2026ApJ..1000...66P

The black hole (BH) X-ray binary 4U 1755-338 underwent an outburst in 2020 after 25 yr of quiescence. The comprehensive spectral analysis revealed that the system possesses a low interstellar neutral hydrogen column density of 0.34 ± 0.01 × 10²² cm^–2. The outburst began with a low mass-accretion rate and was characterized as a low-luminosity outburst. The radius of the inner accretion disc remained constant throughout the outburst. Additionally, a growing neutral medium with constant density was detected in the local environment of 4U 1755-338. The hardness-intensity diagram (HID) did not follow the standard q-shaped pattern, indicating a noncanonical outburst. Instead, the HID showed a correlated evolution of hardness and source flux, suggesting a thermal disc origin of the flux. A wideband spectral analysis was performed using simultaneous NICER-NuSTAR data in two frameworks, based on kerrbb and bhspec models. The results of bhspec (kerrbb) based modeling indicate that 4U 1755-338 is a high-inclination system, at $67.4{4}_{-3.03}^{+9.75}$ ($75.2{5}_{-4.68}^{+5.59}$) degrees, and harbors a moderately spinning BH with a spin parameter of $0.7{8}_{-0.14}^{+0.02}$ ($0.5{0}_{-0.43}^{+0.19}$) and a mass of $3.3{7}_{-1.04}^{+0.45}\,(3.2{8}_{-1.1}^{+1.7})\,{M}_{\odot }$, respectively. The inferred key parameters: BH mass, spin, and system inclination are consistent across both modeling approaches. No reflection features were detected in the spectra of 4U 1755-338. The high spectral index, the blackbody nature (L ∝ T⁴) of the hardness ratio, the absence of reflection signatures, and the weak variability in the power density spectra indicate that the source remained in the high/soft state throughout the outburst.