20 research outputs found
Evidence for Cosmic Ray Acceleration in Cassiopeia A
Combining archival data taken at radio and infrared wavelengths with
state-of-the-art measurements at X-ray and gamma-ray energies, we assembled a
broadband spectral energy distribution (SED) of Cas A, a young supernova
remnant. Except for strong thermal emission at infrared and X-ray wavelengths,
the SED is dominated by non-thermal radiation. We attempted to model the
non-thermal SED with a two-zone leptonic model which assumes that the radio
emission is produced by electrons that are uniformly distributed throughout the
remnant while the non-thermal X-ray emission by electrons that are localized in
regions near the forward shock. Synchrotron emission from the electrons can
account for data from radio to X-ray wavelengths. Much of the GeV-TeV emission
can also be explained by a combination of bremsstrahlung emission and
inverse-Compton scattering (mainly of infrared thermal photons). However, the
model cannot fit a distinct feature at GeV energies. This feature can be well
accounted for by adding a pion-zero emission component to the model, providing
evidence for cosmic ray production in Cas A. We discuss the implications of the
results.Comment: 16 pages, 4 figure
The GMRT EoR Experiment: Limits on Polarized Sky Brightness at 150 MHz
The GMRT reionization effort aims to map out the large scale structure of the
Universe during the epoch of reionization (EoR). Removal of polarized Galactic
emission is a difficult part of any 21 cm EoR program, and we present new upper
limits to diffuse polarized foregrounds at 150 MHz. We find no high
significance evidence of polarized emission in our observed field at mid
galactic latitude (J2000 08h26m+26). We find an upper limit on the
2-dimensional angular power spectrum of diffuse polarized foregrounds of [l^2
C_l/(2 PI)]^{1/2}< 3K in frequency bins of width 1 MHz at 300<l<1000. The
3-dimensional power spectrum of polarized emission, which is most directly
relevant to EoR observations, is [k^3 P_p(k)/(2 PI^2)]^{1/2}
0.03 h/Mpc, k < 0.1 h/Mpc. This can be compared to the expected EoR signal in
total intensity of [k^3 P(k)/ (2 PI^2) ]^{1/2} ~ 10 mK. We find polarized
structure is substantially weaker than suggested by extrapolation from higher
frequency observations, so the new low upper limits reported here reduce the
anticipated impact of these foregrounds on EoR experiments. We discuss Faraday
beam and depth depolarization models and compare predictions of these models to
our data. We report on a new technique for polarization calibration using
pulsars, as well as a new technique to remove broadband radio frequency
interference. Our data indicate that, on the edges of the main beam at GMRT,
polarization squint creates ~ 3% leakage of unpolarized power into polarized
maps at zero rotation measure. Ionospheric rotation was largely stable during
these solar minimum night time observations.Comment: 17 pages, 6 figures, 2 tables; changed figures, added appendices. To
appear in MNRA
Seven-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Planets and Celestial Calibration Sources
(Abridged) We present WMAP seven-year observations of bright sources which
are often used as calibrators at microwave frequencies. Ten objects are studied
in five frequency bands (23 - 94 GHz): the outer planets (Mars, Jupiter,
Saturn, Uranus and Neptune) and five fixed celestial sources (Cas A, Tau A, Cyg
A, 3C274 and 3C58). The seven-year analysis of Jupiter provides temperatures
which are within 1-sigma of the previously published WMAP five-year values,
with slightly tighter constraints on variability with orbital phase, and limits
(but no detections) on linear polarization. Scaling factors are provided which,
when multiplied by the Wright Mars thermal model predictions at 350 micron,
reproduce WMAP seasonally averaged observations of Mars within ~2%. An
empirical model is described which fits brightness variations of Saturn due to
geometrical effects and can be used to predict the WMAP observations to within
3%. Seven-year mean temperatures for Uranus and Neptune are also tabulated.
Uncertainties in Uranus temperatures are 3%-4% in the 41, 61 and 94 GHz bands;
the smallest uncertainty for Neptune is ~8% for the 94 GHz band. Intriguingly,
the spectrum of Uranus appears to show a dip at ~30 GHz of unidentified origin,
although the feature is not of high statistical significance. Flux densities
for the five selected fixed celestial sources are derived from the seven-year
WMAP sky maps, and are tabulated for Stokes I, Q and U, along with polarization
fraction and position angle. Fractional uncertainties for the Stokes I fluxes
are typically 1% to 3%. Source variability over the seven-year baseline is also
estimated. Significant secular decrease is seen for Cas A and Tau A: our
results are consistent with a frequency independent decrease of about 0.53% per
year for Cas A and 0.22% per year for Tau A.Comment: 72 pages, 21 figures; accepted to ApJS; (v2) corrected Mars model
scaling factors, added figure 21, added text to Mars, Saturn and celestial
sources section
Time-dependent modeling of pulsar wind nebulae: Study on the impact of the diffusion-loss approximations
In this work, we present a leptonic, time-dependent model of pulsar wind
nebulae (PWNe). The model seeks a solution for the lepton distribution function
considering the full time-energy dependent diffusion-loss equation. The
time-dependent lepton population is balanced by injection, energy losses, and
escape. We include synchrotron, inverse Compton (IC, with the cosmic-microwave
background as well as with IR/optical photon fields), self-synchrotron Compton
(SSC), and bremsstrahlung processes, all devoid of any radiative
approximations. With this model in place we focus on the Crab nebula as an
example and present its time dependent evolution. Afterwards, we analyze the
impact of different approximations made at the level of the diffusion-loss
equation, as can be found in the literature. Whereas previous models ignored
the escape term, e.g., with the diffusion-loss equation becoming advective,
others approximated the losses as catastrophic, so that the equation has only
time derivatives. Additional approximations are also described and computed. We
show which is the impact of these approaches in the determination of the PWN
evolution. In particular, we find the time-dependent deviation of the
multi-wavelength spectrum and the best-fit parameters obtained with the
complete and the approximate models.Comment: In press in MNRA
Galactic electrons and positrons at the Earth:new estimate of the primary and secondary fluxes
We analyse predictions of the CR lepton fluxes at the Earth of both secondary
and primary origins, evaluate the theoretical uncertainties, and determine
their level of consistency with respect to the available data. For propagation,
we use a relativistic treatment of the energy losses for which we provide
useful parameterizations. We compute the secondary components by improving on
the method that we derived earlier for positrons. For primaries, we estimate
the contributions from astrophysical sources (supernova remnants and pulsars)
by considering all known local objects within 2 kpc and a smooth distribution
beyond. We find that the electron flux in the energy range 5-30 GeV is well
reproduced by a smooth distant distribution of sources with index , while local sources dominate the flux at higher energy. For
positrons, local pulsars have an important effect above 5-10 GeV. Uncertainties
affecting the source modeling and propagation are degenerate and each
translates into about one order of magnitude error in terms of local flux. The
spectral shape at high energy is weakly correlated with the spectral indices of
local sources, but more strongly with the hierarchy in their distance, age and
power. Despite the large theoretical errors that we describe, our global and
self-consistent analysis can explain all available data without over-tuning the
parameters, and therefore without the need to consider any exotic physics.
Though a \emph{standard paradigm} of Galactic CRs is well established, our
results show that we can hardly talk about any \emph{standard model} of CR
leptons, because of the very large theoretical uncertainties. Our analysis
provides details about the impact of these uncertainties, thereby sketching a
roadmap for future improvements.Comment: 34 pages, 14 figures. V2: few changes, results unchanged; matches the
version accepted in Astron. Astrophy
AMI observations of northern supernova remnants at 14-18 GHz
We present observations between 14.2 and 17.9 GHz of 12 reported supernova
remnants (SNRs) made with the Arcminute Microkelvin Imager Small Array (AMI
SA). In conjunction with data from the literature at lower radio frequencies,
we determine spectra of these objects. For well-studied SNRs (Cas A, Tycho's
SNR, 3C58 and the Crab Nebula), the results are in good agreement with spectra
based on previous results. For the less well-studied remnants the AMI SA
observations provide higher-frequency radio observations than previously
available, and better constrain their radio spectra. The AMI SA results confirm
a spectral turnover at ~11 GHz for the filled-centre remnant G74.9+1.2. We also
see a possible steepening of the spectrum of the filled-centre remnant
G54.1+0.3 within the AMI SA frequency band compared with lower frequencies. We
confirm that G84.9+0.5, which had previously been identified as a SNR, is
rather an HII region and has a flat radio spectrum.Comment: 12 pages, 24 figures, accepted MNRA
Low-frequency radio absorption in Cassiopeia A
Cassiopeia A is one of the best-studied supernova remnants. Its shocked
ejecta emits brightly in radio and X-rays. Its unshocked ejecta can be studied
through infrared emission, the radio-active decay of Ti, and low
frequency free-free absorption due to cold gas internal to the shell. Free-free
absorption is affected by the mass, geometry, temperature, and ionisation
conditions in the absorbing gas. Observations at the lowest radio frequencies
constrain a combination of these properties. We use LOFAR LBA observations at
30-77 MHz and L-band VLA observations to compare -matched images with a
common resolution of 17". We simultaneously fit, per pixel, for the emission
measure and the ratio of the emission from the unabsorbed front of the shell
versus the absorbed back of the shell. We explore the effects that low
temperatures and a high degree of clumping can have on the derived physical
properties, such as mass and density. We also compile published radio flux
measurements, fit for the absorption processes that occur in the radio band,
and consider how they affect the secular decline of the source. We find a mass
in the unshocked ejecta of for an assumed gas
temperature of K. This estimate is reduced for colder gas temperatures
and if the ejecta are clumped. We measure the reverse shock to have a radius of
" 6". We also find that a decrease in the amount of mass in the
unshocked ejecta (as more and more material meets the reverse shock and heats
up) cannot account for the observed low frequency behaviour of the secular
decline rate. To reconcile our low frequency absorption measurements with
models that predict little mass in the unshocked ejecta we need the ejecta to
be very clumped, or the temperature in the cold gas to be low ( K).
Both conditions can jointly contribute to the high absorption.Comment: Accepted for publication in A&A v2: including the DOI, language edit