In a recent study published in The Astrophysical Journal, an international research group led by Dr. ZHANG Guobao from Yunnan Observatories of the Chinese Academy of Sciences reported a multiwavelength study of the short orbital period black hole X-ray binary candidate Swift J1753.5–0127, focusing on the final stages of its 12 yr long outburst.

Black hole X-ray transients (BHXTs) are binary systems consisting of a stellar-mass black hole (BH) primary accreting matter from a noncollapsed donor secondary. During outburst, the optical, UV and X-ray flux increases by orders of magnitude, approaching in some cases the Eddington luminosity limit.

Outbursts of BHXTs are thought to be triggered by a thermal-viscous instability in the accretion disk (disk instability model, DIM). However, some BHXTs show several rebrightenings after the outburst peak, which cannot be well reproduced by the DIM.

After being active for about 12 years, J1753 became faint at X-ray, UV, optical, and radio wavelengths, almost reaching the previously measured optical quiescence level. Using a new proposed rebrightening classification method, researchers recorded a mini-outburst and a reflare in the optical light curves.

From optical colors, they found that the temperature of the outer disk was ～11,000 K when the source started to fade rapidly. According to the DIM, this is close to the critical temperature when a cooling wave is expected to form in the disk, shutting down the outburst.

Besides, researchers found that all X-ray binaries that show mini-outbursts following a main outburst are short orbital period systems (<7 hr).

In analogy with another class of short-period binaries showing similar mini-outbursts, the cataclysmic variables of the RZ LMi type, they suggested that mini-outbursts could occur if there is a hot inner disk at the end of the outburst decay.

The newly introduced method can be used to classify rebrightening events quantitatively in neutron star/black hole X-ray binaries in the future work.