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ultracompact x - ray binaries ( ucxbs ) are systems with orbital periods ( @xmath2 ) shorter than @xmath31  hr in which a neutron star or black hole accrete matter from a companion low mass star .
their short periods rule out ordinary hydrogen - rich companion stars , since these stars are too big and do not fit in the roche lobe ( nelson et al .
ucxbs are rare objects and their identification is very difficult because of the difficulty to measure @xmath2 in lmxbs .
the most recent compilation of ultracompact x - ray binaries lists 27 candidates ( in t zand et al . , 2007 ) .
eight out of 52 lmxbs with measured orbital period are in the ultracompact regime ( in t zand et al .
2007 , nelemans & jonker 2006 ) .
the remainder were classified as ultracompact x - ray binaries on the base of some combination of tentative orbital period measurements , deep optical spectra lacking hydrogen emission lines , high ratios of x - ray to optical flux , or persistent emission at low fractions of the eddington rate ( in t zand et al .
2007 and references within ) .
the ibis results of the long monitoring of the ucxbs showed that these sources spend most of the time in the canonical low / hard state , with x - ray luminosities @xmath4 , plasma temperature @xmath5 kev and @xmath6 ( fiocchi et al .
+ 4u  0513 - 40 is an x - ray binary in the galactic globular cluster ngc  1851 with a 17-minute orbital modulation first observed with the _ hubble space telescope _
( zurek et al . 2009 ) .
it is a persistent source showing evidence for variability of a factor of @xmath7 in x - ray luminosity on timescales of @xmath0 weeks , and a factor of more than 20 overall ( maccarone et al .
such variability is unusual for ultracompact x - ray binaries and hence deserve some attention .
table [ jou ] gives a summary of source observations performed with instruments on board _ bepposax _ , _ chandra _ , _ xmm - newton _ and _ integral _ satellites .
the lecs , mecs and pds/_bepposax _ event files and spectra were generated with the supervised standard science analysis ( fiore , guainazzi & grandi 1999 ) . both lecs and mecs spectra
were accumulated in circular regions of @xmath8 radius .
publicly available matrices were used for these instruments .
the pds spectra were extracted using the xas version 2.1 package ( chiappetti & dal fiume 1997 ) .
the background sampling was performed by making use of the default rocking law of the two pds collimators that sample on/+off , on/-off fields for each collimator with a dwell time of 96 s ( frontera et al . 1997 ) .
when one collimator is pointing on - source , the other collimator is pointing toward one of the two off positions .
we used the standard procedure to obtain pds spectra ( dal fiume et al .
chandra _ data were processed with the ciao ( _ chandra _ interactive analysis of observations ) software , version 4.1.2 , i.e. the same version of caldb ( calibration data base ) , provided by the _
chandra _ x - ray center and following the science threads listed on the ciao website .
the ciao routine wavdetect was used to search for x - ray sources on the acis chips . the ciao routine dmextract was used to produce energy spectra and mkacisrmf and mkarf for the response and ancillary files respectively .
we extracted source photons from a circular region centered on the source with an extraction region of 8 arcsec .
for the background , we used circular source - free regions in the same ccd of the studied source .
+ _ xmm - newton _ data have been processed starting from the observation files with sas 7.0.0 .
x - ray events corresponding to patterns 0 - 4 were selected from epic - pn camera .
we used the most updated calibration files available at the time of the reduction for each source data .
source light curves and spectra were extracted from circular regions of 10@xmath9 centered on the source , while background products were obtained from off - set regions close to the source .
the ancillary and detector response matrices were generated using the xmm-_newton _ sas arfgen and rmfgen tasks .
+ the analyzed _ integral _ ( winkler et al .
2003 ) data consist of all public observations in which 4u  051340 was within the field of view of the high - energy detectors .
broad - band spectra , @xmath0580 kev , are obtained using data from the high - energy instruments , jem - x ( lund et al .
2003 ) and ibis ( ubertini et al .  2003 ) .
the ibis and jem - x data have been processed using the off - line scientific analysis ( osa v.  9.0 ) software released by the _
integral _ science data center ( isdc , courvoisier et al .
light curves and spectra are extracted for each individual science windows .
these runs were performed with aves cluster , designed to optimize performances and disk storage for the integral data analysis ( federici et al .
the excellent angular resolution provided by _
chandra _ allows us to resolve the 4u  051340 x - ray emission from the binary pulsar prs j0514 - 4002 .
figure 1 shows the 0.5 - 8.0 kev ( left panel ) and 4 - 8 kev acis image ( right panel ) .
the ciao routine wavdetect was used to search for x - ray sources .
this routine found two sources in the 0.5 - 8.0 kev : the first located at ra=05 14 06.48 and dec=-40 02 38.8 with a positional uncertainty of 0.64 arcsec ( 1-@xmath10 statistical errors ) and the second at ra=05 14 06.79 and dec=-40 02 48.5 with positional uncertainty of 1.3 arcsec ( 1-@xmath10 statistical errors ) .
only ucxb 4u  051340 was detected in the 4 - 8 kev energy range ( see figure 1 ) .
image analysis of the _ chandra _ observation shows that only the ultra compact binary system is emitting at high energy and this suggest only 4u  051340 as the only possible counterpart of the very high energy ( _ bepposax _ and _ integral _ ) object .
in fact the pulsar source is too weak to be the counterparts of the ibis and pds source .
this is in line with the peculiar nature of this binary pulsar , being a radio steep and very faint pulsar ( freire et al .
+    .summary of the x - ray binary 4u0513 - 40 observations [ cols="<,^,^ " , ]      +
in the present paper , we have shown that the binary system 4u  0513 - 40 in ngc  1851 exhibits a clear periodic signal with @xmath11  min in soft x - ray .
this signal is sinusoidal and has an amplitude from @xmath04% to @xmath010% .
it is observed only in two observations ( _ chandra _ and _ bepposax _ after the burst ) when the source is in a typical high / soft state : the energy spectrum is well described as the sum of a comptonized plasma with a temperature of @xmath12 2 - 5 kev and an optical depth of @xmath13  6 and a blackbody component with a thermal temperature of @xmath00.3 - 0.5  kev .
this result is independent from the extrapolated luminosity of the system , which spans from 0.7 to 5.2 @xmath14 erg  s@xmath15 in the 0.5 - 50 kev energy band , for _ chandra _ and _ bepposax _ ( after the burst ) , respectively .
this periodicity is not seen when the source is in a low / hard state ( _ xmm - newton _ and _ integral _ data ) and data is well reproduced by a simple comptonized model with a plasma temperature of @xmath12 15 - 21  kev and an optical depth of @xmath13  2 , without any thermal component . according to our present understanding , the black - body component in the soft state could originate at both the neutron star surface or boundary layer and the surface of an optically - thick accretion disk .
the comptonization component in the hard state may arise from a corona above the disk and/or between the disk and the stellar surface and accretion probably assumes the form of a truncated outer accretion disk ( olive et al .
the origin of these spectral changes is not clear , the thermal temperature ( @xmath00.3 - 0.5 kev ) confirm that the accretion disk is ionized and should not be subject to the standard ionization instability ( done et al .
2007 ) .
the far - ultraviolet photometry obtained with the _ hubble space telescope _ has shown the same periodicity described here ( zurek et al .
these timing properties seen in uv / optical observations and the eclipse observed in the x - ray band imply that the origin of this modulation is of orbital nature and the inclination angle is higher than 80 @xmath16 ( arons and king 1993 ) .
in fact , the orbital motion modulates the soft thermal emission coming from a small region around the neutron star but not the comptonization component generated in a more extended corona above and/or around the neutron star .
the authors acknowledge the asi financial support via asi - inaf contract i/033/10/0 .
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the ultracompact low - mass x - ray binary 4u  051340 in the globular cluster ngc1851 exhibits large amplitude x - ray flux variations with spectral changes from low / hard to high / soft states which have not been reported previously in other ultracompact x - ray binaries . using _ bepposax _ , _ chandra _ and _ xmm - newton _ archival data together with recent _ integral _ observations , we reveal a clear sinusoidal periodic signal with a period of @xmath0 17 minutes when the source is in a typical high / soft state with a dominant soft thermal component . 
 the periodicity disappears when the source is in a low / hard state and the thermal soft component is not required any more to model the data . 
 these properties indicate the orbital nature of the detected signal and imply an high inclination angle of the binary system ( @xmath1 ) .    
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