2001ApJ...559.1054R -
Astrophys. J., 559, 1054-1059 (2001/October-1)
Quiescent thermal emission from the neutron star in Aquila X-1.
RUTLEDGE R.E., BILDSTEN L., BROWN E.F., PAVLOV G.G. and ZAVLIN V.E.
Abstract (from CDS):
We report on the quiescent spectrum measured with Chandra ACIS-S of the transient, type I, X-ray-bursting neutron star Aql X-1, immediately following an accretion outburst. The neutron star radius, assuming a pure hydrogen atmosphere and a hard power-law spectrum, is R∞=13.4+5–4(d/5 kpc) km. Based on the historical outburst record of the Rossi X-Ray Timing Explorer All-Sky Monitor, the quiescent luminosity is consistent with that predicted by Brown, Bildsten, and Rutledge from deep crustal heating, lending support to this theory for providing a minimum quiescent luminosity of transient neutron stars. While not required by the data, the hard power-law component can account for 18%±8% of the 0.5-10 keV thermal flux. Short-timescale intensity variability during this observation is less than 15% rms (3 σ; 0.0001-1 Hz, 0.2-8 keV). Comparison between the Chandra spectrum and three X-ray spectral observations made between 1992 October and 1996 October find all spectra consistent with a pure H atmosphere, but with temperatures ranging from 145 to 168 eV, spanning a factor of 1.87±0.21 in observed flux. The source of variability in the quiescent luminosity on long timescales (greater than years) remains a puzzle. If from accretion, then it remains to be explained why the quiescent accretion rate provides a luminosity so nearly equal to that from deep crustal heating.
Abstract Copyright:
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Journal keyword(s):
Stars: Atmospheres - stars: individual (Aquila X-1) - Stars: Neutron - X-Rays: Binaries
Simbad objects:
2
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