No abstract

We report results from an intensive multiwavelength monitoring campaign on the TeV blazar Mrk 421 over the period of [2003][2004]. The source was observed simultaneously at TeV energies with the Whipple 10 m telescope and at X-ray energies with the Rossi X-Ray Timing Explorer (RXTE) during each clear night within the Whipple observing windows. Supporting observations were also frequently carried out at optical and radio wavelengths to provide simultaneous or contemporaneous coverages. The large amount of simultaneous data has allowed us to examine the variability of Mrk 421 in detail, including cross-band correlation and broadband spectral variability, over a wide range of flux. The variabilities are generally correlated between the X-ray and gamma-ray bands, although the correlation appears to be fairly loose. The light curves show the presence of flares with varying amplitudes on a wide range of timescales at both X-ray and TeV energies. Of particular interest is the presence of TeV flares that have no coincident counterparts at longer wavelengths, because the phenomenon seems difficult to understand in the context of the proposed emission models for TeV blazars. We have also found that the TeV flux reached its peak days before the X-ray flux did during a giant flare (or outburst) in 2004 (with the peak flux reaching $135 mcrab in X-rays, as seen by the RXTE ASM, and $3 crab in gamma rays). Such a difference in the development of the flare presents a further challenge to both the leptonic and hadronic emission models. Mrk 421 varied much less at optical and radio wavelengths. Surprisingly, the normalized variability amplitude in the optical seems to be comparable to that in the radio, perhaps suggesting the presence of different populations of emitting electrons in the jet. The spectral energy distribution of Mrk 421 is seen to vary with flux, with the two characteristic peaks moving toward higher energies at higher fluxes. We have failed to fit the measured spectral energy distributions (SEDs) with a one-zone synchrotron self-Compton model; introducing additional zones greatly improves the fits. We have derived constraints on the physical properties of the X-ray/gamma-ray flaring regions from the observed variability (and SED) of the source. The implications of the results are discussed.

LS I +61 303 is one of only a few high-mass X-ray binaries currently detected at high significance in very high energy -rays. The system was observed over several orbital cycles (between 2006 September and 2007 February) with the VERITAS array of imaging air Cerenkov telescopes. A signal of -rays with energies above 300 GeV is found with a statistical significance of 8.4 standard deviations. The detected flux is measured to be strongly variable; the maximum flux is found during most orbital cycles at apastron. The energy spectrum for the period of maximum emission can be characterized by a power law with a photon index of À ¼ 2:40 AE 0:16 stat AE 0:2 sys and a flux above 300 GeV corresponding to 15%-20% of the flux from the Crab Nebula.

We report a possible detection of TeV gamma rays from the Galactic center by the Whipple 10 m gammaray telescope. Twenty-six hours of data were taken over an extended period from 1995 through 2003 resulting in a total significance of 3.7 j. The measured excess corresponds to an integral flux of Ϫ8 1.6 # 10 ‫ע‬ 0.5 # above an energy of 2.8 TeV, roughly 40% of the flux from the10 (stat) ‫ע‬ 0.3 # 10 (sys) photons m s Crab Nebula at this energy. The 95% confidence region has an angular extent of about 15Ј and includes the position of Sgr A*. The detection is consistent with a point source and shows no evidence of variability.

The multiwavelength observation of the nearby radio galaxy M87 provides a unique opportunity to study in detail processes occurring in active galactic nuclei from radio waves to TeV -rays. Here we report the detection of -ray emission above 250 GeV from M87 in spring 2007 with the VERITAS atmospheric Cerenkov telescope array and discuss its correlation with the X-ray emission. The -ray emission is measured to be pointlike with an intrinsic source radius less than 4.5 0 . The differential energy spectrum is fitted well by a power-law function: dÈ/dE ¼ (7:4 AE 1:3 stat AE 1:5 sys )(E/TeV) (À2:31AE0:17 stat AE0:2 sys ) 10 À9 m À2 s À1 TeV À1 . We show strong evidence for a year-scale correlation between the -ray flux reported by TeV experiments and the X-ray emission measured by the ASM RXTE observatory, and discuss the possible short-timescale variability. These results imply that the -ray emission from M87 is more likely associated with the core of the galaxy than with other bright X-ray features in the jet.