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the discovery of millisecond spin periods of neutron stars in low mass x - ray binaries ( lmxbs ) with the _ rossi x - ray timing explorer _ ( _ rxte _ ) has helped elucidate the nature of these sources . neutron star lmxbs consist of a neutron star accreting from a low mass companion . as material ( mostly h and he ) is accreted onto the star and gets compressed , it eventually ignites and burns unstably ( see * ? ? ? this phenomenon is observed as a type i x - ray burst . type i x - ray bursts have been observed from over @xmath070 lmxbs ( see * ? ? ? * and references therein ) . the recurrence time of these bursts varies but in some cases it can be as frequent as every few hours ( see * ? ? ? * for examples ) . occasionally it is possible to observe the spin of the neutron star modulating the burst emission  the so called `` burst oscillations '' @xcite . burst oscillations have been observed from @xmath018 lmxbs ( see * ? ? ? * for examples ) . x - ray bursts from lmxbs have been discovered which are @xmath01000 times longer , and thus , that much more energetic , than canonical type i x - ray bursts . they are aptly named `` superbursts '' . superbursts are believed to occur by the unstable burning of carbon @xcite . @xcite discovered highly coherent pulsations during a superburst from 4u  1636@xmath253 . the pulse train lasted for @xmath0800  s , as opposed to the @xmath010  s long pulse trains observed in type i x - ray bursts . superbursts are much rarer than type i x - ray bursts . thus far ten have been observed from eight lmxbs ( see * ? ? ? between 2005 march 7 and 2005 july 21 the pca galactic bulge scan survey discovered an outburst from the direction of the globular cluster ngc  6440 . a follow up 1.8-ks long pointed _ rxte _  observation ( observation identification number 91050 - 03 - 07 - 00 ) was initiated on 2005 june 14 . the data presented here were acquired from the proportional counter array ( pca ) on board _ rxte_. the pca consists of five identical and independent proportional counter units ( pcus ) . each pcu is a 90% xenon/10% methane gas filled proportional counter . each pcu has a collimated 1@xmath3@xmath41@xmath3 field - of - view , 256 spectral channels in the 260  kev range , and a limiting temporal resolution of @xmath01  @xmath5s . only two pcus ( pcu  0 and pcu  2 ) were operational throughout the span of the observation . the data were taken in ` e_125us_64m_0_1s ` mode , which returns events to a limiting resolution of 125  @xmath5s and with moderate ( 64 channels as opposed to the full 256 channels ) spectral resolution . this mode was used because it is not as susceptible to buffer overflows during high countrate data as compared to the less restrictive modes ( e.g. ` goodxenon ` ) . using the pointing of the observation ( 17@xmath648@xmath7528 , -202132 ) and the planetary ephemeris de200 , the times of raw events were corrected to the solar system barycenter . binning the events into a 1-s resolution lightcurve reveals a quickly decaying burst ( fig . [ fig : lc ] ) . this burst was reminiscent of canonical type  i x - ray bursts from lmxbs , therefore we searched this event for burst oscillations . we rebinned the raw events using the full spectral band pass into a time series with 0.5/1024  s resolution , which yields an equivalent nyquist frequency of 1024  hz . a leahy normalized ( see * ? ? ? * ) power density spectrum ( pds ) is displayed in figure  [ fig : lc ] ( inset ) . a prominent peak is clearly seen at 442  hz . the probability of this peak occurring by chance after accounting for the number of trials ( the total number of frequency bins in our pds ) is @xmath0@xmath8 . ngc  6440 harbors the bright x - ray transient sax  j1748.9@xmath22021 , which exhibited a 409.7  hz burst oscillation @xcite . however , there are many other x - ray sources in the cluster ( see fig  [ fig : chandra image ] ) . _ chandra x - ray observatory _ ( _ cxo _ ) imaging by @xcite revealed 24 x - ray sources in ngc  6440 , and they concluded that 45 of these sources are likely quiescent lmxbs . thus , the phenomenon we discovered is most likely from a different object in the cluster .    ]      to study the frequency evolution of the pulse we calculated a dynamic @xmath9 statistic .the @xmath9 statistic is analogous to the fourier power spectrum , with the advantage that the data need not be binned , thus allowing us to oversample our data . the @xmath9 statistic is defined as : @xmath10 where @xmath11 is the total number of photons in each interval , @xmath12 is the total number of harmonics that one deems significant , @xmath13 is the frequency searched over , and @xmath14 is the event time . the factor in front of the summations normalizes the @xmath9 statistics in an analogous way to leahy normalizing a pds . we calculated the dynamic @xmath9 power spectrum using a 200-s long window , which was translated across the observation with a step size of 16  s. our dynamic @xmath9 power spectrum is displayed in fig . [ fig : frequency evolution ] . notice that the pulsations where highly significant for 576  s. as is common for canonical x - ray bursts , the frequency drifts ; however , unlike the large @xmath01 - 2  hz drifts seen in those , the pulsation here drifts down in frequency only by @xmath02.1@xmath15  hz in 576  s.        to further quantify this frequency evolution we determined how the phase of the pulsations varies as a function of time . for each interval with @xmath16 we generated a pulse profile by folding the events in that interval on the frequency determined from the @xmath9 statistic . we then cross correlated these pulse profiles with a sinusoid of fixed phase to determine the pulse phase as a function of time . we can model the pulse phase ( @xmath17 ) at a given time ( @xmath18 ) by the following taylor expansion : @xmath19 where @xmath20 is the barycentric frequency , @xmath21 is the frequency derivative , and @xmath22 is some reference epoch . we were able to `` whiten '' our phase residuals with just a frequency derivative ( see fig . [ fig : frequency evolution ] , bottom ) . we find @xmath23  hz , @xmath24  hz  s@xmath25 at @xmath22=53535.463215  mjd  ( utc ) . the phase residuals after subtracting the model given by eq . [ eq : phase ] are shown in fig . [ fig : frequency evolution ]  ( bottom panel ) . notice that the pulse train lasts for 576  s ; this is quite long when compared to those of type i x - ray bursts . we folded all the events within the 576  s pulse train using our best fit @xmath20 and @xmath21 , and the resulting pulse profile is shown in fig . [ fig : profile ] . notice that the pulse profile is highly sinusoidal . fitting a sinusoid to the pulse profile returned a reduced @xmath26 value of 0.76 for 13 degrees of freedom , affirming the absence of any harmonic content . from the fit we obtain a fractional pulse amplitude of 2.1@xmath270.1% . it is possible that the frequency drift we have observed is entirely due to orbital motion . the observed spin frequency of a neutron star in a circular orbit is given by @xmath28 , \label{eq : orbit}\ ] ] where , @xmath29 is the barycentric frequency at time @xmath30 , @xmath31 is the projected velocity of the neutron star , @xmath32 is the orbital period , and @xmath33 is the orbital phase at time @xmath30 . unfortunately we do not have a long enough data set to place interesting constraints on @xmath32 or @xmath31 ; however , we can determine whether orbital modulation is consistent with the observed frequency drift . for example , if we assume , for solely demonstrative purposes , that this system has a projected velocity comparable to the one found by @xcite for 4u  1636@xmath253 ( @xmath34  km  @xmath25 ) , then fitting eq . [ eq : orbit ] to our frequency time series yields a good fit with a reasonable orbital period , i.e. @xmath357@xmath271  hr . hence , binary motion can account for the observed frequency drift . observing another outburst would be essential in ultimately constraining the orbital parameters of this system . the timing analysis seems to suggest that this event is in fact very different from a canonical burst oscillation . to study it further we performed a spectral analysis . we analyzed the spectral evolution of the burst by breaking the burst up into ten intervals . the exposure of each interval was selected by demanding that each one contained an equal number of photons . using the last interval as an estimator of the persistent emission , we fit each interval to a simple photoelectrically absorbed blackbody model while holding the column density fixed to the value found by @xcite for the optical reddening . we find only subtle evidence for spectral softening , significant only at the 1-@xmath36 level .    using the distance estimate to the cluster found by @xcite , @xmath37  kpc , we were able to calculate the luminosity of the burst . at the start of the observation the flux was @xmath02@xmath4@xmath38  erg  s@xmath25 , which is @xmath00.1@xmath39 , where @xmath39 is the eddington luminsosity for a neutron star . the burst lightcurve was well fit by an exponential with a decay timescale of 22  s. unfortunately the observation only caugth the tail end of the burst - like event , we could not therefore characterize the peak luminosity of the burst . the timing properties of the pulsations are reminiscent of those observed during superbursts . in particular the highly coherent pulsations and the long pulse trains . the superburst interpretation would be very exciting given the rarity of such events . however , we could not unambiguously determine this is the case given that we only caught the tail end of the burst . to study this phenomenon further we extrapolated our luminosity model determined in our spectral analysis before the start time of the observation . we find that the burst reaches the eddington luminosity @xmath050  s before the start of the observation . now , if we assume that we caught the tail of the outburst it is very surprising that it dropped from @xmath0@xmath39 down to @xmath0.01@xmath39 in @xmath077  s. this is much too fast for a superburst - like event . if , on the other hand , this is a precursor events , such as those seen in superbursts , than it is unclear why the persistent flux did not rise back to @xmath0@xmath39 shortly after as observed in other superbursts . thus , the spectral analysis of this event suggests it is not a superburst . the timing properties of the pulsations share all the hallmarks of those observed during superbursts , but the spectral analysis makes this interpretation unlikely . if this event is not a superburst , then could it be that these highly coherent pulsations have eminated from an accretion - powered millisecond pulsar ? recently @xcite discovered `` intermittent '' pulsations from the accretion - powered millisecond pulsar ( amp ) hete  j1900.1@xmath22455 . they found that the properties of this pulsar differed from those of the other six known amps . for example , the pulsations were only present in the first few months of its outburst as opposed to other amps which show pulsations throughout ( see * ? ? ? * for a review ) . in addition , the pulse amplitude increased at three points in time that were almost coincident with the times of thermonuclear bursts . the pulsed amplitude subsequently decaded after the bursts . thus , it is plausible that we have observed a similar phenomenon . following the theoretical work of @xcite and @xcite , @xcite suggest that the reason why the pulsations in hete  j1900.1@xmath22455 sometimes `` switch off '' is because the magnetic field is burried by accreted material , thus it can not channel the flow which gives rise to the pulsations in the first place    if the event we have discovered is another example of these intermittent pulsations from an unidentified source in ngc  6440 , then a future outburst should reveal more long pulse trains , and if they happen to be correlated with the times of type - i x - ray bursts then this would solidify the connection between this source and hete  j1900.1@xmath22455 . we discovered a 442  hz pulsation during a burst from an x - ray source in ngc  6440 . we could not establish the energetics of the burst as _ rxte _ only caught the tail end of the event , but based on the timing properties of the pulsation we conclude that it was not a canonical burst oscillation . in particular we find that : the pulse train lasted for 576  s , as opposed to the @xmath010  s long pulse trains observed in type i x - ray bursts . the signal was highly coherent and drifted down only by @xmath40  hz , as opposed to type - i x - ray burst which exhibit drifts which are three orders of magnitude larger . the pulsations share all the properties of those observed during superbursts ; however , the complete energetics of this event make this interpretation implausible . we conclude that the long pulse train is most likely an `` intermittent '' pulsation from an accreting millisecond pulsar such as those seen only in hete  j1900.1 - 2455 thus far @xcite . we note that ngc  6440 is known to harbor a transient x - ray source with a 409.7  hz spin frequency @xcite however , x - ray imaging of the cluster by @xcite has shown that ngc  6440 contains many x - ray sources . thus , the burst presented here most likely emanated from a different object in the cluster . fpg is supported by the nasa postdoctoral program administered by oak ridge associated universities at nasa goddard space flight center . this research has made use of data obtained through the high energy astrophysics science archive research center online service , provided by the nasa / goddard space flight center .
we report on the serendipitous discovery of a 442-hz pulsar during a _ rossi x - ray timing explorer _ ( _ rxte _ ) 
 observation of the globular cluster ngc  6440 . 
 the oscillation is detected following a burst - like event which was decaying at the beginning of the observation . 
 the time scale of the decay suggests we may have seen the tail - end of a long - duration burst . 
 low - mass x - ray binaries ( lmxbs ) are known to emit thermonuclear x - ray bursts that are sometimes modulated by the spin frequency of the star , the so called burst oscillations . 
 the pulsations reported here are peculiar if interpreted as canonical burst oscillations . in particular 
 , the pulse train lasted for @xmath0500 s , much longer than in standard burst oscillations . 
 the signal was highly coherent and drifted down by @xmath02@xmath1hz , much smaller than the @xmath0hz drifts typically observed during normal bursts . 
 the pulsations are reminiscent of those observed during the much more energetic `` superbursts '' , however , the temporal profile and the energetics of the burst suggest that it was not the tail end nor the precursor feature of a superburst . 
 it is possible that we caught the tail end of an outburst from a new ` intermittent' accreting x - ray millisecond pulsar , a phenomenon which until now has only been seen in hete  j1900.1@xmath22455 @xcite . we note that @xcite reported the discovery of a 409.7  hz burst oscillation from sax  j1748.9@xmath22021 , also located in ngc  6440 . 
 however , _ chandra x - ray observatory _ imaging indicates it contains several point - like x - ray sources , thus the 442  hz object is likely a different source .