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Astron. Astrophys. 321, 696-702 (1997)

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6. Relevant X-ray observations

It is well possible that the ROSAT satellite could have already detected neutron stars accreting in dark clusters. Here we mention some observations relevant for our scenario.

The longest pointed observations with the ROSAT PSPC in the direction of the Lockmann Hole (with the lowest neutral hydrogen column density) revealed the presence of 413 resolved sources per square degree at the faintest flux level (Hasinger et al. 1993 ). These sources constitute a statistically complete sample in the hard band with fluxes between [FORMULA] and [FORMULA] erg cm-2 s-1 and a maximum of objects at [FORMULA] erg cm-2 s-1. The [FORMULA] diagram clearly indicates a factor 1.6 more sources than predicted by standard models for AGN X-ray luminosity function. As suggest by Hasinger et al., one way to remedy this discrepancy might be to adopt different parameters or more complicated evolutionary models for AGN. Another possible explanation would be the existence of a new population of faint sources (with a rather steep [FORMULA] function) which cannot be identified with any class of known objects (as stars, BL Lac objects, clusters of galaxies and normal galaxies). Such a population could be practically absent at fluxes greater than a few [FORMULA] erg s-1 cm-2 but could contribute with [FORMULA] sources per square degree at the faint flux level. Therefore, since we estimated that less than [FORMULA] HNS per square degree should be observable, our proposed X-ray emitting HNS can contribute only up to [FORMULA] to this class of unknown objects.

Hasinger et al. (1993 ) also found that about 60% of the XRB is resolved to discrete sources with flux (averaged over the directions to 27 fields at high galactic latitude [FORMULA]) that amounts to [FORMULA] erg cm-2 s-1 deg-2 in the 0.5-2 KeV band. The total XRB intensity (averaged over the analyzed fields) is [FORMULA] erg cm-2 s-1 deg-2. As we have seen at the end of the previous section, the unabsorbed X-ray flux at high galactic latitude 6 from (unresolved) HNS is estimated to be in the range [FORMULA] erg cm-2 s-1 deg-2, depending on the core radius b of the neutron star distribution in the galactic halo (the lower value is for [FORMULA] kpc, the higher one for [FORMULA] kpc). This implies that the contribution of the HNS emission to the XRB excess (with respect to the discrete sources) can be in the range [FORMULA] so that, at least in the the case of HNS distributed as the dark matter, their contribution to the XRB excess can be significant.

Recently, Maoz & Grindlay (1995 ) showed that a new galactic population of faint ([FORMULA] erg s-1) X-ray sources with scale height of a few kpc can explain the XRB excess as well as the [FORMULA] sources per square degree required by the Hasinger et al. analysis. However, these authors have ruled out the possibility that accreting neutron stars could be the new population on the basis of the very soft spectrum (outside the ROSAT energy range) expected for low luminosity objects. This possibility has been reconsidered by Zane et al. (1995 ) which noted that the emission from magnetized neutron stars is harder than that expected from a black body at the neutron star effective temperature, since the accretion is channeled into polar caps so diminishing the emitting area. These authors have also calculated the actual emerging flux from low luminosity objects (by using the method in Zampieri et al. 1995 ) showing that up to [FORMULA] of the unresolved soft excess at high galactic latitude in the XRB can be explained in terms of neutron stars (in the galactic disk) accreting interstellar matter. In analogy, we note that X-rays emitted from HNS of luminosity [FORMULA] erg s-1 and magnetic field [FORMULA] gauss should emit - due to Eq. (7) - X-rays with mean energy of [FORMULA] keV, which is within the ROSAT energy band.

ROSAT observations during the All-Sky Survey and deep pointing, have also revealed a new and distinct class of supersoft X-ray sources with luminosities from a few times [FORMULA] erg s-1 up to [FORMULA] erg s-1 (Kahabka, Pietsch & Hasinger 1994 ). These are among the softest X-ray sources characterized by a black body spectrum with a temperature roughly a few times 10 eV and with most of the X-ray emission below 0.5 keV. While five sources have been detected towards both the LMC and the SMC and 16 have been discovered in M 31, only very few have been found in the Galaxy (Greiner 1996a ).

Supersoft X-ray sources are a collection of different type of objects (single non interacting white dwarfs, central stars of planetary nebulae, PG 1159 stars, symbiotic variables, magnetic cataclysmic variables, active galactic nuclei) which, generally, are recognized after follow-up optical observations.

Some of the sources in the Magellanic Clouds have been optically identified with close binary systems involving mass transfer from a main-sequence star onto a white dwarf burning the accreted matter (van den Heuvel et al. 1992). ROSAT observations established these supersoft X-ray sources as a distinct class which is named SSS (see Greiner 1996a ). As far as M 31 is concerned, the supersoft X-ray sources location distribution suggests that they belong to a disk population, while their identification with any known class of objects is under discussion (Greiner, Supper & Magnier 1996 ).

Another possible class of supersoft X-ray sources are old isolated neutron stars shrouded by super-Eddington accretion rates which emit a soft spectrum due to the presence around them of extended Compton scattering clouds (Greiner, Hasinger & Kahabka 1991 ; Kylafis & Xilouris 1993 ). Two sources located in the galactic disk have been proposed to be such neutron stars and one of these (RX J1856.5-3754) is a bright source also seen by ROSAT (Walter, Wolk & Neuhäuser 1996 ).

Remarkably enough, in our scenario DCNS have the same X-ray luminosity of the supersoft X-ray sources and, if located in the inner regions of dark clusters (with density in excess with respect to the assumed value of [FORMULA] cm-3), they could make a super-Eddington accretion and have Compton scattering clouds. Accordingly, DCNS could emit a spectrum softer than that predicted by Eq. (7) for high luminosity objects. A second reason for a softer spectrum is that DCNS (being born in globular clusters) are expected to have lower magnetic fields with respect to neutron stars ejected from the galactic disk (see discussion in footnote 2). Therefore, it appears natural to suggest that some of the unidentified supersoft X-ray sources can be DCNS, for which we estimated an occurrence of [FORMULA] sources per square degree.

Very recently a systematic search for supersoft X-ray sources has been undertaken using the ROSAT All-Sky Survey data, with emphasis on the galactic population (Greiner 1996b ). The selection criterion based on the hardness ratio led to 143 sources which, after optical identification with any known class of objects, came up to 12 unidentified objects. It is well possible that some of these objects are DCNS (also visible in the optical band due to the presence of a thick atmosphere extending up to large distance around the neutron star). However, it is also possible that the conservative approach adopted to select supersoft X-ray sources (for instance, in the Greiner 1996b search only 7 out of the more than 30 known supersoft X-ray sources are members of the sample) could have made DCNS undetected until now. It is without saying that our proposed identification of DCNS with a distinct class of supersoft X-ray sources is well possible, but in any case more detailed observations are needed.

Finally we mention the detection by ROSAT of shadows in the 1/4 keV X-ray background toward high-latitude interstellar clouds in Draco (Burrows & Mendenhall 1991 , Snowden et al. 1991 ). Data show that about half of the emission originate beyond the clouds. More recent observations show that other halo spots may exist in a region of almost 300 deg2 in Ursa Major (Snowden et al. 1994 ) and in a region of angular size of [FORMULA] deg2 in Eridanus (Snowden et al. 1995 ). These observations are interpreted as a first direct evidence for the presence of a million-degree galactic halo gas. The derived total X-ray luminosity of this diffuse gas (which can only accounts for less than a few percent of the galactic dark matter) is [FORMULA] erg s-1, in agreement with the X-ray observations for other spirals (Fabbiano 1989 ). It is straightforwards to say that in our scenario a population of unresolved HNS and DCNS could significantly contribute to this diffuse X-ray emission.

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Online publication: June 30, 1998