An artist's illustration of hot gas from a low-mass companion star flowing into an accretion disk
around the black hole. Image credit: ESO/L. Calçada. - Download Image
During the past week, NuSTAR performed multiple observations of the black hole X-ray binary (BHXB) GS 1354−64, including one observation coordinated with the JAXA/ESA/NASA mission XRISM. GS 1354−64 is unusual in several respects. Discovered in 1987 during an outburst, it has since undergone two additional outbursts (1997 and 2015), yet in both cases it “failed” to complete the canonical hard-to-soft X-ray spectral transition, observed in outbursts by other BHXB, before fading back into quiescence. In the current episode, however, the most recent NuSTAR observation indicates that the source is now transitioning toward the soft X-ray state. Optical studies of the companion star (BW Cir) have placed the system at a distance of ~27 kpc, which—if correct—would make it among the most distant known Galactic BHXBs. A preliminary analysis of the latest NuSTAR data suggests that, at this distance, the transition would be occurring at an X-ray luminosity of roughly 70% of the Eddington limit (the luminosity at which radiation pressure balances gravity for accreting material). This is striking because BHXBs typically undergo the hard-to-soft transition at ≲10% of the Eddington limit. The NuSTAR dataset will therefore be used to test several possibilities: is the source substantially closer than we thought it was (e.g., 8–10 kpc and thus less luminous), significantly more massive than currently assumed (and thus would have a higher Eddington limit), or otherwise genuinely anomalous? In addition, the combination of the high spectral resolution and broad X-ray sensitivity from the joint NuSTAR+XRISM observation will enable tighter constraints on the binary system’s spin and inclination.
During the past week, NuSTAR performed multiple observations of the black hole X-ray binary (BHXB) GS 1354−64, including one observation coordinated with the JAXA/ESA/NASA mission XRISM. GS 1354−64 is unusual in several respects. Discovered in 1987 during an outburst, it has since undergone two additional outbursts (1997 and 2015), yet in both cases it “failed” to complete the canonical hard-to-soft X-ray spectral transition, observed in outbursts by other BHXB, before fading back into quiescence. In the current episode, however, the most recent NuSTAR observation indicates that the source is now transitioning toward the soft X-ray state. Optical studies of the companion star (BW Cir) have placed the system at a distance of ~27 kpc, which—if correct—would make it among the most distant known Galactic BHXBs. A preliminary analysis of the latest NuSTAR data suggests that, at this distance, the transition would be occurring at an X-ray luminosity of roughly 70% of the Eddington limit (the luminosity at which radiation pressure balances gravity for accreting material). This is striking because BHXBs typically undergo the hard-to-soft transition at ≲10% of the Eddington limit. The NuSTAR dataset will therefore be used to test several possibilities: is the source substantially closer than we thought it was (e.g., 8–10 kpc and thus less luminous), significantly more massive than currently assumed (and thus would have a higher Eddington limit), or otherwise genuinely anomalous? In addition, the combination of the high spectral resolution and broad X-ray sensitivity from the joint NuSTAR+XRISM observation will enable tighter constraints on the binary system’s spin and inclination.
Author: Oluwashina Adegoke (Postdoctoral Scholar, Caltech)
