Broad-band spectral energy distribution of the X-ray transient Swift J1745−26 from outburst to quiescence^⋆

¹ Université de Paris, AIM, 91191 Gif-sur-Yvette, France
e-mail: sylvain.chaty@u-paris.fr
² AIM, CEA, CNRS, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
³ Université de Paris, CNRS, APC, 75013 Paris, France
⁴ Instituto de Astrofísica de Canarias, Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain

Received: 17 April 2019
Accepted: 5 March 2020

Abstract

Aims. We aim to analyse our study of 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 light curve 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 black body emitted by an accretion disc, and a synchrotron power law 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 ≤ ν_break ≤ 3.4 × 10¹⁴ Hz, and a radio spectral break at 10¹² Hz ≤ ν_break ≤ 10¹³ Hz. We also show that the system is compatible with an absorption A_V of ∼7.69 mag, lies within a distance interval of D ∼ [2.6 − 4.8] kpc with an upper limit of orbital period P_orb = 11.3 h, and that the companion star is a late 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 localisation 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 light curve and the broad-band SED of Swift J1745−26, and we plotted 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.