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D. K. Galloway (MIT), E. Kuulkers (ESA-ESTEC), L. Bildsten (ITP, UCSB), D. Chakrabarty (MIT), A. Cumming (UCO/Lick Observatory and UC Santa Cruz)
We present results from analysis of 19 thermonuclear (type I) X-ray bursts from GS 1826-24, observed by the Rossi X-ray Timing Explorer (RXTE) in two epochs between 1997 November - 1998 June, and 2000 June - September. The burst properties were remarkably consistent within each epoch, with no significant variations found in the rise or decay times. However, between 1997-98 and 2000 we measured a significant increase in the burst decay timescale, combined with a weakly-signficant decrease in the peak burst flux, which resulted in a constant integrated burst fluence over all the bursts.
As has been observed previously, the bursts occurred with remarkable regularity. The 7 bursts observed between 1997-98 were consistent with a recurrence time of 5.7 hr; however, the 12 bursts observed in 2000 appeared to recur on a much shorter interval, around 4.1 hr. Pointed RXTE Proportional Counter Array measurements indicate that the persistent intensity of the source increased between 1997-2000 by around 36%; RXTE All-Sky Monitor measurements indicate that this is part of a long-term trend. The decrease in burst interval is exactly as predicted by theoretical models involving complete consumption of the available H/He fuel, for the increase in accretion rate inferred from the persistent flux measurements.
More recent measurements by Chandra and RXTE in 2002 July found an even shorter burst interval of 3.5 hr, as the persistent flux continues to increase, again in line with theoretical predictions. GS 1826-24 is an ideal test case for studies of burst physics in the high-accretion rate regime.
This work was supported in part by the NASA Long Term Space Astrophysics program under grant NAG 5-9184.
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Bulletin of the American Astronomical Society,
© 2003. The American Astronomical Soceity.