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Softening of state during recent X-ray outburst of LMC X-3 as detected by RXTE

ATEL # 2230; Mayukh Pahari (TIFR), Sabyasachi Pal (ICRAR/UWA)
on 6 Oct 2009; 8:19 UT
Password Certification: Sabyasachi Pal (

Subjects: X-ray, Request for Observations, Binaries, Black Holes

The All Sky Monitor (ASM), onboard RXTE, has recently detected the rise in ASM flux (2.0-12.0 keV) from the black hole candidate LMC X-3. The burst started on 29 July 2009 (MJD 55043), when the ASM flux was ~2 cts/s and it reached the peak value on 7 September 2009 (MJD 55084), when the ASM flux showed ~6 cts/s and remained there for 4-5 days and then started decaying. The ASM flux of the source is still showing high luminosity with ~4 cts/s recorded on MJD 55105.

We have analysed all PCA observations of the source taken between 7 July 2009 and 7 September 2009. In hard color-magnitude diagram (HR2=13.0-60.0 keV flux/2.1-5.2 keV flux), 29 July 2009 showed the hardest state where average HR2 was ~ 0.62 and PCA flux ~ 61 cts/s/PCU (2.0 keV-60.0 keV). Then the source gradually moved to higher flux and lower HR2 value and on 7 September 2009, it became softer. The average HR2 value became ~ 0.31 and PCA flux ~ 116 cts/s/PCU. Interestingly, no significant change in soft color (HR1=5.6-12.8 keV flux/2.1-5.2 keV flux) has been observed throughout the evolution of the burst. We have also done the spectral fitting of all standard-2 PCA spectrum with multi-colored disk black body (diskbb) and power-law model in addition with small Gaussian fixed at 6.4 keV. All of them are modified by the presence of interstellar medium having column density ~ 3.2 x 10^20 cm^(-2). On 7 July 2009, the spectrum was steep power-law dominated (PI ~ 3.44 +/- 0.05) with the presence of hard power-law tail. The power-law flux (3.0-26.0 keV) was ~74%-79% of the total flux and diskbb flux ~15%-20% of total flux. While on the higher end, the spectrum of 7 September 2009 showed flatter power-law (PI ~ 4.29 +/- 0.019) with 45% power-law flux and 55% diskbb flux of the total flux. Interestingly, disk temperature remained almost same (~ 0.88 +/- 0.031 keV) throughout the burst.

Multi band observations are encouraged to study interesting features of this source.

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R. E. Rutledge , Editor
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