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supergiant fast x  ray transients ( sfxts ) are a new class of high mass x ray binaries ( hmxbs ) discovered by ( e.g.   * ? ? ? * ) that are associated with ob supergiant stars via optical spectroscopy . in the x rays they display outbursts significantly shorter than those of typical be / x  ray binaries characterized by bright flares with peak luminosities of 10@xmath510@xmath6  erg  s@xmath7 which last a few hours ( as observed by ; * ? ? ? * ; * ? ? ? as their quiescence is characterized by a luminosity of @xmath8  erg  s@xmath7 ( e.g.   * ? ? ? * ; * ? ? ? * ) , their dynamic range is of 35 orders of magnitude . while in outburst , their hard x  ray spectra resemble those of hmxbs hosting accreting neutron stars , with hard power laws below 10kev combined with high energy cut - offs at @xmath930  kev , sometimes strongly absorbed at soft energies @xcite . so , even if pulse periods have only been measured for a few sfxts , it is tempting to assume that all sfxts might host a neutron star . the mechanism producing the outbursts is still being debated , and it is probably related to either the properties of the wind from the supergiant companion @xcite or to the presence of a centrifugal or magnetic barrier @xcite .    was discovered during _ asca _ observations of the scutum arm region performed on 1994 april 12 , and 1999 october 34 as a flaring source which exhibited flux increases by a factor of 10 ( up to @xmath10 erg @xmath3 s@xmath7 ) with rising times on the order of 1hr @xcite , a strong absorption @xmath11 @xmath3 , and coherent pulsations with a period of @xmath12s . a _ chandra _ observation on 2004 may 12 , which provided the coordinates refined to arcsecond accuracy [ ra(j2000@xmath13 , dec(j2000@xmath14 @xmath15 , @xcite ] , found the source at a much fainter level ( @xmath16 erg @xmath3 s@xmath7 ) , and with a spectrum that was fit with an absorbed power - law model [ @xmath17 , @xmath18 @xmath3 ] . a newly discovered source , igr  j18410@xmath00535 , was observed to flare by  on 2004 october 8 @xcite , as it reached @xmath20mcrab in the 2060kev energy range ( integrated over 1700s ) and 20 mcrab in the 60200kev range . the source was also detected in the 2060kev energy range in subsequent observations , at a flux roughly half that of the initial peak . @xcite identified igr  j18410@xmath00535 as . @xcite established that the ir counterpart was 2mass 18410043@xmath00535465 , a reddened star with a weak double - peaked h@xmath21 emission line , initially classified as a be star , which @xcite later reclassified as b1 ib type star ; this corroborated the evidence that  is a member of the sfxt class , as proposed by @xcite . @xcite presented the first broad - band spectrum of this source , obtained with  ( ibis@xmath22jem - x ) , that they fit with an absorbed power - law with @xmath23 , @xmath24 @xmath3 .    in 2007 @xmath25  @xcite observed the outburst of the periodic sfxt igr  j11215@xmath05952 @xcite , which allowed us to discover that the accretion phase during the bright outbursts lasts much longer than a few hours , as seen by lower - sensitivity instruments . this is contrary to what was initially thought at the time of the discovery of this new class of sources . between 2007 october 26 and 2008 november 15 ,  was observed by @xmath25  as part of a sample of 4 sfxts which included igr  j16479@xmath04514 , xte  j1739302 , and . the main aims were to characterize their long - term behavior , to determine the properties of their quiescent state , to monitor the onset of the outbursts and to measure the outburst recurrence period and duration @xcite . approximately two observations per week were collected with the x  ray telescope ( xrt , * ? ? ? * ) and the uv / optical telescope ( uvot , * ? ? ? * ) . during such an intense and sensitive monitoring ,  was the only sfxt that did not go through a bright outburst , although several on - board burst alert telescope ( bat , * ? ? ? * ) detections have been recorded @xcite .    in this paper we report on the observations of the first outburst of  observed by @xmath25   on 2010 june 5 and we compare its properties with those of the prototype of the sfxt class , , which went into a bright outburst on 2010 march 04 .     and count s@xmath7 detector@xmath7 , respectively . the empty circles correspond to bat in event mode ( s / n@xmath26 ) , filled circles to bat survey mode data . , width=321 ] ax  j1841.0@xmath00536 triggered the @xmath25/bat on 2010 june 5 at 17:23:30 ut ( trigger 423958 , * ? ? ? * ; * ? ? ? this is the first outburst of ax  j1841.0@xmath00536 detected by the bat for which @xmath25  performed a slew , thus allowing broad - band data collection . the source was detected in a 1344s bat image trigger , during a pre - planned observation , and there is an indication that the source was already in outburst before this observation began and well after it ended . the xrt began observing the field rather late , at 17:51:50 ut ( @xmath27s ) , after the very long bat image trigger . the automated target ( at , sequences 00423958000 - 001 ) observations lasted for several orbits , until @xmath28ks after the trigger ) . follow - up target of opportunity ( too ) observations for a total of 10.8ks were obtained ( sequences 00030988093101 ) . the data cover the first 11d after the beginning of the outburst . the sfxt prototype  triggered the bat on 2010 march 04 at 23:13:54 ut ( trigger 414875 , * ? ? ? @xmath25  executed an immediate slew , so that the narrow - field instruments ( nfi ) started observing it about 395s after the trigger . the at ran for @xmath29ks and was followed by one too observation ( 00035056149 ) for @xmath30ks until the source went into moon constraint .         for the 2010 march 4 outburst of . , width=321 ]    the xrt data were processed with standard procedures ( xrtpipeline v0.12.4 ) , filtering and screening criteria by using ftools in the heasoft package ( v.6.9 ) , as fully described in e.g.   @xcite . we used the latest spectral redistribution matrices ( 20100930 ) . the bat data were analysed using the standard bat analysis software within ftools . mask - tagged bat light curves were created in several energy bands ( see * ? ? ? * for further details ) . survey data products , in the form of detector plane histograms ( dph ) , are available , and were also analysed with the standard batsurvey software . all quoted uncertainties are given at 90% confidence level for one interesting parameter unless otherwise stated . [ cols="<,<,<,<,<,<,>,<,<,<,<,<,<,<,<",options="header " , ]     @xmath31 is absorbing column density ( @xmath32 @xmath3 ) ; @xmath33 is 210kev observed flux ( @xmath34 erg @xmath3 s@xmath7 ) ; @xmath35 is 210kev luminosity ( @xmath36 erg s@xmath7 ) . pow : absorbed powerlaw . cpl : cutoff powerlaw , energy cutoff e@xmath37 ( kev ) . hct : absorbed powerlaw , high energy cutoff e@xmath37 ( kev ) , e - folding energy e@xmath38 ( kev ) . figure  [ axj1841fig : lcv_campaign ] ( left ) shows the _ swift_/xrt 0.210kev light curve of  throughout our 2008 monitoring program @xcite background - subtracted and corrected for pile - up , psf losses , and vignetting . all data in one observation ( 12ks , typically ) were generally grouped as one point , except for the june 5 outburst , which shows up as a vertical line on the adopted scale [ fig . [ axj1841fig : lcv_campaign ] ( right ) ] . the observed dynamical range of this source in the xrt band is @xmath39 , considering as the lowest point a 3@xmath40 upper limit obtained on mjd  54420 at @xmath41 counts s@xmath7 , and the highest point the peak of the june 5 outburst . [ axj1841fig : lcv_allbands ] shows the detailed light curves during the brightest part ( first orbit ) of the outburst in several energy bands . the bat light curve is rather flat and weak , but significant signal is found at the lower energies ( 1550kev ) .    for the timing analysis we converted the event arrival times to the solar system barycentre with the task barycorr and the _ chandra _ position @xcite . we note that the xrt pc - mode readout frequency slightly undersamples the source period of @xmath424.7 s @xcite with respect to its nyquist frequency , which would guarantee an unambiguous reconstruction of the signal . timing searches were conducted in various time intervals and energy ranges around @xmath424.7s , employing two methods : a fast - folding algorithm and an unbinned @xmath43 test @xcite . in both cases the searches were inconclusive and we could not set meaningful upper limits on the pulsed fraction , because of the red noise and the scalloping due to the poor sampling .      for the 2010 june 5 outburst of , we extracted the mean spectrum of the brightest x - ray emission ( obs . 00423958000 , @xmath441708 to 2390s ) and performed a fit in the 0.310kev band of the data , which were rebinned with a minimum of 20 counts bin@xmath7 to allow @xmath45 fitting . an absorbed power - law model yielded a column of @xmath46 @xmath3 , a photon index @xmath47 ( @xmath48 , 17 degrees of freedom , dof ) , and a 210kev unabsorbed flux of @xmath49 erg @xmath3 s@xmath7 . as there is no strict overlap between the bat event data and the xrt data ( see fig . [ axj1841fig : lcv_allbands ] ) , we extracted one spectrum from the bat event file of the whole observation 00423958000 ( ` event ' ) , and one from the xrt event file in the same observation . there are , however , bat survey data available in the interval @xmath50@xmath51s , and we extracted one ( ` survey ' ) spectrum in this restricted interval . we performed joint fits in the 0.310kev and 14100kev energy bands for xrt and bat , respectively . a factor was included to allow for normalization uncertainties between the two spectra . the broad - band fits performed with the bat ` survey ' spectrum yield consistent results with the ones performed with the bat ` event ' spectrum , albeit with more unconstrained parameters due to the much more limited statistics . therefore in table  [ axj1841:tab : broadspec ] we report the fits performed with the bat ` event ' spectrum . a simple absorbed power - law model is clearly an inadequate representation of the broad band spectrum ( @xmath52 for 29 dof ) , so we also considered other models typically used to describe the x  ray emission from accreting pulsars in hmxbs , such as an absorbed power - law model with an exponential cutoff ( cutoffpl in xspec ) and an absorbed power - law model with a high energy cut - off ( highecut ) . the latter models provide a significantly more satisfactory fit of the broad - band emission , resulting in a hard powerlaw - like spectrum below 10kev , with a roll over of the higher energies when simultaneous xrt and bat data fits are performed . figure  [ axj1841:fig : meanspec ] shows the fits for the highecut model . table  [ axj1841:tab : broadspec ] reports the average 210kev luminosities . two estimates of the distance are available , from ( * ? ? ? * @xmath53kpc ) and ( * ? ? ? * @xmath54kpc ) , so we assumed 5  kpc , which is consistent with both . figure  [ axj1841fig:17544lcv_allbands ] shows the first orbit of the xrt and bat light curves of the 2010 march 04 outburst of . the xrt count rate reaches a peak exceeding 25 counts s@xmath7 , then decreases to about 0.5 counts s@xmath7 and increases again up to about 20 counts s@xmath7 at the end of the first orbit of observations .    the xrt / wt spectrum ( @xmath55@xmath56s ) , extracted with a grade 0 selection to mitigate residual calibration uncertainties at low energies , was fit with an absorbed power law resulting in @xmath57 , @xmath58 @xmath3 ( @xmath59 for 125 dof ) , and @xmath60 erg @xmath3 s@xmath7 ( unabsorbed ) . the xrt / pc spectrum ( @xmath61 to @xmath62s ) yields @xmath63 @xmath3 , @xmath64 ( @xmath65 , 17 dof ) and @xmath66 erg @xmath3 s@xmath7 ( unabsorbed ) . we extracted a bat spectrum strictly simultaneous with the xrt one and fitted them ( 0.310kev , 1450kev ) with the same models as adopted for . the results are in table  [ axj1841:tab : broadspec ] , while figure  [ axj1841:fig:17544meanspec ] shows the fits for the highecut model . for the luminosity calculation we adopted a distance of 3.6  kpc @xcite .     for the 2010 march 4 outburst of . , width=283 ] in this paper we report our analysis of the 2010 june 5 outburst of  and the 2010 march 04 outburst of the sfxt prototype . while in the first case , the image trigger was a very long one and nfi data could be collected only @xmath67s after the trigger , when the source was relatively dim , in the second case , the slew occurred immediately after the trigger , while  was still very bright .    figure  [ axj1841fig : best_sfxts ] ( panels e and g ) shows the full light curves of the outbursts of  and  as they were observed by @xmath25   for 11 and 2 days after the trigger , respectively . the  xrt light curve shows a decreasing trend from the initial bright flare from a maximum of @xmath68 counts s@xmath7 down to @xmath69 counts s@xmath7 during the first day , with several flares superimposed , hence yielding a dynamic range of approximately 900 during this outburst . then , after three days , the source count rate rose again and reached @xmath70 counts s@xmath7 . we estimate that the observed dynamical range of this source in the xrt band , considering the historical data we collected during our monitoring campaign ( * ? ? ? * ; * ? ? ? * see fig . [ axj1841fig : lcv_campaign ] ) is @xmath39 , hence placing it well in the customary range for sfxts . the outburst of  has similar characteristics to the one observed on 2008 march 31 , as the xrt light curve shows a peak at about 25 counts s@xmath7 , decreases to about 0.5 counts s@xmath7 and then increases again up to about 20 counts s@xmath7 at the end of the first orbit ( fig .  [ axj1841fig:17544lcv_allbands ] ) . this behaviour was previously observed in  and , most notably , in igr  j08408@xmath04503 @xcite and sax  j1818.6@xmath01703 @xcite , so that this multiple - peak structure of the light curve could be considered a defining characteristic of the sfxt class and it is likely due to inhomogeneities within the accretion flow ( e.g. * ? ? ? figure  [ axj1841fig : best_sfxts ] compares the light curves of  and   with the outbursts of sfxts as observed during our monitoring campaigns with @xmath25 . the most complete set of x  ray observations of an outburst of a sfxt is the one of the periodic sfxt igr  j11215@xmath05952 @xcite , which was surprisingly long . we now know that such a length of the outburst ( hence the length of the accetion phase ) is a common characteristic of the whole sample of sfxts followed by @xmath25 , and in this respect   fits right in , as its outburst lasted several days . 5952 did not trigger the bat , so it is referred to mjd 54139.94 ) . points denote detections , triangles 3@xmath40 upper limits . red data points ( panels e , g ) refer to observations presented here for the first time , while grey points to data presented elsewhere . where no data are plotted , no @xmath25  data were collected . vertical dashed lines mark time intervals equal to 1 day , up to a week . references : igr  j084084503 ( 2008 - 07 - 05 , * ? ? ? * panel a ) ; igr  j11215@xmath05952 ( 2007 - 02 - 09 , * ? ? ? * panel b ) ; igr  j16479@xmath04514 ( 2005 - 08 - 30 , * ? ? ? * panel c ) ; xte  j1739@xmath0302 ( 2008 - 08 - 13 , * ? ? ? * panel d ) ; sax  j1818.6@xmath01703 ( 2009 - 05 - 06 , * ? ? ? * panel f ) . panels e and g report the 2010 - 03 - 04 outburst of igr  j17544@xmath02619 and the 2010 - 06 - 05 outburst of ax  j1841.0@xmath00536 , respectively ( this work ) . , width=321 ]    we have presented the broad - band ( 0.3100kev ) simultaneous spectroscopy of . this allows us to make a comparison with the findings on the other sfxts that were observed in the same fashion . the soft x ray spectral properties observed during this flare are generally consistent with those observed with _ asca _ during the 1999 flare ( * ? ? ? * @xmath11 @xmath3 , @xmath71 ) . as  was observed relatively late after the trigger , no meaningful information can be derived on variability of the soft spectral parameters during the outburst , such as the absorbing column density . however , we note that the value of @xmath72 in outburst follows the same trend of ` harder when brighter ' as reported in table  4 of @xcite , which was based on out - of - outburst emission .    for the joint bat@xmath22xrt spectrum during the 2010 june 5 outburst , an absorbed power - law model is an inadequate description , and more curvy models are required . we considered an absorbed power - law model with an exponential cutoff and an absorbed power - law model with a high energy cut - off , models typically used to describe the x  ray emission from accreting neutron stars in hmxbs . we obtained a good fit of the 0.3100kev spectrum , characterized by high absorption @xmath73 @xmath3 , a hard power law below 10kev , and a high energy cutoff . these properties of  are reminiscent of those of the prototypes of the sfxt class ,  [ whose data we have presented here and in @xcite ] , and xte  j1739@xmath0302 ( @xcite ) . although no statistically significant pulsations were found in the present data ,  is one of the 4 sfxts with known pulse period @xcite , @xmath74s , the others being igr  j11215@xmath05952 ( 186.78@xmath750.3s , * ? ? ? * ) , igr  j16465@xmath04507 ( 228@xmath756s , * ? ? ? * ) , and igr  j18483@xmath00311 ( 21.0526@xmath750.0005s , * ? ? ? * ) . while lacking the detection of cyclotron lines , which would yield a direct measurement of the magnetic field @xmath76 of the neutron star , an indirect estimate can be obtained by considering the highecut fit to the broad - band spectrum of in outburst . our value of the high energy cutoff @xmath77kev , although loosely constrained , yields a @xmath7810@xmath79  g @xcite this value for @xmath76 , which is indeed similar to the one derived for the prototype of the sfxt class , is inconsistent with a magnetar nature of .    in conclusion , we have shown how ax  j1841.0@xmath00536 nicely fits in the sfxt class , based on the observed properties of  during the 2010 june 5 outburst : a large dynamical range in x  ray luminosity , the similarity of the light curve length and shape to those of the prototype of the class , and the x  ray broad - band spectrum , which we show here for the first time down to 0.3kev , thus constraining both the absorption and the cutoff energy . we thank the @xmath25  team duty scientists and science planners and the remainder of the @xmath25  xrt and bat teams , s.  barthelmy in particular , for their invaluable help and support . this work was supported in italy by contract asi - inaf i/009/10/0 , at psu by nasa contract nas5 - 00136 . pe acknowledges financial support from the autonomous region of sardinia through a research grant under the program po sardegna fse 20072013 , l.r . 7/2007 `` promoting scientific research and innovation technology in sardinia '' . lllllll source & sequence & obs / mode & start time ( ut ) & end time ( ut ) & exposure & time since trigger + & & & ( yyyy - mm - dd hh : mm : ss ) & ( yyyy - mm - dd hh : mm : ss ) & ( s ) & ( s ) + ax  j1841.0@xmath00536 & 00423958000 & bat / evt & 2010 - 06 - 05 17:15:13 & 2010 - 06 - 05 19:01:13 & 1515 & -502 + & 00423958000 & xrt / pc & 2010 - 06 - 05 17:52:02 & 2010 - 06 - 05 18:03:24 & 682 & 1708 + & 00423958001 & xrt / pc & 2010 - 06 - 05 19:02:59 & 2010 - 06 - 06 09:46:49 & 6111 & 5965 + & 00030988093 & xrt / pc & 2010 - 06 - 07 21:07:22 & 2010 - 06 - 07 21:23:58 & 969 & 186228 + & 00030988094 & xrt / pc & 2010 - 06 - 08 14:49:26 & 2010 - 06 - 08 15:05:56 & 977 & 249952 + & 00030988095 & xrt / pc & 2010 - 06 - 09 16:18:40 & 2010 - 06 - 09 16:38:56 & 1212 & 341705 + & 00030988096 & xrt / pc & 2010 - 06 - 10 19:47:57 & 2010 - 06 - 10 21:33:56 & 1414 & 440663 + & 00030988097 & xrt / pc & 2010 - 06 - 11 13:10:31 & 2010 - 06 - 11 13:35:56 & 1503 & 503217 + & 00030988098 & xrt / pc & 2010 - 06 - 13 13:41:14 & 2010 - 06 - 13 15:23:58 & 848 & 677859 + & 00030988099 & xrt / pc & 2010 - 06 - 14 04:09:44 & 2010 - 06 - 14 05:58:57 & 1554 & 729969 + & 00030988100 & xrt / pc & 2010 - 06 - 15 02:39:58 & 2010 - 06 - 15 04:22:57 & 1184 & 810983 + & 00030988101 & xrt / pc & 2010 - 06 - 16 03:55:52 & 2010 - 06 - 16 04:14:58 & 1110 & 901937 + igr  j17544@xmath02619 & 00414875000 & bat / evt & 2010 - 03 - 04 23:10:00 & 2010 - 03 - 04 23:30:02 & 1202 & -239 + & 00414875000 & xrt / wt & 2010 - 03 - 04 23:20:40 & 2010 - 03 - 05 00:47:11 & 421 & 401 + & 00414875000 & xrt / pc & 2010 - 03 - 04 23:27:59 & 2010 - 03 - 05 00:49:12 & 1520 & 840 + & 00414875001 & xrt / wt & 2010 - 03 - 05 01:21:58 & 2010 - 03 - 05 08:48:39 & 783 & 7679 + & 00414875001 & xrt / pc & 2010 - 03 - 05 01:22:03 & 2010 - 03 - 05 09:02:56 & 2502 & 7684 + & 00035056149 & xrt / pc & 2010 - 03 - 06 00:59:19 & 2010 - 03 - 06 01:33:50 & 801 & 92721 +
_ swift _ observed an outburst from the supergiant fast x  ray transient ( sfxt ) ax  j1841.0@xmath00536 on 2010 june 5 , and followed it with xrt for 11 days . 
 the x  ray light curve shows an initial flare followed by a decay and subsequent increase , as often seen in other sfxts , and a dynamical range of @xmath1 . 
 our observations allow us to analyse the simultaneous broad - band ( 0.3100kev ) spectrum of this source , for the first time down to 0.3kev , which can be fitted well with models usually adopted to describe the emission from accreting neutron stars in high - mass x 
 ray binaries , and is characterized by a high absorption ( @xmath2 @xmath3 ) , a flat power law ( @xmath4 ) , and a high energy cutoff . 
 all of these properties resemble those of the prototype of the class , igr  j17544@xmath02619 , which underwent an outburst on 2010 march 4 , whose observations we also discuss . 
 we show how well ax  j1841.0@xmath00536 fits in the sfxt class , based on its observed properties during the 2010 outburst , its large dynamical range in x  ray luminosity , the similarity of the light curve ( length and shape ) to those of the other sfxts observed by _ 
 swift _ , and the x  ray broad - band spectral properties .    [ firstpage ]    x - rays : binaries - x - rays : individual : ax  j1841.0@xmath00536 - x - rays : individual : igr  j17544@xmath02619    facility : _ swift _