Ultraluminous X-ray sources in the group-centric elliptical galaxy NGC 5813

ABSTRACT The number of ultraluminous X-ray sources (ULXs) is observed to be correlated with the current star formation rate in late-type galaxies and with the stellar mass in early-type galaxies (ETGs). Since there is very little gas, dust, or star formation in ETGs, it has been suggested that most of the ULXs associated with them could be high luminosity low mass X-ray binaries (LMXBs) or foreground/background sources. It has been reported that NGC 5813, the central dominant (cD) galaxy in the NGC 5846 group of galaxies, which shows signs of a possible recent merger event, has an unusually high number of ULXs. We have undertaken a multi-epoch spectral study of the persistent ULXs in the galaxy using Chandra and XMM-Newton observations. Of the eight ULXs reported elsewhere, four have been re-identified, two are not consistently detected across all nine Chandra observations, and two are found to be foreground sources. One new persistent ULX has been identified. We present a spectral analysis of the five ULXs with luminosity consistently greater than $10^{39}$ erg s$^{-1}$ in nine Chandra-ACIS observations, and assess their variability, adding data from XMM-Newton. The association of these ULXs with globular clusters was examined: we find one ULX lying within the field of an HST observation within 0.1 arcmin of the centre of a globular cluster. Optical and UV counterparts are found for another ULX. One of the ULXs is found to be variable over the time scale of days, but there is no unambiguous evidence of longer term variability for the remaining ULXs.