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in a recent paper , tananbaum et al . ( 1997 ) have reported a detailed analysis of a rosat pspc observation of j2310 - 43 , a very peculiar and interesting galaxy firstly discovered as a luminous x - ray source ( @xmath4 erg s@xmath5 ) in an _ einstein _
ipc image ( tucker , tananbaum & remillard 1995 ) .
the ipc data showed that the x - ray source is spatially extended .
this , combined with the fact that the galaxy is a cd in cluster ( tucker , tananbaum & remillard 1995 ) supported the hypothesis that the x - ray emission is due to the cluster , rather than to the activity of the galaxy itself . however , on the basis of the x - ray spatial and spectral analysis of the pspc data , tananbaum et al .
( 1997 ) suggested that only 20% of the total x - ray emission comes from the cluster , while the bulk is associated with a nuclear activity of the galaxy .
optical spectroscopy of the galaxy ( tucker , tananbaum & remillard 1995 ) , however , did not reveal the emission lines or the excess blue continuum expected if j2310 - 43 contains an active nucleus .
another interesting possibility considered by tananbaum et al .
( 1997 ) , is that j2310 - 43 is related to the bl lac phenomenon .
the lack of emission lines in the optical spectrum would apparently support this view .
furthermore , the broad band energy distribution of j2310 - 43 , i.e. the position of the object in the @xmath6 plane ( tananbaum et al .
1997 ) , falls at the edge of the region occupied by bl lac objects ( stocke et al .
on the other hand , the same authors show that the ( b - v ) color observed in j2310 - 43 is consistent with that of a normal elliptical galaxy and it is rather different from that of a typical bl lac object .
the resulting picture is rather intriguing with contradicting or inconclusive evidences as for the presence of  nuclear activity " . indeed
tananbaum et al .
( 1997 ) leave open the question whether j2310 - 43 belongs to the tail of the bl lac population or to a different class of objects and point out similarities with the optically dull galaxies with strong nuclear x - ray emission discovered by elvis et al .
( 1981 ) .
the spectroscopic data on j2310 - 43 , however , were limited to the 4700- 6700   interval thus excluding two regions critical for the understanding of the nature of the source : that of the ca ii break ( expected at @xmath74355 ) and of the @xmath0 line ( expected at @xmath77145 ) .
in fact agns showing no @xmath8 and [ oiii ] lines but exhibiting a broad @xmath0 line are known to exist ( stocke et al .
1991 ; see also halpern , eracleous & forster 1997 ) .
for these reasons the possibility that j2310 - 43 is hosting a reddened seyfert nucleus or a bl lac object could not be completely ruled out .    with the aim of further studying j2310 - 43 and understanding its real nature we have therefore secured two optical spectra covering the wavelength range 3700  @xmath9 9800 .
spectroscopy of j2310 - 43 was carried out with the eso 3.6 m telescope on 1996 december 10 .
observations were made with efosc1 in longslit mode , using a 1.5 arcsec wide slit and two different grisms , b300 and r300 , with wavelength coverage from 3700   to 6800   and from 6200   to 9800 , respectively .
the dispersions achieved with the two grisms , scaled to the tek512 ccd detector , were 6.3  / pixel ( b300 ) and 7.5  / pixel ( r300 ) .
the exposure time was of 600 sec with the grism b300 and of 300 sec with the r300 .
the data were reduced using the iraf - longslit package .
the wavelength solution was obtained using a he - ar reference spectrum while the correction for the instrument response was based on the observation of a photometric standard ( ltt 377 ) .
we did not make an absolute flux calibration , thus the flux density scale of the spectra is in arbitrary units .
the calibrated spectra are presented in figure  [ fig1 ] .
the spectra presented in figure  [ fig1 ] cover a wavelength range considerably larger than that of the spectrum discussed in tucker , tananbaum & remillard ( 1995 ) .
in particular , the region of the [ oii ] , caii break ( @xmath2 4355 ) and the region where h@xmath10 is expected ( 7145 ) are fully covered .
no emission lines appear in the spectrum , which shows only the typical absorption features of a `` normal '' early type galaxy . from the main absorption features seen ( ca ii h&k , g band , h@xmath11 , mgi 5175 , na i d ) we have computed a redshift of @xmath12 , confirming the value found by tananbaum et al . (
1997 ) .
the first result worth noting is the absence of h@xmath10 in emission .
the fact that no emission lines are present from [ oii ] to [ nii ] definitively put to rest the possibility that a seyfert nucleus is hiding in j2310 - 43 .
secondly , we note that a pronounced ca ii contrast is detected .
we have computed its amplitude following dressler & shectman ( 1987 ) , i.e. by estimating the average fluxes ( expressed in unit of frequency ) between 3750-3950  ( @xmath13 ) and between 4050-4250   ( @xmath14 ) in the rest - frame of the source ; the contrast is then defined by : @xmath15 we have found , using the central part of the spectrum to minimize the stellar contribution , a ca ii contrast of 38% @xmath1 4.0% , which is below the mean value found for a `` normal '' elliptical galaxy ( @xmath2 50% , dressler & shectman 1987 ) .    if one considers the `` canonical '' limit of 25% for the definition of a bl lac object ( stocke et al .
1991 ) then j2310 - 43 can not be considered a bl lac . on the other end march et al .
( 1996 ) have proposed that objects with a ca ii contrast below 40% are likely to have an extra source of continuum , besides stellar , and consequently they have to be considered as possible low - luminosity bl lac candidates .
the observed contrast can be used to set limits on the presence of such a non - stellar continuum , at least in the wavelength range 3700  @xmath9 4300  ( object rest frame ) . to this end , we have considered the spectrum of a `` normal '' galaxy , showing a ca ii contrast of about 60% .
then , we have `` added '' a non - stellar continuum to the spectrum , in the form of a power - law ( @xmath16 ) with a spectral index ranging from 0 to 2 , and we have computed the ca ii contrast as a function of the fraction of non - stellar over stellar continuum .
our results show that the ca ii contrast is about 40% if the non - stellar contribution is approximately equal to the stellar continuum , ( integrated between 3750  and 4250 ) .
values of the contrast of @xmath17 25% ( the limit used to define a bl lac object ) are obtained when the non - stellar contribution is about 3 times or more higher than the stellar continuum .
thus , in j2310 - 43 , which has a ca ii `` contrast '' of @xmath18 , the non - thermal component can still be present but at an intensity level comparable or lower than the fraction of the stellar continuum falling in the extraction region within the slit aperture .
we have also extracted the spectrum of j2310 - 43 considering only the outer region of the galaxy , thus minimizing the contribution of the nucleus , and we have found that the ca ii `` contrast '' increases to 47%@xmath15% . we consider this as a further evidence that j2310 - 43 harbors in its nucleus a weak source of non - thermal continuum that can be detected only if the stellar contribution falling in the aperture is kept at a minimum .
these results are consistent with the observed color of j2310 - 43 ( determined for the whole galaxy ) that indicates a negligible non - stellar contribution in the optical band , as discussed by tananbaum et al .
( 1997 ) .    in conclusion ,
the spectroscopic observations of the galaxy j2310 - 43 presented here support the interpretation that this object represents the faint tail of the bl lac population , in which the extra source of continuum is present but does not contribute significantly to the optical spectrum .
further observations ( polarization , radio spectral index , high resolution x - ray spectroscopy , etc . )
are obviously needed to characterize the presence of such non - thermal continuum emission in j2310 - 43 .
it is worth noting that a similar object ( e0336 - 248 ) has been recently discovered by halpern et al .
also in this case , the computed ca ii `` contrast '' ( 33% ) does not meet the nominal criterion of @xmath17 25% for classification as a bl lac object . nevertheless , these authors have produced convincing evidence for the presence of a non - stellar continuum in the spectrum of e0336 - 248 and , consequently , for its classification as a weak bl lac object .
clearly , a re - assessment of the ca ii `` contrast '' criterion for the definition of a bl lac is needed .
tananbaum et al .
( 1997 ) ask  how common are sources such as j2310 - 43 ?
" we have reason to believe that they may be more common than currently recognized .
we have recently initiated a survey of radio emitting x - ray sources ( the rex survey , caccianiga et al .
1997a , 1997b ) with the aim of selecting a new large sample of bl lac objects and radio loud quasars . to enter the sample , a source has to be detected in a pointed rosat pspc observation _ and _ in the vla nvss survey , above
well defined flux limits and thresholds . during the optical identification program of the rex sources we have already found six objects that are similar to j2310 - 43 .
the properties of these objects will be presented and discussed in detail elsewhere ; here we recall that , like j2310 - 43 , they have x - ray luminosity in the range @xmath19 ( 0.5 - 2.0 kev ) and radio luminosity in the range @xmath20 ( 1.4 ghz ) .
they are all radio loud ( @xmath21 0.35 ) and they all have a caii  contrast " between 25% and 40% and no emission lines in their spectrum .
they do not seem to lie preferably in a cluster environment .
we note that these six objects have been found out of @xmath22 new spectroscopic identifications .
it is unfortunate however that , given the very low identification rate so far obtained for the rex survey , we can not , at present , make an estimate of the space density of these objects .
we thank h. tananbaum for useful comments on an earlier version of this paper and a. wolter for bringing to our attention the work on e0336 - 2248 . this work has received partial financial support from the italian space agency ( asi contract 95-rs-72 dmd .
161fae/2 , asi ars-96 - 71 dmd .
caccianiga , a. , della ceca , r. , maccacaro , t. , & wolter , a.,1997a , submitted to apj caccianiga , a. , della ceca , r. , maccacaro , t. , ruscica , c. , & wolter , a. 1997b , mem .
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68 , 325 dressler , a. , & shectman s. 1987 , aj , 94 , 899 elvis , m. at al .
1981 , apj , 246 , 20 halpern , j.p . ,
eracleous , m. , & forster , k. 1997 , aj in press march , m.j.m . ,
browne , i.w.a . ,
impey , c.d . , & smith , p.s .
1996 , mnras 281 , 425 stocke , j.t .
1991 , apjs , 76 , 813 tananbaum , h. , tucker , w. , prestwich , a. , & remillard , r. 1997 , apj 476 , 83 tucker , w. , tananbaum , h. , & remillard , r. 1995 , apj , 444 , 532
we present and discuss new spectroscopic observations of the unusual galaxy j2310 - 43 . 
 the observations cover a wide wavelength range , from 3700   to 9800  allowing the study of both the regions where @xmath0 and the ca ii `` contrast '' are expected . 
 no evidence for @xmath0 in emission is found and we thus confirm the absence of emission lines in the spectrum of j2310 - 43 , ruling out the possibility that it may host a seyfert nucleus . 
 the caii break is clearly detected and the value of the contrast ( 38%@xmath14% ) is intermediate between that of a typical elliptical galaxy ( @xmath2 50% ) and that of a bl lac object ( @xmath3 ) . 
 this result imposes limits on the intensity of a possible non - stellar continuum and , in the light of the radio and x - ray loudness of the source , draws further attention to the problem of the recognition of a bl lac object . 
 objects like j2310 - 43 may be more common than previously recognized , and begin to emerge in surveys of radio - emitting x - ray sources .