NICER observes SLX 1746-331 in a rapidly brightening soft state

ATel #15941; G. Mastroserio (INAF-Osservatorio Astronomico di Cagliari), J. Homan (Eureka Scientific), M Bachetti (INAF-Osservatorio Astronomico di Cagliari), K. Gendreau (NASA/GSFC), Z. Arzoumanian (NASA/GSFC)
on 10 Mar 2023; 19:06 UT
Credential Certification: Guglielmo Mastroserio (gullik@caltech.edu)

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The X-ray transient and black hole candidate SLX 1746-331 was detected by the MAXI/GSC nova alert system at 04:21 UT on 2023 March 8 (ATel #15937). An averaged flux of 129 +/- 10 mCrab (4.0-10.0keV, 1 sigma error) was reported. NICER has observed the source between March 8 16:36 UTC and March 10 15:07 UTC for a total exposure of ~6 ks. During this period the 0.5-10 keV count rate increased from ~1250 counts/s to ~2200 counts/s (for 52 detectors). Preliminary spectral fits (1-10 keV) indicate that the spectrum is dominated by a disk blackbody component, with temperatures increasing from 1.42+/-0.01 keV to 1.81+/-0.01 keV. The Nh was found to be ~9.7e21 cm^-2. The spectra of the first few data segments also required the presence of a weak power-law component with spectral index 1.4+/-0.1 The above spectral parameters are suggestive of a soft spectral state. The unabsorbed 0.5-10 keV flux increased from ~1.5e-10 erg/cm^/s to ~3.6e-10 erg/cm^/s.

The power density spectrum of all data combined shows very little variability, with a hint of a broad component at low frequency. The rms computed in the frequency range 0.1-100 Hz is 1.3+/-0.1%, as expected in the soft state.

The fact that SLX 1746-331 already appeared to be in the soft state during the first detections with MAXI (ATel #15937) is unusual. Typically, black hole X-ray transient outbursts first evolve through the spectrally hard and intermediate states on time scales of a few weeks. In SLX 1746-331 this evolution appears to have occurred much faster and/or at a much lower luminosity, similar to what was observed in the most recent outburst of the black hole transient 4U 1543-47 (ATel #14725). NICER will continue to monitor the outburst of SLX 1746-331. Observations at other wavelengths are encouraged.

NICER is a 0.2-12 keV X-ray telescope operating on the International Space Station. The NICER mission and portions of the NICER science team activities are funded by NASA.