Non-thermal emission from old supernova remnants

We study the non-thermal emission from old shell-type supernova remnants (SNRs) on the frame of a time-dependent model. In this model, the time-dependent non-thermal spectra of both primary electrons and protons as well as secondary electron/positron (e^±) pairs can be calculated numerically by taking into account the evolution of the secondary e^± pairs produced from proton–proton (p–p) interactions as accelerated protons collide with the ambient matter in an SNR. The multiwavelength photon spectrum for a given SNR can be produced through leptonic processes such as electron/positron synchrotron radiation, bremsstrahlung and inverse Compton scattering as well as hadronic interaction. Our results indicate that the non-thermal emission of the secondary e^± pairs is becoming more and more prominent when the SNR ages in the radiative phase because the source of the primary electrons has been cut off and the electron synchrotron energy loss is significant for a radiative SNR, whereas the secondary e^± pairs can be produced continuously for a long time in the phase due to the large energy-loss time for the p–p interaction. We apply the model to two old SNRs, G8.7−0.1 and G23.3−0.3, and the predicted results can explain the observed multiwavelength photon spectra for the two sources.     

Neutrino emission of Fermi supernova remnants

The Fermi γ-ray space telescope reported the observation of several Galactic supernova remnants recently, with the γ-ray spectra well described by hadronic pp collisions. The possible neutrino emissions from these Fermi detected supernova remnants are discussed in this work, assuming the hadronic origin of the γ-ray emission. The muon event rates induced by the neutrinos from these supernova remnants on typical km³ neutrino telescopes, such as the IceCube and the KM3NeT, are calculated. The results show that for most of these supernova remnants the neutrino signals are too weak to be detected by the on-going or up-coming neutrino experiment. Only for the TeV bright sources RX J1713.7-3946 and possibly W28 the neutrino signals can be comparable with the atmospheric background in the TeV region, if the protons can be accelerated to very high energies. The northern hemisphere based neutrino telescope might detect the neutrinos from these two sources.     

The secular behavior of X-ray and radio emission from supernova remnants

General models for the secular behavior of the radio and X-ray emission from supernova remnants are examined and compared with the observations. Hot plasma and synchrotron models for the X-ray emission are considered. Among other things, it is concluded that (1) the total kinetic energy released in most supernova outbursts is probably less than about 10⁵¹ ergs; (2) continuous injection probably occurs for at least 10 yr in every case and about 1000 yr in most supernova remnants, in which case the supernova remnants 3C392, W28, Pup A and IC443 should produce 1–10 keV X-ray fluxes 10^–10 ergs/cm² sec; and (3) the X-ray sources in the Crab Nebula, Cas A and Tycho can be explained in terms of a model wherein continuous injection occurs for 300 yr for the Crab Nebula, much less than 250 yr for Cas A and much longer than 400 yr for Tycho. Finally, it is shown that if Tycho and Cas A contain an X-ray star such as NP0532, it is quite possible that the X-ray emission from those sources is predominantly due to the X-ray star.Supported by the Air Force Office of Scientific Research under Contract No. F44620-67-C-0065.     

A possible explanation of photon emission from supernova remnants by jitter radiation

We investigate a possibility that non-thermal X-ray emission in a supernova remnant(SNR) is produced by jitter radiation, which is the analogue of synchrotron radiation in small-scale random magnetic fields. We can fit the multi-wavelength data of SNRs RX J1713.7-3946 (G347.3-0.5) and RX J0852.0-4622 (G266.6-1.2) by constructing pure jitter and inverse Compton (IC) emission models. We find that the physical fit parameters of random magnetic fields take values of several tens of μG strength and of the order of ∼10⁷ cm correlation length. These properties of random magnetic fields in collisionless shock of SNRs are discussed.      

Near-infrared [Fe ii] emission from supernova remnants and the supernova rate of starburst galaxies

In an effort better to calibrate the supernova rate of starburst galaxies as determined from near-infrared [Fe  ii ] features, we report on a [Fe  ii ] λ 1.644 μm line-imaging survey of a sample of 42 optically selected supernova remnants (SNRs) in M33. A wide range of [Fe  ii ] luminosities are observed within our sample (from less than 6 to 695 L_⊙). Our data suggest that the bright [Fe  ii ] SNRs are entering the radiative phase and that the density of the local interstellar medium (ISM) largely controls the amount of [Fe  ii ] emission. We derive the following relation between the [Fe  ii ] λ 1.644 μm line luminosity of radiative SNRs and the electronic density of the post-shock gas, n _e: L _{[Fe  ii ]}     (cm⁻³). We also find a correlation in our data between L _{[Fe  ii ]} and the metallicity of the shock-heated gas, but the physical interpretation of this result remains inconclusive, as our data also show a correlation between the metallicity and n _e. The dramatically higher level of [Fe  ii ] emission from SNRs in the central regions of starburst galaxies is most likely due to their dense environments, although metallicity effects might also be important. The typical [Fe  ii ]-emitting lifetime of a SNR in the central regions of starburst galaxies is found to be of the order of 10⁴ yr. On the basis of these results, we provide a new empirical relation allowing the determination of the current supernova rate of starburst galaxies from their integrated near-infrared [Fe  ii ] luminosity.     

Barrel-like supernova remnants

The origin of cylindrically-symmetric supernova remnants is discussed. The results of numerical simulations of two most distinguished barrel-like remnants SNR 1006 and G296.5+10.0 are presented.     

Broad-band non-thermal emission from molecular clouds illuminated by cosmic rays from nearby supernova remnants

Molecular clouds are expected to emit non-thermal radiation due to cosmic ray interactions in the dense magnetized gas. Such emission is amplified if a cloud is located close to an accelerator of cosmic rays and if energetic particles can leave the accelerator site and diffusively reach the cloud. We consider here a situation in which a molecular cloud is located in the proximity of a supernova remnant which is efficiently accelerating cosmic rays and gradually releasing them in the interstellar medium. We calculate the multiwavelength spectrum from radio to gamma rays which is emerging from the cloud as the result of cosmic ray interactions. The total energy output is dominated by the gamma-ray emission, which can exceed the emission in other bands by an order of magnitude or more. This suggests that some of the unidentified TeV sources detected so far, with no obvious or very weak counterparts in other wavelengths, might be in fact associated with clouds illuminated by cosmic rays coming from a nearby source. Moreover, under certain conditions, the gamma-ray spectrum exhibits a concave shape, being steep at low energies and hard at high energies. This fact might have important implications for the studies of the spectral compatibility of GeV and TeV gamma-ray sources.     

Synchrotron X-ray emission from supernova remnants. Exponential cut-off in the electron spectrum

We present a model which describes the evolution of the energy spectrum of relativistic electrons in supernova remnants, with radiation losses of electrons taken into account. The model can be used to calculate the synchrotron X-ray emission from supernova remnants in the uniform interstellar medium and in the uniform interstellar magnetic field. The importance of various factors in the variations of spatial distributions of nonthermal electrons and their synchrotron emissive capacity is demonstrated. We analyze the errors which arise in the magnetic field strength when it is estimated with the use of the models which ignore the detailed pattern of the evolution of the magnetic field and the electron spectrum behind the shock front in the remnant. The evolution of synchrotron emission spectrum and the ratio between the synchrotron radio and X-ray fluxes from supernova remnants are calculated.     

Synchrotron and thermal X-ray emission from supernova remnants. Low radiation losses of electrons

A model is proposed for the nonthermal synchrotron emission from supernova remnants in the uniform interstellar medium. Some characteristics of nonthermal and thermal emission (luminosity and surface brightness distribution) are compared. The conditions when the nonthermal component can be prominent in the X-ray spectrum are specified. We point out some observational tests which will allow a number of parameters characterizing the cosmic ray injection on supernova remnant shocks to be estimated. The cases when electron radiation losses may be neglected are considered.     

A 95 GHz methanol emission survey toward eight small supernova remnants

We report on a 95 GHz(8_0-7_1A~+) methanol(CH_3OH) emission survey with the Purple Mountain Observatory Delingha 13.7 m telescope. Eight supernova remnants(SNRs) with angular size10′ were observed, but emission was only detected in three SNRs near the Galactic center(Sgr A East,G 0.1–0.1 and G 359.92–0.09). CH_3OH emission mainly surrounds the SNRs and can be decomposed into nine spatial peaks with the velocity range of eight peaks being(-30, 70) km s~(-1), and the other is(70, 120) km s~(-1). They are probably excited by interaction with these SNRs and adjacent molecular gas in the central molecular zone(CMZ), although star formation may play an important role in exciting CH_3OH emission in some regions of CMZ. We infer that tidal action is unlikely to be an excitation source for CH_3OH emission.     

The evolution of supernova remnants

The observational data analysis is based on the selection of 14 shell-type SNR, whose distances are quite reliable. Parameters of those SNR characterizing the values of the mean electron densityn _e, the change ofk ^2/7 H and velocity of expansionV are taken. The other parameters of SNR, such as energy of explosion, energy in magnetic field, and relativistic particles, and its increase during the expansion of SNR, are taken and analysed.     

X-ray spectro-imagery of supernova remnants

Supernovae and their remnants play an essential role in the Interstellar Medium as one of the main sources of mechanical energy and heavy element production and as cosmic-ray accelerators. X-ray spectro-imagery is a key approach to look at these issues, as the collision of high velocity ejecta with the ambient medium generates high temperatures of the order of ten million degrees. I present recent results from the XMM-Newton and Chandra X-ray satellites on the elemental composition of young supernova remnants, on the spatial distribution of these synthesized elements and on the constraints obtained on the efficiency of particle acceleration at the shocks. This revised version was published online in September 2006 with corrections to the Cover Date.     

TeV γ-rays from old supernova remnants

We study the emission from an old supernova remnant (SNR) with an age of around 10⁵ yr and that from a giant molecular cloud (GMC) encountered by the SNR. When the SNR age is around 10⁵ yr, proton acceleration is efficient enough to emit TeV γ-rays both at the shock of the SNR and that in the GMC. The maximum energy of primarily accelerated electrons is so small that TeV γ-rays and X-rays are dominated by hadronic processes,  π⁰  -decay and synchrotron radiation from secondary electrons, respectively. However, if the SNR is older than several 10⁵ yr, there are few high-energy particles emitting TeV γ-rays because of the energy-loss effect and/or the wave-damping effect occurring at low-velocity isothermal shocks. For old SNRs or SNR–GMC interacting systems capable of generating TeV γ-ray emitting particles, we calculated the ratio of TeV γ-ray (1–10 TeV) to X-ray (2–10 keV) energy flux and found that it can be more than  ∼10²  . Such a source showing large flux ratio may be a possible origin of recently discovered unidentified TeV sources.     

Iron needles in supernova remnants?

It has been suggested by Dwek that iron needles could explain the submillimetre emission from the Cas A supernova remnant (SNR) with only a very small total mass. We investigate whether a similar model holds for the Kepler SNR, and find that its emission could indeed be explained by a dust mass of less than  10⁻² M_⊙  , dependent on the axial ratio l / a of the needles – which we constrain to be less than 700. But the implied needle model for Kepler is inconsistent with that suggested for Cas A since either the needles would have to have a resistivity one or two orders of magnitude greater than those in Cas A or the electron density in Kepler's shocked plasma must be 40 times greater than suggested by X-ray observations. An additional problem with the needle model is that the implied thickness of the needles seems to be implausibly small, if the emission properties are calculated under the usual approximations.