Determination of activation volumes of reversal in perpendicular media

We discuss a method for the determination of activation volumes of reversal in perpendicular media. This method does not require correction for the self-demagnetizing field normally associated with these media. This is achieved by performing time dependence measurements at a constant level of magnetization. From the difference in time taken for the magnetization to decay to a fixed value at two fields-separated by a small increment DeltaH, the activation volume can be determined. We report data for both CoCrPt alloy films and a multilayer film, typical of those materials under consideration for use as perpendicular media. We find activation volumes that are consistent with the hysteresis curves of the materials. The activation volume scales qualitatively with the exchange coupling. The alloy films have significantly lower activation volumes, implying that they would be capable of supporting a higher data density

Activation volumes in CoPtCr-SiO2 perpendicular recording media

CoPtCr-SiO2 perpendicular recording media with varying levels of SiO2 were examined by two different methods to determine the activation volume. The first is based on the sweep-rate dependence of the remanence coercivity using Sharrock's equation. The second is based on the measurement of the fluctuation field from time-dependence data, determined using a magneto-optical Kerr effect (MOKE) magnetometer. The values of V-act measured at the coercivity for both methods are almost the same, with the fluctuation field and activation volumes increasing with the SiO2 content. The difference between V-act and the grain volume measured directly from bright-field TEM images decreases as the SiO2 content increases due to the reduction of intergranular exchange coupling. The experimental results indicate that values of V-act obtained from single- and double-layered media are consistent. It was also found that the coercivity and normalized hysteresis loop slope at coercivity varied with SiO2 content, with the coercivity peaking at 8 at % SiO2 (nearly 26 vol% SiO2)

Time dependence in perpendicular media with a soft underlayer

In this paper we describe measurements of magnetic viscosity or time dependence in magnetic thin films suitable for use as perpendicular recording media. Generally, such effects cannot be measured using conventional magnetometry techniques due to the presence of a thin (0.1 mum) soft underlayer (SUL) in the media necessary to focus the head field. To achieve our results we have developed an ultrastable MOKE magnetometer, the construction of which is described. This has enabled us to measure nominally identical films with and without the presence of the SUL. We find that the presence of the SUL narrows the energy barrier distribution in the perpendicular film increasing the nucleation field (H-n), reducing the coercivity (H-c) and results in an increase in the squareness of the loop. This in turn results in an increase in the magnitude of the viscosity in the region of the H-c but that the range of fields over which the viscosity occurs is reduced

A novel remote plasma sputtering technique for depositing high-performance optical thin films

This paper describes a novel remote plasma sputtering technique for depositing optical thin films. This technology is based on generating intensive plasma remotely from the target and then magnetically steering the plasma to the target to realize the sputter deposition. It overcomes several of inherent limitations in conventional sputtering techniques and realizes the fully uniform erosion over the surface of the target and less target poison. This allows a uniform reaction in the plasma phase when performing reactive sputtering, leading to the formation and deposition of material with a uniform stoichiometry and gives pseudo-independence of target current and voltage. This pseudo-independence offers a great deal of flexibility with regard to the control of growth conditions and film properties, the benefits include control of stress, very low deposition rates for ultra thin films. By remote reactive sputtering, dense metal-oxide optical thin films (SiO2, Ta2O5, Nb2O5) with a high deposition rate, excellent optical properties are achieved. High process stability shows an excellent time terminating accuracy for multilayer coating thickness control. Typically, thin film thickness control to <+/- 1% is accomplished simply using time. The multilayer coating, including anti-reflection, dichroic mirror and 2 mu m laser mirrors are presented

Detection of variable VHE gamma-ray emission from the extra-galactic gamma-ray binary LMC P3

Context. Recently, the high-energy (HE, 0.1-100 GeV) $\gamma$-ray emission from the object LMC P3 in the Large Magellanic Cloud (LMC) has been discovered to be modulated with a 10.3-day period, making it the first extra-galactic $\gamma$-ray binary. Aims. This work aims at the detection of very-high-energy (VHE, >100 GeV) $\gamma$-ray emission and the search for modulation of the VHE signal with the orbital period of the binary system. Methods. LMC P3 has been observed with the High Energy Stereoscopic System (H.E.S.S.); the acceptance-corrected exposure time is 100 h. The data set has been folded with the known orbital period of the system in order to test for variability of the emission. Energy spectra are obtained for the orbit-averaged data set, and for the orbital phase bin around the VHE maximum. Results. VHE $\gamma$-ray emission is detected with a statistical significance of 6.4 $\sigma$. The data clearly show variability which is phase-locked to the orbital period of the system. Periodicity cannot be deduced from the H.E.S.S. data set alone. The orbit-averaged luminosity in the $1-10$ TeV energy range is $(1.4 \pm 0.2) \times 10^{35}$ erg/s. A luminosity of $(5 \pm 1) \times 10^{35}$ erg/s is reached during 20% of the orbit. HE and VHE $\gamma$-ray emissions are anti-correlated. LMC P3 is the most luminous $\gamma$-ray binary known so far.Comment: 5 pages, 3 figures, 1 table, accepted for publication in A&

Detailed spectral and morphological analysis of the shell type SNR RCW 86

Aims: We aim for an understanding of the morphological and spectral properties of the supernova remnant RCW~86 and for insights into the production mechanism leading to the RCW~86 very high-energy gamma-ray emission. Methods: We analyzed High Energy Spectroscopic System data that had increased sensitivity compared to the observations presented in the RCW~86 H.E.S.S. discovery publication. Studies of the morphological correlation between the 0.5-1~keV X-ray band, the 2-5~keV X-ray band, radio, and gamma-ray emissions have been performed as well as broadband modeling of the spectral energy distribution with two different emission models. Results:We present the first conclusive evidence that the TeV gamma-ray emission region is shell-like based on our morphological studies. The comparison with 2-5~keV X-ray data reveals a correlation with the 0.4-50~TeV gamma-ray emission.The spectrum of RCW~86 is best described by a power law with an exponential cutoff at $E_{cut}=(3.5\pm 1.2_{stat})$ TeV and a spectral index of $\Gamma$~$1.6\pm 0.2$. A static leptonic one-zone model adequately describes the measured spectral energy distribution of RCW~86, with the resultant total kinetic energy of the electrons above 1 GeV being equivalent to $\sim$0.1\% of the initial kinetic energy of a Type I a supernova explosion. When using a hadronic model, a magnetic field of $B$~100$\mu$G is needed to represent the measured data. Although this is comparable to formerly published estimates, a standard E$^{-2}$ spectrum for the proton distribution cannot describe the gamma-ray data. Instead, a spectral index of $\Gamma_p$~1.7 would be required, which implies that ~$7\times 10^{49}/n_{cm^{-3}}$erg has been transferred into high-energy protons with the effective density $n_{cm^{-3}}=n/ 1$ cm^-3. This is about 10\% of the kinetic energy of a typical Type Ia supernova under the assumption of a density of 1~cm^-3.Comment: accepted for publication by A&

Characterizing the gamma-ray long-term variability of PKS 2155-304 with H.E.S.S. and Fermi-LAT

Studying the temporal variability of BL Lac objects at the highest energies provides unique insights into the extreme physical processes occurring in relativistic jets and in the vicinity of super-massive black holes. To this end, the long-term variability of the BL Lac object PKS 2155-304 is analyzed in the high (HE, 100 MeV 200 GeV) gamma-ray domain. Over the course of ~9 yr of H.E.S.S observations the VHE light curve in the quiescent state is consistent with a log-normal behavior. The VHE variability in this state is well described by flicker noise (power-spectral-density index {\ss}_VHE = 1.10 +0.10 -0.13) on time scales larger than one day. An analysis of 5.5 yr of HE Fermi LAT data gives consistent results ({\ss}_HE = 1.20 +0.21 -0.23, on time scales larger than 10 days) compatible with the VHE findings. The HE and VHE power spectral densities show a scale invariance across the probed time ranges. A direct linear correlation between the VHE and HE fluxes could neither be excluded nor firmly established. These long-term-variability properties are discussed and compared to the red noise behavior ({\ss} ~ 2) seen on shorter time scales during VHE-flaring states. The difference in power spectral noise behavior at VHE energies during quiescent and flaring states provides evidence that these states are influenced by different physical processes, while the compatibility of the HE and VHE long-term results is suggestive of a common physical link as it might be introduced by an underlying jet-disk connection.Comment: 11 pages, 16 figure

The exceptionally powerful TeV gamma-ray emitters in the Large Magellanic Cloud

The Large Magellanic Cloud, a satellite galaxy of the Milky Way, has been observed with the High Energy Stereoscopic System (H.E.S.S.) above an energy of 100 billion electron volts for a deep exposure of 210 hours. Three sources of different types were detected: the pulsar wind nebula of the most energetic pulsar known N 157B, the radio-loud supernova remnant N 132D and the largest non-thermal X-ray shell - the superbubble 30 Dor C. The unique object SN 1987A is, surprisingly, not detected, which constrains the theoretical framework of particle acceleration in very young supernova remnants. These detections reveal the most energetic tip of a gamma-ray source population in an external galaxy, and provide via 30 Dor C the unambiguous detection of gamma-ray emission from a superbubble.Comment: Published in Science Magazine (Jan. 23, 2015). This ArXiv version has the supplementary online material incorporated as an appendix to the main pape

The 2010 very high energy gamma-ray flare & 10 years of multi-wavelength observations of M 87

Abridged: The giant radio galaxy M 87 with its proximity, famous jet, and very massive black hole provides a unique opportunity to investigate the origin of very high energy (VHE; E>100 GeV) gamma-ray emission generated in relativistic outflows and the surroundings of super-massive black holes. M 87 has been established as a VHE gamma-ray emitter since 2006. The VHE gamma-ray emission displays strong variability on timescales as short as a day. In this paper, results from a joint VHE monitoring campaign on M 87 by the MAGIC and VERITAS instruments in 2010 are reported. During the campaign, a flare at VHE was detected triggering further observations at VHE (H.E.S.S.), X-rays (Chandra), and radio (43 GHz VLBA). The excellent sampling of the VHE gamma-ray light curve enables one to derive a precise temporal characterization of the flare: the single, isolated flare is well described by a two-sided exponential function with significantly different flux rise and decay times. While the overall variability pattern of the 2010 flare appears somewhat different from that of previous VHE flares in 2005 and 2008, they share very similar timescales (~day), peak fluxes (Phi(>0.35 TeV) ~= (1-3) x 10^-11 ph cm^-2 s^-1), and VHE spectra. 43 GHz VLBA radio observations of the inner jet regions indicate no enhanced flux in 2010 in contrast to observations in 2008, where an increase of the radio flux of the innermost core regions coincided with a VHE flare. On the other hand, Chandra X-ray observations taken ~3 days after the peak of the VHE gamma-ray emission reveal an enhanced flux from the core. The long-term (2001-2010) multi-wavelength light curve of M 87, spanning from radio to VHE and including data from HST, LT, VLA and EVN, is used to further investigate the origin of the VHE gamma-ray emission. No unique, common MWL signature of the three VHE flares has been identified.Comment: 19 pages, 5 figures; Corresponding authors: M. Raue, L. Stawarz, D. Mazin, P. Colin, C. M. Hui, M. Beilicke; Fig. 1 lightcurve data available online: http://www.desy.de/~mraue/m87

Discovery of the Hard Spectrum VHE γ-Ray Source HESS J1641-463

This Letter reports the discovery of a remarkably hard spectrum source, HESS J1641−463, by the High Energy Stereoscopic System (H.E.S.S.) in the very high energy (VHE) domain. HESS J1641−463 remained unnoticed by the usual analysis techniques due to confusion with the bright nearby source HESS J1640−465. It emerged at a significance level of 8.5 standard deviations after restricting the analysis to events with energies above 4 TeV. It shows a moderate flux level of phgr(E>1 TeV) = (3.64 ± 0.44stat ± 0.73sys) × 10−13 cm−2 s−1, corresponding to 1.8% of the Crab Nebula flux above the same energy, and a hard spectrum with a photon index of Γ = 2.07 ± 0.11stat ± 0.20sys. It is a point-like source, although an extension up to a Gaussian width of σ = 3 arcmin cannot be discounted due to uncertainties in the H.E.S.S. point-spread function. The VHE γ-ray flux of HESS J1641−463 is found to be constant over the observed period when checking time binnings from the year-by-year to the 28 minute exposure timescales. HESS J1641−463 is positionally coincident with the radio supernova remnant SNR G338.5+0.1. No X-ray candidate stands out as a clear association; however, Chandra and XMM-Newton data reveal some potential weak counterparts. Various VHE γ-ray production scenarios are discussed. If the emission from HESS J1641−463 is produced by cosmic ray protons colliding with the ambient gas, then their spectrum must extend close to 1 PeV. This object may represent a source population contributing significantly to the galactic cosmic ray flux around the knee