24,722 research outputs found
Spectral analysis of Markarian 421 and Markarian 501 with HAWC
The Hight Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory monitors the
gamma-ray sky in the energy range from 100 GeV to 100 TeV and has detected two
very high energy (VHE) blazars: Markarian 421 (Mrk 421) and Markarian 501 (Mrk
501) in 1.5 years of observations. In this work, we present the spectral
analysis above 1 TeV of both sources using a maximum likelihood method and an
artificial neural network as an energy estimator. The main objectives are to
constrain the spectral curvature of Mrk 421 and Mrk 501 at 5 TeV using
the EBL models from Gilmore et al. (2012) and Franceschini et al. (2008).Comment: Presented at the 35th International Cosmic Ray Conference (ICRC2017),
Bexco, Busan, Korea. See arXiv:1708.02572 for all HAWC contribution
Electromagnetic Energy, Absorption, and Casimir Forces. Inhomogeneous Dielectric Media
A general, exact formula is derived for the expectation value of the
electromagnetic energy density of an inhomogeneous absorbing and dispersive
dielectric medium in thermal equilibrium, assuming that the medium is well
approximated as a continuum. From this formula we obtain the formal expression
for the Casimir force density. Unlike most previous approaches to Casimir
effects in which absorption is either ignored or admitted implicitly through
the required analytic properties of the permittivity, we include dissipation
explicitly via the coupling of each dipole oscillator of the medium to a
reservoir of harmonic oscillators. We obtain the energy density and the Casimir
force density as a consequence of the van der Waals interactions of the
oscillators and also from Poynting's theorem.Comment: 13 pages, no figures. Updated version with generalization to finite
temperature and added example
The Progenitors of Recent Core-Collapse Supernovae
We present the results of our analysis of Hubble Space Telescope (HST) and deep ground-based images to isolate the massive progenitor stars of the two recent core-collapse supernovae 2008 bk and 2008 cn. The identification of the progenitors is facilitated in one of these two cases by high-precision astrometry based on our HST imaging of SNe at late times
Sub-au imaging of water vapour clouds around four Asymptotic Giant Branch stars
We present MERLIN maps of the 22-GHz H2O masers around four low-mass
late-type stars (IK Tau U Ori, RT Vir and U Her), made with an angular
resolution of ~ 15 milliarcsec and a velocity resolution of 0.1 km s-1. The H2O
masers are found in thick expanding shells with inner radii ~ 6 to 16 au and
outer radii four times larger. The expansion velocity increases radially
through the H2O maser regions, with logarithmic velocity gradients of 0.5--0.9.
IK Tau and RT Vir have well-filled H2O maser shells with a spatial offset
between the near and far sides of the shell, which suggests that the masers are
distributed in oblate spheroids inclined to the line of sight. U Ori and U Her
have elongated poorly-filled shells with indications that the masers at the
inner edge have been compressed by shocks; these stars also show OH maser
flares. MERLIN resolves individual maser clouds, which have diameters of 2 -- 4
au and filling factors of only ~ 0.01 with respect to the whole H2O maser
shells. The CSE velocity structure gives additional evidence the maser clouds
are density bounded. Masing clouds can be identified over a similar timescale
to their sound crossing time (~2 yr) but not longer. The sizes and observed
lifetimes of these clouds are an order of magnitude smaller than those around
red supergiants, similar to the ratio of low-mass:high-mass stellar masses and
sizes. This suggests that cloud size is determined by stellar properties, not
local physical phenomena in the wind.Comment: 21 pages, including 14 figures and 8 tables. Accepted for publication
in MNRA
- ā¦