The minijets-in-a-jet statistical model and the RMS-flux correlation

The flux variability of blazars at very high energies does not have a clear origin. Flux variations on time scales down to the minute suggest that variability originates in the jet, where a relativistic boost can shorten the observed time scale, while the linear relation between the flux and its RMS or the skewness of the flux distribution suggests that the variability stems from multiplicative processes, which are associated in some models with the accretion disk. We study the RMS-flux relation and emphasize its link to Pareto distributions, characterized by a power-law probability density function. Such distributions are naturally generated within a minijets- in-a-jet statistical model, in which boosted emitting regions are isotropically oriented within the bulk relativistic flow of a jet. We prove that, within this model, the flux of a single minijet is proportional to its RMS. This relation still holds when considering a large number of emitting regions, for which the distribution of the total flux is skewed and could be interpreted as being log-normal. The minijets-in-a-jet statistical model reconciles the fast variations and the statistical properties of the flux of blazars at very high energies.Comment: 6 pages, 6 figures, accepted in A

New AGNs discovered by H.E.S.S

During the last year, six new Active Galactic Nuclei (AGN) have been discovered and studied by H.E.S.S. at Very High Energies (VHE). Some of these recent discoveries have been made thanks to new enhanced analysis methods and are presented at this conference for the first time. The three blazars 1ES 0414+009, SHBL J001355.9-185406 and 1RXS J101015.9-311909 have been targeted for observation due to their high levels of radio and X-ray fluxes, while the Fermi/LAT catalogue of bright sources triggered the observation of PKS 0447-439 and AP Librae. Additionally, the BL Lac 1ES 1312-423 was discovered in the field-of-view (FoV) of Centaurus A thanks to the large exposure dedicated by H.E.S.S. to this particularly interesting source. The newly-discovered sources are presented here and in three companion presentations at this conference.Comment: 8 pages, 3 figures, proceeding from the 25th Texas Symposium on Relativistic Astrophysics (Heidelberg, Germany, 2010

Local control of intestinal stem cell homeostasis by enteroendocrine cells in the adult <i>Drosophila</i> midgut

Background: Enteroendocrine cells populate gastrointestinal tissues and are known to translate local cues into systemic responses through the release of hormones into the bloodstream.&lt;p&gt;&lt;/p&gt; Results: Here we report a novel function of enteroendocrine cells acting as local regulators of intestinal stem cell (ISC) proliferation through modulation of the mesenchymal stem cell niche in the &lt;i&gt;Drosophila&lt;/i&gt; midgut. This paracrine signaling acts to constrain ISC proliferation within the epithelial compartment. Mechanistically, midgut enteroendocrine cells secrete the neuroendocrine hormone Bursicon, which acts—beyond its known roles in development—as a paracrine factor on the visceral muscle (VM). Bursicon binding to its receptor, DLGR2, the ortholog of mammalian leucine-rich repeat-containing G protein-coupled receptors (LGR4-6), represses the production of the VM-derived EGF-like growth factor Vein through activation of cAMP.&lt;p&gt;&lt;/p&gt; Conclusions: We therefore identify a novel paradigm in the regulation of ISC quiescence involving the conserved ligand/receptor Bursicon/DLGR2 and a previously unrecognized tissue-intrinsic role of enteroendocrine cells.&lt;p&gt;&lt;/p&gt

Active Galactic Nuclei under the scrutiny of CTA

Active Galactic Nuclei (hereafter AGN) produce powerful outflows which offer excellent conditions for efficient particle acceleration in internal and external shocks, turbulence, and magnetic reconnection events. The jets as well as particle accelerating regions close to the supermassive black holes (hereafter SMBH) at the intersection of plasma inflows and outflows, can produce readily detectable very high energy gamma-ray emission. As of now, more than 45 AGN including 41 blazars and 4 radiogalaxies have been detected by the present ground-based gamma-ray telescopes, which represents more than one third of the cosmic sources detected so far in the VHE gamma-ray regime. The future Cherenkov Telescope Array (CTA) should boost the sample of AGN detected in the VHE range by about one order of magnitude, shedding new light on AGN population studies, and AGN classification and unification schemes. CTA will be a unique tool to scrutinize the extreme high-energy tail of accelerated particles in SMBH environments, to revisit the central engines and their associated relativistic jets, and to study the particle acceleration and emission mechanisms, particularly exploring the missing link between accretion physics, SMBH magnetospheres and jet formation. Monitoring of distant AGN will be an extremely rewarding observing program which will inform us about the inner workings and evolution of AGN. Furthermore these AGN are bright beacons of gamma-rays which will allow us to constrain the extragalactic infrared and optical backgrounds as well as the intergalactic magnetic field, and will enable tests of quantum gravity and other "exotic" phenomena.Comment: 28 pages, 23 figure

A Search for Very High-Energy Gamma Rays from the Missing Link Binary Pulsar J1023+0038 with VERITAS

The binary millisecond radio pulsar PSR J1023+0038 exhibits many characteristics similar to the gamma-ray binary system PSR B1259--63/LS 2883, making it an ideal candidate for the study of high-energy non-thermal emission. It has been the subject of multi-wavelength campaigns following the disappearance of the pulsed radio emission in 2013 June, which revealed the appearance of an accretion disk around the neutron star. We present the results of very high-energy gamma-ray observations carried out by VERITAS before and after this change of state. Searches for steady and pulsed emission of both data sets yield no significant gamma-ray signal above 100 GeV, and upper limits are given for both a steady and pulsed gamma-ray flux. These upper limits are used to constrain the magnetic field strength in the shock region of the PSR J1023+0038 system. Assuming that very high-energy gamma rays are produced via an inverse-Compton mechanism in the shock region, we constrain the shock magnetic field to be greater than $\sim$2 G before the disappearance of the radio pulsar and greater than $\sim$10 G afterwards.Comment: 7 pages, 3 figures, accepted for publication in Ap

VERITAS and Multiwavelength Observations of the BL Lacertae Object 1ES 1741+196

We present results from multiwavelength observations of the BL Lacertae object 1ES 1741+196, including results in the very-high-energy $\gamma$-ray regime using the Very Energetic Radiation Imaging Telescope Array System (VERITAS). The VERITAS time-averaged spectrum, measured above 180 GeV, is well-modelled by a power law with a spectral index of $2.7\pm0.7_{\mathrm{stat}}\pm0.2_{\mathrm{syst}}$. The integral flux above 180 GeV is $(3.9\pm0.8_{\mathrm{stat}}\pm1.0_{\mathrm{syst}})\times 10^{-8}$ m$^{-2}$ s$^{-1}$, corresponding to 1.6% of the Crab Nebula flux on average. The multiwavelength spectral energy distribution of the source suggests that 1ES 1741+196 is an extreme-high-frequency-peaked BL Lacertae object. The observations analysed in this paper extend over a period of six years, during which time no strong flares were observed in any band. This analysis is therefore one of the few characterizations of a blazar in a non-flaring state.Comment: 8 pages, 5 figures. Accepted for publication in MNRA

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

Azimuthal asymmetry in the risetime of the surface detector signals of the Pierre Auger Observatory

The azimuthal asymmetry in the risetime of signals in Auger surface detector stations is a source of information on shower development. The azimuthal asymmetry is due to a combination of the longitudinal evolution of the shower and geometrical effects related to the angles of incidence of the particles into the detectors. The magnitude of the effect depends upon the zenith angle and state of development of the shower and thus provides a novel observable, $(\sec \theta)_\mathrm{max}$, sensitive to the mass composition of cosmic rays above $3 \times 10^{18}$ eV. By comparing measurements with predictions from shower simulations, we find for both of our adopted models of hadronic physics (QGSJETII-04 and EPOS-LHC) an indication that the mean cosmic-ray mass increases slowly with energy, as has been inferred from other studies. However, the mass estimates are dependent on the shower model and on the range of distance from the shower core selected. Thus the method has uncovered further deficiencies in our understanding of shower modelling that must be resolved before the mass composition can be inferred from $(\sec \theta)_\mathrm{max}$.Comment: Replaced with published version. Added journal reference and DO