XTE J1906+090: a persistent low luminosity Be X-ray Binary

Abstract: We present new results from INTEGRAL and Swift observations of the hitherto poorly studied and unidentified X-ray source XTE J1906+090. A bright hard X-ray outburst (luminosity of $\sim$10$^{36}$ erg s$^{-1}$ above 20 keV) has been discovered with INTEGRAL observations in 2010, this being the fourth outburst ever detected from the source. Such events are sporadic, the source duty cycle is in the range (0.8--1.6)% as inferred from extensive INTEGRAL and Swift monitoring in a similar hard X-ray band. Using five archival unpublished Swift/XRT observations, we found that XTE J1906+090 has been consistently detected at a persistent low X-ray luminosity value of $\sim$10$^{34}$ erg s$^{-1}$, with limited variability (a factor as high as 4). Based on our findings, we propose that XTE J1906+090 belongs to the small and rare group of persistent low luminosity Be X-ray Binaries.