IGR J17091-3624 might harbor a rapidly rotating black hole as well as a persistent ultra-fast outflow

ATel #4382; R. C. Reis, J. M. Miller, A. L. King, M . T. Reynolds (University of Michigan)
on 17 Sep 2012; 16:50 UT
Credential Certification: Rubens C. Reis (rdosreis@umich.edu)

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We report on a XMM-Newton Epic-PN observation of the stellar mass black hole candidate IGR J17091-3624. The source was observed (ObsID: 0677980201,) for a total of ~39ks. We extracted a spectrum from a box region having RAWX 20px wide centered on the source and RAWY<140. Background region was extracted from RAWX columns between 3 and 8. This resulted in a good exposure time of 1.14ks. The spectrum was binned to have a minimum of 20 counts per channel and the resolution oversampled by a factor of 2. A simple (absorbed) disk blackbody (DiskBB) plus powerlaw does not provide a satisfactory fit (reduced chi2 = 1.38 for 104 DoF) to the 0.5-10 keV (ignoring 1.7-2.3keV) energy range, with clear presence of a number of potential absorption features including a distinct feature at ~7.1keV. Adding the same photoionization model presented for this source in King et al. (2012, ApJ, 746, L20), improves the fit slightly (Deltachi2 = 7 for 3 DoF) however, a much stronger improvement occurs after the inclusion of a relativistic emission line (LAOR) with reduced chi2 = 1.17 for 96 DoF. An Ftest shows that the improvement over no emission feature is significant at the 99.8% level. The best fit (please see link below for the figure) suggest that the system was caught in a state akin to the soft-intermediate state, with disk (KT = 1.12+\-0.02keV) and powerlaw (gamma = 1.91+0.16-0.19) 0.5-10keV fluxes of (2.00+0.15-0.11)E-9erg/cm2/s and 1.0+\-0.2E-9erg/cm2/s respectively. The ionization parameter and wind-column density are constrained to log(xi) = 3.4+1.1-0.1(erg cm/s) and (2+4-1) E+22 /cm2. With an outflow velocity of 12000+1550-3370 km/s, this XMM-Newton observation taken nearly 4 months prior to the Chandra observation reporting the presence of an ultra-fast outflow (UFO) suggest that the UFO lasted at least this long. From the relativistic emission line (E=6.42+0.25keV; lower limit bound at 6.4keV) we infer that the accretion disk has an inclination less than 62 degrees and emissivity index Q<7.4. Finally, the inner radius is constrained to be <2.3GM/c2, which, assuming to be the radius of the innermost stable circular orbit (ISCO), implies a black hole with a spin a*>~0.9. This high spin is contrary to results obtained by Rao and Vadawale (2012, ApJ, 757, L12) where, based on the continuum fit, the authors find the black hole to be either spinning slowly or possibly even in a retrograde manner. The equivalent width of the emission line is found to be ~500eV which, assuming an iron over abundance of x2 (as found in King et al. 2012) implies a reflection fraction R~1.4, thus supporting the association of this observation with the soft-intermediate state where often the reflection fraction is >1. All errors are at the 90% level of confidence obtained by allowing all parameters to vary.

Figure showing the unfolded spectrum together with the residual to the best-fit model.