688 research outputs found
Effects of Neutral Hydrogen on Cosmic Ray Precursors in Supernova Remnant Shock Waves
Many fast supernova remnant shocks show spectra dominated by Balmer lines.
The H profiles have a narrow component explained by direct excitations
and a thermally Doppler broadened component due to atoms that undergo charge
exchange in the post-shock region. However, the standard model does not take
into account the cosmic-ray shock precursor, which compresses and accelerates
plasma ahead of the shock. In strong precursors with sufficiently high
densities, the processes of charge exchange, excitation and ionization will
affect the widths of both narrow and broad line components. Moreover, the
difference in velocity between the neutrals and the precursor plasma gives rise
to frictional heating due to charge exchange and ionization in the precursor.
In extreme cases, all neutrals can be ionized by the precursor.
In this paper we compute the ion and electron heating for a wide range of
shock parameters, along with the velocity distribution of the neutrals that
reach the shock. Our calculations predict very large narrow component widths
for some shocks with efficient acceleration, along with changes in the broad-
to-narrow intensity ratio used as a diagnostic for the electron-ion temperature
ratio. Balmer lines may therefore provide a unique diagnostic of precursor
properties. We show that heating by neutrals in the precursor can account for
the observed H narrow component widths, and that the acceleration
efficiency is modest in most Balmer line shocks observed thus far.Comment: 9 pages, 3 figure
The relation between post-shock temperature, cosmic-ray pressure and cosmic-ray escape for non-relativistic shocks
Supernova remnants are thought to be the dominant source of Galactic cosmic
rays. This requires that at least 5% of the available energy is transferred to
cosmic rays, implying a high cosmic-ray pressure downstream of supernova
remnant shocks. Recently, it has been shown that the downstream temperature in
some remnants is low compared to the measured shock velocities, implying that
additional pressure support by accelerated particles is present.
Here we use a two-fluid thermodynamic approach to derive the relation between
post-shock fractional cosmic-ray pressure and post-shock temperature, assuming
no additional heating beyond adiabatic heating in the shock precursor and with
all non-adiabatic heating occurring at the subshock. The derived relations show
that a high fractional cosmic-ray pressure is only possible, if a substantial
fraction of the incoming energy flux escapes from the system. Recently a shock
velocity and a downstream proton temperature were measured for a shock in the
supernova remnant RCW 86. We apply the two-fluid solutions to these
measurements and find that the the downstream fractional cosmic-ray pressure is
at least 50% with a cosmic-ray energy flux escape of at least 20%. In general,
in order to have 5% of the supernova energy go into accelerating cosmic rays,
on average the post-shock cosmic-ray pressure needs to be 30% for an effective
cosmic-ray adiabatic index of 4/3.Comment: 9 pages, 6 color figures. This is updated with a corrected figure 5a
and 5b, reflecting an ApJ erratu
Proper Motions of H-alpha filaments in the Supernova Remnant RCW 86
We present a proper motion study of the eastern shock-region of the supernova
remnant RCW 86 (MSH 14-63, G315.4-2.3), based on optical observations carried
out with VLT/FORS2 in 2007 and 2010. For both the northeastern and southeastern
regions, we measure an average proper motion of H-alpha filaments of 0.10 +/-
0.02 arcsec/yr, corresponding to 1200 +/- 200 km/s at 2.5kpc. There is
substantial variation in the derived proper motions, indicating shock
velocities ranging from just below 700 km/s to above 2200 km/s.
The optical proper motion is lower than the previously measured X-ray proper
motion of northeastern region. The new measurements are consistent with the
previously measured proton temperature of 2.3 +/- 0.3 keV, assuming no
cosmic-ray acceleration. However, within the uncertainties, moderately
efficient (< 27 per cent) shock acceleration is still possible. The combination
of optical proper motion and proton temperature rule out the possibility that
RCW 86 has a distance less than 1.5kpc.
The similarity of the proper motions in the northeast and southeast is
peculiar, given the different densities and X-ray emission properties of the
regions. The northeastern region has lower densities and the X-ray emission is
synchrotron dominated, suggesting that the shock velocities should be higher
than in the southeastern, thermal X-ray dominated, region. A possible solution
is that the H-alpha emitting filaments are biased toward denser regions, with
lower shock velocities. Alternatively, in the northeast the shock velocity may
have decreased rapidly during the past 200yr, and the X-ray synchrotron
emission is an afterglow from a period when the shock velocity was higher.Comment: Accepted for publication in MNRA
Temperature Equilibration Behind the Shock Front: an Optical and X-ray Study of RCW 86
We study the electron-proton temperature equilibration behind several shocks
of the RCW 86 supernova remnant. To measure the proton temperature, we use
published and new optical spectra, all from different locations on the remnant.
For each location, we determine the electron temperature from X-ray spectra,
and correct for temperature equilibration between the shock front and the
location of the X-ray spectrum. We confirm the result of previous studies that
the electron and proton temperatures behind shock fronts are consistent with
equilibration for slow shocks and deviate for faster shocks. However, we can
not confirm the previously reported trend of the electron temperature to proton
temperature ratio of 1/v^2.Comment: Accepted for publication by ApJ, 8 pages, 7 figures, 2 table
Chandra observations of SN 1987A: the soft X-ray light curve revisited
We report on the present stage of SN 1987A as observed by the Chandra X-ray
Observatory. We reanalyze published Chandra observations and add three more
epochs of Chandra data to get a consistent picture of the evolution of the
X-ray fluxes in several energy bands. We discuss the implications of several
calibration issues for Chandra data. Using the most recent Chandra calibration
files, we find that the 0.5-2.0 keV band fluxes of SN 1987A have increased by
~6 x 10 ^-13 erg s^-1 cm^-2 per year since 2009. This is in contrast with our
previous result that the 0.5-2.0 keV light curve showed a sudden flattening in
2009. Based on our new analysis, we conclude that the forward shock is still in
full interaction with the equatorial ring.Comment: Accepted for publication by ApJ, 7 pages, 5 figure
A method for the optimal design of composite continuum structures
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76840/1/AIAA-1998-4984-312.pd
Measuring the cosmic ray acceleration efficiency of a supernova remnant
Cosmic rays are the most energetic particles arriving at earth. Although most
of them are thought to be accelerated by supernova remnants, the details of the
acceleration process and its efficiency are not well determined. Here we show
that the pressure induced by cosmic rays exceeds the thermal pressure behind
the northeast shock of the supernova remnant RCW 86, where the X-ray emission
is dominated by synchrotron radiation from ultra-relativistic electrons. We
determined the cosmic-ray content from the thermal Doppler broadening measured
with optical spectroscopy, combined with a proper-motion study in X- rays. The
measured post-shock proton temperature in combination with the shock velocity
does not agree with standard shock heating, implying that >50% of the
post-shock pressure is produced by cosmic rays.Comment: Published in Science express, 10 pages, 5 figures and 2 table
Simultaneous design of structural topology and material properties
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77309/1/AIAA-2000-4913-878.pd
Solar XXI building PV systems: performance during the first two yeras of operation
The purpose of this paper is to present the results obtained in the first two years of operation of the grid-connected photovoltaic (PV) systems installed in the named “Solar XXI” building. One PV system, made with multicrystalline silicon modules, has a peak power of 12 kW and was installed on the façade; another system made with amorphous silicon modules has a peak power of 6 kW and was installed in the surrounding park area near the building. From 1st February 2006 until 31 July 2008, the measured daily average, of the building electrical energy consumption, was about 75 kWh and the two PV systems produced in average about 72 % of this energy. The averaged measured Performance Ratio of the systems was about 0.84 for the PV Façade and about 0.76 for the PV in the Park
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