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the soft x - ray transient ( sxt ) 4u1543 - 47 was discovered on 17 august 1971 by the _ uhuru _ satellite @xcite . since then , additional x - ray outbursts have been observed in 1983 , 1992 and 2002 . the optical counterpart was found during the 1983 outburst by @xcite . dynamical studies of the companion star @xcite show that the compact object is most likely a black hole with a mass of 2.7 @xmath0 m@xmath1 7.5 m@xmath2 . the 2002 outburst began on mjd 52442 ( ut june 16.683 ) and was observed by rxte @xcite . during this outburst the soft x - ray light curve exhibited a classical fast rise , exponential decay profile as observed in many other soft x - ray transients @xcite . the top panel in figure 1 shows the rxte asm light curve from mjd 52390 to 52500 . from early on in the outburst 4u1543 - 47 was in a thermal - dominated @xcite state ( also known as high - soft state ) . between mjd 52457 - 52459 the object briefly transitioned into a low - hard state . after this time , 4u 1543 - 47 went back into a thermal - dominated state until mjd 52476 when the source transitioned into the low - hard state @xcite and remained so until it reached quiescence . we observed the 2002 outburst of 4u 1543 - 47 as part of the yalo ( yale , aura , lisbon , ohio ) consortium . our observations cover most of the outburst cycle in ir and some in optical . a `` secondary maximum '' appeared in the optical and ir ( oir ) light curves which is not apparent in the asm lightcurve . in this paper we present our oir lightcurves and focus our analysis on the secondary maximum . we argue that this secondary maximum is synchrotron emission which originates from a jet and that this may be a common phenomenon of sxts during outburst decline . @xmath3- and @xmath4-band images of 4u1543 - 47 were obtained on a daily basis ( weather permitting ) from mjd 52323.3 ( ut 2002 february 17 ) using the yalo 1.0 m telescope with andicam , a dual - channel imager capable of obtaining oir data simultaneously . data was recorded by a lick / loral-3 2048x2048 ccd on the optical channel and a rockwell 1024x1024 hgcdte `` hawaii '' array on the infrared channel . on mjd 52431 ( ut 2002 june 5 ) the yalo optical ccd failed . however , @xmath4-band observations continued on a daily basis . when the @xmath4-band light curve was observed to have risen by @xmath5 0.5 mag ( @xmath5 mjd 52440 ) we initiated additional observations in the @xmath6-band . optical observations were obtained on a daily basis , when possible , from mjd 52442.9 - 52500.8 ( ut 2002 june 18 - august 15 ) using the 74 inch telescope at mount stromlo observatory with the cassegrain imager ,    a 2k x 4k ccd and @xmath7 and @xmath8 filters . infrared observations ceased when the object was no longer high enough in the night sky to observe . oir data reduction and photometry were performed using the usual routines in iraf . calibration of the optical data was derived by calculating the offsets from a secondary standard , the magnitude of which was provided by jerry orosz ( private communication ) . infrared images were calibrated via observations of the primary standard p9187 , observed on mjd 52520 ( ut 2002 september 2 ) . the optical magnitudes were corrected for airmass extinction using coefficients taken from the ctio extinction table in iraf while the infrared atmospheric extinction coefficients were taken from @xcite . figure 1 shows the oir light curves together with the rxte asm light curve . the @xmath4-band data rises @xmath5 3 - 4 days before the asm , however , this difference will most likely decrease if the analysis is done with the more sensitive pca data instead . it is not clear that there is any noticeable difference in the peak times between the asm and @xmath4-band . after the primary x - ray maximum , the x - rays , oir lightcurves proceed on a steady decline . at mjd @xmath5 52460 the @xmath6-band lightcurve , and to a lesser extend the @xmath4-band , experiences a small reflare . it is interesting to note that just before this reflare 4u 1543 - 47 briefly transitioned into a low state and low - frequency qpos were observed @xcite . the object went back into a thermal - dominated state at mjd 52463 . at mjd 52476 , 4u 1543 - 47 transitioned again into the low state when , again , low - frequency qpos were observed @xcite . no optical or infrared data was obtained during the time of transition , however , soon after a third reflare is observed peaking at mjd 52487 . this third reflare , which we call a secondary maximum , is much stronger in the infrared than in the optical and the strength of the peak increases with wavelength . the reflare is so strong in the @xmath6-band that it s peak surpasses that of the primary maximum . interestingly , no response is seen in the asm lightcurve . this is not to say that there is no response in the x - rays at all . indeed , preliminary analysis of the pca data shows a large decrease in the power - law index , qpo frequency and disk blackbody flux occurring close to the time the secondary maximum begins to rise @xcite . a radio detection was made at the peak of the secondary maximum at a flux density of 3.8 @xmath9 0.6 mjy at 843 mhz ( jeff mcclintock , private communication ) . after the secondary maximum peak , all lightcurves decline . the @xmath4- and @xmath6-bands eventually follow the decrease from the primary peak until reaching quiescent levels . the most extraordinary feature of these lightcurves is the secondary maximum at mjd 52487 and the strength of this feature in @xmath4 and @xmath6 compared to the optical bands . this secondary maximum flux is in addition to the flux from the primary maximum decline since the infrared lightcurves return and follow the primary outburst decline after the reflare . to ascertain the origin of the secondary maximum we fitted a single blackbody , multicolor blackbody @xcite and broken power law to the spectral energy distribution of the secondary maximum . the underlying flux from the primary outburst was removed by fitting a straight line to the data before and after the secondary maximum and subtracting the resulting fit . hence , the sed shown in figure 2 is derived solely from the secondary maximum flux .    in figure 2 we also show our best fits of each model to the sed . the long - dashed line is a single blackbody model of t=1600k . this model can account for the infrared data but none of the optical . at this temperature , the emitting region would have a radius of @xmath5 2 x 10@xmath10 cm . for an orbital period of 1.123 days @xcite , the primary roche lobe is @xmath5 5 x 10@xmath11 cm . hence , if the secondary maximum emission is thermal , the source would be at a distance 10@xmath12 times further than the primary roche lobe , which seems implausible .     = -0.78 @xmath9 0.14 . short - dashed line shows power - law with @xmath13 = 0.062 @xmath9 0.003 , taken to be a lower limit . ] the dot - dashed line in figure 2 represents our best multicolor - blackbody fit . the fit parameters are as follows : @xmath14 = 10@xmath15 ( where @xmath16 was fixed to 3 gravitational radii ) , @xmath17k and m_dot = 10@xmath18 gs@xmath19 ( see mitsuda et al . 1984 for parameter definitions ) . we fixed the mass of the black hole at 5 m@xmath2 @xcite . as for the single blackbody case , the model can account for some but not all of the optical / infrared data and it is clear that the outer temperature given by the best fit is too cold for a physical disk .    the broken power - law fit ( @xmath20 ) is shown as a dashed - solid line in figure 2 . we use this broken power - law to represent emission from a synchrotron source . the solid line is a linear least - squares fit ( in log space ) to the oir data , giving @xmath13 = -0.78 @xmath9 0.14 . since we have only one radio datapoint , and since the sed must break somewhere between the radio and @xmath6-band points , we can only give a lower - limit of @xmath13 @xmath21 0.062 @xmath9 0.003 to the first part of the sed . this is shown as the dashed line in figure 2 . the broken power - law fit is much improved over the blackbody fits . since the oir data are best described by the power - law fit , together with the unphysical parameters given by the blackbody fits , we conclude that the most likely source of the reflare is jet synchrotron emission . this conclusion is supported by the radio detection made at the peak of the secondary maximum . this is not the first time that this type of behavior has been observed during the outbust decline of soft x - ray transients . @xcite observed a large reflare in the oir during the 2000 outburst decline of xte j1550 - 564 . recent observations made by us @xcite of the 2002/2003 outburst of gx 339 - 4 ( see figure 3 ) show the same phenomenon . in both cases , it appears after the sources transition into the low state and the reflare is much stronger in the infrared than optical . past studies of sxt outbursts report a `` plateau '' in optical data during outburst decline ( see * ? ? ? * for example ) . these plateau may be the same phenomenon as we see here . it is now widely accepted that jets are a near - ubiquitous property of sxts during the low state @xcite . this scenario is now increasingly supported by oir observations such as those presented in this paper . indeed , since the infrared data show such strong reflares , they may be used as a reliable trigger for other multiwavelength observations . in doing so we will be able to better constrain when jets appear and make progress in our understanding of jet formation and how this relates to the accretion geometry . the authors would like to thank alfred chen for assisting in the optical data reduction and emrah kalemci and nick morgan for their careful perusal of the manuscript . mb is supported by nsf grant ast-0098421 .
we have taken optical and infrared observations during the 2002 outburst of the soft x - ray transient , 4u 1543 - 47 . 
 a secondary maximum occurs in the lightcurves during the outburst decline . 
 this feature is much stronger at infrared wavelengths than optical . 
 we have applied single blackbody , multicolor blackbody and broken - power law models to the optical / infrared spectral energy distribution of the secondary maximum and find that the broken power - law provides the best fit . 
 we therefore conclude that the secondary maximum emission originates from a jet . 
 we also show the most recent lightcurves of the 2002/2003 outburst of gx 339 - 4 in which a secondary maximum appears . 
 this leads us to the conclusion that secondary maxima may be a common occurrence in soft x - ray transients during outburst decline which appear after the object transitions into the low - hard state . 
 infrared observations of such phenomena will give reliable triggers for multiwavelength observations , allowing us to greatly improve our knowledge of jet formation and behavior , and how this relates to the accretion geometry . 
 address = department of astronomy , yale university , new haven ct 06511 , u.s.a . 
 address = department of astronomy , yale university , new haven ct 06511 , u.s.a .