SIMBAD references

We present X-ray observations of the nearby TeV blazar Mrk 501 obtained with RXTE during 1997, 1998, 1999, 2000, and 2004. The goal of this study is twofold: (1) to characterize the long-term X-ray flux and spectral variability of the source with a model-independent analysis, and (2) to investigate the X-ray and TeV correlation on long timescales (>days). Significant spectral variations were observed during all the observations along with long-term timescale correlations between the X-ray colors and the count rate. Specifically, on long timescales, a typical blazar behavior is observed with the spectrum hardening when the source brightens, and the fractional variability correlating with the energy band. A similar spectral trend is observed also in the majority of the individual flares. The spectral and temporal variability properties appear to be markedly different compared to those of non-jet-dominated radio-loud and radio-quiet AGNs monitored with RXTE. To investigate the X-ray-TeV correlation on long timescales we compared RXTE monitoring data with HEGRA and Whipple historical light curves. We confirm the presence of a direct correlation between X-ray and gamma-ray emissions, which appears to be stronger when the source is brighter. The analysis of individual flares shows that the X-ray-TeV correlated activity is heterogeneous, in the sense that it might be both linear and nonlinear, and some X-ray flares seem to be lacking the TeV counterpart. However, more sensitive TeV observations are necessary to confirm these findings, and to put tighter constraints on jet models.