RXTE observations of PKS 2155-304 during the November 1997 gamma-ray outburst

We present X-ray observations of the nearby BL Lac PKS 2155-304 taken when it was undergoing a GeV/TeV gamma-ray outburst. During the outburst we measured X-ray fluxes in the 2–10 keV band that are the largest ever observed for PKS 2155-304. Comparison of these November 1997 measurements and other X-ray observations made contemporaneously with GeV or TeV gamma-ray observations indicate that X-ray and gamma-ray emissions are correlated. Measurements with X-ray all-sky monitors such as the ASM/RXTE and MOXE can therefore signal the presence of outbursts at gamma-ray energies from PKS 2155-304.