New outburst from the best intermediate mass black hole candidate HLX-1 in ESO 243-49: evidence for an approximate recurrence timescale of a year

ATel #3569; O. Godet (CESR, France), S. Farrell (Univ. of Sydney, Australia), D. Barret, N. Webb (CESR, France) & M. Servillat (Cfa Harvard University, US)
on 18 Aug 2011; 16:15 UT
Credential Certification: Jamie A. Kennea (kennea@astro.psu.edu)

Tweet

We report on new results from our current X-ray monitoring of the best intermediate mass black hole candidate, 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, 721, 102). The source displays spectral hysteresis (transitions from the low/hard state to the thermal state) consistent with those observed in Galactic BH binaries (GBHBs), but with a luminosity 10-100 larger (Godet et al. 2009, ApJ, 705, 109; Servillat et al. 2011, accepted for publication in ApJ). The Swift-XRT light-curve over the past 2.5 years shows 2 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 companion star passing at periastron.

Our Swift-XRT Photon-Counting data collecting on 2011-08-(15-17) show the start of a new outburst. The time of this new outburst is consistent with a recurrence timescale of ~367 days. The count rate increases fast over the past few days reaching around 0.03 c/s on 2011-08-17. The peak count rate for the past two outbursts reached 0.033 c/s. Before this new outburst, the source was in the low/hard state during the past ~170 days. The source was not detected on 2011-08-08; which gives a 3-sigma upper limit of 1e-3 c/s. The Swift-XRT light-curve is posted at the following link: http://www.physics.usyd.edu.au/~farrell/curve_HLX-1.gif

Our spectral analysis of 1.5ks of Swift-XRT data collected on 2011-08-17 showed that the source spectrum appears to be soft. It is well fitted using an absorbed DISKBB model with N_H=4e20 cm^-2 (fixed) and kT=0.20 +0.06/-0.04 keV. The 0.2-10 keV unabsorbed luminosity is then 1.3e42 erg/s. This is similar to that observed at the peak of the previous outburst. Using the same spectral parameters, we found a 0.2-10 keV unabsorbed luminosity from data collected on 2011-08-15 & 2011-08-16 around ~5.9e41 erg/s. Further Swift-XRT data are planned in the next days. 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.