Probing the Geometry of the Comptonizing Coronae of Neutron Star LMXBs

Neutron star (NS) low-mass X-ray binaries (LMXBs) are widely known to exhibit thermal Comptonization phenomena by hot electron coronae both in the low-hard and high-soft states. The Comptonizing coronae are considered to locate in the vicinity of the NS, with a disk-like shape in the soft state (e.g., Inogamov+1999), and nearly spherical in the hard state (e.g., Sakurai+2012). However, further details of the coronal geometry are not yet clear. In this study we aimed to probe possible anisotropy of the coronal shape, by comparing the Comptonization strength (namely the Compton y-parameter) of dipping and non-dipping LMXBs. The difference of these two categories are, the former must be observed from higher inclinations. For this comparison we induce a new parameter Q=(the electron temperature)/(the seed photon temperature), which is a good indicator of the source spectral state. With a wide study of the broadband spectra of dipping and non-dipping LMXBs from Suzaku archived data, we found that all the LMXBs focus towards a common curve on the (Q,y) diagram, while the dippers among them seem to locate in slightly higher positions with larger Compton y-parameters. This suggests that the higher inclination binary systems may exhibit systematically stronger Comptonization. The results further indicate that the Comptonizing coronae may have oblate shapes, thus their optical depths increase when viewed from higher inclinations (Zhang+2014, in preparation).