The Island State of the Atoll Source 4U 1820–30

We study the rapid X-ray time variability in all public data available from the Rossi X-Ray Timing Explorer Proportional Counter Array on the atoll source 4U 1820-30 in the low-luminosity island state. A total of ~46 ks of data were used. We compare the frequencies of the variability components of 4U 1820-30 with those in other atoll sources. These frequencies were previously found to follow a universal scheme of correlations. We find that 4U 1820-30 shows correlations that are shifted by factors of 1.13 ± 0.01 and 1.21 ± 0.02 with respect to those in other atoll sources. These shifts are similar to, but smaller than, the shift factor of ~1.45 previously reported for some accreting millisecond pulsars. Therefore, 4U 1820-30 is the first atoll source that shows no significant pulsations but has a significant shift in the frequency correlations compared with the other three nonpulsating atoll sources.

[1]  W. Lewin,et al.  Compact stellar X-ray sources , 2006 .

[2]  R. Wijnands,et al.  Relations Between Timing Features and Colors in Accreting Millisecond Pulsars , 2004, astro-ph/0410505.

[3]  S. Ransom,et al.  An RXTE Archival Search for Coherent X-Ray Pulsations in the Low-Mass X-Ray Binary 4U 1820–30 , 2004, astro-ph/0407629.

[4]  M. Rupen,et al.  Radio detections of the neutron star X-ray binaries 4U 1820 − 30 and Ser X-1 in soft X-ray states , 2004, astro-ph/0402600.

[5]  G. Nelemans,et al.  Optical spectra of the carbon-oxygen accretion discs in the ultra-compact X-ray binaries 4U 0614+09, 4U 1543-624 and 2S 0918-549 , 2003, astro-ph/0312008.

[6]  M. Klis,et al.  The Atoll Source States of 4U 1608-52 , 2003, astro-ph/0307041.

[7]  W. Kluźniak,et al.  Non-Linear Resonance in Nearly Geodesic Motion in Low-Mass X-Ray Binaries , 2003, astro-ph/0302183.

[8]  W. Harris,et al.  Photospheric radius expansion X-ray bursts as standard candles , 2002, astro-ph/0212028.

[9]  J. Zand,et al.  Discovery of a Second Millisecond Accreting Pulsar: XTE J1751–305 , 2002, astro-ph/0206491.

[10]  P. G. Jonker,et al.  Low- and high-frequency variability as a function of spectral properties in the bright X-ray binary GX 5-1 , 2002, astro-ph/0202420.

[11]  D. Psaltis,et al.  A Unified Description of the Timing Features of Accreting X-Ray Binaries , 2002, astro-ph/0202213.

[12]  S. Corbel,et al.  A Closer Look at the Soft X-Ray Transient X1608–52: Long-Term Optical and X-Ray Observations , 2001, astro-ph/0112465.

[13]  Tod E. Strohmayer,et al.  A Remarkable 3 Hour Thermonuclear Burst from 4U 1820–30 , 2001, astro-ph/0108420.

[14]  M. Klis.  A Possible Explanation for the "Parallel Tracks" Phenomenon in Low-Mass X-Ray Binaries , 2001, astro-ph/0106291.

[15]  L. Bildsten,et al.  A Brown Dwarf Companion for the Accreting Millisecond Pulsar SAX J1808.4–3658 , 2001, astro-ph/0104153.

[16]  Moscow,et al.  New class of low frequency QPOs: Signature of nuclear burning or accretion disk instabilities? , 2000, astro-ph/0011110.

[17]  P. Kaaret,et al.  RXTE Studies of Long-Term X-Ray Spectral Variations in 4U 1820–30 , 2000, astro-ph/0005496.

[18]  M. Klis.  Millisecond Oscillations in X-ray Binaries , 2000, astro-ph/0001167.

[19]  P. Kaaret,et al.  Strong-Field Gravity and X-Ray Observations of 4U 1820–30 , 1999, astro-ph/9905236.

[20]  Rudy WijnandsMichiel van der Klis.  The broadband power spectra of x-ray binaries , 1998, astro-ph/9810342.

[21]  T. Strohmayer,et al.  Correlation between Energy Spectral States and Fast Time Variability and Further Evidence for the Marginally Stable Orbit in 4U 1820–30 , 1998, astro-ph/9804228.

[22]  N. White,et al.  Discovery of Kilohertz Quasi-periodic Oscillations from 4U 1820–303 with Rossi X-Ray Timing Explorer , 1997 .

[23]  William W. Zhang,et al.  In-orbit performance and calibration of the Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) , 1996, Optics & Photonics.

[24]  D. Psaltis,et al.  Sonic-Point Model of Kilohertz Quasi-periodic Brightness Oscillations in Low-Mass X-Ray Binaries , 1996, astro-ph/9609157.

[25]  Edward H. Morgan,et al.  Dead-Time Modifications to Fast Fourier Transform Power Spectra , 1995 .

[26]  Edward H. Morgan,et al.  Laboratory performance of the proportional counter array experiment for the X-ray Timing Explorer , 1993, Optics & Photonics.

[27]  S. Rappaport,et al.  The evolutionary status of 4U 1820-30 , 1987 .

[28]  B. Geldzahler.  RADIO-FREQUENCY OBSERVATIONS OF GALACTIC X-RAY SOURCES. II. , 1987 .

[29]  W. Priedhorsky,et al.  Discovery of a 176 day period in 4U 1820-30 , 1983 .

[30]  Herbert W. Schnopper,et al.  Discovery of intense x-ray bursts from the globular cluster NGC 6624 , 1976 .

[31]  B. Krauskopf,et al.  Proc of SPIE , 2003 .

[32]  J. Paradijs,et al.  The lives of the neutron stars , 1995 .

[33]  L. Stella,et al.  A 685 second orbital period from the X-ray source 4U 1820-30 in the globular cluster NGC 6624. , 1987 .