ASCA Spectroscopy of the Globular Cluster X-ray Binary X1820$-$303

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Session 9 -- Galactic X-Ray/Gamma-Ray Sources
Display presentation, Monday, 30, 1994, 9:20-6:30

[9.05] ASCA Spectroscopy of the Globular Cluster X-ray Binary X1820$-$303

A.~P.~Smale (USRA, NASA/GSFC), T.~Dotani, K.~Mitsuda, \& G.~Zylstra (ISAS)

X1820$-$303 is a low-mass binary situated in the core of the globular cluster NGC6624. Its orbital period of 11 minutes is the shortest known of any LMXB, and in addition the system shows QPO and burst behavior. X1820$-$303 was observed by {\sl ASCA} on 1993 October 6 05:14 UT for 22 hrs. The deadtime-corrected source flux of 3.7$\times$10$^{37}$ erg~cm$^{-2}$~s$^{-1}$ (at 6.4 kpc) indicates that X1820$-$303 was in a low state during the observation. The 3\% orbital modulation of the source was clearly detected in periodogram analysis of the light curve.

The 0.6--11 keV GIS2 and GIS3 spectra were fitted simultaneously with various models, and a reasonable fit ($\chi^2$=696/426=1.63) was obtained using a combination of Comptonization with $kT_{ee}$=3.57$\pm$0.24 keV and $\tau$=13.5$\pm$0.6, plus a blackbody with $kT_{bb}$=0.76$\pm$0.02 keV, the blackbody making up 13\% of the total flux. Iron K emission at 6.7 keV was observed with equivalent width 60$\pm$20~eV, the first unambiguous detection of K-line emission in this source. Phase-selected spectroscopy and a refined ephemeris determination are in progress.

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