240 research outputs found
Multiple synchrotron self-Compton modeling of gamma-ray flares in 3C 279
The correlation often observed in blazars between optical-to-radio outbursts
and gamma-ray flares suggests that the high-energy emission region shall be
co-spatial with the radio knots, several parsecs away from the central engine.
This would prevent the important contribution at high-energies from the Compton
scattering of seed photons from the accretion disk and the broad-line region
that is generally used to model the spectral energy distribution of
low-frequency peaking blazars. While a pure synchrotron self-Compton model has
so far failed to explain the observed gamma-ray emission of a flat spectrum
radio quasar like 3C 279, the inclusion of the effect of multiple
inverse-Compton scattering might solve the apparent paradox. Here, we present
for the first time a physical, self-consistent SSC modeling of a series of
shock-waves in the jet of 3C 279. We show that the analytic description of the
high-energy emission from multiple inverse-Compton scatterings in the
Klein-Nishina limit can fairly well account for the observed gamma-ray spectrum
of 3C 279 in flaring states.Comment: 6 pages, 3 figures, proceedings of "Beamed and Unbeamed Gamma-rays
from Galaxies", 11-15 April 2011, Finland. To be published in the Journal of
Physics: Conference Serie
Atacama compact array observations of the pulsar-wind nebula of SNR 0540-69.3
We present observations of the pulsar-wind nebula (PWN) region of SNR 0540-69.3. The observations were made with the Atacama Compact Array (ACA) in Bands 4 and 6. We also add radio observations from the Australia Compact Array at 3 cm. For 1.449–233.50 GHz, we obtain a synchrotron spectrum Fν∝ν−αν, with the spectral index αν = 0.17 ± 0.02. To conclude how this joins the synchrotron spectrum at higher frequencies, we include hitherto unpublished AKARI mid-infrared data, and evaluate published data in the ultraviolet (UV), optical, and infrared (IR). In particular, some broad-band filter data in the optical must be discarded from our analysis due to contamination by spectral line emission. For the UV/IR part of the synchrotron spectrum, we arrive at αν=0.87+0.08−0.10. There is room for 2.5 × 10−3 M⊙ of dust with a temperature of ∼55 K if there are dual breaks in the synchrotron spectrum, one around ∼9 × 1010 Hz and another at ∼2 × 1013 Hz. The spectral index then changes at ∼9 × 1010 Hz from αν = 0.14 ± 0.07 in the radio to αν=0.35−0.07+0.05 in the millimetre-to-far-IR range. The ACA Band 6 data marginally resolve the PWN. In particular, the strong emission ∼1.′′5 south-west of the pulsar, seen at other wavelengths, and resolved in the 3 cm data with its 0.″8 spatial resolution, is also strong in the millimetre range. The ACA data clearly reveal the supernova remnant shell ∼20–35 arcsec west of the pulsar, and for the shell we derive αν = 0.64 ± 0.05 for the range 8.6–145 GHz
Spectral evolution and polarization of variable structures in the pulsar wind nebula of PSR B0540-69.3
We present high spatial resolution optical imaging and polarization
observations of the PSR B0540-69.3 and its highly dynamical pulsar wind nebula
(PWN) performed with HST, and compare them with X-ray data obtained with the
Chandra X-ray Observatory. We have studied the bright region southwest of the
pulsar where a bright "blob" is seen in 1999. We show that it may be a result
of local energy deposition around 1999, and that the emission from this then
faded away. Polarization data from 2007 show that the polarization properties
show dramatic spatial variations at the 1999 blob position arguing for a local
process. Several other positions along the pulsar-"blob" orientation show
similar changes in polarization, indicating previous recent local energy
depositions. In X-rays, the spectrum steepens away from the "blob" position,
faster orthogonal to the pulsar-"blob" direction than along this axis of
orientation. This could indicate that the pulsar-"blob" orientation is an axis
along where energy in the PWN is mainly injected, and that this is then
mediated to the filaments in the PWN by shocks. We highlight this by
constructing an [S II]-to-[O III]-ratio map. We argue, through modeling, that
the high [S II]/[O III] ratio is not due to time-dependent photoionization
caused by possible rapid Xray emission variations in the "blob" region. We have
also created a multiwavelength energy spectrum for the "blob" position showing
that one can, to within 2sigma, connect the optical and X-ray emission by a
single power law. We obtain best power-law fits for the X-ray spectrum if we
include "extra" oxygen, in addition to the oxygen column density in the
interstellar gas of the Large Magellanic Cloud and the Milky Way. This oxygen
is most naturally explained by the oxygen-rich ejecta of the supernova remnant.
The oxygen needed likely places the progenitor mass in the 20 - 25 Msun range.Comment: Accepted by MNRAS on December 6th 2010, 18 pages, 15 figures. The
article with full resolution figures is available here
ftp://ftp.astro.su.se/pub/peter/papers/pwn0540_2010_corrected.pd
What is the progenitor of the Type Ia SN 2014J?
We report the deepest radio interferometric observations of the closest Type Ia supernova in decades, SN 2014J, which exploded in the nearby galaxy M 82. These observations
represent, together with radio observations of SNe 2011fe, the most sensitive radio studies of a Type Ia SN ever. We constrain the mass-loss rate from the progenitor system of
SN 2014J lower than 7.0 × 10^(−10) M yr^(−1) (for a wind speed of 100 km s^(−1) ). Our deep upper limits favor a double-degenerate scenario–involving two WD stars–for the progenitor system of SN 2014J, as such systems have less circumstellar gas than our upper limits. By contrast,
most single-degenerate scenarios, i.e., the wide family of progenitor systems where a red giant, main-sequence, or sub-giant star donates mass to a exploding white dwarf, are ruled out by our observations. The evidence from SNe 2011fe and 2014J points in the direction of a double-degenerate scenario for both
Variable polarization in the optical afterglow of GRB 021004
We present polarimetric observations of the afterglow of gamma-ray burst
(GRB) 021004, obtained with the Nordic Optical Telescope (NOT) and the Very
Large Telescope (VLT) between 8 and 17 hours after the burst. Comparison among
the observations shows a 45 degree change in the position angle from 9 hours
after the burst to 16 hours after the burst, and comparison with published data
from later epochs even shows a 90 degree change between 9 and 89 hours after
the burst. The degree of linear polarization shows a marginal change, but is
also consistent with being constant in time. In the context of currently
available models for changes in the polarization of GRBs, a homogeneous jet
with an early break time of t_b ~ 1 day provides a good explanation of our
data. The break time is a factor 2 to 6 earlier than has been found from the
analysis of the optical light curve. The change in the position angle of the
polarization rules out a structured jet model for the GRB.Comment: 5 pages, 2 figures. Published in A&A letter
Optical and near-infrared observations of the GRB020405 afterglow
(Abridged) We report on observations of the optical and NIR afterglow of
GRB020405. Ground-based optical observations started about 1 day after the GRB
and spanned a period of ~10 days; archival HST data extended the coverage up to
70 days after the GRB. We report the first detection of the afterglow in NIR
bands. The detection of emission lines in the optical spectrum indicates that
the GRB is located at z = 0.691. Absorptions are also detected at z = 0.691 and
at z = 0.472. The latter system is likely caused by clouds in a galaxy located
2 arcsec southwest of the GRB host. Hence, for the first time, the galaxy
responsible for an intervening absorption system in the spectrum of a GRB
afterglow is identified. Optical and NIR photometry indicates that the decay in
all bands follows a single power law of index alpha = 1.54. The late-epoch VLT
and HST points lie above the extrapolation of this power law, so that a plateau
is apparent in the VRIJ light curves at 10-20 days after the GRB. The light
curves at epochs later than day ~20 after the GRB are consistent with a
power-law decay with index alphaprime = 1.85. We suggest that this deviation
can be modeled with a SN having the same temporal profile as SN2002ap, but 1.3
mag brighter at peak, and located at the GRB redshift. Alternatively, a shock
re-energization may be responsible for the rebrightening. A polarimetric R-band
measurement shows that the afterglow is polarized, with P = 1.5 % and theta =
172 degrees. Optical-NIR spectral flux distributions show a change of slope
across the J band which we interpret as due to the presence of nu_c. The
analysis of the multiwavelength spectrum within the fireball model suggests
that a population of relativistic electrons produces the optical-NIR emission
via synchrotron in an adiabatically expanding blastwave, and the X-rays via IC.Comment: 17 pages, 10 figures, 4 tables, accepted for publication on A&A, main
journa
Measuring energy dependent polarization in soft gamma-rays using Compton scattering in PoGOLite
Linear polarization in X- and gamma-rays is an important diagnostic of many
astrophysical sources, foremost giving information about their geometry,
magnetic fields, and radiation mechanisms. However, very few X-ray polarization
measurements have been made, and then only mono-energetic detections, whilst
several objects are assumed to have energy dependent polarization signatures.
In this paper we investigate whether detection of energy dependent polarization
from cosmic sources is possible using the Compton technique, in particular with
the proposed PoGOLite balloon-experiment, in the 25-100 keV range. We use
Geant4 simulations of a PoGOLite model and input photon spectra based on Cygnus
X-1 and accreting magnetic pulsars (100 mCrab). Effective observing times of 6
and 35 hours were simulated, corresponding to a standard and a long duration
flight respectively. Both smooth and sharp energy variations of the
polarization are investigated and compared to constant polarization signals
using chi-square statistics. We can reject constant polarization, with energy,
for the Cygnus X-1 spectrum (in the hard state), if the reflected component is
assumed to be completely polarized, whereas the distinction cannot be made for
weaker polarization. For the accreting pulsar, constant polarization can be
rejected in the case of polarization in a narrow energy band with at least 50%
polarization, and similarly for a negative step distribution from 30% to 0%
polarization.Comment: 11 pages, 12 figures; updated to match version accepted for
publication in Astroparticle Physics (only minor changes
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