Variable thermal emission from aquila X-1 in quiescence

Robert E. Rutledge, Lars Bildsten, Edward F. Brown, George Pavlov, Vyacheslav E. Zavlin

Research output: Contribution to journalReview article

98 Citations (Scopus)

Abstract

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 kTeff, ∞ = 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.

Original languageEnglish (US)
Pages (from-to)346-358
Number of pages13
JournalAstrophysical Journal
Volume577
Issue number1 I
DOIs
StatePublished - Sep 20 2002

Fingerprint

thermal emission
neutron stars
power law
hydrogen
long-term change
outburst
accretion
timescale
x rays
atmosphere
photosphere
temperature
atmospheres
radii
detectors

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Rutledge, R. E., Bildsten, L., Brown, E. F., Pavlov, G., & Zavlin, V. E. (2002). Variable thermal emission from aquila X-1 in quiescence. Astrophysical Journal, 577(1 I), 346-358. https://doi.org/10.1086/342155
Rutledge, Robert E. ; Bildsten, Lars ; Brown, Edward F. ; Pavlov, George ; Zavlin, Vyacheslav E. / Variable thermal emission from aquila X-1 in quiescence. In: Astrophysical Journal. 2002 ; Vol. 577, No. 1 I. pp. 346-358.
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Rutledge, RE, Bildsten, L, Brown, EF, Pavlov, G & Zavlin, VE 2002, 'Variable thermal emission from aquila X-1 in quiescence', Astrophysical Journal, vol. 577, no. 1 I, pp. 346-358. https://doi.org/10.1086/342155

Variable thermal emission from aquila X-1 in quiescence. / Rutledge, Robert E.; Bildsten, Lars; Brown, Edward F.; Pavlov, George; Zavlin, Vyacheslav E.

In: Astrophysical Journal, Vol. 577, No. 1 I, 20.09.2002, p. 346-358.

Research output: Contribution to journalReview article

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N2 - 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 kTeff, ∞ = 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.

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Rutledge RE, Bildsten L, Brown EF, Pavlov G, Zavlin VE. Variable thermal emission from aquila X-1 in quiescence. Astrophysical Journal. 2002 Sep 20;577(1 I):346-358. https://doi.org/10.1086/342155