document: soft x  ray transients ( sxrts ) , when in outburst , show properties similar to those of persistent low mass x  ray binaries containing a neutron star ( lmxrbs ; white et al . 
   1984 ; tanaka & shibazaki 1996 ; campana et al . 1998 ) . the large variations in the accretion rate that are characteristic of sxrts allow the investigation of a variety of regimes for the neutron stars in these systems which are inaccessible to persistent lmxrbs . 
   while it is clear that , when in outbursts , sxrts are powered by accretion , the origin of the low luminosity x 
   ray emission that has been detected in the quiescent state of several sxrts is still unclear . 
   an interesting possibility is that a millisecond radio pulsar ( msp ) turns on in the quiescent state of sxrts ( stella et al . 
   this would provide a  missing link " between persistent lmxrbs and recycled msps . 
   aql x-1 is the most active sxrt known : more than 30 x  ray and/or optical outbursts have been detected so far . 
   the companion star has been identified with the k1v variable star v1333 aql and an orbital period of 19 hr has been measured ( chevalier and ilovaisky 1991 ) . 
   the outbursts of aql x-1 are generally characterised by a fast rise ( 510 d ) followed by a slow decay , with an @xmath4folding time of 3070  d ( see tanaka & shibazaki 1996 and campana et al . 
   1998 and references therein ) . 
   type i x  ray bursts were discovered during the declining phase of an outburst ( koyama et al . 
   1981 ) , testifying to the presence of a neutron star . 
   ray luminosities are in the @xmath5 range ( for the @xmath6 kpc distance inferred from its optical counterpart ; thorstensen et al . 1978 ) . 
   close to the outburst maximum the x  ray spectrum is soft with an equivalent bremsstrahlung temperature of @xmath7  kev . 
   sporadic observations of aql x-1 during the early part of the outburst decay ( czerny et al . 
   1987 ; tanaka & shibazaki 1996 ; verbunt et al . 
   1994 ) showed that when the source luminosity drops below @xmath8 the spectrum changes to a power law with a photon index of @xmath2 , extending up to energies of @xmath9 kev ( harmon et al . 1996 ) . 
   rosat pspc observations revealed aql x-1 in quiescence on three occasions at a level of @xmath1 ( 0.42.4 kev ; verbunt et al . 
   1994 ) . in this lower energy band 
   the spectrum is considerably softer and consistent with a black body temperature of @xmath10  kev . 
   an outburst from aql x-1 reaching a peak luminosity of @xmath11 ( 210 kev ) was discovered and monitored starting from mid - february , 1997 with the rossixte all sky monitor ( asm ; levine et al . 
   six observations were carried out with the bepposax narrow field instruments ( nfis ) starting from march 8@xmath12 , 1997 ( see table 1 ) , with the aim of studying the final stages of the outburst decay . 
   1a shows the light curve of the aql x-1 outburst as observed by the rossixte asm and bepposax mecs . 
   the first part of the outburst can be fit by a gaussian with sigma @xmath13  d. this is not uncommon in sxrts ( e.g. in the case of 4u 160852 ; lochner & roussel - dupr 1994 ) . 
   the flux decay rate changed dramatically around mjd 50512 ( march 5@xmath12 , 1997 ) . at the time of the first bepposax observation ( which started on march 8@xmath12 , 1997 ) the source luminosity was decreasing very rapidly , fading by about 30% in 11 hr , from a maximum level of @xmath14 . 
   the second observation took place on march 12@xmath12 , 1997 when the source , a factor of @xmath15 fainter on average , reduced its flux by about 25% in 12 hr . 
   in the subsequent four observations the source luminosity attained a constant level of @xmath16 , consistent with previous measurements of the quiescent luminosity of aql x-1 ( verbunt et al . 1994 ) . the sharp decrease after mjd 50512 
   is well described by an exponential decay with an @xmath4folding time @xmath17 d.    the bepposax lecs , mecs and pds spectra during the fast decay phase , as well as those obtained by summing up all the observations pertaining to quiescence , can be fit with a model consisting of a black body plus a power law ( see table 2 and fig . 
   the soft black body component remained nearly constant in temperature ( @xmath18 kev ) , but its radius decreased by a factor of @xmath19 from the decay phase to quiescence . 
   the equivalent radius in quiescence ( @xmath20  km ) is consistent with the rosat results ( verbunt et al . 
   the power law component changed substantially from the decay phase to quiescence : during the decay the photon index was @xmath21 , while in quiescence it hardened to @xmath22 . 
   the two values are different with @xmath23 confidence ( see table 1 ) . the ratio of the 0.510 kev luminosities in the power law and black body components decreased by a factor of five between the first bepposax observation and quiescence . 
   [ cols="^,^,^,^,^,^ " , ]      @xmath25 spectra from the lecs and mecs ( and pds for the first observation ) detectors have been considered . 
   the spectra corresponding to the quiescent state have been summed up , in order to increase the statistics and an upper limit from the summed pds data was also used to better constrain the power law slope .    @xmath26 unabsorbed x  ray luminosity in the energy range 0.510 kev . in the case of the first observation 
   the pds data were included in the fit ( the unabsorbed 0.5100 kev luminosity amounts to @xmath27 ) . 
   the bepposax observations enabled us to follow for the first time the evolution of a sxrt outburst down to quiescence . 
   the sharp flux decay leading to the quiescent state of aql x-1 is reminiscent of the final evolution of dwarf novae outbursts ( e.g. ponman et al . 
   1995 ; osaki 1996 ) , although there are obvious differences with respect to the x  ray luminosities and spectra involved in the two cases , likely resulting from the different efficiencies in the gravitational energy release between white dwarfs and neutron stars . 
   models of low mass x  ray transient outbursts hosting an old neutron star or a black hole are largely built in analogy with dwarf novae outbursts . in particular , van paradijs ( 1996 ) showed that the different range of time - averaged mass accretion rates over which the dwarf nova and low mass x 
   ray transient outbursts were observed to take place is well explained by the higher level of disk irradiation caused by the higher accretion efficiency of neutron stars and black holes . 
   however , the outburst evolution of low mass x  ray transients presents important differences . 
   in particular , the steepening in the x  ray flux decrease of aql x-1 has no clear parallel in low mass x 
   ray transients containing black hole candidates ( bhcs ) . 
   the best sampled light curves of these sources show an exponential - like decay ( sometimes with a superposed secondary outburst ) with an @xmath4folding time of @xmath29 d and extending up to four decades in flux , with no indication of a sudden steepening ( chen et al . 
   in addition , bhc transients display a larger luminosity range between outburst peak and quiescence than neutron star sxrts ( garcia et al . 1998 and references therein ) . 
   being the mass donor stars and the binary parameters quite similar in the two cases , it appears natural to attribute these differences to the different nature of the underlying object : neutron stars possess a surface and , likely , a magnetosphere , while bhcs do not . 
   when in outburst accretion down to the neutron star surface takes place in sxrts , as testified by the similarity of their properties with those of persistent lmxrbs , especially the occurrence of type i bursts and the x  ray spectra and luminosities . the mass inflow rate during the outburst decay decreases , causing the expansion of the magnetospheric radius , @xmath30 . 
   thus , accretion onto the neutron star surface can continue as long as the centrifugal drag exerted by the corotating magnetosphere on the accreting material is weaker than gravity ( illarionov & sunyaev 1975 ; stella et al . 
   this occurs above a limiting luminosity @xmath31 , where @xmath32 is the gravitational constant ; @xmath33 , @xmath34 , @xmath35 g and @xmath36 ms are the neutron star mass , radius , magnetic field and spin period , respectively ( here and in the following we assume @xmath37 and @xmath38 cm ) . as @xmath30 reaches the corotation radius , @xmath39 , accretion onto the surface is inhibited and a lower accretion luminosity ( @xmath40 ) of @xmath41 is released . after this luminosity gap 
   the source enters the propeller regime . 
   if the mass inflow rate decreases further , the expansion of @xmath30 can continue up to the light cylinder radius , @xmath42 , providing a lower limit to the accretion luminosity that can be emitted in the propeller regime @xmath43 . 
   below @xmath44 the radio pulsar mechanism may turn on and the pulsar relativistic wind interacts with the incoming matter pushing it outwards . 
   matter inflowing through the roche lobe is stopped by the radio pulsar radiation pressure , giving rise to a shock front ( illarionov & sunyaev 1975 ; shaham & tavani 1991 ) . 
   clearly these regimes have no equivalent in the case of black hole accretion .      during the february - march 1997 outburst of aql x-1 , rossixte observations led to the discovery of a nearly coherent modulation at @xmath45  hz ( @xmath46 ms ) during a type i x  ray burst . 
   a single qpo peak , with a centroid frequency ranging from @xmath47 to 830 hz , was also observed at two different flux levels , when the persistent luminosity was @xmath48 and @xmath49 ( zhang et al . 
   1998 ) . in the presence of a single qpo peak , the magnetospheric and sonic point beat frequency models ( alpar & shaham 1985 ; miller et al . 1997 ) interpretation is ambiguous in that the qpo peak could represent either the keplerian frequency at the inner disk boundary or the beat frequency . 
   moreover , the burst periodicity at @xmath45  hz may represent the neutron star spin frequency , @xmath50 , or half its value ( zhang et al . 1998 ) . in either cases , 
   the possibility that accretion onto the neutron star surface takes place even in the quiescent state of aql x-1 faces serious difficulties : for a quiescent luminosity of order @xmath51 a magnetic field of only @xmath52 g would be required , in order to fulfill the condition @xmath53 . 
   for such a low magnetic field , aql  x-1 and , by inference , lmxrbs with khz qpos can hardly be the progenitors of recycled msps . 
   more crucially , the marked steepening in the outburst decay that takes place below @xmath54 , is accompanied by a marked spectral hardening , resulting from a sudden decrease of the flux in the black body spectral component . 
   this is clearly suggestive of a transition taking place deep in the gravitational well of the neutron star , where most of the x 
   rays are produced . 
   the most appealing mechanism is a transition to the propeller regime , where most of the inflowing matter is stopped at the magnetospheric boundary ( zhang , yu & zhang 1998 ) . in fig . 
   1a , the luminosity at mjd 50512 is identified with @xmath55 and the lower horizontal lines indicate the luminosity interval during which aql x-1 is likely in the propeller regime .    additional information on the neutron star magnetic field ( and spin ) can be inferred as follows . 
   the observed ratio of the luminosity , @xmath56 , when the qpo at @xmath57  hz were detected and the luminosity @xmath58 when the centrifugal barrier closes is @xmath59 . at @xmath58 the keplerian frequency of matter at the magnetospheric boundary is , by definition , equal to the spin frequency , i.e. @xmath60 or @xmath61  hz for aql x-1 . 
   based on beat - frequency models , at @xmath56 the keplerian frequency at the inner disk boundary can be either @xmath62  hz or @xmath63  hz , depending on whether the single khz qpos observed corresponds to the keplerian or the beat frequency . in the magnetospheric beat - frequency models , simple theory predicts that the keplerian frequency at the magnetospheric boundary is @xmath64 ; in the radiation pressure - dominated regime relevant to the case at hand , the ghosh and lamb ( 1992 ) model predicts instead @xmath65 . 
   therefore we expect @xmath66 and @xmath67 , in the two models , respectively . 
   such a low ratio clearly favors the interpretation in which @xmath62  hz and @xmath68  hz . in the sonic point beat - frequency model ( miller et al . 
   1997 ) , the innermost disk radius is well within the magnetosphere , implying that the keplerian frequency at the magnetospheric boundary is @xmath69 . in this case 
   all possible combinations of @xmath50 and @xmath70 are allowed . by using the observed @xmath58 , a neutron star magnetic field of @xmath71  g ( depending on the adopted model of the disk - magnetosphere interaction ) 
   is obtained in the case @xmath68  hz and @xmath72  g in the case @xmath73  hz . 
   once in the propeller regime , the accretion efficiency decreases further as the magnetosphere expands for decreasing mass inflow rates ( @xmath74 ) . 
   the @xmath75  d exponential - like luminosity decline observed with bepposax is considerably faster than the propeller accretion luminosity extrapolated from the first part of the outburst ( e.g. the gaussian profile shown by the dashed line in fig . 
   we note here that the spectral transition accompanying the onset of the centrifugal barrier may also modify the irradiation properties of the accretion disk , contributing to x  ray luminosity turn off . 
   alternatively , an active contribution of the `` propeller '' mechanism or the neutron star spin - down energy dissipated into the inflowing matter can not be excluded . 
   it is unlikely that the quiescent luminosity of aql  x-1 is powered by magnetospheric accretion in the propeller regime . as shown in fig . 
   1a , the quiescent x  ray luminosity is probably lower than the minimum magnetospheric accretion luminosity @xmath76 allowed in the propeller phase ( this remains true for @xmath77 g if @xmath68 hz , and for @xmath78 g if @xmath73 hz ) . 
   moreover the bepposax x  ray spectrum shows a pronounced decrease in the power law to black body flux ratio together with a flattening of the power law component between the fast decay phase and quiescence , suggesting that a transition to shock emission from the interaction of a radio pulsar wind with the matter outflowing from the companion star has taken place . note that an x  ray spectrum with a slope of @xmath79 has been observed from the radio pulsar psr  b125963 immersed in the wind of its be star companion . 
   models of this interaction predict that a power law x  ray spectrum with a slope around @xmath79 should be produced for a wide range of parameters ( tavani & arons 1997 ) .    the additional soft x 
   ray component observed during the outburst decay ( see table 2 ) might be emitted at the polar caps as a result of the residual neutron star accretion in the propeller phase . 
   note that the equivalent black body radius decreases for decreasing x  ray luminosities , just as it would be expected if the magnetospheric boundary expanded . 
   alternatively , the black body - like spectral component observed in quiescence could be due to the streaming of energetic particles that hit the polar caps , in close analogy to the soft x  ray component observed , in msps , at the weaker level of @xmath80 ( becker & trmper 1997 ) .    assuming a magnetic field in the range derived in section 3.1 ( i.e. @xmath81 g for @xmath82 hz and @xmath83 g for @xmath84 hz ) 
   , we can consistently explain the @xmath1 quiescent x  ray luminosity as powered by a radio pulsar enshrouded by matter outflowing from the companion star , if the conversion efficiency of spin - down luminosity to x  ray is @xmath85% . 
   this is consistent with modeling and observations of enshrouded pulsars ( tavani 1991 ; verbunt et al . 
   1996 ) . 
   there are chances of observing a msp ( a simple scaling from msps implies a signal at 400 mhz of @xmath86 mjy ; see kulkarni et al . 
   1990 ) , even though the emission would probably be sporadic , like in the case of the pulsar psr  174424a due to the large amount of circumstellar matter ( see lyne et al . 1991 ; shaham & tavani 1991 ) .    in summary aql  x-1 appears to provide the first example of an old fast rotating neutron stars undergoing a transition to the propeller regime at first , followed by a transition to the radio pulsar regime , as the transient x  ray emission approaches its quiescent level . 
   therefore , aql x-1 ( and possibly sxrts in general ) likely represents the long - sought `` missing link '' between lmxrbs and recycled msps .
summary: we report on the march - april 1997 bepposax observations of aql x-1 , the first to monitor the evolution of the spectral and time variability properties of a neutron star soft x 
   ray transient from the outburst decay to quiescence . 
   we observed a fast x  ray flux decay , which brought the source luminosity from @xmath0 to @xmath1 in less than 10 days . 
   the x  ray spectrum showed a power law high energy tail with photon index @xmath2 which hardened to @xmath3 as the source reached quiescence . 
   these observations , together with the detection by rossixte of a periodicity of a few milliseconds during an x  ray burst , likely indicate that the rapid flux decay is caused by the onset of the propeller effect arising from the very fast rotation of the neutron star magnetosphere . the x  ray luminosity and hard spectrum that characterise the quiescent emission can be consistently interpreted as shock emission by a turned - on rotation - powered pulsar .