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; λ ≃ 3 − 10 μm) 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 NEOWISE survey offers an excellent data set to study MIR variability when combined with contemporaneous X-ray data from MAXI over a ≈15 yr baseline. Using a new difference imaging pipeline for NEOWISE data, we present the first systematic MIR study of BHXRB outbursts. Analyzing 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 ≈ 1 − 4) in the optically thin regime and indicating emitting regions of few 10 × gravitational radii when evolving towards a flat spectrum, ii) the MIR luminosity (LIR) scales as $L_{IR} \propto L_X^{0.82\pm 0.12}$ with the 2 − 10 keV X-ray luminosity (LX) in the hard state, consistent with a jet, and iii) thermal disk emission dominates the soft states as the jet switches off and suppresses (≳ 10 ×) the MIR emission into an inverted spectrum (spectral index α ≈ −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 colors suggestive of thermal disk emission. 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.