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RXTE/PCA detection of QPO near 0.22 Hz in Aql X-1 during ongoing outburst

ATEL # 2902; Mayukh Pahari (TIFR, India), Sabyasachi Pal (ICRAR-UWA) and Arunava Mukherjee (TIFR, India)
on 4 Oct 2010; 6:40 UT
Password Certification: Sabyasachi Pal (

Subjects: X-ray, Binaries, Neutron Stars, Transients, Stars
Referred to by ATEL #: 2911

We have analyzed eight RXTE/PCA archived observations (2-15 September 2010) of the current outburst of the neutron star Aql X-1 occurred during September 2010 (ATEL #2850, #2871). We have detected QPO near 0.22 Hz in the power density spectrum (with the frequency bin of 1/256 Hz) in one observation on 2nd September 2010 with a significance of 5.1sigma. The other parameters of the detected QPO feature are given below:

Central frequency 0.22+/-0.01 Hz
Width 0.041+/-0.008 Hz
Quality factor 5.36+/-1.11
rms amplitude 0.043+/-0.005

The background noise in the power spectrum is modeled with a constant+power-law combination with the constant value of 1.98+/-0.01 and power-law index of -0.54+/-0.06. The comparison of model combinations, namely, constant+power-law and constant+power-law+gaussian (at the position of QPO) yields the null hypothesis probability of the computed F-statistics to be 3.05 x 10^(-7).

We fitted spectra of all 8 observations. Combination of absorbed black-body and power-law is found to be best model among all other choices. We have detected Fe fluorescence emission line near 6.4 keV with a significance of 5.8sigma. We found that the outburst started in low-hard state with luminosity (calculated between 2.0-60.0 keV with a distance assumption of 5 kpc (Rutledge et al. 2001, ApJ, 559,1054)) of 6.81 x 10^(35) ergs/sec and reached the peak luminosity of 1.76 x 10^(38) ergs/sec on 13 September 2010. From the low-hard to the peak, the blackbody temperature is found to increase from 0.61+/-0.06 keV to 2.23+/-0.25 keV and the power law index is found to increase from 1.75+/-0.05 to 4.32+/-0.17. However, the hydrogen column density is found to increase significantly with the progress of outburst. As a result, from the low-hard to the peak, the relative contribution of the blackbody flux increases from 10% to 15% only. ASM flux (2.0-12.0 keV) reaches its peak on 16th Sept 2010 and currently the flare is in decay stage.

More multi-wavelength observations are encouraged.

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R. E. Rutledge , Editor
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