The curious case of Swift J1753.5−0127: a black hole low-mass X-ray binary analogue to Z cam type dwarf novae

Swift J1753.5−0127 (J1753) is a candidate black hole low-mass X-ray binary (BH-LMXB) that was discovered in outburst in 2005 May. It remained in outburst for ∼12 yr, exhibiting a wide range of variability on various time-scales, before entering quiescence after two short-lived, low-luminosity ‘mini-outbursts’ in 2017 April. The unusually long outburst duration in such a short-period (P_orb ≈ 3.24 hr) source, and complex variability observed during this outburst period, challenges the predictions of the widely accepted disc-instability model, which has been shown to broadly reproduce the behaviour of LMXB systems well. The long-term behaviour observed in J1753 is reminiscent of the Z Cam class of dwarf novae, whereby variable mass transfer from the companion star drives unusual outbursts, characterized by stalled decays and abrupt changes in luminosity. Using sophisticated modelling of the multiwavelength light curves and spectra of J1753, during the ∼12 yr the source was active, we investigate the hypothesis that periods of enhanced mass transfer from the companion star may have driven this unusually long outburst. Our modelling suggests that J1753 is in fact a BH-LMXB analogue to Z Cam systems, where the variable mass transfer from the companion star is driven by the changing irradiation properties of the system, affecting both the disc and companion star.