The Swift-XRT catches a possible rebrightening of the best intermediate mass black hole candidate, ESO 243-49 HLX-1

ATel #4327; O. Godet, N. Webb, D. Barret (IRAP, France), S. Farrell (Univ. of Sydney, Australia), N. Gerhels (NASA/GSFC, USA) & M. Servillat (CEA-Saclay, France)
on 22 Aug 2012; 13:39 UT
Credential Certification: Olivier Godet (godet@cesr.fr)

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We report on new results from our monitoring of HLX-1 with the Swift-XRT designed to catch the rise of the 2012 outburst. HLX-1 (Hyper Luminous X-ray source 1) in the galaxy ESO 243-49 is the best intermediate mass black hole candidate (Farrell et al. 2009, Nature, 460, 73; Wiersema et al. 2010, ApJL, 721, 102). The source displays properties similar to those observed in Galactic BH binaries: i) hard-to-soft transitions, but with a luminosity 10-100 larger (Godet et al. 2009, ApJ, 705, 109; Servillat et al. 2011, ApJ, 743, 6); ii) the first detection of variable radio emission associated with hard-to-soft transitions; which is consistent with discrete jet ejection (Webb et al. 2012, Science, in press). The Swift-XRT light-curve over the past 3 years shows 3 well sampled FRED-like outbursts separated by roughly a year (Lasota et al. 2011, ApJ, 735, 89). The latter authors proposed that HLX-1 is an eccentric binary system with a period of roughly a year in which an IMBH accretes matter from a tidally stripped companion star when passing at its periastron.

From our Swift-XRT monitoring campaign triggered from 2012-07-12, we observe a possible rebrigthening of HLX-1 since 2012-08-(19-21). Before that time, the source was in the low/hard state with a 0.3-10 keV count rate around 0.0005-0.001 c/s. The time of this rebrightening is consistent with a recurrence timescale of 370-380 (Lasota et al. 2011, Servillat et al. 2011). The count rate is (9.5 +/-3.9)e-3 c/s at 2012-08-21. The Swift-XRT light-curve (LC.png) is posted at the following link: http://userpages.irap.omp.eu/~ogodet/ in the directory HLX-1. We used a dynamical binning of at least 20 counts per bin.

We analysed the Swift-XRT data collected from 2012-08-12 to 2012-08-21 (i.e. ~3.7 ks). The background subtracted spectrum reveals 6 counts in the 0.3-1.5 keV band and 5 counts in the 1.5-10 keV band. Assuming an absorbed powerlaw model with N_H=4e20 cm^-2 and Gamma = 2.2 (fixed -- see Servillat et al. 2011; Godet et al. 2012, ApJ, 752, 34), we found a 0.3-10 keV observed flux of (1.2 +0.8/-0.7)e-13 erg/cm^2/s. This corresponds to an unabsorbed 0.2-10 keV luminosity of (1.7 +0.8/-0.7)e41 erg/s using a source distance of 95 Mpc.

Further Swift-XRT data are planned. Follow-up observations at other wavelengths are highly encouraged.