The Luminosity Function of Low Mass X-Ray Binaries in the Globular Cluster System of NGC 1399

Abstract

We present a study of the faint-end of the X-ray Luminosity Function of Low Mass X-ray binaries in the Globular Cluster system of the cD galaxy NGC 1399 by performing a stacking experiment on 618 X-ray undetected GCs, in order to verify the presence of faint LMXBs and to constrain the faint-end slope of the GC-LMXBs XLF below the individual detection threshold of $8\times 10^{37}$ erg s${}^{-1}$ in the $0.5-8$ keV band. We obtain a significant X-ray detection for the whole GC sample, as well as for the red and blue GC subpopulations, corresponding to an average luminosity per GC $<L_X{>}_{GC}$ of $(3.6\pm 1.0)\times 10^{36}erg{s}^{-1}$, $(6.9\pm 2.1)\times 10^{36}erg{s}^{-1}$ and $(1.7\pm 0.9)\times 10^{36}erg{s}^{-1}$, respectively for all, red and blue GCs. If LMXBs in red and blue GCs have the same average intrinsic luminosity, we derive a red/blu ratio $\simeq 3$ of GCs hosting LMXBs ($2.5\pm 1.0$ or $4.1\pm 2.5$ depending on the surveyed region); alternatively, assuming the fractions observed for brighter sources, we measure an average X-ray luminosity of $L_X=(4.3\pm 1.3)\times 10^{37}erg{s}^{-1}$ and $L_X=(3.4\pm 1.7)\times 10^{37}erg{s}^{-1}$ per red and blue GC-LMXBs respectively. In the assumption that the XLF follows a power-law distribution, we find that a low-luminosity break is required at $L_X\le 8\times 10^{37}$ erg s${}^{-1}$ both in the whole, as well as in the color-selected (red and blue) subsamples. Given the bright-end slopes measured above the X-ray completeness limit, this result is significant at $>3\sigma$ level. Our best estimates for the faint end slope are ${\beta }_L=-1.39/-1.38/-1.36$ for all/red/blue GC-LMXBs. We also find evidence that the luminosity function becomes steeper at luminosities $L_X\gtrsim 3\times 10^{39}$ erg s${}^{-1}$, as observed in old ellipticals.

arXiv commentIn press on A&A

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