Observation of the TeV gamma-ray source MGRO J1908+06 with ARGO-YBJ

The extended gamma ray source MGRO J1908+06, discovered by the Milagro air shower detector in 2007, has been observed for about 4 years by the ARGO-YBJ experiment at TeV energies, with a statistical significance of 6.2 standard deviations. The peak of the signal is found at a position consistent with the pulsar PSR J1907+0602. Parametrizing the source shape with a two-dimensional Gauss function we estimate an extension \sigma = 0.49 \pm 0.22 degrees, consistent with a previous measurement by the Cherenkov Array H.E.S.S.. The observed energy spectrum is dN/dE = 6.1 \pm 1.4 \times 10^-13 (E/4 TeV)^{-2.54 \pm 0.36} photons cm^-2 s^-1 TeV^-1, in the energy range 1-20 TeV. The measured gamma ray flux is consistent with the results of the Milagro detector, but is 2-3 times larger than the flux previously derived by H.E.S.S. at energies of a few TeV. The continuity of the Milagro and ARGO-YBJ observations and the stable excess rate observed by ARGO-YBJ along 4 years of data taking support the identification of MGRO J1908+06 as the steady powerful TeV pulsar wind nebula of PSR J1907+0602, with an integrated luminosity above 1 TeV about 1.8 times the Crab Nebula luminosity.Comment: 6 pages, accepted for pubblication by ApJ. Replaced to correct the author lis

Long-term Monitoring on Mrk 501 for Its VHE gamma Emission and a Flare in October 2011

As one of the brightest active blazars in both X-ray and very high energy $\gamma$-ray bands, Mrk 501 is very useful for physics associated with jets from AGNs. The ARGO-YBJ experiment is monitoring it for $\gamma$-rays above 0.3 TeV since November 2007. Starting from October 2011 the largest flare since 2005 is observed, which lasts to about April 2012. In this paper, a detailed analysis is reported. During the brightest $\gamma$-rays flaring episodes from October 17 to November 22, 2011, an excess of the event rate over 6 $\sigma$ is detected by ARGO-YBJ in the direction of Mrk 501, corresponding to an increase of the $\gamma$-ray flux above 1 TeV by a factor of 6.6$\pm$2.2 from its steady emission. In particular, the $\gamma$-ray flux above 8 TeV is detected with a significance better than 4 $\sigma$. Based on time-dependent synchrotron self-Compton (SSC) processes, the broad-band energy spectrum is interpreted as the emission from an electron energy distribution parameterized with a single power-law function with an exponential cutoff at its high energy end. The average spectral energy distribution for the steady emission is well described by this simple one-zone SSC model. However, the detection of $\gamma$-rays above 8 TeV during the flare challenges this model due to the hardness of the spectra. Correlations between X-rays and $\gamma$-rays are also investigated.Comment: have been accepted for publication at Ap

Development of freezing tolerance in different altitudinal ecotypes of Salix paraplesia

Salix paraplesia was used as an experimental model to investigate the effect of short day photoperiod (SD) and low temperature (LT) on development of freezing tolerance and on endogenous abscisic acid (ABA) contents. We characterized differences in SD and LT-induced cold acclimation in three ecotypes from different altitudes. The results demonstrated that cold acclimation could be triggered by exposing the plants to SD or LT alone, and that a combination of the different treatments had an additive effect on freezing tolerance in all ecotypes studied. However, the high altitudinal ecotype was more responsive to SD and LT than the low altitudinal ecotype. Development of freezing tolerance induced by SD and LT was accompanied by changes in ABA contents which were ecotype-dependent. Although the stem had higher initial freezing tolerance, the leaves developed freezing tolerance more quickly than the stem and thus leaves may provide an interesting experimental system for physiological and molecular studies of cold acclimation in woody plants

Effect of increased alkalinity on Na+ and K+ contents, lipid peroxidation and antioxidative enzymes in two populations of Populus cathayana

We compared two populations of Populus cathayana Rehder, originating from altitudes 2 840 m and 1 450 m, to determine whether trees from different altitudes exhibit different tolerance to alkalinity. The tree cuttings were exposed to nutrient solutions with pH 7.9, 8.8, 9.8 and 10.4 and the salt concentration 200 mM. Na+ and K+ contents, and Na+/K+ ratios in leaves and roots were greatly affected by pH values. At pH 10.4, the Na+/K+ ratios in both leaves and roots sharply dropped in the higher altitude population but were maintained at higher levels in the lower altitude population. The patterns of pH-induced changes in contents of malondialdehyde and free proline, and antioxidative enzyme activities indicated that the higher altitude population exhibits greater tolerance to alkalinity stress than does the lower altitude population

pH- and Thermal-Responsive Multishape Memory Hydrogel

A multistimuli-sensitive shape memory hydrogel with dual and triple shape memory properties,was prepared by grafting dansyl groups into the network of polyacrylamide (PAAM). The hydrophobic aggregation of dansyl groups acted as molecular switches, which showed reversible aggregation-disaggregation transition in aqueous solution in response to the pH or temperature change

Component carbon fluxes and their contribution to ecosystem carbon exchange in a pine forest: an assessment based on eddy covariance measurements and an integrated model

We used a combination of eddy flux, canopy, soil and environmental measurements with an integrated biophysical model to analyze the seasonality of component carbon (C) fluxes and their contribution to ecosystem C exchange in a 50-year-old Scots pine forest (Pinus sylvestris L.) in eastern Finland (62degrees47' N, 30degrees58' E) over three climatically contrasting years (2000-2002). Eddy flux measurements showed that the growing Scots pine forest was a sink for CO2, with annual net C uptakes of 131, 210 and 258 g C m(-2) year(-1) in 2000, 2001 and 2002, respectively. The integrated process model reproduced the annual course of daily C flux above the forest canopy as measured by the eddy covariance method once the site-specific component parameters were estimated. The model explained 72, 66 and 68% of the variation in daily net C flux in 2000, 2001 and 2002, respectively. Modeled annual C loss by respiration was 565, 629 and 640 g C m(-2) year(-1), accounting for 77, 77 and 65% of annual gross C uptake, respectively. Carbon fluxes from the forest floor were the dominant contributors to forest ecosystem respiration, with the fractions of annual respiration from the forest floor, foliage and wood being 46-62, 27-44 and 9-10%, respectively. The wide range in daily net C uptake during the growing season was largely attributable to day-to-day fluctuations in incident quantum irradiance. During just a few days in early spring and late autumn, ecosystem net C exchange varied between source and sink as a result of large daily changes in temperature. The forest showed a greater reduction in gross C uptake by photosynthesis than in C loss by respiration during the dry summer of 2000, indicating that interannual variability in ecosystem net C uptake at this site was modified mostly by summer rainfall and vapor pressure deficit

Semi-IPNs with Moisture-Triggered Shape Memory and Self-Healing Properties

Moisture or water has the advantages of being green, inexpensive, and moderate. However, it is challenging to endow water-induced shape memory property and self-healing capability to one single polymer because of the conflicting structural requirement of the two types of materials. In this study, this problem is solved through introducing two kinds of supramolecular interactions into semi-interpenetrating polymer networks (semi-IPNs). The hydrogen bonds function as water-sensitive switches, making the materials show moisture-induced shape memory effect. The host-guest interactions (-cyclodextrin-adamantane) serve as both permanent phases and self-healing motifs, enabling further increased chain mobility at the cracks and self-healing function. In addition, these polyvinylpyrrolidone/poly(hydroxyethyl methacrylate-co-butyl acrylate) semi-IPNs also show thermosensitive triple-shape memory effect