Abstract
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.
Related
17215 NICER sees declined X-ray activity of XB 1732-304 in Terzan 1
17201 SVOM/ECLAIRs detection of a thermonuclear burst in Terzan 1
16124 MAXI/GSC detection of the brightest transient from Terzan 1 in more than 13 years
16108 LEIA detected a new X-ray outburst from MAXI J1810-222, and several outbursts from nearby sources in the Galactic center region
15957 NICER follow-up observations of transient activity from Terzan 1
15955 MAXI/GSC detection of a new X-ray outburst and a burst from the direction of Terzan 1
13991 Swift/XRT observation of Terzan 1
13983 INTEGRAL detection of hard X-ray activity from Terzan 1 (likely XB 1732-304) and XTE J1810-189
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.
Related
17215 NICER sees declined X-ray activity of XB 1732-304 in Terzan 1
17201 SVOM/ECLAIRs detection of a thermonuclear burst in Terzan 1
16124 MAXI/GSC detection of the brightest transient from Terzan 1 in more than 13 years
16108 LEIA detected a new X-ray outburst from MAXI J1810-222, and several outbursts from nearby sources in the Galactic center region
15957 NICER follow-up observations of transient activity from Terzan 1
15955 MAXI/GSC detection of a new X-ray outburst and a burst from the direction of Terzan 1
13991 Swift/XRT observation of Terzan 1
13983 INTEGRAL detection of hard X-ray activity from Terzan 1 (likely XB 1732-304) and XTE J1810-189
| Original language | English |
|---|---|
| Publication date | 2 Jun 2025 |
| Publication status | Published - 2 Jun 2025 |
| Series | The Astronomer's telegram |
|---|---|
| Number | ATel #17215 |
Keywords
- X-ray
- Binary
- Neutron Star
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