arXiv subjects

L. T. Ksenofontov

Publications and source records attributed to L. T. Ksenofontov.

33 records · Page 2Linked to original sources

arXiv2008

Internal dynamics and particle acceleration in Tycho's SNR

The consequences of a newly suggested value for the SN explosion energy 1.2x10^{51} erg are explored for the case of Tycho's supernova remnant (SNR). A nonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs is employed to investigate the properties of Tycho's SNR and their correspondence to the existing experimental data. It is demonstrated that the large mean ratio between the radii of the contact discontinuity and the forward shock is consistent with the very effective acceleration of nuclear energetic particles at the forward shock. It is also argued that consistency of the value E_{sn} = 1.2x10^{51} erg with the gas dynamics, acceleration theory, and the existing gamma-ray measurements requires the source distance to be greater than 3.3 kpc. The corresponding ambient gas number density is lower than 0.4 cm^{-3}. Since the expected gamma-ray flux strongly depends on the source distance, F_γ\propto d^{-7}, a future experimental determination of the actual gamma-ray flux from Tycho's SNR will make it possible to determine the values of the source distance and of the mean ambient gas density. A simple inverse Compton model without a dominant population of nuclear CRs is not compatible with the present upper limit for the gamma-ray emission for any reasonable ambient interstellar B-field. Given the consistency between acceleration theory and overall, as well as internal, gas dynamics, a future gamma-ray detection would make the case for nuclear particle acceleration in Tycho's SNR incontrovertible in our view.

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arXiv2006

New evidence for strong nonthermal effects in Tycho's supernova remnant

For the case of Tycho's supernova remnant (SNR) we present the relation between the blast wave and contact discontinuity radii calculated within the nonlinear kinetic theory of cosmic ray (CR) acceleration in SNRs. It is demonstrated that these radii are confirmed by recently published Chandra measurements which show that the observed contact discontinuity radius is so close to the shock radius that it can only be explained by efficient CR acceleration which in turn makes the medium more compressible. Together with the recently determined new value $E_{sn}=1.2\times 10^{51}$ erg of the SN explosion energy this also confirms our previous conclusion that a TeV gamma-ray flux of $(2-5)\times 10^{-13}$ erg/(cm$^2$s) is to be expected from Tycho's SNR. Chandra measurements and the HEGRA upper limit of the TeV gamma-ray flux together limit the source distance $d$ to $3.3\leq d\leq 4$ kpc.

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arXiv2006

Magnetic field in supernova remnant SN 1987A

A nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants is employed to investigate the properties of the remnant SN 1987A. It is shown that a large downstream magnetic field ~10 mG is required to fit the existing observational data. Such a strong field together with the strong shock modification due to CR backreaction provides the steep and concave radioemission spectrum and considerable synchrotron cooling of high energy electrons which diminish their X-ray synchrotron flux below the observed Chandra flux which has to be considered as an upper limit for nonthermal X-ray emission. The expected gamma-ray energy flux at TeV-energies at the current epoch is 2x10^{-13} erg/(cm^2 s).

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arXiv2006

Gamma-ray emission expected from Kepler's SNR

Nonlinear kinetic theory of cosmic ray (CR) acceleration in supernova remnants (SNRs) is used to investigate the properties of Kepler's SNR and, in particular, to predict the gamma-ray spectrum expected from this SNR. Observations of the nonthermal radio and X-ray emission spectra as well as theoretical constraints for the total supernova (SN) explosion energy E_sn are used to constrain the astronomical and particle acceleration parameters of the system. Under the assumption that Kepler's SN is a type Ia SN we determine for any given explosion energy E_sn and source distance d the mass density of the ambient interstellar medium (ISM) from a fit to the observed SNR size and expansion speed. This makes it possible to make predictions for the expected gamma-ray flux. Exploring the expected distance range we find that for a typical explosion energy E_sn=10^51 erg the expected energy flux of TeV gamma-rays varies from 2x10^{-11} to 10^{-13} erg/(cm^2 s) when the distance changes from d=3.4 kpc to 7 kpc. In all cases the gamma-ray emission is dominated by π^0-decay gamma-rays due to nuclear CRs. Therefore Kepler's SNR represents a very promising target for instruments like H.E.S.S., CANGAROO and GLAST. A non-detection of gamma-rays would mean that the actual source distance is larger than 7 kpc.

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arXiv2005

Dependence of the gamma-ray emission from SN 1006 on the astronomical parameters

We use nonlinear kinetic theory to study the remnant dynamics and the particle acceleration as well as the properties of the nonthermal emission from the supernova remnant SN 1006. The known range of astronomical parameters is examined to determine whether it encompasses the existing synchrotron emission data. Given the present-day spatial extent and expansion rate of the object, it is shown that the hadronic gamma-ray flux is very sensitive to the ambient gas density N_H and that the existing H.E.S.S. upper limit requires N_H < 0.1 cm^{-3}. The strength of the amplified magnetic field downstream of the shock is about 150 μG.

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arXiv2005

Detection of Gamma-rays around 1TeV from RX J0852.0-4622 by CANGAROO-II

We have detected gamma-ray emission at the 6sigma level at energies greater than 500GeV from the supernova remnant RX J0852.0-4622 (G266.2-1.2) using the CANGAROO-II Imaging Atmospheric Cherenkov Telescope (IACT). The flux was 0.12 times of that of Crab at 1TeV. The signal centroid is consistent with the peak of the X-ray emission in the north-west rim of the remnant.

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arXiv2004

Magnetic Field Amplification in Tycho and other Shell-type Supernova Remnants

It is shown that amplification of the magnetic field in supernova remnants (SNRs) occurs in all six objects where morphological measurements are presently available in the hard X-ray continuum at several keV. For the three archetypical objects (SN 1006, Cas A and Tycho's SNR) to which nonlinear time-dependent acceleration theory has been successfully applied up to now, the global theoretical and the local observational field strengths agree very well, suggesting in addition that all young SNRs exhibit the amplification effect as a result of very efficient acceleration of nuclear cosmic rays (CRs) at the outer shock. Since this appears to be empirically the case, we may reverse the argument and consider field amplification as a measure of nuclear CR acceleration and it has indeed been argued that acceleration in the amplified fields allows the CR spectrum from SNRs to reach the knee in the spectrum or, in special objects, even beyond. The above results are furthermore used to investigate the time evolution of field amplification in young SNRs. Although the uncertainties in the data do not allow precise conclusions regarding this point, they rather clearly show that the ratio of the magnetic field energy density and the kinetic energy density of gas flow into the shock is of the order of a few percent if the shock speed is high enough V_s > 10^3 km/s, and this ratio remains nearly constant during the SNR evolution. The escape of the highest energy nuclear particles from their sources becomes progressively important with age, reducing also the cutoff in the π^0 -decay gamma-ray emission spectrum with time after the end of the sweep-up phase. Simultaneously the leptonic gamma-ray channels will gain in relative importance with increasing age of the sources.

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arXiv2004

Detection of Sub-TeV Gamma-Rays from the Galactic Center Direction by CANGAROO-II

We have detected sub-TeV gamma-ray emission from the direction of the Galactic Center (GC) using the CANGAROO-II Imaging Atmospheric Cherenkov Telescope (IACT). We detected a statistically signicant excess at energies greater than 250GeV. The flux was one order of magnitude lower than that of Crab at 1 TeV with a soft spectrum E^{-4.6+-0.5}. The signal centroid is consistent with the GC direction and the observed prole is consistent with a point-like source. Our data suggests that the GeV source 3EG J1746-2851 is identical with this TeV source and we study the combined spectra to determine the possible origin of the gamma-ray emission. We also obtain an upper limit on the cold dark-matter density in the Galactic halo.

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arXiv2003

Confirmation of strong magnetic field amplification and nuclear cosmic ray acceleration in SN 1006

It is shown that the nonlinear kinetic model of cosmic ray (CR) acceleration in supernova remnants (SNRs) fits the shell-type nonthermal X-ray morphology, obtained in Chandra observations, in a satisfactory way. The set of empirical parameters is the same which reproduces the dynamical properties of the SNR and the spectral characteristics of the emission produced by CRs. The extremely small spatial scales of the observed X-ray distribution are due to the large effective magnetic field $B_{\mathrm d}\sim 100$ $μ$G in the interior, which is also required to give a good fit for the spatially integrated radio and X-ray synchrotron spectra. The only reasonably thinkable condition for the production of such a large effective field strength is an efficiently accelerated nuclear CR component. Therefore the Chandra data confirm the inference that SN 1006 indeed accelerates nuclear CRs with the high efficiency required for SNRs to be considered as the main Galactic CR sources.

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arXiv2003

Variation of cosmic ray injection across supernova shocks

The injection rate of suprathermal protons into the diffusive shock acceleration process should vary strongly over the surface of supernova remnant shocks. These variations and the absolute value of the injection rate are investigated. In the simplest case, like for SN 1006, the shock can be approximated as being spherical in a uniform large-scale magnetic field. The injection rate depends strongly on the shock obliquity and diminishes as the angle between the ambient field and the shock normal increases. Therefore efficient particle injection, which leads to conversion of a significant fraction of the kinetic energy at a shock surface element, arises only in relatively small regions near the "poles", reducing the overall CR production. The sizes of these regions depend strongly on the random background field and the Alfven wave turbulence generated due to CR streaming instability. For the cases of SN 1006 and Tycho's SNR they correspond to about 20, and for Cas A to between 10 and 20 percent of the entire shock surface. In first approximation, the CR production rate, calculated under the assumption of spherical symmetry, has therefore to be renormalized by this factor, while the shock as such remains roughly spherical.

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arXiv2003

Cosmic ray production in supernova remnants including reacceleration: the secondary to primary ratio

We study the production of cosmic rays (CRs) in supernova remnants (SNRs), including the reacceleration of background galactic cosmic rays (GCRs) - thus refining the early considerations by Blandford & Ostriker (1980) and Wandel et al. (1987) - and the effects of the nuclear spallation inside the sources (the SNRs). This combines for the first time nuclear spallation inside CR sources and in the diffuse interstellar medium, as well as reacceleration, with the injection and subsequent acceleration of suprathermal particles from the postshock thermal pool. Selfconsistent CR spectra are calculated on the basis of the nonlinear kinetic model. It is shown that GCR reacceleration and CR spallation produce a measurable effect at high energies, especially in the secondary to primary (s/p) ratio, making its energy-dependence substantially flatter than predicted by the standard model. Quantitatively, the effect depends strongly upon the density of the surrounding circumstellar matter. GCR reacceleration dominates secondary CR production for a low circumstellar density. It increases the expected s/p ratio substantially and flattens its spectrum to an almost energy-independent form for energies larger than 100 GeV/n if the supernovae explode on average into a hot dilute medium with hydrogen number density $N_H=0.003$cm$^{-3}$. The contribution of CR spallation inside SNRs to the s/p ratio increases with increasing circumstellar density and becomes dominant for $N_H\gsim 1$ cm$^{-3}$, leading at high energies to a flat s/p ratio which is only by a factor of three lower than in the case of the hot medium. Measurements of the boron to carbon ratio at energies above 100 GeV/n could be used in comparison with the values predicted here as a consistency test for the supernova origin of the GCRs.

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arXiv2003

Search for TeV gamma-rays from SN 1987A in 2001

We searched for TeV gamma-rays from the remnant of SN 1987A around 5400 days after the supernova. The observations were carried out in 2001, from November 16 to December 11, using the CANGAROO-II Imaging Atmospheric Cherenkov Telescope. In total, 708 minutes of ON- and 1019 minutes of OFF-source data were obtained under good conditions. The detection threshold was estimated to be 1 TeV, due to the mean zenith angle of 39$^\circ$. The upper limits for the gamma-ray flux were obtained and compared with the previous observations and theoretical models. The observations indicate that the gamma-ray luminosity is lower than $1\times 10^{37}$ erg s$^{-1}$ at $\sim 10$ TeV.

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arXiv2002

The high energy gamma-ray emission expected from Tycho's supernova remnant

A nonlinear kinetic model of cosmic ray (CR) acceleration in supernova remnants (SNRs) is used to describe the properties of Tycho's SNR. Observations of the expansion characteristics and of the nonthermal radio and X-ray emission spectra, assumed to be of synchrotron origin, are used to constrain the overall dynamical evolution and the particle acceleration parameters of the system, in addition to what is known from independent estimates of the distance and thermal X-ray observations. It is shown that a very efficient production of nuclear cosmic rays, leading to strong shock modification, and a large downstream magnetic field strength B_d approx 240muG are required to reproduce the observed synchrotron emission from radio to X-ray frequencies. This field strength is still well within the upper bound for the effective magnetic field, consistent with the acceleration process. The pi^0-decay gamma-ray flux turns out to be somewhat greater than the inverse Compton (IC) flux off the Cosmic Microwave Background at energies below 1 TeV, dominating it strongly at 10 TeV. The predicted TeV gamma-ray flux is consistent with but close to the very low upper limit recently obtained by HEGRA. A future detection at E_gamma ~ 10 TeV would clearly indicate hadronic emission.

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arXiv2002

Emission of SN 1006 produced by accelerated cosmic rays

The nonlinear kinetic model of cosmic ray (CR) acceleration in supernova remnants (SNRs) is used to describe the properties of the remnant of SN 1006. It is shown, that the theory fits the existing data in a satisfactory way within a set of parameters which is consistent with the idea that SN 1006 is a typical source of Galactic CR nucleons, although not necessarily of CR electrons. The adjusted parameters are those that are not very well determined by present theory or not directly amenable to astronomical observations. The calculated expansion law and the radio-, X-ray and gamma-ray emissions produced by the accelerated CRs in SN 1006 agree quite well with the observations. A rather large interior magnetic field B_d\approx 100 \muG is required to give a good fit for the radio and X-ray synchrotron emission. In the predicted TeV gamma-ray flux from SN 1006, the pi^0-decay gamma-rays, generated by the nuclear CR component, dominate over the inverse Compton (IC) gamma-rays, generated by the CR electrons in the cosmic microwave background. The predicted source morphology in high energy gamma-rays roughly corresponds to that of the synchrotron emission. The predicted integral gamma-ray flux F \propto e^-1 extends up to energies 100 TeV if CR diffusion is as strong as the Bohm limit. Only if the interior magnetic field is much lower in the SNR, B_d\approx 10 \muG, then the observed gamma-ray emission can be due to the accelerated electron component alone. In this case, not plausible physically in our view, the lowest permissible value of the electron to proton ratio is rather high, and the maximum individual energy and total energy content of accelerated nucleons so small, that SN 1006 can not be considered as a typical source of the nuclear Galactic CRs.

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arXiv1997

Spectrum and Chemical Composition of CRs Accelerated in SNRs

Spectrum and chemical composition of cosmic rays accelerated in supernova remnants are studied on a basis of kinetic approach. The cosmic ray transport equation with the Bohm diffusion coefficient has been numerically solved self-consistently with gas dynamic equations for the underlying flow. Comparison with observational results gives some indications that galactic cosmic rays are produced by supernovae shocks which expanded in the relatively low temperature partially ionized interstellar medium.

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