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Hard X-ray spectral and timing properties of IGR J17454-2919 consistent with a black hole in the hard state

ATel #6574; Shriharsh P. Tendulkar (Caltech), Matteo Bachetti (INAF/OAC), John A. Tomsick (SSL/UCB), Jerome Chenevez (DTU Space, Denmark), Fiona Harrison (Caltech) on behalf of the NuSTAR binaries team
on 13 Oct 2014; 20:10 UT
Credential Certification: Shriharsh Tendulkar (spt@astro.caltech.edu)

Subjects: X-ray, Request for Observations, Black Hole, Neutron Star, Transient

Referred to by ATel #: 6602, 7020, 7096

IGR J17454-2919 is a recently discovered X-ray transient that lies 24 arcminutes away from the Galactic Center (ATel #6530). NuSTAR observed the source starting at 2014 October 10, 9.17 h UT, and obtained an exposure time time of 29 ks. An inspection of the 3-79 keV light curve does not show any evidence for bursts or pulsations, but the NuSTAR count rate drops from 2.8 c/s to 2.4 c/s (rates are for one NuSTAR module with no deadtime correction). We performed spectral and timing analysis of the NuSTAR data. The energy spectrum is well described (reduced chi2 = 1.06 for 748 dof) by an absorbed power-law with an exponential cutoff and a broad, asymmetric, iron emission line. The column density is (3.3+/-0.6)e22 cm-2 (using Anders & Grevesse 1989 abundances; errors on all spectral parameters are 90% confidence), the photon index is 1.46+/-0.06, and the e-folding energy is >100 keV (with the "cutoffpl" model in XSPEC). The unabsorbed 3-79 keV flux is 3.96e-10 erg/cm2/s with a 0.8% error. At the Galactic center distance, this corresponds to an isotropic luminosity of 3e36 erg/s. The power spectrum consists of two components: a zero-centered Lorentzian peaking near 1 Hz and a power-law at lower frequencies. The Lorentzian has a width of 2 Hz and a fractional rms of 25+/-3%. The hard power-law index, the high energy of the cutoff, and the level of variability all are consistent with properties expected for an accreting black hole in the hard state. While we cannot completely rule out the possibility of a low magnetic field neutron star, a black hole is more likely.