The 7-year MAXI/GSC Source Catalog of the Low-Galactic-latitude Sky (3MAXI)

We present the first MAXI/GSC X-ray source catalog in the low-Galactic-latitude sky | b| < 10^\circ outside the Galactic center region (| b| < 5^\circ , l < 30°, and l > 330°) based on 7-year data from 2009 August 13 to 2016 July 31. To overcome source confusion in crowded regions, we have accurately calibrated the position-dependent shape of the point-spread function of the MAXI/GSC by analyzing onboard data. We have also taken into account the Galactic ridge X-ray emission. Using a maximum likelihood image fitting method, we have detected 221 sources with a significance threshold >6.5σ, 7 of which are transients only detected in 73-day time-sliced images. The faintest source has a flux of 5.2 × 10^-12 erg cm^-2 s^-1 (or an intensity of 0.43 mCrab) in the 4-10 keV band. We have identified the counterparts for about 81% of the detected sources, by cross-matching with the Swift, Uhuru, RXTE, XMM-Newton, MCXC, and ROSAT all-sky survey catalogs. Our catalog contains the source name, position and its error, flux and detection significance in the 3-4 keV, 4-10 keV, and 10-20 keV bands, hardness ratios, and information on the likely counterpart for the individual detected sources. We have obtained 73-day bin light curves of all the cataloged sources over 7 years and have calculated their periodograms. On the basis of the mean properties of time variability and spectral hardness, we suggest that the majority of the unidentified sources are low-mass X-ray binaries or blazars. Finally, we present the log N-log S relations at different Galactic longitudes and for different source populations.