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 $8\times10^{37}$ 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 $_{GC}$ of $(3.6\pm1.0)\times10^{36}\ erg\ s^{-1}$, $(6.9\pm2.1)\times10^{36}\ erg\ s^{-1}$and$(1.7\pm0.9)\times10^{36}\ erg\ s^{-1}$, 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 ($2.5\pm1.0$or$4.1\pm2.5$depending on the surveyed region); alternatively, assuming the fractions observed for brighter sources, we measure an average X-ray luminosity of$L_{X}=(4.3\pm1.3)\times10^{37}\ erg\ s^{-1}$and$L_{X}=(3.4\pm1.7)\times10^{37}\ erg\ s^{-1}$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$L_{X}\leq 8\times10^{37}$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.Comment: In press on A&