The behavior of subluminous X-ray transients near the Galactic center as observed using the X-ray telescope aboard Swift

In this paper we report on the spectral analysis of seven X-ray transients, which were found to be active during a monitoring campaign of the Galactic center carried out in 2006 and 2007 using the X-ray telescope aboard the Swift satellite. This campaign detected new outbursts of five known X-ray transients and discovered two new systems. Their 2-10 keV peak luminosities range from 10³⁴ to 6 × 10³⁶ erg s^-1, which implies that all seven X-ray transients are subluminous compared to the bright X-ray transients that have peak luminosities of 10^37-39 erg s^-1. Two of the sources discussed in this paper are confirmed neutron star systems (AX J1745.6-2901 and GRS 1741-2853), while the five others have an unknown nature. We discuss the characteristics of the observed outbursts and the duty cycles of the various systems. Several of the detected transients seem to undergo enhanced X-ray activity with levels intermediate between quiescence and full outburst. We discuss the possibility that the subluminous appearance of the eclipsing X-ray burster AX J1745.6-2901 is due to line-of-sight effects. We detected two type-I X-ray bursts with a duration of 50-60 s from AX J1745.6-2901, which we discuss in view of the bursting behavior of low-luminosity X-ray transients. Assuming that we are dealing with accreting neutron stars and black holes, we estimate the time-average accretion rate, < dot{M}_long >, of the transients, which is an important input parameter for binary evolution models that attempt to explain the nature of subluminous X-ray transients. Our estimates lie in the range of 3 × 10^-13 {M}_⊙ yr^-1 ⪉ < dot{M}_long > ⪉ 1 × 10^-10 {M}_⊙ yr^-1, if the systems are neutron star X-ray binaries and between 4 × 10^-14 {M}_⊙ yr^-1 ⪉ < dot{M}_long > ⪉ 2 × 10^-11 {M}_⊙ yr^-1 for a scenario where the accreting object is a black hole. Some of the systems have such low estimated mass-accretion rates that they possibly pose a challenge for binary evolution models.