The Astrophysical Journal, Volume 646, Issue 1, pp. 61-75.

 arXiv:astro-ph/0603693

Abstract:   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.

The Astrophysical Journal, Volume 644, Issue 1, pp. 148-154.

http://arxiv.org/abs/astro-ph/0602258 

Abstract: Galaxy clusters might be sources of TeV gamma rays emitted by high-energy protons and electrons accelerated by large scale structure formation shocks, galactic winds, or active galactic nuclei. Furthermore, gamma rays may be produced in dark matter particle annihilation processes at the cluster cores. We report on observations of the galaxy clusters Perseus and Abell 2029 using the 10 m Whipple Cherenkov telescope during the 2003-2004 and 2004-2005 observing seasons. We apply a two-dimensional analysis technique to scrutinize the clusters for TeV emission. In this paper we first determine flux upper limits on TeV gamma-ray emission from point sources within the clusters. Second, we derive upper limits on the extended cluster emission. We subsequently compare the flux upper limits with EGRET upper limits at 100 MeV and theoretical models. Assuming that the gamma-ray surface brightness profile mimics that of the thermal X-ray emission and that the spectrum of cluster cosmic rays extends all the way from thermal energies to multi-TeV energies with a differential spectral index of -2.1, our results imply that the cosmic ray proton energy density is less than 7.9% of the thermal energy density for the Perseus cluster.

The Astrophysical Journal, Volume 644, Issue 2, pp. 742-747.

http://xxx.arxiv.org/abs/astro-ph/0603013 

Abstract: In April-May 2003, the blazar 1ES 1959+650 showed an increased level of X-ray activity. This prompted a multiwavelength observation campaign with the Whipple 10 m gamma-ray telescope, the Rossi X-ray Timing Explorer, the Bordeaux Optical Observatory, and the University of Michigan Radio Astrophysical Observatory. We present the multiwavelength data taken from May 2, 2003 to June 7, 2003 and compare the source characteristics with those measured during observations taken during the years 2000 and 2002. The X-ray observations gave a data set with high signal-to-noise light curves and energy spectra; however, the gamma-ray observations did not reveal a major TeV gamma-ray flare. Furthermore, we find that the radio and optical fluxes do not show statistically significant deviations from those measured during the 2002 flaring periods. While the X-ray flux and X-ray photon index appear correlated during subsequent observations, the apparent correlation evolved significantly between the years 2000, 2002, and 2003. We discuss the implications of this finding for the mechanism that causes the flaring activity.

The Astrophysical Journal, Volume 641, Issue 2, pp. 740-751.

astro-ph/0512628 

Abstract: We report on a multiwavelength campaign on the TeV γ-ray blazar Mrk 421 performed during 2002 December and 2003 January. These target of opportunity observations were initiated by the detection of X-ray and TeV γ-ray flares with the All Sky Monitor (ASM) on board the Rossi X-Ray Timing Explorer (RXTE) and the 10 m Whipple γ-ray telescope. The campaign included observational coverage in the radio (University of Michigan Radio Astronomy Observatory), optical (Boltwood, La Palma KVA 0.6 m; WIYN 0.9 m), X-ray (RXTE pointed telescopes), and TeV γ-ray (Whipple and HEGRA) bands. At TeV energies, the observations revealed several flares at intermediate flux levels, peaking between 1 and 1.5 times the flux from the Crab Nebula. While the time-averaged spectrum can be fitted with a single power law of photon index Γ=2.8 from dN_γ/dE~E^-Γ, we find some evidence for spectral variability. Confirming earlier results, the campaign reveals a rather loose correlation between the X-ray and TeV γ-ray fluxes. In one case, a very strong X-ray flare is not accompanied by a comparable TeV γ-ray flare. Although the source flux was variable in the optical and radio bands, the sparse sampling of the optical and radio light curves does not allow us to study the correlation properties in detail. We present a simple analysis of the data with a synchrotron self-Compton model, emphasizing that models with very high Doppler factors and low magnetic fields can describe the data.