XMM-Newton and INTEGRAL observations of the very faint X-ray transient IGR J17285-2922/XTE J1728-295 during the 2010 outburst

We report the first broad-band (0.5-150 keV) simultaneous X-ray observations of the very faint X-ray transient IGR J17285-2922/XTE J1728-295 performed with the XMM-Newton and INTEGRAL satellites during its last outburst, started on 2010 August 28. XMM-Newton observed the source on 2010 September 9-10 for 22 ks. INTEGRAL observations were part of the publicly available Galactic bulge program, and overlapped with the times covered by XMM-Newton. The broad-band spectroscopy resulted in a best fit with an absorbed power law displaying a photon index Γ of 1.61 ± 0.01, an absorbing column density N_H of (5.10 ± 0.05) × 10²¹ cm^-2 and a flux of 2.4 × 10^-10 erg cm^-2 s^-1 (1-100 keV), corrected for the absorption. The data did not require either a spectral cut-off (E_c > 50 keV) or an additional soft component. The slopes of the power law fitting the XMM-Newton and INTEGRAL separate spectra were compatible, within the uncertainties. The timing analysis does not show evidence either for X-ray pulsations or for type I X-ray bursts. The broad-band X-ray spectrum as well as the power-density spectrum is indicative of a low-hard state in a low-mass X-ray binary, although nothing conclusive can be said about the nature of the compact object (neutron star or black hole). The results we report here allow us to conclude that IGR J17285-2922 is a low-mass X-ray binary, located at a distance greater than 4 kpc.