Articles related to our science case:


Chandra Observations of the Bursting X-Ray Transient SAX J1747.0-2853
during Low-Level Accretion Activity
Wijnands, Rudy; Miller, Jon M.; Wang, Q. Daniel
http://adsabs.harvard.edu/abs/2002ApJ...579..422W
Abstract:
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^m02.60^s 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 0.5-10 keV luminosity of the source during our observations was ~3×10³⁵ 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 10³⁵ ergs s^-1, a poorly studied accretion rate regime.



Sedimentation and Type I X-Ray Bursts at Low Accretion Rates
Peng, Fang; Brown, Edward F.; Truran, James W.
http://adsabs.harvard.edu/abs/2007ApJ...654.1022P
Abstract:

Neutron stars, with their strong surface gravity, have interestingly short timescales for the sedimentation of heavy elements. Motivated by observations of Type I X-ray bursts from sources with extremely low persistent accretion luminosities, L_X<10³⁶ ergs s^-1 (~=0.01L_Edd), we study how sedimentation affects the distribution of isotopes and the ignition of H and He in the envelope of an accreting neutron star. For local mass accretion rates m˙<~10^-2m˙_Edd (for which the ignition of H is unstable), where m˙_Edd=8.8×10⁴ g cm^-2 s^-1, the helium and CNO elements sediment out of the accreted fuel before reaching a temperature at which H would ignite. Using one-zone calculations of the thermonuclear burning, we find a range of accretion rates for which the unstable H ignition does not trigger unstable He burning. This range depends on the emergent flux from reactions in the deep neutron star crust; for F=(0.1 MeV)(m˙/m_u), the range is 3×10^-3m˙_Edd<~m˙<~10^-2m˙_Edd. We speculate that sources accreting in this range would build up a massive He layer that would later produce an energetic and long X-ray burst. At mass accretion rates lower than this range, we find that the H flash leads to a strong mixed H/He flash. Surprisingly, even at accretion rates m˙>~0.1m˙_Edd, although the H and He do not completely segregate, the H abundance at the base of the accumulated layer is still reduced. While following the evolution of the X-ray burst is beyond the scope of this introductory paper, we note that the reduced proton-to-seed ratio favors the production of ¹²C-an important ingredient for subsequent superbursts.