Discovery of kHz quasi-periodic oscillations in the Z source cygnus X-2

Rudy Wijnands, Jeroen Homan, Michiel Van Der Klis, Erik Kuulkers, Jan Van Paradijs, Walter H G Lewin, Frederick K. Lamb, Dimitrios Psaltis, Brian Vaughan

Research output: Contribution to journalArticle

38 Citations (Scopus)

Abstract

During observations with the Rossi X-Ray Timing Explorer from 1997 June 31 to July 3 we discovered two simultaneous kHz quasi-periodic oscillations (QPOs) near 500 and 860 Hz in the low-mass X-ray binary and Z source Cygnus X-2. In the X-ray color-color diagram and hardness-intensity diagram (HID), a clear Z track was traced out, which shifted in the HID within 1 day to higher count rates at the end of the observation. Z track shifts are well known to occur in Cyg X-2; our observation for the first time catches the source in the act. A single kHz QPO peak was detected at the left end of the horizontal branch (HB) of the Z track, with a frequency of 731 ± 20 Hz and an amplitude of 4.7-0.6+0.8% rms in the energy band 5.0-60 keV. Further to the right on the HB, at somewhat higher count rates, an additional peak at 532 ± 43 Hz was detected with an rms amplitude of 3.0-0.7+1.0%. When the source moved down the HB, thus when the inferred mass accretion rate increased, the frequency of the higher frequency QPO increased to 839 ± 13 Hz, and its amplitude decreased to 3.5-0.3+0.4% rms. The higher frequency QPO was also detected on the upper normal branch (NB) with an rms amplitude of 1.8-0.4+0.6% and a frequency of 1007 ± 15 Hz; its peak width did not show a clear correlation with inferred mass accretion rate. The lower frequency QPO was most of the time undetectable, with typical upper limits of 2% rms; no conclusion on how this QPO behaved with mass accretion rate can be drawn. If the peak separation between the QPOs is the neutron star spin frequency (as required in some beat-frequency models), then the neutron star spin period is 2.9 ± 0.2 ms (346 ± 29 Hz). This discovery makes Cyg X-2 the fourth Z source that displays kHz QPOs. The properties of the kHz QPOs in Cyg X-2 are similar to those of other Z sources. Simultaneous with the kHz QPOs, the well-known horizontal-branch QPOs (HBOs) were visible in the power spectra. At the left end of the HB, the second harmonic of the HBO was also detected. We also detected six small X-ray bursts. No periodic oscillations or QPOs were detected in any of them, with typical upper limits of 6%-8% rms.

Original languageEnglish (US)
JournalAstrophysical Journal
Volume493
Issue number2 PART II
StatePublished - 1998
Externally publishedYes

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oscillation
oscillations
diagram
accretion
neutron stars
hardness
diagrams
color-color diagram
beat frequencies
x rays
X Ray Timing Explorer
energy bands
power spectra
bursts
rate
low frequencies
harmonics
shift

Keywords

  • Accretion, accretion disks
  • Stars: individual (Cygnus X-2)
  • Stars: neutron
  • X-rays: stars

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

Wijnands, R., Homan, J., Van Der Klis, M., Kuulkers, E., Van Paradijs, J., Lewin, W. H. G., ... Vaughan, B. (1998). Discovery of kHz quasi-periodic oscillations in the Z source cygnus X-2. Astrophysical Journal, 493(2 PART II).

Discovery of kHz quasi-periodic oscillations in the Z source cygnus X-2. / Wijnands, Rudy; Homan, Jeroen; Van Der Klis, Michiel; Kuulkers, Erik; Van Paradijs, Jan; Lewin, Walter H G; Lamb, Frederick K.; Psaltis, Dimitrios; Vaughan, Brian.

In: Astrophysical Journal, Vol. 493, No. 2 PART II, 1998.

Research output: Contribution to journalArticle

Wijnands, R, Homan, J, Van Der Klis, M, Kuulkers, E, Van Paradijs, J, Lewin, WHG, Lamb, FK, Psaltis, D & Vaughan, B 1998, 'Discovery of kHz quasi-periodic oscillations in the Z source cygnus X-2', Astrophysical Journal, vol. 493, no. 2 PART II.
Wijnands R, Homan J, Van Der Klis M, Kuulkers E, Van Paradijs J, Lewin WHG et al. Discovery of kHz quasi-periodic oscillations in the Z source cygnus X-2. Astrophysical Journal. 1998;493(2 PART II).
Wijnands, Rudy ; Homan, Jeroen ; Van Der Klis, Michiel ; Kuulkers, Erik ; Van Paradijs, Jan ; Lewin, Walter H G ; Lamb, Frederick K. ; Psaltis, Dimitrios ; Vaughan, Brian. / Discovery of kHz quasi-periodic oscillations in the Z source cygnus X-2. In: Astrophysical Journal. 1998 ; Vol. 493, No. 2 PART II.
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title = "Discovery of kHz quasi-periodic oscillations in the Z source cygnus X-2",
abstract = "During observations with the Rossi X-Ray Timing Explorer from 1997 June 31 to July 3 we discovered two simultaneous kHz quasi-periodic oscillations (QPOs) near 500 and 860 Hz in the low-mass X-ray binary and Z source Cygnus X-2. In the X-ray color-color diagram and hardness-intensity diagram (HID), a clear Z track was traced out, which shifted in the HID within 1 day to higher count rates at the end of the observation. Z track shifts are well known to occur in Cyg X-2; our observation for the first time catches the source in the act. A single kHz QPO peak was detected at the left end of the horizontal branch (HB) of the Z track, with a frequency of 731 ± 20 Hz and an amplitude of 4.7-0.6+0.8{\%} rms in the energy band 5.0-60 keV. Further to the right on the HB, at somewhat higher count rates, an additional peak at 532 ± 43 Hz was detected with an rms amplitude of 3.0-0.7+1.0{\%}. When the source moved down the HB, thus when the inferred mass accretion rate increased, the frequency of the higher frequency QPO increased to 839 ± 13 Hz, and its amplitude decreased to 3.5-0.3+0.4{\%} rms. The higher frequency QPO was also detected on the upper normal branch (NB) with an rms amplitude of 1.8-0.4+0.6{\%} and a frequency of 1007 ± 15 Hz; its peak width did not show a clear correlation with inferred mass accretion rate. The lower frequency QPO was most of the time undetectable, with typical upper limits of 2{\%} rms; no conclusion on how this QPO behaved with mass accretion rate can be drawn. If the peak separation between the QPOs is the neutron star spin frequency (as required in some beat-frequency models), then the neutron star spin period is 2.9 ± 0.2 ms (346 ± 29 Hz). This discovery makes Cyg X-2 the fourth Z source that displays kHz QPOs. The properties of the kHz QPOs in Cyg X-2 are similar to those of other Z sources. Simultaneous with the kHz QPOs, the well-known horizontal-branch QPOs (HBOs) were visible in the power spectra. At the left end of the HB, the second harmonic of the HBO was also detected. We also detected six small X-ray bursts. No periodic oscillations or QPOs were detected in any of them, with typical upper limits of 6{\%}-8{\%} rms.",
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AU - Wijnands, Rudy

AU - Homan, Jeroen

AU - Van Der Klis, Michiel

AU - Kuulkers, Erik

AU - Van Paradijs, Jan

AU - Lewin, Walter H G

AU - Lamb, Frederick K.

AU - Psaltis, Dimitrios

AU - Vaughan, Brian

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N2 - During observations with the Rossi X-Ray Timing Explorer from 1997 June 31 to July 3 we discovered two simultaneous kHz quasi-periodic oscillations (QPOs) near 500 and 860 Hz in the low-mass X-ray binary and Z source Cygnus X-2. In the X-ray color-color diagram and hardness-intensity diagram (HID), a clear Z track was traced out, which shifted in the HID within 1 day to higher count rates at the end of the observation. Z track shifts are well known to occur in Cyg X-2; our observation for the first time catches the source in the act. A single kHz QPO peak was detected at the left end of the horizontal branch (HB) of the Z track, with a frequency of 731 ± 20 Hz and an amplitude of 4.7-0.6+0.8% rms in the energy band 5.0-60 keV. Further to the right on the HB, at somewhat higher count rates, an additional peak at 532 ± 43 Hz was detected with an rms amplitude of 3.0-0.7+1.0%. When the source moved down the HB, thus when the inferred mass accretion rate increased, the frequency of the higher frequency QPO increased to 839 ± 13 Hz, and its amplitude decreased to 3.5-0.3+0.4% rms. The higher frequency QPO was also detected on the upper normal branch (NB) with an rms amplitude of 1.8-0.4+0.6% and a frequency of 1007 ± 15 Hz; its peak width did not show a clear correlation with inferred mass accretion rate. The lower frequency QPO was most of the time undetectable, with typical upper limits of 2% rms; no conclusion on how this QPO behaved with mass accretion rate can be drawn. If the peak separation between the QPOs is the neutron star spin frequency (as required in some beat-frequency models), then the neutron star spin period is 2.9 ± 0.2 ms (346 ± 29 Hz). This discovery makes Cyg X-2 the fourth Z source that displays kHz QPOs. The properties of the kHz QPOs in Cyg X-2 are similar to those of other Z sources. Simultaneous with the kHz QPOs, the well-known horizontal-branch QPOs (HBOs) were visible in the power spectra. At the left end of the HB, the second harmonic of the HBO was also detected. We also detected six small X-ray bursts. No periodic oscillations or QPOs were detected in any of them, with typical upper limits of 6%-8% rms.

AB - During observations with the Rossi X-Ray Timing Explorer from 1997 June 31 to July 3 we discovered two simultaneous kHz quasi-periodic oscillations (QPOs) near 500 and 860 Hz in the low-mass X-ray binary and Z source Cygnus X-2. In the X-ray color-color diagram and hardness-intensity diagram (HID), a clear Z track was traced out, which shifted in the HID within 1 day to higher count rates at the end of the observation. Z track shifts are well known to occur in Cyg X-2; our observation for the first time catches the source in the act. A single kHz QPO peak was detected at the left end of the horizontal branch (HB) of the Z track, with a frequency of 731 ± 20 Hz and an amplitude of 4.7-0.6+0.8% rms in the energy band 5.0-60 keV. Further to the right on the HB, at somewhat higher count rates, an additional peak at 532 ± 43 Hz was detected with an rms amplitude of 3.0-0.7+1.0%. When the source moved down the HB, thus when the inferred mass accretion rate increased, the frequency of the higher frequency QPO increased to 839 ± 13 Hz, and its amplitude decreased to 3.5-0.3+0.4% rms. The higher frequency QPO was also detected on the upper normal branch (NB) with an rms amplitude of 1.8-0.4+0.6% and a frequency of 1007 ± 15 Hz; its peak width did not show a clear correlation with inferred mass accretion rate. The lower frequency QPO was most of the time undetectable, with typical upper limits of 2% rms; no conclusion on how this QPO behaved with mass accretion rate can be drawn. If the peak separation between the QPOs is the neutron star spin frequency (as required in some beat-frequency models), then the neutron star spin period is 2.9 ± 0.2 ms (346 ± 29 Hz). This discovery makes Cyg X-2 the fourth Z source that displays kHz QPOs. The properties of the kHz QPOs in Cyg X-2 are similar to those of other Z sources. Simultaneous with the kHz QPOs, the well-known horizontal-branch QPOs (HBOs) were visible in the power spectra. At the left end of the HB, the second harmonic of the HBO was also detected. We also detected six small X-ray bursts. No periodic oscillations or QPOs were detected in any of them, with typical upper limits of 6%-8% rms.

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