205 research outputs found
Polarization of synchrotron emission from relativistic reconfinement shocks with ordered magnetic fields
We calculate the polarization of synchrotron radiation produced at the
relativistic reconfinement shocks, taking into account globally ordered
magnetic field components, in particular toroidal and helical fields. In these
shocks, toroidal fields produce high parallel polarization (electric vectors
parallel to the projected jet axis), while chaotic fields generate moderate
perpendicular polarization. Helical fields result in a non-axisymmetric
distribution of the total and polarized brightness. For a diverging downstream
velocity field, the Stokes parameter U does not vanish and the average
polarization is neither strictly parallel nor perpendicular. A distance at
which the downstream flow is changing from diverging to converging can be
easily identified on polarization maps as the turning point, at which
polarization vectors switch, e.g., from clockwise to counterclockwise.Comment: 10 pages, 6 figures, accepted for publication in A&
Early phase observations of extremely luminous Type Ia Supernova 2009dc
We present early phase observations in optical and near-infrared wavelengths
for the extremely luminous Type Ia supernova (SN Ia) 2009dc. The decline rate
of the light curve is , which is one of the
slowest among SNe Ia. The peak -band absolute magnitude is mag even if the host extinction is mag. It reaches
mag for the host extinction of mag as
inferred from the observed Na {\sc i} D line absorption in the host. Our
-band photometry shows that the SN is one of the most luminous SNe Ia
also in near-infrared wavelengths. These results indicate that SN 2009dc
belongs to the most luminous class of SNe Ia, like SN 2003fg and SN 2006gz. We
estimate the ejected Ni mass of \Msun for no host
extinction case (or 1.6 0.4 M for the host extinction of
mag). The C {\sc ii} 6580 absorption line keeps visible
until a week after maximum, which diminished in SN 2006gz before its maximum
brightness. The line velocity of Si {\sc ii} 6355 is about 8000 km
s around the maximum, being considerably slower than that of SN 2006gz,
while comparable to that of SN 2003fg. The velocity of the C {\sc ii} line is
almost comparable to that of the Si {\sc ii}. The presence of the carbon line
suggests that thick unburned C+O layers remain after the explosion. SN 2009dc
is a plausible candidate of the super-Chandrasekhar mass SNe Ia
The characterization of the distant blazar GB6 J1239+0443 from flaring and low activity periods
In 2008 AGILE and Fermi detected gamma-ray flaring activity from the
unidentified EGRET source 3EG J1236+0457, recently associated with a flat
spectrum radio quasar GB6 J1239+0443 at z=1.762. The optical counterpart of the
gamma-ray source underwent a flux enhancement of a factor 15-30 in 6 years, and
of ~10 in six months. We interpret this flare-up in terms of a transition from
an accretion-disk dominated emission to a synchrotron-jet dominated one. We
analysed a Sloan Digital Sky Survey (SDSS) archival optical spectrum taken
during a period of low radio and optical activity of the source. We estimated
the mass of the central black hole using the width of the CIV emission line. In
our work, we have also investigated SDSS archival optical photometric data and
UV GALEX observations to estimate the thermal-disk emission contribution of GB6
J1239+0443. Our analysis of the gamma-ray data taken during the flaring
episodes indicates a flat gamma-ray spectrum, with an extension of up to 15
GeV, with no statistically-relevant sign of absorption from the broad line
region, suggesting that the blazar-zone is located beyond the broad line
region. This result is confirmed by the modeling of the broad-band spectral
energy distribution (well constrained by the available multiwavelength data) of
the flaring activity periods and by the accretion disk luminosity and black
hole mass estimated by us using archival data.Comment: 30 pages, 7 figures, 4 tables MNRAS Accepted on 2012 June 1
GRB 091208B: First Detection of the Optical Polarization in Early Forward Shock Emission of a Gamma-Ray Burst Afterglow
We report that the optical polarization in the afterglow of GRB 091208B is
measured at t = 149 - 706 s after the burst trigger, and the polarization
degree is P = 10.4% +/- 2.5%. The optical light curve at this time shows a
power-law decay with index -0.75 +/- 0.02, which is interpreted as the forward
shock synchrotron emission, and thus this is the first detection of the
early-time optical polarization in the forward shock (rather than that in the
reverse shock reported by Steele et al. (2009). This detection disfavors the
afterglow model in which the magnetic fields in the emission region are random
on the plasma skin depth scales, such as amplified by the plasma instabilities,
e.g., Weibel instability. We suggest that the fields are amplified by the
magnetohydrodynamic instabilities, which would be tested by future observations
of the temporal changes of the polarization degrees and angles for other
bursts.Comment: 12 pages, 4 figures, accepted for publication in ApJ Letter
Modelling spectral and timing properties of accreting black holes: the hybrid hot flow paradigm
The general picture that emerged by the end of 1990s from a large set of
optical and X-ray, spectral and timing data was that the X-rays are produced in
the innermost hot part of the accretion flow, while the optical/infrared (OIR)
emission is mainly produced by the irradiated outer thin accretion disc. Recent
multiwavelength observations of Galactic black hole transients show that the
situation is not so simple. Fast variability in the OIR band, OIR excesses
above the thermal emission and a complicated interplay between the X-ray and
the OIR light curves imply that the OIR emitting region is much more compact.
One of the popular hypotheses is that the jet contributes to the OIR emission
and even is responsible for the bulk of the X-rays. However, this scenario is
largely ad hoc and is in contradiction with many previously established facts.
Alternatively, the hot accretion flow, known to be consistent with the X-ray
spectral and timing data, is also a viable candidate to produce the OIR
radiation. The hot-flow scenario naturally explains the power-law like OIR
spectra, fast OIR variability and its complex relation to the X-rays if the hot
flow contains non-thermal electrons (even in energetically negligible
quantities), which are required by the presence of the MeV tail in Cyg X-1. The
presence of non-thermal electrons also lowers the equilibrium electron
temperature in the hot flow model to <100 keV, making it more consistent with
observations. Here we argue that any viable model should simultaneously explain
a large set of spectral and timing data and show that the hybrid
(thermal/non-thermal) hot flow model satisfies most of the constraints.Comment: 26 pages, 13 figures. To be published in the Space Science Reviews
and as hard cover in the Space Sciences Series of ISSI - The Physics of
Accretion on to Black Holes (Springer Publisher
PKS 1502+106: a new and distant gamma-ray blazar in outburst discovered by the Fermi Large Area Telescope
The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope
discovered a rapid (about 5 days duration), high-energy (E >100 MeV) gamma-ray
outburst from a source identified with the blazar PKS 1502+106 (OR 103, S3
1502+10, z=1.839) starting on August 05, 2008 and followed by bright and
variable flux over the next few months. Results on the gamma-ray localization
and identification, as well as spectral and temporal behavior during the first
months of the Fermi all-sky survey are reported here in conjunction with a
multi-waveband characterization as a result of one of the first Fermi
multi-frequency campaigns. The campaign included a Swift ToO (followed up by
16-day observations on August 07-22, MJD 54685-54700), VLBA (within the MOJAVE
program), Owens Valley (OVRO) 40m, Effelsberg-100m, Metsahovi-14m, RATAN-600
and Kanata-Hiroshima radio/optical observations. Results from the analysis of
archival observations by INTEGRAL, XMM-Newton and Spitzer space telescopes are
reported for a more complete picture of this new gamma-ray blazar.Comment: 17 pages, 11 figures, accepted for The Astrophysical Journa
- âŠ