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low mass x - ray binaries are for the majority of the time in a state of quiescensce with faint or non - detected x - ray emission , though optical or near - infrared ( nir ) counterparts may be visible due to emission from the donor star , or possibly the jet , hot spot , or outer accretion disk . they are often only discovered when they enter an active state of outburst when  powered by an increased level of accretion onto the central , compact object ( black hole or neutron star )  there is a dramatic increase of the x - ray , optical / nir and radio flux . during these outbursts the systems have been observed to go through a number of high energy spectral states before returning to a quiescent state , usually on times scales of weeks , months or even longer . these states are a generally low intensity , power - law dominated , _ hard _ state followed by a usually , higher intensity , _ thermal - dominant _ , _ soft _ state which decreases in flux , via a late hard state , over time . additionally , the hard states are associated with aperiodic variability of the light curve not present in the soft state ( see @xcite for a fuller description of the various possible states ) . xtej1752 - 223 , a new x - ray transient and black hole candidate @xcite in the galactic center region , was detected on 2009 - 10 - 23 at 19:55 ut ( mjd 55128.33 ) by rxte and on 2009 - 10 - 24 at 14:18:50 ut by the bat instrument on board the _ swift _ satellite @xcite . the high energy , variable emission of the source was confirmed in the following days by _ swift_/xrt @xcite and rxte @xcite as well as by maxi / gsc @xcite and _ fermi_/gbm @xcite . an optical and nir counterpart was proposed by @xcite while a radio source coincident with the x - ray position @xcite was observed by the australia telescope compact array ( atca ; @xcite ) . the source was observed to have undergone a state transition in mid - january 2010 ( mjd @xmath0 ; @xcite ) , from a spectrally hard to a spectrally soft x - ray state . this was identified by a de - coupling of high energy ( @xmath1kev ) and low energy ( @xmath2kev ) light curves @xcite which had previously traced each other , and by the dominance of a thermal component . the source was observed to have reverted to a hard state at the end of march 2010 ( mjd @xmath3 ; @xcite ) , after the low energy light curve decreased to trace once more the high energy light curve ; the thermal component was no longer dominant . it has also been suggested that the turn - on of a compact jet has been observed in this system @xcite during this transition back to the low / hard state .    in this article we present the _ swift _  burst alert telescope , x - ray telescope and ultraviolet / optical telescope  monitoring observations of xtej1752 - 223 , obtained over the period of the outburst . based on these data , we identify the periods of the various states and compare the behavior of the major photometric , spectral and timing parameters during these states to those expected from black hole x - ray binaries . in section [ section : observations ] we introduce the observations and reduction methods , while in section [ section : analysis ] we present the results of our photometric , spectral and timing analyses of the data . we summarise our findings in section [ section : conclusion ] . _ swift _ @xcite observed xtej1752 - 223 with its narrow field instruments , the ultraviolet / optical telescope ( uvot ; @xcite ) and the x - ray telescope ( xrt ; @xcite ) , on 55 epochs from october 25 to november 03 2009 and from february 04 2010 to june 5 2010 . these observations ( under target ids 31532 , 31640 and 31688 ) coincide with the period of the outburst . observations with the narrow field instruments were interrupted for 3 months due to the source becoming constrained by the position of the sun . wide field instruments also became sun constrained but at a later date and for a shorter period of time , depending on the satellite and instrument in question ( e.g. maxi was constrained from december 15 to 24 ) . during the period of the outburst , as it had been on previous occasions , the field was also being observed ( though constrained from december 13 to 28 ) by _ swift _ s burst alert telescope ( bat ; @xcite ) which , in combination with xrt and uvot , affords quasi - simultaneous observations from 0.002 to 150 kev . reduction of _ swift _ data was carried out using version 3.4 of the _ swift _ software , released as part of heasoft 6.7 . all uncertainties are quoted at the @xmath4 confidence level . uvot data have been pre - processed at the _ swift _ data center ( see @xcite ) and require only minimum user processing . as the pointing of _ swift _ is only accurate to a few arcseconds , the standard pre - process pipeline provides an astrometric correction by matching sources with the usno - b catalog . however , in this case the automatic matching failed because the field differs significantly between the ultraviolet and the usno - b bandpass . to overcome this , we created our own astrometric reference catalog by selecting blue and bright field stars in the gsc-2.3 catalog @xcite to determine astrometric corrections for the data . the image data of each filter , from each observation sequence , i.e. , with a given observation i d , were summed using uvotimsum . all sequences were then combined in an individual fits file , using fappend , which was again summed using uvotimsum to obtain a deep image of the field in each filter . from these images , two possible optical counterparts of xtej1752 - 223 are identified , consistent with the x - ray position @xcite ; source a , consistent with the counterpart proposed by @xcite , is within @xmath4 and source b is within @xmath5 of the x - ray position . the positions ( j2000 ; 0.31error ) of the possible counterparts were derived from such a deep ( 1853s ) @xmath6 band image summed between mjd 55130 and 55138 ( figure [ fig : uvot_v ] ) , using the uvotdetect command , as : +    [ cols= " < , < , < , < " , ]      the xrt windowed timing mode ( wt ; @xcite ) event data [ 0.3 - 10kev ] were analysed using the tools presented by @xcite . these correct for pile up and the affect of bad ccd columns . version v011 response matrices were used for the spectral analysis . the best xrt position of 17:52:15.14 @xmath722:20:33.8 ( j2000 ; 3.1error ) is as previously reported @xcite . background subtracted xrt count rate light curves are binned per observing sequence ( figure[fig : xray_lc ] ; combined with maxi observations ) . unbinned source light curves ( not background subtracted ) , for each sequence , were used to produce power density spectra ( pds ) and to calculate the rms variability with the ftools , powspec and lcstats respectively . within each of these the binning was set to be n times the minimum bin size ( 1.766ms ) where n=1 for the pds and n=8 ( 14.128ms ) for the rms variability , giving frequency ranges of 0.02 - 570hz and 0.02 - 70hz respectively . the average pds were calculated , within powspec , from an input list of the light curves of the required sequences , using the same binning . lags between the standard xrt low [ 0.3 - 1.5kev ] and high [ 1.5 - 10kev ] energy band unbinned light curves were calculated with crosscor and a bin size set to n=8 ( 14.128ms ) . bat detector plane histograms ( dphs ) were obtained in survey mode ; these are 2 dimensional images with full spectral data and coarse time binning . spectra and light curves were extracted from these dphs using mask weighting ( also known as ray - tracing ) to account for the position of the source on the coded aperture mask . the data were initially processed with the ftool , batsurvey which applies data quality cuts and uses the tools batfftimage and batcelldetect to extract 8-channel spectra for the source . we then used custom software to format the spectra for xspec , and batdrmgen to generate the response files . the light curve was derived using a similar method . bat , as a survey telescope , had been observing the field of xtej1752 - 223 since the start of _ swift _ operations in january 2006 , though not monitoring the source . the extracted bat light curve ( 15 - 150kev ) of the source from january 12 2006 through to the present , displays no activity until the 2009 outburst . the joint xrt - bat spectra were fit within xspec ( 12.5.1 ) , using c - statistics and interstellar absorption modeled by tbabs ; initially we fit an absorbed power law ( pow ) to the spectra before adding a thermal component ( diskbb ) . we note that these are phenomenological models for the respective spectral states , not a suggestion that these are necessarily the best physical models ; they should not be used to deduce physical constraints but only as an indication of the spectral state of the source .    the early data ( mjd 55131 - 55138 ) are well described by the power law of average photon index , @xmath8 , ( figure[fig : spectral ] ) and do not require a thermal component - clearly indicating that the source is in the hard state . the absorption of a simultaneous fit of these sequences is @xmath9@xmath10 , greater than the galactic value of @xmath11@xmath10 but lower than the @xmath12@xmath10 found by @xcite . data observed from mjd 55234 - 55280 additionally support a soft state thermal component decreasing from a temperature of @xmath13kev to @xmath14kev over the period , as well as decreasing normalisation . due to the degeneracy between the power law and the thermal components and lack of counts from the bat , the power law index is poorly constrained in a manner not obvious from the nominal fit errors but causing a high level of variability in the upper panel of figure[fig : spectral ] ; these values are not reliable .    from mjd 55283 the data no longer support a thermal component , though the signal to noise at these low counts rates can not rule out a thermal component . additionally , as can be seen from figure[fig : spectral ] , the power law indices are decreasing until mjd 55328 , signaling the transitional period between the soft and hard states , in agreement with the increasing hardness ratio ( figure[fig : xray_lc ] ) . from mjd 55328 to the end of our observations , on mjd 55352 , the spectra are described by a power law of relatively constant photon index , the average of which is @xmath15 . the power law indices and the hardness ratios at these late times fail to return to the original , rising hard state values ( @xmath8 ) , indicating that , though the count rate is lower than it was in that rising hard state , the source has not yet returned to its original hard state or that the late hard state has marginally different spectral properties . an optical and nir counterpart was proposed by @xcite based on a comparison between their images and those from the digitized sky survey , 2mass and ukidss . we have confirmed uvot source a as this optical counterpart on the basis of a significant , though low amplitude variability in the @xmath6 , @xmath16 and @xmath17 bands ( figure[fig : uvot_lc ] ) . furthermore , these optical magnitudes , or count rates , exhibit a power - law correlation with the x - ray count rate , as observed by xrt at times after mjd 55232 ( figure[fig : uvot - xrt ] ) , i.e. , when the source is in the soft state and later , during the transition to the late hard state . the earlier , hard state data does not fit on this correlation , but displays hysteretical behaviour and for the given x - ray count rate it spans has a significantly higher magnitude in all bands . this is similar to the hysteretical behaviour observed in the nir for a number of transients @xcite , most notably xtej1550 - 564 , where the additional hard state emission is attributed to optically thin synchrotron emission from a jet @xcite , which would be quenched in the soft state ( e.g. , @xcite ) , and weak at low x - ray luminosities , leading to the hysteresis . though this hysteresis is not normally observed in the optical bands , the optical data presented here is in good agreement with the synchrotron emitting jet making a significant contribution to the optical emission in the rising hard state . while the observed @xmath6 band brightening from mjd 55292 corresponds to the suggested turn - on , in nir , of a compact jet between mjd 55293 and 55298 @xcite , it is not corroborated by the other bands and is not statistically significant , though it may indicate the turn - on of a jet and the transition to the hard state ; at such low x - ray luminosity levels , the jet contribution to the optical would be minimal .    from a simultaneous fit of the @xmath6 , @xmath16 and @xmath17 bands the correlation is found to be @xmath18 with observed colours of @xmath19 and @xmath20 ( @xmath21 ) , though the individual bands display indices of @xmath22 , @xmath23 and @xmath24 respectively , hinting at possible spectral evolution . such a correlation is more usually associated with the hard state @xcite but we can not say if the early , hard state data follows another correlation ( expected to be @xmath25 ; @xcite ) due to the very limited range of magnitudes and x - ray count rates over this period . though we find a correlation between the optical and x - ray , we can not imply causality or say which leads without a study of the phase lag between the two bands , which is not possible given the time resolution of the optical data . similarly a timing analysis to investigate variability of the optical data is not possible . the correlation may be due to reprocessing of the x - rays or through both originating from the accretion disk ; it is also possible that optical emission in the soft state originates from a jet , the state of which traces the spectral states @xcite    the quality of the uvot spectral data does not support any spectral evolution in the optical , nor does it enable us to specify the most appropriate spectral model for the source such as a power law associated with the above suggested jet emission . hence , we model the data phenomenologically in xspec , by a power law ( @xmath26 ) and a dust extinction component ( zdust ) , though we note that this is not a particularly good fit . given that an unknown amount of galactic extinction , @xmath27 @xcite , effects the data we can only place limits on the spectral index of @xmath28 , though the nominal best fit is at @xmath29 for @xmath30 . given the uncertainties of the spectral slope and the lack of phase lag information , we are unable to make a meaningful comparison to the predictions of the various models .      the hardness - intensity diagrams ( hid ) for both the xrt data [ 1.5 - 10kev / 0.3 - 1.5kev ] and the quasi - simultaneous ( same day ) bat / maxi data starting at the same time are shown in figure[fig : hid ] . due to the fact that the bat / maxi data are only quasi - simultaneous , the hardness ratio ( hr ) of bat [ 15 - 150kev ] with respect to maxi [ 4 - 10kev ] at low bat count rates ( @xmath31 ) , where the relative variability over a day is greater , are underestimated by the error bars . in general , hids follow a canonical trajectory ( e.g. , @xcite ) : increasing intensity at a high hr to a maximum in the hard state , indicating the power law shape of the spectra ; decrease of hr at a high intensity through an intermediate state as the thermal component of the spectra becomes more dominant ; decrease of intensity at the low hr level during the soft state ; before the hr increases at low intensity , through an intermediate state , to the original value . this is consistent with both the xrt data , though a significant amount of the trajectory in the first intermediate state is missing due to the source being sun constrained for a period of 3 months , and the bat / maxi data in which the final phase is obscured due to the large error bars on the hr . it is clear from the xrt hid that the hardness has yet to increase to the original value , indicating that the source has not yet returned to its original hard state , though the count rate is lower than it was in that rising hard state , or that the late hard state has different properties . these tracks are common among black hole binary sources  though are also observed in neutron star binary sources such as aquilax-1 ( e.g. , @xcite )  and further confirm the spectral state of the system over the periods in question . the rms variation of the light curves exhibit a similar evolution to hardness ratio ( figure[fig : xray_lc ] ) , from average rms @xmath32 per cent in the observed period of the rising hard state ( mjd 55131 - 55138 ) to @xmath33 percent during the soft state observations and increasing to the original levels at later times . examining the light curves in the standard xrt low [ 0.3 - 1.5kev ] and high [ 1.5 - 10kev ] energy bands we find that , during the rising hard state this variability is more pronounced in the low band with an average ratio between the two of @xmath34 . this suggests an energy dependency of rms variation , in contrast to @xcite who find a flat rms spectrum @xcite from xte data at energies in the range 2 - 10kev . the lag between the two bands , calculated via a cross correlation , is consistent with zero in all cases though is not tightly constrained .    examining the average power density spectra of the states ( figure[fig : powspec ] ) we see that the hard state has a high level of power in low frequency ( @xmath35hz ) variability , though there is no significant sign of quasi - periodic oscillations ( qpos ) . the soft state , on the other hand , has a much lower lever of power and only at frequencies @xmath36hz , as is manifested by the @xmath33 percent limit of rmss during this period ( mjd 55234 - 55289 ) . after mjd 55321 , where the xrt count rate drops below 10 cts / s , the light curves and pdss are heavily affected by background counts ( figure[fig : powspec_back ] ) and hence the rms values for these observations are unreliable and not plotted in figure[fig : xray_lc ] . the excess of low energy variability in the rising hard state is similar to the behaviour of the hard state variability in swiftj1753.5 - 0127 and gx339 - 4 as reported by @xcite . within the framework of their model , enhanced low energy variability ( at frequencies @xmath37hz ) is due to a contribution from the thermal emission of the accretion disk . the excess variability is attributed to intrinsic disk variability  due to accretion rate fluctuations  as opposed to reprocessing of the spectral power - law component of the emission . though a thermal component is not supported by the spectra of xtej1752 - 223 in the rising hard state , the excess variability we observe suggests that one might contribute to the flux at low energies . more detailed spectral fits using the methods introduced in @xcite are required to test this possibility . data , over a greater energy range , which better constrains column density and photon index may also be necessary . xtej1752 - 223 was observed by _ swift _ in a relatively low intensity , hard state from 2009 - 10 - 26 to 2009 - 11 - 03 ( mjd 55131 - 55138 ) which was signified by hard power law energy spectra , and high levels ( @xmath38 per cent ) of rms variability in the x - ray light curve . xtej1752 - 223 then became constrained by the position of the sun and was unobservable until 2010 - 02 - 04 ( mjd 55234 ) by which time it had experienced a ten fold increase in the flux observed by xrt [ 0.3 - 10.0kev ] , which proceeded to decrease to the original levels over a period of 2 months . the spectra during this period clearly exhibited a strong thermal component and the variability and hardness ratios of the light curves had dropped significantly , consistent with the source being in a soft state until 2010 - 03 - 25 ( mjd 55280 ) . observations from 2010 - 03 - 28 ( mjd 55283 ) to 2010 - 05 - 12 ( mjd 55328 ) do not support a thermal component but do display a decreasing photon index along with increasing hardness and rms variability . this suggests a transitional state before reverting to the hard state , with a relatively constant photon index , in which the system is observed until the end of observations on 2010 - 06 - 05 ( mjd 55352 ) . the high energy hardness - intensity diagrams over two separate bands follow the canonical behavior associated with a black hole binary , further confirming the spectral states of the system during the outburst . our timing analysis shows that in the hard state there is significant variability below 10hz which is more pronounced at low energies , while during the soft state the level of variability is consistent with being minimal . we are able to confirm that the uvot source at the position of the proposed optical and near - infrared counterpart is associated with xtej1752 - 223 due to the observed low amplitude variability and , in the soft state and later , correlation with the x - ray emission as measured by xrt . however , we can not state that the optical flux is causally connected to the x - ray emission via reprocessing of the x - rays or through both originating from the accretion disk ; given the uncertainties of the spectral slope and the lack of phase lag information , we are unable to make a meaningful comparison to the predictions of the various models . the optical counterpart also displays hysteretical behaviour not normally observed in the optical bands ; for a given x - ray count rate , the magnitude in the rising hard state is significantly higher than that in the soft state . this is similar to the hysteretical behaviour observed in the nir for a number of transients where the additional hard state emission is attributed to optically thin synchrotron emission from a jet , which would be quenched in the soft state and weak at low x - ray luminosities . though this hysteresis is not normally observed in the optical bands , it is in good agreement with the synchrotron emitting jet making a significant contribution to the optical emission in the rising hard state . we thank the referee for constructive comments and paul kuin for useful discussions on uvot . pac , pae and cb acknowledge support from stfc . tjm thanks the eu fp7 for support through grant number itn 215212 ` black hole universe ' . pc acknowledges funding via a eu marie curie intra - european fellowship under contract no . 2009 - 237722 . this research has made use of : swift data supplied by the uk swift science data centre at the university of leicester ; and maxi data provided by riken , jaxa and maxi teams . , j.  e. , et  al . 2004 , in society of photo - optical instrumentation engineers ( spie ) conference series , vol . 5165 , society of photo - optical instrumentation engineers ( spie ) conference series , ed . k.  a. flanagan & o.  h.  w. siegmund , 217
we present _ swift _ broadband observations of the recently discovered black hole candidate , x - ray transient , xtej1752 - 223 , obtained over the period of outburst from october 2009 to june 2010 . from _ 
 swift_-uvot data we confirm the presence of an optical counterpart which displays variability correlated , in the soft state , to the x - ray emission observed by _ swift_-xrt . 
 the optical counterpart also displays hysteretical behaviour between the states not normally observed in the optical bands , suggesting a possible contribution from a synchrotron emitting jet to the optical emission in the rising hard state . 
 we offer a purely phenomenological treatment of the spectra as an indication of the canonical spectral state of the source during different periods of the outburst . 
 we find that the high energy hardness - intensity diagrams over two separate bands follows the canonical behavior , confirming the spectral states . 
 our xrt timing analysis shows that in the hard state there is significant variability below 10hz which is more pronounced at low energies , while during the soft state the level of variability is consistent with being minimal . 
 these properties of xtej1752 - 223 support its candidacy as a black hole in the galactic centre region . 
 [ firstpage ]    x - rays : binaries  x - rays : bursts  binaries : close  stars : individual : xtej1752 - 223