EVIDENCE FOR GAMMA-RAY EMISSION FROM THE LOW-MASS X-RAY BINARY SYSTEM FIRST J102347.6+003841

Abstract

The low-mass X-ray binary (LMXB) system FIRST J102347.6+003841 hosts a newly\nborn millisecond pulsar (MSP) PSR J1023+0038 that was revealed as the first and\nonly known rotation-powered MSP in a quiescent LMXB. While the system is shown\nto have an accretion disk before 2002, it remains unclear how the accretion\ndisk has been removed in order to reveal the radio pulsation in 2007. In this\nLetter, we report the discovery of gamma-rays spatially consistent with FIRST\nJ102347.6+003841, at a significance of 7 standard deviations, using data\nobtained by the Fermi Gamma-ray Space Telescope. The gamma-ray spectrum can be\ndescribed by a power law (PL) with a photon index of 2.9+-0.2, resulting in an\nenergy flux above 200 MeV of (5.5+-0.9)x10^{-12} erg cm^{-2}s^{-1}. The\ngamma-rays likely originate from the MSP PSR J1023+0038, but also possibly from\nan intrabinary shock between the pulsar and its companion star. To complement\nthe gamma-ray study, we also re-investigate the XMM-Newton data taken in 2004\nand 2008. Our X-ray spectral analysis suggests that a broken PL with two\ndistinct photon indices describes the X-ray data significantly better than a\nsingle PL. This indicates that there exists two components and that both\ncomponents appear to vary with the orbital phase. The evidence for gamma-ray\nemission conforms with a recent suggestion that gamma-rays from PSR J1023+0038\nmay be responsible for ejecting the disk material out of the system.\n