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in 1996 september , a new x - ray transient was discovered with the wide field cameras ( wfcs ) on board the _ bepposax _ satellite and the source was designated sax j1808.43658 ( in t zand et al . type i x - ray bursts were detected demonstrating that the compact object in this system is a neutron star and that the source is a low - mass x - ray binary ( lmxb ) . from the bursts , an initial distance estimate of 4 kpc was determined but this was later revised to 2.5 kpc ( in t zand et al . 1998 , 2001 ) . the maximum luminosity of sax j1808.43658 was only approximately @xmath3 erg s@xmath4   indicating that the source was a faint neutron - star x - ray transient . the outburst lasted about three weeks after which the source returned to quiescence . in april 1998 , the _ rossi x - ray timing explorer _ ( _ rxte _ ) satellite scanned , using the proportional count array ( pca ) , the region on the sky where sax j1808.43658 is located and the source was found to be active again ( marshall 1998 ) . follow up _ rxte_/pca observations showed that sax j1808.43658 exhibits coherent millisecond x - ray oscillations with a frequency of approximately 401 hz ( wijnands & van der klis 1998 ) , making this source the first ( and so far the only ) accretion - powered millisecond x - ray pulsar discovered . the source was also detected in the optical ( giles , hill , & greenhill 1999 , with an optical position of r.a . = 18@xmath5 08@xmath6 27.54@xmath7@xmath80.015@xmath9 and dec . 36  58@xmath1 44.3@xmath10@xmath80.2@xmath10 ; all coordinates in the paper are for j2000.0 ) and in the radio ( gaensler , stappers , & getts 1999 ) . also this outburst lasted only for a few weeks ( e.g. , gilfanov et al . 1998 ) .    during quiescence , sax j1808.43658 was observed with the narrow field instruments ( nfi ) on board _ bepposax _ and near the position of sax j1808.43658 a faint ( a few times @xmath11 erg s@xmath4 ) x - ray source was discovered ( stella et al . 2000 ) , which was identified with the pulsar based on presumed positional coincidence . an _ asca _ observation about half a year later resulted in a detection for sax j1808.43658 in quiescence but with a luminosity at least 4 times lower than measured with _ ( dotani , asai , wijnands 2000 ) . this low luminosity makes sax j1808.43658 the dimmest quiescent neutron - star lmxb x - ray transient detected so far ( see , e.g. , asai et al . 1996 , 1998 ) . sax j1808.43658 stayed dormant until 2000 january 23 , when _ rxte _ found the source to be active again ( van der klis et al 2000 ) . due to solar constraints , the source was unobservable before this date ( from the end of november 1999 ) and it is unclear when exactly the source had become active . the source was only detected at low luminosities ( up to a few times @xmath12 erg s@xmath4 ) with highly variable activity lasting up to 13 may 2000 ( wijnands et al . 2000 , 2001 ) . here we report on _ bepposax_/nfi observations during this outburst . to study the 0.2100 kev x - ray spectrum of sax j1808.43658 , we had _ bepposax_/nfi target - of - opportunity observations planned during its anticipated 2000 outburst . these observations were triggered and we obtained 3 observations in march 2000 ( see tab .  [ tab : log ] ; for a total of 89.1 ksec ) . the data were analyzed using several different approaches , but here we report only on the maximum - likelihood ratio ( mlr ) method which is the most sensitive one ( the results of the other approaches were consistent with the results obtained from the mlr analysis ) . the details of this method are described in kuiper et al . ( 1998 ) and in t zand et al . ( 2000 ; see also , e.g. , kuulkers et al . 2000 ) . as already mentioned above in section  [ section : introduction ] , the source was highly variable during the 2000 outburst ( see wijnands et al . 2001 ) and unfortunately all our _ bepposax_/nfi observations were taken during times when the source had very low luminosities and consequently only the data obtained with the mecs were useful .    to optimize the sensitivity towards detecting sources , we combined the data of the three observations . the resulting mlr image is shown in figure  [ fig : image ] . in the center of this image four sources are detected ; their coordinates are listed in table  [ tab : fov ] ( using the latest spatial calibration ; perri & capalbi 2001 , private communication ; the positions obtained by using the three observations individually varied only by @xmath010@xmath10 ) . the brightest source in the field is the one closest to the position of sax j1808.43658 . sufficient photons were detected from this source to extract its spectrum , which could satisfactorily be modeled with a power - law with photon index of 1.65@xmath80.10 . assuming a column density @xmath13 of 1.22 @xmath14 @xmath15 ( using the @xmath16 from wang et al . 2001 and the relation between @xmath13 and @xmath17 from predehl & schmitt 1995 ) , the unabsorbed fluxes are 1.2@xmath80.2 @xmath18 ( 0.55 kev ) and 1.9@xmath80.3 @xmath18 ( 0.510 kev ) erg s@xmath4 @xmath15 . using a black - body , we obtained a temperature @xmath19 of 1.05@xmath80.03 kev and unabsorbed fluxes of 0.9@xmath80.2 @xmath18 ( 0.55 kev ) and 1.2@xmath80.2 @xmath18 ( 0.510 kev ) erg s@xmath4 @xmath15 . however , the power - law fit is preferred over the black - body fit ( reduced @xmath20 is 1.1 vs. 1.9 , respectively ) . when fitting a two component model ( e.g. , power - law plus a black - body ) , the fit parameters could not usefully be constrained . the position of this brightest source , however , is located 1.63@xmath1 from the position of the optical counterpart of the millisecond x - ray pulsar , which is considerably larger than the 0.55@xmath1 error radius ( 68% confidence level ( 1@xmath21 ) ; conservatively calculated as the direct sum of the statistical ( 18@xmath10 ) and systematical ( 15@xmath10 ) 1@xmath21 errors ) on the source position . the millisecond x - ray pulsar is located at the 3@xmath21 location contour of this brightest _ bepposax _ source , and the probability that this source can still be identified with the millisecond x - ray pulsar is @xmath00.0029 . although this would indicate that it is unlikely that this source can be identified with the millisecond pulsar , it is possible that the systematics in the position reconstruction for _ bepposax _ are not fully understood and we can not rule out completely the possibility that this source is indeed sax j1808.43658 . until it is conclusively demonstrated that this source is the pulsar or not , we will refer to this source as `` the _ bepposax _ source '' . we have reanalyzed the 1999 march 1719 _ bepposax_/nfi quiescent observation of sax j1808.43658 . we confirm the presence of the source reported by stella et al . ( 2000 ) , however , the position of this source ( r.a . = 18@xmath5 08@xmath6 32.9@xmath9 ; dec . = 36  58@xmath1 07.5@xmath10 ; error radius 0.95@xmath1 , 68% confidence level ) and its count rates ( 2.0@xmath80.5 @xmath22 counts s@xmath4 ; 1.310.0 kev ; see also stella et al . 2000 whom obtained very similar coordinates and count rates ) are almost identical to those obtained for the _ bepposax _ source during the combined 2000 march observations , suggesting that it is the same source ( the detected source is @xmath01.2@xmath1 away from the position of the pulsar ) . if this source can not be identified with the pulsar , then during all the _ bepposax_/nfi observations ( both the 1999 quiescent observation and the 2000 outburst ones ) , the millisecond x - ray pulsar was not detected . we have reanalyzed the quiescent _ asca _ data of sax j1808.43658 ( dotani et al . 2000 ; see tab . [ tab : log ] ) to investigate if the four detected _ bepposax _ sources are also detected with _ we used two different methods to analyze the _ asca _ data ; the method applied by dotani et al . ( 2000 ; hereafter referred to as the standard method ) , but also the mlr method . the morphology of the _ asca_/sis images obtained with both methods ( see fig .  [ fig : ascasis_image ] for the mlr image and dotani et al . 2000 for the image obtained with the standard method ) are very similar to each other , and a source was detected in the center of the images ( at @xmath23 significance , using the mlr method ) and its position is consistent with the position of sax j1808.43658 ( see tab . [ tab : fov ] ; using an updated spatial calibration of _ asca _ ; gotthelf et al . 2000 ; we only list the positions obtained with the mlr method ) . we also applied the mlr method to the combined gis2 and gis3 data . although the coarse image binning and relatively high background prevented us from applying the standard analysis ( dotani et al . 2000 ) , the mlr method may be better suited to reconstruct such a noisy , coarse - rebinned image . the resulting mlr image is shown in figure  [ fig : ascasis_image ] . we could detect the millisecond x - ray pulsar at a significance of @xmath24 . the _ asca_/sis count rate ( 0.55.0 kev ) obtained using the mlr method is 2.4@xmath80.3 @xmath22 counts s@xmath4 , which is significantly higher than the one we obtained using the standard method ( 1.1@xmath80.4 @xmath22 counts s@xmath4 ) . however , for faint sources it is important to correctly estimate the local background level at the source position , which is not trivial due to the extended _ asca _ point spread - function . to investigate the effect of a more structured background on the source count rate , we modeled the background near the pulsar in terms of including 3 extra point sources at `` hot '' positions in the mlr image , which are defined as the positions of remaining excesses in a residual mlr image after the detected sources were modeled out . using this background model , the count rate of the millisecond x - ray pulsar decreases to 1.6@xmath80.3 @xmath22 counts s@xmath4 . this count rate is still @xmath050% higher than obtained with the standard method , but the error bars are such that the count rates are consistent with each other . clearly , to obtain the correct _ asca_/sis count rates for sax j1808.43658 , it is very important to understand the local background level .    using the mlr method , we were able to extract a spectrum of the source . however , the spectrum is of low statistical quality and multiple models could be fitted to the data equally well . the spectrum can be fitted using a blackbody model ( resulting in @xmath19 of @xmath00.65 kev ) or with a power - law model ( with a photon index of @xmath25 ) . we have also investigated the effects of the uncertainties in the column density @xmath15 . ] towards sax j1808.43658 by fitting the spectrum with an @xmath13 between 0.6 and 6 @xmath14 @xmath15 . a range of fluxes was obtained and we quote the typical unabsorbed fluxes , which include statistical errors ( as due to the fit procedure ) and systematic errors ( i.e. , the uncertainties in the background and in the column density ; using different spectral shapes like black - body or power - law ) : 7@xmath84 ( 0.55 kev ) and 10@xmath86 @xmath26 erg s@xmath4 @xmath15  ( 0.510 kev ) . within the errors , those fluxes are consistent with what has been reported previously by dotani et al . ( 2000 ) .    at the position of the _ bepposax _ source , no source could be detected , with a 3@xmath21 upper limit on the count rate of @xmath27 counts s@xmath4 ( 0.55.0 kev ; single sis ; depending on the spectral shape and the column density this would result in an unabsorbed 0.510 kev flux range of @xmath28 erg s@xmath4 @xmath15 ) . two of the remaining three _ bepposax _ sources were detected during the _ asca _ observation ( fig . [ fig : ascasis_image ] ) . sax j1809.03659 was both detected in the sis ( although at the edge of the ccd detectors ; 5@xmath21 using the mlr method ) and gis detectors ( 4.5@xmath21 ) . sax j1808.23654 was outside the field of view ( fov ) of the sis but inside that of the gis and was detected by this instrument ( 3.5@xmath21 ) . the positions of those two sources are consistent with those obtained using _ bepposax _ [ tab : fov ] ) . sax j1808.53703 was also outside the fov of the sis . the gis did not detect the source . an additional source was detected with the gis at a position of r.a . = 18@xmath5 08@xmath6 02.8@xmath9 and dec . = 37  02@xmath1 41.7@xmath10 ( @xmath29 ; designated ax j1808.03702 ) , although it might not be a point source but an extended source ( note that this causes additional systematical errors on the source position which can be as large as 2@xmath1 ) . no pronounced counterpart of this source is visible in the _ bepposax_/mecs image ( fig . [ fig : image ] ) , but it might be present at a 3@xmath21 level . we have obtained _ bepposax_/nfi observations of the millisecond x - ray pulsar during its 2000 outburst . due to the extreme variability during this outburst ( see wijnands et al . 2001 ) , the _ bepposax _ observations were unfortunately taken when the source had very low luminosities . we detected four field sources during our observations . one source is 1.63@xmath1 away from the position of the millisecond x - ray pulsar , considerably larger than the 1@xmath21 error of 0.55@xmath1 . this source can only be identified with sax j1808.43658 if we assume that the _ bepposax _ positional reconstruction is not completely understood . if indeed this source can not be identified with the millisecond x - ray pulsar , then it should be designated sax j1808.63658 .    a possible non - detection of sax j1808.43658 during our _ bepposax _ observations would result in an 3@xmath21 upper limit on the count rate of @xmath30 counts s@xmath4 ( 1.310 kev ; both mecs units active ) . when assuming an @xmath13 of 1.22 @xmath31 @xmath15 and a power - law shaped spectrum with index 2 , this would result in an upper limit on the luminosity of @xmath32 erg s@xmath4  ( 0.510 kev ; for a distance of 2.5 kpc ) . in our _ asca _ observations we could not detect a source on the best position of the _ bepposax _ source with a range of upper limits on its x - ray flux of @xmath33 erg s@xmath4 @xmath15  ( 0.510.0 kev ; unabsorbed ) . if the column density is indeed @xmath34 cm@xmath4 then the upper limit determined with ( @xmath36 erg s@xmath4 @xmath15 ) was lower then flux detected during the _ bepposax _ observations ( 1.9@xmath80.3 @xmath18 erg s@xmath4 @xmath15 ) . this discrepancy can mean that indeed the _ bepposax _ source is the millisecond pulsar or also this source is variable . in the latter case , the fact that this source is detected during all _ bepposax_/nfi observations ( including the 1999 march 17 ) and not during the only _ asca _ observation should then be regarded as a coincidence . the _ asca _ fluxes obtained ( using w3pimms ) for the other three _ bepposax _ sources were consistent with the values obtained from the _ bepposax _ fluxes ( taking into account the uncertainties in the spectra , the internal absorption of the sources , and in calculating the fluxes using w3pimms ) . the nature of those field sources is unknown , but they could be , e.g. , agn .    if indeed the _ bepposax _ source is a previously unknown source , its proximity to the millisecond pulsar raises the question if all the outbursts so far observed from the pulsar are indeed due to this source and not from this extra source . it is clear that the source which exhibited the 1998 and 2000 outburst was the millisecond x - ray pulsar because during both outbursts the 401 hz pulsations were detected ( wijnands & van der klis 1998 ; van der klis et al . furthermore , during both outbursts the optical counterpart increased considerable in luminosity ( giles et al . 1999 ; wachter & hoard 2000 ; wachter et al . 2000 ; note , that this secures the sub - arcsecond accurate position of the pulsar ) . however , the _ bepposax_/wfc positional errors of the persistent source detected during the 1996 outburst are consistent with both the position of the millisecond pulsar and the _ bepposax _ source . still , there are two measurements which provide convincing evidence that the 1996 outburst was also due to the millisecond x - ray pulsar . first , the 99% error regions of the three bursts detected during that outburst are all consistent with the pulsar , while that of one burst is clearly inconsistent with the other source . we note that all three bursts were observed in a small time window during the 1996 outburst and are most likely due to same source . second , there is a marginal detection of a 401 hz oscillation during one of the three bursts ( in t zand et al . 2000 ) . independent whether the _ bepposax _ source can be identified with the millisecond x - ray pulsar or that it is an unrelated field source , it is clear that sax j1808.43658 has very low luminosities in quiescence . as discussed in detail by wijnands et al . ( 2001 ) , the very low luminosity of sax j1808.43658 during the _ bepposax _ observations demonstrate the highly variable nature of the source during its 2000 outburst . for example , the source luminosity was below a few times @xmath11 erg s@xmath4  on 2000 march 56 and 8 , it reached @xmath37 erg s@xmath4  on 2000 march 11 as observed with the _ rxte_/pca , but the source was very dim again with _ bepposax_/nfi on 2000 march 2223 . also , on several occasions after 2000 march the source could again be detected with the _ rxte_/pca . therefore , although the x - ray luminosity was very low during our _ bepposax _ observations , the clear long - term activity of the source demonstrated that these observations can not be regarded as true quiescent observations . we reanalyzed the _ observation of sax j1808.43658 reported by dotani et al . we confirm the detection of the millisecond x - ray pulsar . due to the uncertainties in its exact _ asca _ count rate ( see section  [ section : asca ] ) and its exact spectral shape , the exact quiescent luminosity of the millisecond pulsar is difficult to constrain but it is most likely in the range 0.31@xmath38 erg s@xmath4  ( 0.510 kev ) . despite this large uncertainty in its quiescent luminosity , the source is clearly fainter than the other neutron - star lmxb x - ray transients when they are in quiescence ( which are larger than a few times @xmath39 erg s@xmath4 ) . if the quiescent luminosity of sax j1808.43658 is indeed as low as a few times @xmath40 erg s@xmath4 , then it is unclear whether the found distinction ( e.g. , garcia et al . 2001 and references therein ) between the quiescent luminosity of neutron - star transients and black - hole transients ( a few times @xmath41 erg s@xmath4 ) can hold . if certain neutron - star x - ray transients can indeed be as dim in quiescence as the black - hole systems , then the reasons to invoke the presence of event horizons in the black - hole systems will disappear ( garcia et al . 2001 ) . more observations of neutron - star transients in quiescence are required to assess the validity of reported quiescent luminosity difference between the two types of systems . such observations will also give important information to assess the question whether or not the millisecond x - ray pulsar is not only unique with respect to its pulsating nature , but also with respect to its quiescent behavior . brown , bildsten , & rutledge ( 1998 ) proposed that the quiescent x - ray luminosities of neutron - star lmxb transients depend on their time - averaged mass accretion rates . compared with the other systems , the time - averaged accretion rate of the millisecond x - ray pulsar is low and brown et al . ( 1998 ) suggested that this source should have a low quiescent x - ray luminosity . within the uncertainties of their model and our observations , the predicted quiescent luminosity is consistent with the detected one . it is also possible that this low quiescent luminosity is related to the fact that the source is so far the only accretion - driven millisecond x - ray pulsar known . for example , if this uniqueness is due to a higher neutron - star magnetic field than that of the other sources , then during quiescence the so - called propeller mechanism might inhibit residual accretion onto the neutron star . indications for residual accretion in quiescence have been found for the other neutron - star systems , which is mainly inferred from the power - law tail in their quiescent x - ray spectrum and the variability of several sources in quiescence ( however , see ushomirsky & rutledge 2001 for a discussion about x - ray variability due to deep crustal heating ) . however , only at most @xmath050% of the quiescent x - ray flux of those sources is due to this power - law component , suggesting that the low quiescent luminosity of sax j1808.43658 can only in part be due to the lack of residual accretion in this system . it is very well possible that the very low quiescent luminosity is due to a combination of the two above mentioned effects . the low time - averaged accretion rate of sax j1808.43658 will not heat - up its neutron star to the same temperature as in the other systems , causing a lower thermal luminosity from its surface . also , when residual accretion is inhibited due to its magnetic field , the power - law component is absent and the total quiescent x - ray luminosity of sax j1808.43658 could be very low compared to the other neutron - star x - ray transients in quiescence . high quality quiescent x - ray spectra ( e.g. , with _ xmm - newton _ or _ chandra _ ) of sax j1808.43658 will help considerably to understand its low quiescent luminosity . _ note added in manuscript : _ after submission of our paper , campana et al . ( 2001 ) reported preliminary results on their _ xmm - newton _ observation of sax j1808.43658 in quiescence . they detected the source at a luminosity of approximately @xmath42 erg s@xmath4  ( 0.510 kev ) , consistent with our quiescent _ asca _ luminosity . they also reported that at the position of the _ bepposax _ source no source could be detected . however , the _ xmm - newton _ observation was not taken at the same time as our _ bepposax _ observations during the 2000 outburst and variability of this _ bepposax _ source can not be ruled out . analysis of the ddt _ xmm - newton _ observation taken on 8 march 2000 , will definitely settled this issue because it was taken very close in time to our _ bepposax _ observations . this work was supported by nasa through chandra postdoctoral fellowship grant number pf9 - 10010 awarded by cxc , which is operated by sao for nasa under contract nas8 - 39073 . mk acknowledges support by the netherlands organization for scientific research ( nwo ) . we thank jon miller and erik kuulkers for helpful discussions and carefully reading a previous version of this paper . we thank the referee , josh grindlay , for useful comments on our paper . campana , s. , stella , l. , gastaldell , f. , israel , g. , mereghetti , s. , burderi , l. , di salvo , t. 2001 , poster presented at `` new visions of the x - ray universe in the xmm - newton and chandra era '' , 2630 november 2001 , noordwijk , the netherlands                                                    cccccc + sax j1808.43658 & optical & 18@xmath5 08@xmath6 27.54@xmath9 & 36  58@xmath1 44.3@xmath10 & 0.2@xmath10 & + & _ asca_/sis & 18@xmath5 08@xmath6 26.9@xmath9 & 36  58@xmath1 35.9@xmath10 & 0.24@xmath1 & 2.4@xmath80.3 + & & & & & 1.6@xmath80.3 + & _ asca_/gis & 18@xmath5 08@xmath6 29.8@xmath9 & 36  58@xmath1 27.6@xmath10 & 0.57@xmath1 & + _ bepposax _ source & _ bepposax _ & 18@xmath5 08@xmath6 35.1@xmath9 & 36  58@xmath1 07.8@xmath10 & 0.55@xmath1 & 1.9@xmath80.3 + sax j1809.03659 & _ bepposax _ & 18@xmath5 09@xmath6 02.9@xmath9 & 36  59@xmath1 42.3@xmath10 & 0.56@xmath1 & 1.9@xmath80.3 + & _ asca_/sis & 18@xmath5 09@xmath6 02.3@xmath9 & 36  59@xmath1 39.3@xmath10 & 0.23@xmath1 & 2.8@xmath80.5 + & _ asca_/gis & 18@xmath5 09@xmath6 02.1@xmath9 & 36  59@xmath1 19.5@xmath10 & 0.53@xmath1 & + sax j1808.23654 & _ bepposax _ & 18@xmath5 08@xmath6 12.9@xmath9 & 36  54@xmath1 42.7@xmath10 & 0.59@xmath1 & 1.6@xmath80.2 + & _ asca_/gis & 18@xmath5 08@xmath6 10.3@xmath9 & 36  54@xmath1 11.3@xmath10 & 0.63@xmath1 & 1.2@xmath80.5 + sax j1808.53703 & _ bepposax _ & 18@xmath5 08@xmath6 32.0@xmath9 & 37  03@xmath1 03.1@xmath10 & 0.73@xmath1 & 0.8@xmath80.2 + & _ asca_/gis & & & & @xmath431.6 +
we have observed the millisecond x - ray pulsar sax j1808.43658 on three occasions during its 2000 outburst with the _ bepposax _ satellite . 
 the source was highly variable and erratic during this outburst , and by coincidence we obtained data only during times when the source had very low luminosities . during our observations 
 , we detected four faint sources . 
 the source closest to the position of sax j1808.43658 is still @xmath01.6@xmath1 away . 
 this source can only be identified with sax j1808.43658 if we assume that the _ bepposax _ positional reconstruction is not completely understood . 
 we also reanalyzed a _ 
 bepposax _ observation taken in march 1999 when the source was in quiescence and during which the source was thought to have been detected ( stella et al . 
 2000 ) . based on the similarities ( position and luminosity ) of this source with the above mentioned source @xmath2 away from sax j1808.43658 , 
 it is possible that they are the same source . 
 if this source is not the millisecond pulsar , then during all _ bepposax _ observations of sax j1808.43658 ( the 2000 outburst ones and the 1999 quiescent one ) , the millisecond pulsar was not detected . 
 a reanalysis of the _ asca _ quiescent data of sax j1808.43658 ( dotani , asai , & wijnands 2000 ) confirms that during this observation the source was securely detected in quiescence . 
 we discuss our results for sax j1808.43658 in the context of the quiescent properties of low - mass x - ray binary transients .