5 1032 f erg s-1. Observed and modeled luminosity are consistent for (d/100pc)2 = 20f, e.g. for a distance of 100 pc yielding f = 0.05, Lbol = 2.6 1031 erg s-1 and M_= 1.2 1011 g s-1. For this distance the derived luminosity yields a lower limit for the ambient interstellar gas density (assuming spherical accretion with a relative neutron star velocity of 0, see Eq. 21 in Blaes & Madau 1993) of about 0.05 cm-3. For a typical gas density of 1 cm-3 a relative velocity of around 15 km s-1 is expected. Because the density rises steeply with the relative neutron star velocity, unrealistic high densities are derived for velocities in excess of 100 km s-1. The only solution would be a lower luminosity and hence a lower distance which is probably in contradiction to the values derived from the absorption, if it is mainly of interstellar origin. However the emitted spectrum might deviate from a blackbody (Zampieri et al. 1995, Zavlin et al. 1996) and the parameters derived above should be treated critically.

Isolated neutron stars with internal frictional heating only are expected to cool to the observed temperature of 8 105 K within about 4 105 yr (Umeda et al. 1993, Becker 1996). This is a lower limit for the age of a neutron star in RX J0720.4-3125 and the real age is depending on how much the accretion onto the magnetic poles re-heats the star. The case of a purely cooling neutron star is improbable for RX J0720.4-3125. Unlike young cooling neutron stars RX J0720.4-3125 has no radio or gamma-ray counterpart or association with a supernova remnant and the spin period would be exceptionally long.

An estimate of the age of the neutron star may come from the group of 5-9 s X-ray pulsars which has been suggested as the result of common envelope evolution of a high mass X-ray binary (van Paradijs et al. 1995). In this picture a neutron star is accreting from a massive disk, the remnant of the companion star. These pulsars show apart from their very narrow period distribution other similarities: their X-ray spectra are much softer than those of `normal' pulsars, their pulse period increases with time, their locations are well confined to the galactic plane (within [FORMULA] 100 pc) like for high mass X-ray binaries and their X-ray luminosities are in the range of 1035 to  10 [FORMULA] erg s-1 and relatively constant on time scales of days to about 10 years. The fact that the pulse period of RX J0720.4-3125 fits into the period distribution of the 5-9 s pulsars is conspicuous and could indicate a relation between the objects, but we stress that it may be purely accidential as the relation between the 5-9 s pulsars is not proven yet. However if there is a relation, and the required low space velocity of RX J0720.4-3125 and its low distance to the galactic plane may be arguments for an evolution from a high mass X-ray binary, RX J0720.4-3125 could be the finally evolved single neutron star having lost the disk or at most have only a tenuous remainder as the much lower X-ray luminosity of RX J0720.4-3125 indicates. In the latter case RX J0720.4-3125 might be relatively young ( [FORMULA] yr), but modeling the spin evolution of the neutron star through the disk accretion phase and beyond is required to yield better age estimates. The evolutionary scenario outlined above could even hold if the 5-9 s pulsars turn out to be a group of unrelated objects with a different evolution. Isolated neutron stars which were going through a common envelope evolution in a high mass X-ray binary, perhaps via a Thorne-Zytkow object (the neutron star spirals into the center of the massive star) are expected to have a relatively low space velocity of around 50 km s-1 (Podsiadlowski 1995). They can accrete more efficiently from the interstellar medium than high velocity neutron stars and are therefore brighter and more easily to detect.

The estimate of the X-ray luminosity and the pulse period allow to constrain the magnetic field strength of the neutron star in RX J0720.4-3125, if the X-ray emission is powered by accretion. For accretion to significantly occur, the Kepler co-rotation radius should be larger than the Alfven radius, which implies for an X-ray luminosity of around  10 [FORMULA] erg s-1 a magnetic field strength of less than 1010 G. According to current ideas the decay of neutron star magnetic fields is strongly related with the accretion of matter. Observations of binary radio pulsars with low-mass companions are consistent with increasing field decay with increasing amount of matter accreted (van den Heuvel & Bitzaraki 1995). Evolution via a Thorne-Zytkow object could easily provide the required mass for accretion to explain the systematically low magnetic fields in the 5-9 s X-ray pulsars of the order of 1011 G (van Paradijs et al. 1995, White et al. 1996) and the even lower field required for RX J0720.4-3125. A similar evolutionary scenario was proposed by Lorimer et al. (1995) for low-velocity radio pulsars with low magnetic field. On the other hand an old isolated neutron star evolved from a single star can not be ruled out from the magnetic field strength arguments. Lyne et al. (1985) present field decay time scales of [FORMULA] yr from observations of pulsars, but Bhattacharya et al. (1992) argue for no significant decay in single radio pulsars over 108 yr and Kulkarni (1986) derives evidence for a long-lived component of [FORMULA] G at least for neutron stars in binaries. It is therefore not clear how the magnetic field of an isolated neutron star decays with time, but 109 yr may be sufficient for the field to decay from typical 1012 G to 109 G and also spin down a fast rotating neutron star to the observed spin period of RX J0720.4-3125.

RX J0720.4-3125 and RX J1856.5-3754 are both close to the galactic plane and have a low space velocity, if our picture of accreting neutron stars is correct. These facts however do neither favour the evolution from a high mass X-ray binary nor from a star which left a single low-velocity neutron star. The derived low magnetic field strength in the case of RX J0720.4-3125 may suggest that it has undergone periods of large matter accretion in favour of a high mass X-ray binary evolution. The temporal evolution of neutron star magnetic fields however needs to be better understood before a definite statement can be made. The low magnetic field in RX J0720.4-3125 may be in turn the reason for the still relatively short spin period as the propeller mechanism which can slow down the rotation becomes inefficient. This may suggest that at least RX J0720.4-3125 is not belonging to the expected large class of isolated old neutron stars. The latter may have larger space velocities, closer to that of radio pulsars which are distributed around 450 km s-1 (Lyne & Lorimer 1994) or low mass X-ray binaries (van Paradijs & White 1995), as assumed in the estimates for the number of old neutron stars detectable in the ROSAT survey (e.g. Blaes & Madau 1993). Faster neutron stars are expected to be fainter because spherical accretion (Bondi & Hoyle 1944) strongly depends on the relative velocity of the matter to the neutron star ( [FORMULA] (v [FORMULA] +c [FORMULA] ) [FORMULA] ). In this case even nearby (a few 100 pc) old isolated neutron stars would be very faint in the ROSAT survey data (e.g. for only a factor 2 higher space velocity, the count rate of RX J0720.4-3125 would decrease to 0.2 counts s-1 ). However, it can again not be excluded that we see old objects (evolved from a single star) from the very low end of their velocity distribution. In particular at low velocities this distribution is not certain and the number of isolated neutron stars detected in the ROSAT all-sky survey may help to constrain it. Further observations to determine the pulse frequency change and proper motion of RX J0720.4-3125 will gain further insight into this object. The upper limits derived from optical identifications of ROSAT survey sources in selected fields (Motch et al. 1997) on the number of isolated neutron star observed by ROSAT however still needs to be explained by the models.

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© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997