"Cas A — Multiwavelength SNR" (I) (II) (III) (IV) Hist. SNe und ihre Überreste Our Milky Way Galaxy
Cassiopeia A - A Supernova Remnant Cas A - Type IIB Cassiopeia A Fact Sheet Determination of Ejecta Structure and Explosion Asymmetry Where was the Iron Synthesized in Cassiopeia A K1.1 The Cas A Supernova K1.2 Sternenleiche schwebt in Eisenwolken K1.3 Is the Compact Source at the Center of Cas A Pulsed? K1.4 The SNR Cas A and Its Compact Source K1.5 Cassiopeia A: Deepest Image of Exploded Star Uncovers Bipolar Jets K3 XMM-Newton Abundance maps of Cassiopeia A K4 Is the Compact Source at the Center of Cas A Pulsed? K5 VHE gamma-rays from Cas A Literatur	Cassiopeia A (3C 461) and Supernova Remnant G111.7-2.1 Data: Distance: d = 3.4 kpc; Radius r = 1.7 pc ; Apparent Dim: 5 (arc min) , Vis brightness 6 (mag) Expansion velocity of gas: v_exp = 6·10³ km s^-1 Max. velocity of knots: V_max ~ 14 × 10³ km s-1 column density of N_H ~ 2 × 10²² X-ray luminosity: L_X = 1·10³⁶ erg s^-1 Precursor mass: M = 20 M Multiwavelength Views of Cas A Großbild (all) Großbild (tricolor) A color composite of Mid-IR by SST (red), Visible by HST (green), and X-ray by Chandra (blue). (O. Krause, 2005)
Three Chandra Views of Cassiopeia A This sequence shows three different sets Chandra observations of Cassiopeia A. The first image is Chandra's "First Light" image, which was released in August 1999 as the observatory's first major science image. This 5,000-second-long observation then dissolves into another image created from Chandra that contained data from 50,000 seconds of X-ray data released in 2002. Finally, the new one-million-second observation of Cassiopeia A is seen, revealing spectacular new detail and complexity to the supernova remnant. (Credit: NASA/CXC/GSFC/U.Hwang et al.) SUPERNOVA EXPLOSION - with Dissolve to Cassiopeia A This animation of a supernova explosion dissolves into the Chandra First Light Image, Cassiopeia A. (Animation: CXC/D.Berry & A.Hobart)

Cassiopeia A (I)

Cas A (3C 461) — Anatomie eines SNR (G111.7-2.1)

K1.1 The Cas A Supernova

Cassiopeia A and Supernova 1680
Cassiopeia A was the first target to be photographed by Nasa's new Chandra X-ray Observatory (CXO), then also known as AXAF; this is also image here.

CXO / Hughes et al. 1999

This is the youngest known supernova remnant in our Milky Way Galaxy, and the strongest extrasolar radio source in the sky. Calculating its expansion back, astronomers have found that the supernova must have blown up around the year 1667. Strangely, it was not widely noticed by that epoch's astronomers. However, as astronomy historian William Ashworth found out in 1980, it was perhaps observed by John Flamsteed on August 16, 1680, who cataloged a star near its position as "3 Cassiopeiae". However, he did not recognize it as a supernova, or "New Star", or anything particular else, and simply cataloged it as ordinary star. As this star was not noticed elsewhere, it cannot have been much brighter than 6th magnitude.

Later astronomers didn't find any sufficiently bright star near Flamsteed's position, and classified his catalog entry as erroneus.

As the supernova was rather close, its observed maximum brightness was extremely faint for a supernova, only about 250,000 solar luminosities, fainter than the brighter "normal" stars. This indicates that it was heavily obscured by interstellar matter.

The supernova remnant was found among the earliest discrete radio sources, in 1947 by radio astronomers from Cambridge, England, and is the strongest radio source in the sky beyond the solar system. This radio source was first named Cassiopeia A and later cataloged 3C 461. Its optical counterpart coudn't be found until a more precise position was obtained, by radio interferometry in 1950. Consequently, David Dewhirst of Cambridge obtained first deep optical photos of this region in the sky and discovered a strange faint nebula, which was then investigated by Walter Baade and Rudolph Minkowski with the then-new Palomar 5-meter telescope (Baade & Minkowski, 1954).

The youngest known supernova remnant in our MWG

A supernova occurs when a massive star has used up its nuclear fuel and the pressure drops in the central core of the star. The matter in the core is crushed by gravity to higher and higher densities, and temperatures reach billions of degrees. Under these extreme conditions, nuclear reactions occur violently and catastrophically reversing the collapse. A thermonuclear shock wave races through the now expanding stellar debris, fusing lighter elements into heavier ones and producing a brilliant visual outburst.

About every fifty years in our galaxy, a massive star explodes. The shell of matter thrown off by the supernova creates a bubble of multi-million degree gas called a supernova remnant. Cas A is a prime example. The hot gas will expand and produce X-rays for thousands of years.

The nature of the explosion that produced Cas A has been an enigma. Although radio, optical and x-ray observations of the remnant indicate that it was a powerful event, the visual brightness of the outburst was much less than a normal supernova. Apparently Cas A was produced by the explosion of an unusual massive star that had previously ejected most of its outer layers.

Probing Cas A Mysteries with NASA's Chandra X-ray Observatory

Chandra's spectacularly vivid images of Cas A allow scientists to trace the dynamics of the remnant and its collision with any material ejected by the star before it exploded. Chandra detectors provide scientists with precise x-ray spectra- measurements of the energies of individual x-rays-from the Cas A remnant. These measurements make it possible to identify which heavy elements are present and in what quantities. Chandra's observations should help astronomers to resolve the long-standing mystery as to the nature and origin of Cas A.

A related mystery is whether the explosion that produced Cas A left behind a neutron star, black hole, or nothing at all. This "First Light" Chandra image of Cas A shows a bright object near the center of the remnant! Longer observations with Chandra can determine if this is the long sought for neutron star or black hole.

Importance of Supernova

The study of remnants of exploded stars, or supernovae, is essential for our understanding of the origin of life on Earth. The cloud of gas and dust that collapsed to form the sun, Earth and other planets was composed mostly of hydrogen and helium, with a small amount of heavier elements such as carbon, nitrogen, oxygen and iron. The only place where these and other heavy elements necessary for life are made, is deep in the interior of a massive star. There they remain until a catastrophic explosion spreads them throughout space.

Supernovae are the creative flashes that renew the galaxy. They seed the interstellar gas with heavy elements, heat it with the energy of their radiation, stir it up with the force of their blast waves and cause new stars to form.

Spectroscopic observations soon confirmed its nature as the rapidly expanding shell of a supernova remnant, also cataloged as G111.7-2.1. Within two years, American astronomers were able to determine its angular expansion rate, and calculated back that the expansion must have started around the year AD 1667, as mentioned above. Nevertheless, until Ashworth' publication of 1980, it was thought that the supernova had not been observed because of heavy obstruction. Even now there are some doubts, as the position of "3 Cassipeiae" does not exactly co-incide with that of Cassiopeia A, and some historians think Flamsteed may simply have cataloged an erroneous position of another star.

K1 Sternenleiche schwebt in Eisenwolken

Chandra X-Ray Observatory — the first light observation
	Authors: John P. Hughes, Cara E. Rakowski, David N. Burrows, and Patrick O. Slane
	Journal-ref: ApJ 528 (2000) L109-L113 [astro-ph/9910474 ]
	Title: Nucleosynthesis and Mixing in Cassiopeia A
Abstract: We present results from the first light observations of the Cassiopeia A supernova remnant (SNR) by the Chandra X-Ray Observatory. Based on representative spectra from four selected regions, we investigate the processes of nucleosynthesis and mixing in Cas A. We make the first unequivocal identification of iron-rich ejecta produced by explosive silicon burning in a young Galactic SNR. Elsewhere in the remnant, we see silicon-rich ejecta from explosive oxygen burning. The Fe-rich ejecta lie outside the Si-rich material, indicating that bulk motions were extensive and energetic enough in Cas A to cause a spatial inversion of a significant portion of the supernova core. It is likely that this inversion was caused by "Fe"-rich ejecta emerging in plumes from the rising bubbles in the neutrino-driven convection layer during the supernova explosion. In addition, the radioactive decay energy from ⁵⁶Ni may have contributed to the subsequent evolution of the material. We have also discovered faint, well-defined filaments with featureless X-ray spectra that are possibly sites of cosmic-ray acceleration in Cas A. 1. INTRODUCTION Young supernova remnants (SNRs) are the critical link between the nucleosynthetic processes that occur in stars and essentially all the metals that exist in the Universe. Accurate knowledge of the nucleosynthetic yields from exploded stars is essential for studies of the evolution of the interstellar medium, external galaxies, and even clusters of galaxies.


		Fig. 1. — Chandra X-ray image of Cas A, encoding information on the intrinsic X-ray spectrum. The figure was constructed from images in three different energy bands: 0.6 – 1.65 ( red ), 1.65 – 2.25 ( green ) and 2.25 – 7.50keV ( blue ). The separate images were each adaptively smoothed to a signal-to-noise ratio per pixel of 7, which mainly had the effect of rendering the faint outer regions of the remnant visible; the bright, high spatial frequency parts of the images were unaffected. Lupe: ROSAT Image Image credit: Chandra / Hughes et al. 1999 At a distance of 3.4 kpc, the 2' radius optical shell corresponds to a physical size of 1.7 pc. In the optical band, Cas A displays complex variations of composition with position and velocity, and it is one of the brightest SNRs at X-ray and radio frequencies.

A color image of Cas A, encoding information on the intrinsic X-ray spectrum of the remnant, is shown in Figure 1.

Image credit: Chandra / Hughes et al. 1999

Fig. 2. —
Broadband unsmoothed Chandra X-ray image of Cas A using a square-root intensity scaling. The spectral extraction regions in our study are indicated.

Regions that appear red indicate places where Cas A shows more emission from the energy band (0.61.65 keV) that contains the K-shell lines (transitions to the 1s electronic level) of O, Ne, and Mg and the L-shell lines (transitions to the 2s electronic level) of Ca and Fe.
Green regions are relatively enhanced in the K-shell emission lines of Si (energy band 1.652.25 keV), while blue regions are weighted toward higher energy emission (2.257.50 keV) that includes the S, Ar, Ca, and Fe K-shell emission lines.
Regions with comparable levels of emission in the three bands appear white.

The red material on the left outer edge is enriched in iron, whereas the bright greenish white region on the lower left is enriched in silicon and sulphur. In the blue region on the right edge, low and medium energy X rays have been filtered out by a cloud of dust and gas in the remnant.

All three bands also contain emission from thermal bremsstrahlung as well as other nonthermal continuum components. Figure 1 shows that the X-ray spectral character of Cas A varies on all angular scales down to the resolution limit of the telescope, corresponding to physical scales of 0.02 pc.

As we illustrate below, using spectra from the four representative regions indicated in Figure 2 (a broadband X-ray image), spectral differences arise from variations in the underlying radiation process, composition, excitation conditions, and the column density of absorbing gas and dust in the line of sight toward Cas A.


Image credit: Chandra / Hughes et al. 1999 Fig. 3. — Cas A Spectra		Fig. 3. — Energy spectra from several regions in Cas A as indicated in Fig. 2. The horizontal error bars show the widths of the energy bins, and the vertical ones indicate the statistical error on the measured event rate; systematic errors are not included. Superposed on the data points are smooth curves of simulated Chandra ACIS-S spectra. The simulations for regions A, B, and C are of a shock-heated plasma with NEI fractions absorbed by line-of-sight interstellar material. The dotted curves in regions A and C and the solid curve in region B assume abundances corresponding to explosive incomplete Si burning. A considerably better match for region A uses O-burning abundances (solid curve). The solid curve for region C is more Fe-rich; i.e., the Si, S, Ar, and Ca abundances are reduced by factors of 5 or more from their values in incomplete Si burning. The solid curves for regions A, B, and C have temperatures of 2.5, 2.5, and 2.8 keV, ionization timescales of 2.5 × 10¹⁰,7.9 × 10¹⁰, and 7.9 × 10¹⁰ cm^-3 s^-1, and column densities of N_H = 0.9 × 10²², 2.3 × 10²², and 1.5 × 10²² atoms cm^-2, respectively. All the models for regions A, B, and C also include significant amounts of continuum emission from material with a lower atomic number. The solid curve for region D is an absorbed power-law model with a photon index of 2.6 and a column density of 1.3 × 10²² atoms cm^-2.

NASA/ CXC/ GSFC

Chandra-Bild von Cas A: Die Schockwelle der Supernova hat einen Durchmesser von zehn Lichtjahren. Rot, Grün und Blau markieren niedrige, mittlere und hohe Strahlungsenergien

NASA/ CXC/ GSFC

Cassiopeia A: 15 Millionen Grad heiße Silizium-Atome senden Röntgenstrahlung aus

Jets aus Silizium, Wolken aus Eisen und eine stille Sternenleiche: Vor 10.000 Jahren hat Cassiopeia A in einer gewaltigen Explosion ihr Leben ausgehaucht. Von ihren Resten hat die Nasa jetzt das bisher detailreichste Foto einer Supernova überhaupt geschossen.

Cassiopeia A ist für Astronomen eine alte Bekannte: Die Überbleibsel der 10.000 Lichtjahre entfernten Supernova, die vor 340 Jahren auf der Erde zu sehen war, werden seit langem intensiv studiert. Dennoch hat die Nasa die Sternentrümmer jetzt erneut mit dem Chandra-Röntgenteleskop ins Visier genommen - und erstaunliche Ergebnisse erzielt. "Es ist die genaueste Beobachtung der Reste eines explodierten Sterns, die jemals angestellt wurde", sagt Martin Laming, Mitglied des Forscherteams des Goddard Space Flight Centers der Nasa. "In dieser Daten-Goldmine werden Astronomen jahrelang schürfen."
Das neue Bild enthält fast 200-mal so viele Daten wie die erste, fünf Jahre alte Chandra-Aufnahme von Cassiopeia A. Es zeigt nach Angaben der Nasa, dass die Explosion des Sterns weit komplizierter verlief als bisher vermutet. Bei einer Belichtungszeit von einer Million Sekunden, was in etwa elfeinhalb Tagen entspricht, tauchten zwei große Jets auf, die in entgegen gesetzter Richtung zehn Lichtjahre weit vom Zentrum der Explosion in den Weltraum hineinreichen. Zudem entdeckten die Forscher Wolken aus nahezu reinem Eisen.
"Die Existenz bipolarer Jets legt nahe, dass sie bei relativ normalen Supernovae weit verbreitet sein könnten", kommentierte Teamleiterin Una Hwang. Die Röntgenspektren zeigen demnach, dass die Jets reich an Siliziumatomen sind und kaum Eisen enthalten. Säulen aus gasförmigem Eisen breiten sich dagegen nahezu lotrecht zu den Jets aus.
Die Wissenschaftler schließen daraus, dass die massiven Silizium-Jets nicht die unmittelbaren Auslöser der Explosion waren, da sie sonst große Mengen von Eisen aus dem heißen Inneren des Sterns enthalten müssten. Stattdessen nehmen Hwang und ihre Kollegen an, dass die Explosion Hochgeschwindigkeits-Jets hervorbrachte, ähnlich denen, die bei Hypernovae entstehen und so genannte Gammablitze auslösen.
Nach dem feurigen Ende wurde es still um Cassiopeia A. Im Zentrum der Explosionswolke ruht eine Leiche: Der Neutronenstern leuchtet nur schwach und zeigt kaum Anzeichen von Aktivität, ganz im Gegensatz zu seinen schnell rotierenden Artgenossen im Krebsnebel oder in den Überresten der Vela-Supernova.

K2 The SNR Cas A and Its Compact Source

Chandra — A Million-Second View — bipolar structure of the Si-rich ejecta
	Authors: Una Hwang, J. Martin Laming, Carles Badenes, Fred Berendse, John Blondin, Denis Cioffi, Tracey DeLaney, Daniel Dewey, Robert Fesen, Kathryn A. Flanagan, Christopher L. Fryer, Parviz Ghavamian, John P. Hughes, Jon A. Morse, Paul P. Plucinsky, Robert Petre, Martin Pohl, Lawrence Rudnick, Ravi Sankrit, Patrick O. Slane, Randall K. Smith, Jacco Vink, Jessica S. Warren
	Journal-ref: ApJ 615 (2004) L117-L120 [astro-ph/0409760 ]
	Title: A Million-Second Chandra View of Cassiopeia A
Abstract: 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 (NE jet and SW 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.

[30.8.2004] Eine extrem tiefe Aufnahme des Supernovarests Cassiopeia A
ist mit dem Röntgensatelliten Chandra entstanden: Rund eine Million Sekunden oder fast zwei Wochen lang wurden Photonen gesammelt - dann war klar, daß aus der inetwa kugelförmigen Explosionswolke zwei einander gegenüberliegende riesige Röhren herausragen. Auch scheinbar gewöhliche Supernovareste können also eine bipolare Struktur haben, wenn man nur tief genug nachschaut - in welchem zeitlichen oder kausalen Zusammenhang mit der Explosion des massereichen Sterns vor rund 340 Jahren die beiden Jetstrukturen allerdings stehen, ist noch unklar.

Credit: NASA/CXC/GSFC/U. Hwang et al.

Chandra-Bild von Cassiopeia A. Rot, Grün und Blau markieren niedrige, mittlere und hohe Strahlungsenergien (X-ray). Im Zentrum der Explosionswolke leuchtet schwach ein Neutronenstern. Each panel is 8 arcmin per side (~3.3 pc).

The SNR Cas A and Its Compact Source

Explanation:
One million seconds of x-ray image data were used to construct this view of supernova remnant Cassiopeia A, the expanding debris cloud from a stellar explosion.

The stunningly detailed image from the Chandra Observatory will allow an unprecedented exploration of the catastrophic fate that awaits stars much more massive than the Sun. Seen in false-color, Cas A's outer green ring, 10 light-years or so in diameter, marks the location of the expanding shock from the original supernova explosion.

At about 10 o'clock around the ring, a structure extends beyond it, evidence that the initial explosion may have also produced energetic jets. Still glowing in x-rays, the tiny point source near the center of Cas A is a neutron star, the collapsed remains of the stellar core. While Cas A is about 10,000 light-years away, light from the supernova explosion first reached Earth just over 300 years ago.

K2a Cassiopeia A: Deepest Image of Exploded Star Uncovers Bipolar Jets

The spectacular NASA's Chandra X-ray Observatory image of Cassiopeia A released [August 23, 2004] has nearly 200 times more data than the "First Light" Chandra image of this object made five years ago. The new image reveals clues that the initial explosion was far more complicated than suspected.

This spectacular image of the supernova remnant Cassiopeia A is the most detailed image ever made of the remains of an exploded star.
The one-million-second image shows a bright outer ring (green) ten light years in diameter that marks the location of a shock wave generated by the supernova explosion. A large jet-like structure that protrudes beyond the shock wave can be seen in the upper left.
In the accompanying image, specially processed to highlight silicon ions, a counter-jet can be seen on the lower right.

Credit: NASA/CXC/GSFC/U.Hwang et al.

Chandra X-ray Image of Cassiopeia A. Each panel is 8 arcmin per side.

Surprisingly, the X-ray spectra show that the jet and counter-jet are rich in silicon atoms and relatively poor in iron atoms. This indicates that the jets formed soon after the initial explosion of the star; otherwise, the jets should have contained large quantities of iron from the star's central regions.

The bright blue fingers located near the shock wave on the lower left are composed almost purely of iron gas. This iron was produced in the central, hottest regions of the star and somehow ejected in a direction almost perpendicular to the jets.

The bright source at the center of the image is presumed to be a neutron star created during the supernova. Unlike the rapidly rotating neutron stars in the Crab Nebula and Vela supernova remnants that are surrounded by dynamic magnetized clouds of electrons called pulsar wind nebulas, this neutron star is quiet, faint, and so far shows no evidence for pulsed radiation.

A working hypothesis is that the explosion that created Cassiopeia A produced high-speed jets similar to but less energetic than the hypernova jets thought to produce gamma-ray bursts. During the explosion, the neutron star may have developed an extremely strong magnetic field that helped to accelerate the jets.

This strong magnetic field later stifled any pulsar wind activity, so the neutron star today resembles other strong-field neutron stars (a.k.a. "magnetars") in lacking a pulsar wind nebula.

K3 XMM-Newton Abundance maps of Cassiopeia A

Cas A — XMM-Newton — X-ray spectroscopy — M_ini ~ 30 M.
	Authors: Willingale, R., Bleeker, J. A. M., van der Heyden, K. J., & Kaastra, J. S.
	Journal-ref: A&A 398 (2003) 1021-1028 [astro-ph/0207273 ]
	Title: The mass and energy budget of Cassiopeia A
Abstract: Further analysis of X-ray spectroscopy results recently obtained from the MOS CCD cameras on-board XMM-Newton provides a detailed description of the hot and cool X-ray emitting plasma in Cas A. Measurement of the Doppler broadening of the X-ray lines is consistent with the expected ion velocities, ~1500 km/s along the line of sight, in the post shock plasma. Assuming a constant total pressure throughout the remnant we estimate the total remnant mass as M_SNR = 10 M and the total thermal energy as 7 × 10⁵⁰ ergs. We derive the differential mass distribution as a function of ionisation age for both X-ray emitting components. This distribution is consistent with a hot component dominated by swept up mass heated by the primary shock and a cool component which are ablated clumpy ejecta material which were and are still being heated by interaction with the preheated swept up material. We calculate a balanced mass and energy budget for the supernova explosion giving 1 × 10⁵¹ ergs in ejected mass; approximately 0.4 M of the ejecta were diffuse with an initial rms velocity of 15000 km/s while the remaining ~ 1.8 M were clumpy with an initial rms velocity of ~2400 km/s. Using the Doppler velocity measurements of the X-ray spectral lines we can project the mass into spherical coordinates about the remnant. This provides quantitative evidence for mass and energy beaming in the supernova explosion. The mass and energy occupy less than 4.5 sr (<40 % of the available solid angle) around the remnant and 64 % of the mass occurs in two jets within 45 degrees of a jet axis. We calculate a swept up mass of 7.9 M in the emitting plasma and estimate that the total mass lost from the progenitor prior to the explosion could be as high as M_evap ~ 20 M.

Abundance maps
for the elements included in the Cassiopeia A spectral analysis of the data provided by XMM-Newton
All are plotted on the logarithmic scale indicated by the bar at the bottom.

Image courtesy R.Willingale

Cassiopeia A (Cas-A) is a young shell-shaped supernova remnant
some 15 light years in diameter
situated some 10 thousand light years away.
It is the remains of a massive star which, having exhausted all its hydrogen fuel, exploded 320 years ago.
The core of such a collapsing star can give rise to a neutron star or black hole. Its external parts are blown apart projecting stellar material, glowing in X-rays, into the surrounding interstellar medium.
The stellar material contains many heavy elements which have been forged from lighter elements in the progenitor star, and during the explosion process.
All the chemical elements in our human bodies have their origins in such stellar explosions and the resulting primordial broth.

K4 Is the Compact Source at the Center of Cas A Pulsed?

Zum Thema: CXOU J232327.9+584842
CCOs in SNRs	Table

CXOU J232327.9+584842 —

Authors: S.S. Murray, S.M. Ransom, M. Juda, U. Hwang, S.S. Holt

Journal-ref: ApJ 566 (2002) 1039 [astro-ph/0106516

]

Title: Is the Compact Source at the Center of Cas A Pulsed?

Chandra

Abstract: A 50 ksec (12 h) observation of the Supernova Remnant Cas A was taken using the Chandra X-Ray Observatory High Resolution Camera (HRC) to search for periodic signals from the compact source located near the center. In an extensive analysis of the HRC data, several possible candidate signals are found using various algorithms. Of the candidate periods, none is at a high enough confidence level to be particularly favored over the rest. When combined with other information, however (e.g., spectra, total energetics, and the historical age of the remnant), a 12 ms candidate period seems to be more physically plausible than the others, and we use it for illustrative purposes in discussing the possible properties of a putative neutron star in the remnant. We emphasize that this is not necessarily the true period, and that a follow-up observation, scheduled for the fall of 2001, is required.
Analysis of these data for the central object shows that the spectrum is consistent with several forms, and that the emitted X-ray luminosity (in the 0.1 -10 keV band) is 10³³ - 10³⁵ erg cm^-2 s^-1 depending on the spectral model and the interstellar absorption along the line of sight to the source.

The first high resolution X-ray image of Cas A was obtained using the Einstein HRI. A search for a point source near the center of the remnant resulted only in an upper limit on its luminosity.
Other X-ray observations using ROSAT and ASCA also failed to detect a central point source. This situation changed dramatically with the first light image from the Chandra X-ray Observatory taken on 1999, August 20.
In a relatively short (6ksec) Advanced CCD Imaging Spectrometer (ACIS) observation, the power of the Chandra high resolution X-ray telescope was demonstrated by revealing immediately the existence of a point-like object near the center of the remnant.

Image credit: Chandra / S. Murray

Chandra X-Ray Image of central region (50 ks)

This object is likely to be either a neutron star or black hole left over from the explosion of the progenitor star.
Subsequent searches in the radio and optical have yet to detect a point source at the center of Cas A.
One distinguishing characteristic of a rotating neutron star would be the presence of periodic pulses.
On average there are only about 300 HRC counts per 10 ksec from the source (counts within a 1 arc second radius), and Chakrabarty et al. placed an upper limit of 35% pulsed fraction for periods longer than 20msec.
Spectral analysis of the public ACIS data by both Chakrabarty et al. and Pavlov result in only marginal conclusions regarding the nature of the central point source.
Here we present a deeper observation of Cas A of 50 ksec with the HRC for timing purposes, and 50 ksec with the ACIS for higher accuracy spectral analysis than was possible from the calibration data sets.

K5 VHE gamma-rays from Cas A

Zum Thema
HEGRA: Evidence for TeV gamma ray emission

—
	Authors: J. Albert, et al, for the MAGIC collaboration
	Journal-ref: A&A 474 (2007) 937 [0706.4065 ]
	Title: Observation of VHE gamma-rays from Cassiopeia A with the MAGIC telescope
Abstract: We searched for very high energy (VHE) gamma-ray emission from the supernova remnant Cassiopeia A. The shell-type supernova remnant Cassiopeia A was observed with the 17 meter MAGIC telescope between July 2006 and January 2007 for a total time of 47 hours. The source was detected above an energy of 250 GeV with a significance of 5.2s and a photon flux above 1 TeV of f_TeV = 7.3 × 10^-13 photons cm^-2 s^-1. The photon spectrum is compatible with a power law dN/dE = A e^-G with a photon index G = 2.3. The source is point-like within the angular resolution of the telescope. 1. Introduction Cassiopeia A (Cas A), is a prominent shell type supernova remnant and a bright source of synchrotron radiation observed at radio frequencies, and in the X-ray band. The remnant results from the youngest known Galactic supernova, whose explosion took place around 1680. Its distance was estimated at 3.4 kpc. High resolution X-ray images from the Chandra satellite, see Hughes et al. (2000), reveal a shell-type nature of the remnant and the existence of a central object. The progenitor of Cas A was probably a Wolf-Rayet star, as discussed in Fesen et al. (1991). The progenitor’s initial mass was large, estimated to be between 15 and 25 M, see Young et al. (2006). The morphology of the remnant as seen in optical, X-ray and IR wavelength consists on a patchy and irregular shell with a diameter of 4’ (4 pc at 3.4 kpc). The supernova blast wave is expanding into a wind bubble formed from the previous wind phases of the progenitor star; this plays an important role in shock acceleration of CR. At TeV energies, Cas A was detected by the HEGRA Stereoscopic Cherenkov Telescope System, which accumulated 232 hours of data from 1997 to 1999. TeV g-ray emission was detected at 5s level and a flux of F_g = (5.8 ± 1.2_stat±1.2_sys) × 10^-3 ph cm^-2 s^-1 above 1 TeV was derived, as discussed in Aharonian et al. (2001). The spectral distribution between 1 and 10 TeV was found to be consistent with a power law with a differential spectral index of G = -2.5±0.4_stat±0.1_sys. Upper limits at TeV energies have been set also by Whipple, References Aharonian, F. et al. 2001, A&A 112, 307 Hughes, J.P., Rakowski, C.E., Burrows, D.N. & Slane, P.O., 2000, ApJ 528, L109. Hwang, U. et al., 2004, ApJ Letters 615, 117.

Literatur zu "SNR Cas A"
Hughes, J., et al.	2000	ApJ 528, L109-L113	"Nucleosynthesis and Mixing in Cassiopeia A "
Laming, M. & Hwang, U.	2003	ApJ 597, 347-361	"Determination of Ejecta Structure and Explosion Asymmetry from the X-ray Knots of Cas A"
Hwang, U. & Laming, M.	2003	ApJ 597, 362-373	"Where was the Iron Synthesized in Cassiopeia A?"
Hwang, U., et al.	2004	ApJ 615, L117-L120	"A Million-Second Chandra View of Cassiopeia A"
Willingale, R., et al.	2003	A&A 398, 1021-1028	"The mass and energy budget of Cassiopeia A"
Dunne, L., et al.	2003	Nature 424, 285-287	"Type II supernovae as a significant source of interstellar dust"
Krause, O., G.H. Rieke, S.M. Birkmann, et al.	2005	Science 308, 1604	"Infrared Echoes near the Supernova Remnant Cassiopeia A"
J. Albert, et al, for the MAGIC collaboration	2007	A&A 474, 937	"Observation of VHE gamma-rays from Cassiopeia A with the MAGIC telescope"

H. Heintzmann

( Eintrag vom 2.6.2008)

— Nr:

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