J. Vink

A MILLION-SECOND CHANDRA VIEW OF CASSIOPEIA A

We introduce a million second observation of the supernova remnant Cassiopeia A with the Chandra X-Ray Observatory. The bipolar structure of the Si-rich ejecta (northeast jet and southwest counterpart) is clearly evident in the new images, and their chemical similarity is confirmed by their spectra. These are most likely due to jets of ejecta as opposed to cavities in the circumstellar medium, since we can reject simple models for the latter. The properties of these jets and the Fe-rich ejecta will provide clues to the explosion of Cas A.

Measuring the Cosmic-Ray Acceleration Efficiency of a Supernova Remnant

Cosmic Shock Waves Cosmic rays are high-energy charged particles that bombard Earth from all directions in the sky; those originating from within our Galaxy are thought to be accelerated in the shockwaves produced by supernova explosions. Helder et al. (p. 719, published online 25 June; see the Perspective by Raymond) measured the velocity of a section of the blast wave created by supernova RCW 86, an exploding star believed to have been witnessed by Chinese astronomers in 185 A.D., and the post-shock proton temperature. The post-shock proton temperature was much lower than would be expected without any cosmic ray acceleration, which implies that the pressure induced by cosmic ray exceeds the thermal pressure behind the shock. The pressure induced by cosmic rays produced by the explosion of a star exceeds the thermal pressure behind the shock wave. 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 ultrarelativistic 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 postshock proton temperature, in combination with the shock velocity, does not agree with standard shock heating, implying that >50% of the postshock pressure is produced by cosmic rays.

Supernova remnant energetics and magnetars: no evidence in favour of millisecond proto-neutron stars

It is generally accepted that anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are magnetars, i.e. neutron stars with extremely high surface magnetic fields (B > 1014 G). The origin of these high magnetic fields is uncertain, but a popular hypothesis is that magnetars are born with an initial spin period not much exceeding the convective overturn time (∼3 ms), which results in a powerful dynamo action, amplifying the seed magnetic field to ≳1015 G. Part of this rotation energy is then expected to power the supernova through rapid magnetic braking. It is therefore possible that magnetar creation is accompanied by supernovae that are an order of magnitude more energetic than normal supernovae, provided their initial spin period is ∼1 ms. However, we list here evidence that the explosion energies of these supernova remnants associated with AXPs and SGRs – Kes 73 (AXP 1E 1841−045), CTB 109 (AXP 1E2259+586) and N49 (SGR 0526−66) – are close to the canonical supernova explosion energy of 1051 erg, suggesting an initial spin period of P0≳ 5 ms.

Constraining supernova models using the hot gas in clusters of galaxies

Context: The hot X-ray emitting gas in clusters of galaxies is a very large repository of metals produced by supernovae. During the evolution of clusters, billions of supernovae eject their material into this Intra-Cluster Medium (ICM). Aims: We aim to accurately measure the abundances in the ICM of many clusters and compare these data with metal yields produced by supernovae. With accurate abundances determined using this cluster sample we will be able to constrain supernova explosion mechanisms. Methods: Using the data archive of the XMM-Newton X-ray observatory, we compile a sample of 22 clusters. We fit spectra extracted from the core regions and determine the abundances of silicon, sulfur, argon, calcium, iron, and nickel. The abundances from the spectral fits are subsequently fitted to supernova yields determined from several supernova type Ia and core-collapse supernova models. Results: We find that the argon and calcium abundances cannot be fitted with currently favoured supernova type Ia models. We obtain a major improvement of the fit, when we use an empirically modified delayed-detonation model that is calibrated on the Tycho supernova remnant. The two modified parameters are the density where the sound wave in the supernova turns into a shock and the ratio of the specific internal energies of ions and electrons at the shock. Our fits also suggest that the core-collapse supernovae that contributed to the enrichment of the ICM had progenitors which were already enriched. Conclusions: .The Ar/Ca ratio in clusters is a good touchstone for determining the quality of type Ia models. The core-collapse contribution, which is about 50% and not strongly dependent on the IMF or progenitor metallicity, does not have a significant impact on the Ar/Ca ratio. The number ratio between supernova type Ia and core-collapse supernovae suggests that binary systems in the appropriate mass range are very efficient (~5-16%) in eventually forming supernova type Ia explosions.

A new radiative cooling curve based on an up-to-date plasma emission code

This work presents a new plasma cooling curve that is calculated using the SPEX package. We compare our cooling rates to those in previous works, and implement the new cooling function in the grid-adaptive framework “AMRVAC”. Contributions to the cooling rate by the individual elements are given, to allow for the creation of cooling curves tailored to specific abundance requirements. In some situations, it is important to be able to include radiative losses in the hydrodynamics. The enhanced compression ratio can trigger instabilities (such as the Vishniac thin-shell instability) that would otherwise be absent. For gas with temperatures below 10 4 K, the cooling time becomes very long and does not affect the gas on the timescales that are generally of interest for hydrodynamical simulations of circumstellar plasmas. However, above this temperature, a significant fraction of the elements is ionised, and the cooling rate increases by a factor 1000 relative to lower temperature plasmas.

Detection of hot gas in the filament connecting the clusters of galaxies Abell 222 and Abell 223

Context. About half of the baryons in the local Universe are invisible and – according to simulations – their dominant fraction resides in filaments connecting clusters of galaxies in the form of low density gas with temperatures in the range of 10 5 < T < 10 7 K. This warm-hot intergalactic medium has never been detected indisputably using X-ray observations. Aims. We aim to probe the low gas densities expected in the large-scale structure filaments by observing a filament connecting the massive clusters of galaxies A 222 and A 223 (z = 0.21), which has a favorable orientation approximately along our line-of-sight. This filament has been previously detected using weak lensing data and as an over-density of colour-selected galaxies. Methods. We analyse X-ray images and spectra obtained from a deep observation (144 ks) of A 222/223 with XMM-Newton. Results. We present observational evidence of X-ray emission from the filament connecting the two clusters. We detect the filament in the wavelet-decomposed soft-band (0.5–2.0 keV) X-ray image with a 5σ significance. Following the emission down to the 3σ significance level, the observed filament is ≈1.2 Mpc wide. The temperature of the gas associated with the filament, determined from the spectra, is kT= 0.91±0.25 keV, and its emission measure corresponds to a baryon density of (3.4±1.3)×10 −5 (l/15 Mpc) −1/2 cm −3 , where l is the length of the filament along the line-of-sight. This density corresponds to a baryon over-density of ρ/ � ρC �≈ 150. The properties of the gas in the filament are consistent with results of simulations of the densest and hottest parts of the warm-hot intergalactic medium.

THE SLOW TEMPERATURE EQUILIBRATION BEHIND THE SHOCK FRONT OF SN 1006

We report on the observation of O VII Doppler line broadening in a compact knot at the edge of SN 1006 detected with the reflective grating spectrometer on board XMM-Newton. The observed line width of σ = 3.4 ± 0.5 eV at a line energy of 574 eV indicates an oxygen temperature of kT = 528 ± 150 keV. Combined with the observed electron temperature of ~1.5 keV, the observed broadening is direct evidence for temperature nonequilibration in high Mach number shocks and slow subsequent equilibration. The O VII line emission allows an accurate determination of the ionization state of the plasma, which is characterized by a relatively high forbidden line contribution, indicating log net 9.2.

Detection of the 67.9 keV and 78.4 ke.V lines associated with the radio-active decay of 44 Ti in Cassiopeia A

We report the detection of the 44Sc nuclear decay lines at 67.9 and 78.4 keV associated with the nuclear decay of 44Ti in Cassiopeia A. The line emission was observed by the Phoswich Detection System instrument on board BeppoSAX, which recently observed the supernova remnant for over 500 ks. The detection of the line emission with a flux of (2.1 ± 0.7) × 10-5 photons cm-2 s-1 in each line (90% confidence) is at the 5 σ significance level, if we can assume that the 12-300 keV continuum is adequately represented by a single power law. However, as the nature of the continuum is not clear, we investigate various other possibilities. A more conservative estimate of the line flux is made by assuming that a power-law continuum is at least a good approximation to the continuum emission for a narrower 30-100 keV energy range. With this limitation, the measured line flux is (1.9 ± 0.9) × 10-5 photons cm-2 s-1, with the detection still at the 3.4 σ significance level. We suggest that together with the Compton Gamma Ray Observatory/COMPTEL measurement of the 44Ca line at 1157 keV of (3.3 ± 0.6) × 10-5 photons cm-2 s-1, a flux for all three lines of (2.5 ± 1.0) × 10-5 photons cm-2 s-1 for Cas A can be adopted. This implies an initial 44Ti mass of (0.8-2.5) × 10-4 M☉.

Observational Signatures of Particle Acceleration in Supernova Remnants

We evaluate the current status of supernova remnants as the sources of Galactic cosmic rays. We summarize observations of supernova remnants, covering the whole electromagnetic spectrum and describe what these observations tell us about the acceleration processes by high Mach number shock fronts. We discuss the shock modification by cosmic rays, the shape and maximum energy of the cosmic-ray spectrum and the total energy budget of cosmic rays in and surrounding supernova remnants. Additionally, we discuss problems with supernova remnants as main sources of Galactic cosmic rays, as well as alternative sources.

The signature of 44Ti in Cassiopeia a Revealed by IBIS/ISGRI on INTEGRAL

We report the detection of both the 67.9 and 78.4 keV 44 Sc g-ray lines in Cassiopeia A with the INTEGRAL IBIS/ISGRI instrument. Besides the robustness provided by spectroimaging observations, the main improvements compared to previous measurements are a clear separation of the two 44 Sc lines together with an improved significance of the detection of the hard X-ray continuum up to 100 keV. These allow us to refine the determination of the 44 Ti yield and to constrain the nature of the nonthermal continuum emission. By combining COMPTEL, BeppoSAX PDS and ISGRI measurements, we find a line flux of