Abstracts of Interest
Selected by:
Gavin Rowell
Abstract: 1810.05409
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Title: Radio Galaxies at VHE energies
(Submitted on 12 Oct 2018)
Abstract: Radio Galaxies have by now emerged as a new $\gamma$-ray emitting source class on the extragalactic sky. Given their remarkable observed characteristics, such as unusual gamma-ray spectra or ultrafast VHE variability, they represent unique examples to probe into the nature and physics of AGN in general. This review provides a compact summary of their observed characteristics at very high $\gamma$-ray energies (VHE; $> 100$ GeV) along with a discussion of their possible physics implications. A particular focus is given to a concise overview of fundamental concepts concerning the origin of variable VHE emission, including recent developments in black hole gap physics.
Abstract: 1810.05154
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Title: Efficient Nonthermal Particle Acceleration by the Kink Instability in Relativistic Jets
(Submitted on 11 Oct 2018)
Abstract: Relativistic magnetized jets from active galaxies are among the most powerful cosmic accelerators, but their particle acceleration mechanisms remain a mystery. We present a new acceleration mechanism associated with the development of the helical kink instability in relativistic jets, which leads to the efficient conversion of the jet's magnetic energy into nonthermal particles. Large-scale three-dimensional ab initio simulations reveal that the formation of highly tangled magnetic fields and a large-scale inductive electric field throughout the kink-unstable region promotes rapid energization of the particles. The energy distribution of the accelerated particles develops a well-defined power-law tail extending to the radiation-reaction limited energy in the case of leptons, and to the confinement energy of the jet in the case of ions. When applied to the conditions of well-studied bright knots in jets from active galaxies, this mechanism can account for the spectrum of synchrotron and inverse Compton radiating particles, and offers a viable means of accelerating ultra-high-energy cosmic rays to $10^{20}$ eV.
Abstract: 1810.05055
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Title: Imaging Stellar Radio Photospheres with the Next Generation Very Large Array
(Submitted on 11 Oct 2018)
Abstract: We perform simulations of the capabilities of the next generation Very Large Array to image stellar radio photospheres. For very large (in angle) stars, such as red supergiants within a few hundred parsecs, good imaging fidelity results can be obtained on radio photospheric structures at 38 GHz employing standard techniques, such as disk model fitting and subtraction, with hundreds of resolution elements over the star, even with just the ngVLA-classic baselines to 1000 km. Using the ngVLA Rev B plus long baseline configuration (with baselines out to 9000 km, August 2018), we find for main sequence stars within $\sim$ 10 pc, the photospheres can be easily resolved at 85 GHz, with accurate measures of the mean brightness and size, and possibly imaging large surface structures, as might occur on e.g., active M dwarf stars. For more distant main sequence stars, we find that measurements of sizes and brightnesses can be made using disk model fitting to the u,v-data down to stellar diameters $\sim$ 0.4 mas in a few hours. This size would include M0 V stars to a distance of 15 pc, A0 V stars to 60 pc, and Red Giants to 2.4 kpc. Based on the Hipparcos catalog, we estimate that there are at least 10,000 stars that will be resolved by the ngVLA. While the vast majority of these (95\%) are giants or supergiants, there are still over 500 main sequence stars that can be resolved, with $\sim$ 50 to 150 in each spectral type (besides O stars). Note that these are lower limits, since radio photospheres can be larger than optical, and the Hipparcos catalog might not be complete. Our initial look into the Gaia catalog suggests these numbers might be pessimistic by a factor few.
Abstract: 1810.04951
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Title: Solar Physics with the Square Kilometre Array
(Submitted on 11 Oct 2018)
Abstract: The Square Kilometre Array (SKA) will be the largest radio telescope ever built, aiming to provide collecting area larger than 1 km$^2$. The SKA will have two independent instruments, SKA-LOW comprising of dipoles organized as aperture arrays in Australia and SKA-MID comprising of dishes in South Africa. Currently the phase-1 of SKA, referred to as SKA1, is in its late design stage and construction is expected to start in 2020. Both SKA1-LOW (frequency range of 50-350 MHz) and SKA1-MID Bands 1, 2, and 5 (frequency ranges of 350-1050, 950-1760, and 4600-15300 MHz, respectively) are important for solar observations. In this paper we present SKA's unique capabilities in terms of spatial, spectral, and temporal resolution, as well as sensitivity and show that they have the potential to provide major new insights in solar physics topics of capital importance including (i) the structure and evolution of the solar corona, (ii) coronal heating, (iii) solar flare dynamics including particle acceleration and transport, (iv) the dynamics and structure of coronal mass ejections, and (v) the solar aspects of space weather. Observations of the Sun jointly with the new generation of ground-based and space-borne instruments promise unprecedented discoveries.
Abstract: 1810.04644
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Full Text: [ PostScript, PDF]
Title: Resonant Particle Backscattering of a Shock Wave
(Submitted on 10 Oct 2018)
Abstract: A collisionless shock wave is treated as a streaming plasma instability in the interstellar medium (ISM). We assume that in a steady state, this instability propagates through the ISM as a self-driven plasma instability, whereby the parameters of the instability are determined such that it causes density and velocity jumps as well as isotropization of the particle velocities, which then must be in accordance with MHD theory. The process of resonant interaction and particle scattering off such instability and their backscattering to an upstream region is simulated. We find that some ISM particles bounce off a shock and thus become suprathermal and eligible to enter the process of acceleration to cosmic ray energies by the mechanism of diffusive shock acceleration.
Abstract: 1810.04641
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Full Text: [ PostScript, PDF]
Title: VHE $γ$-ray discovery and multi-wavelength study of the blazar 1ES 2322-409
(Submitted on 10 Oct 2018)
Abstract: A hotspot at a position compatible with the BL Lac object 1ES 2322-409 was serendipitously detected with H.E.S.S. during observations performed in 2004 and 2006 on the blazar PKS 2316-423. Additional data on 1ES 2322-409 were taken in 2011 and 2012, leading to a total live-time of 22.3h. Point-like very-high-energy (VHE; E>100GeV) $\gamma$-ray emission is detected from a source centred on the 1ES 2322-409 position, with an excess of 116.7 events at a significance of 6.0$\sigma$. The average VHE $\gamma$-ray spectrum is well described with a power law with a photon index $\Gamma=3.40\pm0.66_{\text{stat}}\pm0.20_{\text{sys}}$ and an integral flux $\Phi(E>200GeV) = (3.11\pm0.71_{\rm stat}\pm0.62_{\rm sys})\times10^{-12} cm^{-2} s^{-1}$, which corresponds to 1.1$\%$ of the Crab nebula flux above 200 GeV. Multi-wavelength data obtained with Fermi LAT, Swift XRT and UVOT, RXTE PCA, ATOM, and additional data from WISE, GROND and Catalina, are also used to characterise the broad-band non-thermal emission of 1ES 2322-409. The multi-wavelength behaviour indicates day-scale variability. Swift UVOT and XRT data show strong variability at longer scales. A spectral energy distribution (SED) is built from contemporaneous observations obtained around a high state identified in Swift data. A modelling of the SED is performed with a stationary homogeneous one-zone synchrotron-self-Compton (SSC) leptonic model. The redshift of the source being unknown, two plausible values were tested for the modelling. A systematic scan of the model parameters space is performed, resulting in a well-constrained combination of values providing a good description of the broad-band behaviour of 1ES 2322-409.
Abstract: 1810.04516
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Full Text: [ PostScript, PDF]
Title: H.E.S.S. first public test data release
(Submitted on 10 Oct 2018)
Abstract: The High Energy Stereoscopic System (H.E.S.S.) is an array of ground-based imaging atmospheric Cherenkov telescopes in Namibia. For the first time, the H.E.S.S. collaboration is releasing a small dataset of event lists and instrument response information. This is a test data release, with the motivation to support the ongoing efforts to define open high-level data models and associated formats, as well as open-source science tools for gamma-ray astronomy. The data are in FITS format. Open-source science tools that support this format exist already. The release data consists of 27.9 hours in total of observations of the Crab nebula, PKS 2155-304, MSH 15-52 and RX J1713.7-3946 taken with the H.E.S.S. 1 array. Most data are from 2004, the PKS 2155-304 data are from 2006 and 2008. In addition, 20.7 hours of off observations of empty fields of view are included. The targets and observations were chosen to be suitable for common analysis use cases, including point-like and extended sources for spectral and morphology measurements, as well as a variable source (PKS 2155-304) and the off dataset for background studies. The total size of the files in this data release is 42.8 MB. This is a very small subset of the thousands of hours of H.E.S.S. 1 observations taken since 2004. The quality of this dataset, and measurements derived from this data, does not reflect the state of the art for H.E.S.S. publications, e.g. the event reconstruction and gamma-hadron separation method used here is a very basic one.
Abstract: 1810.04139
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Full Text: [ PostScript, PDF]
Title: The Feasibility of Magnetic Reconnection Powered Blazar Flares from Synchrotron Self-Compton Emission
(Submitted on 9 Oct 2018)
Abstract: Order of magnitude variability has been observed in the blazar sub-class of Active Galactic Nuclei on minute timescales. Such fast high energy flares are often difficult to explain with conventional shock acceleration models due to the small size of the emitting region inferred by the light crossing time. Recent particle-in-cell (PIC) simulations have shown that magnetic reconnection may offer a viable explanation. In this paper, we present a macroscopic emission model physically motivated by PIC simulations, where the energy for particle acceleration originates from the reconnecting magnetic field. We track the radial growth and relative velocity of a reconnecting plasmoid, modelling particle acceleration and radiative losses, which occur via synchrotron and synchrotron self-Compton (SSC) emission. We assume the reconnection layer consists of merging plasmoids, which grow as they merge. We use our model to successfully fit to the 2016 TeV flare of BL Lacertae, demonstrating that reconnection is able to produce TeV flares on the observed timescales. We find that the flaring light curve depends on the merge timescale and the magnetisation of the emitting plasma. However, the SEDs produced by our model are all synchrotron-dominated, and observable reconnection plasmoids have radii comparable to the jet radius. We conclude that while reconnection powered SSC flares can produce flares on the correct timescale, the primary source of emission cannot be SSC and the size of plasmoids required may be implausibly large.
Abstract: 1810.02823
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Full Text: [ PostScript, PDF]
Title: Active Galactic Nuclei and the Origin of IceCube's Diffuse Neutrino Flux
(Submitted on 5 Oct 2018)
Abstract: The excess of neutrino candidate events detected by IceCube from the direction of TXS 0506+056 has generated a great deal of interest in blazars as sources of high-energy neutrinos. In this study, we analyze the publicly available portion of the IceCube dataset, performing searches for neutrino point sources in spatial coincidence with the blazars and other active galactic nuclei contained in the Fermi 3LAC and the Roma BZCAT catalogs, as well as in spatial and temporal coincidence with flaring sources identified in the Fermi Collaboration's All-Sky Variability Analysis (FAVA). We find no evidence that blazars generate a significant flux of high-energy neutrinos, and conclude that no more than 5-15% of the diffuse flux measured by IceCube can originate from this class of objects. While we cannot rule out the possibility that TXS 0506+056 has at times generated significant neutrino emission, we find that such behavior cannot be common among blazars, requiring TXS 0506+056 to be a rather extreme outlier and not representative of the overall blazar population. The bulk of the diffuse high-energy neutrino flux must instead be generated by a significantly larger population of less-luminous sources, such as non-blazar active galactic nuclei.
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