CHANDRA OBSERVATIONS OF THE BURSTING X-RAY TRANSIENT SAX J1747.0-2853 DURING LOW-LEVEL ACCRETION ACTIVITY

摘要

We present Chandra/ACIS observations of the bursting X-ray transient SAX J1747.0-2853 performed on 2001 July 18. We detected a bright source at the position of R.A. = 17~h47~m02~S.60 and decl. = -28°52'58''.9 (J2000.0, with a 1 σ error of ～0''.7), consistent with the BeppoSAX and ASCA positions of SAX J1747.0-2853 and with the Ariel V position of the transient GX +0.2-0.2, which was active during the 1970s. The 0.5-10keV luminosity of the source during our observations was ～ 3 x 10~(35) ergs s~(-1) (assuming a distance of 9 kpc), demonstrating that the source was in a low-level accretion state. We also report on the long-term light curve of the source as observed with the all-sky monitor on board the Rossi X-Ray Timing Explorer. After the initial 1998 outburst, two more outbursts (in 2000 and 2001) were detected with peak luminosities about 2 orders of magnitude larger than our Chandra luminosity. Our Chandra observation falls between those two outbursts, making the outburst history for SAX J1747.0-2853 complex. Those bright 2000 and 2001 outbursts, combined with the likely extended period of low-level activity between those outbursts, strongly suggest that the classification of SAX J1747.0-2853 as a faint X-ray transient was premature. It might be possible that the other faint X-ray transients can also exhibit bright, extended outbursts that would eliminate the need for a separate subclass of X-ray transients. We discuss our results also in the context of the behavior of X-ray binaries accreting at low levels with luminosities around 10~(35) ergs s~(-1), a poorly studied accretion rate regime.