ESO 243-49 HLX-1: Possible outburst delayed by nearly a month

ATel #5439; O. Godet, N. Webb, D. Barret (IRAP, France), S. Farrell (Univ. of Sydney, Australia), N. Gehrels (GFSC, USA), M. Servillat (CEA, France), R. Soria (Curtin Inst. of Radio Astronomy, Australia)
on 3 Oct 2013; 13:17 UT
Credential Certification: Olivier Godet (godet@cesr.fr)

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We report on new results from our Swift-XRT monitoring of the best intermediate mass black hole candidate HLX-1 (Hyper Luminous X-ray source 1 - Farrell et al. 2009; Wiersema et al. 2010) in the galaxy ESO 243-49. HLX-1 displays properties similar to those observed in Galactic BH binaries: i) hard-to-soft transitions (Godet et al. 2009; Servillat et al. 2011); ii) the first detection of a radio jet-ejection emission associated with hard-to-soft transitions (Webb et al. 2012). The Swift-XRT light-curve over the past 4 years shows 4 well sampled FRED-like outbursts separated by nearly a year (from ~350 to ~370 days). The duration of the outbursts have decreased annually from ~170 days in 2009 to ~93 days in 2012. The average peak count rate also appears to have decreased from ~0.03 cts/s in 2009 to ~0.02 cts/s in 2012. Between the outbursts, the source stays in the low/hard state. In Lasota et al. (2011), we proposed that the X-ray lightcurve is due to mass transfer episodes when a companion star orbiting the IMBH in a ~1-yr eccentric orbit passes at periapse and is tidally stripped. From our Swift-XRT monitoring campaign triggered from mid-July 2013, we observe a possible re-brightening of the source on 2nd October 2013. The 0.3-10 keV count rate at this date is equal to 0.0096 +/- 0.0036 (1 sigma) ct/s. So, at the 2 sigma level, the count rate is consistent with the count rate measured so far in the low/hard state (0.001-0.002 ct/s). The background subtracted spectrum reveals 5 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, we found a 0.3-10 keV observed flux of < 3e-13 erg/cm^2/s. If the data point on 2nd October 2013 really marks the beginning of the outburst, this outburst is delayed compared to the previous outbursts by about a month (the 2012 outburst occurred 407 days ago). This appears to be difficult to explain using the eccentric binary scenario described above. We thank the Swift team for scheduling these observations. Further Swift-XRT data are planned. Follow-up observations at other wavelengths are highly encouraged.