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Recent Swift XRT Observations of GX 339-4

ATEL # 1588; A.K.H. Kong (NTHU, Taiwan)
on 24 Jun 2008; 19:33 UT
Password Certification: Albert Kong (akong@cfa.harvard.edu)

Subjects: X-ray, Black Holes, Transients

Following the discovery of the unusual optical and X-ray flaring activity in GX 339-4 (ATel #1586), we analyzed the two recent Swift XRT observations of the source taken on 2008 June 20 and 24. GX 339-4 is clearly detected in both observations. The first observation is 1985 sec long and the spectrum can be fit with an absorbed power-law model with a hydrogen column density of (4.1+/-1.1)e21 cm^-2 and a photon index of 1.45+/-0.15. The unabsorbed 2-10 keV flux is 4e-11 erg/s/cm^2. From the first orbit of the second observation (546 sec exposure time), the spectrum can be fit with an absorbed power-law model with a hydrogen column density of (3.5+/-2.0)e21 cm^-2 and a photon index of 1.44+/-0.25. The unabsorbed 2-10 keV flux is 5.9e-11 erg/s/cm^2. Assuming a distance of 8 kpc, the 2-10 keV luminosity is (3-4.5)e35 erg/s.

The X-ray spectral fits are consistent with the "off" state of the source and the flux is now at the upper end of the "off" state (Kong et al. 2000; Corbel et al. 2003). Since the optical flux shows sign of increasing activity (ATel #1586) and if the X-ray flux continues to increase at the current rate, the source is expected to enter the low/hard state in about 10 days. Observations at other wavelengths are encouraged.


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