A NICER and AstroSat view of the neutron star low-mass X-ray binary 1A 1246-588

Abstract

Neutron star (NS) low-mass X-ray binary (LMXB) systems depict a variety of X-ray spectral and timing features, which can be useful to probe the accretion-ejection mechanism in the strong gravity regime. Here, we study the relatively unexplored and faint NS LMXB 1A 1246-588, which is also an ultra-compact X-ray binary (UCXB) with a white dwarf donor. We investigate its temporal and spectral behavior using pointed NICER and AstroSat observations, supported by long-term MAXI/GSC monitoring. The MAXI light curve shows modest, recurrent outburst-like enhancements, providing the long-term flux context for interpreting the pointed observations. During the AstroSat observations in 2017, the source exhibits an absorbed 0.4-20 keV flux of $(1.18 \pm 0.02)$ x $10^{-10}$ $erg$ $cm^{-2}$ $s^{-1}$, while during the NICER observations in 2019, it spans an absorbed 0.5-10 keV flux range of $(0.7-3.7)$ x $10^{-10}$ $erg$ $cm^{-2}$ $s^{-1}$ and traces an atoll-like pattern in the hardness-intensity diagram. Broadband spectral modeling shows that the emission is well described by a soft blackbody and a hard Comptonized component, with no statistically required multicolor disk contribution. The blackbody temperature increases from 0.28 to 0.39 keV, with an emitting radius consistent within 6.9-13.8 km, while the Comptonization photon index varies from 1.8 to 2.3. We find that the observed spectral-state evolution is driven by a redistribution of accretion power between thermal emission from the NS boundary layer and Comptonized emission, consistent with atoll-type behavior. These results provide the first quantitative, multi-epoch view of accretion-state evolution in 1A 1246-588, revealing systematic changes in the thermal boundary-layer emission and the Comptonizing region in this UCXB system.