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

The mid-infrared (MIR; λ3 - 10μm) bands offer a unique window into understanding accretion and its interplay with jet formation in Galactic black hole X-ray binaries (BHXRBs). Although extremely difficult to observe from the ground, the NEOWISE time domain survey offers an excellent data set to study MIR variability when combined with contemporaneous X-ray data from the MAXI all-sky survey over a ≈15 yr baseline. Using a new forced photometry pipeline for NEOWISE data, we present the first systematic study of BHXRB MIR variability in outburst. Analyzing a sample of 16 sources detected in NEOWISE, we show variability trends in the X-ray hardness and MIR spectral index wherein i) the MIR bands are typically dominated by jet emission during the hard states, constraining the electron power spectrum index to p ≈ 1-4 in the optically thin regime and indicating emitting regions of a few tens of gravitational radii when evolving towards a flat spectrum, ii) the MIR luminosity (LIR) scales as LIR∝ LX0.82±0.12 with the $2-10$ keV X-ray luminosity (LX) in the hard state, consistent with its origin in a jet, and iii) the thermal disk emission dominates the soft state as the jet switches off and dramatically suppresses (10×) the MIR emission into a inverted spectrum (α≈ -1, where F_ν∝ν-α). We highlight a population of `mini' BHXRB outbursts detected in NEOWISE (including two previously unreported episodes in MAXI J1828-249) but missed in MAXI due to their faint fluxes or source confusion, exhibiting MIR spectral indices suggestive of thermal emission from a large outer disk. We highlight that upcoming IR 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.