Abstract 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 × 10 22 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 ⊙ , 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 ∝ T 4 ) 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.
Prabhakar et al. (Thu,) studied this question.