541 research outputs found
A Deep Chandra Observation of Kepler's Supernova Remnant: A Type Ia Event with Circumstellar Interaction
We present initial results of a 750 ks Chandra observation of the remnant of
Kepler's supernova of AD 1604. The strength and prominence of iron emission,
together with the absence of O-rich ejecta, demonstrate that Kepler resulted
from a thermonuclear supernova, even though evidence for circumstellar
interaction is also strong. We have analyzed spectra of over 100 small regions,
and find that they fall into three classes. (1) The vast majority show Fe L
emission between 0.7 and 1 keV and Si and S K alpha emission; we associate
these with shocked ejecta. A few of these are found at or beyond the mean blast
wave radius. (2) A very few regions show solar O/Fe abundance rations; these we
associate with shocked circumstellar medium (CSM). Otherwise O is scarce. (3) A
few regions are dominated by continuum, probably synchrotron radiation.
Finally, we find no central point source, with a limit about 100 times fainter
than the central object in Cas A. The evidence that the blast wave is
interacting with CSM may indicate a Ia explosion in a more massive progenitor.Comment: Accepted by ApJ Letter
Constraints on the kinematics of the 44Ti ejecta of Cassiopeia A from INTEGRAL/SPI
The medium-lived 44Ti isotope is synthesised by explosive Si-burning in
core-collapse supernovae. It is extremely sensitive to the dynamics of the
explosion and therefore can be used to indirectly probe the explosion scenario.
The young supernova remnant Cassiopeia A is to date the only source of
gamma-ray lines from 44Ti decay. The emission flux has been measured by
CGRO/COMPTEL, BeppoSAX/PDS and INTEGRAL/IBIS. The high-resolution spectrometer
SPI on-board the INTEGRAL satellite can provide spectrometric information about
the emission. The line profiles reflect the kinematics of the 44Ti in
Cassiopeia A and can thus place constraints on its nucleosynthesis and
potentially on the associated explosion process. Using 4 years of data from
INTEGRAL/SPI, we have searched for the gamma-ray signatures from the decay of
the 44Ti isotope. The overwhelming instrumental background noise required an
accurate modelling and a solid assessment of the systematic errors in the
analysis. Due to the strong variability of the instrumental background noise,
it has not been possible to extract the two lines at 67.9 and 78.4keV.
Regarding the high-energy line at 1157.0keV, no significant signal is seen in
the 1140-1170keV band, thereby suggesting that the line signal from Cassiopeia
A is broadened by the Doppler effect. From our spectrum, we derive a ~ 500km/s
lower limit at 2sigma on the expansion velocity of the 44Ti ejecta. Our result
does not allow us to constrain the location of 44Ti since the velocities
involved throughout the remnant, derived from optical and X-ray studies, are
all far above our lower limit.Comment: 7 pages, 8 figures, published in A&
Accelerated Electrons in Cassiopeia A: An Explanation for the Hard X-ray Tail
We propose a model for the hard X-ray (> 10 keV) emission observed from the
supernova remnant Cas A. Lower hybrid waves are generated in strong (mG)
magnetic fields, generally believed to reside in this remnant, by shocks
reflected from density inhomogeneities. These then accelerate electrons to
energies of several tens of keV. Around 4% of the x-ray emitting plasma
electrons need to be in this accelerated distribution, which extends up to
electron velocities of order the electron Alfven speed, and is directled along
magnetic field lines. Bremsstrahlung from these electrons produces the observed
hard x-ray emission. Such waves and accelerated electrons have been observed in
situ at Comet Halley, and we discuss the viability of the extrapolation from
this case to the parameters relevant to Cas A.Comment: 20 pages, 3 figures, aasTeX502, accepted in Ap
Recommended from our members
Collisional Ionization Equilibrium for Optically Thin Plasmas
Reliably interpreting spectra from electron-ionized cosmic plasmas requires accurate ionization balance calculations for the plasma in question. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and their reliability are often highly suspect. We have utilized state-of-the-art calculations of dielectronic recombination (DR) rate coefficients for the hydrogenic through Na-like ions of all elements from He to Zn. We have also utilized state-of-the-art radiative recombination (RR) rate coefficient calculations for the bare through Na-like ions of all elements from H to Zn. Using our data and the recommended electron impact ionization data of Mazzotta et al. (1998), we have calculated improved collisional ionization equilibrium calculations. We compare our calculated fractional ionic abundances using these data with those presented by Mazzotta et al. (1998) for all elements from H to Ni, and with the fractional abundances derived from the modern DR and RR calculations of Gu (2003a,b, 2004) for Mg, Si, S, Ar, Ca, Fe, and Ni
Non-Maxwellian Proton Velocity Distributions in Nonradiative Shocks
The Balmer line profiles of nonradiative supernova remnant shocks provide the
means to measure the post-shock proton velocity distribution. While most
analyses assume a Maxwellian velocity distribution, this is unlikely to be
correct. In particular, neutral atoms that pass through the shock and become
ionized downstream form a nonthermal distribution similar to that of pickup
ions in the solar wind. We predict the H alpha line profiles from the
combination of pickup protons and the ordinary shocked protons, and we consider
the extent to which this distribution could affect the shock parameters derived
from H alpha profiles. The Maxwellian assumption could lead to an underestimate
of shock speed by up to about 15%. The isotropization of the pickup ion
population generates wave energy, and we find that for the most favorable
parameters this energy could significantly heat the thermal particles.
Sufficiently accurate profiles could constrain the strength and direction of
the magnetic field in the shocked plasma, and we discuss the distortions from a
Gaussian profile to be expected in Tycho's supernova remnant.Comment: 13 pages, 6 figure
Line intensity enhancements in stellar coronal X-ray spectra due to opacity effects
Context. The I(15.01 A)/I(16.78 A) emission line intensity ratio in Fe XVII
has been reported to deviate from its theoretical value in solar and stellar
X-ray spectra. This is attributed to opacity in the 15.01 A line, leading to a
reduction in its intensity, and was interpreted in terms of a geometry in which
the emitters and absorbers are spatially distinct. Aims. We study the I(15.01
A)/I(16.78 A) intensity ratio for the active cool dwarf EV Lac, in both flare
and quiescent spectra. Methods. The observations were obtained with the
Reflection Grating Spectrometer on the XMM-Newton satellite. The emission
measure distribution versus temperature reconstruction technique is used for
our analysis. Results. We find that the 15.01 A line exhibits a significant
enhancement in intensity over the optically thin value. To our knowledge, this
is the first time that such an enhancement has been detected on such a sound
statistical basis. We interpret this enhancement in terms of a geometry in
which the emitters and absorbers are not spatially distinct, and where the
geometry is such that resonant pumping of the upper level has a greater effect
on the observed line intensity than resonant absorption in the line-of-sight.Comment: accepted for publication in A&
Recommended from our members
New Collisional Ionization Equilibrium Calculations for Optically Thin Plasmas
Reliably interpreting spectra from electron-ionized laboratory and cosmic plasmas requires accurate ionization balance calculations for the plasma in question. However, much of the atomic data needed for these calculations have not been generated using modern theoretical methods and their reliability are often highly suspect. We have carried out state-of-the-art calculations of dielectronic recombination (DR) rate coefficients for the hydrogenic through Mg-like ions of all elements from He to Zn as well as for Al- like to Ar-like ions of Fe. We have also carried out state-of-the-art radiative recombination (RR) rate coefficient calculations for the bare through Na-like ions of all elements from H to Zn. Using our data and the most recently recommended electron impact ionization data, we present improved collisional ionization equilibrium (CIE) calculations. Here, as an example, we present our calculated fractional ionic abundances for iron using these data and compare them with those from the previously recommended CIE calculations
HST-COS Observations of Hydrogen, Helium, Carbon and Nitrogen Emission from the SN 1987A Reverse Shock
We present the most sensitive ultraviolet observations of Supernova 1987A to
date. Imaging spectroscopy from the Hubble Space Telescope-Cosmic Origins
Spectrograph shows many narrow (dv \sim 300 km/s) emission lines from the
circumstellar ring, broad (dv \sim 10 -- 20 x 10^3 km/s) emission lines from
the reverse shock, and ultraviolet continuum emission. The high signal-to-noise
(> 40 per resolution element) broad LyA emission is excited by soft X-ray and
EUV heating of mostly neutral gas in the circumstellar ring and outer supernova
debris. The ultraviolet continuum at \lambda > 1350A can be explained by HI
2-photon emission from the same region. We confirm our earlier, tentative
detection of NV \lambda 1240 emission from the reverse shock and we present the
first detections of broad HeII \lambda1640, CIV \lambda1550, and NIV]
\lambda1486 emission lines from the reverse shock. The helium abundance in the
high-velocity material is He/H = 0.14 +/- 0.06. The NV/H-alpha line ratio
requires partial ion-electron equilibration (T_{e}/T_{p} \approx 0.14 - 0.35).
We find that the N/C abundance ratio in the gas crossing the reverse shock is
significantly higher than that in the circumstellar ring, a result that may be
attributed to chemical stratification in the outer envelope of the supernova
progenitor. The N/C abundance ratio may have been stratified prior to the ring
expulsion, or this result may indicate continued CNO processing in the
progenitor subsequent to the expulsion of the circumstellar ring.Comment: 12 pages, 8 figures. ApJ - accepte
- …