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Register a new userThe X-ray Ribs Within the Cocoon Shock of Cygnus A
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We use new and archival Chandra observations of Cygnus A, totalling $sim$1.9 Ms, to investigate the distribution and temperature structure of gas lying within the projected extent of the cocoon shock and exhibiting a rib-like structure. We confirm that the X-rays are dominated by thermal emission with an average temperature of around 4 keV, and have discovered an asymmetry in the temperature gradient, with the southwestern part of the gas cooler than the rest by up to 2 keV. Pressure estimates suggest that the gas is a coherent structure of single origin located inside the cocoon, with a mass of roughly $2times10^{10} M_{odot}$. We conclude that the gas is debris resulting from disintegration of the cool core of the Cygnus A cluster after the passage of the jet during the early stages of the current epoch of activity. The 4 keV gas now lies on the central inside surface of the hotter cocoon rim. The temperature gradient could result from an offset between the centre of the cluster core and the Cygnus A host galaxy at the switch-on of current radio activity.
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The study of $gamma$-ray emission from galactic sources such as supernova remnants (SNR) may provide key insights into their potential role as accelerators of cosmic rays up to the knee ($sim 10^{15}$ eV). The VERITAS Observatory is sensitive to galactic and extragalactic $gamma$-ray sources in the 100 GeV to 30 TeV energy range. We report here on VERITAS observations of the vicinity of the cocoon of freshly accelerated cosmic rays reported by Fermi, which lies between potential accelerators in the Cygnus OB2 association and the $gamma$-Cygni SNR. A particular focus is placed on the source VER J2019 +407 in $gamma$-Cygni.
The extended TeV gamma-ray source ARGO J2031+4157 (or MGRO J2031+41) is positionally consistent with the Cygnus Cocoon discovered by $Fermi$-LAT at GeV energies in the Cygnus superbubble. Reanalyzing the ARGO-YBJ data collected from November 2007 to January 2013, the angular extension and energy spectrum of ARGO J2031+4157 are evaluated. After subtracting the contribution of the overlapping TeV sources, the ARGO-YBJ excess map is fitted with a two-dimensional Gaussian function in a square region of $10^{circ}times 10^{circ}$, finding a source extension $sigma_{ext}$= 1$^{circ}$.8$pm$0$^{circ}$.5. The observed differential energy spectrum is $dN/dE =(2.5pm0.4) times 10^{-11}(E/1 TeV)^{-2.6pm0.3}$ photons cm$^{-2}$ s$^{-1}$ TeV$^{-1}$, in the energy range 0.2-10 TeV. The angular extension is consistent with that of the Cygnus Cocoon as measured by $Fermi$-LAT, and the spectrum also shows a good connection with the one measured in the 1-100 GeV energy range. These features suggest to identify ARGO J2031+4157 as the counterpart of the Cygnus Cocoon at TeV energies. The Cygnus Cocoon, located in the star-forming region of Cygnus X, is interpreted as a cocoon of freshly accelerated cosmic rays related to the Cygnus superbubble. The spectral similarity with Supernova Remnants indicates that the particle acceleration inside a superbubble is similar to that in a SNR. The spectral measurements from 1 GeV to 10 TeV allows for the first time to determine the possible spectrum slope of the underlying particle distribution. A hadronic model is adopted to explain the spectral energy distribution.
The origin of Galactic cosmic rays remains unconfirmed, but promising candidates for their sources are found in star-forming regions. We report a series of X-ray observations, with Suzaku, toward the nearby star-forming region of Cygnus X. They aim at comparing diffuse X-ray emissions on and off the $gamma$-ray cocoon of hard cosmic rays revealed by Fermi LAT. After excluding point sources and small-scale structures and subtracting the non-X-ray and cosmic X-ray backgrounds, the 2--10~keV X-ray intensity distribution is found to monotonically decrease with increasing Galactic latitude. This indicates that most of the extended emission detected by Suzaku originates from the Galactic ridge. In two observations, we derive upper limits of $3.4 times 10^{-8}~{rm erg~s^{-1}~cm^{-2}~sr^{-1}}$ and $1.3 times 10^{-8}~{rm erg~s^{-1}~cm^{-2}~sr^{-1}}$ to X-ray emission in the 2--10 keV range from the gamma-ray cocoon. These limits exclude the presence of cosmic-ray electrons with energies above about 50 TeV at a flux level capable of explaining the gamma-ray spectrum. They are consistent with the emission cut-off observed near a TeV in gamma rays. The properties of Galactic ridge and local diffuse X-rays are also discussed.
We present the results of numerical simulations of the prompt emission of short-duration gamma-ray bursts. We consider emission from the relativistic jet, the mildly relativistic cocoon, and the non-relativistic shocked ambient material. We find that the cocoon material is confined between off-axis angles 15<theta<45 degrees and gives origin to X-ray transients with a duration of a few to ~10 seconds, delayed by a few seconds from the time of the merger. We also discuss the distance at which such transients can be detected, finding that it depends sensitively on the assumptions that are made about the radiation spectrum. Purely thermal cocoon transients are detectable only out to a few Mpc, Comptonized transients can instead be detected by the FERMI GBM out to several tens of Mpc.
Context: Physics behind the soft X-ray light curve asymmetries in Cygnus X-3, a well-known microquasar, was studied. AIMS: Observable effects of the jet close to the line-of-sight were investigated and interpreted within the frame of light curve physics. METHODS: The path of a hypothetical imprint of the jet, advected by the WR-wind, was computed and its crossing with the line-of-sight during the binary orbit determined. We explore the possibility that physically this imprint is a formation of dense clumps triggered by jet bow shocks in the wind (clumpy trail). Models for X-ray continuum and emission line light curves were constructed using two absorbers: mass columns along the line-of-sight of i) the WR wind and ii) the clumpy trail, as seen from the compact star. These model light curves were compared with the observed ones from the RXTE/ASM (continuum) and Chandra/HETG (emission lines). Results: We show that the shapes of the Cygnus X-3 light curves can be explained by the two absorbers using the inclination and true anomaly angles of the jet as derived in Dubus et al. (2010) from gamma-ray Fermi/LAT observations. The clumpy trail absorber is much larger for the lines than for the continuum. We suggest that the clumpy trail is a mixture of equilibrium and hot (shock heated) clumps. Conclusions: A possible way for studying jets in binary stars when the jet axis and the line-of-sight are close to each other is demonstrated. The X-ray continuum and emission line light curves of Cygnus X-3 can be explained by two absorbers: the WR companion wind plus an absorber lying in the jet path (clumpy trail). We propose that the clumpy trail absorber is due to dense clumps triggered by jet bow shocks.
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