Suzaku observation of the symbiotic X-ray binary IGR J16194-2810

We observed IGR J16194-2810 in the low/hard state with the Suzaku X-ray satellite in 2009. The source is a Symbiotic X-ray Binary (SyXB) classified as a category of a Low-Mass X-ray Binary (LMXB), since the system is composed of an M-type giant and probably a neutron star (NS). We detected the 0.8-50 keV signal with the XIS and HXD-PIN. The 2-10 keV luminosity was L ∼ 7 × 10³⁴ erg s^-1 corresponding to ∼ 10^-3L_Edd, where L_Edd is the Eddington luminosity of a 1.4 M_⊙ NS and a source distance of 3.7 kpc is assumed. The luminosity is similar to those of past observations. The spectral analysis showed that there are two emission components below and above ∼ 2 keV. The hard emission component is represented by a Comptonized blackbody emission model with seed-photon temperature ∼ 1.0 keV and emission radius ∼ 700 m. The seed photon is considered to come from a small fraction of the NS surface. The soft component is reproduced by either a raw blackbody (∼ 0.4 keV, ∼ 1.7 km) or a Comptonized emission (∼ 0.1 keV, ∼ 75 km). We think that the origin is the emission from another part of the NS surface or the accreting stream. The physical parameters of the hard emission component of IGR J16194-2810 are compared with those of an SyXB (4U 1700+24) and two LMXBs (Aql X-1 and 4U 0614+091). This comparison reveals that these SyXBs in the low/hard state have a smaller radiation region (< 1 km) on the NS surface with a higher seed-photon temperature (∼ 1 keV) than the comparison LMXBs.