The origin of UV/optical emission in the black hole low-mass X-ray binary Swift J1753.5-0127

TL;DR

This study analyzes multi-wavelength data from the black hole binary Swift J1753.5-0127 over 12 years, revealing that UV/optical emission correlates with X-ray emission during hard states, primarily originating from a viscously heated accretion disc.

Contribution

It provides the first detailed correlation analysis between UV/optical and X-ray emissions in Swift J1753.5-0127, highlighting the disc's role in emission during outbursts.

Findings

UV/optical and X-ray emissions are strongly correlated during hard states.

The power-law index of the correlation increases with decreasing wavelength.

Deviations from the correlation are linked to inner disc emission during low-intensity X-ray peaks.

Abstract

The emission from the accreting black holes (BHs) in low-mass X-ray binaries (LMXBs) covers a broad energy band from radio to X-rays. Studying the correlations between emission in different energy bands during outbursts can provide valuable information about the accretion process. We analyse the simultaneous optical, ultraviolet (UV) and X-ray data of the BH-LMXB Swift J1753.5-0127 during its $\sim$ 12-year long outburst with the {\it Neil Gehrels Swift Observatory}. We find that the UV/optical and X-ray emission are strongly correlated during the hard states of the outburst. We fit the relation with a power-law function $F_{UV/optical} \propto F_{X}^{\beta}$ and find that the power-law index $\beta$ increases from $\sim$ 0.24 to $\sim$ 0.33 as the UV/optical wavelength decreases from $\sim$ 5400 \r{A} (V) to $\sim$ 2030 \r{A} (UVW2). We explore the possible reasons for this and suggest that in Swift J1753.5-0127 the UV/optical emission is dominated by a viscously heated accretion disc at large radii. We find that the data that deviate from the correlation correspond to the low-intensity peaks appeared in the X-ray band during the outburst, and suggest that these deviations are driven by the emission from the inner part of the accretion disc.