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1.
The synchrotron radio maps of supernova remnants (SNRs) in a uniform interstellar medium and interstellar magnetic field (ISMF) are analysed, allowing for different 'sensitivity' of the injection efficiency to the obliquity of the shock. The very-high-energy γ-ray maps arising from inverse Compton processes are also synthesized. The properties of images in these different wavelength bands are compared, with particular emphasis on the location of the bright limbs in bilateral SNRs. Recent High-Energy Stereoscopic System (HESS) observations of SN 1006 show that the radio and inverse Compton γ-ray limbs coincide, and we found that this may happen if (i) injection is isotropic but the variation of the maximum energy of electrons is rather fast to compensate for differences in the magnetic field, or (ii) the obliquity dependence of injection (either quasi-parallel or quasi-perpendicular) and the electron maximum energy are strong enough to dominate the magnetic field variation. In the latter case, the obliquity dependences of the injection and the maximum energy should not be opposite. We argue that the position of the limbs alone, and even their coincidence in radio, X-rays and γ-rays, as discovered by HESS in SN 1006, cannot be conclusive as regards the dependence of the electron injection efficiency, the compression/amplification of the ISMF and the electron maximum energy on the obliquity angle.  相似文献   

2.
Measurement sensitivity in the energetic γ-ray region has improved considerably and is about to increase further in the near future, motivating a detailed calculation of high-energy (HE; ≥100 MeV) and very high-energy (VHE; ≥100 GeV) γ-ray emission from the nearby starburst galaxy NGC 253. Adopting the convection–diffusion model for energetic electron and proton propagation, and accounting for all the relevant hadronic and leptonic processes, we determine the steady-state energy distributions of these particles by a detailed numerical treatment. The electron distribution is directly normalized by the measured synchrotron radio emission from the central starburst region; a commonly expected theoretical relation is then used to normalize the proton spectrum in this region. Doing so fully specifies the electron spectrum throughout the galactic disc and, with an assumed spatial profile of the magnetic field, the predicted radio emission from the full disc matches well the observed spectrum, confirming the validity of our treatment. The resulting radiative yields of both particles are calculated; the integrated HE and VHE fluxes from the entire disc are predicted to be   f (≥100 MeV) ≃ (1.8+1.5−0.8) × 10−8 cm−2 s−1  and   f (≥100 GeV) ≃ (3.6+3.4−1.7) × 10−12 cm−2 s−1  , with a central magnetic field value   B 0≃ 190 ± 10 μ  G. We discuss the feasibility of measuring emission at these levels with the space-borne Fermi and ground-based Cherenkov telescopes.  相似文献   

3.
The presence of dust at high redshift requires efficient condensation of grains in supernova (SN) ejecta, in accordance with current theoretical models. Yet observations of the few well-studied supernovae (SNe) and supernova remnants (SNRs) imply condensation efficiencies which are about two orders of magnitude smaller. Motivated by this tension, we have (i) revisited the model of Todini & Ferrara for dust formation in the ejecta of core collapse SNe, and (ii) followed, for the first time, the evolution of newly condensed grains from the time of formation to their survival – through the passage of the reverse shock – in the SNR. We find that  0.1–0.6  M⊙  of dust form in the ejecta of 12–40 M⊙ stellar progenitors. Depending on the density of the surrounding interstellar medium, between 2 and 20 per cent of the initial dust mass survives the passage of the reverse shock, on time-scales of about  4–8 × 104  yr  from the stellar explosion. Sputtering by the hot gas induces a shift of the dust size distribution towards smaller grains. The resulting dust extinction curve shows a good agreement with that derived by observations of a reddened QSO at   z = 6.2  . Stochastic heating of small grains leads to a wide distribution of dust temperatures. This supports the idea that large amounts (∼0.1 M⊙) of cold dust  ( T ∼ 40   K)  can be present in SNRs, without being in conflict with the observed infrared emission.  相似文献   

4.
We present a new determination of the local volume-averaged star formation rate from the 1.4-GHz luminosity function of star forming galaxies. Our sample, taken from the   B ≤12  Revised Shapley–Ames catalogue (231 normal spiral galaxies over an effective area of 7.1 sr) has ≃100 per cent complete radio detections and is insensitive to dust obscuration and cirrus contamination. After removal of known active galaxies, the best-fitting Schechter function has a faint-end slope of  −1.27±0.07  in agreement with the local H α luminosity function, characteristic luminosity   L ∗=(2.6±0.7)×1022 W Hz−1  and density   φ ∗=(4.8±1.1)×10−4 Mpc−3.  The inferred local radio luminosity density of  (1.73±0.37±0.03)×1019 W Hz−1 Mpc−3  (Poisson noise, large-scale structure fluctuations) implies a volume-averaged star formation rate ∼2 times larger than the Gallego et al. H α estimate, i.e.   ρ 1.4 GHz=(2.10±0.45±0.04)×10−2 M⊙ yr−1 Mpc−3  for a Salpeter initial mass function from  0.1–125 M⊙  and Hubble constant of 50 km s−1 Mpc−1. We demonstrate that the Balmer decrement is a highly unreliable extinction estimator, and argue that optical–ultraviolet (UV) star formation rates (SFRs) are easily underestimated, particularly at high redshift.  相似文献   

5.
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.  相似文献   

6.
Using 2D magnetohydrodynamic (MHD) numerical simulations performed with two different finite-difference Eulerian codes, we analyse the effect that a toroidal magnetic field has on low-mass planet migration in non-turbulent protoplanetary discs. The presence of the magnetic field modifies the waves that can propagate in the disc. In agreement with a recent linear analysis, we find that two magnetic resonances develop on both sides of the planet orbit, which contribute to a significant global torque. In order to measure the torque exerted by the disc on the planet, we perform simulations in which the latter is either fixed on a circular orbit or allowed to migrate. For a     planet, when the ratio β between the square of the sound speed and that of the Alfven speed at the location of the planet is equal to 2, we find inward migration when the magnetic field   B φ  is uniform in the disc, reduced migration when   B φ  decreases as   r −1  and outward migration when   B φ  decreases as   r −2  . These results are in agreement with predictions from the linear analysis. Taken as a whole, our results confirm that even a subthermal stable field can stop inward migration of an earth-like planet.  相似文献   

7.
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.  相似文献   

8.
We present the results of a 22.5 ks pointed ROSAT PSPC observation of the 3.4-h period eclipsing polar MN Hya (RX J0929.1−2404). The X-ray light curve exhibits a 'double-humped' shape, with a secondary minimum occuring at φ∼ 0.45, a morphology consistent with two-pole accretion. Strong aperiodic flaring activity, with flux enhancements of ∼ 6 × the quiescent level, is also observed. A pre-eclipse 'dip' occurs in the phase interval φ= 0.87–0.95 with the X-rays becoming harder, indicative of photoelectric absorption by the pre-shock flow. There is also evidence of a secondary spectrally hard 'dip' near φ = 0.45–0.55, which might be associated with a second accretion stream flowing to the other magnetic pole.   The X-ray spectrum is best represented by a combination of a ∼50 eV blackbody and a thermal bremsstrahlung component of kT 1.6 keV, with a total absorption column of N H  = 2.9 × 1020 cm−2.   The primary maximum (φ∼ 0.65) has a slightly larger column and normalization compared to the secondary maximum. Although there are few photons, the dip spectrum is very flat in comparison to other phases, and is best represented by a single bremsstrahlung component. This is indicative of the spectral hardening seen in the light curves attributed to photoabsorption. The ratio of unabsorbed bremsstrahlung and blackbody luminosities is ∼ 0.1 for the best-fitting average spectral models. This implies a magnetic field strength  30 MG on the basis of the empirical L hard/ L soft −  B relationships, although consideration of the cyclotron flux and aspect effects could allow for an even higher field (55 MG).  相似文献   

9.
Chandra X-ray Observatory observations of the powerful, peculiar radio galaxy 3C 123 have resulted in an X-ray detection of the bright eastern hotspot, with a 1-keV flux density of ∼5 nJy. The X-ray flux and spectrum of the hotspot are consistent with the X-rays being inverse-Compton scattering of radio synchrotron photons by the population of electrons responsible for the radio emission ('synchrotron self-Compton emission') if the magnetic fields in the hotspot are close to their equipartition values. 3C 123 is thus the third radio galaxy to show X-ray emission from a hotspot which is consistent with being in equipartition. Chandra also detects emission from a moderately rich cluster surrounding 3C 123, with L X(2–10 keV)=2×1044 erg s−1 and kT ∼5 keV, and absorbed emission from the active nucleus, with an inferred intrinsic column density of 1.7×1022 cm−2 and an intrinsic 2–10 keV luminosity of 1044 erg s−1.  相似文献   

10.
We present X-ray, broad-band optical and low-frequency radio observations of the bright type IIP supernova SN 2004et. The Chandra X-ray Observatory observed the supernova at three epochs, and the optical coverage spans a period of ∼470 d since explosion. The X-ray emission softens with time, and we characterize the X-ray luminosity evolution as   L X∝ t −0.4  . We use the observed X-ray luminosity to estimate a mass-loss rate for the progenitor star of  ∼2 × 10−6 M⊙ yr−1  . The optical light curve shows a pronounced plateau lasting for about 110 d. Temporal evolution of photospheric radius and colour temperature during the plateau phase is determined by making blackbody fits. We estimate the ejected mass of 56Ni to be  0.06 ± 0.03 M⊙  . Using the expressions of Litvinova & Nadëzhin we estimate an explosion energy of  (0.98 ± 0.25) × 1051 erg  . We also present a single epoch radio observation of SN 2004et. We compare this with the predictions of the model proposed by Chevalier, Fransson & Nymark. These multiwavelength studies suggest a main-sequence progenitor mass of  ∼20 M⊙  for SN 2004et.  相似文献   

11.
We investigate the brightest regions of the kpc-scale jet in the powerful radio galaxy 3C 346, using new optical Hubble Space Telescope ( HST ) ACS/F606W polarimetry together with Chandra X-ray data and 14.9 and 22.5 GHz Very Large Array (VLA) radio polarimetry. The jet shows a close correspondence between optical and radio morphology, while the X-ray emission shows a  0.80 ± 0.17 kpc  offset from the optical and radio peak positions. Optical and radio polarimetry show the same apparent magnetic field position angle and fractional polarization at the brightest knot, where the jet undergoes a large kink of almost 70° in the optical and radio images. The apparent field direction here is well aligned with the new jet direction, as predicted by earlier work that suggested the kink was the result of an oblique shock. We have explored models of the polarization from oblique shocks to understand the geometry of the 3C 346 jet, and find that the upstream flow is likely to be highly relativistic  (βu= 0.91+0.05−0.07)  , where the plane of the shock front is inclined at an angle of  η= 51°± 11°  to the upstream flow which is at an angle  θ= 14+8−7  deg to our line of sight. The actual deflection angle of the jet in this case is only 22°.  相似文献   

12.
We examine the status of various dark energy models in light of the recently observed SN 1997ff at   z ≈1.7  . The modified data still fit a pure cosmological constant Λ or a quintessence with an equation of state similar to that of Λ. The kinematical Λ models,  Λ∼ S -2  and  Λ∼ H 2  , also fit the data reasonably well and require less dark energy density (hence more matter energy density) than is required by the constant Λ model. However, the model  Λ∼ S -2  with low energy density becomes unphysical as it cannot accommodate higher redshift objects.
We also examine an alternative explanation of the data, namely the absorption by the intervening whisker-like dust, and find that the quasi-steady state (QSS) model and the Friedmann–Robertson–Walker (FRW) model  Ωm0=0.33  without any dark energy also fit the data reasonably well.
We notice that the addition of SN 1997ff to the old data has worsened the fit to most of the models, except a closed FRW model with a constant Λ and a closed quintessence model with   ω φ =-0.82  , and the models have started departing from each other as we go above   z =1  . However, to make a clear discrimination possible, a few more supernovae with   z >1  are required.
We have also calculated the age of the Universe in these models and find that, in the models with a constant Λ, the expansion age is uncomfortably close to the age of the globular clusters. Quintessence models show even lower age. The kinematical Λ models are, however, interesting in this connection (especially the model  Λ∼ H 2)  , as they give a remarkably large age of the Universe.  相似文献   

13.
Synchrotron X-ray emission components were recently detected in many young supernova remnants (SNRs). There is even an emerging class – SN 1006, RX J1713.72−3946, Vela Jr and others – that is dominated by non-thermal emission in X-rays, also probably of synchrotron origin. Such emission results from electrons/positrons accelerated well above TeV energies in the spectral cut-off regime. In the case of diffusive shock acceleration, which is the most promising acceleration mechanism in SNRs, very strong magnetic fluctuations with amplitudes well above the mean magnetic field must be present. Starting from such a fluctuating field, we have simulated images of polarized X-ray emission of SNR shells and show that these are highly clumpy with high polarizations up to 50 per cent. Another distinct characteristic of this emission is the strong intermittency, resulting from the fluctuating field amplifications. The details of this 'twinkling' polarized X-ray emission of SNRs depend strongly on the magnetic field fluctuation spectra, providing a potentially sensitive diagnostic tool. We demonstrate that the predicted characteristics can be studied with instruments that are currently being considered. These can give unique information on magnetic field characteristics and high-energy particle acceleration in SNRs.  相似文献   

14.
A model for the angular momentum transfer within the convection zone of a rapidly rotating star is introduced and applied to the analysis of recent observations of temporal fluctuations of the differential rotation on the young late-type stars AB Doradus (AB Dor) and LQ Hydrae (LQ Hya). Under the hypothesis that the mean magnetic field produced by the stellar dynamo rules the angular momentum exchanges and that the angular velocity depends only on the distance s from the rotation axis and the time, the minimum azimuthal Maxwell stress  | BsB φ|  , averaged over the convection zone, is found to range from ∼0.04 to  ∼0.14 T2  . If the poloidal mean magnetic field   B s   is of the order of 0.01 T, as indicated by the Zeeman–Doppler imaging maps of those stars, then the azimuthal mean field   B φ  can reach an intensity of several teslas, which significantly exceeds equipartition with the turbulent kinetic energy. Such strong fields can account also for the orbital period modulation observed in cataclysmic variables and RS Canum Venaticorum systems with a main-sequence secondary component. Moreover, the model allows us to compute the kinetic energy dissipation rate during the maintenance of the differential rotation. Only in the case of the largest surface shear observed on LQ Hya may the dissipated power exceed the stellar luminosity, but the lack of a sufficient statistic on the occurrence of such episodes of large shear does not allow us to estimate their impact on the energy budget of the convection zone.  相似文献   

15.
The cluster 3C 129 is classified as a rich cluster. An analysis of the properties of the cluster 3C 129 from ROSAT PSPC and HRI, Einstein IPC, and EXOSAT ME observations is presented. The mean temperature from a joint fit of the ROSAT PSPC and EXOSAT ME data is 5.5(±0.2) keV. The luminosity is 0.6×1044 erg s−1 in 0.2–2.4 keV and 2.7×1044 erg s−1 in 0.2–10 keV. We find a cooling flow with a rate of ∼84 M⊙ yr−1. The central gas density is 6×10−3 cm−3, and the ICM mass is 3.6×1013 M⊙. The total cluster mass is ∼5×1014 M⊙. The X-ray morphology shows an east–west elongation, which is evidence for a recent merger event. The radio source 3C 129.1 is located near the X-ray centre. Another cluster member galaxy (the radio galaxy 3C 129) is a prototype of head-tailed radio galaxies, and is located in the west part of the cluster. The tail points along the gradient of intracluster gas pressure. There are no significant point X-ray sources associated with the AGNs of the two radio galaxies.  相似文献   

16.
We present measurements of the clustering properties of galaxies in the field of redshift range 0.5 ≲ z ≲ 1.5 Ultra Steep Spectrum radio sources selected from the Sydney University Molonglo Sky Survey and the National Radio Astronomy Observatories Very Large Array Sky Survey. Galaxies in these USS fields were identified in deep near-infrared observations, complete down to   K s= 20  , using the IRIS2 instrument at the Anglo-Australian Telescope. We used the redshift distribution of   K s < 20  galaxies taken from Cimatti et al. (2002) to constrain the correlation length r 0. We find a strong correlation signal of galaxies with   K s < 20  around our USS sample. A comoving correlation length   r 0= 14.0 ± 2.8  h −1 Mpc  and γ= 1.98 ± 0.15 are derived in a flat cosmological model universe.
We compare our findings with those obtained in a cosmological N -body simulation populated with galform semi-analytic galaxies. We find that clusters of galaxies with masses in the range   M = 1013.4–14.2  h −1 M⊙  have a cluster–galaxy cross-correlation amplitude comparable to those found between the USS hosts and galaxies. These results suggest that distant radio galaxies are excellent tracers of galaxy overdensities and pinpoint the progenitors of present day rich clusters of galaxies.  相似文献   

17.
We have observed the   z =0.78  cluster MS 1137.5+6625 with the Ryle Telescope (RT) at 15 GHz. After subtraction of contaminating radio sources in the field, we find a Sunyaev–Zel'dovich flux decrement of  -421±60 μJy  on the ≈0.65 k λ baseline of the RT, spatially coincident with the optical and X-ray positions for the cluster core.
For a spherical King-profile cluster model, the best fit to our flux measurement has a core radius   θ C=20 arcsec  , consistent with previous X-ray observations, and a central temperature decrement  Δ T =650±92 μK  .
Using this model, we calculate that the cluster has a gas mass inside a     radius of  2.9×1013 M⊙  for an  Ω M =1  universe and  1.6×1013 M⊙  for  Ω M =0.3  ,  ΩΛ=0.7  . We compare this model with existing measurements of the total mass of the cluster, based on gravitational lensing, and estimate a gas fraction for MS 1137.5+6625 of ≈8 per cent.  相似文献   

18.
The theory of polar magnetic burial in accreting neutron stars predicts that a mountain of accreted material accumulates at the magnetic poles of the star, and that, as the mountain spreads equatorward, it is confined by, and compresses, the equatorial magnetic field. Here, we extend previous, axisymmetric, Grad–Shafranov calculations of the hydromagnetic structure of a magnetic mountain up to accreted masses as high as   M a= 6 × 10−4 M⊙  , by importing the output from previous calculations (which were limited by numerical problems and the formation of closed bubbles to   M a < 10−4 M⊙  ) into the time-dependent, ideal-magnetohydrodynamic code zeus-3d and loading additional mass on to the star dynamically. The rise of buoyant magnetic bubbles through the accreted layer is observed in these experiments. We also investigate the stability of the resulting hydromagnetic equilibria by perturbing them in zeus-3d . Surprisingly, it is observed that the equilibria are marginally stable for all   M a≤ 6 × 10−4 M⊙  ; the mountain oscillates persistently when perturbed, in a combination of Alfvén and acoustic modes, without appreciable damping or growth, and is therefore not disrupted (apart from a transient Parker instability initially, which expels <1 per cent of the mass and magnetic flux).  相似文献   

19.
Using recently published estimates — based on high spatial resolution spectroscopy — of the mass M BH of nuclear black holes for a sample of nearby galaxies, we explore the dependence of galaxy nucleus emissivity at various wavelengths on M BH. We confirm an almost linear scaling of the black hole mass with the baryonic mass of the host spheroidal galaxy. A remarkably tight relationship is also found with both nuclear and total radio centimetric flux, with a very steep dependence of the radio flux on M BH ( P  ∝  M 2.5BH). The high-frequency radio power is thus a very good tracer of a supermassive black hole, and a good estimator of its mass. This, together with the lack of significant correlations with the low-energy X-ray and far-IR flux, supports the view that advection-dominated accretion is ruling the energy output in the low accretion rate regime. Using the tight dependence of total radio power on M BH and the rich statistics of radio emission of galaxies, we derive an estimate of the mass function of remnants in the nearby Universe. This is compared with current models of quasar and active galactic nucleus (AGN) activity and of the origin of the hard X-ray background (HXRB). As for the former, continuous long-lived AGN activity is excluded by the present data with high significance, whereas the assumption of a short-lived, possibly recurrent, activity pattern gives remarkable agreement. The presently estimated black hole mass function also implies that the HXRB has been produced by a numerous population (∼ 10−2 Mpc−3) of moderately massive ( M BH ∼ 107 M⊙) black holes.  相似文献   

20.
The problem of the efficiency of particle acceleration for a paraboloidal poloidal magnetic field is considered within the approach of steady axisymmetric magnetohydrodynamic (MHD) flow. For the large Michel magnetization parameter σ it is possible to linearize the stream equation near the force-free solution and to solve the problem self-consistently as was done by Beskin, Kuznetsova & Rafikov for a monopole magnetic field. It is shown that, on the fast magnetosonic surface (FMS), the particle Lorentz factor γ does not exceed the standard value  σ1/3  . On the other hand, in the supersonic region, the Lorentz factor grows with the distance z from the equatorial plane as  γ≈ ( z / R L)1/2  up to the distance   z ≈σ2 R L  , where   R L= c /ΩF  is the radius of the light cylinder. Thus, the maximal Lorentz factor is  γmax≈σ  , which corresponds to almost the full conversion of the Poynting energy flux into the particle kinetic one.  相似文献   

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