Eddington-limited X-ray bursts as distance indicators. I. Systematic trends and spherical symmetry in bursts from 4U 1728-34

被引:63
作者
Galloway, DK [1 ]
Psaltis, D
Chakrabarty, D
Muno, MP
机构
[1] MIT, Ctr Space Res, Cambridge, MA 02139 USA
[2] MIT, Dept Phys, Cambridge, MA 02139 USA
关键词
equation of state; nuclear reactions; nucleosynthesis; abundances stars : individual (4U 1728-34); stars : neutron; X-rays : bursts; X-rays : individual (4U 1728-34);
D O I
10.1086/375049
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
We investigate the limitations of thermonuclear X-ray bursts as a distance indicator for the weakly magnetized accreting neutron star 4U 1728 - 34. We measured the unabsorbed peak flux of 81 bursts in public data from the Rossi X-Ray Timing Explorer (RXTE). The distribution of peak fluxes was bimodal: 66 bursts exhibited photospheric radius expansion ( presumably reaching the local Eddington limit) and were distributed about a mean bolometric flux of 9.2 x 10(-8) ergs cm(-2) s(-1), while the remaining ( non - radius expansion) bursts reached 4.5 x 10(-8) ergs cm(-2) s(-1), on average. The peak fluxes of the radius expansion bursts were not constant, exhibiting a standard deviation of 9.4% and a total variation of 46%. These bursts showed significant correlations between their peak flux and the X-ray colors of the persistent emission immediately prior to the burst. We also found evidence for quasi-periodic variation of the peak fluxes of radius expansion bursts, with a timescale of similar or equal to40 days. The persistent flux observed with RXTE/ASM over 5.8 yr exhibited quasiperiodic variability on a similar timescale. We suggest that these variations may have a common origin in reflection from a warped accretion disk. Once the systematic variation of the peak burst fluxes is subtracted, the residual scatter is only similar or equal to3%, roughly consistent with the measurement uncertainties. The narrowness of this distribution strongly suggests that ( 1) the radiation from the neutron star atmosphere during radius expansion episodes is nearly spherically symmetric and ( 2) the radius expansion bursts reach a common peak flux that may be interpreted as a standard candle intensity. Adopting the minimum peak flux for the radius expansion bursts as the Eddington flux limit, we derive a distance for the source of 4.4 - 4.8 kpc (assuming R-NS = 10 km), with the uncertainty arising from the probable range of the neutron star mass M-NS = 1.4 - 2M(.).
引用
收藏
页码:999 / 1007
页数:9
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