Variable thermal emission from aquila X-1 in quiescence

We obtained four Chandra/ACIS-S observations beginning 2 weeks after the end of the 2000 November outburst of the neutron star (NS) transient Aql X-1. Over the 5 month span in quiescence, the X-ray spectra are consistent with thermal emission from a NS with a pure hydrogen photosphere and R _∞ = 15.9_-2.9^+0.8 (d/5 kpc) km at the optically implied X-ray column density. We also detect a hard power-law tail during two of the four observations. The intensity of Aql X-1 first decreased by 50% ± 4% over 3 months, then increased by 35% ± 5% in 1 month, and then remained constant (<6% change) over the last month. These variations in the first two observations cannot be explained by a change in either the power-law spectral component or the X-ray column density. Presuming a pure hydrogen atmosphere and that R_∞ is not variable, the long-term changes can only be explained by variations in the NS effective temperature, from kT_{eff, ∞} = 130_-5^-3 eV, down to 113_-4^-3 eV, and finally increasing to 118 _-4^-9 eV for the final two observations. During one of these observations, we observe two phenomena that were previously suggested as indicators of quiescent accretion onto the NS: short-timescale (<10 ⁴ s) variability (at 32_-6⁺⁸% rms) and a possible absorption feature near 0.5 keV. The possible absorption feature can potentially be explained as being due to a time-variable response in the ACIS detector. Even so, such a feature has not been detected previously from a NS and, if confirmed and identified, can be exploited for simultaneous measurements of the photospheric redshift and NS radius.