NICER sees declined X-ray activity of XB 1732-304 in Terzan 1
ATel #17215; G. K. Jaisawal, J. Chenevez (DTU Space), K. C. Gendreau, Z. Arzoumanian (NASA GSFC), A. Sanna (UniCA), D. Altamirano (University of Southampton), J. B. Coley (Howard University, NASA GSFC, CRESST II), E. C. Ferrara (University of Maryland, NASA GSFC, CRESST II)
on 2 Jun 2025; 15:10 UT
Credential Certification: Gaurava Kumar Jaisawal (gaurava.jaisawal@gmail.com)
Subjects: X-ray, Binary, Neutron Star
Following the detection of a type I X-ray burst from the globular cluster Terzan 1 by ECLAIRs onboard SVOM on 2025-05-20 (ATel #17201), NICER observed the low-mass X-ray binary XB 1732-304 (also known as XB 1733-30, SLX 1732-304, and 1RXS J173546.9-302859) between May 24 and May 27, 2025, accumulating a total exposure of 2.7 ks. The NICER light curve in the 0.5-10 keV band shows a gradual decline in count rate from (85 ± 3) cts/s to (46 ± 4) cts/s, indicating a steady decrease in source intensity throughout the observations. We did not observe any significant variability in the power density spectra. No X-ray burst was detected with NICER.
The averaged NICER spectrum can be fitted with an absorbed power-law model in the 0.5-10 keV energy range. The best-fit spectral parameters are: hydrogen column density nH = (1.8 ± 0.1) × 1022 cm-2, photon index = 1.89 ± 0.02, power-law normalization of 0.11 ± 0.01, and reduced chi^2 of 1.03 for 684 degrees of freedom. We obtained an averaged unabsorbed flux of (5.78 ± 0.05) × 10-10 erg/s/cm2 in the 0.5-10 keV band. Considering the light curve evolution, we also performed time-resolved spectroscopy to examine the spectral evolution. The time-resolved analysis revealed a flux decline from (8.3 ± 0.2) × 10-10 erg/s/cm2 (near the start of the observation) to (4.8 ± 0.1) × 10-10 erg/s/cm2 (at the end of the observation), accompanied by changes in the spectral parameters during the observations. This corresponds to a luminosity decrease from 3.2 × 1036 erg/s to 1.9 × 1036 erg/s, assuming a distance of 5.7 ± 0.2 kpc (Baumgardt & Vasiliev 2021, MNRAS, 505, 5957; Heinke et al. 2024, arXiv:2407.18867). Uncertainties on parameters are reported at the 90% confidence level.
Based on the 2-20 keV MAXI light curve, the initial intensity from Terzan 1 (likely associated with XB 1732-304) was around (33 ± 6) mCrab on April 7, 2025 (MJD 60772), which gradually increased, peaking at (80 ± 5) mCrab on May 15, 2025 (MJD 60810). This was followed by a gradual decline, reaching (15 ± 5) mCrab on June 1, 2025 (MJD 60827). This behavior closely matches the intensity evolution observed during NICER observations, suggesting the fading of the outburst from XB 1732-304.
NICER is a 0.2-12 keV X-ray telescope operating onboard the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.
