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          Institute: MPI für extraterrestrische Physik     Collection: High Energy Astrophysics at MPE     Display Documents

ID: 11016.0, MPI für extraterrestrische Physik / High Energy Astrophysics at MPE
Variable thermal emission from aquila X-1 in quiescence
Authors:Rutledge, Robert E.; Bildsten, Lars; Brown, Edward F.; Pavlov, George G.; Zavlin, Vyacheslav E.
Date of Publication (YYYY-MM-DD):2002-09-20
Title of Journal:The Astrophysical Journal
Journal Abbrev.:Astrophys. J.
Issue / Number:1
Start Page:346
End Page:358
Review Status:Peer-review
Audience:Experts Only
Abstract / Description: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-infinity = 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-&INFIN; is not variable, the long-term changes can only be explained by variations in the NS effective temperature, from kT(eff,&INFIN;) = 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.
Free Keywords:stars : atmospheres; stars : individual ( Aquila X-1); stars : neutron; X-rays : binaries
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für extraterrestrische Physik
External Affiliations:CALTECH, Div Phys Math & Astron, MS 130-33, Pasadena, CA 91125; USA; CALTECH, Div Phys Math & Astron, Pasadena, CA 91125 USA; Univ Calif Santa Barbara, Inst Theoret Phys, Santa Barbara, CA 93106 USA; Univ Calif Santa Barbara, Dept Phys, Santa Barbara, CA 93106 USA; Univ Chicago, Enrico Fermi Inst, Chicago, IL 60637 USA; Penn State Univ, Davey Lab 525, University Pk, PA 16802 USA; Max Planck Inst Extraterr Phys, D-85748 Garching, Germany
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