Correlated mid-infrared and X-ray outbursts in black hole X-ray binaries: a new route to discovery in infrared surveys

ABSTRACT The mid-infrared (MIR; $\lambda \simeq 3\!\!-\!\!10\ \mu \mathrm{ m}$) bands offer a unique window into understanding accretion and jet formation in Galactic black hole X-ray binaries (BHXRBs). Although difficult to observe from the ground, the Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) survey offers an excellent data set to study MIR variability when combined with contemporaneous X-ray data from Monitor of All-sky X-ray Image (MAXI) mission over an $\approx\!\! 15$ yr baseline. Using a new difference imaging pipeline for NEOWISE data, we present the first systematic MIR study of BHXRB outbursts. Analysing a sample of 16 sources, we show variability trends wherein (i) the MIR bands are dominated by jet emission during the hard states, constraining the electron power spectrum index ($p\approx 1\!\!-\!\!4$) in the optically thin regime and indicating emitting regions of few $10\times$ gravitational radii when evolving towards a flat spectrum, (ii) the MIR luminosity ($L_{\mathrm{ IR}}$) scales as $L_{\mathrm{ IR}} \propto L_\mathrm{ X}^{0.82\pm 0.12}$ with the $2\!\!-\!\!10$ keV X-ray luminosity ($L_\mathrm{ X}$) in the hard state, consistent with a jet, and (iii) thermal disc emission dominates the soft states as the jet switches off and suppresses ($\gtrsim\!\! 10\times$) the MIR emission into an inverted spectrum (spectral index $\alpha \approx -1$). We highlight ‘mini’ outbursts detected in NEOWISE (including two previously unreported episodes of MAXI J1828−249 from September 2015 and September 2016) but missed in MAXI due to their faint fluxes’ confusion, exhibiting MIR colours suggestive of thermal disc emission. We highlight that upcoming infrared surveys and the Rubin observatory will be powerful discovery engines for the distinctively large-amplitude and long-lived outbursts of BHXRBs, as an independent discovery route to X-ray monitors.