The Reawakening of 4U 1755-338 after 25 Years of Quiescence: Spectro-temporal Analysis Using Multi-instrument X-ray Data

Abstract

The black hole X-ray binary 4U 1755$-$338 underwent an outburst in 2020 after 25 years of quiescence. The comprehensive spectral analysis revealed that the system has a low interstellar neutral hydrogen column density of $0.34\pm0.01 \times$10$^{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$-$338.The hardness-intensity diagram (HID) did not follow the standard q-shaped pattern, indicating a non-canonical outburst. Instead, the HID showed a correlated evolution of hardness and source flux, suggesting a thermal disc origin of the flux. A wideband spectral analysis was performed using simultaneous NICER-NuSTAR data in two frameworks, based on kerrbb and bhspec. The results of bhspec (kerrbb) based modeling indicate that 4U 1755$-$338 is a high-inclination system, $67.44_{-3.03}^{+9.75}$ ($75.25_{-4.68}^{+5.59}$) degrees, and harbors a moderately spinning black hole with a spin parameter of $0.78_{-0.14}^{+0.02}$ ($0.50_{-0.43}^{+0.19}$) and a mass of $3.37_{-1.04}^{+0.45} (3.28_{-1.1}^{+1.7})M_{\odot}$ respectively. The inferred key parameters: black hole 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\propto 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.