Context :

Aims : We study the X-ray transient Swift J1745-26 , using observations obtained from its outburst in September 2012 , up to its decay towards quiescence in March 2013 .

Methods : We obtained optical and infrared observations , through override programme at ESO/VLT with FORS2 and ISAAC instruments , and added archival optical ( VLT/VIRCAM ) , radio and X-ray ( Swift ) observations , to build the lightcurve and the broad-band Spectral Energy Distribution ( SED ) of Swift J1745-26 .

Results : We show that , during its outburst and also during its decay towards quiescence , Swift J1745-26 SED can be adjusted , from infrared up to X-rays , by the sum of both a viscous irradiated multi-colour blackbody emitted by an accretion disk , and a synchrotron powerlaw at high energy .
In the radio domain , the SED arises from synchrotron emission from the jet .
While our SED fitting confirms that the source remained in the low/hard state during its outburst , we determine an X-ray spectral break at frequency 3.1 \leq \nu _ { break } \leq 3.4 \times 10 ^ { 14 } Hz , and a radio spectral break at 10 ^ { 12 } Hz \leq \nu _ { break } \leq 10 ^ { 13 } Hz .
We also show that the system is compatible with an absorption A\ _ { V } of \sim 7.69 \mbox { magnitudes } , lies within a distance interval of D \sim [ 2.6 - 4.8 ] \mbox { kpc } , with an upper limit of orbital period P _ { orb } = 11.3 hours , and that the companion star is a late-type star of spectral type in the range K0 – M0 V , confirming that the system is a low-mass X-ray binary .
We finally plot the position of Swift J1745-26 on an optical-infrared – X-ray luminosity diagram : its localization on this diagram is consistent with the source staying in the low-hard state during outburst and decay phases .

Conclusions : By using new observations obtained at ESO/VLT with FORS2 and ISAAC , and adding archival optical ( VLT/VIRCAM ) , radio and X-ray ( Swift ) observations , we built the lightcurve and the broad-band SED of Swift J1745-26 , and we plot its position on an optical-infrared – X-ray luminosity diagram .
By fitting the SED , we characterized the emission of the source from infrared , via optical , up to X-ray domain , we determined the position of both the radio and X-ray spectral breaks , we confirmed that it remained in the low-hard state during outburst and decay phases , and we derived its absorption , distance interval , orbital period upper limit , and the late-type nature of companion star , confirming Swift J1745-26 is a low-mass X-ray binary .