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

TL;DR

This study analyzes the 2020 outburst of the black-hole X-ray binary 4U 1755-338, observed with NICER and NuSTAR, to characterize its spectro-temporal evolution after 25 years of quiescence. The source exhibits a non-canonical, disc-driven high/soft-state behavior with a low accretion rate and no detectable reflection features. Two broad-band continuum-fitting approaches, using kerrbb and bhspec (with simpl), constrain a BH mass near 3.3 M⊙, a spin $a \,\sim\,$0.5–0.78, and an inclination of roughly $67^ ext{o}$–$75^ ext{o}$ at a distance of ~8.5 kpc, with L/L_Edd ≈ 0.04. The HID’s linear flux–hardness trend and the sustained soft-state spectra, alongside a gradually evolving neutral absorber, indicate a disc-dominated outburst with minimal timing variability, contributing valuable BH parameter constraints for a low-luminosity, high-inclination system.

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.

Figures (9)

• Figure 1: The MAXI/GSC lightcurve of 4U 1755$-$338 during the 2020 outburst in the energy bands (a) 2$-$10 keV and (b) 10$-$20 keV. (c) The ratio of fluxes in $4-10$ keV to $2-4$ keV defines the hardness. The four different NICER observation windows considered for this study, spanning the outburst period, are shown in yellow, red, blue, and green colors. The blue dashed line marks the single NuSTAR observation on day 8 of the outburst.
• Figure 2: Fitted spectrum of the NICER observation (ID: 3201080122) on day 127 of the outburst using the model $tbabs \times tbpcf \times diskbb$. The top panel shows the modelled data, the middle panel represents the unfolded spectrum, and the bottom panel shows the fitting residual.
• Figure 3: (a) The NICER lightcurve of 2020 outburst of 4U 1755$-$338 in different energy bands, 0.3$-$2 keV, 2$-$10 keV, and 0.3$-$10 keV represented in cyan colored stars, orange triangles, and violet squares respectively. The four windows of NICER observation are marked with their respective colors as previously discussed. (b) Hardness-Intensity Diagram of 4U 1755$-$338 using NICER observations. The colors yellow, red, blue, and green are used to distinguish between the four windows of NICER observation. The uncertainties are within the 90 per cent confidence range.
• Figure 4: Evolution of the diskbb(a, b) and tbpcf(c, d) parameters obtained from the spectral modeling of NICER observations using the model M$_1$. Panel (a), the inner disc temperature, $T_{in}$(b) the diskbb norm, (c) the equivalent hydrogen column density, $N_H^{pc}$ and (d) the partial covering fraction, pcf. The error bars represent 90$\%$ uncertainty.
• Figure 5: Wideband NICER-NuSTAR fitted spectra using the model $tbabs \times tbpcf \times simpl \times kerrbb \times constant$ for $i=80^{\circ}$ and $D=8$$kpc$. The NICER spectrum is shown in black color, and FPMA and FPMB spectra of NuSTAR are shown in red and green colors, respectively.