The structural properties of the multi-layer graphene/4H-SiC(000-1) system as determined by Surface X-ray Diffraction

We present a structural analysis of the multi-layer graphene-4HSiC(000-1}) system using Surface X-Ray Reflectivity. We show for the first time that graphene films grown on the C-terminated (000-1}) surface have a graphene-substrate bond length that is very short (0.162nm). The measured distance rules out a weak Van der Waals interaction to the substrate and instead indicates a strong bond between the first graphene layer and the bulk as predicted by ab-initio calculations. The measurements also indicate that multi-layer graphene grows in a near turbostratic mode on this surface. This result may explain the lack of a broken graphene symmetry inferred from conduction measurements on this system [C. Berger et al., Science 312, 1191 (2006)].Comment: 9 pages with 6 figure

Tev Observations of the Variability and Spectrum of Markarian 421

Markarian 421 was the first extragalactic source to be detected with high statistical certainty at TeV energies. The Whipple Observatory gamma-ray telescope has been used to observe the Active Galactic Nucleus, Markarian 421 in 1996 and 1997. The rapid variability observed in TeV gamma rays in previous years is confirmed. Doubling times as short as 15 minutes are reported with flux levels reaching 15 photons per minute. The TeV energy spectrum is derived using two independent methods. The implications for the intergalactic infra-red medium of an observed unbroken power law spectrum up to energies of 5 TeV is discussed.Comment: 4 pages, 4 figures, to appear in proceedings of 25 ICRC (Durban

TeV Observations of the Variability and Spectrum of Markarian 501

Markarian 501 is only the second extragalactic source to be detected with high statistical certainty at TeV energies; it is similar in many ways to Markarian 421. The Whipple Observatory gamma-ray telescope has been used to observe the AGN Markarian 501 in 1996 and 1997, the years subsequent to its initial detection. The apparent variability on the one-day time-scale observed in TeV gamma rays in 1995 is confirmed and compared with the variability in Markarian 421. Observations at X-ray and optical wavelengths from 1997 are also presented.Comment: 4 pages, 2 figures, to appear in proceedings of 25th ICRC (Durban

The Flux Variability of Markarian 501 in Very High Energy Gamma Rays

The BL Lacertae object Markarian 501 was identified as a source of gamma-ray emission at the Whipple Observatory in March 1995. Here we present a flux variability analysis on several times-scales of the 233 hour data set accumulated over 213 nights (from March 1995 to July 1998) with the Whipple Observatory 10 m atmospheric Cherenkov imaging telescope. In 1995, with the exception of a single night, the flux from Markarian 501 was constant on daily and monthly time-scales and had an average flux of only 10% that of the Crab Nebula, making it the weakest VHE source detected to date. In 1996, the average flux was approximately twice the 1995 flux and showed significant month-to-month variability. No significant day-scale variations were detected. The average gamma-ray flux above ~350 GeV in the 1997 observing season rose to 1.4 times that of the Crab Nebula -- 14 times the 1995 discovery level -- allowing a search for variability on time-scales shorter than one day. Significant hour-scale variability was present in the 1997 data, with the shortest, observed on MJD 50607, having a doubling time of ~2 hours. In 1998 the average emission level decreased considerably from that of 1997 (to ~20% of the Crab Nebula flux) but two significant flaring events were observed. Thus, the emission from Markarian 501 shows large amplitude and rapid flux variability at very high energies as does Markarian 421. It also shows large mean flux level variations on year-to-year time-scales, behaviour which has not been seen from Markarian 421 so far.Comment: 19 pages, 3 figures, to appear in ApJ, June 20, 1999, Vol. 518 #