Comparative spectro-temporal study between 1A 1246$-$588 and 4U 0614$+$091 using AstroSat

Abstract

We present a comparative broadband spectro-temporal study of the ultra-compact X-ray binary (UCXB) candidates 1A 1246$-$588 and 4U 0614$+$091 using AstroSat observations. Spectral modelling shows that the spectrum of 1A 1246$-$588 is well described by thermal Comptonization and a soft thermal disk component. We identify a candidate narrow quasi-periodic oscillation (QPO) at $993\pm4$ Hz with a single-detector significance of $ ~ 3.7σ$, tentatively consistent with the upper kHz QPO previously reported from this source. An independent cross-instrument check does not confirm this feature, and we therefore treat it as a tentative, unconfirmed signature rather than a secure detection. In contrast, across the 4 observations, 4U 0614$+$091 evolves from a Comptonization-dominated hard state to a thermally dominated soft state, with the electron temperature decreasing from ~ 19 keV to ~2.3 keV, while the disk and blackbody temperatures increase. This evolution is accompanied by an increase in the disk contribution to the total flux from ~ 18% to ~ 50% and the appearance of an Fe emission line near 6.97 keV in the soft state. The inner disk radius remains approximately constant at ~13- 15 $R_g$, suggesting that the observed evolution is driven primarily by changes in the thermal properties of the accretion flow rather than substantial variations in disk truncation. When the source is in its hardest state, it displays the richest timing behaviour, with variability components spanning from ~ 12 Hz to nearly 1 kHz, whereas the softer observations are dominated by low frequency variability, along with a persistent component near ~140 Hz and a prominent ~ 100 Hz feature in O4. These results suggest that progressive cooling of the Comptonizing corona, along with enhanced thermal emission from the disk and boundary layer, play a key role in shaping the spectral and timing evolution of 4U 0614$+$091.