ASCA Observations of the Two Nearest Globular Clusters, M 4 and NGC 6397

Abstract

Abstract We report on ASCA observations of the two nearby globular clusters, M 4 and NGC 6397. Although 14 and 8 sources were detected at more than the $5\,\sigma$ confidence level from each cluster field, respectively, the majority of them, except for the cores of both clusters, were found unlikely to be associated with the clusters. The core of NGC 6397 shows optically thin thermal plasma emission with a temperature of $kT \gtrsim 12 \,\mathrm{keV}$. This result supports the results of HST, ROSAT, and Chandra, indicating that the main constituent of the NGC 6397 core is cataclysmic variables (CVs). The high temperature suggests it to be magnetic CVs, which supports the result of the HST spectroscopy. The core of M 4 was unambiguously detected at a $\gtrsim 5\,\sigma$ confidence level for the first time. Although we cannot carry out a detailed spectral analysis on the M 4 core due to a statistical limitation, the hardness ratio of the detected photons is also consistent with CVs. We compared the ASCA fluxes of the core sources to those from other X-ray observations. The absorption-corrected 0.5–10keV flux of the NGC 6397 core with ASCA was obtained to be $2.5 \times 10^{-12} \,\mathrm{erg} \,\mathrm{cm}^{-2} \,\mathrm{s}^{-1}$, and had been constant within a factor of $\sim 2$ during 1993–2001. The M 4 core flux obtained under the same condition is $2.6 \times 10^{-13} \,\mathrm{erg} \,\mathrm{cm}^{-2} \,\mathrm{s}^{-1}$, which seems to be $\sim 4$ times as high as the ROSAT HRI flux, although the possibility of a constant flux between the ASCA and HRI observations cannot be excluded due to a statistical limitation. Spectral and temporal properties of other out-of-core sources, including the radio galaxy PKS 1620$-$260 and the G0V star V972 Sco, are also described.