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ATEL # 2821; O. Godet, D. Barret, N. Webb (CESR, France) & S. Farrell (Univ. of Leicester, England)
on 30 Aug 2010; 20:18 UT
Password Certification: Jamie A. Kennea (firstname.lastname@example.org)
Subjects: X-ray, Black Holes
We report on results from our current X-ray monitoring of the Hyper Luminous X-ray source (HLX-1) in the galaxy ESO 243-49 with the Swift-XRT (Farrell et al. 2009, Nature, 460, 73, Wiersema et al. 2010, ApJL, in press, arXiv:1008.4125). The Swift-XRT Photon-Counting light-curve exhibits large variability. During our first Swift-XRT observation on 2008-10-24, the source count rate was around 0.013 c/s. Our second Swift-XRT observation on 2009-05-08 revealed that HLX-1 became much fainter with a count rate around 0.0012 c/s before rebrightening and peaking at 0.033 c/s on 2009-08-16. The source count rate then decreased up to 2010-01-29 showing a small rebrightening around 2010-01-08 and then dropped suddenly. The source count rate went as low as 0.0006 c/s between 2010-02-10 and 2010-08-13. Our recent data collected on Sunday 29th of August 2010 reveals that HLX-1 recently became brighter reaching a count rate of ~0.030 c/s. The Swift-XRT Photon-Counting light-curve is shown at the following link: http://www.star.le.ac.uk/~saf28/Data/hlx1lc.gif . It was produced using the Swift-XRT light-curve generator with a count binning set at 40 counts per bin (Evans et al. 2007, A&A, 469, 379).
The source also appears to be spectrally softer than in the low count rate observations. The use of an absorbed disk blackbody model to fit the spectrum gives a good fit (chi^2/dof = 11.1/9) with kT = 0.23 +0.03/-0.02 keV and an unabsorbed 0.2-10 keV luminosity of ~1.3 x 10^42 erg/s for an assumed N_H of 4 x 10^20 cm^-2. This luminosity and temperature are consistent with those obtained during the last luminosity peak of the source in August 2009 (Godet et al. 2009, ApJL, 705, 109).
Follow-up observations at other wavelengths are highly encouraged.
We thank Neil Gehrels and the Swift team for scheduling the XRT monitoring of HLX-1.