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To: stefano@bicar.it
From: atel@astronomerstelegram.org
Subject: ATel 17215, 17216
Message-Id: <20250603070920.A112328807AB@server.astronomerstelegram.org>
Date: Tue, 3 Jun 2025 03:09:20 -0400 (EDT)

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ATEL #17215							     ATEL #17215

Title:	NICER sees declined X-ray activity of XB 1732-304 in Terzan
		1 
Author:	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)
		
Queries:	gaurava@space.dtu.dk
Posted:	2 Jun 2025; 15:10 UT
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 &plusmn; 3) cts/s to (46
&plusmn; 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 &plusmn; 0.1) &times; 1022
cm-2, photon index = 1.89 &plusmn; 0.02, power-law normalization
of 0.11 &plusmn; 0.01, and reduced chi^2 of 1.03 for 684 degrees of freedom.
We obtained an averaged unabsorbed flux of (5.78 &plusmn; 0.05) &times;
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 &plusmn; 0.2) &times; 10-10 erg/s/cm2
(near the start of the observation) to (4.8 &plusmn; 0.1) &times; 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 &times; 1036 erg/s to 1.9 &times;
1036 erg/s, assuming a distance of 5.7 &plusmn; 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 &plusmn; 6) mCrab
on April 7, 2025 (MJD 60772), which gradually increased, peaking at (80
&plusmn; 5) mCrab on May 15, 2025 (MJD 60810). This was followed by a gradual
decline, reaching (15 &plusmn; 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.

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Password Certification: Gaurava Kumar Jaisawal (gaurava.jaisawal@gmail.com)
https://www.astronomerstelegram.org/?read=17215
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ATEL #17216							     ATEL #17216

Title:	Dramatic changes in the timing behavior of SXP31.0 as seen
		by NuSTAR and SRG/ART-XC during the ongoing type II outburst
Author:	Alexander Salganik (Turku U.), Sergey S. Tsygankov (Turku
		U.), Sergey Molkov (IKI RAS), Alexander Lutovinov (IKI RAS), Alexander
		Mushtukov (Oxford U.), Juri Poutanen (Turku U.)
Queries:	alsalganik@gmail.com
Posted:	2 Jun 2025; 17:04 UT
Subjects:X-ray, Neutron Star, Pulsar

The Be/X-ray pulsar SXP31.0 (Chakrabarty et al. 1998) is currently exhibiting
a Type II outburst in April--June 2025 (ATel 17147, ATel 17163), reaching
a bolometric luminosity of ~4E+38 erg/s, the brightest one since its discovery
in 1998. The outburst evolution closely resembles that of the 1998 event.

Two NuSTAR and two SRG/ART-XC observations were triggered by our team during
the outburst and revealed significant timing changes. At the time of the
first NuSTAR and ART-XC observations (MJD 60796-60798) the source flux
was around (1.2-1.4)E-10 erg/s/cm2 in the 4-12 keV band,  the pulsed fraction
(PF) was ~10% below 20 keV, rising to ~30% at higher energies. Significant
low-frequency QPO was detected by both instruments at frequencies near
0.8 +/- 0.1 mHz (see also ATel 17204). The energy spectra were described
by a power law with an exponential cutoff (Gamma = 0.55+/- 0.04, Efold
= 8.6+/-0.1 keV) plus a blackbody component with the temperature of kT=1.31+/-0.03
keV. An iron line at 6.4 keV was also detected in all observations.

In the second ART-XC observation on MJD 60813, we found that QPO was dissipated.
At the same time, the PF increased to ~25% in the 4-25 keV energy range,
and  the flux rose to 1.9E-10 erg/s/cm2 in 4-12 keV.  The spectrum was
described by the same model with Gamma = 1.1 +/- 0.3, Efold=7+/-1 keV,
kT=1.44+/-0.05 keV.

During the second NuSTAR observation on MJD 60826, the 4-12 keV flux was
1.5E-10 erg/s/cm2, comparable to the first NuSTAR observation. No QPO was
detected in this observation, and the PF remained consistent with the second
ART-XC observation when luminosity was 1.2 times higher. The spectrum was
described by Gamma = 0.28 +/- 0.04, Efold=9.5+/-0.2 keV, kT=0.96+/-0.04
keV. Also during our observations the spin period decreased from 30.451
to 30.394 s over 30 days (MJD 60796--60826).

Thanks to joint fits with Swift/XRT, we also consistently detect a soft
excess (Yokogawa et al. 2000) with the temperature kT approximately 0.2
keV and blackbody radius of R~100-200 km.

We continue to monitor the source using the XRT, NICER and ART-XC telescopes.
We also thank the NuSTAR and SRG/ART-XC teams for accepting the TOO observations
and for the quick execution of the observing program. We are also grateful
to the Swift and NICER teams for rapid scheduling of the monitoring program.

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Password Certification: Alexander Salganik (alsalganik@gmail.com)
https://www.astronomerstelegram.org/?read=17216
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