Back to Business: SLX 1746–331 after 13 Years of Silence

The black hole candidate system SLX 1746–331 was back to business in 2023, after a long silence of roughly 13 years. An outburst was observed thoroughly by Insight-HXMT and NICER. The outburst is characterized by spectral dominance of the soft state, where the joint Insight-HXMT and NICER spectral analysis shows the temperature dependence of the disk flux follows Tin3.98 , and thus suggests that the inner disk reaches its innermost stable circular orbit during almost the entire outburst. By assuming 0.3 L Edd for the peak flux and an inclination angle of zero degrees, the lower limit of the compact object hosted in this system is estimated as 3.28 ± 2.14 M ⊙. We also look into the relation between the disk temperature and disk flux for a sample of black hole systems, and by taking the disk temperature derived in the outburst of SLX 1746–331, such a relation results in a mass estimation of 5.2 ± 4.5 M ⊙. Finally, the spin of the compact object is constrained to be larger than 0.8 with the spectral model KERRBB.