Eddington-limited X-ray bursts as distance indicators. II. Possible compositional effects in bursts from 4U 1636-536

Duncan K. Galloway, Dimitrios Psaltis, Michael P. Muno, Deepto Chakrabarty

Research output: Contribution to journalArticle

43 Citations (Scopus)

Abstract

We analyzed 123 thermonuclear (type I) X-ray bursts observed by the Rossi X-Ray Timing Explorer (RXTE) from the low-mass X-ray binary 4U 1636-536. All but two of the 40 radius expansion bursts in this sample reached peak fluxes normally distributed about a mean of 6.4 × 10-8 ergs cm -2 s-1, with a standard deviation of 7.6%. The remaining two radius-expansion bursts reached peak fluxes a factor of 1.69 ± 0.13 lower than this mean value; as a consequence, the overall variation in the peak flux of the radius-expansion bursts was a factor of ≈2. This variation is comparable to the range of the Eddington limit between material with solar H fraction (X = 0.7) and pure He. Such a variation may arise if, for the bright radius-expansion bursts, most of the accreted H is either eliminated by steady hot CNO burning or expelled in a radiatively driven wind. However, steady burning cannot exhaust the accreted H for solar composition material within the typical ≈2 hr burst recurrence time, nor can it result in sufficient elemental stratification to allow selective ejection of the H only. An additional stratification mechanism appears to be required to separate the accreted elements and thus allow preferential ejection of the hydrogen. We found no evidence for a gap in the peak flux distribution between the radius-expansion and non-radius-expansion bursts, previously observed in smaller samples. Assuming that the faint radius-expansion bursts reached the Eddington limit for H-rich material (X ≈ 0.7), and the brighter bursts the limit for pure He (X = 0), we estimate the distance to 4U 1636-536 (for a canonical neutron star with MNS = 1.4 M, RNS = 10 km) to be 6.0 ± 0.5 kpc, or for MNS = 2 M at most 7.1 kpc.

Original languageEnglish (US)
Pages (from-to)1033-1038
Number of pages6
JournalAstrophysical Journal
Volume639
Issue number2 I
DOIs
StatePublished - Mar 10 2006

Fingerprint

bursts
stratification
expansion
x rays
radii
erg
hydrogen
ejection
indicator
effect
material
X Ray Timing Explorer
neutron stars
standard deviation
estimates
distribution

Keywords

  • Stars: distances
  • Stars: individual (4U 1636-536)
  • Stars: neutron
  • X-rays: bursts

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

Eddington-limited X-ray bursts as distance indicators. II. Possible compositional effects in bursts from 4U 1636-536. / Galloway, Duncan K.; Psaltis, Dimitrios; Muno, Michael P.; Chakrabarty, Deepto.

In: Astrophysical Journal, Vol. 639, No. 2 I, 10.03.2006, p. 1033-1038.

Research output: Contribution to journalArticle

Galloway, Duncan K. ; Psaltis, Dimitrios ; Muno, Michael P. ; Chakrabarty, Deepto. / Eddington-limited X-ray bursts as distance indicators. II. Possible compositional effects in bursts from 4U 1636-536. In: Astrophysical Journal. 2006 ; Vol. 639, No. 2 I. pp. 1033-1038.
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abstract = "We analyzed 123 thermonuclear (type I) X-ray bursts observed by the Rossi X-Ray Timing Explorer (RXTE) from the low-mass X-ray binary 4U 1636-536. All but two of the 40 radius expansion bursts in this sample reached peak fluxes normally distributed about a mean of 6.4 × 10-8 ergs cm -2 s-1, with a standard deviation of 7.6{\%}. The remaining two radius-expansion bursts reached peak fluxes a factor of 1.69 ± 0.13 lower than this mean value; as a consequence, the overall variation in the peak flux of the radius-expansion bursts was a factor of ≈2. This variation is comparable to the range of the Eddington limit between material with solar H fraction (X = 0.7) and pure He. Such a variation may arise if, for the bright radius-expansion bursts, most of the accreted H is either eliminated by steady hot CNO burning or expelled in a radiatively driven wind. However, steady burning cannot exhaust the accreted H for solar composition material within the typical ≈2 hr burst recurrence time, nor can it result in sufficient elemental stratification to allow selective ejection of the H only. An additional stratification mechanism appears to be required to separate the accreted elements and thus allow preferential ejection of the hydrogen. We found no evidence for a gap in the peak flux distribution between the radius-expansion and non-radius-expansion bursts, previously observed in smaller samples. Assuming that the faint radius-expansion bursts reached the Eddington limit for H-rich material (X ≈ 0.7), and the brighter bursts the limit for pure He (X = 0), we estimate the distance to 4U 1636-536 (for a canonical neutron star with MNS = 1.4 M⊙, RNS = 10 km) to be 6.0 ± 0.5 kpc, or for MNS = 2 M⊙ at most 7.1 kpc.",
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T1 - Eddington-limited X-ray bursts as distance indicators. II. Possible compositional effects in bursts from 4U 1636-536

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AU - Muno, Michael P.

AU - Chakrabarty, Deepto

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N2 - We analyzed 123 thermonuclear (type I) X-ray bursts observed by the Rossi X-Ray Timing Explorer (RXTE) from the low-mass X-ray binary 4U 1636-536. All but two of the 40 radius expansion bursts in this sample reached peak fluxes normally distributed about a mean of 6.4 × 10-8 ergs cm -2 s-1, with a standard deviation of 7.6%. The remaining two radius-expansion bursts reached peak fluxes a factor of 1.69 ± 0.13 lower than this mean value; as a consequence, the overall variation in the peak flux of the radius-expansion bursts was a factor of ≈2. This variation is comparable to the range of the Eddington limit between material with solar H fraction (X = 0.7) and pure He. Such a variation may arise if, for the bright radius-expansion bursts, most of the accreted H is either eliminated by steady hot CNO burning or expelled in a radiatively driven wind. However, steady burning cannot exhaust the accreted H for solar composition material within the typical ≈2 hr burst recurrence time, nor can it result in sufficient elemental stratification to allow selective ejection of the H only. An additional stratification mechanism appears to be required to separate the accreted elements and thus allow preferential ejection of the hydrogen. We found no evidence for a gap in the peak flux distribution between the radius-expansion and non-radius-expansion bursts, previously observed in smaller samples. Assuming that the faint radius-expansion bursts reached the Eddington limit for H-rich material (X ≈ 0.7), and the brighter bursts the limit for pure He (X = 0), we estimate the distance to 4U 1636-536 (for a canonical neutron star with MNS = 1.4 M⊙, RNS = 10 km) to be 6.0 ± 0.5 kpc, or for MNS = 2 M⊙ at most 7.1 kpc.

AB - We analyzed 123 thermonuclear (type I) X-ray bursts observed by the Rossi X-Ray Timing Explorer (RXTE) from the low-mass X-ray binary 4U 1636-536. All but two of the 40 radius expansion bursts in this sample reached peak fluxes normally distributed about a mean of 6.4 × 10-8 ergs cm -2 s-1, with a standard deviation of 7.6%. The remaining two radius-expansion bursts reached peak fluxes a factor of 1.69 ± 0.13 lower than this mean value; as a consequence, the overall variation in the peak flux of the radius-expansion bursts was a factor of ≈2. This variation is comparable to the range of the Eddington limit between material with solar H fraction (X = 0.7) and pure He. Such a variation may arise if, for the bright radius-expansion bursts, most of the accreted H is either eliminated by steady hot CNO burning or expelled in a radiatively driven wind. However, steady burning cannot exhaust the accreted H for solar composition material within the typical ≈2 hr burst recurrence time, nor can it result in sufficient elemental stratification to allow selective ejection of the H only. An additional stratification mechanism appears to be required to separate the accreted elements and thus allow preferential ejection of the hydrogen. We found no evidence for a gap in the peak flux distribution between the radius-expansion and non-radius-expansion bursts, previously observed in smaller samples. Assuming that the faint radius-expansion bursts reached the Eddington limit for H-rich material (X ≈ 0.7), and the brighter bursts the limit for pure He (X = 0), we estimate the distance to 4U 1636-536 (for a canonical neutron star with MNS = 1.4 M⊙, RNS = 10 km) to be 6.0 ± 0.5 kpc, or for MNS = 2 M⊙ at most 7.1 kpc.

KW - Stars: distances

KW - Stars: individual (4U 1636-536)

KW - Stars: neutron

KW - X-rays: bursts

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