科研之友
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摘要
We report on continued monitoring observations of the Galactic center carried out by the X-ray telescope aboard the Swift satellite in 2008 and 2009. This campaign revealed activity of the five known X-ray transients AX J1745.6-2901, CXOGC J174535.5-290124, GRS 1741-2853, XMM J174457-2850.3 and CXOGC J174538.0-290022. All these sources are known to undergo very faint X-ray outbursts with 2-10 keV peak luminosities of L-X,L-peak similar to 10(34-36) erg s(-1), although the two confirmed neutron star low-mass X-ray binaries AX J1745.6-2901 and GRS 1741-2853 can also become brighter (L-X,L-peak similar to 10(36-37) erg s(-1)). We discuss the observed long-term lightcurves and X-ray spectra of these five enigmatic transients. In 2008, AX J1745.6-2901 returned to quiescence following an unusually long accretion outburst of more than 1.5 years. GRS 1741-2853 was active in 2009 and displayed the brightest outburst ever recorded for this source, reaching up to a 2-10 keV luminosity of L-X similar to 1 x 10(37) (D/7.2 kpc)(2) erg s(-1). This system appears to undergo recurrent accretion outbursts approximately every 2 years. Furthermore, we find that the unclassified transient XMM J174457-2850.3 becomes bright only during short episodes (days) and is often found active in between quiescence (L-X similar to 10(32) erg s(-1)) and its maximum outburst luminosity of L-X similar to 10(36) erg s(-1). CXOGC J174535.5-290124 and CXOGC J174538.0-290022, as well as three other very-faint X-ray transients that were detected by Swift monitoring observations in 2006, have very low time-averaged mass-accretion rates of <(M)over dot >(long) less than or similar to 2 x 10(-12) M-circle dot yr(-1). Despite having obtained two years of new data in 2008 and 2009, no new X-ray transients were detected.
- 出版日期2010-12
