Dr. Sayantan Bhattacharya

X-ray binaries (XRBs), hosting either a neutron star or black hole accreting from a stellar companion, emit across the electromagnetic spectrum, making them key laboratories for studying accretion and disk–jet coupling. This talk presents an overview of their multi-wavelength behavior during accretion outbursts, thermonuclear (Type-I) X-ray bursts, and general spectral variations in high-mass and low-mass X-ray binaries (HMXBs and LMXBs). During outbursts, XRBs undergo hard-to-soft spectral state transitions accompanied by correlated variability from radio to X-rays, linking compact jets, accretion disks, and reprocessing in optical/infrared bands (even multi-wavelength pulsations !). Thermonuclear bursts in neutron-star LMXBs produce rapid X-ray flashes followed by optical/UV counterparts that probe disk structure and the binary system geometry. HMXBs, dominated by stellar-wind accretion, show strong photoionized lines and their orbital modulation, while all transient disk-fed XRBs exhibit disk emission and outbursting activity. Coordinated multi-wavelength observations provide a unified picture of accretion flow, outflows, and the physical environments surrounding compact objects.