3,268 research outputs found
Dynamic instability in resonant tunneling
We show that an instability may be present in resonant tunneling through a
quantum well in one, two and three dimensions, when the resonance lies near the
emitter Fermi level. A simple semiclassical model which simulates the resonance
and the projected density of states by a nonlinear conductor, the Coulomb
barrier by a capacitance, and the time evolution by an iterated map, is used.
The model reproduces the observed hysteresis in such devices, and exhibits a
series of bifurcations leading to fast chaotic current fluctuations.Comment: 7 pages, 2 figure
The Unusual Super-Luminous Supernovae SN 2011kl and ASASSN-15lh
Two recently discovered very luminous supernovae (SNe) present stimulating
cases to explore the extents of the available theoretical models. SN 2011kl
represents the first detection of a supernova explosion associated with an
ultra-long duration gamma ray burst. ASASSN-15lh was even claimed as the most
luminous SN ever discovered, challenging the scenarios so far proposed for
stellar explosions. Here we use our radiation hydrodynamics code in order to
simulate magnetar powered SNe. To avoid explicitly assuming neutron star
properties we adopt the magnetar luminosity and spin-down timescale as free
parameters of the model. We find that the light curve (LC) of SN 2011kl is
consistent with a magnetar power source, as previously proposed, but we note
that some amount of 56^Ni (> 0.08 M_sun) is necessary to explain the low
contrast between the LC peak and tail. For the case of ASASSN-15lh we find
physically plausible magnetar parameters that reproduce the overall shape of
the LC provided the progenitor mass is relatively large (a mass of the ejecta
approx 6 M_sun). The ejecta hydrodynamics of this event is dominated by the
magnetar input, while the effect is more moderate for SN 2011kl. We conclude
that a magnetar model may be used for the interpretation of these events and
that the hydrodynamic modeling is necessary to derive the properties of
powerful magnetars and their progenitors.Comment: Accepted by Astrophysical Journal Letters, 5 pages, 5 figure
Leptonic secondary emission in a hadronic microquasar model
Context: It has been proposed that the origin of the very high-energy photons
emitted from high-mass X-ray binaries with jet-like features, so-called
microquasars (MQs), is related to hadronic interactions between relativistic
protons in the jet and cold protons of the stellar wind. Leptonic secondary
emission should be calculated in a complete hadronic model that include the
effects of pairs from charged pion decays inside the jets and the emission from
pairs generated by gamma-ray absorption in the photosphere of the system. Aims:
We aim at predicting the broadband spectrum from a general hadronic microquasar
model, taking into account the emission from secondaries created by charged
pion decay inside the jet. Methods: The particle energy distribution for
secondary leptons injected along the jets is consistently derived taking the
energy losses into account. We also compute the spectral energy distribution
resulting from these leptons is calculated after assuming different values of
the magnetic field inside the jets. The spectrum of the gamma-rays produced by
neutral pion-decay and processed by electromagnetic cascades under the stellar
photon field. Results: We show that the secondary emission can dominate the
spectral energy distribution at low energies (~1 MeV). At high energies, the
production spectrum can be significantly distorted by the effect of
electromagnetic cascades. These effects are phase-dependent, and some
variability modulated by the orbital period is predicted.Comment: 8 pages, 5 figures. Accepted for publication in Astronomy &
Astrophysic
The Canada-France High-z Quasar Survey: 1.2mm Observations
We report 250 GHz (1.2 mm) observations of a sample of 20 QSOs at redshifts
5.8<z<6.5 from the the Canada-France High-z Quasar Survey (CFHQS), using the
Max-Planck Millimeter Bolometer (MAMBO) array at the IRAM 30-metre telescope. A
rms sensitivity <~ 0.6 mJy was achieved for 65% of the sample, and <~ 1.0 mJy
for 90%. Only one QSO, CFHQS J142952+544717, was robustly detected with
S_250GHz = 3.46 +/-0.52 mJy. This indicates that one of the most powerful known
starbursts at z~6 is associated with this radio loud QSO. On average, the other
CFHQS QSOs, which have a mean optical magnitude fainter than previously studied
SDSS samples of z~6 QSOs, have a mean 1.2 mm flux density = 0.41
+/-0.14 mJy; such a 2.9-sigma average detection is hardly meaningful. It would
correspond to ~ 0.94+/-0.32 10^12 Lo, and an average star formation
rate of a few 100's Mo/yr, depending on the IMF and a possible AGN contribution
to . This is consistent with previous findings of Wang et al. (2011) on
the far-infrared emission of z~6 QSOs and extends them toward optically fainter
sources.Comment: 6 pages, 1 figure, A&A in pres
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