We apply the model of subsonic settling accretion on to isolated neutron stars accreting from the interstellar medium (AINS). We show that in this regime the expected mean X-ray luminosity from AINS turns out to be 2-3 orders of magnitude as small as the maximum possible Bondi value, i.e. $10^{ 27}-10^{ 28}$~erg s$^{ -1}$. The intrinsically unstable character of settling accretion due to long plasma cooling time leads to regular appearance of X-ray flares with a duration of about one hour and a maximum luminosity of about the Bondi value, $\sim10^{ 31}$~erg~s$^{ -1}$. This feature can be used to distinguished AINS from other dim X-ray sources. With the sensitivity of the forthcoming all-sky X-ray surveys the expected number of the potentially detectable AINS can be from a few to ten.
Categories: astro-ph.HE