2002ApJ...577..346R -
Astrophys. J., 577, 346-358 (2002/September-3)
Variable thermal emission from Aquila X-1 in quiescence.
RUTLEDGE R.E., BILDSTEN L., BROWN E.F., PAVLOV G.G. and ZAVLIN V.E.
Abstract (from CDS):
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+0.8–2.9(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 kTeff,∞=130+3–5eV, down to 113+3–4eV, and finally increasing to 118+9–4eV 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 (<104 s) variability (at 32+8–6% 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.
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Journal keyword(s):
Stars: Atmospheres - Stars: Individual: Constellation Name: Aquila X-1 - Stars: Neutron - X-Rays: Binaries
Simbad objects:
7
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