SPECTRAL SOFTENING BETWEEN OUTBURST AND QUIESCENCE IN THE NEUTRON STAR LOW-MASS X-RAY BINARY SAX J1750.8-2900

Tracking the spectral evolution of transiently accreting neutron stars between outburst and quiescence probes relatively poorly understood accretion regimes. Such studies are challenging because they require frequent monitoring of sources with luminosities below the thresholds of current all-sky X-ray monitors. We present the analysis of over 30 observations of the neutron star low-mass X-ray binary SAX J1750.8-2900 taken across four years with the X-ray telescope aboard Swift. We find spectral softening with decreasing luminosity both on long (similar to 1 yr) and short (similar to days to week) timescales. As the luminosity decreases from 4 x 10(36) erg s(-1) to similar to 1 x 10(35) erg s(-1) (0.5-10 keV), the power law photon index increases from 1.4 to 2.9. Although not statistically required, our spectral fits allow an additional soft component that displays a decreasing temperature as the luminosity decreases from 4 x 10(36) to 6 x 10(34) erg s(-1). Spectral softening exhibited by SAX J1750.8-2900 is consistent both with accretion emission whose spectral shape steepens with decreasing luminosity and also with being dominated by a changing soft component, possibly associated with accretion onto the neutron star surface, as the luminosity declines.