Tsing Hua Univ, Dept Engn Phys, Beijing 100084, Peoples R China
; Tsing Hua Univ, Ctr Astrophys, Beijing 100084, Peoples R China
; Tsing Hua Univ, Dept Phys, Beijing 100084, Peoples R China
; Chinese Acad Sci, Inst High Energy Phys, Key Lab Particle Astrophys, Beijing 100049, Peoples R China
Hardness ratios are commonly used in X-ray photometry to roughly indicate spectral properties. They are usually defined as the ratio of counts in two different wave bands. This definition, however, is problematic when the counts are very limited. Here we instead define the hardness ratio using the lambda-parameter of Poisson processes and develop an estimation method via Bayesian statistics. Our Monte Carlo simulations show the validity of our method. Based on this new definition we can estimate the hydrogen column density for the photoelectric absorption of X-ray spectra in the case of low counting statistics.