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Detection of 27 Hz QPO and type-I X-ray burst during state transition from recent RXTE-PCA observation of 4U 1820-303

ATEL # 2167; Mayukh Pahari (TIFR) and Sabyasachi Pal (UWA)
on 20 Aug 2009; 11:59 UT
Password Certification: Sabyasachi Pal (

Subjects: Binaries, Neutron Stars, Variables

A recent state transition of low mass X-ray binary 4U 1820-303, from long low-hard state to high-soft state, has been detected by the RXTE All Sky Monitor (ASM) and Proportional Counter Array (PCA) observations between 12 June 2009 (MJD 54994) and 16 June 2009 (MJD 54998). The source remained in low hard state for two months from 10 April 2009 (ATEL #2071) when ASM counts were ~8 and PCA measured ~ 355 counts/s/PCU in 2-60 keV. The light curve was more or less constant during low hard state except the detection of one 16 second duration type-I X-ray burst on 27 April 2009 (MJD 54948) with corresponding peak counts of ~ 4600 counts/s/PCU. The power density spectrum, observed on 27 April, showed high low-frequency noise below 1 Hz and no low frequency quasi-periodic oscillation (QPO) has been detected up to 3-sigma significance.

Analysing the data of 15 June 2009 (MJD 54497), when the source regained its high soft state, we have detected 27.34 Hz QPO with 4-sigma significant level. Very broad low-frequency noise was also found which was peaked near 10 Hz after lorentzian fitting. The QPO was also visible in 20 June 2009 (MJD 54502) PCA observation with ~ 3.5 sigma significance. Highly variable count rate (ASM count rate ~ 30 and PCA measures ~ 940 counts/s/PCU in 2-60 keV) in high soft state demonstrate emission instability. On 12 June 2009 (MJD 54495) another type-I burst with ~ 20 second duration and ~ 5500 counts/s/PCU peak value has been detected.

The PCA spectral analysis has been performed using two additive components, cutoff power-law (cutoffpl) and black-body (bbody). Both of these components were modified by interstellar absorption with nH ~ 1.1 x 10^22/cm^2 and a small gaussian ~ 6.4 keV has been added for better fit. 2% systematic error has been introduced during fitting. On 27 April 2009, when the source was entirely in low hard state, the spectral fit showed the photon index to be 1.74 +/- 0.05 and high energy cut-off at 12 +/- 1 keV and the black-body temperature was found to be 3.73 +/- 0.08 keV with normalization value 0.0033. When the source moved to high soft state on 15 June 2009, the photon index became 3.12 +/- 0.02 keV, the cut off energy shifted to 25 +/- 2 keV and the black body temperature became 2.31 +/- 0.07 keV with normalization value 0.0236. Reduced chi-square of the fit was found to be ~1.2. During state transition, a shift in HR2 values (defined as 13.0-60.0 keV/2.0-6.0 keV) was found from ~ 1.2 in low-hard state to ~ 0.8 in high-soft state.

Further observations of the source at all possible frequencies are encouraged for detail understanding of the evolution of the source.

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R. E. Rutledge , Editor
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